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PREVENTION OF PERIPROSTHETIC BONE LOSS BY THE USE OF A CARBON COMPOSITE FEMORAL STEM.

South Wales

The purpose of this study: is to test the hypothesis that there is little or no stress shielding afforded by a carbon composite femoral hip prosthesis when implanted in the human subject, and to investigate the possibility that a hydroxyapatite coating would prevent loosening.

The need for this development: is that loosening remains a problem for young patients who need a long term, reliable fixation of hip replacements, and it appears that if a solution exists to this problem then it probably lies away from the traditional cemented metal varieties.

One of the causes of loosening is stress shielding caused by rigid metal implants and a carbon composite femoral stem has been developed to overcome this. Paradoxically, flexible stems result in increased micro-motion at the prosthetic-bone interface and as a result they tend to loosen more frequently than metal ones. To overcome this, the carbon stem has been coated on its proximal third with hydroxyapatite, in order to get a secure fixation to the upper femur, but left bare distally to minimise weight transfer within the lower shaft.

The Study: 50 patients have entered the trial to date, and the detailed results of the first 35, which have been followed up for an average of 4 years will be presented. Stress shielding by the prosthesis was assessed, clinically, radiologically, and by dexa-scanning (usinga Hologic scanner with metal exclusion software). Two dexa-scan studies were carried out on each patient, at 1 and 2 years post operatively, and the bone mineral density of the implanted bone was compared with that of the normal contralateral side, using the Gruen zones as the basis of comparison. These results were compared with published figures for metal stems, and also with a small series of our own metal stems.

Results: 13 males and 22 females entered the trial, with an average age of 61.

1. Bone density around the carbon composite hip was found to increase by an average of 2% between the measurements carried out at 1 and 2 years post-op.
   
2. In the contralateral hip, bone density remained unchanged over the period.
   
3. Bone density around comparable metal stems reduced by an average of 3% in our cases, but losses over 20% are quoted by others especially for zones 1&7.
   
4. Follow up is very short for responsible prognosis to be offered regarding loosening, but to date the function of the hips remains good.
   

Conclusion: it appears that this prosthesis is fulfilling the predictions made for it, and although there is a spread of responses to it, the average patient is showing a steady increase in periprosthetic bone mineral density and is

Clinically asymptomatic. Progress to a wider trial can now be recommended.

The abstracts were prepared by Mr Ray Moran. Correspondence should be addressed to him at the Irish Orthopaedic Association, Secretariat, c/o Cappagh Orthopaedic Hospital, Finglas, Dublin