Ever wonder the use of our patella?

These are some concepts that has been lingering in my minds for quite some time. It was not until I picked up a book from the library that I finally realised the immerse stress that our knee is undergoing each day.I have summarised the facts as followed. Exam coming next Wednesday.


2 Biomechanical functions of the knee

1) Aids in knee extension by producing anterior displacement of the quadriceps tendon throughout the entire range of motion, thereby lengthening the lever arm of the quadriceps muscle force.
2) Allows a wider distribution of compressive stress on the femur by increasing the area of contact between the patella tendon and the femur.
The contribution of the patella to the length of the quadriceps muscle force lever arm varies from full flexion to full extension of the knee.
At full flexion:
Patella is in the intercondylar groove, it produces little displacement of the quadriceps tendon, and it contributes the least to the length of the quadriceps muscle force lever arm (10% of the total length).

As knee is extended (up to 45 degrees),
Patella rises from the intercondylar groove, producing significant displacement of the tendon. (lengthen the lever arm up to about 30%)

Knee extension beyond 45 degrees,
Length of the lever arm is diminished slightly. With this decrease in its lever arm, the quadriceps muscle force must increase for the torque about the knee to remain the same.

Studies show that the quadriceps force required to increase the knee the last 15 degree increased by approximately 60%.


If patella is removed from a knee,
The patellar tendon lies closer to the center of motion of the tibiofemoral joint than in an intact knee. Acting with a shorter lever arm, the quadriceps muscle must produce even more force than is normally required in order for a certain torque about the knee to be maintained during the last 45 degrees of extension. This increase in force may be beyond the capacity of some patients, particularly those who have intra-articular disease.


Quadriceps muscle lever (represented by broken line) in a normal knee from which the patella has been removed. The lever arm is the perpendicular distance between the force exerted by the quadriceps muscle through the patellar tendon and the instant centre of the tibiofemoral joint for the last 2 degress of extension.




Statics and Dynamics of the PATELLOFEMORAL JOINT

In general, the greater the muscle force, the greater the joint reaction force. In this joint, the quadriceps muscle force increases with knee flexion.

During relaxed upright standing,
Minimal quadriceps muscle force are required to counterbalance the small flexion moments about the patellofemoral joint because the centre of gravity of the body above the knee is almost directly above the centre of rotation of this joint.

During knee flexion,
The centre of gravity shifts farther away from the centre of rotation, thereby greatly increasing the flexion moments to be counterbalanced by the quads muscle force. As the quads muscle force rises, so does the patellofemoral joint reaction force.

Level walking requires little knee flexion, the reaction force was low. The peak value, in the middle of stance phase when flexion is the greatest, was one half the body weight.

During stairs climbing and descent, at the point when knee flexion reached a maximum of about 60 degrees, the peak value for joint reaction force equaled 3.3 times the body weight.

When knee is extended, the lower part of patella rests against the femur. As the knee is flexed to 90 degrees, the contact surface between the two surface between the patella and femur shifts cranially and its size increases. This increase in contact surface with knee flexion to some extent compensated for the large patellofemoral joint.

For a tendon rupture of weight lifer lifting a barbell of 175g:
The torque on the knee joint was 550 Nm and the quads muscle force was 10,300N at the instant of tendon rupture when knee was flexed to 90 degrees!!!

Therefore, patients with patellofemoral joint derangements experience increased pain when performing activities that requires large amount of knee flexion.


Q: Quadriceps tendon
P:Patella tendon
J:Patellofemoral joint

Throughout the knee bend the patellofemoral joint reaction force remained higher than the quadriceps.


Some other facts:
During the different phases of walking:
1) Heel Strike :
*Hip Abductors, Extensors : 4x Bodyweight
2)Push off:
*Hip Adductors, Flexors : 7x Bodyweight

Most stable position for the hip:
Abduction,lateral rotation,extension
(frog leg position)


The two main moments acting around the centre of motion of the tibiofemoral jt (solid dot) are designated on the free body diagram of the lower leg during stair climbing. Since the lower leg is in equilibrium, the extending moment produced by the patellar tendon force P times its lever arm (b) counterbalances the flexing moment prodcued by the ground reaction force W times its lever arm (a). The weight of the lower limb is disregarded.


Joint reaction force in terms of body weight transmitted through the tibial plateau during walking, one gait cycle (12 subjects). The muscle forces producing the peak magnitudes of this force are designated.



With thanks to Margareta Nordin - Basic Biomechanics of the Musculoskeletal system.