Accuracy of mechanical torque-limiting gauges for mini-screw placement

The purpose of this investigation was to determine and compare the accuracy of four available mechanical torque-limiting gauges (MTLGs) for mini-screw placement.

The torque outputs of six randomly obtained MTLGs, either of the screwdriver or torque ratchet type of four mini-screw manufacturers were obtained. Mounted on a joint, a universal testing machine applied perpendicular force to a lever arm with a crosshead speed of 1 mm/min. For each device, 10 repetitions of the corresponding target torque level were recorded after initial sterilisation (1) and after 5, 10, 20, 50 and 100 times to evaluate its potential influence on MTLGs. The breakpoints (N cm) were calculated for comparison of the groups. Descriptive statistics and mean breakpoints values for each MTLG computed and compared with the reference values indicated on the respective torque gauges provided by the producer.

The mean torque values for the AbsoAnchor MTLG devices were significantly below torque levels, but provide consistent torque values. All but one obtained values for the Spider Screw, MTLG of the screw driver type, were within the indicated moment range during the first 50-times of sterilization process. But after 100-times of steam sterilization all mean breakpoint values were relevantly higher than the indicated torque range values. Each individual MTLG produced independently constant breakpoint torque values, but differed significantly from each other. For all but the Spider Screw MTLG, the sterilisation process had a statistically significant different influence at the various breakpoint torque levels.

After application of the manufacturers' preset torque levels, significant variations were observed between individual devices. The torque output of each individual device deviated in varying degrees from target torque values and was influenced by various degrees by the sterilisation process over time.