DESIGN RATIONALE FOR A LARGE DIAMETER CERAMIC ON CERAMIC HIP BEARING

Abstract

Current CoC hip bearing ranges are typically from 28mm to 36mm diameter, but to improve stability and range of motion, a novel large diameter hip bearing is introduced, with bearing diameter from 32mm to 48mm. To minimise acetabular bone loss, a low profile acetabular component is required and achieved with a ceramic wall thickness of 3.5mm and metal shell thickness of 1.5mm at the acetabular rim. This paper presents some of the testing required to develop this novel design.

Finite element (FE) modelling was performed to simulate the standard 46kN burst loading of the acetabular cup for 5 different geometries of ceramic liner and metal (titanium) shell. By smoothing the facets on the back of the ceramic and thinning the metal shell, the stresses in the ceramic were reduced by 20% and failure was not predicted for the burst test. Reducing the thickness of the metal shell increased the stresses in the metal, but these were kept below the yield strength of the material. When assembled, the hoop stresses in the titanium shell caused a greater volume of the ceramic to be in compression and the strength of the assembled cup was therefore increased.

To assess the effect of fatigue loading on the ceramic/titanium taper-lock, cups were loaded at 45° to the horizontal for 10,000 cycles in Ringer’s solution at 37°C. The load required to push the ceramic from the metal shell were recorded after the test and compared to the push out load of unloaded specimens. There was no significant decrease in the push-out load (mean 2kN) indicating that the taper lock retains its strength during fatigue loading.

The new CoC acetabular cup design is assembled under controlled conditions before packaging. To demonstrate the effectiveness of this, the new device was compared to a commercially available intraoperatively assembled Ti/ceramic device which had a metal shell thickness of 5mm. The cups were placed in reamed cavities of polyurethane foam and the rims impacted with increasing impact energy until the ceramic came loose from the metal shell. An average impact energy of 4J (1kg dropped from 400mm) was necessary to separate the ceramic from the metal liner of the new design compared to 2J for the commercially available design. The thicker titanium wall thickness and intraoperative assembly method of the commercially available design limited the amount of shell deformation/hoop stress generated, and therefore limited the ‘grip’ of the Ti/ceramic interface. The thinner titanium shell (1.5mm) and controlled assembly load of the new design allowed greater shell deformation/hoop stress which produced a two-fold improvement in interface strength. Further effects of assembly in vivo, in particular the effects of periprosthetic or lavage fluids, remain to be investigated. In any case, incomplete ceramic liner seating has been reported in 16% of procedures in vivo (Lang-down, JBJS Br 2007) and the preassembled design therefore represents a notable and necessary improvement to current technology.