Knee dislocation

The knee joint is formed where the thighbone above meets the shinbone below. Knee dislocation means these two bones have completely moved out of their normal alignment. It usually occurs with a high-force impact, such as a traffic accident or a fall from a height, but cases have also been reported with relatively low-force injuries, such as severe bending or twisting during sports or a fall in someone with a high body weight.

The phrase “the knee popped out” can be confusing because it is also used in everyday conversation when the kneecap (patella) shifts to the side. In patellar dislocation, the kneecap moves out of its groove; in knee dislocation, the entire knee joint becomes displaced. The structures injured and the assessments needed are therefore completely different.

The knee is supported by the medial and lateral collateral ligaments, the anterior and posterior cruciate ligaments, the joint capsule, and the surrounding muscles and tendons. A force strong enough to completely displace the joint can stretch or tear several of these structures at once, rather than just one. That is why multiple ligaments, including both cruciate ligaments, need to be assessed.

The area behind the knee, called the popliteal region, requires particularly careful assessment. The popliteal artery and vein, which carry blood to the lower leg and foot, pass through this area, along with nerves responsible for sensation and movement in the lower leg and foot. The popliteal artery in particular follows a relatively fixed path around the knee and is described as being vulnerable to stretching or compression when the bones become severely displaced. A systematic review also identifies assessment for blood vessel and nerve injuries as a key priority after knee dislocation.

Dislocations are classified as anterior, posterior, medial, lateral, or rotational, depending on the direction in which the bones are displaced. These categories provide a framework for describing the mechanism of injury. The actual damage cannot be determined from the direction alone, so it is carefully assessed with imaging.

In many cases, the injury is immediately apparent. The knee is extremely painful, the shape of the joint is visibly altered, and no weight can be put on the leg. However, the displaced bones sometimes return to their normal position on their own. In that situation, the knee may not look very different, making it easy to mistake the injury for a simple sprain. But the bones returning to position does not mean the ligaments or blood vessels are unharmed. If the knee becomes very swollen and unstable after being severely bent or twisted, it needs a thorough assessment at a hospital.

Care generally proceeds in the following order.

• If the bones are displaced, a reduction is performed to return them to their normal position, and the knee is immobilized.
• The pulse in the foot, skin color and temperature, and sensation and movement in the toes are checked. If needed, vascular tests are used to assess the popliteal artery.
• Imaging tests such as X-rays and MRI are used to clearly determine the extent of bone and ligament injuries.
• Depending on their condition, injured ligaments may be surgically reconstructed. The specific timing and approach vary from person to person.

This is followed by rehabilitation in stages. The process starts with gentle joint movement exercises and progresses toward restoring strength and balance. The treating medical team and physical therapist plan this together.

After a major knee injury, go to the emergency department without delay if you notice any of the following signs.

• The foot is cold and pale or turns bluish.
• The pulse on the top of the foot or behind the ankle is difficult to feel.
• The toes are tingling or numb, and difficult to move.
• The knee is visibly deformed, or you cannot put any weight on the leg.