Abstract
Introduction
Halo traction, either on bed or with an accompanying vest is used commonly in Spine surgery, in a variety of clinical situations. The pins are inserted into the skull in safe anatomic zones, using wrenches that are either pre-torqued or exhibit a torque gauge to allow measurable torque application. A torque of 6 – 8 inch pounds is considered ideal for optimal pin tightening. Lesser torques may hypothetically lead to pin loosening and pin track infections, while, excessive torques could lead to pin penetration through the skull.
Aim
To test the accuracy and consistency of different torque wrenches used for Halo pin insertion, using a standardised calibration device.
Methodology
Three different types of torque wrenches were tested, using the MTS Systems 858 Mini Bionix II calibration device at the School of Engineering laboratory in Cardiff. Each type had four samples. Three more samples of one type, which were already in use at the Spine unit were also tested, making a total sample size of fifteen. The calibration device had a declared average error of 0.5 %. Each torque wrench was tested twenty five times to a pre set value of 6 inch pounds (0.67790897 N-m), resulting in 375 independent observations. Data was recorded electronically and was analysed for error, percentage of error and variability for each device.
Results
All wrenches, regardless of model or make, failed to deliver accurate torque at the pre set value on repeated testing. There were both negative and positive errors. The average torque delivered by all wrenches together at a pre set value of 0.67790897 N-m was 0.721337 (+/- 0.116919) N-m. The average percent error for individual wrenches ranged from 4 % to 34 %. When grouped by model, the average percent error (model specific) ranged from 5 % to 29 %.
When assessed for consistency, the wrenches revealed a range of values over a minimum of 0.053303 N-m to a maximum of 0.846512 N-m. The variability of all wrenches of one model type was however similar, though not identical. The best performing model had an average variation over 0.165531 N-m while the value for the worst was 0.685794 N-m.
Conclusion
Torque wrenches used to tighten Halo pins appear to be neither accurate nor consistent. They deliver torques that are either less than or more than their pre set levels. Different wrenches of one model displayed a similar pattern of variability.
While this study does not seek to relate this lack of accuracy and consistency to the development of specific complications like loosening or penetration, it does suggest that extreme care is advisable while using torque wrenches to minimise clinical mishaps.
