Introduction
The length and quality of service provided by a partial denture depend on the main- tenance of both the denture itself and the patient’s supporting dentition. As tooth units are lost, it may be possible simply to augment an existing prosthesis by adding denture teeth and pink acrylic. However, if supporting teeth require either a new or replacement extra-coronal restoration, the adaptation of the new restoration to the denture may be problematic, particularly for an existing alloy denture and if dentists are not aware of all the options. If the existing denture is replaced, it is expensive. A reasonable short-term solution is simply to cut off clasps and rests, but loss of support, retention, and fit, may eventually have unwanted consequences. A better option is to create a new extra-coronal restoration which conforms accu- rately to the existing alloy denture and to the occlusion. This is not an over-challenging task and allows a satisfactory denture to remain in service. Therefore, this chapter provides an outline of techniques for how to do this. When carried out successfully, they can save time for the dentist and expenditure for the patient. In addition to the various impression techniques for the new restoration, will invariably be required an accurate opposing impression and an appropriate interocclusal record.
Since Killebrew first published on this topic in 1965, many revised tech- niques have been described; some of them are so complex and technique- sensitive that they can be tricky to execute precisely. Conversely, some are simple but may require extensive adjustment of the final extra-coronal restoration prior to cementation. Others, whilst working effectively, have been superseded by more user-friendly techniques, with the introduction of new materials and new crown production methods. Many earlier reported techniques involve recording informa- tion about the original form of the tooth in question , but little or no information was gathered about the denture itself. Nowadays, much greater efforts are made to record the features of the existing denture.
Since Killebrew first published on this topic in 1965, many revised tech- niques have been described; some of them are so complex and technique- sensitive that they can be tricky to execute precisely. Conversely, some are simple but may require extensive adjustment of the final extra-coronal restoration prior to cementation. Others, whilst working effectively, have been superseded by more user-friendly techniques, with the introduction of new materials and new crown production methods. Many earlier reported techniques involve recording informa- tion about the original form of the tooth in question , but little or no information was gathered about the denture itself. Nowadays, much greater efforts are made to record the features of the existing denture.
A range of techniques illustrative of the many available will be described includ- ing two preferred methods currently used at several UK dental schools.
Simple Method
The simplest technique is to record an impression for the new crown with the den- ture in place. In this way, the denture is picked up within the impression and sent to the laboratory. Before pouring-up and to allow the denture to be easily removed from the cast, any undercuts are carefully blocked out. The denture can later be seated on and off the cast and a closely adapted wax-up made for the new crown. A major disadvantage with this method is that patients are left without the denture between appointments. However, most of the other techniques described below allow patients to keep the denture.
Where only the fit surface of the denture needs to be accommodated against a new crown, we recommend the following technique: A working impression of the preparation is recorded with the denture in situ. However, instead of sending the denture to the laboratory, it is removed from the impression and returned to the patient. The technical process is outlined in Fig. 25.1 and involves the lab mak- ing an acrylic matrix representing the denture’s fitting surface in the region of the crown preparation. To do this, the technician flows acrylic pattern resin into the impression space previously occupied by the denture. To aid subsequent location of the matrix, acrylic is also flowed into the lingual aspects of the impression spaces of adjacent teeth. Once set, the acrylic matrix is removed and relocated on the stone cast allowing adaptation of the restoration pattern to the matrix and full access to the preparation margins.
An alternative which works well when adapting new anterior crowns to an exist- ing partial denture is to incorporate the adjacent teeth in the acrylic denture replica. This approach gives very positive reseating of the acrylic matrix against the under- lying stone cast (Fig. 25.2) and avoids having fragile stone teeth anteriorly which may fracture easily.
Simple Method
The simplest technique is to record an impression for the new crown with the den- ture in place. In this way, the denture is picked up within the impression and sent to the laboratory. Before pouring-up and to allow the denture to be easily removed from the cast, any undercuts are carefully blocked out. The denture can later be seated on and off the cast and a closely adapted wax-up made for the new crown. A major disadvantage with this method is that patients are left without the denture between appointments. However, most of the other techniques described below allow patients to keep the denture.
