Why large-head metal-on-metal hip replacements are painful: the anatomical basis of psoas impingement on the femoral head-neck junction; Cobb JP, Davda K, Ahmad A, Harris SJ, Masjedi M, Hart AJ; Journal of Bone and Joint Surgery. British Volume 93 (7), 881-5 (Jul 2011)

Large-head metal-on-metal total hip replacement has a failure rate of almost 8% at five years, three times the revision rate of conventional hip replacement. Unexplained pain remains a feature of this type of arthroplasty. All designs of the femoral component of large-head metal-on-metal total hip replacements share a unique characteristic: a subtended angle of 120° defining the proportion of a sphere that the head represents. Using MRI, we measured the contact area of the iliopsoas tendon on the femoral head in sagittal reconstruction of 20 hips of patients with symptomatic femoroacetabular impingement. We also measured the articular extent of the femoral head on 40 normal hips and ten with cam-type deformities. Finally, we performed virtual hip resurfacing on normal and cam-type hips, avoiding overhang of the metal rim inferomedially. The articular surface of the femoral head has a subtended angle of 120° anteriorly and posteriorly, but only 100° medially. Virtual surgery in a normally shaped femoral head showed a 20° skirt of metal protruding medially where iliopsoas articulates. The excessive extent of the large-diameter femoral components may cause iliopsoas impingement independently of the acetabular component. This may be the cause of postoperative pain with these implants.

My View

• the big femoral head is a fact of life for all hip resurfacings.
• the femoral heads with >50 mm diameter are favoured as they reduce the risk of dislocation.
• BUT they often hurt.
• seems the reason is that during the normal range of movement the larger head pokes (impinges) the iliopsoas muscle – see the picture opposite – it is the muscle that links the lower lumbar spine to the leg – so quite important, you might say, and not too distant from some nerves or two!
• So – practicalities of artificial hips bump into anatomy – bloody inconvenient really!

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