Technical Errors May Affect Accuracy of Torque Limiter in Locking Plate Osteosynthesis
In locking plate osteosynthesis, proper surgical technique is crucial in reducing potential pitfalls, and use of a torque limiter makes it possible to control insertion torque.
We conducted a study of the ways in which different techniques can alter the accuracy of torque limiters. We tested 22 torque limiters (1.5 Nm) for accuracy using hand and power tools under different rotational scenarios: hand power at low and high velocity and drill power at low and high velocity. We recorded the maximum torque reached after each torque-limiting event.
Use of torque limiters under hand power at low velocity and high velocity resulted in significantly (P < .0001) different mean (SD) measurements: 1.49 (0.15) Nm and 3.73 (0.79) Nm. Use under drill power at controlled low velocity and at high velocity also resulted in significantly (P < .0001) different mean (SD) measurements: 1.47 (0.14) Nm and 5.37 (0.90) Nm. Maximum single measurement obtained was 9.0 Nm using drill power at high velocity.
Locking screw insertion with improper technique may result in higher than expected torque and subsequent complications. For torque limiters, the most reliable technique involves hand power at slow velocity or drill power with careful control of insertion speed until 1 torque-limiting event occurs.
In Scenario 3, the device was attached to a power drill braced to the flat working surface and rotated at low velocity under power until 1 torque-limiting event occurred.
In Scenario 4, the device was again attached to a power drill braced to the flat working surface. The completed system was rotated at high velocity under power until multiple torque-limiting events occurred.
After each trial, we recorded maximum torque achieved before each device’s torque-limiting event. Either an orthopedic surgery resident or a qualified medical student tested each torque-limiting device in each standardized testing scenario.
Statistical Analysis
Experiments for each torque limiter were repeated for 3 trials of each of the 4 different scenarios. For comparative statistics between experiments, maximum torque measurements were expressed as means and SDs; 95% confidence interval (95% CI) was calculated and reported to determine extent of variation within a single group. One-way analysis of variance (ANOVA) and Tukey post hoc tests were performed between groups for comparison of the normally distributed data. Significance was set at P ≤ .05.
Results
During simulation, we successfully measured maximum torque achieved with each torque limiter under the 4 different scenarios. All testing was done by 2 operators. ANOVA demonstrated significant (P ≤ .001) differences in torque among the scenarios.
In scenario 1, mean (SD) maximum torque under hand power at low velocity was 1.49 (0.15) Nm (95% CI, 1.43-1.55), near the advertised maximum torque of 1.5 Nm, with relatively minimal variation between devices. This scenario confirmed proper calibration of properly used torque limiters. Mean maximum torque ranged from 1.25 to 1.93 Nm.
In scenario 2, mean (SD) maximum torque under hand power at high velocity was 3.73 (0.79) Nm (95% CI, 3.33-4.13), a 2.5-fold increase compared with scenario 1 (P < .0001) (Figure 3). There also was an increase in variation of maximum torque between trials of individual devices and between different devices. Mean maximum torque ranged from 2.27 to 5.53 Nm.
In scenario 3, mean (SD) maximum torque under drill power at controlled low velocity was 1.47 (0.14) Nm (95% CI, 1.37-1.56), again near the advertised maximum torque of 1.5 Nm, with relatively minimal variation. Mean maximum torque ranged from 1.10 to 1.73 Nm.
In scenario 4, mean (SD) maximum torque under drill power at full power/high velocity was 5.37 (0.90) Nm (95% CI, 4.92-5.83), a 3.65-fold increase compared with scenario 3 (P < .0001) (Figure 3). Mean maximum torque measured in 3 tests ranged from 3.40 to 6.92 Nm.
There was no significant difference in mean maximum torque between the scenarios of hand power at low velocity and drill power at low velocities (P = .999) (Figure 4). Highest maximum torque from any device was 9.0 Nm (drill at full power). Results are summarized in the Table. There was no statistical significance in the test between the 2 test operators.
Discussion
Maximum torque was measured using a torque-limiting attachment under 4 different simulated scenarios. Our goals were to determine if varying practice and rotational velocity would affect maximum insertional torque and to measure consistency among torque limiters. We designed the scenarios to mimic practice patterns, including hand insertion and power insertion of locking screws. Results demonstrated that misuse of a torque-limiting device may inadvertently produce insertional torque substantially higher than recommended. Highest maximum torque was 9.0 Nm, which is 6.0-fold higher than expected for a locking screw using a 1.5-Nm torque limiter.
Our study results showed that insertion under controlled hand power (and low-velocity drill power) until 1 torque-limiting event occurred produced the most consistent and predictable results. Insertion under drill power or high-velocity hand power produced multiple sequential torque-limiting events, yielding inaccurate insertion torque. Low-velocity insertion under hand power, or carefully controlled drill power, consistently produced torque similar to advertised values.
Manufacturers’ technique guides are available for proximal humerus locking compression plate (LCP) systems, small-fragment LCP systems, the Proximal Humeral Interlocking System (PHILOS; DePuy Synthes), and the LISS. These technique guides clearly state that insertion can be performed under power. Only the PHILOS and LISS guides state that insertion should be performed under power until a single click is heard or that final tightening should be completed under hand power. The proximal humerus LCP guide states that surgeons should insert the locking screw under power until the torque-limiting device clicks. The small-fragment LCP guide states that insertion under power should always be completed with the torque-limiting attachment; there is no mention of reducing power or a single click (this may give the surgeon a false sense of security).
