Abstract
Introduction
Geometric variations of the hip joint can give rise to abnormal joint loading causing increased stress on the articular cartilage, which may ultimately lead to degenerative joint disease. In-vitro simulations of total hip replacements (THRs) have been widely reported in the literature, however, investigations exploring the tribology of two contacting cartilage surfaces, and cartilage against metal surfaces using complete hip joint models are less well reported.
The aim of this study was to develop an in-vitro simulation system for investigating and comparing the tribology of complete natural hip joints and hemiarthroplasties with THR tribology. The simulation system was used to assess natural porcine hip joints and porcine hemiarthroplasty hip joints. Mean friction factor was used as the primary outcome measure to make between-group comparisons, and comparisons with previously published tribological studies.
Method
In-vitro simulations were conducted on harvested porcine tissue. A method was developed enabling natural acetabula to be orientated with varying angles of version and inclination, and natural femoral heads to be potted centrally with different orientations in all three planes. Acetabula were potted with 45° of inclination and in the complete joint studies, natural femoral heads were anatomically matched and aligned (n=5). Hemiarthroplasty studies (n=5) were conducted using cobalt chrome (CoCr) heads mounted on a spigot (Figure 1), size-matched to the natural head. Natural tissue was fixed using PMMA (polymethyl methacrylate) bone cement.
A pendulum friction simulator (Simulator Solutions, UK), with a dynamic loading regime of 25–800N, ± 15° flexion-extension (FE) at 1 Hertz was used. The lubricant was a 25% (v/v) bovine serum. Axial loading and motion was applied through the femoral head and frictional torque was measured using a piezoelectric transducer, from which the friction factor was calculated.
Results
The correct anatomical orientation and positioning was achieved enabling in-vitro simulation testing to be conducted on hemiarthroplasty and complete hip joint samples for two-hours. Mean friction increased rapidly followed by a continued gradual increase to ≈0.03 ± 0.00 in the complete joints, with the hemiarthroplasty group plateauing at ≈0.05 ± 0.01 (Figure 2). Mean friction factor was significantly lower (t-test; p < 0.05) in the complete natural joint group.
Discussion
An in-vitro simulation system for the natural hip joint with controlled orientation of the femur and acetabulum was successfully developed and used to measure friction in complete porcine hip joints and porcine hip hemiarthroplasties. A non-linear increase in friction indicative of biphasic lubrication was observed in both groups with slower exudation of fluid from the complete joints compared to the hemiarthroplasties, inferring a quicker move towards solid-phase lubrication. Higher friction in the hemiarthroplasties, which was similar to that measured in-vitro in metal-on-polyethylene THRs, was most likely due to variable clearances between the non-conforming spherical metal head and aspherical acetabulum, causing poorer congruity and distribution of the load. This could in time lead to abrasive wear and cartilage degradation. This methodology could have an important role when investigating associations between hip geometric variations, interventions for hip disease/pathology, and risk factors for cartilage degeneration.
