Patella - patellar mechanics
Submitted by admin on March 14, 2008 - 11:18am.
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When talking about the mechanics of the patella, doctors often refer to the 'extensor mechanism' - meaning the whole muscle/tendon/bone structure of which the patella is a part. 'Extension' means 'straightening', and the extensor mechanism is comprised of the bones, muscles and tendons which act to straighten (extend) the knee. The main extensor muscle is the quadriceps muscle (the 'quads' or your 'lap') - it attaches above to the pelvis and the top of the femur (thighbone), and below to the tibia (thighbone) at an important lumpy spot about 1.5 inches below the bottom of the patella - the 'tibial tubercle'. In this image (an MRI scan with reverse contrast) you are looking at the knee from the side. It has been digitally cut in half so that you can see the components of the extensor mechanism in relation to the underlying bones. You can see the realtionship of the patella to the patellar tendon and the quads tendon, and the point where the extensor mechanism attaches to the tibia bone. |
Q angle
Anatomical conditions affecting the extensor mechanism and tending to pull the patella to the outer side include a twisted femur (femoral torsion) or a tibial tubercle which is too far over to the outer side. A measure of the likely stress on the patella is the 'Q-angle' 'quads' angle (see image). The bigger this angle, the greater the force trying to pull the patella to the outer side, and the restraints and the trochlear groove are not always sufficient to keep the patella where it should be.
Normally, when the knee is straight (extension) the kneecap sits at the shallow section of the trochlea. As the knee flexes ('bends'), it is pulled into the deeper part of the trochlea at about 30 degrees of flexion, then runs centrally in the trochlear groove during the rest of bending. Should the walls of the groove be defective ('trochlear dysplasia') or the patella sit too high on the femur ('patella alta'), then the patella may sometimes come out of the groove, partially ('subluxation') or fully ('dislocation').
Fibrous Restraints
The capsular structures of the knee (the internal casing of the joint) play a local role in maintaining patellar alignment. On the lateral (outer side) is a tight area called the 'lateral retinaculum' and it tends to restrain the patella on the outer side, usually counteracted on the medial side by the medial patellofemoral ligament and a strong quads muscle, especially that portion of it known as the VMO. The medial capsular structures are less tight, so if the VMO becomes weak there may be a tendency for the patella to be pulled to the lateral side.
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