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Extensor mechanism of the knee is the strongest muscle tendon unit in human body. This unit is composed of the quadriceps (thigh muscle), quadriceps tendon, the patella (kneecap) and the patellar tendon.

This unit transfer forces generated by the quadriceps muscle across the knee to the tibia. These forces have been estimated at up to 1,350 lbs. This unit also is responsible for knee extension. Injury to any part of this unit will interfere with extension of the knee and will lead to significant functional disability.

The mechanism of injury usually are either direct trauma such as a direct blow to the front of the knee or forced hyper flexion such as landing from a jump or falling down the stairs.

Forces that generally cause disruption of the extensor mechanism is more than the above estimated number of 1,350 lbs.

There are conditions where individual is predisposed to these injuries such as prior injection of corticosteroids, chronic tendonitis and some medical conditions such as malfunction of the parathyroid glands.


Usually, severe pain is associated with disruption of the quadriceps or patellar tendon, at times patients describe a loud “pop”, swelling, discoloration, palpation of a gap at the rupture site by the examiner.

There may be visible contusion or abrasions. Patients also describe an involuntary sensation of “giving way”.


A careful examination by a physician, radiographic studies (x-ray), ultrasonography and ultimately magnetic Resonance Imaging (MRI) will confirm the diagnosis.


In most situations and especially cases of complete disruption of these structures treatment is surgical and require repair and at times reconstruction of the quadriceps or patellar tendon.

Return to normal activities and especially sport may need to be delayed at times up to 6-9 months.

Complete healing and remodeling should take place before returning to competitive sport in order to prevent re rupture of these structures.

 Extensor mechanism

49-year-old power lifter with traumatic rupture of the left quadriceps, 9 months following a successful reconstruction wins the world championship by lifting over 600 lbs.