Evaluation of a new method to achieve optimal passivity of implant-supported superstructures

Statement of problem. Passive fit of implant-supported superstructures has been suggested as a prerequisite for maintenance of osseo-integration and for successful prosthetic reconstruction.

This study evaluated the degree of fit of superstructures cemented onto titanium abutments, compared to structures cast onto gold cylinders.

An impression was taken and a master cast made of five implants mounted in a stainless steel model. Six similar porcelain fused to metal superstructures with standardised dimensions were then fabricated. One group featured castings onto gold UCLA abutments, while in the other group the structures were cemented onto tapering titanium abutments. Vertical discrepancies between implants and both types of superstructures were measured on at six locations around each implant using a reflex microscope. Measurements were taken after securing the structures to the model with a single screw (torqued to 32 Ncm) placed in various locations and also after tightening all the retaining screws simultaneously to 10 Ncm.

With the new single-screw procedure, no significant difference in mean gap size was noted between the two groups. The discrepancies were significantly larger for certain screw locations (1 and 5), indicating that the one-screw test might be an inaccurate way of assessing fit. After tightening all the retaining screws simultaneously to 10 Ncm, the mean gaps were 11.9 microns for the cemented group and 17.8 microns for the cast group.

Although not statistically significant, cementing of superstructures onto titanium abutments improved the fit up to 50% compared to casting the structures onto gold UCLA-abutments. Tightening one terminal abutment screw and evaluating the discrepancies at the other abutments (one-screw test) distorted the results when evaluating the fit of long-span, implant-supported ceramo-metal superstructures.

The results of this study indicate that cementing superstructures onto titanium abutments will lead to an improved degree of fit, compared to casting superstructures onto gold cylinders. In addition, the one-screw test may not be an accurate method of determining the level of passivity of long-span, ceramo-metal, implant-supported superstructures.