MACROSCOPIC AND MICROSCOPIC CONTACT ANALYSIS OF RETRIEVED KNEE PROSTHESIS USING FINITE ELEMENT METHOD

Abstract

The wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in knee and hip prostheses is one of the major restriction factors on the longevity of theses implants. Despite quite a number of studies on the wear of UHMWPE, the wear mechanism is not clear yet. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE. Especially for the artificial knee joint with anatomical design, the contact stresses in the UHMWPE tibial insert are generally higher than the yield stress of the material during normal gait. In addition, the predominant types of wear on reported simulator-tested and retrieved UHMWPE tibial inserts are delamination and pitting. These facts suggest that the fatigue fracture that causes micro-cracks both on and below the surface of the UHMWPE tibial insert and the generation of wear particles as fatigue type are closely related to the repeated plastic deformation. On the metallic femoral components of the retrieved knee prostheses with anatomical design, a number of microscopic scratches caused by various factors were observed. It is thought that microscopic surface asperities caused by this surface damage contribute to increasing and/or accelerating wear of the UHMWPE tibial insert. The primary objective of this study was to investigate the factors influencing the wear mechanism of UHMWPE tibial insert in knee prosthesis.

In this study, macroscopic and microscopic elasto-plastic contact analyses of the UHMWPE tibial insert based on macroscopic and microscopic geometrical measurements from retrieved knee prosthesis were performed using finite element method (FEM) in order to investigate the mechanical state, plastic deformation behavior in the UHMWPE tibial insert and microscopic wear of the polyethylene caused by microscopic surface asperity. For this purpose, the determinative method of the contact position between the femoral component and the UHMWPE tibial insert for the retrieved knee prosthesis was developed. The three-dimensional FEM model of the retrieved knee prosthesis with worn contact surfaces was produced. Three-dimensional microscopic surface profile measurements of damaged surface of a retrieved metallic femoral component by using a laser microscope and reproduction of the femoral component surface by using 3D CAD software were performed in order to produce the 3D FEM models of the microscopic asperity based on actual measurement data.

The analytical findings of this study suggest that maximum plastic strain below the surface is closely related to subsurface crack initiation and delamination of the retrieved UHMWPE tibial insert. The worn surface whose macroscopic geometrical congruity had been improved due to wear after joint replacement showed lower contact stress at the macroscopic level. The aspect ratio, shape ratio and indentation depth of the microscopic asperity have a significant effect on increasing and/or accelerating wear on the UHMWPE. Higher aspect ratios, shape ratios and indentation depths cause higher contact stresses and plastic strains in the UHMWPE. These are therefore significant factors influencing the wear mechanism of UHMWPE.