Sathasivam, Shivani; (1998) Optimisation of the bearing surface design of total knee replacements. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A total knee replacement consists of a double-convex, cobalt-chrome layer which resurfaces the femur, and a cobalt-chrome tibial tray with a double-concave polyethylene insert. The longevity of knee replacements is limited, as the polyethylene suffers from fatigue resulting in delamination wear. Previous researchers have analysed the stresses produced in tibial inserts of different designs. However, these studies do not consider the relative displacements and rotations which occur between the prosthetic components during gait. Various designs of knee replacement are currently available and the aim of this study was to investigate the effect of bearing surface geometry on durability. Rigid body analyses were used to predict the kinematics of different designs of knee replacements during one level walking cycle. The effects of soft tissue restraints were included to compare designs of different conformities, and friction had to be included to obtain realistic motion. These theoretical results were validated by a knee simulating machine. Finite element analyses were carried out at intervals during the walking cycle, to calculate the stresses associated with the orientations of the prosthetic components predicted by the rigid body analyses. Damage functions were formulated which accumulated the fluctuations and average magnitudes of the stresses produced in each element of the tibial inserts. The maximum shear stress damage function to predict subsurface cracking in the polyethylene was equivalent to the strain energy density criterion, used by material scientists to predict crack growth. Using the damage function values, damage maps were generated to indicate the regions of the polyethylene which were most prone to fatigue. Varying the geometry of the prosthetic bearing surfaces to cover the range of knee replacement designs currently available produced significant differences in the values of the damage functions, suggesting differences in their durabilities. Furthermore the damage functions were used to optimise the prosthetic bearing surface geometries so that the least destructive fatigue mechanisms in the polyethylene would be induced while conserving the laxity of the natural knee.

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