Where only the fit surface of the denture needs to be accommodated against a new crown, we recommend the following technique: A working impression of the preparation is recorded with the denture in situ. However, instead of sending the denture to the laboratory, it is removed from the impression and returned to the patient. The technical process is outlined in Fig. 25.1 and involves the lab mak- ing an acrylic matrix representing the denture’s fitting surface in the region of the crown preparation. To do this, the technician flows acrylic pattern resin into the impression space previously occupied by the denture. To aid subsequent location of the matrix, acrylic is also flowed into the lingual aspects of the impression spaces of adjacent teeth. Once set, the acrylic matrix is removed and relocated on the stone cast allowing adaptation of the restoration pattern to the matrix and full access to the preparation margins.
An alternative which works well when adapting new anterior crowns to an exist- ing partial denture is to incorporate the adjacent teeth in the acrylic denture replica. This approach gives very positive reseating of the acrylic matrix against the under- lying stone cast (Fig. 25.2) and avoids having fragile stone teeth anteriorly which may fracture easily.
Accommodating Clasps and Rests
Whilst the previously described methods are useful, they are not a good solution for rests and clasps. The relationship of these more intricate elements to the preparation is best recorded directly. A convenient way of doing this is with a silicone matrix. This is made by syringing a silicone mousse material, normally used for occlusal registration, around the preparation with the denture in place (see Fig. 25.3). Importantly, the denture is not picked up within the working impression for the new crown. Instead, the working impression is recorded as normal without the denture in place. This impression along with the separate silicone index is sent to the labora- tory. A provisional restoration for the preparation is made in the usual way (see Chap. 23), but to prevent it being dislodged, take care to ease it where it fits tightly against the partial denture.
In the lab, a removable replica of the clasps and occlusal rests is made by flowing acrylic pattern resin into the silicone index located on the stone die. Once set, this replica forms an index which is used to shape the wax pattern for the crown. The replica must be reliably relocated on the cast to ensure clasps and rests are correctly orientated. One way of creating a stable location for the replica is to extend the resin into two or more holes strategically drilled into the working cast. Another way, which we prefer, is simply to extend the resin mesially and distally onto suitable features already present on the stone surface.
There are several similar techniques for dealing with clasps and rests (see Box 25.1).
Accommodating Precision Attachments
Although this presentation is comparatively rare, it may occasionally be necessary to consider retrofitting a new extra-coronal restoration to a denture precision attach- ment. This is described most recently by Uludag—although the precision attach- ment described in the paper is probably only available in and around Turkey. Nonetheless, the method may be adapted for retrofitting crowns having an extra- coronal attachment (patrix) which engages with a resilient matrix within a free-end partial denture. This sort of resilient matrix allows a small amount of hinge move- ment of the denture under occlusal loading. Essentially, the denture is picked up within the crown impression and a master cast poured up. The denture is then removed from the cast and a proprietary patrix pattern inserted in the resilient matrix within the denture. With the patrix pattern correctly located, it can then be waxed into the coping for the new crown and then cast. A disadvantage of the technique is that patients need to be without their denture between appointments, but this may be a small price to pay compared with having a sophisticated partial denture remade.
CAD/CAM
Marchack [9] describes a method of creating a durable zirconia coping designed to contact the denture components, whilst the aesthetic parts of the crown are built in weaker sintered ceramic (see Chap. 14). The clinic and laboratory procedures are as follows:
In the clinic, the denture is seated and provisional crown composite syringed over the tooth preparation and under the denture components. Once set, the result- ing composite coping captures the denture’s guide planes. It also captures the fit surfaces of the rest seats and clasp assemblies. The composite coping is removed and an impression recorded of the tooth preparation without the denture in place. Both the impression and coping are sent to the laboratory.
In the laboratory, the cast is poured. The die is then trimmed and scanned. Before placing the coping on the die, it is cut back to accommodate the aesthetic sintered ceramic. A further scan is then made with the composite coping in situ. The coping is then finalised on screen and machined using CAD/CAM. Finally, it is veneered with ceramic, avoiding layering over the active elements which will engage with the denture. This technique could easily be adapted for use with crowns made from mono- lithic zirconia which would avoid the need for the coping to be cut back.
