Prosthetic Role in Peri-Implant Soft Tissue Management: Prosthetic Phase

Restoration Design (Screw Vs. Cement Retention)
Fixed implant restorations can either be screw or cement retained (Fig. 8.1). Each design has its merits and drawbacks. Screw-retained restorations are retrievable. Retightening a loose abutment screw can therefore be performed easily. They do not depend on cement for retention. Therefore, the problem of removing excess cement does not exist. However, they require special technical skills and are usually more expensive than cement-retained restorations as they may require additional components and procedures.

Cement-retained restorations do not have a screw access channel; therefore, they are more esthetic and provide better occlusal anatomy contacts (Fig. 8.2). They are usually easier to fabricate especially if ready-made stock abutments are used. When multiple cement-retained restorations are used, passivity becomes an advantage. The presence of a cement gap space, the amount of which can be controlled, ensures passivity of the restoration, which is important to avoid undue stresses on implants when abutment screws are tightened . However, if excess cement is not com- pletely removed, adverse peri-implant soft tissue reactions can occur.
 
The choice between screw and cement retention depends on many factors, including interocclusal distance, location of the screw access channel, esthetic demands, need for restoration retrievability, frequency of screw loosening, labora- tory efficiency, need for passivity of fit, combining natural tooth and implant sup- port, and finish line location.

It is important to mention that the choice between using a screw- or cement- retained restoration should be decided during the treatment plan phase and not after implant placement. Most of the factors of choice between both designs can be deter- mined during treatment planning. Therefore, the restoration design and means of retention are always predetermined, and the treatment plan and implant positioning and alignment are always geared toward achieving a predictable restoration.

Interocclusal Distance
The bond strength of cement retention depends on several factors, the most impor- tant being the surface area for cement bonding . The larger the surface of the abutment, the greater the bond strength between the abutment and restoration.

Generally, in cases with a wide inter-arch distance allowing for a longer abutment, screw and cement retention are equally acceptable. However, in cases with narrow inter-arch distance, screw retention is more favorable as it depends on the tightness of the abutment/restoration screw rather than abutment surface area.

Unlike cement-retained abutments, those for screw retention usually cannot be modified or shortened. Interarch distance should therefore be measured, and the minimal height of a screw-retained abutment should be selected according to manu- facturer guidelines (Fig. 8.3).

Location of the Screw Access Channel
Screw-retained restorations require a screw access channel that should be located on the palatal surface of anterior teeth (Fig. 8.4) and in the center of the occlusal sur- face of posterior teeth (Fig. 8.5) so as not to interfere with the esthetic and biome- chanical qualities of the restoration. Screw access channels are usually blocked with the composite resin of a similar shade to the restoration (Fig. 8.6).

However, the shade of the composite does not always match the shade of the restoration, especially if the restoration has a metal component, such as porcelain fused to metal. The shadow of the metal usually interferes with the esthetic qualities of the restoration (Fig. 8.7). Locating a screw access channel on an incisal edge or a functional cusp tip might weaken the restoration and therefore should be avoided (Fig. 8.8). This requires implants to be properly aligned during placement to ensure a favorable access hole location. An alternative would be to use angled screw- retained abutments or cement-retained restorations that do not have a screw access channel on the final restoration.

Abutment/Restoration Finish Line Position

Abutment/restoration finish line location can either be supragingival, subgingival, or equigingival. Supragingival finish lines have many advantages. They are easier during impression making as there would be no need for soft tissue retraction that might compromise peri-implant soft tissue attachment. Peri-implant soft tissue does not collapse between visits if a temporary restoration is not used, finish line visibility makes lab work procedures simpler, detection of restoration abutment marginal fit is more predictable, and excess cement can easily be removed. The main drawback of supragingival finish lines is the unesthetic appearance of the abut- ment collar especially if a metal abutment is used (Fig. 8.9).

For cement-retained restorations, excess cement can be completely removed without any residues that may compromise the peri-implant soft tissue health. Even for screw-retained restorations, supragingival finish lines are preferable from a bio- logical point of view. A micro gap always exists at the abutment restoration junc- tion, the amount of which varies depending on the technique and accuracy of prosthesis construction [12]. A micro gap of considerable size that may result from a restoration with inaccurate margins can harbor bacteria that may cause peri-implant soft tissue problems. Supragingival location of the micro gap is there- fore desirable. When selecting a supragingival finish line, both designs would be similar from a biological point of view.

