It is well established that implant integration is fundamental to successful reconstruction. Nevertheless, osseointegration is only one of a number of steps required for implant-based reconstruction of the occlusion. A vital part of this process is determining the appropriate prosthetic tooth position in the reconstruction, which affects implant placement within the alveolar bone.1 Prosthetically guided implant positioning must be accomplished to promote predictable, functional, aesthetic, and hygienically maintainable restorations.2
It is essential to capture the three-dimensional position of the dental implants and associated peri-implant tissues, and then accurately transfer this information to the laboratory master cast.3 This is particularly important when fabricating a superstructure or fixed partial framework casting.4 This is also required when fabricating an implant-supported fixed partial denture (FPD), as any inaccuracies in the transfer of information from the patient to the laboratory can lead to the fabrication of a nonpassive fitted casting, and/or a casting that moves when seated or screwed into place.5 Either of the aforementioned problems will have adverse effects on the implants by placing non-compressional forces on the fixtures. These forces will be detrimental to the long-term success of the implant-restoration complex.
Unfortunately, often the first indication that the exact spatial position of the implant abutments was not precisely transferred to the master cast is at the framework try-in appointment.6 At this appointment, if the framework does not fit passively or does not completely seat, the ill-fitting casting must be sectioned, indexed with a material such as Duralay (Dental Mfg Co), then captured in an impression. The casting must then be soldered into the new indexed position in order to achieve a passively-fitting framework.7
From the perspective of the laboratory technician, this technique for the correction of an ill-fitting casting has inherent drawbacks.8 For example, the solder joint may fail because of porosities in this joint9 and/or the weaker nature of the soldered metal, as compared with the metal used for the FPD framework.10 This inherent drawback may be obviated by the use of laser welding, which when used to join two pieces of an FPD framework, produces a union that is usually as strong as the original casting. However, there is generally an additional cost involved with the use of this technique.11
When the aforementioned procedures must be performed because of an ill-fitting casting, not only is there an increased cost, but the patient's confidence in the dentist may be affected. One method that can prevent the fabrication of ill-fitting castings because of an inaccurate master cast and laboratory working model is the use of an extraorally fabricated verification stent (EOVS).
This article reviews the utilization of a preprosthetic EOVS that can be used by the clinician to ensure the accuracy of the master cast, and ultimately the final restoration. From this master cast the laboratory working model is used to construct the implant-supported prosthesis framework.
The fabrication of a verification stent is an efficient, simple, inexpensive, but valuable procedure that confirms the proper duplication of the three-dimensional intraoral position of the dental implants as they relate to the master cast, prior to initiating the fabrication of the superstructure or FPD framework casting.12 Therefore, the verification stent gives the clinician a high degree of confidence that the master cast is an accurate representation of the patient's intraoral condition, which would allow the fabrication of a properly fitting casting.
The case to be described illustrates the utilization of the EOVS system. This case had root form dental implants placed in the mandibular arch. Four months were allowed for osseointegration before second-stage surgery was performed to uncover the dental implants, and then healing abutments were placed. Approximately 6 weeks later, the healing abutments were removed and the impression copings seated on the implant platforms. Radiographs were taken to ensure that these prosthetic components were properly seated.13 An impression of the dental arches and implant impression coping assemblies was taken, utilizing a tray material and a syringeable medium body polyvinylsiloxane impression material. Following removal of the impression trays, the impression copings were removed, healing abutments were again placed on the dental implants, and metal implant analogs were attached to the impression copings by a fixation screw. The analog-coping assemblies were then properly reinserted into their respective positions in the impression. Lastly, the laboratory fabricated the master casts for the case.14
Because a superstructure for an implant-supported overdenture type prosthesis was to be fabricated, prior to placing the dental stone into the impression with the impression coping analog assemblies, a clear soft liner was first poured into the impression, followed shortly thereafter by the dental stone, which produced a "soft tissue" master cast.15 Regardless of whether a master cast is made with or without the "soft tissue" replica, a small expansion dental stone (ADA Product Classification 3) is used.16 It is important that the individual who mixes the dental stone used to produce a master cast follows the manufacturer's suggested guidelines regarding the amount of distilled water to be used.17 For example, using an insufficient amount of distilled water causes increased expansion of the dental stone during setting.18
|Figure 1. Master cast that should be an exact replica of the three-dimensional position of the dental implants in the patient's mouth.||Figure 2. EOVS fabricated on the working model (duplicate of master cast) with light-cured material on the provisional prosthetic abutments.|
The fabrication of an EOVS and its duplicate working model begins subsequent to the creation of the master cast (Figure 1). In this case, temporary titanium prosthetic abutment heads were placed on each implant laboratory analog on the working lab model (Figure 2). Triad TranSheet light-cure acrylic material (DENTSPLY) was placed completely around these abutments. In order to not interfere with the seating of these components or the visualization of the prosthetic component-implant platform interface, the Triad material was limited to the middle portion of the prosthetic component. The Triad material was then immediately light cured.
In order to minimize potential detrimental effects caused by "curing shrinkage" inherent with all acrylics,19 the fully cured Triad material connecting all the provisional prosthetic abutments (forming the EOVS) was sectioned using a separating disc or 558 bur (Figure 3). These cuts in the EOVS were made between each implant-supported provisional prosthetic abutment.
|Figure 3. EOVS that was sectioned between each dental implant.||Figure 4. The EOVS on the working model with the previously sectioned pieces of the EOVS luted together.|
To accurately and securely reattach these segments, the sections were luted together by syringing Triad Gel over and between the edges of the cut sections (Figure 4). This step ensured that the EOVS fit passively on the master cast and/or working lab model, and was not affected by curing shrinkage.20 These steps are performed subsequent to the fabrication of the master cast to ensure that the EOVS precisely and passively capture the intraoral orientation of the implants.
