The patella is the largest sesamoid bone in the body and is situated in the tendon of the quadriceps femoris muscle. It articulates against the anterior articular surface of the distal femur. It holds the patellar tendon off the distal femur thus improving the angle of approach of the tendon to its distal insertion on the tibial tuberosity, so increasing the power generation of the quadriceps mechanism by 30%. The anterior surface of the patella is convex. The superior border is thick and gives attachment to the tendinous fibres of the rectus femoris and vastus intermedius muscles. The lateral and medial borders are thinner and receive the tendinous fibres of the vastus lateralis and vastus medialis muscles respectively. These two borders converge inferiorly to the pointed lower pole of the patella which gives attachment to the patellar ligament.
The articulation between the patella and femoral trochlea forms the patello-femoral knee joint compartment. The articular surface of the patella is described as possessing seven facets. Both the medial and lateral facets are divided vertically into approximately equal thirds, whereas a seventh or odd facet lies along the extreme medial border of the patella. Overall the medial facet is smaller and slightly convex, the lateral facet which consists of roughly two thirds of the patella has both sagittal convexity and coronal concavity.
Six variants of the patella have been described. Types one and two are stable whereas the other variants are more likely to result in lateral subluxation as a result of unbalanced forces. The facets of the patella are covered by the thickest hyaline cartilage in the body which may measure up to 6.5mm in thickness.
The patella fits in the trochlea of the femur imperfectly with the contact area between the patella and femur varying with position of flexion. The area of contact never exceeds about one third of the total patella articular surface. At 10 to 20° of flexion at the distal pole the patella first contacts the trochlea in a narrow band across the medial and lateral facet. As flexion increases the contact area moves proximally and laterally. The most extensive contact is made at about 45° where the contact area is an elipse across the central portion of the medial and lateral facet. By 90° the contact area is shifted to the upper part of the medial and lateral facets. With further flexion the contact area separates into two distinct medial and lateral patches. Because the odd facet only makes contact with the femur in extreme flexion (such as in the act of squatting) this facet is habitually a non contact zone in humans in Western countries.
The main biomechanical function of the patella is to increase the momentum of the quadriceps mechanism. The load across the joint rises as flexion increases but because the contact area also increases, higher forces are dissipated over a larger area. However, if extension against resistance is performed, the force increases while the contact area shrinks. Straight leg raises eliminate forced transmission across the patello-femoral joint because in full extension the patella has not yet engaged the trochlea.
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