The influence of size, clearance, cartilage properties, thickness and hemiarthroplasty on the contact mechanics of the hip joint with biphasic layers

Abstract

Computational models of the natural hip joint are needed to examine and optimise tissue sparing interventions where the natural cartilage remains part of the bearing surfaces. Although the importance of interstitial fluid pressurisation in the performance of cartilage has long been recognized, few studies have investigated the time dependent interstitial fluid pressurisation in a three dimensional natural hip joint model. The primary aim of this study was to develop a finite element model of the natural hip incorporating the biphasic cartilage layers that was capable of simulating the joint response over a prolonged physiological loading period. An initial set of sensitivity studies were also undertaken to investigate the influence of hip size, clearance, cartilage properties, thickness and hemiarthroplasty on the contact mechanics of the joint. The contact stress, contact area, fluid pressure and fluid support ratio were calculated and cross-compared between models with different parameters to evaluate their influence. It was found that the model predictions for the period soon after loading were sensitive to the hip size, clearance, cartilage aggregate modulus, thickness and hemiarthroplasty, while the time dependent behaviour over 3000s was influenced by the hip clearance and cartilage aggregate modulus, permeability, thickness and hemiarthroplasty. The modelling methods developed in this study provide a basic platform for biphasic simulation of the whole hip joint onto which more sophisticated material models or other input parameters could be added in the future.

Fig. 1

The three dimensional finite element model of the hip joint. A–The entire model, B–Lateral view of acetabulum. C–Oblique view of acetabular cartilage with hexahedral elements.

Fig. 2

Verificaiton of the constitutive properties (left–model; right–results). Indentation model of a creep test using a quarter-symmetry model. Material properties and geometric parameters were taken from a previous study (Pawaskar, 2010). The biphasic model with neo-Hookean solid phase in FEBio behaves nearly identically to the biphasic model with linearly elastic solid phase in Abaqus (Maas and Weiss, 2007). The experimental results from Pawaskar (2010) are also shown.

Fig. 3

Contours of fluid pressure (MPa) and contact stress (MPa) of the acetabular cartilage for the original model at 1 s and 3000 s. On the acetabular cartilage surface, the peak contact stress is slightly higher than the peak fluid pressure. Obvious cartilage consolidation can be detected. The change in the fluid pressure is greater than that in the contact stress.

Fig. 4

The results of the parametric tests for all models at 1 s and 3000 s. Both the short-term and long-term behaviour of the models depend on the size, clearance, hemiarthroplasty, cartilage thickness and stiffness. Cartilage permeability has almost no influence on the short-term behaviour, but greatly affects the long-term performance of the model.

Fig. 5

Cross-sectional view of fluid pressure (MPa) in the cartilage of the acetabulum (1) and femoral head and (2) of the original model at 1 s. Fluid pressure distribution was similar for the femoral head cartilage and acetabular cartilage. There was no marked difference in the fluid pressure across the thickness of the cartilage.