Collateral Ligament Sprain of the knee is caused due to sudden thrust being applied to the knee joint along the frontal plane commonly from the lateral side. The medial and lateral collateral ligaments of the knee are very important structures that predominantly prevent valgus and varus forces, respectively. As with other ligamentous injuries, knee collateral ligament sprains can be classified in three grades of injury. With a first-degree sprain, there is localized tenderness without frank instability. Anatomically, only a minimal number of fibers are torn. On physical examination, the joint space opens less than 5 mm. With a moderate, or second-degree sprain, there is more generalized tenderness without frank instability. Grade 2 sprains can cover the gamut from a few fibers torn to nearly all fibers torn. The joint may gap 5 to 10 mm when force is applied. A severe or grade 3 sprain is a complete disruption of all ligamentous fibers with the joint space gapping greater than 10 mm upon stressing the ligament. Medial collateral ligament and Resultant Instability The medial collateral ligament (MCL) also known as the tibial collateral ligament is the most commonly injured ligament of the knee. Usually this ligament is injured when valgus forces are applied to the knee. Contact injuries often produce grade 3 MCL deficits, whereas noncontact MCL injuries typically result in lower-grade injuries. Although MCL injury can occur in isolation, valgus forces typically instigate injury to other medial structures.
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