A spring network model for the analysis of load transfer and tissue reactions in intra-medullary fixation

Objective. A spring network can be used to represent the load transfer from a prosthetic stem into its surrounding bone. The study seeks to test the hypothesis that clinical patterns of bone remodelling can be simulated using a feedback that modifies the properties of the network depending on the load transfer. Design. A mathematical model is used to simulate the initial properties of the linear system and its subsequent remodelling behaviour. Background. A stable and pain-free transfer of physiological forces is essential for a clinically successful arthroplasty. Following surgery, bone remodelling and osteolysis can modify this load transfer. Methods. The combined effect of all factors that influence prosthesis–bone load transfer are summarised in the properties of ‘inter-linkʹ springs that connect springs representing the prosthesis and bone in the linear network. It is on these inter-links that a remodelling feedback operates, and their properties can be varied with time in response to deformation or force values. Results. Reducing inter-link stiffness leads to a broad distribution of load transfer, whilst an iso-elastic stem concentrates this transfer through the proximal and distal portions of a prosthesis. Physiological patterns of bone resorption and osteolysis become apparent in a time-series analysis of the feedback in the linear system. Specifically, osseo-integration requires a fixation of sufficient stiffness otherwise loosening will occur. Simulated osteolysis following osseo-integration loosens the implant from a distal to a proximal direction. Conclusions. Complex physiological bone remodelling patterns can emerge from a simple feedback within a linear system.