1. Most Dislocations Are Lateral
The patella, or kneecap, sits above a groove on the end of the femur, called the trochlea. As the knee flexes, the patella slides down the center of the trochlea. For an example of this, feel your kneecap with your knee extended straight. You can grab your kneecap and move it side to side. As you bend your knee, the kneecap no longer has as much mobility, because it begins to engage the trochlea. As a result, most dislocations happen as the knee is in a relatively straight position. That is also the position in which it will pop back in. Oftentimes, the patella will self-reduce as the knee is brought straight, into extension.
Patellar dislocations or subluxations (partial dislocations) tend to go toward the lateral side (away from the other knee). Very rarely does it go medially.
2. Risk Factors
Risk factors include the following: patella alta (higher kneecap), flat trochlea, generalized ligamentous laxity, increased Q angle, valgus knees (knock-knees), increased femoral anteversion (intoeing). Patella alta means that the kneecap is starting off higher, up away from the trochlea. This means that the patella doesn't lock into its more stable position in the trochlea until the knee is bent farther. Usually associated with this is a flatter trochlea, meaning that the groove is more shallow and doesn't lock in the patella as well. People who are "loose-jointed" may have generalized ligamentous laxity, where the ligaments and joints are more "stretchy," again leading to decreased stability. The Q angle is the quadriceps angle, which is formed by the quadriceps muscles and the patellar tendon. People with wider hips will have increased Q angles. In these cases, there is a higher force vector pulling the patella laterally. People who are "knock-kneed" also have a higher Q angle. People who intoe also place an increased laterally directed force on the patella. Often a person has all these factors.
3. Bone Bruising and Ligament Tears
As the patella slides off the lateral edge of the trochlea, the medial portion of the patella usually bumps up against the lateral trochlea on its way out. There can be bone bruising, or even chips of bone knocked off the patella, which can then become loose within the joint. For the patella to go all the way out of the trochlea, something medially has to be ripped. It is often the medial patellofemoral ligament (MPFL).
4. About 50 Percent Improve Without Surgery
In patients with a traumatic dislocation, without a loose body inside the joint, without the risk factors listed above, about half will improve without surgery. As the swelling subsides, physical therapy is beneficial in regaining the motion and strengthening the muscles to stabilize the patella. A patellar-stabilizing brace may help as well. It's a knee brace with a hole cut out for the patella, with an extra bolster on the lateral side of the patella to help keep it in place.
5. Surgery for Instability or Recurrent Dislocations
When there is a loose body, or there is continued instability, surgical intervention may be beneficial. There are numerous ways of stabilizing the patella, which involve any numbers of techniques to release tight ligaments, tighten loose ligaments, cut bones to realign the tendons, reattach muscles and/or reconstruct torn ligaments.
