COMPUTER-MODEL TO PREDICT SUBSURFACE DAMAGE IN TIBIAL INSERTS OF TOTAL KNEES

Two designs of total knee replacements were analysed to determine how the geometry of their bearing surface would affect the susceptibility of their ultra high molecular weight polyethylene tibial inserts to de lamination. Orientations of the femoral components on the tibial surfa ces were calculated with use of rigid body analysis for discrete inter vals during the stance phase of gait. For each successive orientation, finite element analysis was used to compress the components together to determine the stresses in the tibial inserts. A damage function ana logous to strain energy density was defined to account for the accumul ated amplitudes and frequencies of the maximum shear stress cycles and hence to predict fatigue failure, The damage function was applied to each polyethylene element in the tibial insert, and the highest value calculated for each design was its damage score. One knee had a damage score more than three times less than that of the other because of lo wer stresses and because the contact points moved in the medial-latera l as well as anterior-posterior directions during internal-external ro tation. The femoral and tibial components of this knee had large outer frontal radii and close conformity in the frontal plane. We propose t hat this method, which accounts for the motions and stresses endured d uring walking, makes different predictions regarding the likelihood of dt lamination compared with the predictions made by conventional stat ic compression tests performed when the knee is in a neutral position.