ALUMINA CERAMICS IN JOINTS PROSTHESIS

Abstract

Biomaterials improve the quality of life for an ever increasing number of people each year. The range of applications is vast and includes such things as joint and limb replacements, artificial arteries and skin, contact lenses and dentures.

Ceramic biomaterials can be divided roughly into three main types governed by their in vivo behaviour and tissue response. In broad terms, there are the bioresorbable ceramics (b-tricalciumphosphate), bioreactive (hydroxyapatite, fluorapatite and bioglass) and bioinert (alumina, zirconia and pyrolytic carbon). The resorbable ones are incorporated into the surrounding tissue, or may even dissolve completely over a period of time. The bioreactive ones, like hydroxyapatite (used for coatings on metallic pins), encourage bonding to surrounding tissue with, for example, new bone growth being stimulated. The bionert ceramics are mostly used for structural components. Alumina and Zirconia are known for their general chemical inertness and hardness. These properties are exploited for implant purposes, where they are used as an articulating surface in hip and knee joints. Their ability to be polished to a high surface finish make them an ideal candidate for this wear application, where they operate against materials such as ultra high molecular weight polyethylene (UHMWPE).

Alumina is a highly inert material and resistant to most corrosive environments, including the highly dynamic environment that is the human body. Under physiological conditions, it is classed as nearly inert, with evidence of any response from surrounding tissues and remaining essentially unchanged after many tyears of service. However, the body does recognise it as a foreign material and does attempt to isolate it by forming a layer of non adherent fibrous tissue around the implant where possible. Porous alumina may also be used to replace large sections of bone that have been removed for reasons such as cancer.

Alumina has been used in dental applications. Specifically, it has been used for tooth replacements. The term high alumina ceramics is referred to materials that have the minimal content of 97% of alumina. If the percentage of minimal alumina is of 99% it is called high purity alumina ceramics. In its α phase (better famous like corundum), characterized from its particular structure and stability, the high purity alumina is used in orthopaedics, in the articulations of the hip and knee prostheses.

From more than 30 years, the alumina has been successfully used. Today, more than 3,5 million of ball-heads e and 350 thousand of inserts of alumina BIOLOXA8 have been implanted confirming, in clinical use, the characteristics of low wear and biocompatibility that has allowed to reduce the problems of osteolisis induced by the polyethylene. The increase of the mechanical characteristics, the new shapes and the conical fixation have raised the reliability of the ball-heads and inserts of alumina.

The BIOLOXA8forte (in commerce from 1994) is an high purity alumina (ca 99,7 %) with a small percentage of magnesium oxide (MgO). Approximately 50 years ago, magnesium oxide was introduced in the phase of sintering of the alumina, because it was discovered that a small amount of this additive prevented the increase of grains of alumina during the sintering process. It was therefore possible to have a more homogenous and dense microstructure, both characteristic directly correlated with the mechanical resistance. The suffix ‘forte’ derives from the increased mechanical characteristic caught up with the continuous optimization of the fabrication technology.

Many laboratory tests and clinical cases have shown that the wear rate of alumina-alumina bearing coupling is extremely low (0.001 mm/year). If compared with metal-polyethylene (0,2 mm/year), it evidences the drastic reduction of particles of debris and therefore of the osteolysis problem.

One of the main factors that the reduction of the wear rate involves is the characteristic molecular structure of alumina. Its superficial layer is composed of oxygen atoms that create a residual electric power which interacts with polarized molecules of the lubricant, binding it to the surface by strong Van der Waals ties. It is therefore guaranteed the presence of a fluid film that reduces the coefficient of clutch between the two surfaces involved during the articulation.

The colour of alumina components is subjected to variations. Originally it is ivory, but it can easy stretch to the brown after sterilization with gamma beams that interact with the free valences introduced by the MgO. This change of colour does not induce changes of the mechanical characteristics.

Currently the systems are completely modular and allow a wide choice of couplings. Ceramic acetabulum has been abandoned and replaced by ceramic inserts. In 1984 and subsequently in 1995, the introduction of ISO standards for the production of ceramics ball-heads and inserts and the concept of conical fixation has allowed to catch up higher reliability. The third generation of alumina has reduced the complications rates to values around 0.01% (for the 28 mm ball-heads and inserts), maintaining the excellent tribology and wear characteristics.

Today, the alumina BIOLOXA8forte components are prepared in clean-room, sintered with high quality control processes, marked by laser and accurately inspected and tested. The dimension of grains of the microstructure, currently reduced to inferior values of 2 B5m, has allowed to raise the value of the mechanical resistance of about 45% (580 Mpa) of the value requested by ISO standard (400 Mpa). The tolerances between ceramics (ball-heads and inserts) and metallic parts (taper and metal shell) are fundamental for lengthening the implant reliability. It is important to control and certificate the stems and cups which the ceramic parts are applied on. Correct assembling and the respect of the compatibilities between parts (angle, material, producer) guarantee the longevity of the implants.

Actually, in the orthopaedic field, the alumina application is mainly used in standard applications of the hip prostheses. Ball-heads of 22 milimeters of diameter, lengths of neck type XL, and the knee prostheses are not possible because of the mechanical characteristics of alumina not allowing to catch up the elevate stress values requested for these special applications.

Between 1975 and 1977, the first studies issued that the strenght of alumina could be reinforced by the introduction of ceramic oxides. It was discovered that the strenght and toughness of alumina could endure a remarkable increment through the realization of composites with oxide of zirconium (zirconia). In the zirconia, during the phase of cooling from temperatures over 1170A1C, the grains endure a change of phase (from tetragonal to monoclinic), with an increase of 3% of volume. At ambient temperature the phase monoclina is stable. This transformation is martensitic, with energy absorption, and involves a heat-proof change of the simmetry of the structure. In the case of dispersed grains of zirconia in the alumina matrix, the transformation absorbs the energy of the crack and the tenacity of the ceramics increases. The Yttria (Y2O3) use, as stabilizing of the zirconia, has allowed to exceed the problem of the defects of the structure. It was introduced a percentage of zirconia stabilized with yttria (Y-TZP) in the alumina matrix and other mixed oxides to counterbalance the reduction of the hardness caused by particles of zirconia and to create lengthened particles during the sintering.

All this studies have been used to create the new ceramics BIOLOXA8delta. Tests of biocompatibility in agreement with norms EN 30993 have been carried out allowing the implants of these new composite ceramics. The BIOLOXA8delta has a bending strenght around 1000 MPa, that is more than the double of the alumina standard (400 MPa). In the minimum fracture load test, ball-heads of 28 mm AF millimeter (neck L) have caught up values around 100 KN, very beyond the 46 KN requested by the FDA.

Multiple cycles of sterilization in autoclaves have demonstrated that the BIOLOXA8delta does not endure alterations of the mechanical and tribological characteristics.

On the basis of these results, BIOLOXA8delta will allow the realization of medical ceramics devices, already in study phase, like knee prosthesis, 22 mm ball-heads, thinner wallthickness of inserts, whose realization was not possible with the ceramic materials up to now available.