Early-Onset Degenerative Bone Changes as a Manifestation of Alkaptonuria: A Case Report

Alkaptonuria is a rare inherited metabolic disorder characterized by progressive pigment deposition in connective tissues, leading to early degenerative joint disease and tendon fragility. Although uncommon, its clinical manifestations may closely resemble more prevalent musculoskeletal conditions, often resulting in delayed diagnosis. We present the case of a 53-year-old woman with long-standing, disabling osteoarticular complaints initially attributed to common degenerative disease, but markedly premature for her age. Due to worsening bilateral gonarthrosis, she underwent left total knee arthroplasty, during which intraoperative black discoloration of cartilage was observed, consistent with ochronosis. Physical examination revealed bluish pigmentation of the sclerae, auricular cartilage, and tympanic membranes, notably asymmetric, as well as urine that darkened after air exposure. Histology confirmed ochronotic pigment with severe cartilage degeneration and chronic synovitis. Gas chromatography-mass spectrometry demonstrated markedly elevated urinary homogentisic acid (7,016 µmol/mmol creatinine), confirming the diagnosis. Additional findings included lumbar disc disease, osteoporosis, and a non-obstructive renal calculus, although a causal relationship with alkaptonuria could not be established. She later sustained a low-energy intertrochanteric fracture, surgically managed. This case reinforces how common musculoskeletal presentations may mask rare metabolic disorders, emphasizing the importance of recognizing subtle pigmentary clues, including asymmetric otologic involvement, to avoid diagnostic delay through timely collaboration between primary and hospital care.