Patella sleeve avulsion is an important diagnosis not to miss. Note that there is no periosteum at the patella - the thin sclerotic line represents epiphyseal cartilage, bone-forming physeal cartilage, and zone of provisional calcification ¹.

In this case, there is minimal cortical elevation in a relatively mature knee (no substantial unossified cartilage). The patella does not appear high-riding, nor is there significant effusion to suggest intra-articular extension.

However, when there is substantial unossified cartilage, sleeve avulsion often continues posteriorly to involve articular cartilage. Displacement of potentially bone-forming cartilage risks elongation or duplication of the patella, as bone growth continues from the displaced chondral fragment. Orthopedic fixation of the displaced chondral fragment is recommended to avoid this complication.

A high index of suspicion is therefore required, even though there may be minimal or no visible avulsed bone. MRI should be suggested to define the size and displacement of the radiolucent chondral fragment.