Michael Gross, M.D., FRCSC
Halifax, NS

The ideal bearing surface is one that demonstrates advanced tribological properties, i.e.: wears very slowly, generates debris in small quantities which is biologically inert in the body. The coefficient of friction should be equivalent to or less than articular cartilage and not change over time. Complications of the bearing surfaces should be quantifiable and manageable by the implanting orthopaedic surgeons or colleagues.  

I do not think that there could be much argument with such specifications for bearing surfaces.

Ceramic bearings made from aluminium oxide have been used as bearing surfaces for over 25 years in Europe. The technological advances in ceramics have resulted in higher density products with smaller crystal sizes and a lower incidence of failure through crack propagation. The use of the ceramic from the major manufacturer, Biolox forte™, has resulted in a sharp reduction in the incidence of fractures, down to zero in published data. The wear rates of alumina ceramic couples are extremely low¹.

There have been three iterations of ceramic development, and papers published reflect this^2,3. There have been problems with fixation of ceramic components particularly the acetabulum, resulting in higher revision rates due to loosening of the components. The third generation of the ceramic components now allows for modularity and therefore can be used with whatever acetabular shell or femoral component that the surgeon wishes to use^3,4.

The main disadvantage of ceramic bearings is their intolerance for any impingement or subluxation. This places great emphasis on the correct placement of the components by the surgeon, particularly when replacing hips in young active individuals who can be expected to demand full range of motion of the hip. However, the advantage of the ceramic bearings is that large femoral heads can be used thus increasing range of motion and stability of the hip.

If ceramic bearings do fail, the failure is usually obvious and identified by the patient or the surgeon. The patient knows immediately if the femoral head has disintegrated, and the x-ray is obvious. More subtle changes can occur when a liner fails, whether by cracking through impingement or by excessive wear through subluxation. A component that allows for implantation of alternative bearings, such as polyethylene, will allow the surgeon to revise the liner and not the metal shell. There is newer evidence that ceramic heads can be replaced on the trunion of the neck of the well fixed and not revised femoral component with no increased failure rate of the new ceramic femoral head.

The potential benefit to the patient from a well-placed hip replacement is the long-term survival of that hip replacement: 20 years or more, particularly in young active individuals. Wear will occur, but the literature shows that in well fixed prostheses the wear products are minimal and not biologically active. We do not yet know what will cause failure in these long lasting prostheses, as most patients die with their hips intact. Retrievals of well fixed hips show a small wear pattern and no other abnormalities. In particular, there is no evidence of any biological effect on bone that could cause loosening of the components.

When considering other bearing surfaces, it is essential to consider how complications will affect the patient and how the surgeon will handle them. It becomes a difficult management problem when the surgeon is dependant on other health care professionals and when the issues are not clear to either the patient or the surgeon. This problem bedevils the metal-on-metal debate.

In addition, with new technology, patients need either to know what the results are or what the potential benefits and pitfalls could be. Not all new technologies have outcome results available and it is appropriate that the patients be part of a clinical trial that will gather the answers to these questions. The major problem with the new X-linked polyethylenes is that they are not all equivalent in terms of manufacturing process and that the laboratory wear results may be better that the results in the human body⁵. Use of these new polyethylenes must be subjected to trials to determine the actual in vivo wear results.

My personal preference for ceramic bearings has prompted me to be a co-investigator in a RCT comparing ceramic bearings to conventional polyethylene. This study will give us valuable information as the years progress. I also have an ongoing appreciation of the success of these bearings in Europe in experienced surgeons' hands. The demands of young patients and the historical failure rate of standard metal and polyethylene bearing surfaces has made me comfortable with offering alternative bearings such as ceramics. At present I have two cracked liners in over 100 acetabuli, one due to impingement and one recently identified in an ex alcoholic with AVN. I have not had to revise any patients yet. Although I consider these two patients as failed, they are still walking with no pain and the latter patient is asymptomatic (his is a radiographic diagnosis). I am confident that I can handle any complications that might ensue in this patient population, such as described as above. I feel that these patients do require extremely precise attention to detail to prevent impingement and the complications that arise from this.

References

1. Fruh, H.J. Wear characteristics of ceramic on ceramic for hip endoprosthesis. Biomaterials.1B ,12,p873-6 1997.
   
   
2. Jazrawi, L.M. et al. Wear rates of ceramic on ceramic bearing surfaces in total hip implants: a 12 year follow up study. J Arthroplasty. 14. 781-7. 1999.
   
   
3. D'Antonio, J. et al. New experience with alumina on alumina ceramic bearings for total hip arthroplasty. J Arthroplasty. 17. 390-7. 2002.
   
   
4. Garino, J.P. Modern ceramic on ceramic total hip systems in the United States: early results. Clin. Orthop. P41-7.2000.
   
   
5. Muratoglu, O.K. et al. The comparison of the wear behaviour of four different types of crosslinked acetabular components. Abstract AAOS 2000.