To reduce the amount of micro gap between prosthetic components, it is always recommended to torque the abutment screw according to manufacturer’s instruc- tions by using a torque wrench. According to studies, this significantly reduces the micro gap and consequently peri-implant soft tissue problems.

 

Subgingival finish lines are mainly indicated in the esthetic zone to avoid abut- ment collar display that can compromise esthetics, especially if a metal abutment is used. In such cases, the subgingival depth of the finish line must be controlled to avoid peri-implant soft and hard tissue problems.

 

Cement-Retained Restorations with a Screw Access Channel 

Cement-retained restorations have many advantages; however, the difficulty encountered in the complete removal of excess cement precludes their routine use. To solve the problem of excess cement, a modified cement-retained restoration was advocated combining the advantages of screw and cement retention. The modification involves the creation of a screw access channel on the occlusal surface of the cement-retained restoration (Fig. 8.10). After complete intraoral cementation (Fig. 8.11), the restoration/abutment assembly can be removed by unscrewing the abutment screw through the screw access channel and the excess cement can be completely removed extra orally (Fig. 8.12a–c). The restoration after cementation becomes permanently attached to the abutment and can be screw retained onto the implant. The screw access channel can later on be filled with a composite of similar shade [1] (Fig. 8.13a,b).

The advantage of this design is that it combines the benefits of both screw and cement retention. The limitations, however, include that it is not recommended if the screw access channel is not in a favorable position from an esthetic or biome- chanical point of view (Fig. 8.14).

Studies have been conducted to evaluate the effect of creating a screw access channel on the structural durability and the fracture resistance of the restoration. Different restorative materials were tested, including zirconia, lithium disilicate, and porcelain fused to metal. In spite of the variability of the results between various studies and tested materials, most of the studies did not show significant weakening of the restorations. In multiunit restorations where passivity of the restoration is of utmost importance, this technique can also be applied pro- vided that implants are parallel or very close to being parallel [18–20] (Fig. 8.15a,b).

 

Peri-implant Soft Tissue Thickness/Finish Line Location
Locating the finish line in the desired position is achieved by first measuring the peri-implant soft tissue thickness from the implant platform to the free gingival margin, then selecting an abutment with a suitable collar height that can be shorter than the soft tissue thickness to position the finish line subgingivally or greater than the soft tissue thickness to position the finish line supragingivally. It is therefore important to give the soft tissue a chance to heal and stabilize after second-stage surgery to be able to properly locate the restoration finish line [21] (Fig. 8.16). Measuring peri-implant soft tissue thickness can be performed by using many instruments, the easiest of which is a plastic periodontal probe (Fig. 8.17).

Cement-retained restorations have always been guilty of causing peri-implant soft tissue irritation and inflammation due to the excess cement that is difficult to completely remove. The main cause of peri-implant soft tissue problems associated with cement-retained restorations is the inappropriate finish line location resulting from improper abutment collar height selection or designing. A cement-retained restoration with a supragingival finish line does not cause any soft tissue problems as excess cement can be completely removed. Even if a 1 mm subgingival finish line is used, excess cement can still be removed conveniently. However, in the case of deep subgingival finish lines greater than 2 mm, excess cement removal becomes very difficult, especially interproximally, as it is associated with bleeding and pain preventing proper cement removal. Residual cement causes peri-implant soft tissue problems that can advance to peri-implantitis (Fig. 8.18).

Relation Between Finish Line Contour and Peri-implant Soft Tissue Contour

Measuring peri-implant soft tissue thickness is usually performed at the midfacial region. However, peri-implant soft tissue thickness may vary between the midfacial and interproximal regions. The difference would be less in cases associated with loss of interdental papillae and more in cases with well-preserved interdental papil- lae (Fig. 8.19a,b).

Stock abutments usually have a finish line that has a straight contour. To achieve a 1-mm subgingival finish line position, the abutment collar height should be 1 mm shorter than the peri-implant mucosa thickness at the midfacial region. The problem usually exists at the proximal surfaces, where the 1-mm subgingival midfacial finish line becomes much deeper interproximally due to the thicker peri-implant mucosa at the interdental papilla region (Fig. 8.20a,b). When cement-retained restorations are used over stock abutments with a 1-mm subgingival midfacial finish line, excess cement would be easily removed facially but would be very difficult to remove interproximally. In such cases, custom-made cement-retained abutments are pre- ferred and recommended [22, 23] (Fig. 8.21). Stock abutments can be acceptably used in cases with missing interdental papillae where the soft tissue contour is flat rather than curved (Fig. 8.16).

