Development of a Minimally Invasive Drilling Methodology for Strong Fixation in Human Femur

Abstract

Surgical drilling procedures are important to be optimized, to ensure the strongest fixation of an implant possible when used in bone fracture treatment and reconstruction surgery. There is currently a lack of control of the large drilling forces and in-situ temperature that are created during implant fixation. These uncontrolled large drilling forces cause micro crack formation and mechanical damage to the surrounding bone. Because of the selection of in-appropriate drilling parameters, the in-situ bone temperature reaches above 47º C and causes an irreversible thermal osteonecrosis. The mechanistic force models that are currently used are only suitable for bovine bones, human spines and human mandibles. These models are not relevant to human long bones and therefore not applicable to the surgical drilling of human femurs. Also, there is no method currently available to predict the in-situ temperature of the human femurs during drilling. This thesis addresses this lack of information, and aimed to develop the predictive models for drilling human femurs at different principal directions in order to ensure the strongest implant fixation by using cortical bone screws. Two different predictive models namely one for drill thrust force and another one for in-situ drilling temperature were developed. In this work, optimal bone drilling parameters for in situ temperature as well as thrust force were found specific to the drilling direction on the account of apparent density variations and direction of osteons. In order to avoid the generation of micro crack, mechanical damage and thermal osteonecrosis, simultaneous minimization of both in-situ temperature and drill thrust forces are essential to be controlled using the optimal bone drilling parameters specific to the drilling direction. So, multi-objective optimization was also carried out to drill the holes without any adverse-effect. Then pull-out tests were conducted to ensure the strong implant fixation.