In a time when all-ceramic systems are touted for their esthet- ics, it is not unrealistic to ask: Why do we continue to teach dental students and train dental laboratory technicians how to fabricate metal-ceramic restorations? Isn’t it fair to say that more time is needed to master the techniques required to achieve a high level of esthetics with metal-ceramic tech- nology compared to all-ceramic technology? The answers to these questions may rest with the fact that when well planned and executed, the rewards for using metal-ceramic technology are restorations with a track record of success spanning decades. Of course, such longevity is attainable and should only be expected when clinicians and dental laboratory technicians under- stand their respective responsibilities and partner well in the process.
Therefore, this chapter has been added for the benefit of the emerging generations of dentists and dental labora- tory technicians who otherwise might not be aware of how the addition of a porcelain margin, in the hands of skilled clinicians and ceramists, can result in metal-ceramic resto- rations that are not only durable but esthetic. In the process, today’s clinicians can expand their armamentarium and select different types of restorations, be they all-ceramic or metal-ceramic, to meet the unique functional demands of each patient situation without compromising their esthet- ics expectations.
Clinical Performance of Metal-Ceramic Restorations
In dentistry, long-term studies (those greater than 10 years) dealing with the clinical performance of various materials or restorations shed much-needed light on the survival rates and longevity of treatment outcomes. Reports of 5-year survival rates for metal-ceramic restorations are meaning- ful, but analyses spanning longer time periods hold even greater significance for dentists, dental laboratory techni- cians, and patients alike.
In 2013, Walton published a report on the survival and clinical performance of 2,340 high gold metal-ceramic sin- gle crowns at 10 years (97.1%) and up to 25 years (85.4%) provided for 670 patients in his private prosthodontic prac- tice.1 Biologic rather than technical factors accounted for the majority of the 133 reported failures (5.7% over- all failure rate) with patient complaints of “unaccept- able esthetics” attributed to just 22 (0.9%) crowns of the 2,340 metal-ceramic restorations placed between 1984 and 2008.1 Readers who question the place of metal-ceramic technology in a contemporary dental practice may find this report of interest.
Metal-Ceramic Versus All-Ceramic Restoration
Less than ideal esthetic results are not unique to metal- ceramic restorations. In reality, they can be seen when the best of all-ceramic systems are used (Fig 10-1). Why is that, and what factors come into play with the different ceramic materials that might influence the appearance of a particu- lar product for different clinical applications? It is likely that several factors are at work that influence the final outcome.
Important aspects of care
When a patient presents for treatment or retreatment, it is important to analyze the existing situation and devise a strategy for success that addresses the patient’s chief complaint and avoids a repeat of past failures or limitations (Fig 10-2). One way to organize that strategy, before pro- ceeding directly to actual patient treatment, is to examine three aspects of care: (1) treatment planning, (2) execution of clinical procedures, and (3) dental laboratory outcomes. The first two variables are the responsibility of the dentist, while the third is a reflection of the skill and talent of the dental laboratory technician working within any constraints posed by the first two variables.
Treatment planning
During an examination of defective or unesthetic metal- ceramic restorations, the initial response may be to criticize the choice of restorative materials rather than to step back for a moment and objectively critique the clinical and labora- tory execution (see Fig 10-2a). For example, after a compre- hensive examination, it may become apparent that the poor result could have been avoided with an alternate treatment approach. Perhaps a patient’s occlusion, interocclusal space limitations, periodontal health, oral hygiene, orthodontic sta- tus, or endodontic needs were not evaluated and managed appropriately. After all, proper treatment planning is essential to avoid an unattractive metal-ceramic crown that is too long, too wide, and too bulky facially, or to avoid the need for post- operative appointments to adjust premature occlusal contacts that were not identified initially (see Fig 10-2a).
All too frequently, if a definitive restoration’s overall appearance does not blend with the adjacent natural teeth or existing restorations, it is likely to become a source of complaint for patients. In other words, should the treat- ment planning process itself be lacking, there is nothing to prevent a new restoration from reflecting those clinical flaws left unaddressed or undiagnosed prior to the com- pletion of care. It may be fair to say that compared to cur- rently available all-ceramic systems, a metal-ceramic res- toration requires more attention to clinical detail and a higher degree of technical skill in the fabrication process, even when all the requisite clinical procedures are per- formed well. Either way, the treatment-planning phase is the time to determine the type of restoration to recommend (ie, metal-ceramic or all-ceramic).
Execution of clinical procedures
A frequent challenge facing dental laboratory technicians is management of a case where the tooth to be restored has not been properly prepared for the intended resto- ration. There may be an inadequate amount of tooth struc- ture removed from the facial and lingual surfaces; a lack of two-plane tooth reduction; insufficient incisal or occlusal tooth reduction for the chosen restorative materials to allow the re-creation of adequate primary and secondary tooth morphology; or tooth finish lines that are irregular, unread- able, and/or appear to violate the biologic width. Perhaps the definitive impression does not adequately capture the finish line, contains defects (ie, voids) in critical areas, fails to include enough of the surrounding dentition to permit proper articulation of the master cast, or there are indica- tions it is distorted. Should the opposing cast be derived from a hastily mixed irreversible hydrocolloid (ie, alginate) impression that was not processed in a timely manner, then even accurate jaw relation records will not permit the estab- lishment of the correct occlusal relationship between the master cast and that opposing cast.
In instances where several of these clinical variables converge, you have a situation in which any expectations for an ideal esthetic outcome are unlikely to be realized, regardless of the type of restorative system involved. If these deficiencies are not identified and corrected prior to fabricating the definitive restoration, how could one expect any type of restoration to overcome them?
Patients need to be made aware at the initial visit that, should some aspect of their treatment be unsatisfactory or less than ideal, they should mention their concerns prior to cementation of a definitive restoration (see Fig 10-2a). After all, the form, shade, and overall appearance of a crown will not improve over time. Likewise, the shade of the ceramic does not lighten or darken intraorally like natural teeth. Cer- tainly, coronal tooth structure can become more exposed over time if gingival recession occurs due to violation of the biologic width, mechanical abrasion, or a poorly contoured restoration. In other words, the variables or conditions that tend to make a restoration unesthetic, or less than ideal, are likely present and readily identifiable at the try-in appoint- ment. Then it begs the question of whether the respon- sibility for esthetic failure rests with the restorative mate- rials used or with the clinical and laboratory procedures required to produce that restoration.
Clinicians who were not involved in the initial treatment and only later evaluated restorations may inadvertently attri- bute esthetic failures to limitations in metal-ceramic materi- als rather than flawed treatment planning and/or less than ideal clinical execution and management. Therefore, simply switching to an all-ceramic restoration may not be the answer and will not always result in an improved outcome, unless any underlying problems are diagnosed and addressed during retreatment. Suffice it to say that once treatment or retreatment is initiated, it is important to adhere to the tooth preparation guidelines for the restorative system selected, make an accurate definitive impression, and obtain a qual- ity master cast for the dental laboratory (Fig 10-3).
Dental laboratory outcomes
For many clinical situations, a dental laboratory tech- nician with a modest amount of training and experience may find it easier to produce a very acceptable defini- tive all-ceramic restoration as compared to a metal-ceramic restoration for the same tooth. The fabrication processes for many of the newer all-ceramic materials vary, but gen- erally speaking, they can be easier for ceramists to exe- cute and require less time compared to traditional pro- cesses required to produce a metal-ceramic restoration. But what may go unrecognized is that a highly skilled cera- mist can create metal-ceramic restorations of esthetic qual- ity akin to all-ceramic materials while retaining the structural benefits and enjoying the long-term success associated with metal-ceramic technology (Fig 10-4).
The Metal-Ceramic Restoration with a Porcelain Margin
The preceding chapters presented basic techniques involved in the production of metal-ceramic restorations. Throughout that fabrication process, every effort was made to mask the presence of the underlying metallic substruc- ture. Nowhere is the problem of unwanted metal display more pronounced and troublesome than in the facial and interproximal surfaces, particularly with highly visible ante- rior restorations where soft tissue management and appear- ance are so critical.
An unintended display of metal—one of the leading con- cerns with metal-ceramic restorations—can be attributed to any number of issues, ranging from inadequate tooth prepa- ration (ie, clinical variable) to substructure design errors (ie, laboratory variable) to postoperative gingival recession (a clinical and/or patient variable). The display of metal at the restoration margin may be evident at the try-in appointment or appear long after cementation and final placement. If the amount of tooth reduction prepared for both metal and por- celain is inadequate, laboratory attempts to mask a metal margin with porcelain are certain to result in a poorly con- toured crown and may contribute to chronic gingival irri- tation and inflammation of the surrounding tissues. Con- versely, in efforts to achieve ideal esthetics, clinicians may overcompensate and prepare an unnecessarily deep sub- gingival facial finish line. Unfortunately, such a preparation is more likely to cause irreparable damage to the surround- ing periodontal tissues, doing more harm than good.
It also is important to point out that all-ceramic resto- rations are not always appropriate alternatives because contemporary ceramic materials have their own require- ments for case selection and tooth preparation to meet esthetic expectations. Thus, in situations when all-ceramic restorations are perhaps ill-advised, it may be advanta- geous to recommend a metal-ceramic restoration with a porcelain margin as a treatment alternative (Fig 10-5). At the very minimum, the facial and interproximal portions of these metal-ceramic crowns can be reproduced in porce- lain, leaving metal for the less critical and esthetically sen- sitive areas (see Fig 10-4b).
Soft tissue response to dental porcelain
An important added benefit of any restoration with a porce- lain margin is the favorable gingival response to a ceramic material. It is generally recognized that metals have high surface energy and, therefore, are more likely to attract and retain a higher volume of dental plaque than do ceramics. Dental porcelain is a nonreactive, biocompatible material with low surface energy. With proper home care, a patient’s soft tissue response should be quite favorable to a properly designed and placed metal-ceramic restoration with a por- celain margin (see Fig 10-5b).
