Any set of collagen fibres joining one bone of an articulating pair to the other is called a ligament. Thus, the articular bursal wall is a ligament, called either the fibrous capsule or the joint capsule. WHAT IS LIGAMENT ?
Contrary to the opinion of earlier anatomists, ligaments are not normally responsible for holding joint surfaces together. This is because a set of collagen fibres, like a string, can exert a reactive force only if stretched and tightened by some tensile stress. Normally, the bones at a joint are pressed together (when at rest) by the action of muscles or by gravity. An individual ligament can stop a movement that tightens it. Such a movement will loosen the ligaments that would be tightened by the opposite movement. The one exception to this case is the movement that brings a joint into the close-packed position. This movement is brought about by a combination of a swing with a spin of the moving bone. Experiments show that the combination of movement screws the articular surfaces firmly together so that they cannot be separated by traction and that the capsule and most of the ligaments are in simultaneous maximal tautness.
BIOMECHANICS OF THE KNEE JOINT AXIS MOTION CLOSE- PACKED POSITION tibio-femoral lateral longitudinal flexion/extension tibial rotation extension
Ligaments function to limit joint motion Certain motions elongate certain ligaments. When they elongate, ligaments develop force. The force resists further lengthening of the ligament, and resists or limits further joint motion.
during knee flexion: tibia rolls posteriorly, elongating ACL. ACL's pull on tibia causes it to glide posteriorly. during knee extension: tibia rolls anteriorly, elongating PCL. PCL's pull on tibia causes it to glide anteriorly.
Development of passive (viscoelastic) force HUMAN TISSUES, INCLUDING MUSCLES, BEHAVE LIKE SPRINGS; THEY ARE ELASTIC. At lengths greater than their resting length (l0), they develop tension or force. This force is passive, since it exists whether or not the muscle is active. Passive force, like active force, acts in a direction from the muscle’s points of attachment toward its center.
Development of active force Active muscle force is proportional to number of active actin/myosin binding sites. The number of sites available for actin/myosin binding depends on the muscle's length.
Length-tension properties of muscle A muscle's length-tension curve illustrates how its force comes from two sources: Actice tension derives from the interaction between myosin and actin active tension. Passive tension can develop in the muscle's complex connective tissue.
The distinctive stiffness properties of any soft tissue are illustrated by its length-tension curve. The knee's anterior cruciate ligament has a distinct length-tension curve.
The knee is a commonly injured joint, Within recent years, the increase in ACL, anterior cruciate ligament, injuries has been remarkable, as it is one of the most commonly injured ligaments of the knee. Trauma to this ligament within the knee can be serious, and injuries to it usually occur during high-risk athletic activities such as basketball, football, skiing, and soccer etc. This injury is so common that it is currently estimated that approximately 200,000 ACL’s are injured annually, with 100,000 ACL reconstructions performed each year.
A 1st degree sprain involves damage to few ligament fibres. A 2nd degree injury means damage to a larger muscle group with the ligaments still intact A 3rd degree sprain involves rupture of the whole ligament. Other structures such as the knee, cartilage and the anterior cruciate ligament can also be damaged due to the force applied to it.
Mechanism of Injury Direct blow to Anterior tibia Hyperextension injury Dashboard injury Fall onto a flexed knee with foot In plantarflexion