The kneecap connects all the muscles in the thigh to the shinbone (tibia). As the leg is bent or straightened, the kneecap is pulled up or down. The thighbone has a V-shaped notch (trochlear groove) at the lower end to accommodate the moving kneecap.
In a normal knee, the kneecap fits nicely in the groove. A dislocated kneecap (patella) occurs when the kneecap (patella) pops out from its normal position.
- A kneecap that slides out of the groove partway is called a subluxation.
- A kneecap that moves fully outside the groove is called a dislocation.
The kneecap usually pops out to the outer aspect of the knee i.e. laterally.
Kneecap dislocation can occur due to laxity of the ligaments surrounding the knee, or due to a developmental condition that leads to an improper alignment of the kneecap with the distal thighbone (femur).
It can also be due to a sharp blow to the kneecap, as in a fall or from a twisting injury to the knee. It usually occurs after a sudden change in direction when the leg is planted. This puts the kneecap under stress.
Most dislocations first occur in active teenagers and young adults from the ages of 14-20 years old.
Contact sports like soccer are commonly involved, although a significant number of dislocations occur from simple falls or a wide variety of activities such as gymnastics, dancing, or netball.
Kneecap dislocations tend to be more common in girls compared to boys.
Signs & Symptoms
Symptoms of kneecap dislocation include:
- Knee appears to be deformed
- Knee is bent and cannot be straightened out
- Kneecap (patella) dislocates to the outside of the knee
- Knee pain and tenderness
- Knee swelling
- “Sloppy” kneecap — you can move the kneecap too much from right to left (hypermobile patella)
The first few times this occurs, it is quite painful (especially the first time) and inability to walk. However, if dislocations continue to occur and are untreated, there may be less pain and less immediate disability. This is not a reason to avoid treatment. Kneecap dislocation damages the knee joint.
A clinical examination of the knee is conducted (Patella Apprehension Test). This may confirm that the kneecap is dislocated.
A knee x-ray and, sometimes, MRIs will be done to make sure that the dislocation did not cause a broken bone or cartilage damage.
If tests show that there is no damage:
- The knee may be placed in a brace, splint, or cast for several weeks.
- May need to use crutches at first so that not too much weight is placed on the knee.
- Most people recover fully within 6 to 8 weeks.
- After this time, physical therapy can help build back muscle strength and improve the knee’s range of motion.
If there is damage to the bone and cartilage, or if the kneecap continues to be unstable, surgery may be needed to stabilize the kneecap. This may be done using arthroscopic or open surgery.
This surgery includes lengthening the tight ligaments and tightening the overstretched ligaments that surround the kneecap. If a more aggressive approach is needed, moving the patellar tendon medially will help the knee cap to track in a straight line. The extent of the surgical procedure is determined by the surgeon during the procedure. It is necessary to be aggressive enough to avoid future dislocations, but conservative enough to minimize recovery.