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Maximizing Final Impression and Dental Cast Articulation Accuracy for Kennedy Class I and II Cases

SIAMAK ABAI, DDS, MMedSc

by Siamak Abai, DDS, MMedSc


When a patient presents for implant treatment to replace two or more teeth in either of the distal-most residual regions of the arch, it can be difficult to capture the accurate final impression and bite registration needed to properly fabricate and articulate the casts for production of the dental restorations. This condition, known as a Kennedy Class I when present on both ends of the arch and Class II when it occurs unilaterally, leaves patients with an edentulous, open-ended space in the posterior area of the jaw.

Due to the lack of tooth support at the end of the arch, preventing the impression material from collapsing when recording a bite registration can be very challenging. Inaccurate interocclusal records frequently result, hindering the articulation of the master casts and the design of the implant crowns (Fig. 1). Further, when multiple implants are placed next to each other, capturing a final impression can be complicated by the divergent angles of the implants. This is due to flexure in the impression material, which can allow the transfer copings to move, resulting in an inaccurate representation of the inter-implant positions.

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Figure 1: When extra measures are not taken to ensure an accurate bite registration for Kennedy Class I and II cases, restorations with inaccurate occlusion are often the result.

Clinicians have adopted various methods to overcome these obstacles, with varying degrees of success. The following case report demonstrates a straightforward means of addressing both of these issues by fabricating and utilizing an occlusal verification jig and implant verification jig. Using materials and components that are relatively common to the offices of dentists restoring implants, practitioners can produce a final impression and bite registration that result in accurate and correctly articulated casts upon which the prosthetic designs for their Kennedy Class I and II cases can be determined. Taking these extra steps makes certain that both the positioning of the implants and the interocclusal relationship of the arches are properly represented on the master cast, helping to ensure an accurate fit, bite and contacts for the final restorations.

Taking these extra steps makes certain that both the positioning of the implants and the interocclusal relationship of the arches are properly represented on the master cast.

Case Report

A female patient presented with edentulous spaces in the areas of teeth #18 and #19, as well as gross decay that had rendered tooth #20 non-restorable (Figs. 2a, 2b). After being offered the restorative options of a removable partial denture or dental implant treatment, the patient opted for the latter. A treatment plan was developed calling for the extraction of tooth #20, the placement of three dental implants in the areas of teeth #18–20, and the delivery of screw-retained implant crowns.

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Figures 2a, 2b: Preoperative occlusal and retracted lateral views of non-restorable tooth #20 and edentulous spaces in the areas of #18 and #19.

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Figures 2a, 2b: Preoperative occlusal and retracted lateral views of non-restorable tooth #20 and edentulous spaces in the areas of #18 and #19.

Following implant placement, healing of the soft tissue, and osseointegration of the implants, an implant verification jig and occlusal verification jig would be used during final impressions to maximize the accuracy of the definitive restorative designs. These tools would be crucial in overcoming the obstacles presented by the divergent angulation of adjacent implants and the lack of a mandibular occlusal stop resulting from the patient’s Kennedy Class II condition.

At the next appointment, the patient’s tooth #20 was extracted using periotomes and dental elevators (Fig. 3). Tooth removal was accomplished atraumatically, leaving the socket intact. Thus, treatment progressed to the healing phase without any bone grafting.

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Figure 3: Atraumatic extraction of tooth #20.

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Figures 4a, 4b: The patient exhibited healthy soft tissue four months following tooth extraction.

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Figures 4a, 4b: The patient exhibited healthy soft tissue four months following tooth extraction.

After four months of healing, the patient returned for digital intraoral impressions and cone-beam computed tomography (CBCT) scanning. Gingival healing had progressed nicely at the extraction site (Figs. 4a, 4b). The data generated from the digital impression and CBCT scans were used to evaluate the quality and quantity of bone underlying the implant sites. A digital treatment plan was developed, including the ideal depth and angulation of implants needed to support three screw-retained crowns.

Surgery commenced by using a tissue punch to gain access to the osteotomy sites (Fig. 5). The osteotomy for each implant site was created using a series of progressively wider drills (Fig. 6). Then, three Inclusive® Tapered Implants (Glidewell Direct; Irvine, Calif.) were delivered (Fig. 7). Custom healing abutments were placed to establish esthetic emergence profiles during the period of osseointegration (Figs. 8a, 8b).

The patient was allowed to heal for a period of four months before returning for the preliminary final impression. After removing the custom healing abutments, closed-tray impression copings were seated in the implants and the prosthetic screws tightened with a hand driver (Fig. 9). Vinyl polysiloxane (VPS) impressions of the edentulous spaces and opposing arch were made and sent to the lab. Lab implant analogs were inserted into the impression copings and placed into the mandibular impression, the varying angles of which illustrated why an implant verification jig would be needed to ensure an accurate final impression (Figs. 10a, 10b).

Next, the lab fabricated and trimmed the soft-tissue model based on the preliminary final impression (Fig. 11). Preliminary working casts were then articulated for use in the fabrication of the implant and occlusal verification jigs (Figs. 12a, 12b).

The implant verification jig was fabricated first. VPS material is subject to deformation, which introduces risk when adjacent implants are being restored, as the relative positioning of the impression copings can become distorted. The implant verification jig addresses this by connecting the transfer copings and keeping their relative positions fixed, thus ensuring an accurate representation of the implants in the final impression. Note that while the occlusal verification jig is indicated specifically for Kennedy Class I and II cases, the implant verification jig is a valuable tool for any case involving adjacent implant restorations.

