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The Hip: Anatomy and Total Replacements

Published in N°266 - August/September 2017
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The Hip: Anatomy and Total Replacements

By Philippe Chiron in category SURGICAL TECHNIQUE
CHU Toulouse - France

Total hip replacements are now a reliable procedure. Advances in the materials, forms and surgical techniques have made it possible to improve outcomes and survival and reduce the risk of short- and long-term complications. Orthopaedic surgeons can be proud of having developed one of the most useful surgical procedures of the past fifty years. However, let us take a step back and be humble for a moment! When performing a total hip replacement, should we be trying to replicate the native anatomy? Should we endeavour to preserve the elements that were intact prior to the operation? Can intraoperative anatomical damage create a risk of severe complications that could jeopardise function or even be life threatening for the patient? We do in fact have the answer to all these questions, but it can be useful to consider them all together.

What are we replacing?

Materials

The joint surfaces:

The cartilage, made of collagen, proteoglycans, calcium, chondrocytes and water, is replaced with chrome, cobalt, zinc, molybdenum, ceramic and polyethylene.

The femoral head, made of vascularised hydroxyapatite, collagen, osteoblasts, osteoclasts, H2O and marrow is replaced with avascular chrome, cobalt, molybdenum, zinc and titanium which is therefore not accessible to antibiotics or biofilm-support media. Although these materials are biocompatible, they are nevertheless susceptible to corrosion and fretting and may leach particles that cannot be lysed by cytokines, or carried by macrophages if they measure between 1 and 10 μ in diameter. The leached particles are responsible for local reactions such as ALVAL (aseptic lymphocyte-dominated vasculitis-associated lesion) (1) or more simply bone lysis that leads to loosening (2,3) (Fig. 1).

 

Figure 1: Wear debris is resistant to organic chemistry.

 

Passive stability components

The hip is an enarthrodial joint with three degrees of freedom and in which, the acetabulum and head diameter help maintain stability. We replace an ogival acetabular socket of varying shape with a rigid spherical cup (4). Destruction of the subchondral bone affects stress distribution (5,6). We...

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