Blackish Discoloration of Articular Cartilage during Total Knee Replacement; a Case Report

Abstract

Introduction: Alkaptonuria is a rare metabolic disorder that follows an autosomal recessive genetic inheritance pattern. It is distinguished by the buildup of homogentisic acid in tissues due to deficient homogentisic acid oxidase enzyme activity. The excess homogentisic acid is expelled through urine, darkening it on oxidation. Moreover, it deposits in connective tissues, imparting a characteristic blue-black pigmentation. This condition is known as Ochronosis or black bone disease. This accumulation over time renders cartilage brittle, potentially leading to ochronotic arthropathy.

Case report: A 46-year-old lady presented with long-standing bilateral knee pain for 8 years with a predominant focus on the right side. Recurrent swelling and pain episodes in the right knee were noted, occurring without significant traumatic events. Despite various conservative treatments attempted to alleviate her knee pain, the patient experienced minimal relief. On examination, the patient demonstrated difficulty in ambulating, with severe tenderness along the joint line and a restricted range of motion. Standing knee X-rays revealed severe tricompartmental osteoarthritis and a correctable valgus deformity of 5. She underwent total knee replacement. A provisional diagnosis of ochronosis was made intraoperatively, later confirmed on histopathological examination of the tissue, and the patient was evaluated retrospectively.

Conclusion: Ochronotic arthropathies are rare conditions- characterized by articular cartilage damage. They are typically diagnosed post-surgery, often when surgeons observe the dark coloration of cartilage. However, joint replacement surgery can safely and effectively manage these conditions, leading to favorable outcomes, including improved functionality, pain alleviation, and heightened patient satisfaction.

Keywords: Ochronosis; alkaptonuria; ochronotic arthropathy; total knee arthroplasty.

Figure 1

X-ray anteroposterior and lateral views showing tricompartmental osteoarthritis.

Figure 2

Intraoperative image after tibial and femoral cuts.

Figure 3

Resected specimen of articular cartilage and synovium showing blackish discoloration.

Figure 4

Post-operative X-ray.

Figure 5

Follow-up X-ray at 2 years.

Figure 6

Hematoxylin and eosin staining showing ochronotic bodies and degenerative changes.

Figure 7

Negative pearls Prussian blue staining.