Ochronosis Masquerading as Osteoarthritis

Abstract

Ochronosis is a rare metabolic disorder resulting from alkaptonuria due to a deficiency of homogentisate 1,2-dioxygenase, leading to accumulation and deposition of homogentisic acid in connective tissues. It commonly presents with progressive arthropathy in adulthood and may clinically mimic degenerative joint diseases such as osteoarthritis, often resulting in delayed diagnosis. We report a case of a 60-year-old male who presented with progressive left hip pain of 6 months’ duration, associated with difficulty in ambulation and daily activities. Clinical and radiological evaluation suggested osteoarthritis of the left hip. Routine laboratory investigations were within normal limits; however, urine examination revealed darkening on standing and tested positive for homogentisic acid. The patient underwent left total hip replacement. Gross examination of the femoral head showed diffuse black pigmentation involving the articular cartilage and subchondral bone. Histopathological examination revealed characteristic brown-black pigment deposition within cartilage and elastic tissues, consistent with ochronosis. Ochronotic arthropathy typically manifests in the fourth to fifth decade and may masquerade as primary osteoarthritis. Recognition of clinical clues, such as darkening of urine and characteristic pigment deposition on gross and microscopic examination, is crucial for diagnosis. There is no definitive curative therapy; management is largely symptomatic, with joint replacement indicated in advanced disease. This case highlights the importance of considering ochronosis in the differential diagnosis of early or atypical osteoarthritis. Awareness of this rare entity and correlation of clinical, biochemical, and histopathological findings are essential for accurate diagnosis and appropriate management.