Purpose : Most mechanical failures in implant treatment are screw breakage or loosening. One cause is excessive or insufficient screw tightening torque, so it is essential to use a torque wrench to provide an accurate tightening torque. However, the detailed characteristics of torque wrench are still unknown. The purpose of the present study was to clarify the accuracy and reproducibility of typical torque wrenches.

Materials and methods : Torque values of torque wrenches of 12 manufacturers (8 beam type, 2 preset type, and 2 digital type wrenches) were measured with a torque gauge. One examiner applied the recommended torque value of each manufacturer's screws five times clockwise. Average measured torque, standard deviation, bias, and coefficient of variation were calculated. The statistical method used was the Tukey method with the target torque value as the factor and the difference between the measured torque value and the target torque value as the dependent variable.

Results : The torque wrench used in this study showed a maximum value of 10% and a minimum value of －8% when the target torque value was about 10 Ncm. When the target torque value was about 25 Ncm, the maximum value was 0.6% and the minimum value was －2.3%. The maximum and minimum values were 2.4% and －7.2%, respectively, when the target torque value was about 50 Ncm.

Conclusion : The accuracy and reproducibility of the torque wrench depended on the manufacturer. It was revealed that a smaller target torque value had poorer accuracy and reproducibility, suggesting that prosthetic screws should be tightened carefully to about 10 to 15 Ncm.