DESCRIPTION

A 55-year-old white female complained of progressive pain in the left hip mainly while walking. Since the age of 40 she had suffered from back pain and stiffness. In 2000 she consulted a rheumatologist who noticed brown pigmentation on the sclera and external ear, and diagnosed alkaptonuria. A quantitative essay of homogentisic acid in the urine confirmed a high amount of metabolite (4 g/24 h). On examination, hip joints presented a marked reduction in internal and external rotation. Radiography of the hips revealed worsening of osteoarthritic lesions within the last year, particularly on the left side, characterised by joint space narrowing, subchondral sclerosis and marginal osteophyte formation (fig 1). The patient underwent total left hip replacement. Gross examination of the femoral head showed dark pigmentation of the cartilage that was heavily fibrillated and eroded in the weight-bearing areas exposing the subchondral bone (fig 2).

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Figure 1

Radiographic evolution of ochronotic arthropathy of the left hip.

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Figure 2

Gross appearance of the ochronotic femoral head, characterised by dark pigmentation of the cartilage.

Alkaptonuria is a rare autosomal recessive disorder characterised by the inability to metabolise homogentisic acid.1 Affected individuals excrete homogentisic acid in the urine, causing a characteristic dark colour when the urine is alkalinised or oxidised.2 Ochronosis is a condition in which brown–black pigment is deposited in connective tissue, such as cartilage, skin and sclerae. Musculoskeletal involvement is the most serious complication of this condition, leading to a severe and sometimes crippling form of destructive arthropathy.3