The Influence of Process Variation on a Cortical Bone Interference Fit Pin Connection

The interference fit is a common method for creating mechanical assemblies. When manufacturing the individual components to be assembled in this method, close dimensional control of the mating components is required in order to ensure that the amount of interference is sufficient to create a secure assembly, but not so great as to cause excessive stresses or failure of the individual components. In this work, we study interference fit connections in an assembly of human (cadaveric) cortical and cancellous bone, i.e., an allograft, used in spinal fusion surgeries. A difficulty encountered in this application is that, in addition to the machining steps, the assembly must go through subsequent sterilization and lyophilization, or freeze drying, processes that may affect the quality of the interference fit. This report examines the quality of the allograft interference fits using dimensional measurements of manufactured components at all stages of the manufacturing process, followed by examination for cracking and measurement of the pull-apart forces for assemblies. The experimental results are compared to finite element models of the interference fit and also to Monte Carlo models of the assembly using a simple thick-wall cylinder model. Experimental results show that the lyophilization process significantly affects the component dimensions, resulting in a much greater spread in interference values and likely leading to cracking and∕or loss of interference.