Removal torque and force to failure of non-axially tightened implant abutment screws.

STATEMENT OF PROBLEM: Components have been introduced that allow the screw channel of an implant crown to be angled lingually and the screws to be tightened in a non-axial direction to the implant. Information is lacking as to how the removal torque value (RTV) and force to failure (FTF) of these components compare with those of conventional screws.

PURPOSE: The purpose of this in vitro study was to evaluate and compare the RTV and FTF values of cyclically loaded implant-supported restorations. Specifically, values for conventional axially tightened gold screws were compared to those for non-axially tightened screws aligned at 3 different angulations.

MATERIAL AND METHODS: A total of 28 external hexagon implants were embedded in acrylic resin and divided into 4 groups. Simulated restorations were fabricated on abutments capable of different screw channel angulations. Dynamic abutments (DA) were waxed at different angulations and then cast. Simulated restorations were placed on the implants and tightened: group 0GS: 0-degree angulation gold screw tightened to 35 Ncm (control group); group 0DAS, 0-degree angulation with dynamic abutment (DAS) screw; group 20DAS: 20-degree angulation with DA screw; group 28DAS: 28-degree angulation with DAS screw. In groups 0DAS, 20DAS, and 28DAS, the DAS screw was used and tightened to 25 Ncm. Screw removal torque values were recorded by using a digital torque gauge at baseline and, after reaching cyclic fatigue, by using a dual-axis mastication simulator for 1200000 cycles. The fracture strength (FS) of the implant restorations was tested under compression until failure by using a universal testing machine. Differences between baseline and removal torque (ΔRT) were calculated. Statistical analysis was performed by using 1-way ANOVA for ΔRT and FS separately (α=.05).

RESULTS: ΔRT and FS values were not significantly different among the groups (P>.05). The screw fractured in 5 of 28 specimens (17.8%); the remaining specimens failed with fracture of the implant.

CONCLUSIONS: The removal torque and FS values of the angulated abutment screw were comparable to those of the gold screw. Angulation of the abutment had no significant influence on the screw removal torque values.