Abstract 20

Computer assisted tightening of cancellous bone screws

Trevor C. Hearn, Karen J. Reynolds, Tammy M. Cleek

Flinders University of South Australia

Internal fixation of fractures often requires the tightening of bone screws to stabilise fragments. Inadequate torque can leave the fracture unstable, while over-tightening results in thread stripping and loss of fixation. The optimal amount of screw torque is specific to each application and is difficult to attain in practice due to the wide variability in bone properties. The aim of this project was to develop an automated system for sensing the properties of a material through its interaction with a bone screw, and to use this data to determine an appropriate level of tightening. A custom test rig was designed and built for bone screw experiments. We established that differences in synthetic bone material density of 0.1 gm/cc in range corresponding to osteoporotic bone could be automatically detected through effects on the rotational characteristics of a cancellous bone screw. Based on this detection, an electric motor controller was demonstrated to change driver speed and stop implant tightening at a variable level corresponding to material density. Ovine cancellous bone specimens were then tested to evaluate the method given the continuously variable density and interface characteristics of bone. Results indicated that plateau current measured during screw insertion is directly related to bone density and is a strong predictor of peak current. Based on these relationships, a control system was programmed and subsequent shutoff tests results were encouraging. We have demonstrated that bone density can be automatically detected through screw rotational characteristics, establishing the basis for adaptive surgical control of screw tightening.

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