A MINIMAL PARAMETRIC MODEL OF THE FEMUR TO DESCRIBE AXIAL ELASTIC STRAIN IN RESPONSE TO LOADS

Evaluating the state of stress/strain for a given geometry and load in femurs can be done both experimentally, measuring strain at a limited number of locations, and theoretically with finite element models. An other approach is to describe the state of strain with a few synthetic indices. For this purpose the reverse elastic problem (i.e. bone para meters are estimated given the strain distribution and loads) needs to be solved as opposed to the finite element direct problem. Such rever se models can be then used: (1) to describe simply the strain distribu tion by means of few synthetic indices, (2) to explain the state of st rain; and (3) to predict the strain distribution under different loadi ng conditions. Various linear models, characterized by two to five bon e-related parameters, were tested on (1) 12 femurs, (2) a finite eleme nt model, and (3) data taken from the literature, for a total of 43 lo ading cases. Three and four-parameter models were able to fit the expe rimental strain distributions with mean squared residuals smaller than 5% of the strain range. The consistency of the model was proved by th e repeatability of the parameters estimate for identical femurs. Furth ermore the bone-related coefficients were able to detect the stiffenin g effect of the implantation of an uncemented stem. Finally, the model can be used for predictive purposes if the parameter estimates are us ed with different loading conditions. Copyright (C) 1996 Elsevier Scie nce Ltd for IPEMB.