The advantage of building zirconia features to accommodate clasps and rests is that it should reduce the risk of ceramic chipping or fracture. For the same reason, where ceramo-metal crowns are being made, it is best to accommodate the rest seats and guide planes in metal. Nevertheless, aesthetic considerations may sometimes preclude areas of clasp engagement on crowns being made in metal
Conclusion
Instead of replacing an existing but satisfactory alloy partial denture, it is always worth considering retrofitting a new restoration. There are many techniques available, and we have highlighted two uncomplicated methods which appear to work well within UK dental schools. Critical to any of the methods is good com- munication between clinician and technician, ensuring from the outset that both parties are clear about the process.
Whilst the previously described methods are useful, they are not a good solution for rests and clasps. The relationship of these more intricate elements to the preparation is best recorded directly. A convenient way of doing this is with a silicone matrix. This is made by syringing a silicone mousse material, normally used for occlusal registration, around the preparation with the denture in place (see Fig. 25.3). Importantly, the denture is not picked up within the working impression for the new crown. Instead, the working impression is recorded as normal without the denture in place. This impression along with the separate silicone index is sent to the labora- tory. A provisional restoration for the preparation is made in the usual way (see Chap. 23), but to prevent it being dislodged, take care to ease it where it fits tightly against the partial denture.
In the lab, a removable replica of the clasps and occlusal rests is made by flowing acrylic pattern resin into the silicone index located on the stone die. Once set, this replica forms an index which is used to shape the wax pattern for the crown. The replica must be reliably relocated on the cast to ensure clasps and rests are correctly orientated. One way of creating a stable location for the replica is to extend the resin into two or more holes strategically drilled into the working cast. Another way, which we prefer, is simply to extend the resin mesially and distally onto suitable features already present on the stone surface.
There are several similar techniques for dealing with clasps and rests (see Box 25.1).
Accommodating Precision Attachments
Although this presentation is comparatively rare, it may occasionally be necessary to consider retrofitting a new extra-coronal restoration to a denture precision attach- ment. This is described most recently by Uludag—although the precision attach- ment described in the paper is probably only available in and around Turkey. Nonetheless, the method may be adapted for retrofitting crowns having an extra- coronal attachment (patrix) which engages with a resilient matrix within a free-end partial denture. This sort of resilient matrix allows a small amount of hinge move- ment of the denture under occlusal loading. Essentially, the denture is picked up within the crown impression and a master cast poured up. The denture is then removed from the cast and a proprietary patrix pattern inserted in the resilient matrix within the denture. With the patrix pattern correctly located, it can then be waxed into the coping for the new crown and then cast. A disadvantage of the technique is that patients need to be without their denture between appointments, but this may be a small price to pay compared with having a sophisticated partial denture remade.
CAD/CAM
Marchack [9] describes a method of creating a durable zirconia coping designed to contact the denture components, whilst the aesthetic parts of the crown are built in weaker sintered ceramic (see Chap. 14). The clinic and laboratory procedures are as follows:
In the clinic, the denture is seated and provisional crown composite syringed over the tooth preparation and under the denture components. Once set, the result- ing composite coping captures the denture’s guide planes. It also captures the fit surfaces of the rest seats and clasp assemblies. The composite coping is removed and an impression recorded of the tooth preparation without the denture in place. Both the impression and coping are sent to the laboratory.
In the laboratory, the cast is poured. The die is then trimmed and scanned. Before placing the coping on the die, it is cut back to accommodate the aesthetic sintered ceramic. A further scan is then made with the composite coping in situ. The coping is then finalised on screen and machined using CAD/CAM. Finally, it is veneered with ceramic, avoiding layering over the active elements which will engage with the denture. This technique could easily be adapted for use with crowns made from mono- lithic zirconia which would avoid the need for the coping to be cut back.
The advantage of building zirconia features to accommodate clasps and rests is that it should reduce the risk of ceramic chipping or fracture. For the same reason, where ceramo-metal crowns are being made, it is best to accommodate the rest seats and guide planes in metal. Nevertheless, aesthetic considerations may sometimes preclude areas of clasp engagement on crowns being made in metal
Conclusion
Instead of replacing an existing but satisfactory alloy partial denture, it is always worth considering retrofitting a new restoration. There are many techniques available, and we have highlighted two uncomplicated methods which appear to work well within UK dental schools. Critical to any of the methods is good com- munication between clinician and technician, ensuring from the outset that both parties are clear about the process.
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