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
- See-Seong Chang1
- Eugene T Ek2
- Vicki Pliatsios2
- 1 Goulburn Valley Health, Shepparton, VIC.
- 2 St Vincent’s Health, Melbourne, VIC.
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- 2. La Du BN. Alkaptonuria. In: Scriver CR, Beaudet AL, Valle D, Sly WS, et al, editors. The metabolic and molecular bases of inherited diseases. 8th ed. New York: McGraw Hill, 2001: 2109-2123.
- 3. Garrod AE. The Croonian lectures on inborn errors of metabolism. III. Alcaptonuria. Lancet 1908; 2: 73.
- 4. Phornphutkul C, Introne WJ, Perry MB. Natural history of alkaptonuria. N Engl J Med 2002; 347: 2111-2121.