Statement of problem: A unidiameter abutment attached to a large-diameter implant has been reported to result in an unexpectedly high failure rate, inconsistent with the general understanding of dental implant mechanics. Purpose: The purpose of this finite element analysis study was to investigate the mechanical mechanism underlying these unexpected failures with the hypothesis that the cold welding or interference fit interface between abutment and implant increases the failure probability of a large-diameter implant system with a unidiameter abutment. Material and methods: A conical implant system with different abutment gingival heights and implant diameters was analyzed for 3 contact conditions of the abutment-implant interface (bond and frictional coefficients of 0.3 and 0.7). A computer model was created using computed tomography images, and an oblique load of 100 N was applied to the abutment to determine the mechanical effect of the implant diameter and gingival height under the 3 contact conditions. Results: When the abutment-implant interface was bonded, the peak stress of the abutment increased and that of the bone decreased with increasing implant diameter. When friction was applied to the abutment-implant interface, the peak stress of the implant, screw, and bone decreased with increasing implant diameter. Furthermore, the peak stress of the implant system and bone increased when the abutment gingival height increased under all contact conditions. Conclusions: Cold wielding or interference fit at the abutment-implant interface can prevent a screw fracture; however, it puts high stress on the unidiameter abutment neck when the implant diameter is increased. Screw loosening may lead to a slide between the abutment and implant, considerably increasing the stress of the screw. A system with a narrow diameter implant may cause an implant fracture rather than an abutment fracture when friction is applied to the abutment-implant interface.
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