Fractographic analysis of fractured fixation screws in implant-supported restorations.

OBJECTIVES

The aim of this study was to fractographically examine clinically fractured fixation screws used to fix implant supported restorations and to estimate stress at failure.

METHODS

20 clinically fractured titanium implant fixation screws were collected and analyzed under scanning electron microscope in order to locate critical crack origin and calculate stress at fracture. Principles of fractographic analysis were applied using relative equations and cause and conditions of screw fracture were calculated. X-rays and patients' records were collected after approval of necessary consent forms.

RESULTS

Fractographic analysis revealed location and dimensions of critical crack size for every fractured screw examined. Two patterns of screw fracture were identified; the first was related to improper seating of fixation screw leading to damage at screw threads and eventual fracture of screw body. The second was related to slow crack growth propagation causing final fracture of screw body. The calculated stress at failure ranged from 755 to 804 MPa. Patients' records revealed that 75% of fractured screws were related to single unit screw retained restorations.

SIGNIFICANCE: Single unit screw retained restorations and improperly seated implant abutments impose high stress concentration on fixation screw which may result in fixation screw fracture.