Dislocations of the knee are rare. They are true orthopedic emergencies and have a significant association with soft tissue injuries and neurovascular compromise. A dislocated knee occurs most commonly after a major force is applied to the knee joint from motor vehicle trauma, pedestrian–vehicle collisions, bicycle collisions, or motorcycle collisions. The forces necessary to cause a dislocation of the knee joint often fracture the bones of the leg.
Complete dislocation of the knee joint results in a gross deformity that is confirmed by plain radiographs. Reduction by the Emergency Physician (EP) may be reasonable if the Orthopedic Surgeon is not immediately available and/or if the injured extremity shows signs of distal neurologic or vascular compromise.
A careful examination of the distal extremity must be performed and documented. It must include an assessment of the capillary refill, the dorsalis pedis pulse, the posterior tibial pulse, peroneal nerve function, and tibial nerve function.
A knee dislocation is the displacement of the tibiofemoral articulation (Figure 88-1). It involves the rupture of the anterior cruciate ligament, the posterior cruciate ligament, the joint capsule, and/or the collateral ligaments of the knee. Anterior knee dislocations are the most common type of knee dislocation. This injury is defined as anterior displacement of the tibia relative to the femur (Figure 88-1A). It results from an acute hyperextension injury to the knee joint that ruptures the anterior cruciate ligament completely, the posterior cruciate ligament partially, and the posterior joint capsule. The distal femur is driven posterior to the proximal tibia. The collateral ligaments usually remain intact. Tibial spine fractures, osteochondral fractures of the tibia or femur, and meniscal injuries are avulsion-type fractures resulting from the rupture of the anterior cruciate ligament. Distal femoral epiphyseal separation, rather than complete dislocation, as a result of a hyper-extension injury is more common in children.
The classification of knee dislocations. A. Anterior. B. Posterior. C. Lateral. D. Medial. E. Rotary.
An anterior knee dislocation is associated with a popliteal artery injury in 30% to 40% of patients.1 The popliteal artery is at particular risk for injury because it is anchored proximally at the adductor hiatus and distally at the soleus arch. The collateral circulation around the knee joint is relatively poor. Therefore disruption of the popliteal artery may result in distal ischemia and limb loss if the reduction is delayed. It is important to note that the presence of distal peripheral pulses and capillary refill does not preclude an arterial injury.
The peroneal nerve is tethered as it winds around the fibular neck. With knee dislocations, the peroneal nerve is at risk. Peroneal nerve injury may occur in up to 23% of patients with knee dislocations. Nearly one-half of the patients with peroneal nerve injuries have a permanent deficit.10