Historical perspective
An understanding of the history and evolution of this mod- ified metal-ceramic restoration helps one appreciate how contemporary dental products lessen any number of techni- cal challenges and bring this treatment option well within the capability of aspiring ceramists. Furthermore, the potential for an esthetic ceramic margin has led to a metal-ceramic restoration with a variety of names, several suggested fab- rication techniques, and the development of high-fusing shoulder or margin porcelains specifically designed for the gingival area of a metal-ceramic restoration.
Naming the restoration
What is identified here as a porcelain-margin restoration or technique has been described in the literature and dental textbooks by any number of alternate labels: buccal butt, metal-ceramic crown with an all-porcelain labial margin, collarless metal-ceramic restoration, collarless veneered crown, complete porcelain margin, porcelain-butt margin restoration, porcelain-shoulder margin restoration, and the list goes on.
Over the years, several different techniques have been proposed to add a porcelain margin to a metal-ceramic res- toration. When first introduced, the focus was on replacing just the facial and interproximal areas with porcelain, a fact reflected in the first two designations listed above. As use of this treatment option gained greater clinical acceptance, technicians modified the design of the substructure, so the entire crown margin can be reproduced in dental porcelain with no metal visible externally (Fig 10-6). Although men- tioned here, the techniques and technical skills required to produce a 360-degree ceramic margin for a metal-ceramic restoration exceed the scope of this book.
Classification of porcelain-margin techniques
The laboratory procedures recommended over the years to create a ceramic margin are varied, so it is helpful to orga- nize and classify them to appreciate their relative similar- ities and differences. For example, there are two general methods to fabricate these restorations: indirect (not on the master die) and direct (on the master die) (Table 10-1). The indirect method involves the use of a high-heat refractory die, but the quality of the marginal fit cannot be determined until that die material is removed. On the other hand, five processes have evolved over the years using a direct fabri- cation method; they include the platinum foil technique and four direct-lift techniques: with conventional dentin porce- lain, porcelain-wax, porcelain-resin, and shoulder porce- lain. The platinum foil approach is a hybrid of the direct and indirect methods, as is explained later. The remain- ing four direct-lift techniques share a common benefit in that they all rely on lifting the unfired porcelain margin off a master die. But unlike the platinum foil technique, mar- ginal fit can be evaluated immediately after each porce- lain firing cycle.
Accuracy of porcelain margins
Two studies published in 1985 evaluated different labora- tory procedures used to produce a ceramic margin with a metal-ceramic restoration. Belser et al reported that a porcelain marginal opening (ie, fit) of less than 50 μm was achievable on a consistent basis using the platinum-foil technique. Cooney et al compared the platinum-foil meth- odology (using a stone die and a silver-plated die) to the porcelain-wax and another direct-lift technique.12 They reported an average facial marginal opening of 70 μm for the direct-lift technique and 81 μm for the porcelain-wax technique, but 38 μm for the platinum-foil technique on a plated die and 32 μm using the platinum-foil technique on a gypsum die. An important distinction to point out was that these investigators used conventional body porcelain for the direct-lift technique. Furthermore, in both studies, the researchers measured marginal openings directly (ie, from the frontal view) using photomicrographs taken at an origi- nal magnification of ×17011 or ×80, respectively. A subsequent report published in 1991 compared metal- ceramic crowns with porcelain margins made using the direct-lift techniques with porcelain-resin or shoul- der porcelain to a metal-ceramic crown with a conven- tional metal collar. The facial margin discrepancies of the two direct-lift techniques varied but were not statistically different. However, the mean marginal gap of a conven- tional metal margin (45.86 μm) was significantly less than either the shoulder porcelain (68. μm) or porcelain-resin (88.34 μm) margins. The mean lingual marginal openings were not statistically different: metal margin (49.83 μm), shoulder porcelain (48.96 μm), and porcelain-resin (40.34 μm). The authors contended that the marginal discrepan- cies with the three techniques varied, but all the outcomes were considered to be in a clinically acceptable range.
A few years later, Boyle et al arrived at entirely differ- ent outcomes for marginal openings in their comparison of three techniques: the direct-lift technique using high-fusing shoulder porcelain (8.2 μm), platinum-foil technique with conventional body porcelain (11.3 μm), and the direct-lift technique with conventional body porcelain-wax (13.7 μm).14 Unlike some of the earlier studies, marginal openings in this 1993 investigation were measured in cross section at a magnification of ×100. Interestingly, the cross-section perspective allowed the authors to identify the formation of positive and negative marginal rounding with shoulder por- celain, as the ceramic flowed on heating and conformed to the natural rounding of the gypsum die margin.14 This finding led Boyle et al to conclude that any “lack of mar- ginal sharpness of porcelain facial margins may be influ- enced more by the die material” than the actual fabrica- tion technique. Furthermore, the restorations made with the high-fusing shoulder porcelain had the least amount of marginal opening, just 8.2 μm. By way of comparison, oth- ers later reported a mean marginal gap of 27.93 μm (stan- dard deviation of 15.84 μm) for shoulder porcelain com- pared to 42.43 μm (standard deviation of 24.12 μm) for a feather-edge metal margin.
Metal Substructure Design
Irrespective of the method (ie, indirect or direct) chosen to create a porcelain-margin restoration, the master die should be trimmed (ie, ditched) and prepared in the cus- tomary manner. The entire finish line must be discernable and clearly marked in red or another highly visible color and then carefully sealed with a very thin layer of cyanoac- rylate cement or gypsum die hardener.
Labial surface
The metal substructure should be designed according to the recommendations in chapter 4 and as illustrated in Fig 10-7a with an opaqued substructure for a maxillary cen- tral incisor. As a brief review, the labial surface of the cast metal substructure should extend to the axiogingival line angle of the shoulder.
Some authors have recommended alternate substruc- ture designs in which the facial axial metal terminates short of the axiogingival line angle by as little as 0.5 mm to as much as 2.0 mm (Fig 10-7b). According to the results of a 1991 laboratory study by Belles et al, the facial marginal openings using this modified substructure design (0.5 mm short) were no different for restorations in which the axial wall of the coping extended to the base of the shoulder. McLaren suggested that a 2.0-mm cutback in height results in less shadowing of the marginal area.17 He expressed the view that when transilluminated, the optical properties were similar to a natural tooth. Lehner et al evaluated substruc- tures with three configurations for the facial metal: (1) full support (ie, metal to the junction of the shoulder), (2) 1.0 mm short axially, and (3) 2.0 mm short axially.18 In their comparison of the fracture strength of a restoration with the standard coping design to those with 0.5 to 2.0 mm of unsupported porcelain, they did not find a statistical differ- ence for these modified coping designs when a 90-degree shoulder preparation was used.
Shoulder configuration
As for the facial shoulder itself, ideally it should have a sharp external line angle, a slightly rounded internal line angle, a uniform dimension, and a sufficient amount of tooth reduc- tion to provide space for the opaqued metal substructure and a shoulder porcelain “scaffold” veneered with dentin porcelain (Fig 10-8).
Porcelain-metal junction axially
The lingual metal should extend to the point where the shoulder begins in the interproximal areas (usu- ally lingual to the proximal contact area) with a defi- nite vertical porcelain-to-metal junction (see Fig 7-18). This porcelain-to-metal junction is generally designed to be perpendicular to the finish line. Alternatively, the shoul- der can be prepared through the interproximal areas with the substructure designed as in Figs 4-2b or 10-4b, so the ceramic margin terminates on the lingual aspect of the tooth. If the restoration is to have porcelain on the lin- gual or occlusal surface, the lingual collar (see Figs 4-12b and 4-13b) should be at least 2.0 mm in height for rigidity and to prevent distortion of the metal substructure.
Alloy selection, casting, and metal finishing
The wax coping should be cast in a ceramic alloy that is known to be compatible with the brand of opaque, den- tin, and shoulder porcelain selected for use. As with any casting, always inspect the intaglio surface of the sub- structure under magnification (Fig 7-1) to remove any raised defects that could damage a gypsum die and pre- vent complete seating of the casting. Follow the proto- col described in chapter 7 for fitting the cast restoration and make any required internal adjustments. Then begin the recommended metal finishing steps to develop a uni- form width of exposed shoulder. Refine the lingual metal to establish a 90-degree interface with the porcelain (ie, a dis- tinct porcelain-metal junction). If this area of the substruc- ture has a rounded edge, refresh the external finish line with a no. 8 round carbide bur (see Fig 7-19e).
Indirect Method
The designation indirect identifies a technique in which the porcelain margin is not constructed on the master gypsum die itself but indirectly on a duplicate die. As a result, it is only after the porcelain margin has been created that the restoration can be transferred to the master die to evalu- ate fit and marginal accuracy. The refractory die technique is just such a method, and it represents one of the earliest approaches to creating a porcelain margin. It is included primarily to provide a historical perspective.
Refractory die technique
This technique called for the production of a duplicate mas- ter die created in a high-heat refractory material on which the marginal area was created in porcelain.3 To achieve this, several flawless nonaqueous elastomeric impres- sions of the ditched master stone die had to be made.
Multiple pours of the refractory material were needed, because voids in the impressions, fracture of the refractory dies during removal from the impression mold, and chip- ping/fracturing of the delicate margins would render a die unusable.
