Introduction

In total hip arthroplasty ceramic on ceramic bearing couples are used more and more frequently and on a wordwide basis. The main reason of this choice is reduction of wear debris and osteolysis. The tribological properties and the mechanical behaviour of the implanted ceramic must remain the same throughout the patient's life.

The aim of this study was to evaluate the resistance of Alumina Matrix Composite to environmental degradation.

Dislocation remains one of the most common complications after total hip arthroplasty.

Precise cup position appears to be a main factor as significant variations occur for frontal and sagittal acetabular tilt and anteversion according to sitting or standing positions.

An innovative dual mobility ceramic-on-ceramic joint has been developed to solve these problems.

The dual mobility ceramic-on-ceramic joint allows to move the rotation center much deeper inside the insert in order to increase the joint stability without negative impact on the ROM. This device revealed higher torques against subluxation in comparison to the classical Al-Al systems, even with 36mm head diameters, or 41 mm metal on metal bearings.

The additional outer-bearing surface motion creates a second “adjustable acetabulum” due to the eccentration between the rotation center of the ball head and the rotation center of the bipolar head. This offset creates a resultant force that rotates the bipolar component.

Using two bearing ceramic surfaces, the intermediate component acts as a “self adjusting cup”, dealing with the variations of pelvic orientation and acetabulum anteversion.

The use of the dual mobility ceramic-on-ceramic joint seems an interesting alternative when facing difficult or unexpected situations for cup adjustment and cases with hip instability In a hip simulator in micro separation condition, the wear of the dual mobility ceramic-on-ceramic was less than 0.01 mm3/million cycles, the detection limit for wear measurement. There was no change in the surface roughness of the inserts.

The design of the joint with the mobile ceramic head prevented edge loading of the head on the edge of the cup. No stripe wear was observed.

Since 2006 more than 2000 dual mobility ceramic-on-ceramic systems have been implanted in Europe and clinical studies are conducted. The aim is to demonstrate the resistance to dislocation in primary total hip arthroplasty. Previous results over 125 patients in a prospective multicentric study show a Harris and Womac score equivalent to a standard hip prosthesis. No dislocations have been reported. No ceramic breakage or “squeaking” phenomenon appears.

Dislocation and microseparation are major causes of failure for ceramic-ceramic hip prosthesis. When no ideal solution has been found for acetabular implantation, the dual mobility ceramic-on-ceramic device is a real alternative. The exclusive design of the bipolar head give the high resistance to wear and stripe wear to the dual mobility ceramic-on-ceramic joint. Reducing the risk of dislocation and reducing wear drastically are two advantages that can place the dual mobility ceramic-on-ceramic joint as the best choice in primary Total Hip Arthroplasty. Obviously this choice applies to recurrent dislocation also.

The introduction of ceramics in total hip arthroplasty contributed significantly to the wear reduction of poly-ethylene and in consequence reduced osteolysis and loosening. This great benefit has been demonstrated in several clinical observations. In a recent study from Norway, the wear of a 28mm alumina and a CrCo ball head against Ultra High Molecular Weight Polyethylene (UHMWPE) after 10 years is compared using the RSA method of wear measurement.

It was concluded that the considerable reduced wear for ceramic ball heads in comparison to CrCo ball heads is a great advantage in hip arthroplasty.

A first prospective, randomized study with a 15 years follow up has been presented recently in the EFORT 2009. The comparison of wear of polyethylene between alumina and metal ball head shows a reduction of 44% penetration (linear wear) with the alumina-polyethylene bearing surface. In order to offer improved mechanical resistance and tribological qualities than alumina whilst maintaining structural stability, a new generation of alumina matrix composite (BIOLOX^®delta) has been used in orthopedics since 2001. The topic of this study is to demonstrate the excellent wear performance of the alumina ceramic composite against polyethylene, compared to alumina/PE in vivo.

Methods: The BIOLOX^®delta-PE bearing has been tested on a six station hip simulator (Endolob, Rosenheim) according to ISO/DIS 14242. The newborn calf serum was replaced every 0.5 million cycles and the test was stopped after 5 million cycles. Weight was measured using a high precision balance (Sartorius BP 211D)

Results and Discussion: After 5 million cycles, the insert surface appeared polished with fine scratching on the whole contact area. The wear rates calculated by linear interpolation were 13,52 mg per million cycles. (Standard deviation 0,60). The wear rate measured for BIOLOX^®delta against UHMWPE was 13,52 mg per million cycles.

In general, the wear rate can be regarded as small compared to other hip simulator tests using ceramic against polyethylene couplings. When comparing the results for BIOLOX^®forte on polyethylene with the same 28mm diameter and same testing parameter, we observed 26,57 +/− 3,55mg/million and 16,08+/−2,31 mg/million, respectively. The BIOLOX^®delta on UHMWPE bearing shows improved wear behavior with a much lower wear rate.

Conclusion: This study demonstrates the very low in vitro wear of the Alumina ceramic composite on UHMPE compared to ball heads made of pure alumina. Based on this results and the clinical performance of the alumina-UHMPE bearing from the literature, we can expect a further reduction of wear for the BIOLOX^®delta on UHMWPE in vivo that will increase the survival rate of the total hip arthroplasty.

Demand for ceramic bearings is increasing rapidly because of excellent clinical results. Alumina offers advantages such as chemical resistance, excellent bio-inertness and tribology. However, alumina has limited strength, therefore the applications are restricted to certain designs. Zirconia materials have been used clinically for ten years, they reveal problems due to poor hydrothermal stability. Thus, there is a strong need for new bearing material that combine strength and stability.

The new ceramic named Alumina Matrix Composite (AMC) uses the following principle of transformation toughening: Firstly, the dispersing of small particles of Y-TZP Zirconia in the alumina matrix and secondly the reinforcement by introduction of an anisotropic crystal like whiskers. This process dissipates the crack energy that is associated with an increase of strength. The examination of the tribological situation of AMC, especially under challenging conditions of hydrothermal ageing and under severe micro separation, shows the aptitude of this material in wear applications.

Alumina Matrix Composite offers a better mechanical resistance than alumina while maintaining the structural stability and equivalent tribological qualities. This is a material that has been very thoroughly evaluated and tested as a permanent implant material for the last 9 years. The results of this evaluation and testing process have been included in the manufacturer’s Master File at the Food and Drug Administration and approved.

The substantial improvement in mechanical properties and the excellent wear behaviour, even under severe microseparation conditions, make this material a promising new addition to the orthopaedic surgical community and a possible solution to the longevity problems seen with many total joint systems in young and active patients. No complications have been reported yet at six-year follow-up, with more than 310,000 components (heads and inserts) implanted. Additionally, due to the enhanced mechanical behaviour, new applications in orthopaedics are possible.