Ochronotic arthropathy mimicking degenerative osteoarthritis in an older adult with alkaptonuria: diagnostic and perioperative lessons from a case report.

BACKGROUND: Alkaptonuria is a rare inherited metabolic disorder caused by impaired homogentisic acid metabolism. Ochronotic arthropathy is a late musculoskeletal manifestation and may mimic ordinary degenerative osteoarthritis in older adults. This diagnostic overlap may delay recognition of the underlying systemic metabolic disease.

CASE PRESENTATION: A 67-year-old woman presented with progressive pain and restricted motion of the left knee and hip for four years. Preoperative imaging suggested advanced degenerative arthropathy. However, she had a long-standing history of urine darkening after standing and imaging evidence of multi-joint and spinal involvement. During left total knee arthroplasty, diffuse black-brown pigmentation of the articular cartilage and synovium was observed. Histopathological examination demonstrated ochronotic pigment deposition. The patient subsequently underwent staged left total knee and total hip arthroplasty. At five-year follow-up, knee and hip function were satisfactory, and radiographs showed stable prosthesis positioning without loosening.

CONCLUSION: Ochronotic arthropathy should be considered in older patients with atypical multi-joint degenerative changes, particularly when urine darkening, spinal involvement, or intraoperative black-brown cartilage pigmentation is present. Although biochemical or genetic confirmation is ideal, clinicopathological recognition remains important. Staged joint arthroplasty may provide durable pain relief and functional improvement in advanced ochronotic arthropathy.