A dislocated knee, technically known as a tibiofemoral dislocation, is an extremely severe and uncommon orthopedic injury and a medical emergency. It involves the complete separation of the thigh bone (femur) from the shin bone (tibia) at the main knee joint, distinguishing it from a kneecap (patellar) dislocation. The violent force required causes extensive damage to supporting structures and frequently compromises the limb’s blood supply and nerve function.
Recognizing the Visual Deformity
The most striking sign of a true knee dislocation is a grossly abnormal alignment of the leg, which presents as a severe deformity of the joint. The lower leg may appear twisted, bent at an unnatural angle, or visibly shortened compared to the uninjured limb. Depending on the direction of displacement—anterior, posterior, lateral, or medial—the knee joint itself will look distinctly “out of place,” often with a prominent bump where the bone has been displaced.
Rapid, massive swelling (edema) develops around the joint due to internal bleeding. Bruising (ecchymosis) also develops quickly, sometimes presenting as a subtle indentation or fullness behind the knee, which can indicate an associated artery injury. However, the initial visual presentation can be misleading because nearly half of all tibiofemoral dislocations spontaneously reduce, or “pop back in,” before the patient reaches the emergency department.
Even if the bones relocate themselves, the injury remains serious because the internal structures have been catastrophically damaged. In cases of spontaneous reduction, the knee will still look severely swollen, feel grossly unstable, and the limb will be immobile due to extensive ligament tears. This instability results from the disruption of at least three of the four major stabilizing ligaments: the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL).
The Mechanism of Injury
The disruption of the tibiofemoral joint requires a massive amount of force, typically associated with high-energy trauma. Common causes include high-speed motor vehicle accidents, such as a dashboard impact pushing the tibia backward (posterior dislocation), or a pedestrian being struck by a car. Falls from height or severe industrial accidents also generate sufficient force to overcome the knee’s natural stability.
In a sports context, a direct, high-impact collision or a severe hyperextension injury can cause dislocation. Anterior dislocations, the most common type, are often caused by forced hyperextension, while posterior dislocations result from a direct force to the front of the tibia. Less common medial or lateral dislocations occur from powerful rotational or sideways forces.
Immediate Risks and Sensory Warning Signs
The severe nature of the dislocation places the limb at immediate risk due to damage to the neurovascular bundle behind the knee. The popliteal artery, the main blood vessel supplying the lower leg, is injured in 20% to 40% of all knee dislocations because it is susceptible to stretching or tearing. Loss of arterial blood flow for more than eight hours dramatically increases the risk of amputation to over 80%.
Signs of arterial compromise include a loss or weakening of the pulses below the knee. The foot and lower leg may appear pale, feel cool to the touch, and exhibit delayed capillary refill, indicating poor circulation. Furthermore, the common peroneal nerve is frequently damaged, potentially in up to one-third of cases.
Peroneal nerve injury manifests as sensory loss, such as numbness or tingling (paresthesia), on the top of the foot and outer lower leg. Motor function may also be affected, leading to “foot drop,” which is the inability to lift the front part of the foot. Even if a pulse is present, a vascular injury can still exist, making persistent, severe pain and any sensory changes a major warning sign.
Emergency Response and Next Steps
A dislocated knee is a dire emergency, and the immediate response must prioritize calling emergency medical services (911) right away. The most important action is to avoid any attempt to move the limb, push the joint back into place, or otherwise manipulate the leg. Trying to reduce the dislocation without medical training can cause further tearing of the artery or nerves.
While awaiting professional help, the limb should be gently stabilized and immobilized exactly as it lies to prevent accidental movement. Use pillows, rolled towels, or clothing to support the leg above and below the joint without applying direct pressure to the knee itself. If possible, check the color and warmth of the foot compared to the uninjured side, as these are indicators of circulation that medical responders will need.
Upon arrival at the hospital, the joint will be urgently reduced, often under sedation, to restore alignment and relieve pressure on the vessels. Following reduction, specialized imaging, such as a CT angiogram, is often necessary to definitively assess for hidden popliteal artery damage. Definitive treatment almost universally requires surgery to repair the torn ligaments and any damaged vessels, as delaying vascular repair significantly impacts the chance of saving the limb.