These are classified by AO/OTA as 34A1 fractures.
Extraarticular avulsion fractures generally occur in adolescents, or young adults. This fracture type is biomechanically equivalent to patellar tendon rupture. This avulsion fracture may be simple or multifragmentary.
They may be due to hyperflexion of the knee.
Occasionally a thin shell of bone may be avulsed from the distal patella together with the patellar tendon (“sleeve” fracture). This may be subtle on the radiographs. A clue to the injury may be found by detection of a high-riding patella.
The clinical findings of impaired function of the extensor mechanism and tenderness and swelling in the region of the inferior patella should lead to suspicion of this injury in adolescents or young adults.