DISLOCATION OF KNEE JOINT
Dislocation of the knee joint must be distinguished from dislocation of the patella. Whereas a patella dislocation involves the patellofemoral joint, a knee dislocation involves the tibiofemoral articulation. Any dislocation is an emergency, and dislocation of the knee is no exception. Reduction should be achieved as soon as possible. Striking the knee against the dashboard during an automobile accident is the most common cause of injury, but athletic injuries are also common causes. The popliteal artery and its branches are often damaged during dislocation of the knee. Therefore, arterial injury must be suspected in every knee dislocation. A thorough neurovascular examination should be performed before and after reduction, and an ankle- brachial index (ABI) should be obtained as well. If there remains any question of arterial damage, the patient frequently will undergo arteriography or CT angiography and any necessary arterial repair should be done immediately.
Classification of knee dislocations is based on the
position of the tibia in relation to the femur. In an anterior dislocation of
the knee the tibia is anterior to the femur, whereas in a posterior dislocation
the tibia is posterior to the femur. Lateral, medial, and rotational
dislocations may also occur, as may combination patterns such as anterolateral and
posterolateral. Associated vascular injuries are more common with anterior
dislocations, whereas the peroneal nerve is more likely to be injured in
posterolateral dislocations.
Diagnosis of dislocation of the knee is based on the
patient’s history and typical clinical findings. If the dis- location has not
spontaneously reduced before the patient is examined, the diagnosis is clear
because the deformity is obvious and impressive. However, spontaneous reduction
of knee dislocations is common. When gross dislocation is not detected by
physical examination or radiography but there is a history of significant knee
injury, a dislocation that has spontaneously reduced may be suspected. A large
effusion or hemarthrosis may not develop because large tears in the joint
capsule allow the fluid to escape into the soft tissues about the knee.
The initial treatment of a knee dislocation is
straight-forward but must begin without delay. Reduction is performed using
gentle longitudinal traction and is frequently accomplished with little or no
sedation. If any difficulty at all is encountered, anesthesia should be induced
promptly. After reduction, the neurovascular status of the limb must be
carefully monitored.
Many knee dislocations are treated with splinting or
casting, but because the reduced knee is so unstable, it is difficult to keep the joint surfaces in proper
apposition without surgical stabilization. Once the patient is hemodynamically
stable, it is common to obtain an MRI to assess for ligamentous and soft tissue
injury. Early surgical repair of torn ligaments and joint capsule may then be
performed. In cases of severe instability, the knee can temporarily be
stabilized with an external fixator or internally with pins or plates. Surgical
repair maintains the reduction and may provide long-term stability after this
devastating injury.
The trifurcation of the popliteal artery is tethered to the leg where the anterior tibial artery goes through a gap in the interosseous membrane. In anterior dislocations, stretching of the artery and vein is severe and often results in vascular injury. If dislocation is not reduced and if the vascular supply is cut off for hours, reduction and restoration of vascular flow may be accompanied by the development of a compartment syndrome, which is a serious complication with often irreversible consequences.
