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Knee Extensor Mechanism Injuries

By Danielle Campagne, MD , Assistant Clinical Professor, Department of Emergency Medicine, University of San Francisco - Fresno

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(Quadriceps Tendon Tear; Patellar Tendon Tear; Patellar Fracture; Tibial Tubercle Fracture)

Knee extensor mechanism injuries can involve the quadriceps tendon, patellar tendon, patella, or tibial tubercle. Surgical repair is usually required.

Extension of the knee involves the quadriceps muscles, which are attached to the patella by the quadriceps tendon; the patella is connected to the tibial tubercle by the patellar tendon. Forced flexion at the knee with a contracted quadriceps muscle can damage these structures. Injuries include

  • Quadriceps tendon tears

  • Patellar tendon tears

  • Patellar fractures

  • Tibial tubercle fractures

In healthy people, significant force is required to injure these structures; normal tendons are strong enough that the patella often fractures transversely before a tendon tears. However, certain people are at risk of tendon tears. They include the elderly and people who take certain drugs (eg, fluoroquinolones, corticosteroids). In these people, the injury can result from minor trauma (eg, when descending stairs). The quadriceps tendon is injured more often than the patellar tendon, particularly in the elderly.

The affected area is painful and swollen.

Patients with complete tendon tears cannot stand, do a straight leg raise while lying on their back, or extend their knee while seated. Long-term complications (eg, loss of motion, weakness) are common.


  • Clinical evaluation

  • X-rays

  • MRI

Examination of the knee can suggest which structure is injured:

  • Quadriceps tendon tear: The patella is palpably displaced inferiorly.

  • Patella tendon tear: The patella is displaced superiorly.

  • Transverse patellar fracture: There is often a palpable gap between the 2 bone fragments.

However, swelling in the area can be significant and mask these findings so that the injury may be misinterpreted as a ligamentous knee joint injury with hemarthrosis.

Pearls & Pitfalls

  • Always test active knee extension if patients have knee swelling and pain after an injury.

Routine knee x-rays are taken. X-rays often show displacement or fracture of the patella but may appear normal. MRI confirms the diagnosis.


  • Surgical repair

Treatment is surgical repair as soon as possible.

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* This is the Professional Version. *