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 h ypothesis that clinical patterns of bone remodelling can be simulated using a feedback that modifies the properties of the network depending on the lo ad 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 esse ntial for a clinically successful arthroplasty. Following surgery, bone rem odelling 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-link' 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 thei r properties can be varied with time in response to deformation or force Va lues. Results. Reducing inter-link stiffness leads to a broad distribution of loa d transfer, whilst an iso-elastic stem concentrates this transfer through t he 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 require s a fixation of sufficient stiffness otherwise loosening will occur. Simula ted osteolysis following osseo-integration loosens the implant from a dista l to a proximal direction. Conclusions. Complex physiological bone remodelling patterns can emerge fro m a simple feedback within a linear system. Relevance Implant loosening is presented here as an adverse response of a s table dynamic system caused by mechanical or biological stimuli. (C) 2001 E lsevier Science Ltd. All rights reserved.