The extensor mechanism consists of a chain of anatomical structures including the quads muscle and kneecap that work together to straighten the knee.

rectus femoris part of quadriceps muscle

The rectus femoris is the central part of the quadriceps (quads) muscle and is a key part of the extensor mechanism. The patella (kneecap) is embedded in its tendon. The complex reaches from the hip, through the femur and patella to the tibia.


extensor mechanism


The patella is unusual in being a sesamoid bone, which means that it is integrated within a tendon.


Injury of the extensor mechanism

The four heads of the quadriceps all exert tension through their common tendon (quadriceps tendon), then the embedded patella and its patellar tendon. As the muscles contract, the patella acts as a fulcrum making the contraction more efficient, and straightening the tibia bone. Injury to any part of this chain of structures may result in decreased extension or weakness of extension. A tear of the muscle, or either tendon or a fracture of the patella or avulsion of the bony attachments may all have similar outcomes. 


The patella - important anatomical relationships

The kneecap or patella is part of a chain of structures that help to straighten the knee.

Reader-compiled list of online links about quads or patellar tendon rupture.

An explanation of factors affecting measurement of the Q-angle.



Extensor Mechanism Disruption after Total Knee Arthroplasty: A Case Series and Review of Literature. Vaishya R,Amit Kumar Agarwal AK and Vijay V. Cureus. 2016 Feb; 8(2): e479.

Anterior avulsion fracture of the tibial tuberosity in adolescents – Two case reports. da Silva Júnior AT, da Silva LJ, da Silva Filho UC, Teixeira EM, Araújo HRS, and de Moraes FB. Rev Bras Ortop. 2016 Sep-Oct; 51(5): 610–613.

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