Once cast, the metal substructure was adjusted on the master stone die until the desired fit was obtained. The most accurate and complete refractory die was heated in a porcelain furnace in a true degassing process to elimi- nate ammonia gas by-products from the refractory material. Then the finished and oxidized metal casting was carefully placed on the degassed refractory die. With the casting in place, the porcelain margin was created with the applica- tion and firing of successive layers of metal conditioner, opaque, and body porcelain until the porcelain margin had the desired emergence profile and axial contours. Once the facial margin was complete, the refractory die could be removed, allowing the restoration, with its delicate por- celain margin, to be returned to the master gypsum die. Assuming the desired margin seat and seal were obtained, the remainder of the porcelain buildup was completed in the usual manner until the restoration was finalized.
Challenges
The first challenge was obtaining an intact refractory die with the porcelain-margin area accurately reproduced. A major limitation was the fact that the fit of the porcelain mar- gin could not be evaluated until the restoration was returned to the original gypsum die. And before the margin could be evaluated, the refractory die had to be destroyed. In the event the facial margin was less than ideal for any reason, the porcelain shoulder had to be removed so the entire pro- cess could be repeated using a new, degassed refractory die. Not surprisingly, this indirect method lost favor over time once improved direct techniques were introduced.
Direct Methods
A direct method is one in which a porcelain margin is formed on the master die (typically a gypsum die) produced from the final impression of the prepared tooth. That ceramic margin is added only after the metal substructure has been cast, finished, oxidized, and opaqued but before any den- tin and enamel body porcelains have been applied.
At least five direct techniques have emerged over the years (see Table 10-1): (1) platinum foil, (2) direct-lift with con- ventional dentin porcelain, (3) direct-lift with porcelain-wax, (4) direct-lift with porcelain-resin, and (5) direct-lift with shoulder porcelain.
It should be pointed out that with the platinum-foil tech- nique, the restoration must remain on the working die until the ceramic margin has been formed completely. With the remaining four direct-lift methods, the casting is removed (ie, manually “lifted” off the die) periodically during fabrica- tion to permit an assessment of marginal fit and contour.
The general advantages and disadvantages of all five direct methods are listed in Table 10-2
Platinum-foil technique
The platinum-foil technique shares features of both the indirect and direct methods. In one respect it is partially a direct technique, because the porcelain margin is typically created directly on a secondary gypsum die with no facial undercuts.19 But the technique also shares the principal feature with the indirect method because the marginal fit cannot be evaluated until after the ceramic margin has been created and the platinum foil has been removed.
Placement of the platinum foil
The two roles of the platinum foil were: (1) to serve as a separating component (between the underlying die and the porcelain veneer) and (2) to provide a matrix on which to construct that ceramic margin.2,4,8,19 Although the foil is quite thin (25 µm thick), space had to be created for it by relieving either the gypsum die or the intaglio surface of the casting. The foil had to be well adapted to the facial shoulder preparation and burnished carefully so it extended 2.0 mm incisal to the axiogingival line angle and 2.0 mm apical to the finish line. Once the foil was positioned in this fashion, it was either spot welded or glued to the coping to establish a firm attachment to the intaglio surface.
Traditionally, it was recommended that the restoration, with foil attached, be cleaned ultrasonically in hydrofluoric acid, rinsed thoroughly, and oxidized. (As mentioned before, hydrofluoric acid poses a health risk and is not recommended for use in the dental laboratory.) Opaque porcelain was then applied to the porcelain-bearing areas of the substructure but not on the platinum foil, and the work was sintered. After firing, the coping was returned to the gypsum die, because the foil needed to be readapted to the marginal area. Next, a thin layer of separating medium had to be applied to the shoulder area to prevent porcelain from attaching to the foil matrix during the next firing cycle.
Porcelain application
At the time this technique was introduced, high-fusing shoulder porcelains had not been developed, so conventional dentin porcelain was used to create the ceramic margin. After placing a thin layer of separating medium on the foil covering the shoulder, dentin porcelain of the desired tooth shade was mixed and used to form the ceramic mar- gin. Enamel porcelain was then added to the body buildup to complete the remainder of the restoration. Once condensed, the area between the platinum foil and the dentin porcelain had to be delicately ditched for the length of the shoulder using a no. 11 scalpel blade. Separating the platinum foil from the porcelain was essential to prevent the porcelain from lifting the foil off the shoulder, because the porcelain would shrink during the firing cycle.
Once cooled, the fired restoration was returned to the gypsum die so the foil could be readapted again, after which additional dentin porcelain had to be placed in the ditched space and condensed. The restoration had to be inspected closely because any excess porcelain past the facial margin of the shoulder needed to be removed before firing the restoration again. After the second sintering, the entire ditched area was reexamined. Any remaining discrepancies in the marginal area or the restoration contours were then corrected in a third firing.
It is important to mention that the high temperature set- ting for each firing cycle should be at least 10°C lower than the last highest temperature. Lowering the high temperature for each subsequent firing is critical to protect the structural integrity of the porcelain margin irrespective of technique.
As with the refractory die method, it is only after the restoration has been finalized and glazed that the platinum foil can be removed to reveal a completed restoration with a porcelain-labial margin.
Challenges
The laboratory procedures just described could be challenging in their own right and demand considerable laboratory time and skill, especially for those not experienced in manipulating plating foil. In other words, it required technical skill just to adapt, fit, and attach the foil to the metal substructure. More importantly, the quality and completeness of the porcelain margin could not be evaluated until after the crown had been glazed and the platinum foil removed. This delay in assessing the ceramic margin was unavoidable because the foil supported the porcelain mar- gin during all the firing cycles.
But once the foil had been removed, the marginal area of the restoration could be inspected for fit (seat and seal) and the presence of any excess porcelain. Overextensions could be ground away using a flexible diamond disk or finishing diamond instrument. The restoration could be returned to the gypsum die as many times as needed until the margin porcelain exhibited the desired fit and it was ready for a clinical try-in.
Those ceramists who do not wish to deal with the technical steps required with platinum foil can turn to one of the four direct-lift techniques.
Direct-lift Techniques
As the name itself suggests, a direct-lift technique entails fabricating the porcelain margin directly on the master gyp- sum die with repeated removal (ie, lifting) of the casting off that die for each porcelain firing cycle. Furthermore, nothing is interposed between the die and the dental porcelain other than a thin layer of a liquid separating medium.
The gypsum die and metal substructure
For best results, the shoulder preparation of the tooth and the resulting stone die should have a width of approximately 1.2 mm.20 This dimension provides between 0.3 and 0.5 mm of space for metal and 0.2 to 0.3 mm for opaque porcelain, thus allowing the porcelain-margin material to have a uniform thickness of at least 0.7 mm20 and as much as 1.0 mm21 (see Fig 10-8). It is important to stress that these dimensions are only approximate because highly skilled ceramists invariably can achieve esthetic outcomes for clinical cases in which the prepared shoulder is actually less than 1.2 mm. In the event the prepared shoulder is not uniform in width because of extensive dental caries, restorative material, or previous over-preparation for a crown, modifications can be made in the substructure. In other words, wax can be added to the labial surface of the wax pattern to create that desired uniform shoulder dimension. Any needed adjustments can be made before the substructure is invested and cast (see Fig 4-29). The exposed shoulder should allow for a uniform thickness of shoulder porcelain. From this point forward, the substructure is adjusted and finished, cleaned, oxidized, and opaqued as described in chapter 8.
It is only after a uniform opaque porcelain veneer has been applied and fired that the porcelain labial margin is created. According to some authors, the relative simplicity of a direct-lift technique is its principal advantage. As explained in the following sections, ceramists have a choice of materials from which to choose when creating the actual ceramic margin.
Direct-lift technique with conventional dentin porcelain
This early version of the porcelain-margin restoration was introduced nearly 40 years ago and called for extending the opaque layer from the metal substructure over the prepared shoulder on the gypsum die right to the axiogingival line angle.5 After the moist opaque was condensed, a con- cavity had to be created in the opaque layer, and the restoration was carefully lifted off the die. Opaque that did not separate and remained on the gypsum die was brushed away but replaced on the underside of the opaque margin. The casting, complete with its shoulder of opaque porce- lain, was dried slowly and vacuum fired. Any marginal gaps were also filled with additional opaque porcelain, and the restoration was refired.
What was different about this technique was the need to capture the entire shoulder in a concave foundation of opaque porcelain. Dentin porcelain did not come into play until it was applied over the concave shoulder of fired opaque to the external margin. Conventional dentin porcelain was applied to the remainder of the coping with a veneer of enamel porcelain added to complete the first body buildup. The restoration was fired, and additional bakes were made as needed. Minor marginal deficiencies were remedied using a mixture of correction and dentin porcelains in a 1:3 ratio. The author claimed a marginal gap of only 6.0 µm was achievable with this technique.
Challenges
The ceramic margin foundation had to be created in opaque. In actual practice, users found that lifting the crown off the die with wet opaque porcelain intact was not easy to accomplish. Nor was the task of adding opaque to the underside of the ceramic shoulder when small amounts of opaque did not separate cleanly. Despite the application of a separating medium, small areas of porcelain often were left behind on the die shoulder.5 Technicians also had to exercise great care to avoid fired opaque from being visible externally as part of the ceramic margin. This was a concern for the dentist because exposed porcelain should not be in contact with the gingival tissues. Fired opaque is rough, cannot be polished or glazed, and should be veneered with a body porcelain. With the other direct-lift techniques, conventional body porcelain or a high-fusing shoulder porcelain create the entire ceramic margin, so there is no similar concern.
Direct-lift technique with porcelain-wax
The same preliminary steps for preparing the gypsum master die and the opaqued metal casting should be followed for the direct-lift technique with shoulder porcelain. To create the ceramic margin, a dental wax, rather than distilled water or modeling liquid, was melted and mixed with con- ventional body (ie, dentin) porcelain. Initially, users had to incorporate dentin powder of the appropriate shade for the definitive restoration into the molten wax.21 Proponents of this approach believed the wax-porcelain suspension facilitated the fabrication process compared to the handling required of other direct-lift techniques prevailing at the time. Perhaps the principal benefit to ceramists was that the rigidity of the cooled wax-porcelain mix made separation of the margin porcelain from the gypsum die relatively easy, assuming the die was properly lubricated with a separating medium. To facilitate fabrication, this technique lent itself to the use of an electric wax spatula. The adjustable temperature of the spatula aided heating, facilitated placement, and extended the working time slightly for the porcelain-wax shoulder materials.
