Loose Body in the Knee

Q: What is the commonest site of osteochondritis dissecans in the knee?

A: The lateral (intercondylar) aspect of the medial femoral condyle, adjacent to the notch and outside the principal weight-bearing zone — best profiled on the tunnel or intercondylar view. This is distinct from SONK, which involves the weight-bearing surface of the medial femoral condyle in an older patient.

Q: How do you distinguish primary from secondary synovial chondromatosis on a radiograph?

A: By the uniformity of the bodies and the state of the joint. Primary shows numerous bodies of relatively uniform size with ring-and-arc chondroid calcification in a joint with preserved space, typically aged 30 to 50. Secondary shows fewer bodies of varying size and irregular shape in a joint with obvious degenerative change — narrowing, sclerosis and osteophytes.

Q: A patient has primary synovial chondromatosis. Does removal of the bodies need to be accompanied by synovectomy?

A: It depends on the phase, and Milgram's answer is more nuanced than the usual teaching. He described three phases: (1) active intrasynovial disease with no loose bodies; (2) transitional, with both intrasynovial proliferation and free bodies; (3) multiple free bodies with no demonstrable intrasynovial disease. In phase 3, synovectomy may not be necessary. And in phases 1 and 2 synovectomy does not guarantee against recurrence, because extrasynovial intra-articular cartilaginous lesions can persist afterwards and grow. Phase the disease at arthroscopy and document every recess.

Q: A posterolateral ossicle behind the lateral femoral condyle appears on every lateral film — what is it?

A: The fabella, a sesamoid in the lateral head of gastrocnemius. It is extracapsular, present in roughly one in five knees and often bilateral, and it never migrates. The cyamella, within the popliteus tendon, is the rarer analogous mimic.

Q: Which MRI features indicate an unstable osteochondritis dissecans fragment?

A: Four. A high-signal line at the fragment–bone interface on T2; subchondral cysts greater than 5 mm; fluid tracking through a full-thickness breach in the overlying cartilage; and a displaced fragment. Any one shifts management towards operative fixation, or excision with treatment of the defect.

Q: Which condition presents with recurrent atraumatic haemarthrosis, bone erosions on both sides of the joint, preserved joint space and NO calcification?

A: Pigmented villonodular synovitis. MRI shows blooming artefact on gradient-echo sequences from haemosiderin. Treatment is synovectomy, with radiosynoviorthesis or targeted CSF1R therapy considered for diffuse or recurrent disease.

Q: Which single plain-film feature most reliably confirms a fragment is a true loose body?

A: A change in its position between two radiographs taken at different times or in different degrees of flexion. Note the corollary though — a normal radiograph does not exclude a fragment: in children after a first patellar dislocation, 44 per cent of MRI-confirmed osteochondral fractures were reported as radiograph-negative (Seeley).