Effect of screw-access hole and mechanical cycling on fracture load of 3-unit implant-supported fixed dental prostheses

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Statement of problem.

The effect of screw-access holes and mechanical cycling on fracture resistance of 3-unit screw-retained zirconia-based implant-supported fixed dental prostheses (ISFDPs) is unknown.


The purpose of this in vitro study was to evaluate the effect of screw-access holes on the maximum fracture load (Lf) of 3-unit ISFDPs fabricated with the same abutment design for screw (s)-retained and cement (c)-retained with either a metal- or zirconia-based framework, testing the hypotheses that ISFDPs with screw-access holes have the lowest Lf and that mechanical cycling decreases the Lf of screw-retained ISFDPs.

Material and methods.

Models with fiberglass bases with 2 implants were fabricated. Special abutments were designed to allow for both s- and c-retained ISFDPs. The study considered the type of framework (s-retained metal-based and s-retained zirconia-based), fixation (s and c), and aging (a) with mechanical cycling, resulting in the following experimental groups (n=12): c-retained metal-based ISFDP; s-retained metal-based ISFDP (Ms); s-retained zirconia-based ISFDP aged by mechanical cycling (Zsa); and s-retained metal-based ISFDP aged by mechanical cycling (Msa). All ISFDPs were porcelain veneered and subjected to compressive load to failure. Fracture load data were statistically analyzed using 1-way ANOVA and Tukey range tests (α=.05). Fractography was used to evaluate the fracture surfaces.


Cement-retained ISFDP (Zc and Mc) showed the greatest Lf values (P<.001). Zs prostheses showed higher Lf values than those for Ms. Aging significantly decreased the Lf of Zsa but had no effect on Msa. Chipping was the failure mode most frequently found. Only Zs and Zsa ISFDPs showed catastrophic failures.


ISFDPs with screw-access holes showed lower Lf values, regardless of the framework type. Mechanical cycling affected the Lf of screw-retained zirconia-based ISFDPs.

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