The patella and the 'extensor mechanism'

There are quite a lot of medical words that need to be absorbed before one can confidently talk about the patella and its problems, and the extensor mechanism falls into that group.

This illustration shows the patella and its relationship to the rectus femoris muscle (one of the four heads of the quadriceps muscle group). The rectus femoris is only one of the four quadriceps muscles but it is the most powerful and this illustration showing just the one muscle allows you more clearly to see the concept than if they were all shown.

You can easily see that if this big muscle contracts the knee will straighten. There might also be some flexion at the hip. So if the muscle contracts and the force of the contraction straightens the knee, then the patella is passively part of this process - because it exists within the tendon of the muscle.

This whole joined-up unit is known as the extensor mechanism - that is from the hip where the muscle strongly attaches, through the femur where some of the muscle attaches, plus the muscle itself and the patella, the tendon above and below the patella and the bump (tibial tubercle) on the tibia where the patellar tendon attaches, and the tibia bone itsel are all components of the extensor mechanism.

Abnormalities of any of these structures - if they alter the normal force that straightens the knee - are said to alter the extensor mechanism.