Biomechanical Analysis of Fixation and Bone

Abstract

Long-term survivorship of a total knee replacement (TKR) relies on the strength of bone around the implant and its initial stability. Aseptic component loosening caused by mechanical factors is a recognised failure mode for knee prostheses. Bone resorption due to “stress-shielding” of the stiff stemmed implants will potentially lead to weakened bone strength, and presents a challenge for revision TKR surgery. The aim of this study was to develop analytical methodologies for the investigation of fixation performance of TKR, and to gain a better understanding of the prosthetic design requirements, addressing two major mechanical problems of bone remodelling and aseptic loosening. Patient-specific finite element (FE) modelling incorporated with a strain-adaptive bone remodelling theory was used to simulate bone remodelling responses of the postoperative tibial fixation. The choice of cementing technique was found to influence the remodelling behaviour; cemented fixationmodelled as a firm anchorage of the prosthesis onto the bone, was predicted to induce greater stress-shielding effect consequently leading to severe proximal bone resorption; for a fixation relying on biological attachment of bony ingrowthmodelled as a less firmly anchored boneprosthesis interface, lesser proximal bone resorption was predicted. The consideration of bone remodelling in FE simulations for fixation analyses is paramount as it influenced the risk prediction of aseptic loosening between prosthesis designs. The cement tensile stresses and bone-prosthesis interface micromotions predicted were different prior to and after bone adaptation. FE predictions of the MIS mini-keel and standard stemmed prosthesis fixations after simulating six months of bone adaptation correlated well with the RSA measurements at a similar period. A modified in-vitro technique of measuring bone-prosthesis relative micromotion was developed for relating initial stability of the cemented and cementless (press-fit) tibial prostheses fixations to late aseptic loosening. The developed computational and invitro methods should be applicable to other joint replacements.