Anterior cruciate ligament (ACL) injuries are becoming more and more common in sports. There are numerous reasons why an ACL injury occurs and unfortunately, not all of these are very well understood. A recent study indicates that 3 out of every 4 ACL injuries are non-contact in nature. While this is true of both males and females, females are 2 to 8 times more likely to experience an ACL injury during sport.Kansas City Chiefs safety, Juan Thornhill is the latest high profile athlete to suffer a non-contact disruption of the ACL ligament. We wanted to share some information about the causes of ACL tears and expected recovery from an ACL injury for athletes. While not everyone has access to the resources at the disposal of professional players, many of the principals and timelines still apply to athletes at all levels. Most ACL injuries occur due to a rotational and shearing force being applied to the knee. This force is often generated when the foot on the injured side is planted and the body moves forward or rotates forcefully about this stationary lower leg. As you can probably imagine, the ACL ligament’s main function is the prevention of excessive movements in these directions. The amount of force that today’s athletes can generate plays a large role in the number of ACL injuries that occur. Think about how much control it takes to land, cut, slow down or change directions in today’s sports. Athletes who are bigger, stronger and faster generate more force and place more strain on the ligaments of the knee and the subsequent movements that those ligaments are trying to prevent. The extent of the tear/involvement of associated tissues and return-to-sport requirements are the two biggest factors that will affect the length of recovery in athletes. In the case of Juan Thornhill we don’t know if the medial collateral ligament (MCL) and/or the medial meniscus are involved with his injury. Because of the close proximity of these structures to the ACL and their associated roles in controlling knee movement, they are often involved with an ACL injury. Having “only” an ACL injury can be a good thing, when thought of in that framework, as in most instances, having only an ACL disruption and reconstruction will result in a shortened recovery. The position of Safety in the NFL requires a great deal of acceleration, deceleration and change of direction in order to be effective. Because of these requirements an athlete playing that position might experience a more extended length of recovery when compared to an offensive lineman, whose role requires slightly less change of direction force throughout the lower body. What is the timetable for returning to play for this type of injury? Assuming that there was no MCL or meniscus involvement and assuming no complications from the reconstruction/surgery, it’s likely that a full recovery could be expected in 9-12 months.