Analysis of bone drilling characteristics for the enhancement of safety and the evaluation of bone strength

Bone drilling is a major part of modern orthopaedic surgery associated with the principles of internal fixation of fractured bones. At present, information related to drilling forces, rate of drill bit penetration and drill bit rotational speed is not available to orthopaedic surgeons, clinicians and researchers as bone drilling is performed manually. This research demonstrates that orthopaedic surgery involving the drilling of bone can greatly benefit from the technology of automation/ mechatronics, which allows the collection and storage of the drilling data for analysis as well as for the improvement of the drilling procedure. The research also represents a significant contribution to the development of a drilling system for the enhancement of safety and/or as a diagnostic tool for the evaluation of bone strength. A novel automated experimental rig, which enables drilling tests to be carried out in a controlled environment, has been developed. The investigation for the enhancement of safety involves the detection of drill bit break-through on a femoral shaft in the presence of system compliance and inherent fluctuation of drilling forces. Since these two factors affect the detection of drill bit break-through, a robust and reliable method based on a modified Kalman filter has been developed. When applied to the force difference between successive samples and the rotational speed, the modified Kalman filter has been found to be very effective in establishing trends and ironing out major fluctuations caused by the system compliance and inherent drilling force fluctuation. The evaluation of bone strength related to the cancellous bone at the proximal femur has resulted in the establishment of a positive relationship between the average drilling force and bone mineral density (BMD), obtained from bone densitometry, which is used to estimate bone strength in clinical practice. The correlation has been found to depend on the direction of drilling. This is indicated by a linear relationship obtained in the anterior-posterior direction (perpendicular to the cervical axis), which is not interchangeable with the relationship in the direction of the cervical axis. Findings of this research have indicated that analysis of bone drilling forces has the potential to provide additional information about the strength of bone.