Biomechanics of the Knee

1. Biomechanics of the Knee • Basics • Meniscus • Patella Motion • Collateral ligaments

2. The hips are wider apart than the ankles thereby making the mechanical axis of the lower limb running obliquely inferiorly and medially and forms an angle of 3 degrees with the vertical. The wider the pelvis, the greater the angle, as in the case of females. This also explains why the physiological valgus of the knee is more marked in women than in men. Basics

3. The structure of the knee makes axial rotation impossible when the knee is fully extended, the axis of the leg coincides with the mechanical axis of the lower limb and axial rotation occurs not at the knee but at the hip, which is complimentary to the knee. Basics

4. There is strictly no absolute extension. It is possible to achieve passive extension (5-10 degrees) from the position of reference, this is wrongly called ‘hyperextension’. Active flexion attains a range of 140 degrees if the hip is already fixed and only 120 degrees if the hip is extended. This difference is due to the fact that the hamstrings lose some of their efficiency with extension of the hip. Basics

5. Passive flexion of the knee attains a range of 160 degrees and allows the heel to touch the buttock. Pathologically, passive flexion is limited by retraction of the extensor apparatus - essentially the quadriceps - or by shortening of the capsular ligaments. Basics

6. Extension, the menisci are pulled forward by the meniscopatellar fibers, which are stretched by the anterior movement of the patella and this draws the transverse ligament forward. The posterior horn of the lateral meniscus also is pulled anteriorly by the tension developed in the meniscofemoral ligament, as the posterior cruciate ligament becomes taut. Meniscus Medial Lateral

7. In flexion, the medial meniscus is drawn posteriorly by the semimembranosus expansion which is attached to its posterior edge, while the anterior horn is pulled anteriorly by the fibers of the anterior cruciate ligament attached to it The lateral meniscus is drawn posteriorly by the popliteus. Medial Lateral Meniscus

9. During axial rotation, the menisci follow exactly the displacements of femoral condyles. Starting from the neutral position they can be seen to move on the tibial condyles in the opposite direction Lateral rotation - the lateral meniscus is pulled towards the anterior part of the tibial condyle while the medial meniscus is drawn posteriorly. Medial rotation - the medial meniscus moves forward while the lateral meniscus recedes. Lateral rotation Medial rotation

10. The menisci become distorted about their attachments of the horns. The total range of movement of the lateral meniscus is twice as great as that of the medial meniscus. These displacement of the menisci during axial rotation are mostly passive Lateral rotation Medial rotation

11. The extensor apparatus of the knee slides on the lower end of the femur like a cable on a pulley. During flexion the patella is vertically displaced Normally the patella moves only in the vertical plane and not transversely. At the end of extension this appositional force is diminished. In hyperextension it tends to be reversed - separating the patella from the femur because the quadriceps tendon and the ligamentum patellae form an obtuse angle laterally. Patella Motion

12. During flexion and extension the patella recedes while moving along the arc of a circle. During movements of axial rotation the patella moves relative to the tibia in a frontal plane Patella Motion to the Tibia

13. Neutral position the ligamentum patellae runs slightly obliquely inferiorly and laterally. In medial rotation, the femur is laterally rotates relative to the tibia and this drags the patella laterally, the ligamentum patella now runs inferiorly and medially. Lateral rotation the femur draws the patella medially so that the ligamentum patellae runs obliquely inferiorly and laterally

15. Collateral ligaments strengthen the articular capsule medially and laterally. Collateral Ligaments Medial Lateral

16. Collaterals become taut during extension. Collateral Ligaments Medial Lateral

17. Collaterals slacken during flexion. Collateral Ligaments Medial Lateral