First, temporary abutments, which would ultimately serve as open-tray impression copings, were attached to the implant analogs (Fig. 13). The temporary abutments were connected using Triad® Ivory Light Provisional Material (Dentsply; York, Pa.) (Fig. 14). After curing the provisional material, fabrication of a custom tray began by adding a layer of wax over the implant verification jig and model teeth (Fig. 15). Pink custom tray material was then molded over the wax (Figs. 16a, 16b). The custom tray was cured on the preliminary cast, removed and cured again. After cleaning, trimming and creating three access holes in the custom tray, it was reseated over the implant verification jig (Fig. 17). With fabrication of the custom tray complete, the implant verification jig was sectioned using a carbide disc and numbered (Fig. 18).

Next, the occlusal verification jig, which would address the significant challenge of obtaining an accurate bite registration by providing a stop for the impression material as it set, was fabricated. To begin, scanning abutments were seated in the preliminary cast (Fig. 19). Alternatively, temporary abutments and other prosthetic components can be used for this purpose. The screw channels were blocked out using cotton-ball sticks, and the scanning abutments were connected using PATTERN RESIN™ (GC America; Alsip, Ill.) (Fig. 20). Note that the cast was lubricated to prevent the acrylic resin from sticking to the restorative area of the cast. The scanning abutments were thoroughly luted, leaving only the screw access channels exposed and taking care to keep the jig out of occlusion (Figs. 21a, 21b).

After receiving the occlusal verification jig, implant verification jig, and custom tray from the lab, the patient was called in for the final impression. First, the individual sections of the implant verification jig were seated (Fig. 22). The sections were then luted together using Camouflage® NanoHybrid Composite (Glidewell Direct). The custom tray was seated over the implant verification jig, verifying that the open-tray impression copings were positioned to protrude through the openings (Fig. 23). After intraoral injection of EXAFAST™ NDS medium-body impression material (GC America) to capture the details of the soft tissue, the custom tray was filled with heavy-body impression material and seated over the implant verification jig in the patient’s mouth. After allowing the VPS material to set, the prosthetic screws of the transfer copings were loosened and the impression was removed, picking up the implant verification jig within the custom tray (Fig. 24). The result was an extremely accurate final impression.

Next, the occlusal verification jig was seated and checked for contact with the opposing dentition (Fig. 25). Contact occurred in this situation, indicating that the articulation of the preliminary working casts was inaccurate. Thus, the occlusal verification jig was adjusted using a fine diamond bur (Fig. 26). After reducing the acrylic resin of the jig until there was no interocclusal contact, a bite registration was taken using Capture® Clear Bite impression material (Glidewell Direct) (Fig. 27). The occlusal verification jig provided the stable platform needed to support the registration material as it set, resulting in the accurate interocclusal record needed to properly articulate the final master casts.

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Figure 25: The occlusal verification jig was seated in the patient’s mouth and examined for any interocclusal contact.

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Figure 26: The occlusal verification jig was reduced with a fine diamond bur to eliminate contact with the opposing dentition.

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Figure 27: An accurate final bite registration was taken with the occlusal verification jig in place, which provided the mandibular stop needed for the impression material in the absence of the three distal-most teeth.

The final master cast was fabricated from the final impression containing the implant verification jig (Fig. 28). This helped ensure that the relative positioning of the implant analogs aligned with that of the implants in the patient’s mandibular arch. The final master casts were articulated, with the accuracy of the interocclusal relationship ensured by the occlusal verification jig and final bite registration, which were seated on the mandibular cast prior to articulation (Fig. 29).

With the precise restorative information needed to produce accurate implant crowns captured, the final master casts were scanned, and dental CAD software was used to design three screw-retained crowns. The crowns were fabricated from BruxZir® Solid Zirconia to provide the maximum level of durability needed for the posterior region of the mouth. The final crowns were milled and then seated on the master cast to verify the accuracy of the restorative designs (Figs. 30a, 30b).

The final restorations fit perfectly … overcoming the prosthetic challenges presented by adjacent implants in the posterior-most region of the mouth.

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Figure 28: The implant verification jig ensured that the comparative positions of the implant analogs in the final master cast were accurate.

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Figure 29: The occlusal verification jig and bite registration were seated before articulating the final master casts, ensuring an accurate representation of the patient’s bite.

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Figures 30a, 30b: The final BruxZir Solid Zirconia screw-retained crowns were digitally designed, fabricated and seated on the master cast to ensure an accurate fit prior to patient delivery.

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Figures 30a, 30b: The final BruxZir Solid Zirconia screw-retained crowns were digitally designed, fabricated and seated on the master cast to ensure an accurate fit prior to patient delivery.

The patient returned for final delivery of the BruxZir screw-retained crowns in the areas of teeth #18–20. The fit of the final restorations was accurate, establishing optimal emergence profiles, contacts and occlusion despite the patient’s open-ended posterior residual ridge and the disparate angles of the three adjacent implants (Figs. 31a, 31b). The extra steps taken to record accurate final impressions were essential in setting up a predictable, esthetic and functional outcome.

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Figures 31a, 31b: The final restorations fit perfectly and established ideal occlusal contacts upon delivery, overcoming the prosthetic challenges presented by adjacent implants in the posterior-most region of the mouth.

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Figures 31a, 31b: The final restorations fit perfectly and established ideal occlusal contacts upon delivery, overcoming the prosthetic challenges presented by adjacent implants in the posterior-most region of the mouth.

Conclusion

Accounting for the relationship between the accuracy of the final impression and the fit of the final restoration, the occlusal verification jig and implant verification jig precisely capture the patient’s bite and the relative positioning of the implants. Although these tools are employed here to address challenges specific to Kennedy Class I and Class II cases, they are an excellent example of how implant therapy is best practiced with a restorative-driven approach, mindful of how each step will affect the final prosthetic outcome.

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