A non-contact ACL tear isn’t entirely uncommon for an NFL player. A dislocated knee and an ACL tear without contact almost never happens.
Former NFL athletic trainer Mike Ryan, who now serves as NBC Sports Medicine Analyst, explained via email that the dislocation likely occurred after the ACL tear made the entire joint unstable.
“Mechanically speaking, the knee joint is formed by the long thigh bone (femur) resting on top of the main shin bone (tibia),” Ryan explained. “Because the knee joint is structurally unstable and very dependent on ligaments for stability, the rupturing of a main ligament(s) leaves the knee joint vulnerable to a dislocation. The Anterior Cruciate Ligament’s two main functions are to stabilize the shin bone directly under the thigh bone and, secondly, to minimize internal rotation of that shin bone. Therefore, when the ACL is torn, the knee ‘buckles’ inward and quickly becomes grossly unstable. As in the case of Teddy’s injury, the stress of a collapsing body over a now-unstable knee joint is enough to dislocate the knee joint.”
Ryan explained, based on his experience treating multiple players having dislocated knees, that “this type of an injury can quickly escalate from an orthopedic injury to a medical emergency because of the high risk of damage to local nerves and blood vessels.” The good news, if there is any, for the Vikings is that it appears that Bridgewater suffered no nerve or artery damage.