It was possible to create your own porcelain-wax mix- ture. When doing so, it was essential to heat the wax until it was fluid, add in the correct volume of porcelain pow- der, and quickly incorporate the powder into the molten wax. Users soon learned that once the wax had cooled, the mixed material was ready for use, and no addition porce- lain powder could be added without heating the wax again. Recommendations for the powder-to-wax ratio (by weight) ranged from 6:122 to a suggested maximum ratio if 8:1.
Challenges
The most significant challenges associated with the porcelain-wax technique were dealing with wax as the car- rying vehicle for the powder, and the fact that the porcelain and wax mixture could not be condensed. In fact, once the mixed material was applied to the labial margin and the wax had cooled, the margin was set. The impact of this lim- itation was described in a study that reported that the addi- tion of wax (or resin) to the porcelain powder reduced both the density and the apparent tensile strength of fired shoul- der porcelain as well as conventional body porcelain; the reductions were statistically significant.
Direct-lift technique with porcelain-resin
Combining porcelain with a visible light–activated, unfilled acrylic resin had its advantages. It allowed a ceramic mar- gin to be formed in the shoulder area quite rapidly, and light activation set the margin once the ceramist established the desired shape and position. Users found the porcelain-resin mix to be easy to place and polymerize, and restorations could be lifted off their respective master dies immediately after polymerization was complete.
This methodology facilitated the simultaneous removal of multiple retainers of a fixed partial denture, which oth- erwise would be a challenge even when using con- ventional high-fusing shoulder porcelain. Furthermore, the porcelain-resin combination was seen as especially helpful when dealing with preparation margins that were not uniform—irregular in shape and uneven in width. How- ever, the mean marginal openings for restorations report- edly ranged from 21.95 to 148.35 μm.
Challenges
In time it was discovered that the addition of either a resin or a wax to conventional body porcelain significantly reduced both the density and the apparent tensile strength of the fired porcelain.24 In addition, the optical properties of the ceramic margin could also be compromised. The areas in the porcelain mix occupied by unfilled resin pre- vented condensation of the porcelain. Without the ability to condense the marginal porcelain, the numerous resin-filled areas became voids in the fired margin porcelain. Nonethe- less, the porcelain-resin technique did facilitate the fabri- cation process by permitting the capture of irregular shoul- der margins and made removal of a restoration from its die far easier.
Direct-lift technique with shoulder porcelain
The introduction of commercially available high-fusing shoulder (ie, margin) porcelains heralded a major advance- ment and was a marked departure from those earlier tech- niques that relied on either opaque porcelain or con- ventional dentin porcelain to create a ceramic margin. Once dental manufacturers made available kits of spe- cial high-fusing shoulder porcelains, there was no longer a need to improvise and experiment with different materi- als and techniques. Ceramists could achieve an esthetic ceramic margin on metal-ceramic restorations with a clin- ically acceptable fit on a routine basis. Three advantages quickly became apparent.
First, the shoulder porcelains undergo minimal firing shrinkage (if properly condensed), as compared to con- ventional dentin porcelain. Yet, like all metal-ceramic por- celains, shrinkage will occur during sintering, so a ceramic margin typically cannot be produced with a single firing. Multiple bakes are needed to achieve acceptable marginal accuracy and fit.
Second, shoulder porcelains have relatively high fus- ing temperatures, and that is by design. In fact, many are heated as much as 20°C to 50°C higher than the subse- quent body bakes. Consequently, this type of ceramic margin tends to remain dimensionally stable when a res- toration is fired at the temperatures recommended for the body buildup and even the final glazing cycle. The obvious advantage here is that the ceramic margin remains intact and does not slump or round for the balance of the fabri- cation process.
Third, numerous dental manufacturers market high-fusing shoulder porcelain, so ceramists do not have to mix differ- ent components or otherwise create their own unique mar- gin porcelain for each clinical case. They can simply dis- pense the needed materials from commercial products designed to provide them with a greater assurance of com- patibility, color matching, and reliability in the production of metal-ceramic restorations with a porcelain margin.
Challenges
Many of the deficiencies and technical problems associ- ated with the techniques previously described have been eliminated or overcome with the direct-lift technique with shoulder porcelain, making simplicity its principal advan- tage.22 Furthermore, the technical procedures required with shoulder porcelain can be learned relatively quickly, so producing this type of restoration is within the reach of many dental laboratory technicians. Perhaps the sin- gle greatest challenge is not to be found in the laboratory but does impact any laboratory technique: the need for a well-prepared shoulder that is flat with a distinct external finish line (see Fig 10-7a). These clinical variables remain under the control of the prescribing dentist, and if not well executed, make it difficult for any dental laboratory to achieve an ideal porcelain margin irrespective of the mate- rials selected or the technique used.
Laboratory Procedures: Direct-Lift Technique with Shoulder Porcelain
As with the other direct-lift techniques, the master gyp- sum die is ditched in the usual manner with the finish line marked in red (or other bright color). Seal the shoulder and finish line with a thin layer of cyanoacrylate cement or die hardener (Figs 10-9a and 10-9b). Once the sealed mar- gins are dry, a light application of a commercial separat- ing medium (or mineral oil) should be applied to the gyp- sum margin (Fig 10-9c) to facilitate separation and removal of the delicate, unfired shoulder porcelain from the master die. While these procedures are being performed the metal substructure can be opaqued (Figs 10-9d and 10-9e) as described in chapter 8.
Fabrication of the ceramic margin
The fabrication process begins with the return of the opaqued casting to the master die and the incremental placement of mixed shoulder porcelain directly on the lubri- cated margin area (Figs 10-9f to 10-9w). You should antic- ipate the need for at least two shoulder porcelain appli- cations and two firings to obtain a ceramic margin with acceptable seat and seal.
For that second margin buildup, start with the incremen- tal placement of smaller amounts of porcelain across the entire shoulder area, seat the work, condense the porce- lain, and shape the material to the proper contour before lifting the casting off the die, cleaning the intaglio surface, and sintering the restoration (Figs 10-9x to 10-9hh). Do not overbuild the margin. Err on the cautious side and reapply a thin layer of the mineral oil separating medium between porcelain applications until this technique has been mas- tered. Users new to these procedures should be pre- pared for a third porcelain application and firing, particu- larly if marginal discrepancies are noted (see Figs 10-9w and 10-9gg). If no gaps or defects are detected and no additional applications of shoulder porcelain are needed, proceed with the body buildup and complete the restoration in the usual manner (Fig 10-9ii).
Use of colloidal silica
Although either distilled water or modeling liquid is usu- ally mixed with a shoulder porcelain, some have suggested substituting colloidal silica (ie, phosphate-bonded invest- ment liquid) to increase the strength and decrease the shrinkage.25 However, Brackett et al found no increase in strength and reported that colloidal silica actually adversely affected the quality and appearance of the sintered por- celain. They noted that the fired porcelain had silica inclusions, increased surface roughness, and undesirable changes to its appearance once glazed.
Management of porcelain shrinkage
As shown in Figs 10-9r, 10-9s, and 10-9dd, inspecting and cleaning the interior of any casting following each ceramic application should be routine before the shoulder porce- lain is actually fired. Expect the greatest amount of porce- lain shrinkage to occur with the first firing despite proper condensation. Shoulder porcelains, like conventional body porcelains, are going to shrink toward bulk when heated. If the first buildup is overbuilt, too much of the shoulder por- celain will be drawn farther away from the marginal area during the sintering process. This will result in a larger than normal marginal opening (see Fig 10-9w). Even under ideal circumstances when the correct amount of shoulder por- celain is applied, expect shrinkage to be more substan- tial on the initial firing than the second and subsequent fir- ings (see Figs 10-9gg and 10-9hh). After all, the volume of unfired margin porcelain is greatest for the first bake and decreased substantially for the second application and fir- ing. A minor third corrective bake may be necessary to achieve a satisfactory marginal seal for dental laboratory technicians new to performing these procedures.
Case Presentations
A series of additional clinical cases are presented in which esthetic outcomes have been achieved using metal-ceramic technology for single anterior crowns (Figs 10-10 to 10-12), single posterior crowns (Figs 10-13 to 10-15), a posterior fixed partial denture (Fig 10-16), and multiple individual anterior crowns (Fig 10-17). Suffice it to say that a well-conceived treatment plan, coupled with capable clinical and laboratory execution, can result in porcelain-margin metal-ceramic restorations with esthetic outcomes rivaling many of the all-ceramic alternatives
Summary
The direct-lift techniques can be used to produce a porcelain-margin metal-ceramic restoration, but the qual- ity of the end result will vary in accordance with the knowl- edge and skill of the dental laboratory technician, not to men- tion the limitations inherent with the materials used and the clinical variables under the control of the prescribing den- tist. Dental laboratory technicians may find they are able to obtain very satisfactory results by avoiding the more tech- nically demanding platinum-foil and flawed porcelain-wax or porcelain-resin methods and focusing their efforts on mastering the direct-lift technique using a commercial high- fusing shoulder porcelain.
Users who are new to these materials and techniques should bear in mind that shoulder porcelain is intended to provide a visually undetectable scaffold or foundation to be veneered by dentin porcelain (see Fig 10-8). Familiarity with the different fabrication options and the various mate- rials available to produce porcelain-margin restorations is important because not all prepared shoulders are text- book in dimension and geometry. Therefore, dental labora- tory technicians will benefit from developing an expanded armamentarium of techniques to create a definitive resto- ration when variables outside their control must be dealt with in the dental laboratory. It is hoped that the information and examples presented in this final chapter illustrate how creating lifelike fixed res- torations may not be so much a question of which materi- als are selected, but more a matter of how the materials selected are actually used.
