Second Clinical Appointment
Preparation for Master Impressions
Multi-unit Abutments attached to the mandibular implant analogs
Multi-unit angle correction abutments are utilized to correct the divergence of the angulated posterior mandibular implants. 30° Multi-unit angle correction abutments are attached to the posterior implant analogs on the cast and are aligned such that they are parallel to the remaining implants and themselves. Straight multi-unit abutments are attached to the anterior mandibular implant analogs on the cast. Note: The multi-unit abutments on the anterior implants are used to keep all the implant platforms at the same level (not to correct angulation of implants) and maintain consistency while ordering implant components.
Splinting Impression Copings
Accurate transfer of the spatial relationships of the implants from the oral cavity to the master cast is a very critical first step for fabricating a well-fitting and passive implant framework and prosthesis.*23A poorly fitted implant framework/prosthesis will exert uneven occlusal loads and stresses on the implants leading to marginal bone loss, failure of implants, loosening of screws, and fatigue fractures of implant components.24, 25
The open tray impression copings (pre-ordered) are attached to the maxillary implant analogs and mandibular multi-unit abutments on the primary casts. The open-tray master impression can be made by attaching the open-tray impression copings to the implants/abutments in the mouth and picking them directly in the master impression. Alternatively, they (copings) can be splinted to provide more rigid fixation of the copings within the impression. Splinting of the open tray impression copings while making a master impression aids in making an accurate impression by minimizing the movement/rotation of the copings during impression making, removal, and pouring of the impression. Splinting of the copings aids in generating an impression index. Various materials may be used for splinting the impression copings including auto polymerizing resin [DuraLay (Reliance) or Pattern Resin LS (GC AMERICA INC)], dual-cured resins, plaster, and prefabricated resin bars.
Fabricating an impression index and custom trays
The maxillary open tray copings may be splinted with prefabricated resin bars and auto polymerizing acrylic resin (having minimum polymerization shrinkage) on the primary cast. The mandibular copings may be splinted with floss and auto polymerizing acrylic resin (having minimum polymerization shrinkage) on the primary cast. The impression index generated is sectioned between adjacent implants and opposing implants (Figure 5). A layer of spacer wax is adapted over the splinted copings and the cast, and custom trays are fabricated (Figure 6). The screw axis holes are created on the custom trays to enable the pick-up of the copings in the impression.
Figure 5. Impression index sectioned between opposing and adjacent implants.
During the clinical appointment, healing abutments are removed. The 30° multi-unit angle correction abutments (retrieved from the casts) are attached to the two posterior mandibular implants in the predetermined position and the straight multi-unit abutments are attached to the anterior mandibular implants. All the abutments are torqued as per the manufacturer’s recommendations (Figure 7).
Figure 7. Multi-unit abutments attached to the mandibular implants.
The impression index sections (attached to the impression copings) are attached to the implants/abutments in the mouth in their predetermined positions. The split sections of the index are reconnected to each other with auto polymerizing resin (having minimum polymerization shrinkage). Upon polymerization of the resin material, the passivity of the index may be tested by performing the one screw test (Sheffield test).
Sheffield test/ one screw test: The index is said to be passive if all the impression copings are completely seated on the implant/abutment platform when only one of the impression copings is attached to the implant/abutment (the distal-most implant) with a screw. If the junction of the impression coping and the implant platform is subgingival, the seating of the impression copings may be verified with a radiograph (periapical or a panoramic radiograph.)
Border molding procedures are performed for both the maxillary and mandibular arches. Most restorative dentists do not perform border molding procedures while fabricating a fixed prosthesis; however, if the treatment plan is altered to a removable prosthesis at the time of try-in, one may have to repeat all the steps starting from master impression if the border molding procedures were not performed. The maxillary and mandibular master impressions may be made using vinyl polysiloxane (VPS) or polyether impression material (Figure 8). Following the complete polymerization of the impression material, the impression copings are detached from the maxillary implants and mandibular abutments by loosening the screws through the screw access perforations in the tray. The impressions are removed from the oral cavity and examined for detail. The impression indices with the copings are picked up in the impressions. The healing abutments and healing caps are attached to the maxillary implants and mandibular multi-unit abutments respectively. Appropriately sized maxillary implant analogs and mandibular multi-unit abutment analogs are attached to the maxillary and mandibular impression copings respectively (on the impression). Tissue forming material is injected around the copings and the impressions are beaded, boxed, and poured with Type IV die stone to generate implant and abutment level maxillary and mandibular casts respectively.
Note: Digital impressions have become very popular in the last twenty years, however, the literature reports that intraoral scans for complete arch prosthesis are not very accurate and should be restricted to short spans.
Figure 8. Master Impressions.