Patellar Instability

The knee is the most complex joint in your body and uses a delicately balanced system of bones, ligaments, and other structures to allow you to run, jump, perform cutting movements, and other advanced activities. The knee is made up of three bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). The two joints of the knee are the tibiofemoral joint (between the femur and tibia bones, this is the main knee joint) and the patellofemoral joint (between the kneecap and the trochlea, a groove on the front of the femur). The fibula bone is a thin bone on the outside of your lower leg that does not contribute to the knee joint, but is important to ankle stability.  

The patellofemoral joint relies on bony geometry and soft tissue structures to function. The medial patellofemoral ligament (MPFL) is the primary restraint to the patella as it moves from fully straight to bent at 30 degrees of flexion. This ligament runs from the inside of the patella to the femur bone. The trochlea is the ridge where the patella rides and takes over as the restraint to the patella as the knee fully bends.

Patellar instability can occur due to a subluxation or dislocation event. A subluxation is when the patella slides towards the outside of the knee and then slides back to the center. A dislocation event is when the patella slides to the outside of the knee and stays there and may require a reduction. When an instability event occurs, the MPFL is damaged and may be fully torn.

Risk Factors for Patellar Instability

Anatomic factors are the primary reason behind patellar instability. Patients with a shallow or flat trochlea (groove for the patella), knock knees, or with a high patella location are at an increased risk for patellar instability. Patient who have had one patellar subluxation or dislocation event are at a higher risk to have a subsequent event. Finally, patients who are loose jointed are also at a higher risk of having patellar instability

Symptoms & Diagnosis

Patients who sustain a patellar instability episode will have pain in the knee and will notice a large and swollen knee after the injury. After the injury, patients will have difficulty walking due to the knee swelling and may have limited range of motion. Patients may also have a sense of apprehension or anxiousness with certain knee movements and in particular, during a physical examination where the patella is pushed outward to test the stability of the patella.

Patellar instability is diagnosed using physical examination and imaging studies to confirm the diagnosis. The physical examination is used to identify areas of pain, swelling, change in range of motion, laxity (or looseness) of the patella, and any limb alignment abnormalities. X-rays are used to look at the bones of the knee and the location of the patella. MRI (magnetic resonance imaging) is an important part of the diagnosis and will allow evaluation of all the knee ligaments, meniscus, cartilage, and any other structures that may have been injured.

Treatment

Patients with patellar instability can be treated with non-surgical or surgical means. Patients with a first-time dislocation may be candidates for non-surgical treatment and a discussion will be had with the patient and their family to discuss the options. Non-surgical treatment options include:

• Rest and activity modification: All sports and active pursuits should be stopped until the knee inflammation resolves. Occasionally, patients require several weeks of crutch use to regain a normal gait.

• Non-steroidal anti-inflammatory medications (NSAIDs) can help decrease inflammation

• Brace: Use of a knee brace can help the patient feel more confident and secure when walking

• Physical therapy: This is an important tool to help you strengthen and retrain the muscles of the knee to work better together. Patients will work with their therapist over a period of several months to work towards returning to sports.

Surgical treatment is used in some patients with a first-time dislocation who meet specific criteria and those with recurring patellar instability episodes. Several procedures exist to treat patellar instability and are each used in specific patients.

• MPFL Reconstruction: This procedure is performed through small incisions along the inside of the knee. A hamstring graft is taken either from the patient’s own leg (autograft) or from a deceased donor (allograft). The graft is then secured to the patella and femur to recreate the torn MPFL.

• Tibial Tubercle Osteotomy: This procedure is performed through an open incision on the front of the lower leg and can be combined with an MPFL reconstruction or other procedure to address cartilage damage. This procedure is used for patients with patellar instability due to a bony malalignment. The patella is realigned by cutting the bony attachment on the top of the tibia and moving the attachment to restore alignment of the patella. The bony piece is held in place with two screws while it heals. These screws may bother the patient after the surgery and can be removed months later if needed.

Treatment of patellar instability is very individualized based on each patient’s age, activity level, history of instability, physical examination, and imaging findings. The above treatment options are discussed to provide a framework for understanding how this condition is treated. Dr. Kew will discuss your specific treatment options and plan during your visit, as well as answer any questions you may have.