Resumo

LEARMONTH, Ian D. The rise and fall of large bore metal-on-metal hip prostheses. SA orthop. j. [online]. 2014, vol.13, n.3, pp.21-27. ISSN 2309-8309.

ABSTRACT Metal-on-metal was the first articular couple to be successfully used in total hip replacement. Large heads and high frictional torque resulted in a high incidence of implant loosening. This led Charnley to introduce the low friction arthroplasty where the torque was reduced by using small heads and low friction hard-on-soft bearings. Osteoarthritis is a disease of cartilage, and resurfacing arthroplasty is therefore intuitively the logical conservative surgical solution. The early resurfacing devices with metal-on-plastic bearings were associated with catastrophic wear of the plastic. In the 1990s resurfacing was revisited, and metal-on-metal was selected to resolve the challenge of the bearing surface. Various factors were not initially recognised as being critical for the successful outcome of these implants. These include the importance of component orientation, the significance of an adequate 'arc of cover', the prevalence of edge loading, and the impact of microseparation. In addition the biological response - pseudotumour/ALVAL etc. - was unpredicted and unexpected. Femoral neck fractures in patients with a resurfacing arthroplasty were treated by revision to a stemmed implant with a large head. However this option was seen to have a wider application than resurfacing, was perceived as a 'high performance bearing' and addressed the problem of dislocation - one of the commonest causes of revision. These large bearings provided a dislocation 'safety net' to surgeons, many of whom consequently became less meticulous about component orientation. Liberation of metallic debris by fretting and corrosion at the taper junction between the large head and the taper of the stem - totally unforeseen by the bioengineers - emerged as a real problem in these hips. This paper attempts to scientifically place some of these issues in perspective against the backdrop of hysteria created by legal colleagues and the media.

Palavras-chave : hip; large metal bearings; failure.

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