Metal Information Center

Alumina pairing in total hip replacement

November 7, 2007 – 11:07 am

In 1971, Boutin introduced the concept of an alumina-on-alumina bearing in total hip replacement and emphasised polyethylene wear as the major limitation of a metal-on-polyethylene articulation.1,2 This was further confirmed by Willert, Bertram and Buchorn.3 Many pioneers from around the world followed this initial experience but long-term results and good methodology were lacking. Recently, a number of reports4-6 have focused attention again on an alumina-on-alumina combination. More than 30 years after its introduction into clinical practice, this article considers the current information on the alumina-on-alumina combination from both experimental and clinical standpoints.

Material properties and design

The process of manufacturing alumina is critical. Experience is needed and the methods of preparing the material precisely should be appropriately controlled in order to avoid catastrophic consequences. Thus, it is still only available from a limited number of companies from Japan, Austria, Germany, the United Kingdom and France. The alumina (Al^sub 2^O^sub 3^) used in modern arthroplasties is a dense, polycrystalline ceramic, obtained from aluminium oxide powder and pressed in a mould at a very high temperature (1600°C). It is a very stable and chemically inert material, unlike zirconia which needs to be chemically stabilised. Currently, alumina must fulfil standards for various parameters including purity, density and grain size

With more modern alumina ceramics the physiological loads are less than the level required for a crack to propagate. Jumping, running and strenuous activities are allowed in those patients whose arthroplasties are of modern design. Flaws in the material can be avoided by well-controlled manufacturing standards and strict quality control. However, this is batch control rather than total control, and the risk of fracture cannot be completely eliminated. The precise stresses which a material will have to endure in vivo are not known and may differ from those studied in a laboratory setting. Consequently, a number of guidelines should be observed: 1) a 32-mm femoral head is safe, although modern manufacturing processes appear also to allow the safe use of 28-mm heads; 2) the connection between the head and the taper, or between the acetabular shell and the alumina insert, must allow a large contact area which is slightly roughened but perfectly clean. Perfect circularity of the taper, and a smaller 12/14 size rather than 14/16, combined with the design of the inner aspect of the alumina femoral head, are also critical; 3) the thickness of the alumina acetabular insert must be at least 6 mm with a minimum outer diameter of the acetabular component of 50 mm for a 32-mm head. A smaller acetabular component needs a 28-mm femoral head. The thickness of the metallic shell should also be controlled and adapted. 4) Different options are available for the angle of the taper, 5.4° (Ceraver-Ostéal, Roissy, France) or 16° (Ceramtech, Flochigen, Germany). It is our view that the larger contact area provided by the 5.4° taper is safer for long-term use.

The operative technique must also take material characteristics into consideration. In order to avoid initiation of a crack the use of a hammer to impact the femoral head onto the cone should be avoided. The placement of the acetabular component should be chosen to maximise surface contact between the bearing surfaces, and a more horizontal placement with an angle of abduction of

Tribological properties

The superb tribological properties of an alumina-on-alumina combination depend upon both the properties of the material and the manufacturing processes.8 Alumina is a wettable material. This can be measured with the contact angle, which describes the shape of a liquid on a solid surface. The contact angle of alumina is 44°, whereas the contact angle of polyethylene is 80°. This property plays a major part in the lubrication process.

In order to achieve optimal tribological performance, a proper manufacturing process is crucial for both the femoral head and the acetabular component. Modern manufacturing processes allow the production of an extra-smooth material with a sphericity of less than 1 µm. Clearance between the femoral head and the acetabular component must be perfectly controlled in order to achieve optimal lubrication. This clearance should be between 20 and 50 µm. When these conditions are met, experimental data show that minimal wear will occur.9-12 Wear performance of an alumina-on-alumina combination follows a biphasic behaviour, with a run-in period which lasts less than one million cycles followed by a steady-state phase. Oonishi et al,12 in an experimental study of 28-mm femoral heads, calculated a mean wear during the run-in period of approximately 1.2 mm^sup 3^ per million cycles, whereas the mean wear during the steady-state phase was only 0.02 mm^sup 3^ per million cycles.6 In vivo, several authors have found a linear wear rate of 0.025 to 5 µm when the acetabular component was secure.13-15 Under similar circumstances the wear rate of a metal-on-polyethylene combination was 100 to 200 µm. Volumetric wear is, therefore, between 2000 and 4000 times less for an alumina-on-alumina combination than for metal-on-polyethylene.16 Recent data highlighted a specific wear pattern with a limited area of stripe wear which is thought to occur during edge loading activities such as rising from a chair or climbing high steps.17 Dennis et al18 measured separation in vivo by fluoroscopy while Tipper et al19 showed experimentally that, when microseparation was introduced, they were able to reproduce stripe wear and that wear increased from 0.1 mm^sup 3^ per million cycles to 1.24 mm^sup 3^ per million cycles. The quality of the alumina is also vital. As it improved with time, Prudhommeaux et al14 and Nevelos et al20 demonstrated from retrieval studies that a decrease