Changes in abutment screw dimensions after off-axis loading of implant-supported crowns: a pilot study.

ABSTRACT

INTRODUCTION: Loss of screw tightness resulting in abutment movement and displacement of the crown or prosthesis may occur from plastic deformation of the screw. This study correlates changes in screw length and diameter with previously reported loss of screw tightness. MATERIALS AND METHODS: Samples consisting of a 4 × 10-mm implant (potted in polymethylmethacrylate resin), a standard abutment, and the cemented crown were loaded with a 200-N force for 1 × 10 cycles. Torque audits were performed and the screws retrieved. Measurements were made of the shank length and diameter for 4 groups of implants-BioLok, external connection; Zimmer, internal connection; NobelBiocare, external connection; and AstraTech, internal connection. RESULTS: Elongation of the screw shanks were observed for NobelBiocare (77.9 µm) and Zimmer (52.5 µm) systems. This correlated with a loss of tightness of 50% (15 N cm). The BioLok system did not exhibit loss of screw tightness or shank lengthening. The Astra Tech system showed no change in screw shank length, but all the screws loosened multiple times. However, no changes in midshank width were observed for any of the systems. CONCLUSIONS: For both internal connection and external hexed systems, loss of screw tightness can be correlated with plastic deformation of the screw. This does not seem to be true, however, for a conical interface implant system.