Abstract
Localised femoral endosteal bone lysis at or distal to the level of the lesser trochanter can occur soon after cemented hip arthroplasty or as long as 15 years later in a hip that has otherwise functioned perfectly well. The first important question about these lesions is why they occur, and the second, why they occur where they do. Particulate debris, particularly from wear of ultra-high molecular weight poly-ethylene, is commonly regarded as the cause, but changes in hydrostatic pressure may play a more important role than previously thought.
Because the femur bows anteriorly and posteriorly, deficiencies in the cement mantle are particularly likely to occur in relation to the interior aspect of the stem at the level of the lesser trochanter and in relation to the tip of the stem posteriorly.
We suggest that localised lytic lesions occur at the sites of defects in the cement mantle. There is evidence that with pressure changes, joint fluid and whatever particles it contains come into contact with the endosteal surface of the femur at the sites of these mantle defects. Such lesions occur only rarely when polished stems are used. With matt stems, abrasive wear enlarges the internal dimension of the cement mantle, increasing the size of the fluid conduit between stem and cement. Matt surfaced stems retrieved from patients with localised lysis show evidence of both abrasive wear and slurry wear, ‘wear caused by hard particles carried in fluid’. The nature of the changes due to slurry wear shows that the flow of fluid along these stems is from distal to proximal. As the stem becomes increasingly unstable inside the cement mantle owing to wear, the hydrostatic effects on increased stem movement become magnified and may on their own produce bone lysis.
The abstracts were prepared by Professor M. B. E. Sweet. Correspondence should be addressed to him at The Department of Orthopaedic Surgery, Medical School, University of Witwatersrand, 7 York Road, Parktown, Johannesburg, 2193 South Africa
