The most likely diagnosis is: c) osteochondritis dissecans

Figure 1 demonstrates a small 3-4 mm rounded focus of abnormal increased signal intensity in the mid-subarticular region of the medial femoral condyle, in keeping with cystic marrow change. Careful inspection of the same image will also reveal a linear area of abnormal signal involving the articular hyaline cartilage immediately below the cystic focus (increased signal and thickness compared to the articular cartilage along the lateral femoral condyle). The combination of these appearances, as well as the location along the medial femoral condyle, is characteristic for localized osteochondritis dissecans (OCD).

Osteochondritis dissecans is a form of osteochondrosis, which affects the articular cartilage and subchondral bone of immature joints. Although the exact etiology is unclear, it’s proposed that a combination of repetitive trauma, and cartilaginous or subchondral ischemia play large roles. Abnormal ossification and genetic factors may also be involved in the pathogenesis. Trauma is the most likely primary insult, resulting in secondary local ischemic changes. The injured hyaline articular cartilage is thought to become permeable to synovial joint fluid, resulting in subchondral cystic change (as shown in Figure 1). The subchondral cyst, and ongoing trauma, likely prevents healing of the cartilage, resulting in creation of an osteochondral defect and small osteochondral fragments, which may become free and produce symptoms as loose joint bodies. The articular damage may result in flattening of the articular surface and eventually premature osteoarthritis years later.

The knee is the most commonly affected joint, typically along the inside aspect of the medial femoral condyle, but the condition has been noted in many other joints, including the elbow (capitellum), ankle (talar dome), hip (femoral head), shoulder (humeral head or glenoid) and wrist (scaphoid).

Patients typically complain of pain and joint swelling with excess use. Immobilization may result in full healing in some cases, although advanced cases often require arthroscopic intervention to debride frayed cartilage and anchor osteochondral fragments back to the parent bone. Microscopic drilling of the defect site may result in increased local blood flow and healing, and this has even been successfully performed percutaneously under ultrasound guidance.

Premature osteoarthritis would be extremely uncommon in a 14-year-old boy, especially with no prior history of injury or insult. In addition, there’s no medial compartment joint space narrowing, which would accompany that diagnosis. The medial meniscus is the triangular black (hypointense) structure immediately below and to the left of the OCD defect in Figure 1 and is intact. Juvenile rheumatoid arthritis (JRA) is the most common chronic arthritis of children and usually involves multiple joints, producing a range of inflammatory changes in the affected joints, with synovial thickening, joint effusion, joint space narrowing, diffuse periarticular osteopenia and subchondral marrow changes. The clinical presentation and MRI findings in Figure 1 are not characteristic of JRA. Epiphyseal giant cell tumours (osteoclastomas) represent about 20% of primary bone tumours and are the most common tumour of epiphyses in skeletally mature individuals with closed growth plates. They’re locally aggressive lytic lesions, growing up to 4-10 cm in size. These lesions only develop after the closure of the physis (growth plate). Since the growth plates are open in this young male, this diagnosis should not be considered, regardless of MRI appearance.

References:

1. Sofka CM, Bogner E. Hospital for Special Surgery Journal 2008;4:71-3.
2. Boutin RD, et al. American Journal of Roentgenology 2003;180:641-5.
3. Azouz EM, et al. Skeletal Radiology 1993;22:17-23.