STATEMENT OF PROBLEM: Within the elastic limit of the screw, the greater the preload, the tighter and more secure the screw joint. However, additional tensile forces can incur plastic deformation of the abutment screw when functional loads are superimposed on preload stresses, and they can elicit the loosening or fracture of the abutment screw. Therefore, it is necessary to find the optimum preload that will maximize fatigue life and simultaneously offer a reasonable degree of protection against loosening. Another critical factor in addition to the applied torque which can affect the amount of preload is the joint connection type between implant and abutment. PURPOSE: The purpose of this study was to evaluate the influence of tightening torque on the implant-abutment screw joint stability. MATERIAL AND METHODS: Respectively, three different amount of tightening torque (20, 30, and 40 Ncm) were applied to implant systems with three different joint connections, one external butt joint and two internal cones. The initial removal torque value and the postload (cyclic loading up to 100,000 cycles) removal torque value of the abutment screw were measured with digital torque gauge. Then rate of the initial and the postload removal torque loss were calculated for the comparison of the effect of tightening torques and joint connection types between implant and abutment on the joint stability. Results and CONCLUSION: 1. Increase in tightening torque value resulted in significant increase in initial and postload removal torque value in all implant systems (P < .05). 2. Initial removal torque loss rates in SS II system were not significantly different when three different tightening torque values were applied (P > .05), however GS II and US II systems exhibited significantly lower loss rates with 40 Ncm torque value than with 20 Ncm (P < .05). 3. In all implant systems, postload removal torque loss rates were lowest when the torque value of 30 Ncm was applied (P < .05). 4. Postload removal torque loss rates tended to increase in order of SS II, GS II and US II system. 5. There was no correlation between initial removal torque value and postload removal torque loss rate (P > .05).
Fatigue ; Joints ; Plastics ; Torque