• Typically occurs in patients >40 years old.
  • Lateral X-rays with 30° of knee flexion used to define patella baja in complete tears.
  • If a patent cannot extend their knee, it is a marker of a surgical injury: quadriceps tendon rupture, patellar tendon rupture, or patellar fracture.

Evaluating extensor mechanism:

  1. Check function by asking patient to extend at the knee. Ensure there is a smooth arc motion without extension lag. Extension lag is a delay in the final 15-20° of extension which indicates accessory muscle use. The presence of extension lag indicates loss of normal extensor function.
  2. If struggling to overcome gravity, patient can be turned on their side to evaluate extensor mechanism function.


Loss of extension of the knee.