Knee dislocations are a rare but very serious injury. In a knee dislocation, the hinge joint of the knee, made up by the thigh bone (femur) articulating on the shin bone (the tibia) is disrupted. In order for the knee joint to dislocate, at least two major ligaments about the knee must tear and more often three or four of the primary stabilizing ligaments of the knee tear. There is usually an obvious deformity of the knee after a knee dislocation on the playing field. However, about 50 percent of knee dislocations will spontaneously reduce, meaning that the knee joint dislocates but then re-aligns itself without manipulation moments later. For those dislocations that don’t reduce spontaneously, I would try to gently reduce the knee on the field. If this is unsuccessful, I would then take the patient to surgery to emergently reduce the knee.
When a knee joint dislocates, there is a significant risk of injury to the blood vessels and/or nerves that supply the lower leg. The risk of injury to the popliteal artery, which provides blood flow to the lower leg and foot, occurs at an average rate of 16 percent. If the popliteal artery blood flow is not reestablished within eight hours, most patients will require amputation of their affected leg. Some years ago, I had a skier dislocate their knee and injure popliteal artery. I immediately engaged the help of a vascular surgeon who was able to repair the artery and re-establish blood flow to the injured leg.
Nerves can also be injured when the knee dislocates. Typically, nerve injuries involve the peroneal nerve, which is the nerve that supplies the muscles that act to raise our toes and raise our foot. The peroneal nerve is injured in 25 percent of knee dislocations. Sometimes this is simply a stretch injury to the nerve, and these injuries will recover over 3-5 months. However, I have cared for patients who have transected their peroneal nerve as a result of a knee dislocation. These patients have a foot drop, meaning they are unable to raise their foot or toes. As such they have to wear a brace on their foot to hold their foot in a neutral position. In some cases, the patient may be a candidate for a tendon transfer, in which a working muscle and tendon from the back of the leg can be rerouted to work as a muscle that raises the foot once again.
After a patient sustains a knee dislocation and the knee is reduced, I splint the injured knee holding it lined up and reduced. Plain xrays and an MRI are obtained. In cases where there is an associated fracture or vascular injury, a CT scan would also be obtained. An MRI would help determine the extent of the ligament injuries as well as any associated cartilage or tendon tears.
Athletes who dislocate their knee will ultimately require surgery to repair the damaged ligaments, tendons and/or cartilage. However, this surgery is typically done in a delayed and staged fashion. If certain tendons or cartilages are torn, these are repaired first. The knee is then rehabilitated to allow for swelling to go down and to restore range of motion to the joint. Ligament tears to the ACL, PCL, LCL, and/or MCL are typically repaired secondarily.
Knee disclocations are very serious injuries that occur in athletes and others. They require urgent care and treatment in order to prevent serious complications.
Dr. Richard Cunningham, M.D. is a board-certified, fellowship-trained orthopedic surgeon and knee and shoulder specialist with Vail-Summit Orthopaedics.