Patella (Knee Cap) Dislocation
Updated: Sep 18, 2020
Mechanism of Injury
Usually occurs as a non-contact injury with a planted foot on the ground. Many times the athlete is shifting directions, where the body is moving in one direction, and the knee is moving in the opposite direction. Usually the athlete will report a "pop". In these instances, the individual may report that it felt as if the knee cap shifted, or they even directly visualized the knee cap shifting to the outside of the knee. In general, a patella dislocation will occur towards the outer surface of the knee. The knee cap will be forcibly shifted out of its groove. The position of a dislocated patella is depicted below. During the dislocation, the knee can sustain damage to the groove, as well as the under surface of the knee cap, where smooth cartilage is found. An impaction injury can also occur when the knee cap strikes the outer portion of the femur.
At the immediate time of injury, it is possible that the knee cap can be stuck out to the side, at which point they will need immediate treatment to replace the knee cap to its original position (see below). Other times, the knee cap will dislocate and then return back to its proper position on its own. If this is a first time occurrence, athletes will usually have dramatic swelling and fluid over the entire knee. The swelling can be described as "like a water balloon" or "like a grapefruit" and should be apparent within the first 24 hours of the injury. If this is a repeat episode, the swelling and fluid may not be as dramatic.
Dislocated Patella Reduction Technique
X-rays should be obtained to confirm that the knee cap has returned to its proper position and to determine if any fracture has occurred due to the shifting and impact of the knee cap. Although the diagnosis can be confidently made by physical exam, when the pain and swelling are significant, an MRI is generally accepted as the appropriate test to make sure there is no cartilage damage. However, if there is minimal swelling and/or it is a repeat injury, an MRI may not be recommended.
Sunrise View X-ray
Sunrise view x-ray of the knee showing the knee cap in the appropriate position within the patellar groove with no associated fractures of the bone seen.
In general, the recommendation for an uncomplicated patella dislocation is bracing and physical therapy. However, if an athlete is having multiple episodes of dislocation even with physical therapy and bracing, then surgical stabilization should be considered. Also, if abnormalities are seen on the MRI, such as a loose piece of bone or a cartilage injury, surgery may also be indicated.
Return to Play
An athlete who is treated without surgery is expected to be able to return back to full activity in approximately 4 to 6 weeks. On the other hand, those who undergo surgery will typically be able to return back to sports at 5 to 6 months, based on functional ability.
The current data on patella dislocation is clear that once an athlete has had one episode, they are much more likely to have a repeat episode, even with proper treatment. Most athletes will return to sports with a patella stabilizing brace that helps to keep the knee cap in position. Those at high risk for re-injury would be athletes involved in cutting, pivoting and change of direction sports. Athletes who have experienced an episode while in season may choose to proceed conservatively with rehab and bracing, and then undergo surgery after the season is over.