Patellar Dislocation / Instability
The patella (kneecap) is a small bone located at the front of the knee. The patella serves an important function in protecting the knee from injuries. It is also part of a pulley system called the extensor mechanism that helps straighten and flex the knee. It sits in a groove on the thighbone (the femur) and slides up and down as the knee moves. Tough ligaments keep the patella in its groove.
What is an unstable kneecap?
Patellar instability is a condition characterized by partial or total dislocation of the kneecap as a result of injury, structural abnormalities, or ligament laxity. When the patella does not track properly in the groove, it may totally or partially dislocate. Patellar dislocations account for 3% of all knee injuries with the majority of dislocations occurring in young individuals.
After an acute dislocation, 15% to 45% of patients will develop recurrent patellar instability. Patients with a history of two or more dislocations have a 50% chance of recurrent dislocations. Chronic instability is both functionally limiting and painful. Structural abnormalities in women tend to run in families.
What causes an unstable kneecap?
Patellar instability can be caused by:
- A structural abnormality such as an irregularly shaped groove which predisposes the kneecap to dislocate partially or completely causing pain and recurrent dislocations. This poor tracking (maltracking) patella usually affects young individuals, particularly girls ages 10-16 and tends to run in families with women.
- A traumatic injury can also cause the kneecap to be displaced. Acute traumatic instability commonly affects young athletes in their 20s and 30s. The mechanism is commonly a non-contact twisting injury of the lower leg with the knee extended, or a direct blow to the knee.
What happens when the kneecap dislocates?
When the kneecap dislocates it stretches or tears the ligaments surrounding it. The articular cartilage on the underside of the patella and at end of the thighbone where the kneecap sits can also be damaged and ultimately lead to knee osteoarthritis.
What are the symptoms of patella instability?
- Pain in the front of the knee, particularly when walking downstairs or sitting for long periods of time
- Swelling and stiffness
- Catching and locking
- Creaky or snapping sensations in the knee with movement
- Inability of the knee to support your weight which results in knee buckling
- Feeling like the knee is giving way or is about to give way
- Inflammation and swelling in the knee
How is patellar instability diagnosed?
Dr. Harrison will take review your medical history focusing on how the kneecap first dislocated, the circumstances of the dislocation and the number of instances of instability/dislocations. He will perform a physical examination assessing ligament laxity and examining the knee for symptoms of bruising, pain, swelling and patellar function.
He may evaluate your gait and test your ability to walk and straighten the knee. Dr. Harrison may order imaging studies to evaluate potential causes of patellar instability including x-rays to assess the patella alignment, and damage to the patella, femur, and tibia. He may order an MRI to evaluate damage to the articular cartilage, tendons, and ligaments to rule out other possible causes of your pain; and at CT scan to evaluate the impact of patellar instability.
Early diagnosis of patellar maltracking is important to prevent further injury and long-term damage like fractures, cartilage damage and osteoarthritis. Call Dr. Harrison to schedule a consultation.
Dr. Harrison is a board-certified fellowship trained orthopedic and sports medicine surgeon who specializes in arthroscopic knee and shoulder surgery. He is the Head Team Physician for the United States Alpine Ski Team and the Head Team Physician for Weber State University for over 20 years. Dr. Harrison completed his undergraduate degree at Baylor University and received his medical degree from the University of Arizona College of Medicine. He completed his orthopedic residency at the University of Utah followed by a fellowship in sports medicine at the Cincinnati Sports Medicine and Orthopedic Center. He finished his formal training with an additional AO Trauma Fellowship in Bern, Switzerland.
At a Glance
Dr. Jeffrey Harrison
- Board-Certified, Fellowship-Trained Orthopedic Surgeon
- Head Team Physician US Women's Alpine Team and Weber State University
- Performs over 800 surgeries per year
- Learn more