Bone drilling is an essential part of many orthopaedic surgical procedures, including those for internal fixation and for attaching prosthetics. Drilling into bone is a fundamental skill that can be both very simple, such as drilling through long bones, or very difficult, such as drilling through the vertebral pedicles where incorrectly drilled holes can result in nerve damage, vascular damage or fractured pedicles. Also large forces experienced during bone drilling may promote crack formation and can result in drill overrun, causing considerable damage to surrounding tissues. Therefore, it is important to understand the effect of bone material quality on the bone drilling forces to select favourable drilling conditions, and improve orthopaedic procedures.

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Top screw pullout is a significant problem in anterior scoliosis correction, with rates of 5-15% reported in the literature. The Mater Misericordiae Hospital in Brisbane currently has a series of 125 patients with scoliosis treated by thoracoscopic anterior fusion, instrumentation and correction between April 2000 and August 2007. In this series 11 top screws are known to have pulled out (a rate of 8.8%), with six occurring in the first week, and all within 6 weeks, suggesting that the problem is one of excessive static force rather than fatigue. This thesis describes a biomechanical investigation into the mechanics of vertebral body screw pullout in anterior scoliosis surgical constructs. Previous biomechanical studies of vertebral body screws have evaluated their resistance to either straight pullout or cephalo-caudad compression forces, however the aim of this study was to assess screw resistance to more realistic loading conditions, namely pullout of initially angled screws, and pullout where the motion path is an arc rather than a straight axial pullout, as would be expected in a single rod anterior construct. The first series of experiments involved straight and angled pullout tests using synthetic bone. In the angled tests, both locked and free-to-pivot configurations were tested. The second series of experiments tested the effect of cephalo-caudad pre-compression (the actual deformity correction step performed during surgery) on subsequent axial pullout strength. A third series of experiments performed arc pullouts using synthetic bone, and the final series of experiments tested the pullout resistance of a newly proposed screw position configuration against the standard screw positioning using ovine lumbar vertebrae. Synthetic bone testing revealed that for initially angled pullout, resistance is greatest as the screw angle approaches 0 (ie a direct axial pullout). Cephalo-caudad pre-compression reduced subsequent pullout strength for cases where a staple was not used under the screw head, but if a staple was used the pre-compression did not decrease pullout force significantly. Arc pullout resistance was greatest when the screw was angled at 10 cephalad, and the mean pullout strength for the proposed screw configuration using ovine lumbar vertebrae (1864N) was almost double that of the standard screw positioning (993N). The clinical implication of this study is that top screw pullout resistance can be maximised by placing the top screw as close as possible to the top endplate and the bottom screw as close as possible to the bottom endplate, although this will have detrimental effects on the pullout of the second screw should the top screw pull out. Screw angulation is a less important factor but any angulation should be in a cephalad direction and around 10º in magnitude. The experimental results also suggest that the use of a staple may play a role in preventing cephalo-caudad pre-compression forces from reducing screw resistance to subsequent pullout forces.

Bone drilling is a major part of orthopaedic surgery performed during the internal fixation of fractured bones. At present, information related to drilling force, drilling torque, 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 bone drilling force data if recorded in-vivo, during the repair of bone fractures, can provide information about the strength/quality of the bone. Drilling force does not give a direct measure of bone strength; therefore it has been correlated with the shear strength and screw pullout strength to determine the efficacy in estimating the bone strength. Various synthetic bone material densities and animal bones have been tested to demonstrate the use of drilling force data. A novel automated experimental test rig, which enables drilling tests, screw insertion and screw pullout tests to be carried out in a controlled environment, has been developed. Both drilling and screw pullout tests have been carried out in a single setting of the specimen to reduce the experimental errors and increase repeatability of the results. A significantly high value of correlation (r² > 0.99) between drilling force & shear strength and also between drilling force & normalised screw pullout strength in synthetic bone material was found. Furthermore, a high value of correlation (r² = 0.958 for pig bones and r² = 0.901 for lamb bones) between maximum drilling force & normalised screw pullout strength was also found. The result shows that drilling data can be used to predict material strength. Bone screws are extensively used during the internal fixation of fractured bones. The amount of screw been tightened is one of the main factor which affects the bone-screw fixation quality. Over tightening of screw can result into the loss of bone-screw fixation strength, whereas under tightening can result in the screw loosening. Therefore, optimum tightening of the screw is important to achieve the maximum bone-screw fixation strength. At present, optimum tightening of the screw is entirely dependent upon the skill and judgment of the surgeon, which is predominantly based on the feel of the screw tightening torque. Various studies have been reported in the literature to develop an algorithm to set an optimum tightening torque value to be used in surgery. A method which is based on the use of rotation angle of the screw while tightening, rather than using screw insertion/tightening torque, to optimise the bone-screw fixation strength is proposed in this research. The effectiveness of the proposed method has been successfully demonstrated on the synthetic bone material using the designed test rig. The optimum angle for the tested screw was found to be 120° which is equivalent to 33% of the screw pitch.