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.
Screw overtightening and head/thread stripping can make screw removal challenging.10 Removal rates for LISS plates range from 8% to 26%, and removal is often reported as taking longer than the index procedure, with complication rates as high as 47%.11-13 Bae and colleagues3 reported significant difficulty in removing 24 of 279 self-tapping locking screws (3.5 mm).
It is important to note that these complications, most notably cold welding, are mostly associated with titanium locking plate and screw constructs. Although stainless steel constructs have gained favor, titanium constructs are still widely used around the world.14,15
In 10% of cases in a laboratory setting, insertion of a 3.5-mm locking screw at 4 to 6 Nm damaged the screw.9 Removal of 3.5-mm locking screws had a stripping rate of 8.6%, and use of the torque limiter did not make removal easy all the time.3 Torque limiters are set specific to each screw diameter to reduce the risk of damage/stripping or even overtightening. Even when a surgeon intends to stop a drill before locking, final tightening often inadvertently occurs under power.3
Cold welding is often described as a cause of difficult implant removal.3,12 According to a newer definition, this process is independent of temperature and can occur when 2 metallic surfaces are in direct contact.16 High contact pressures between 2 similar metals can lead to this solid state welding.17 Theoretically, improper use of torque limiters can increase the risk of welding; however, it appears to be associated only with titanium locking plate and screw constructs.
Locked plating osteosynthesis is a valuable tool for fracture management, but improper use can have significant consequences, including morbid implant removal procedures, which are more difficult and time-consuming than the index surgery. We determined that proper use of torque limiters involves insertion under hand or power control at slow velocity until 1 torque-limiting event occurs. Many orthopedic surgeons may assume that torque limiters are accurate no matter how screws are inserted into locking plates. In addition, they may be unaware guidelines exist, as these are often deeply embedded within text. Therefore, we must emphasize that torque limiters can be inaccurate when used improperly.
One limitation of this study is that it tested only the Synthes 1.5-Nm torque-limiting attachment, though we can speculate that torque limiters designed for larger screws and limiters manufactured by different companies will behave similarly. Another limitation is that we did not obtain the hospitals’ service records for the tested equipment and assumed the equipment was properly checked for accuracy by the providing company. However, we hypothesized that, if maintenance were an issue, then our results would not be similar across all sites tested.
These tests involved a torque limiter linked to a torque-measuring device and may not perfectly represent actual torque measured at the locked screw–thread interface. However, we think our construct accurately determines the torque produced at the level of the driver tip. Also, we can speculate that the torque produced with improper use will lead to the complications mentioned and demonstrated in previous studies. Welding of the screw–plate interface may simply be a result of improper trajectory and cross-threading. However, if we assume that torque limiters prevent excessive torque no matter how they are used, high insertion speeds may compound the effect of welding. Additional biomechanical studies with full locked plate osteosynthesis constructs on bone specimens are planned to further characterize the potential complications of this issue.