Therefore, this chapter has been added for the benefit of the emerging generations of dentists and dental labora- tory technicians who otherwise might not be aware of how the addition of a porcelain margin, in the hands of skilled clinicians and ceramists, can result in metal-ceramic resto- rations that are not only durable but esthetic. In the process, today’s clinicians can expand their armamentarium and select different types of restorations, be they all-ceramic or metal-ceramic, to meet the unique functional demands of each patient situation without compromising their esthet- ics expectations.
Clinical Performance of Metal-Ceramic Restorations
In dentistry, long-term studies (those greater than 10 years) dealing with the clinical performance of various materials or restorations shed much-needed light on the survival rates and longevity of treatment outcomes. Reports of 5-year survival rates for metal-ceramic restorations are meaning- ful, but analyses spanning longer time periods hold even greater significance for dentists, dental laboratory techni- cians, and patients alike.
In 2013, Walton published a report on the survival and clinical performance of 2,340 high gold metal-ceramic sin- gle crowns at 10 years (97.1%) and up to 25 years (85.4%) provided for 670 patients in his private prosthodontic prac- tice.1 Biologic rather than technical factors accounted for the majority of the 133 reported failures (5.7% over- all failure rate) with patient complaints of “unaccept- able esthetics” attributed to just 22 (0.9%) crowns of the 2,340 metal-ceramic restorations placed between 1984 and 2008.1 Readers who question the place of metal-ceramic technology in a contemporary dental practice may find this report of interest.
Metal-Ceramic Versus All-Ceramic Restoration
Less than ideal esthetic results are not unique to metal- ceramic restorations. In reality, they can be seen when the best of all-ceramic systems are used (Fig 10-1). Why is that, and what factors come into play with the different ceramic materials that might influence the appearance of a particu- lar product for different clinical applications? It is likely that several factors are at work that influence the final outcome.
Important aspects of care
When a patient presents for treatment or retreatment, it is important to analyze the existing situation and devise a strategy for success that addresses the patient’s chief complaint and avoids a repeat of past failures or limitations (Fig 10-2). One way to organize that strategy, before pro- ceeding directly to actual patient treatment, is to examine three aspects of care: (1) treatment planning, (2) execution of clinical procedures, and (3) dental laboratory outcomes. The first two variables are the responsibility of the dentist, while the third is a reflection of the skill and talent of the dental laboratory technician working within any constraints posed by the first two variables.
Treatment planning
During an examination of defective or unesthetic metal- ceramic restorations, the initial response may be to criticize the choice of restorative materials rather than to step back for a moment and objectively critique the clinical and labora- tory execution (see Fig 10-2a). For example, after a compre- hensive examination, it may become apparent that the poor result could have been avoided with an alternate treatment approach. Perhaps a patient’s occlusion, interocclusal space limitations, periodontal health, oral hygiene, orthodontic sta- tus, or endodontic needs were not evaluated and managed appropriately. After all, proper treatment planning is essential to avoid an unattractive metal-ceramic crown that is too long, too wide, and too bulky facially, or to avoid the need for post- operative appointments to adjust premature occlusal contacts that were not identified initially (see Fig 10-2a).
All too frequently, if a definitive restoration’s overall appearance does not blend with the adjacent natural teeth or existing restorations, it is likely to become a source of complaint for patients. In other words, should the treat- ment planning process itself be lacking, there is nothing to prevent a new restoration from reflecting those clinical flaws left unaddressed or undiagnosed prior to the com- pletion of care. It may be fair to say that compared to cur- rently available all-ceramic systems, a metal-ceramic res- toration requires more attention to clinical detail and a higher degree of technical skill in the fabrication process, even when all the requisite clinical procedures are per- formed well. Either way, the treatment-planning phase is the time to determine the type of restoration to recommend (ie, metal-ceramic or all-ceramic).
Execution of clinical procedures
A frequent challenge facing dental laboratory technicians is management of a case where the tooth to be restored has not been properly prepared for the intended resto- ration. There may be an inadequate amount of tooth struc- ture removed from the facial and lingual surfaces; a lack of two-plane tooth reduction; insufficient incisal or occlusal tooth reduction for the chosen restorative materials to allow the re-creation of adequate primary and secondary tooth morphology; or tooth finish lines that are irregular, unread- able, and/or appear to violate the biologic width. Perhaps the definitive impression does not adequately capture the finish line, contains defects (ie, voids) in critical areas, fails to include enough of the surrounding dentition to permit proper articulation of the master cast, or there are indica- tions it is distorted. Should the opposing cast be derived from a hastily mixed irreversible hydrocolloid (ie, alginate) impression that was not processed in a timely manner, then even accurate jaw relation records will not permit the estab- lishment of the correct occlusal relationship between the master cast and that opposing cast.
In instances where several of these clinical variables converge, you have a situation in which any expectations for an ideal esthetic outcome are unlikely to be realized, regardless of the type of restorative system involved. If these deficiencies are not identified and corrected prior to fabricating the definitive restoration, how could one expect any type of restoration to overcome them?
Patients need to be made aware at the initial visit that, should some aspect of their treatment be unsatisfactory or less than ideal, they should mention their concerns prior to cementation of a definitive restoration (see Fig 10-2a). After all, the form, shade, and overall appearance of a crown will not improve over time. Likewise, the shade of the ceramic does not lighten or darken intraorally like natural teeth. Cer- tainly, coronal tooth structure can become more exposed over time if gingival recession occurs due to violation of the biologic width, mechanical abrasion, or a poorly contoured restoration. In other words, the variables or conditions that tend to make a restoration unesthetic, or less than ideal, are likely present and readily identifiable at the try-in appoint- ment. Then it begs the question of whether the respon- sibility for esthetic failure rests with the restorative mate- rials used or with the clinical and laboratory procedures required to produce that restoration.
Clinicians who were not involved in the initial treatment and only later evaluated restorations may inadvertently attri- bute esthetic failures to limitations in metal-ceramic materi- als rather than flawed treatment planning and/or less than ideal clinical execution and management. Therefore, simply switching to an all-ceramic restoration may not be the answer and will not always result in an improved outcome, unless any underlying problems are diagnosed and addressed during retreatment. Suffice it to say that once treatment or retreatment is initiated, it is important to adhere to the tooth preparation guidelines for the restorative system selected, make an accurate definitive impression, and obtain a qual- ity master cast for the dental laboratory (Fig 10-3).
Dental laboratory outcomes
For many clinical situations, a dental laboratory tech- nician with a modest amount of training and experience may find it easier to produce a very acceptable defini- tive all-ceramic restoration as compared to a metal-ceramic restoration for the same tooth. The fabrication processes for many of the newer all-ceramic materials vary, but gen- erally speaking, they can be easier for ceramists to exe- cute and require less time compared to traditional pro- cesses required to produce a metal-ceramic restoration. But what may go unrecognized is that a highly skilled cera- mist can create metal-ceramic restorations of esthetic qual- ity akin to all-ceramic materials while retaining the structural benefits and enjoying the long-term success associated with metal-ceramic technology (Fig 10-4).
The Metal-Ceramic Restoration with a Porcelain Margin
The preceding chapters presented basic techniques involved in the production of metal-ceramic restorations. Throughout that fabrication process, every effort was made to mask the presence of the underlying metallic substruc- ture. Nowhere is the problem of unwanted metal display more pronounced and troublesome than in the facial and interproximal surfaces, particularly with highly visible ante- rior restorations where soft tissue management and appear- ance are so critical.
An unintended display of metal—one of the leading con- cerns with metal-ceramic restorations—can be attributed to any number of issues, ranging from inadequate tooth prepa- ration (ie, clinical variable) to substructure design errors (ie, laboratory variable) to postoperative gingival recession (a clinical and/or patient variable). The display of metal at the restoration margin may be evident at the try-in appointment or appear long after cementation and final placement. If the amount of tooth reduction prepared for both metal and por- celain is inadequate, laboratory attempts to mask a metal margin with porcelain are certain to result in a poorly con- toured crown and may contribute to chronic gingival irri- tation and inflammation of the surrounding tissues. Con- versely, in efforts to achieve ideal esthetics, clinicians may overcompensate and prepare an unnecessarily deep sub- gingival facial finish line. Unfortunately, such a preparation is more likely to cause irreparable damage to the surround- ing periodontal tissues, doing more harm than good.
It also is important to point out that all-ceramic resto- rations are not always appropriate alternatives because contemporary ceramic materials have their own require- ments for case selection and tooth preparation to meet esthetic expectations. Thus, in situations when all-ceramic restorations are perhaps ill-advised, it may be advanta- geous to recommend a metal-ceramic restoration with a porcelain margin as a treatment alternative (Fig 10-5). At the very minimum, the facial and interproximal portions of these metal-ceramic crowns can be reproduced in porce- lain, leaving metal for the less critical and esthetically sen- sitive areas (see Fig 10-4b).
Soft tissue response to dental porcelain
An important added benefit of any restoration with a porce- lain margin is the favorable gingival response to a ceramic material. It is generally recognized that metals have high surface energy and, therefore, are more likely to attract and retain a higher volume of dental plaque than do ceramics. Dental porcelain is a nonreactive, biocompatible material with low surface energy. With proper home care, a patient’s soft tissue response should be quite favorable to a properly designed and placed metal-ceramic restoration with a por- celain margin (see Fig 10-5b).