The EOVS system (consisting of the provisional prosthetic implant abutments connected by the Triad light-cured material) was then removed from the master cast, and inserted in the patient's mouth (Figure 5). This important appointment allowed an intraoral verification that the master cast duplicated the three-dimensional intraoral position of the dental implants, as evidenced by the passive fit of the EOVS system on the dental implant platforms.
The fit between the implant platforms and their respective provisional prosthetic implant abutments, which were connected by the Triad light-cured material as previously described, were inspected thoroughly to ensure that the EOVS fit passively and properly prior to being secured with a prosthetic abutment fixation screw. There was no gap between the implant platforms and their respective provisional prosthetic abutments, nor was there any rocking movement of this interface.21
|Figure 5. The EOVS on top of the implant platforms without any fixation screws in order to check for proper and passive fit.||Figure 6. A prosthetic fixation screw inserted into one of the distal most prosthetic abutments and implant. Note the gap evident between some of the dental implant platforms and prosthetic abutments that was then apparent.|
|Figure 7. The EOVS was sectioned between the two implants closest to the patient's dental midline to assure that each section fits passively.||Figure 8. The EOVS reluted, removed from the patient's mouth, which then had implant analogs attached to each of the implants.|
Next, a prosthetic fixation screw was inserted into the most distal provisional abutment and screwed down with finger pressure only (Figure 6). In this case a poor fit was detected (ie, a gap or rocking movement was observed when the system was gently seated), which indicated that the master model was not an accurate representation of the intraoral condition.22 Therefore, at this point the EOVS system was sectioned near the middle of the stent, as close to the facial midline as possible, using a disc or high-speed handpiece with a 558 bur (Figure 7). Once again, as previously described, the remaining prosthetic fixation screws were placed, and radiographs were taken to ensure there was no space between the implant fixture platform and provisional prosthetic abutment. When confirmed, the two sectioned pieces were joined with Triad Gel as previously described (Figure 8). Subsequent to the reconnection of the sectioned pieces of the EOVS system, the fixation screws were removed and the verification stent system was lifted off the implant fixture platforms and removed from the patient's mouth. An implant analog was then affixed to each of the provisional prosthetic abutments of the EOVS system with prosthetic fixation screws.
The EOVS system was placed into dental stone to fabricate a new master cast (known as the verification cast), the accuracy of which had been precisely confirmed (Figure 9). From this new master cast, the dental laboratory proceeded with the fabrication of the overdenture superstructure for this implant-supported bar overdenture case.
|Figure 9. Properly fitting EOVS on the new master cast.||Figure 10. The implant-supported superstructure casting being inserted into the implant platforms in the patient's mouth. A prosthetic fixation screw is being inserted into one of the terminal implants, followed by a careful inspection of the prosthesis framework casting to assure proper fit.|
Subsequent to the fabrication of this implant-supported superstructure, the patient was seen for a framework try-in appointment. At this appointment, the cast superstructure was carefully inserted onto the dental implant platforms and inspected for passive fit, or any discrepancy in fit, ie, gapping or rocking either before or after the terminal abutment is secured to one of the most distal abutments with a fixation screw23 (Figure 10).
It is important for the clinician to be cognizant of the fact that there should have been no change in position of the casting or its fit on the implant fixture platforms as each screw was tightened. If the casting fits properly, when the fixation screws are turned into place they should not bind when tightened. It is also important to ask the patient if he or she feels any pressure, or a pushing sensation when the framework is screwed into place. If these inquiries do not elicit a positive response, and inspection does not reveal an improper fit, a periapical radiograph of each fixture-abutment complex is taken to assure that the superstructure casting was completely seated before the prosthetic fixation screws were secured into place using the appropriate torque wrench.24 It is imperative to ask the patient if he or she feels any discomfort in the jaw or experiences any tension within the bone once the casting is secured into place.
|Figure 11. The finished implant-supported overdentures.|
Even though the superstructure fits properly in the mouth as it did on the verified master cast, errors could occur if the prosthetic teeth were set prior to verification of the properly fitted superstructure.25 With this in mind, the superstructure framework casting with the artificial teeth positioned in wax was tried in the mouth, and the jaw relationship records and the occlusal contacts were re-checked before completion of the prosthesis. This allowed delivery to the patient of an aesthetic, hygienically maintainable, and properly functioning prosthesis (Figure 11).
The utilization of an EOVS can provide the clinician relative certainty that the laboratory master cast was an accurate representation of the three-dimensional intraoral position of the dental implant fixtures. The result will be a healthy, maintainable, aesthetic, and functional implant-supported restoration (see Table).
It is important to emphasize that the accuracy of the master model is especially important when fabricating a fixed partial denture framework or an implant-supported superstructure casting. The utilization of the described extraoral verification system can be effectively and routinely used by the clinician. Its use provides a high degree of confidence that the working model on which the laboratory will fabricate the superstructure or fixed partial denture framework castings is an accurate representation of the intraoral environment. The use of an EOVS ensures that the implants to be restored, and their position in the jaw, is accurately duplicated outside the mouth so the laboratory technician can fabricate an ideal restoration.
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Dr. Shatz currently maintains a full-time private practice, limited to periodontics and implant-related surgery at Kennestone Periodontics, in Atlanta, Ga. Dr. Shatz is also an assistant clinical professor in the Department of Periodontics, Medical College of Georgia, in Augusta, Ga. He may be reached at (770) 432-9373 or thegumdoc@ earthlink. net.