Preparation for Master Impressions
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).
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.