Optimising surgery using bone quality metrics

Abstract

The overall goal of this thesis is to address the growing burden of periprosthetic fractures and revision surgery by aiming to reduce fractures during surgery and by reducing fractures and revision surgery in the long term by reducing stress-shielding. Bone quality determines whether a patient breaks a bone when they sustain an injury and also plays a key role in any operation involving bone. A key component of bone quality is bone stiffness, particularly in joint replacement surgery. There are three main variables that a surgeon needs to know to safely insert a cementless joint replacement implant: The bone stiffness, the impaction force and the implant stiffness. Reliably measuring bone stiffness pre-operatively is currently an unsolved clinical problem. The impaction force is also an unknown variable. In addition, variation in the implant stiffness can potentially affect bone ingrowth, as well as the initial stability of the implant. Microindentation, computed tomography (CT) imaging and ultrasound were used to measure cortical bone stiffness. Microindentation bone elastic modulus measurements, using a large, spherical, indenter tip, correlated with the apparent elastic moduli measured by compression testing. Cortical porosity, measured by CT, correlated strongly with compression testing and ultrasound properties. Cadaveric testing was used to develop a representative in vitro hip replacement impaction testing rig by replicating the boundary conditions of in vivo testing. The importance of implant stiffness in determining bone ingrowth was explored experimentally by using scaffolds of varying degrees of stiffness in a large animal model. This thesis begins to set out a vision for the future: Surgeons armed with reliable information about a patient’s bone stiffness will be able to determine a safe impaction force range for cementless joint replacement. They will then deliver that safe impaction force to an implant that has mechanical properties optimised for that individual patient.