Keywords: implant design, hip replacement, finite elements, wear, metal-backed cup, polyethylene.

The artificial hip joint consists of a ball and a socket and is mostly made of a material combination such as an ultra high molecular weight polyethylene (UHMWPE) acetabular cup, articulating against either a metallic (cobalt-chromium or steel) or ceramic (alumina) femoral head. Although artificial bearing joints perform to a high level, the wear due to friction can lead to the failure of the prosthesis.

Computational tools have been developed to evaluate the joint performance (Maxian et al. [1]). These models take into consideration the loads and the displacements applied in the joint to compute the contact pressure and estimate the wear and heat generation (Fialho et al. [2]).

Usually, the presence of the pelvic bone is neglected in the computational analysis. The objective of this work is to analyze how the inclusion of the pelvic bone in the model can influence the computed acetabular contact pressure. Thus, the influence of the pelvic bone is discussed, in particular how it depends on the acetabular component stiffness.

This study was performed using a three dimensional, non-linear contact finite element model (FEM) of the total hip replacement solved in ABAQUS (version 6.7). The acetabular component was considered to be made of UHMWPE with and without metal-backing. In the present work, a CoCr acetabular component was also tested. To evaluate the effect of the pelvic bone modeling on the computed pressure on cup's surface, two different support conditions were considered. In one model, the posterior area of the acetabular cup was rigidly fixed (the pelvic bone was neglected), while in the other the acetabular cup was attached to the deformable pelvic bone.

The maximum contact pressure for metal-backed acetabular cup with polyethylene liner is 8.68 MPa without pelvic bone and 8.56 MPa with pelvic bone. These values correspond to a maximum linear wear of 0,092 mm/year and 0.091 mm/year, respectively. For an all-polyethylene acetabular cup the contact pressure obtained is 8.16 MPa without pelvic bone and 7.34 MPa with pelvic bone. These values correspond to a maximum linear wear of 0.088 mm/year and 0.081 mm/year respectively.

These results show that, the pressure values obtained for the metal-backed cups (for both polyethylene and CoCr liner) are almost independent of the pelvic bone. Thus, the study of the hip joint pressure and wear can be performed without a patient-specific analysis where the quality of the pelvic bone for a specific patient is included. On the other hand, the analysis of all-polyethylene components leads to different contact pressures with and without the pelvic bone. In that case, patient-specific studies are justified since changes in pelvic bone stiffness can lead to different implant performance.

1

T.A. Maxian, T.D. Brown, D.R. Pedersen, J.J. Callaghan, "A sliding-distance-coupled finite element formulation for polyethylene wear in total hip arthroplasty", Journal of Biomechanics 29 (5), 687-692, 1996. doi:10.1016/0021-9290(95)00125-5

2

J.C. Fialho, P.R. Fernandes, L. Eça, J. Folgado, "Computational hip joint simulator for wear and heat generation", Journal of Biomechanics 40, 2358-2366, 2007. doi:10.1016/j.jbiomech.2006.12.005

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £95 +P&P)