Patellar Instability

Acute kneecap dislocation and patellar instability is frequently the result of a sports injury, a fall, or a direct impact to the joint that forces the patella out of its normal “track.” Frequently, this is the result of an underlying preexisting condition such as an abnormally high kneecap (patella alta), a shallow groove (trochlear dyplasia), or misalignment of the kneecap and tibia (increased q angle, rotational deformities, etc.), among other abnormalities.

Women and girls have naturally loose ligaments, which predisposes them to lateral patellar dislocation, a common injury in young, active girls and women. Lateral patellar dislocations are the most common knee dislocation injury among young adults, leading to an injury to the MPFL.

The risk of a second dislocation is high after an initial injury, dislocation, or subluxation (partial dislocation) of the patella. Approximately, 15 to 44% of patients who suffer a traumatic dislocation will have recurrent dislocations. Two or more dislocations increase the recurrence rate to over 50%.

Repeated dislocations are not the result of trauma. Instead, they are caused by a shallow groove (trochlear dysplasia) (Image 1) a kneecap that sits too high (patella alta) (Image 2), bone abnormalities (Image 3), lax muscles (Image 4), ligament injuries including a torn or stretched MPFL, weak quadriceps muscles, and tight hip flexors. Additional risk factors for recurrent instability include young age and sports-related injuries.

Chronic patellar instability is very painful and can significantly limit participation in physical activities due to fear of recurrent dislocations. People who have kneecap instability typically have symptoms that include:

• Swelling and bruising
• Pain while sitting and standing up.
• Pain in the front of the knee
• Joint stiffness
• Knee buckling and catching.
• A creaking sound with knee motion
• Difficulty walking

A combination of a comprehensive history of prior patellar instability episodes, physical examination, x-rays, and an MRI or CT are used to help diagnosis patellar instability. Dr. Jorge Chahla and his team will perform a physical exam that will include observing how the kneecap moves through a range of motion.  An X-ray will be obtained to further assess for bony abnormalities predisposing you to patellar instability, including patella alta. An MRI will be ordered to evaluate for soft tissue damage, including a torn MPFL or LPFL. The MRI will also identify any loose bodies within the joint or bone bruising that has resulted from contact between the edge of the knee cap (patellar facet) and the side of the thigh bone (femoral condyle) during a patellar instability episode. Sometimes a CT scan can be used to determine if the bones are not aligned properly.