Abstract
Introduction
Contact between the femoral head and rim of the acetabular liner in total hip replacements has been linked to adverse tribological performance that may potentially shorten the lifespan of the prosthesis. Predicting the size and location of the contact area can be done computationally, however, experimental validation of these models is challenging due to the conforming nature of the bearing surfaces.
This study aimed to develop a method of accurately determining the in-vitro contact area between the femoral head and acetabular cup in metal-on-polyethylene and ceramic-on-polyethylene bearings under different component orientations.
Method
Metal-on-polyethylene and ceramic-on-polyethylene samples, with a nominal diameter of 36mm (DePuy Synthes, Leeds, UK), were tested with the cups orientated using a combination of inclination (equivalent to 45°, 55° and 65° in-vivo) and version (−20°, 0°, 20° and 40°) angles. The liners, which were first gold hard-coated (EMSCOPE SC 500, Quarum Technologies, UK), were inserted into a Pinnacle® titanium shell, and femoral heads were mounted on a vertical spigot (Figure 1). A single-station multi-axis electromechanical hip joint simulator (Prosim, Simulator Solutions, UK) was used to position the samples with 18.7° flexion, 6.2° adduction and 8.3° external rotation, congruous with just after heel strike (ISO 14242-1), and apply a 3kN static axial load through the centre of the femoral head.
The contact area was generated by manually turning the head about the vertical axis of the centre of rotation of the applied load, removing the gold hard-coating from the contacting areas. The contact area was determined from photographs of the acetabular cup using SolidWorks (Dassault Systèmes, US) and ImageJ (National Institutes of Health, US) software packages. Three repeats under each combination of cup angles were completed, and the mean contact area and 95% confidence limits were determined for each bearing under all cup angle combinations.
Results
The metal-on-polyethylene and ceramic-on-polyethylene bearings generated non-spherical, irregular shaped contact areas (Figure 2) ranging between 350mm2 and 470mm2 when tested under a range of conditions (Figure 3). Total mean contact area decreased significantly with ceramic-on-polyethylene bearings (t-test; p=0.001) and non-significantly in the metal-on-polyethylene bearings (t-test; p=0.68), when tested with 65° compared to 55° of inclination. Version angle had the greatest effect on contact area when combined with 65° of inclination and the least effect when combined with the 45° inclination angle. Evidence of the contact zone reaching the edge of the cup was observed on samples tested with steep inclination angles for both bearing combinations.
Discussion
A well-demarcated area in the gold hard-coating was quantifiable using the methods described above, enabling the contact area in conforming hip replacement bearings, with the head and liner centres coincident, to be measured experimentally. The polyethylene liners showed a degree of deformation under load resulting in an irregular-shaped contact area, the size and position of which was affected by cup version and inclination angles. This data can be used to validate contact mechanics computational models.
