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.


METHODS
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-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 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. Relevance. Implant loosening is presented here as an adverse response of a stable dynamic system caused by mechanical or biological stimuli.
Egan
J M
JM
E-Tech Limited, P.O. Box 970, S26 1QR, Sheffield, UK. jegan@dial.pipex.com
Marsden
D C
DC
D019644:Arthroplasty, Replacement, Hip / Q000379:methods*; D001696:Biomechanical Phenomena; D016723:Bone Remodeling / Q000502:physiology; D005594:Fracture Fixation, Intramedullary / Q000379:methods*; D006620:Hip Fractures / Q000601:surgery*; D006621:Hip Joint / Q000502:physiology; D006801:Humans; D008962:Models, Theoretical; D016348:Osseointegration / Q000502:physiology*; D011474:Prosthesis Design; D017755:Prosthesis Fitting; D016474:Weight-Bearing