Intraoperative Diagnosis of Ochronotic Arthropathy With Unexpected Medial Collateral Ligament Laxity Managed During Total Knee Arthroplasty.

Ochronosis is a rare metabolic disorder caused by alkaptonuria, resulting in the accumulation of homogentisic acid within connective tissues and progressive degeneration of large joints. Clinical presentation often mimics primary osteoarthritis, leading to delayed or missed diagnosis. We report the case of a 54-year-old male who presented with progressive knee pain and was scheduled for total knee arthroplasty for presumed osteoarthritis. No preoperative clinical or radiological features suggested ochronosis. Intraoperatively, extensive black pigmentation of the articular cartilage, menisci, and synovium was observed, raising suspicion of ochronotic arthropathy, which was later confirmed on histopathological examination. During intraoperative gap assessment, unexpected medial collateral ligament laxity was identified, for which no preoperative plan had been made. The instability was successfully managed using available implants, including semitendinosus autograft augmentation secured with titanium interference screws and a cancellous cannulated screw with a washer. Ochronotic arthropathy may present as an unexpected intraoperative finding during total knee arthroplasty, even in the absence of typical preoperative features. Recognition of characteristic intraoperative pigmentation is essential for diagnosis. Medial collateral ligament laxity may be encountered intraoperatively and may require adaptive management. In this case, a satisfactory range of motion and independent ambulation were achieved at follow-up, highlighting the importance of surgical awareness and preparedness. This case highlights that ochronotic arthropathy may present as an intraoperative diagnostic surprise and may be associated with unanticipated ligamentous instability. Awareness of this possibility and readiness to adapt surgical strategy using available resources are essential for achieving optimal outcomes during total knee arthroplasty.