Historical perspective
An understanding of the history and evolution of this mod- ified metal-ceramic restoration helps one appreciate how contemporary dental products lessen any number of techni- cal challenges and bring this treatment option well within the capability of aspiring ceramists. Furthermore, the potential for an esthetic ceramic margin has led to a metal-ceramic restoration with a variety of names, several suggested fab- rication techniques, and the development of high-fusing shoulder or margin porcelains specifically designed for the gingival area of a metal-ceramic restoration.
Naming the restoration
What is identified here as a porcelain-margin restoration or technique has been described in the literature and dental textbooks by any number of alternate labels: buccal butt, metal-ceramic crown with an all-porcelain labial margin, collarless metal-ceramic restoration, collarless veneered crown, complete porcelain margin, porcelain-butt margin restoration, porcelain-shoulder margin restoration, and the list goes on.
Over the years, several different techniques have been proposed to add a porcelain margin to a metal-ceramic res- toration. When first introduced, the focus was on replacing just the facial and interproximal areas with porcelain, a fact reflected in the first two designations listed above. As use of this treatment option gained greater clinical acceptance, technicians modified the design of the substructure, so the entire crown margin can be reproduced in dental porcelain with no metal visible externally (Fig 10-6). Although men- tioned here, the techniques and technical skills required to produce a 360-degree ceramic margin for a metal-ceramic restoration exceed the scope of this book.
Classification of porcelain-margin techniques
The laboratory procedures recommended over the years to create a ceramic margin are varied, so it is helpful to orga- nize and classify them to appreciate their relative similar- ities and differences. For example, there are two general methods to fabricate these restorations: indirect (not on the master die) and direct (on the master die) (Table 10-1). The indirect method involves the use of a high-heat refractory die, but the quality of the marginal fit cannot be determined until that die material is removed. On the other hand, five processes have evolved over the years using a direct fabri- cation method; they include the platinum foil technique and four direct-lift techniques: with conventional dentin porce- lain, porcelain-wax, porcelain-resin, and shoulder porce- lain. The platinum foil approach is a hybrid of the direct and indirect methods, as is explained later. The remain- ing four direct-lift techniques share a common benefit in that they all rely on lifting the unfired porcelain margin off a master die. But unlike the platinum foil technique, mar- ginal fit can be evaluated immediately after each porce- lain firing cycle.
Accuracy of porcelain margins
Two studies published in 1985 evaluated different labora- tory procedures used to produce a ceramic margin with a metal-ceramic restoration. Belser et al reported that a porcelain marginal opening (ie, fit) of less than 50 μm was achievable on a consistent basis using the platinum-foil technique. Cooney et al compared the platinum-foil meth- odology (using a stone die and a silver-plated die) to the porcelain-wax and another direct-lift technique.12 They reported an average facial marginal opening of 70 μm for the direct-lift technique and 81 μm for the porcelain-wax technique, but 38 μm for the platinum-foil technique on a plated die and 32 μm using the platinum-foil technique on a gypsum die. An important distinction to point out was that these investigators used conventional body porcelain for the direct-lift technique. Furthermore, in both studies, the researchers measured marginal openings directly (ie, from the frontal view) using photomicrographs taken at an origi- nal magnification of ×17011 or ×80, respectively. A subsequent report published in 1991 compared metal- ceramic crowns with porcelain margins made using the direct-lift techniques with porcelain-resin or shoul- der porcelain to a metal-ceramic crown with a conven- tional metal collar. The facial margin discrepancies of the two direct-lift techniques varied but were not statistically different. However, the mean marginal gap of a conven- tional metal margin (45.86 μm) was significantly less than either the shoulder porcelain (68. μm) or porcelain-resin (88.34 μm) margins. The mean lingual marginal openings were not statistically different: metal margin (49.83 μm), shoulder porcelain (48.96 μm), and porcelain-resin (40.34 μm). The authors contended that the marginal discrepan- cies with the three techniques varied, but all the outcomes were considered to be in a clinically acceptable range.
A few years later, Boyle et al arrived at entirely differ- ent outcomes for marginal openings in their comparison of three techniques: the direct-lift technique using high-fusing shoulder porcelain (8.2 μm), platinum-foil technique with conventional body porcelain (11.3 μm), and the direct-lift technique with conventional body porcelain-wax (13.7 μm).14 Unlike some of the earlier studies, marginal openings in this 1993 investigation were measured in cross section at a magnification of ×100. Interestingly, the cross-section perspective allowed the authors to identify the formation of positive and negative marginal rounding with shoulder por- celain, as the ceramic flowed on heating and conformed to the natural rounding of the gypsum die margin.14 This finding led Boyle et al to conclude that any “lack of mar- ginal sharpness of porcelain facial margins may be influ- enced more by the die material” than the actual fabrica- tion technique. Furthermore, the restorations made with the high-fusing shoulder porcelain had the least amount of marginal opening, just 8.2 μm. By way of comparison, oth- ers later reported a mean marginal gap of 27.93 μm (stan- dard deviation of 15.84 μm) for shoulder porcelain com- pared to 42.43 μm (standard deviation of 24.12 μm) for a feather-edge metal margin.
Metal Substructure Design
Irrespective of the method (ie, indirect or direct) chosen to create a porcelain-margin restoration, the master die should be trimmed (ie, ditched) and prepared in the cus- tomary manner. The entire finish line must be discernable and clearly marked in red or another highly visible color and then carefully sealed with a very thin layer of cyanoac- rylate cement or gypsum die hardener.
Labial surface
The metal substructure should be designed according to the recommendations in chapter 4 and as illustrated in Fig 10-7a with an opaqued substructure for a maxillary cen- tral incisor. As a brief review, the labial surface of the cast metal substructure should extend to the axiogingival line angle of the shoulder.
Some authors have recommended alternate substruc- ture designs in which the facial axial metal terminates short of the axiogingival line angle by as little as 0.5 mm to as much as 2.0 mm (Fig 10-7b). According to the results of a 1991 laboratory study by Belles et al, the facial marginal openings using this modified substructure design (0.5 mm short) were no different for restorations in which the axial wall of the coping extended to the base of the shoulder. McLaren suggested that a 2.0-mm cutback in height results in less shadowing of the marginal area.17 He expressed the view that when transilluminated, the optical properties were similar to a natural tooth. Lehner et al evaluated substruc- tures with three configurations for the facial metal: (1) full support (ie, metal to the junction of the shoulder), (2) 1.0 mm short axially, and (3) 2.0 mm short axially.18 In their comparison of the fracture strength of a restoration with the standard coping design to those with 0.5 to 2.0 mm of unsupported porcelain, they did not find a statistical differ- ence for these modified coping designs when a 90-degree shoulder preparation was used.
Shoulder configuration
As for the facial shoulder itself, ideally it should have a sharp external line angle, a slightly rounded internal line angle, a uniform dimension, and a sufficient amount of tooth reduc- tion to provide space for the opaqued metal substructure and a shoulder porcelain “scaffold” veneered with dentin porcelain (Fig 10-8).
Porcelain-metal junction axially
The lingual metal should extend to the point where the shoulder begins in the interproximal areas (usu- ally lingual to the proximal contact area) with a defi- nite vertical porcelain-to-metal junction (see Fig 7-18). This porcelain-to-metal junction is generally designed to be perpendicular to the finish line. Alternatively, the shoul- der can be prepared through the interproximal areas with the substructure designed as in Figs 4-2b or 10-4b, so the ceramic margin terminates on the lingual aspect of the tooth. If the restoration is to have porcelain on the lin- gual or occlusal surface, the lingual collar (see Figs 4-12b and 4-13b) should be at least 2.0 mm in height for rigidity and to prevent distortion of the metal substructure.
Alloy selection, casting, and metal finishing
The wax coping should be cast in a ceramic alloy that is known to be compatible with the brand of opaque, den- tin, and shoulder porcelain selected for use. As with any casting, always inspect the intaglio surface of the sub- structure under magnification (Fig 7-1) to remove any raised defects that could damage a gypsum die and pre- vent complete seating of the casting. Follow the proto- col described in chapter 7 for fitting the cast restoration and make any required internal adjustments. Then begin the recommended metal finishing steps to develop a uni- form width of exposed shoulder. Refine the lingual metal to establish a 90-degree interface with the porcelain (ie, a dis- tinct porcelain-metal junction). If this area of the substruc- ture has a rounded edge, refresh the external finish line with a no. 8 round carbide bur (see Fig 7-19e).
Indirect Method
The designation indirect identifies a technique in which the porcelain margin is not constructed on the master gypsum die itself but indirectly on a duplicate die. As a result, it is only after the porcelain margin has been created that the restoration can be transferred to the master die to evalu- ate fit and marginal accuracy. The refractory die technique is just such a method, and it represents one of the earliest approaches to creating a porcelain margin. It is included primarily to provide a historical perspective.
Refractory die technique
This technique called for the production of a duplicate mas- ter die created in a high-heat refractory material on which the marginal area was created in porcelain.3 To achieve this, several flawless nonaqueous elastomeric impres- sions of the ditched master stone die had to be made.
Multiple pours of the refractory material were needed, because voids in the impressions, fracture of the refractory dies during removal from the impression mold, and chip- ping/fracturing of the delicate margins would render a die unusable.
Once cast, the metal substructure was adjusted on the master stone die until the desired fit was obtained. The most accurate and complete refractory die was heated in a porcelain furnace in a true degassing process to elimi- nate ammonia gas by-products from the refractory material. Then the finished and oxidized metal casting was carefully placed on the degassed refractory die. With the casting in place, the porcelain margin was created with the applica- tion and firing of successive layers of metal conditioner, opaque, and body porcelain until the porcelain margin had the desired emergence profile and axial contours. Once the facial margin was complete, the refractory die could be removed, allowing the restoration, with its delicate por- celain margin, to be returned to the master gypsum die. Assuming the desired margin seat and seal were obtained, the remainder of the porcelain buildup was completed in the usual manner until the restoration was finalized.
