Ceramics as an Important Factor to Reduce In Vivo Complications in Total Hip Arthroplasty

Recently, the endoprosthetic treatment of younger and more active patients has become more prominent in the orthopaedic society. This patient group wants to live an active life, which places increasing demands on the mechanical and tribological properties of artificial hip implants. As a consequence, the assumption of an old, less mobile patient also has to be revised. In a clinical study, Wollmerstedt et al.¹ measured the mean daily movement of patients (mean age 70 years) with total hip arthroplasty (THA). They observed a mean number of two million load cycles per year, which is double the basis for simulator studies (one million cycles per year). On the other hand, an increased level of activity should not increase the risk of a wear-induced osteolysis. The authors concluded that the bearing material plays an even more important role than has been acknowledged until now.

Wear-induced aseptic loosening is still one of the main indications for revision surgery in THA. Addressing the problems created by wear particles in metal or polyethylene (PE) bearings is the first step to minimising the creation of wear debris and avoiding resulting complications. PE particles are responsible for a large number of post-prosthetic osteolyses. In terms of shells and inserts made of PE, the size of the ball head and the activity level of the patient are factors in the creation of wear debris. It is common consensus that due to their enhanced range of motion (ROM), large bearing diameters prevent subluxation, dislocation and impingement and have a positive impact on joint stability. Post-operative dislocation rates of up to 10% indicate that the problem of dislocation is still a major concern, and that implant stability together with a sufficient ROM is an important success factor in THA. Nevertheless, a larger diameter of the bearing couple in connection with PE inserts results in increased wear volume.

For ceramic bearing couples, the question of wear volume is more or less resolved. Clinical experience states that they show the lowest particle emission and osteolytic potential of all bearing materials in use. Consequently, ceramic implants are well-suited for risk patients, e.g. people with an allergy to metal. Furthermore, ceramic bearing couples show no increase of wear volume with increased bearing diameter. An evaluation of the English–Welsh 2000–2006 registry, displaying the reasons for 8,554 revision surgeries, can be seen in Figure 1. Similar results can be seen in the Emilia-Romagna registry, evaluated from 2000 to 2007, with 6,259 revision surgeries (see Figure 2).

The ceramic bearing couples used in orthopaedics are made of pure alumina (Al₂O ₃). ^2–4 Since 1971, more than five million components have been implanted. For a breakdown of the usage of the different bearing materials, see Figure 3. ^5–7 Enhancements regarding the microstructure and the reliability of ceramic materials have been reached mainly by improving the production process, leading to a significant increase in mechanical strength. Composite ceramics with even more improved mechanical properties offer new areas of application such as larger bearing couples for better ROM and joint stability. In the following article, the main characteristics and the mechanical behaviour of a special ceramic composite material is analysed. Its clinical potential is assessed as being very promising. ⁸

Description of the Zirconia-toughened, Platelet-reinforced Alumina Ceramic BIOLOX® delta
BIOLOX®delta is an alumina-based composite ceramic. Approximately 80% of the matrix consists of fine-grained high-purity alumina, which is similar to the well-known material BIOLOX® forte . As is the case in any other composite material, its basic physical properties such as stiffness, hardness, thermal conductivity, etc. are mainly pre-determined from the dominating phase. The basic idea behind the development of the new material was to preserve all of the desirable properties of BIOLOX® forte – an excellent bioceramic with more than 30 years of clinical experience – but to increase its strength and toughness. These properties have been substantially improved by implementation of reinforcing elements. Figure 4 shows the microstructure of BIOLOX®delta.

Two reinforcing components are integrated into BIOLOX®delta . Seventeen per cent of the matrix consists of tetragonal zirconia particles. The average grain size of the zirconia is around 0.27μm. As a further reinforcing element, approximately 3% of the matrix is built from platelet-shaped crystals of the ceramic composition strontium aluminate. The platelets stretch to a maximum length of approximately 5μm with an aspect ratio of 5–10. In order to further reinforce the components, stabilising elements are also doped to the material. Chromium is added, which is soluble in the alumina matrix and increases the hardness of the composite. The small amount of chromium is the reason for the pink colour of the material. Furthermore, yttrium is added to the composite, which is absorbed in the zirconia and supports the stabilisation of the tetragonal phase.