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Titanium and zirconia have enabled the restorative dental team to restore a patient's dentition more economically and expeditiously than ever before. Modern technology can aid in the manufacturing when employing these materials and, when combined with the vision and skill of a talented restorative team including a master dental technician, will function and endure beyond expectations. As precise as these materials are, there are inherent challenges since post-manufacturing correction is either difficult or, as with zirconia, simply not possible. There is also the additional factor that since dental implants are ankylose in the bone without a periodontal ligament, the fit must be precise and passive. Distortions in both impression materials and in gypsum, along with clinical and laboratory errors, can all add to compromising smooth case progression.
There must be a means by which distortions and errors can be brought to light and corrected. Verification jigs are utilized as a way to check both the accuracy of the materials and of the hand work in the early stage of building successful prosthetics. While the techniques described are not perfect, they are a means to minimize potential errors. Much of prosthetic dentistry has limitations and each clinician and technician should familiarize themselves with various techniques and incorporate strategies to permit an accurate workflow.
A verification jig should be designed so that it is rigid, correctable, and cost effective. It needs to be able to confirm the 3-dimensional position of the implant analogs in the dental cast. It can also be utilized to correct the spatial and rotational position of a poorly positioned laboratory analog. There are many ways to accomplish this utilizing varying components and materials. Whichever technique is used, the process begins with an open- or closed-tray impression of the implant transfer components properly seated on the implants. This may be implant-body level, abutment level, or a combination. After the proper laboratory analogs are attached, a soft-tissue cast is appropriately manufactured. This dental cast is then ready for intraoral verification.
The first and possibly oldest technique of creating a verification jig utilizes pattern resin and dental floss. After the soft tissue cast is manufactured, the impression components are removed and the technician will attach the appropriate non-engaging temporary cylinders to each analog (Figure 1). Fit is visually confirmed on the cast. The technician will then wrap unwaxed dental floss around each component and tie it off at the end (Figure 2). Then, using the Nealon technique, pattern resin is applied to each temporary cylinder and across the floss to connect each component together. This acrylic should be approximately 1 mm away from the soft tissue crests (Figure 3). Once polyermized, this splinted component is removed from the dental cast and fit is evaluated intraorally. If there is a mis-fit, the offending temporary cylinder can be sectioned out and reattached in the correct position. If the rotational position also needs to be transferred, the non-engaging temporary cylinder is replaced with an engaging temporary cylinder or open-tray impression component. Although this is a popular design, the pattern resin can be somewhat flexible. Rigidity of the structural component of the jig is paramount to the verification process. When utilizing this design, it is common for the technician or dentist to build the resin thicker to increase the structural rigidity. This, however, can affect the setting of the resin as it distorts volumetrically.
The next variation on making an implant analog verification jig is by using a hybrid of pattern resin and UDMA light-cured tray material. This technique begins at the same point—a dental cast with implant analogs and non-engaging temporary components in place. After confirmation that all the components are properly seated on the analogs, a rectangle-shaped bar is formed with the UDMA light-cured resin following the lingual of the component channel (Figure 4). This bar is not attached to the components but seated passively against the components and then properly polyermized according to manufacturer's instructions (Figure 5). After cooling, the bar is attached to each implant temporary component, one at a time, with pattern resin (Figure 6 and Figure 7). This minimizes the setting distortion of the pattern resin. When all the components are connected, the jig is ready for intraoral verification and correction, if necessary, in the same manner as the pattern-resin-only design. This hybrid design allows for more rigidity than the one using pattern resin only, while also providing a means of correction.
These first two designs allow for the jig to be utilized as a cast correction device, a procedure often referred to as "cast surgery." If during intraoral verification there is a slight mis-fit with one or two implants, the verification jig may be altered to transfer the correct implant position. There are two types of components that are used to complete this procedure depending on the requirements of the case. The first type of restoration would be where the anti-rotational aspect of each implant in not engaged. An example is a screw-retained fixed partial denture without any abutment coronal to the analog. The second type of restoration is a design that requires an abutment to be connected in the laboratory, such as a custom abutment or multi-unit abutment. There are different considerations when working with components like custom abutments that index to the anti-rotational aspect of the dental implant.
For the first restoration type, it is not necessary to transfer the rotational timing of the implant. Once the offending temporary cylinder is ascertained, it is carefully removed from the jig (Figure 8). The remaining components still attached are seated and fit is confirmed. In this design, rotational timing is not relevant, so the same non-engaging temporary component is reconnected to the implant, seating is confirmed, and then the component is reattached to the main structure of the jig with a small amount of pattern resin (Figure 9). Once polyermized, it can be unscrewed from the mouth and used to correct the cast.
For the second restoration type, accurate rotational timing is critical. The offending component is identified and sectioned away from the main jig structure, as previously described. In this situation, an engaging piece such as an open-tray impression component will be replaced here in the same manner (Figure 10).
If intraoral correction is necessary, the modified jig is used to correct the analog position on the cast. The incorrect implant analog is trephined from the dental cast (Figure 11) and, after residual stone is removed, reattached to the newly joined component on the corrected jig (Figure 12). This entire assembly is then connected to the cast with the corrected analog passively positioned in the space created by the removal of the misaligned laboratory analog (Figure 13 and Figure 14). With all screws in place and the soft tissue component properly seated, a mix of type IV dental stone is poured into the void, reattaching the analog to the now-accurate cast (Figure 15 and Figure 16). Clinical judgment will determine if further verification is required.
The third design of dental implant verification jig utilizes low-expansion type IV dental stone as the structural material. The advantage here is two-fold: first, the expansion distortion is considerably less than pattern resin, even in small volumes; and, second, the stone, being brittle, is sensitive to torqueing. If there is a slight mis-fit on intraoral verification, the gypsum will fracture. Thus, there is no chance of material distortion. Manufacturing begins at the same point with the non-engaging metal temporary components properly attached to the analogs on the cast. A wax matrix is formed to contain the gypsum material at the proximal, gingival, buccal, and lingual aspects (Figure 17 and Figure 18). A proper mix of the gypsum material is then poured into the matrix, taking care not to allow the stone to get into the screw channels (Figure 19). When set, the appliance is carefully unscrewed from the cast and the wax is removed (Figure 20). The cast can then be used for intraoral verification; if there is a mis-fit, the stone will crack, requiring a new impression and subsequent verification. While some consider this technique the "gold standard," the author feels that the inherent inability for correction limits the functionality of this method.
Zirconia is a very strong but brittle material, making even the slightest mis-fit an inevitable failure. Being certain the cast is accurate is of the utmost importance. While every effort should be made to accurately transfer the 3-dimensional accuracy of each implant position to a working cast, the additional step of creating a verification jig for the cast is time well spent.
The author would like to thank Dess USA for providing the components used in making these jigs.
About the Author
William Baum, MDT, CDT
Baum Dental Studio, Inc.
Lindenhurst, New York
New York University
College of Dentistry
New York, New York