Custom-made abutments can be constructed by using conventional techniques or CAD-CAM technology. They can have a custom-made emergence profile together with a finish line contour that matches the peri-implant soft tissue contour. A 1-mm subgingival finish line can be maintained all around the restoration margins, making excess cement removal feasible and predictable (Fig. 8.21).

Screw-retained restorations are often preferred over cement-retained ones from a peri-implant soft tissue health point of view. Excess cement, when not completely removed, has always been a frequent reason for peri-implant soft and hard tissue problems [2]. However, a big part of the problem is related to the location of the abutment restoration finish line. Properly constructed cement-retained restorations with proper finish line location can be as good as screw-retained ones.

Prosthetic Materials and Their Relation to Peri-implant Soft Tissue Health
The increasing demand for esthetic restorations has triggered the advancement and improvement of tooth-colored materials to match the high biomechanical and esthetic expectations. Tooth-colored materials are usually nonmetallic, either ceramic based or polymer based. However, metals are still being used with specific indications mainly related to their biomechanical superiority. Commonly used materials include metals such as gold, titanium, and cobalt chromium. Nonmetallic materials include ceramics, zirconia, resin nano ceramics, and BioHPP. Acrylics can also be used, however, mainly as temporary restorative materials.

Implant restorations can be constructed by using conventional techniques or CAD-CAM technology that is taking over prosthodontics to a new dimension. All prosthetic materials are currently being produced for CAD-CAM fabrication, which has improved the accuracy and reduced the time needed for prosthesis construction. Improving the accuracy involves reducing the micro gap between prosthetic compo- nents that in turn improves the peri-implant soft tissue health and reduces complications.

The selection of prosthetic materials is usually based on many factors, including biomechanical, esthetic, and biological. Despite the importance of the biomechani- cal and esthetic features and properties of restorative materials, their biological interaction with peri-implant soft tissues is of major importance. In many situations, the prosthetic material becomes in direct contact with peri-implant soft tissues, especially with subgingival finish lines (Fig. 8.22).

Polished metals especially titanium have produced acceptable results. Glazed ceramic has also shown good soft tissue response. Acrylics are used with caution as they are porous and therefore less hygienic than other definitive materials. They are also stained easily and therefore need to be changed frequently. When acrylic is included in a definitive implant restoration, it is always advisable to fabricate the restoration screw retained.

Many studies were conducted to evaluate the peri-implant soft tissue response to different prosthetic materials . Brunot-Gohin et al investigated soft tissue response to lithium disilicate with three different surface treatments: raw surface treatment, hand-polished surface treatment, and glazed surface treatment. Lithium disilicate polished ceramic provided better adhesion and proliferation than lithium disilicate glazed ceramic (Fig. 8.23). Van Brakel et al compared the peri- implant soft tissue response to titanium versus zirconia abutments. No differences in soft tissue health were seen in peri-implant mucosa adjacent to zirconia and tita- nium abutment surfaces. Similar results were demonstrated in an animal study by Blanco et al, where peri-implant soft tissue response to implant abutments made of zirconia and titanium was similar after 9 months of healing.

CAD-CAM Technology

CAD-CAM technology has revolutionized prosthodontics. Digital workflows have replaced conventional procedures in almost all prosthetic work. The reason for that is that CAD-CAM technology improved the accuracy and predictability of pros- thetic restorations and reduced the time and steps needed to fabricate restorations [23, 27, 33, 34]. The impact of CAD-CAM technology on peri-implant tissues is related to acquisition procedures together with marginal accuracy and passivity of restorations. Acquisition procedures involve digital impressions obtained by using scan bodies attached to implants or abutments [27, 34, 35]. When properly seated, scan bodies lightly retract peri-implant soft tissues, eliminating the need for soft tissue retraction associated with conventional impressions (Fig. 8.24).

Restorations performed by using CAD-CAM technology are machine depen- dent, eliminating the personal variation associated with dental technicians. Restoration margins and passivity are therefore more predictable with their positive impact on peri-implant soft and hard tissues [33, 36] (Fig. 8.25a,b).

