High purity alumina bioceramics have been developed as an alternative to surgical metal alloys for total hip prosthesis and tooth implants. The high hardness, low friction coefficient and excellent corrosion resistance of alumina offers a very low wear rate at the articulating surfaces in orthopaedic applications. Medical grade alumina has a very low concentration of sintering additives (<0.5 wt.%), very small grain size (<7 µm) and a narrow grain size distribution. Such a microstructure is capable of inhibiting static fatigue and slow crack growth while the ceramic is under load. The average grain size of current medical grade aluminas is 1.4 µm, and surface finish is usually controlled to a roughness of less than 0.02 µm. However, unless its surface modified or used directly in articulating areas, alumina has a fundamental limitation as an implant material in that, like other “inert” biomaterials a nonadherent fibrous membrane may develop at the interface. In certain circumstances interfacial failure can occur, leading to loosening, as was observed in some earlier dental implant designs.
Alumina in Total Hip Replacements
Alumina currently is used for orthopaedic and dental implants, and has the ability to be polished to a high surface finish and high hardness. It has been utilised in wear bearing environments such as the total hip arthroplasties (THA) as the femoral head generating reductions in wear particles from ultrahigh molecular weight polyethylene (UHMWPE).
History of Alumina Implants
Since early seventies more than 2.5 million femoral heads and nearly 100,000 liners have been implanted worldwide. And most importantly alumina-on-alumina implants have been FDA monitored and over 3000 implants have been successfully implemented since 1987.
Other Applications of Alumina Implants
Other applications for alumina encompass porous coatings for femoral stems, porous alumina spacers (specifically in revision surgery), knee prosthesis and in the past as polycrystalline and single crystal forms in dental applications as tooth implants.
Mechanical and Wear Behaviour of Alumina Implant Materials in Hip Replacements
The mechanical behaviour of alumina ceramics in simulated physiological environments has lead to long-term survival predictions for these materials when subjected to subcritical stresses. The stress for a 50-year 99.9% survival probability for medical grade alumina is estimated to be 112 MPa. Considering the tensile stresses encountered in many implants, in a ceramic hip joint ball, this type of material can be reliably employed. In a recent work by Oonishi et al., it was reported that wear on alumina / UHMWPE, total hip arthroplasties was decreased by 25-30% of that on metal/UHMWPE in hip simulator tests and clinical results. Wear on total hip replacement prostheses of alumina / alumina was observed to be near zero in a similar hip simulator test.
Mechanical and Wear Behaviour of Alumina Implant Materials in Knee Replacements
In knee simulator tests, UHMWPE wear against alumina decreased to 1/10 of that against metal. They further reported that clinically no revisions were carried out due to the polyethylene wear problems during the last 23 years. In retrieved cases, UHMWPE surface against alumina was very smooth, however in a comparative study on UHMWPE surface against metal, many fibrils and scratches were found, showing extremely good performance of alumina ceramics against UHMWPE.
More Evidence Supporting the Use of Alumina-on-Alumina Implants
Several in vitro and in vivo studies using larger than 28 mm femoral heads demonstrated the advantage of using alumina-on-alumina pairing in young patients, and patients with high-demand body function.
Note: A complete list of references is available by referring to the original text.