Ochronosis of the hip

Clinical History

A patient presented with a rapidly deteriorating pain in his right hip which he had had for the past 6 months.

Imaging Findings

The patient presented with a rapidly deteriorating pain in his right hip which he had had for the past 6 months. Ten year ago, he underwent a left hybrid total hip replacement. It was noticed at the time of surgery that the cartilage had an abnormal blue tinge, and this raised the possibility of the patient having metabolic bone disease. The patient had an older brother with a confirmed diagnosis of alkaptonuria with a typical radiographic finding of the spine and other large joints. As an infant, it was noted that the nappies had an abnormal colouration. However, a diagnosis of alkaptonuria had not been made. There was no other relevant medical history. The diagnosis was later confirmed by doing an urine analysis for homogentisic acid. Plain radiographs of the hips taken with a 6 month gap showed a rapidly progressive destructive arthropathy affecting both the acetabular and femoral components of the right hip. MRI investigation showed extensive destruction of the right femoral head with a thickened synovium. Heterogenous areas within the right hip were consistent with fragmentation and destruction of the cartilage with extensive interarticular debris.

Discussion

Alkaptonuria is a rare, inherited enzyme deficiency of homogentisic acid oxidase causing the accumulation of homogentisic acid when tyrosine and phenylalanine are metabolized. This is excreted in the urine and is oxidized on standing to form a black pigment. Homogentisic acid has a predilection for connective tissue and cartilage. Abnormal pigmentation of the sclera, ears and skin is usually evident by the time a person reaches his twenties. Clothes may be stained due to excessive pigment being excreted in the sweat. Alkaptonuria is a multisystem disorder affecting the cardiovascular system with aortic valve disease (mean age:54), coronary artery calcification (mean age:59), renal stones (mean age:64) and arthropathy leading to joint replacements (mean age:55) . The first manifestations of musculoskeletal symptoms are in the 30s with pain and stiffness of the back . The hips, knees and shoulders are usually involved some years later. The radiographic changes are similar to that of premature osteoarthritis (OA). Loss of articular cartilage and bony sclerosis are prominent. Features that favour alkaptonuria over OA are small or absent osteophytes, collapse and fragmentation with loose intra-articular loose bodies and tendon calcification and ossification.

Differential Diagnosis List

Final Diagnosis

Alkaptonuria of the right hip.

URL:	https://www.eurorad.org/case/3249
DOI:	10.1594/EURORAD/CASE.3249
ISSN:	1563-4086