Challenges
The first challenge was obtaining an intact refractory die with the porcelain-margin area accurately reproduced. A major limitation was the fact that the fit of the porcelain mar- gin could not be evaluated until the restoration was returned to the original gypsum die. And before the margin could be evaluated, the refractory die had to be destroyed. In the event the facial margin was less than ideal for any reason, the porcelain shoulder had to be removed so the entire pro- cess could be repeated using a new, degassed refractory die. Not surprisingly, this indirect method lost favor over time once improved direct techniques were introduced.
Direct Methods
A direct method is one in which a porcelain margin is formed on the master die (typically a gypsum die) produced from the final impression of the prepared tooth. That ceramic margin is added only after the metal substructure has been cast, finished, oxidized, and opaqued but before any den- tin and enamel body porcelains have been applied.
At least five direct techniques have emerged over the years (see Table 10-1): (1) platinum foil, (2) direct-lift with con- ventional dentin porcelain, (3) direct-lift with porcelain-wax, (4) direct-lift with porcelain-resin, and (5) direct-lift with shoulder porcelain.
It should be pointed out that with the platinum-foil tech- nique, the restoration must remain on the working die until the ceramic margin has been formed completely. With the remaining four direct-lift methods, the casting is removed (ie, manually “lifted” off the die) periodically during fabrica- tion to permit an assessment of marginal fit and contour.
The general advantages and disadvantages of all five direct methods are listed in Table 10-2
Platinum-foil technique
The platinum-foil technique shares features of both the indirect and direct methods. In one respect it is partially a direct technique, because the porcelain margin is typically created directly on a secondary gypsum die with no facial undercuts.19 But the technique also shares the principal feature with the indirect method because the marginal fit cannot be evaluated until after the ceramic margin has been created and the platinum foil has been removed.
Placement of the platinum foil
The two roles of the platinum foil were: (1) to serve as a separating component (between the underlying die and the porcelain veneer) and (2) to provide a matrix on which to construct that ceramic margin.2,4,8,19 Although the foil is quite thin (25 µm thick), space had to be created for it by relieving either the gypsum die or the intaglio surface of the casting. The foil had to be well adapted to the facial shoulder preparation and burnished carefully so it extended 2.0 mm incisal to the axiogingival line angle and 2.0 mm apical to the finish line. Once the foil was positioned in this fashion, it was either spot welded or glued to the coping to establish a firm attachment to the intaglio surface.
Traditionally, it was recommended that the restoration, with foil attached, be cleaned ultrasonically in hydrofluoric acid, rinsed thoroughly, and oxidized. (As mentioned before, hydrofluoric acid poses a health risk and is not recommended for use in the dental laboratory.) Opaque porcelain was then applied to the porcelain-bearing areas of the substructure but not on the platinum foil, and the work was sintered. After firing, the coping was returned to the gypsum die, because the foil needed to be readapted to the marginal area. Next, a thin layer of separating medium had to be applied to the shoulder area to prevent porcelain from attaching to the foil matrix during the next firing cycle.
Porcelain application
At the time this technique was introduced, high-fusing shoulder porcelains had not been developed, so conventional dentin porcelain was used to create the ceramic margin. After placing a thin layer of separating medium on the foil covering the shoulder, dentin porcelain of the desired tooth shade was mixed and used to form the ceramic mar- gin. Enamel porcelain was then added to the body buildup to complete the remainder of the restoration. Once condensed, the area between the platinum foil and the dentin porcelain had to be delicately ditched for the length of the shoulder using a no. 11 scalpel blade. Separating the platinum foil from the porcelain was essential to prevent the porcelain from lifting the foil off the shoulder, because the porcelain would shrink during the firing cycle.
Once cooled, the fired restoration was returned to the gypsum die so the foil could be readapted again, after which additional dentin porcelain had to be placed in the ditched space and condensed. The restoration had to be inspected closely because any excess porcelain past the facial margin of the shoulder needed to be removed before firing the restoration again. After the second sintering, the entire ditched area was reexamined. Any remaining discrepancies in the marginal area or the restoration contours were then corrected in a third firing.
It is important to mention that the high temperature set- ting for each firing cycle should be at least 10°C lower than the last highest temperature. Lowering the high temperature for each subsequent firing is critical to protect the structural integrity of the porcelain margin irrespective of technique.
As with the refractory die method, it is only after the restoration has been finalized and glazed that the platinum foil can be removed to reveal a completed restoration with a porcelain-labial margin.
Challenges
The laboratory procedures just described could be challenging in their own right and demand considerable laboratory time and skill, especially for those not experienced in manipulating plating foil. In other words, it required technical skill just to adapt, fit, and attach the foil to the metal substructure. More importantly, the quality and completeness of the porcelain margin could not be evaluated until after the crown had been glazed and the platinum foil removed. This delay in assessing the ceramic margin was unavoidable because the foil supported the porcelain mar- gin during all the firing cycles.
But once the foil had been removed, the marginal area of the restoration could be inspected for fit (seat and seal) and the presence of any excess porcelain. Overextensions could be ground away using a flexible diamond disk or finishing diamond instrument. The restoration could be returned to the gypsum die as many times as needed until the margin porcelain exhibited the desired fit and it was ready for a clinical try-in.
Those ceramists who do not wish to deal with the technical steps required with platinum foil can turn to one of the four direct-lift techniques.
Direct-lift Techniques
As the name itself suggests, a direct-lift technique entails fabricating the porcelain margin directly on the master gyp- sum die with repeated removal (ie, lifting) of the casting off that die for each porcelain firing cycle. Furthermore, nothing is interposed between the die and the dental porcelain other than a thin layer of a liquid separating medium.
The gypsum die and metal substructure
For best results, the shoulder preparation of the tooth and the resulting stone die should have a width of approximately 1.2 mm.20 This dimension provides between 0.3 and 0.5 mm of space for metal and 0.2 to 0.3 mm for opaque porcelain, thus allowing the porcelain-margin material to have a uniform thickness of at least 0.7 mm20 and as much as 1.0 mm21 (see Fig 10-8). It is important to stress that these dimensions are only approximate because highly skilled ceramists invariably can achieve esthetic outcomes for clinical cases in which the prepared shoulder is actually less than 1.2 mm. In the event the prepared shoulder is not uniform in width because of extensive dental caries, restorative material, or previous over-preparation for a crown, modifications can be made in the substructure. In other words, wax can be added to the labial surface of the wax pattern to create that desired uniform shoulder dimension. Any needed adjustments can be made before the substructure is invested and cast (see Fig 4-29). The exposed shoulder should allow for a uniform thickness of shoulder porcelain. From this point forward, the substructure is adjusted and finished, cleaned, oxidized, and opaqued as described in chapter 8.
It is only after a uniform opaque porcelain veneer has been applied and fired that the porcelain labial margin is created. According to some authors, the relative simplicity of a direct-lift technique is its principal advantage. As explained in the following sections, ceramists have a choice of materials from which to choose when creating the actual ceramic margin.
Direct-lift technique with conventional dentin porcelain
This early version of the porcelain-margin restoration was introduced nearly 40 years ago and called for extending the opaque layer from the metal substructure over the prepared shoulder on the gypsum die right to the axiogingival line angle.5 After the moist opaque was condensed, a con- cavity had to be created in the opaque layer, and the restoration was carefully lifted off the die. Opaque that did not separate and remained on the gypsum die was brushed away but replaced on the underside of the opaque margin. The casting, complete with its shoulder of opaque porce- lain, was dried slowly and vacuum fired. Any marginal gaps were also filled with additional opaque porcelain, and the restoration was refired.
What was different about this technique was the need to capture the entire shoulder in a concave foundation of opaque porcelain. Dentin porcelain did not come into play until it was applied over the concave shoulder of fired opaque to the external margin. Conventional dentin porcelain was applied to the remainder of the coping with a veneer of enamel porcelain added to complete the first body buildup. The restoration was fired, and additional bakes were made as needed. Minor marginal deficiencies were remedied using a mixture of correction and dentin porcelains in a 1:3 ratio. The author claimed a marginal gap of only 6.0 µm was achievable with this technique.
Challenges
The ceramic margin foundation had to be created in opaque. In actual practice, users found that lifting the crown off the die with wet opaque porcelain intact was not easy to accomplish. Nor was the task of adding opaque to the underside of the ceramic shoulder when small amounts of opaque did not separate cleanly. Despite the application of a separating medium, small areas of porcelain often were left behind on the die shoulder.5 Technicians also had to exercise great care to avoid fired opaque from being visible externally as part of the ceramic margin. This was a concern for the dentist because exposed porcelain should not be in contact with the gingival tissues. Fired opaque is rough, cannot be polished or glazed, and should be veneered with a body porcelain. With the other direct-lift techniques, conventional body porcelain or a high-fusing shoulder porcelain create the entire ceramic margin, so there is no similar concern.
Direct-lift technique with porcelain-wax
The same preliminary steps for preparing the gypsum master die and the opaqued metal casting should be followed for the direct-lift technique with shoulder porcelain. To create the ceramic margin, a dental wax, rather than distilled water or modeling liquid, was melted and mixed with con- ventional body (ie, dentin) porcelain. Initially, users had to incorporate dentin powder of the appropriate shade for the definitive restoration into the molten wax.21 Proponents of this approach believed the wax-porcelain suspension facilitated the fabrication process compared to the handling required of other direct-lift techniques prevailing at the time. Perhaps the principal benefit to ceramists was that the rigidity of the cooled wax-porcelain mix made separation of the margin porcelain from the gypsum die relatively easy, assuming the die was properly lubricated with a separating medium. To facilitate fabrication, this technique lent itself to the use of an electric wax spatula. The adjustable temperature of the spatula aided heating, facilitated placement, and extended the working time slightly for the porcelain-wax shoulder materials.
