Diagnosis

Extraarticular avulsion fracture (AO/OTA 34A1)

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1. Extraarticular avulsion fracture 
2.  "Sleeve" fracture

1. Extraarticular avulsion fracture 

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.

These are classified by AO/OTA as 34A1 fractures.

They may be due to hyperflexion of the knee.

2.  "Sleeve" fracture

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 (see patient asessment ).
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.