Anodic Oxidation of Titanium Abutments
Titanium abutments have been used successfully over dental implants for many years. Peri-implant soft tissue response to polished titanium abutment collars has shown favorable results [37]. Their main drawback is their unesthetic color espe- cially when used in the esthetic zone in cases with thin gingival biotype. Several attempts have been made to change the abutment color by using different materials and coatings. Gold, metal-ceramic, nitride-treated titanium, composite resin-coated titanium, thermal oxidation, chemical oxidation, and anodic-oxidized titanium have been used to mask the unesthetic grayish abutment color, especially the abutment collar [26, 38–43] (Fig. 8.26).

Anodic oxidation is a surface modification technique for changing the abutment color. Titanium is spontaneously coated with an oxide surface layer as soon as it is exposed to atmospheric air. Anodic oxidation promotes the production of a thick oxide surface layer based on the voltage used. Specific colors can be produced by controlling the variation in oxide layer thickness that interacts differently with light. Studies have been conducted to evaluate the esthetic outcome of anodically oxi- dizing titanium abutment collars. Pink collars have been shown to improve gingival esthetics by masking the grayish titanium color (Fig. 8.27a,b). In addition, anodic oxidation alters the surface characteristics of titanium. Increasing the thickness of the surface oxide layer has also been reported to improve its surface hardness and corrosion resistance.

In an in vitro study, Wang et al reported that anodization increased the grain formation, surface roughness, and hydrophilicity of titanium. Although smooth machined titanium surfaces have been recommended for their improved soft tissue response, a degree of roughness is desirable. While the optimal degree of roughness is unclear, the proliferation of human gingival fibroblasts, their viability, and cell morphology have been reported to be similar on anodized and unanodized surfaces. In a split-mouth study, Farrag and Khamis [44] investigated the effect of anodized titanium on peri-implant soft tissue health and esthetics. Pink anodized titanium abutment collars did not produce a clinically significant effect on the health of peri- implant soft tissues. Anodic oxidation of titanium abutment collars is therefore an effective method to mask their grayish unesthetic color, especially in cases with thin gingival biotypes, without clinically compromising peri-implant soft tissues.

Prosthetic Complications with a Biological Impact
Achieving patient satisfaction is always the ultimate goal. However, complications and even implant failure are possible. Implant failure is not something that happens overnight. Failure is usually preceded by a complication that, if not recognized and properly handled, will eventually propagate to failure.

Complications can happen in every step of implant treatment if the procedure is not performed in a correct manner. Problems can result from improper patient selec- tion, which involves prosthetic and surgical considerations, psychological evalua- tion, and even financial considerations. Complications can occur during implant placement, during the healing period, during the prosthetic procedures, and later on after months and years of use. They can also be related to the soft tissue, bone, or the prosthesis itself.

Prosthetic complications with a biological impact on peri-implant bone and soft tissue include restorations with open contacts or margins, improper occlusion, can- tilevers that are too long, screw loosening, non-passive restorations, and connecting implants to natural teeth.

Restorations with open contacts result in food impaction, causing peri-implant soft tissue inflammation known as peri-implant mucositis. If not properly treated, it can propagate to peri-implantitis (Fig. 8.28a–c). Restorations with open margins or micro gaps at the implant abutment or abutment restoration interfaces can also harbor food and bacteria, resulting in soft tissue problems and later on bone loss, especially if the restoration margins or micro gaps are subgingival (Fig. 8.29). Restorations that are not passive, or those with too long cantilevers, or improper occlusion in the form of high points, deflective occlusal contacts, or restorations with sharp cusps can cause undue stress transmitted to the bone, causing bone resorption that is usually associated with implant surface exposure causing soft tis- sue problems (Fig. 8.30a,b).
 
Abutment screw loosening is a frequent complication. Reasons include improper screw torquing according to the manufacturer’s instructions, non-passive restora- tions, improper occlusion, and inaccurate fit between implant components [2]. Screw loosening is associated with biomechanical problems together with micro gaps causing food accumulation that ends up with soft tissue problems [50]. Restoration and abutment micro movement and later on macro movement can also cause mechanical irritation to peri-implant soft tissues with resultant problems (Fig. 8.31).

Connecting implants to natural teeth is a debatable issue. Movement of natural teeth due to the compressibility of the periodontal ligament results in undue stresses transmitted to implants, causing peri-implant bone loss and later on soft tissue prob- lems (Figs. 8.32,8.33). Most of the studies advocate not connecting implants to natural teeth except in very limited conditions when it is absolutely necessary to avoid biomechanical and biological problems.