A 77-year-old man with no significant medical history presented to the orthopaedic clinic of a metropolitan hospital with longstanding left mechanical knee pain. Examination and x-ray revealed features consistent with degenerative arthritis (Figure, A), and he subsequently underwent total knee arthroplasty. During surgery, a bluish-black pigmentation of the bone and the cartilage of the knee joint was noted (Figure, B). The patient’s surgery and recovery were uneventful. It was later discovered that he had a previously undiagnosed rare metabolic disorder, alkaptonuria, which affects one in 250 000 to 1 000 000 people worldwide.1
Alkaptonuria was one of the first inborn errors of metabolism to be described, in 1908.2 It is caused by mutations in the homogentisate 1,2-dioxygenase (HGD) gene, which results in a deficiency of HGD, which catabolises homogentisic acid (HGA). This leads to accumulation and deposition of HGA in cartilaginous tissues, causing a bluish-black discolouration (ochronosis).3 Ochronosis is generally asymptomatic, but ochronotic arthropathy due to deposition of pigments in the joints is common.3