It was possible to create your own porcelain-wax mix- ture. When doing so, it was essential to heat the wax until it was fluid, add in the correct volume of porcelain pow- der, and quickly incorporate the powder into the molten wax. Users soon learned that once the wax had cooled, the mixed material was ready for use, and no addition porce- lain powder could be added without heating the wax again. Recommendations for the powder-to-wax ratio (by weight) ranged from 6:122 to a suggested maximum ratio if 8:1.
Challenges
The most significant challenges associated with the porcelain-wax technique were dealing with wax as the car- rying vehicle for the powder, and the fact that the porcelain and wax mixture could not be condensed. In fact, once the mixed material was applied to the labial margin and the wax had cooled, the margin was set. The impact of this lim- itation was described in a study that reported that the addi- tion of wax (or resin) to the porcelain powder reduced both the density and the apparent tensile strength of fired shoul- der porcelain as well as conventional body porcelain; the reductions were statistically significant.
Direct-lift technique with porcelain-resin
Combining porcelain with a visible light–activated, unfilled acrylic resin had its advantages. It allowed a ceramic mar- gin to be formed in the shoulder area quite rapidly, and light activation set the margin once the ceramist established the desired shape and position. Users found the porcelain-resin mix to be easy to place and polymerize, and restorations could be lifted off their respective master dies immediately after polymerization was complete.
This methodology facilitated the simultaneous removal of multiple retainers of a fixed partial denture, which oth- erwise would be a challenge even when using con- ventional high-fusing shoulder porcelain. Furthermore, the porcelain-resin combination was seen as especially helpful when dealing with preparation margins that were not uniform—irregular in shape and uneven in width. How- ever, the mean marginal openings for restorations report- edly ranged from 21.95 to 148.35 μm.
Challenges
In time it was discovered that the addition of either a resin or a wax to conventional body porcelain significantly reduced both the density and the apparent tensile strength of the fired porcelain.24 In addition, the optical properties of the ceramic margin could also be compromised. The areas in the porcelain mix occupied by unfilled resin pre- vented condensation of the porcelain. Without the ability to condense the marginal porcelain, the numerous resin-filled areas became voids in the fired margin porcelain. Nonethe- less, the porcelain-resin technique did facilitate the fabri- cation process by permitting the capture of irregular shoul- der margins and made removal of a restoration from its die far easier.
Direct-lift technique with shoulder porcelain
The introduction of commercially available high-fusing shoulder (ie, margin) porcelains heralded a major advance- ment and was a marked departure from those earlier tech- niques that relied on either opaque porcelain or con- ventional dentin porcelain to create a ceramic margin. Once dental manufacturers made available kits of spe- cial high-fusing shoulder porcelains, there was no longer a need to improvise and experiment with different materi- als and techniques. Ceramists could achieve an esthetic ceramic margin on metal-ceramic restorations with a clin- ically acceptable fit on a routine basis. Three advantages quickly became apparent.
First, the shoulder porcelains undergo minimal firing shrinkage (if properly condensed), as compared to con- ventional dentin porcelain. Yet, like all metal-ceramic por- celains, shrinkage will occur during sintering, so a ceramic margin typically cannot be produced with a single firing. Multiple bakes are needed to achieve acceptable marginal accuracy and fit.
Second, shoulder porcelains have relatively high fus- ing temperatures, and that is by design. In fact, many are heated as much as 20°C to 50°C higher than the subse- quent body bakes. Consequently, this type of ceramic margin tends to remain dimensionally stable when a res- toration is fired at the temperatures recommended for the body buildup and even the final glazing cycle. The obvious advantage here is that the ceramic margin remains intact and does not slump or round for the balance of the fabri- cation process.
Third, numerous dental manufacturers market high-fusing shoulder porcelain, so ceramists do not have to mix differ- ent components or otherwise create their own unique mar- gin porcelain for each clinical case. They can simply dis- pense the needed materials from commercial products designed to provide them with a greater assurance of com- patibility, color matching, and reliability in the production of metal-ceramic restorations with a porcelain margin.
Challenges
Many of the deficiencies and technical problems associ- ated with the techniques previously described have been eliminated or overcome with the direct-lift technique with shoulder porcelain, making simplicity its principal advan- tage.22 Furthermore, the technical procedures required with shoulder porcelain can be learned relatively quickly, so producing this type of restoration is within the reach of many dental laboratory technicians. Perhaps the sin- gle greatest challenge is not to be found in the laboratory but does impact any laboratory technique: the need for a well-prepared shoulder that is flat with a distinct external finish line (see Fig 10-7a). These clinical variables remain under the control of the prescribing dentist, and if not well executed, make it difficult for any dental laboratory to achieve an ideal porcelain margin irrespective of the mate- rials selected or the technique used.
Laboratory Procedures: Direct-Lift Technique with Shoulder Porcelain
As with the other direct-lift techniques, the master gyp- sum die is ditched in the usual manner with the finish line marked in red (or other bright color). Seal the shoulder and finish line with a thin layer of cyanoacrylate cement or die hardener (Figs 10-9a and 10-9b). Once the sealed mar- gins are dry, a light application of a commercial separat- ing medium (or mineral oil) should be applied to the gyp- sum margin (Fig 10-9c) to facilitate separation and removal of the delicate, unfired shoulder porcelain from the master die. While these procedures are being performed the metal substructure can be opaqued (Figs 10-9d and 10-9e) as described in chapter 8.
Fabrication of the ceramic margin
The fabrication process begins with the return of the opaqued casting to the master die and the incremental placement of mixed shoulder porcelain directly on the lubri- cated margin area (Figs 10-9f to 10-9w). You should antic- ipate the need for at least two shoulder porcelain appli- cations and two firings to obtain a ceramic margin with acceptable seat and seal.
For that second margin buildup, start with the incremen- tal placement of smaller amounts of porcelain across the entire shoulder area, seat the work, condense the porce- lain, and shape the material to the proper contour before lifting the casting off the die, cleaning the intaglio surface, and sintering the restoration (Figs 10-9x to 10-9hh). Do not overbuild the margin. Err on the cautious side and reapply a thin layer of the mineral oil separating medium between porcelain applications until this technique has been mas- tered. Users new to these procedures should be pre- pared for a third porcelain application and firing, particu- larly if marginal discrepancies are noted (see Figs 10-9w and 10-9gg). If no gaps or defects are detected and no additional applications of shoulder porcelain are needed, proceed with the body buildup and complete the restoration in the usual manner (Fig 10-9ii).
Use of colloidal silica
Although either distilled water or modeling liquid is usu- ally mixed with a shoulder porcelain, some have suggested substituting colloidal silica (ie, phosphate-bonded invest- ment liquid) to increase the strength and decrease the shrinkage.25 However, Brackett et al found no increase in strength and reported that colloidal silica actually adversely affected the quality and appearance of the sintered por- celain. They noted that the fired porcelain had silica inclusions, increased surface roughness, and undesirable changes to its appearance once glazed.
Management of porcelain shrinkage
As shown in Figs 10-9r, 10-9s, and 10-9dd, inspecting and cleaning the interior of any casting following each ceramic application should be routine before the shoulder porce- lain is actually fired. Expect the greatest amount of porce- lain shrinkage to occur with the first firing despite proper condensation. Shoulder porcelains, like conventional body porcelains, are going to shrink toward bulk when heated. If the first buildup is overbuilt, too much of the shoulder por- celain will be drawn farther away from the marginal area during the sintering process. This will result in a larger than normal marginal opening (see Fig 10-9w). Even under ideal circumstances when the correct amount of shoulder por- celain is applied, expect shrinkage to be more substan- tial on the initial firing than the second and subsequent fir- ings (see Figs 10-9gg and 10-9hh). After all, the volume of unfired margin porcelain is greatest for the first bake and decreased substantially for the second application and fir- ing. A minor third corrective bake may be necessary to achieve a satisfactory marginal seal for dental laboratory technicians new to performing these procedures.
Case Presentations
A series of additional clinical cases are presented in which esthetic outcomes have been achieved using metal-ceramic technology for single anterior crowns (Figs 10-10 to 10-12), single posterior crowns (Figs 10-13 to 10-15), a posterior fixed partial denture (Fig 10-16), and multiple individual anterior crowns (Fig 10-17). Suffice it to say that a well-conceived treatment plan, coupled with capable clinical and laboratory execution, can result in porcelain-margin metal-ceramic restorations with esthetic outcomes rivaling many of the all-ceramic alternatives
Summary
The direct-lift techniques can be used to produce a porcelain-margin metal-ceramic restoration, but the qual- ity of the end result will vary in accordance with the knowl- edge and skill of the dental laboratory technician, not to men- tion the limitations inherent with the materials used and the clinical variables under the control of the prescribing den- tist. Dental laboratory technicians may find they are able to obtain very satisfactory results by avoiding the more tech- nically demanding platinum-foil and flawed porcelain-wax or porcelain-resin methods and focusing their efforts on mastering the direct-lift technique using a commercial high- fusing shoulder porcelain.
Users who are new to these materials and techniques should bear in mind that shoulder porcelain is intended to provide a visually undetectable scaffold or foundation to be veneered by dentin porcelain (see Fig 10-8). Familiarity with the different fabrication options and the various mate- rials available to produce porcelain-margin restorations is important because not all prepared shoulders are text- book in dimension and geometry. Therefore, dental labora- tory technicians will benefit from developing an expanded armamentarium of techniques to create a definitive resto- ration when variables outside their control must be dealt with in the dental laboratory. It is hoped that the information and examples presented in this final chapter illustrate how creating lifelike fixed res- torations may not be so much a question of which materi- als are selected, but more a matter of how the materials selected are actually used.
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