June 22, 2023

This is a 41-year-old male with joint pain and generalized structural OA.
The knee joint shows marked full thickness cartilage loss particularly at the medial tibia (A). The lesion has an unusual character as not traumatic and the meniscus looks intact; there are sharp edges of the lesions supporting a recent development. In addition, there is a delaminating component (arrow in B) and large bone marrow lesions at the medial compartment (asterisk in A). This is non-specific but likely a reflection of more rapid progression or a problem with cartilage quality of this patient in general. In addition, there were striking findings of diffuse and advanced intervertebral disc disease with marked spinal degeneration and calcifications of the intervertebral spaces (C and D). This finding is very unusual in a 41-year old male without any external risk factors like prior trauma or long-standing overuse or loading. In the general work-up and after detailed lab analyses a diagnosis of ochronosis, i.e. primary alkaptonuria could be established.
Ochronosis is a syndrome caused by the accumulation of homogentisic acid in connective tissues. The condition is most often associated with alkaptonuria that is caused by a lack of homogentisate oxidase enzyme but can also be secondary (hydroquinone-induced). In the MSK system it will lead to early joint (and particularly spine) degeneration. Tendinopathy, and osteopenia/osteoporosis are other manifestations. Ochronosis is a rare differential diagnosis for early onset structural OA with spinal predominance.