A dislocated knee cap is when the triangle-shaped bone covering the knee (patella) moves or slides out of place. The problem usually occurs toward the outside of the leg. See dislocation.

A careful vascular examination is essential, as popliteal artery injury occurs in 7-45% of all knee dislocations. The popliteal artery may be damaged severely in both closed and open dislocations, and such injury must be ruled out in knees that have relocated spontaneously. Palpation of the dorsalis pedis and posterior tibial arteries along with capillary refill evaluation is necessary. The presence of normal pulses does not rule out the presence of significant vascular injury. Coexistent peroneal nerve injury occurs in 25-35% of patients and manifests with decreased sensation at the first webspace with impaired dorsiflexion of the foot.

Causes

Dislocation may also occur as a direct result of injury. When it is dislocated, the knee cap may slip sideways and around to the outside of the knee.
The first few times this occurs, you will feel pain and be unable to walk. However, if dislocations continue to occur and are untreated, you may feel less pain and have less immediate disability. This is not a reason to avoid treatment. Knee cap dislocation damages your knee joint.

Symptoms and Signs

Swelling and muscle spasm progress over the first few hours. With 2nd-degree sprains, pain is typically moderate or severe. With 3rd-degree sprains, pain may be mild, and surprisingly, some patients can walk unaided. An audible pop suggests an anterior cruciate tear but is uncommon. An effusion suggests injury to the anterior cruciate and possibly other intra-articular structures. However, with severe 3rd-degree tears of the medial collateral ligament or anterior cruciate, no effusion may be apparent because these tears can result in an open joint capsule, allowing blood to exit the joint.

Diagnosis

X-rays may be recommended to see how the kneecap fits in its groove. Your doctor will also want to eliminate other possible reasons for the pain, such as a tear in the cartilage or ligaments of the knee. The following imaging procedures may be used to see how the include:

x-ray - a diagnostic test that uses invisible electromagnetic energy beams to produce images of internal tissues, bones, and organs onto film.
magnetic resonance imaging (MRI) - a diagnostic procedure that uses a combination of large magnets, radiofrequencies, and a computer to produce detailed images of organs and structures within the body.

Prevention

Use proper technique when exercising or playing sports. Maintain strength and flexibility of the knee. Some cases of knee dislocation may not be preventable, especially if anatomic factors predispose you to dislocation.
Treatment

Non-operative

Normal care of patellar dislocations, when a loose fragment has not been created is the immobilization of the knee for a short period of time (seven to 10 days). During this time, the swelling is reduced and the acute discomfort of the dislocation decreases. Slow mobilization of the knee and of the patellofemoral joint is then begun, and usually full recovery can be expected within a three to six week period. This period of time is significantly lengthened when the patellar dislocation is recurrent.

Initial treatment consists of a knee immobilizer or cylinder cast, followed by gentle active range-of-motion (ROM) exercises. Physical therapy should be involved to help regain joint and leg strength, especially the quadriceps muscles. Taping techniques have been explored with variable results.