The ACL helps to limit anterior translation of the tibia on the femur. It works with the posterior cruciate ligament to control gliding and gliding of the tibia on the femur during normal flexion and extension. It provides secondary restraint limiting internal rotation of the tibia.
The ligament is made up of two bands. The twisted configuration of the ACL fibres and shape of the femoral condyles allow for the screw-home mechanism of the knee during the final 20 degrees of extension when the tibia extremely rotates on the femur. The ligament is under varying degrees of tension in all positions of motion, but maximal tibial anterior translation is observed at 30-degree flexion when both the ACL bands display their minimum tension.
The « miserable misalignment syndrome » of high Q-angle, increased pelvic width, anteverted femur, valgus knee and pronated foot is often quoted as an explanation for increased knee injury.
The Q-angle is described as being formed between the vectors for combined pull of the quadriceps femoris muscle and the patellar tendon. There is no strict agreement regarding standardised reference values, but Q-angles exceeding 15 degrees in males and 20 degrees in females are considered are considered abnormal.
Pronation is defined as a combined motion involving subtalar eversion, foot abduction and ankle dorsiflexion. Subtalar pronation and internal rotation of the tibia occur concurrently in the contact phase of the gait cycle and the ACL becomes taut with tibial rotation. Abnormal pronator are found to have increased passive knee rotation at 5 degrees of knee flexion and that there is an important relationship between pronation and rotational knee joint laxity. It has been concluded that prolonged pronation of the subtalar joint produced increased internal tibial rotation, which turn the stresses to the medial structure of the knee.
A specific ski orthotic can provide the necessary amount of support to control the subtalar pronation and align the foot within the boot. This optimum alignment will install the maximum ankle, knee, hip and pelvic join congruency and proprioception. Good balance on skis is as important as muscle power. Postural ski Alignment will involve the foot, knee and hip and will mean freedom of movements within those joins, transmitting power to the ski.