The Alumina Matrix Composite After Six Years of Use in Total Hip Arthroplasty

Over the past 20 years, applications for ceramics in orthopaedics have continuously increased in Europe and the US. The first application of ceramics in this field was a pure medical-grade alumina,^1–3 and has been in clinical use since 1971, and up to five million components have been implanted.^4–6 The performance and reliability of the material have been improved by optimising the manufacturing process to improve reliability and performance. To fulfil the increasing requirements of patients and surgeons, ceramists have also developed a new ceramic composite, the alumina matrix composite (AMC). This material combines the reinforcement mechanisms of ceramics with its excellent tribological qualities and has enhanced mechanical strength and fracture toughness compared with alumina.

Improvements of Ceramics in Orthopaedics
Optimisation of the manufacturing processes has enabled alumina ceramics to improve considerably in terms of their mechanical behaviour. In 1992, the first such improvement was the use of a better-quality alumina powder, with a much finer granulometry and a higher degree of purity. The polishing process of the ceramics has been improved in order to guarantee better tribological results for alumina-on-alumina bearing couples. The engraving of implants using a diamond weakened the material and created tensions on the surface.^7,8 Today, laser engraving does not have this side effect. In the past, complications were frequent; numerous publications describe former ceramics with obsolete designs (e.g. skirted heads).⁹

The mechanical resistance of a ceramic component depends on the size of the minimal default in its microstructure. ^10,11 In other words, reducing the average size of the grains increases the mechanical resistance of the ceramic due to the fact that a crack always grows intergranularly. The practical application of this reinforcement principle is induced by the hot isostatic pressing technology. This high-pressure sintering technique combined with classic sintering techniques has improved the mechanical resistance of alumina. The average size of the grains has been reduced from 3.2 to 1.8μm, and the bending strength values have been increased to 631Mpa for modern alumina ceramics. Today, third-generation alumina ceramics have again improved compared with the values described above.¹² Table 1 summarises the different mechanical characteristics and their evolution.

Alumina Matrix Composite Material
In the 1970s, the basics for the composite ceramic material were developed. This was because companies began investigating the principle of transformation toughening as a means to improve the mechanical strength of alumina materials. One form of ceramic composite is the reinforced alumina BIOLOX®delta, which is composed of an alumina matrix representing 82% by volume of the overall material. Nanoparticles of zirconia oxide are added to the alumina matrix, representing 1% by volume. These zirconia particles are stabilised in the tetragonal phase, which is the phase representing the best mechanical performance of zirconia.¹³ The two reinforcement mechanisms used in this material are:

• the addition of homogeneously dispersed small zirconia particles in the alumina matrix, creating a transformation toughening; and
• in situ formation of elongated oxide crystals acting as crack barriers.

Transformation Toughening
The first mechanical reinforcement mechanism is due to the presence of a small percentage of zirconia oxide in its tetragonal state (17% by volume) in the alumina matrix. ^14,15 The excellent structural stability that
is the main characteristic of alumina is maintained. Figure 2 shows the microstructure of the AMC. Yttria-stabilised zirconia (Y-TZP) grains are homogeneously distributed throughout the microstructure and are therefore isolated from one to another to provide an independent individual transformation ability. These metastable nanoparticles dispersed homogeneously in the alumina matrix will transform if
microcracks appear between the alumina grains. This phase transformation from the tetragonal to the monoclinic phase is accompanied by a 4% increase in volume. Due to this, the phase transformation induces a compressive stress field in the vicinity of the particles. The zirconia particles act like an airbag, absorbing the energy of the crack (see Figure 3). The result of this transformation toughening mechanism is a significant increase in the fracture toughness of the material.