Patellar Instability — Kneecap Dislocation or Subluxation

The knees are the joints that connect the upper and lower bones of your legs. The name for the kneecap is the patella. It is embedded within the tendon of the quadriceps muscle on the front of the femur (thigh bone) and sits over the front of the knee joint. This tendon attaches to the tibia (shin bone), the lower bone of the leg. 

When the quadriceps contracts (or tightens), the knee straightens and the patella helps the tendon move smoothly over the knee joint. When it is functioning normally, the patella slides up and down within a groove called the trochlear groove. This allows movements like standing, walking and running.

Patellar instability is a name for the condition that happens when the kneecap slips out of the trochlear groove.

Types of Patellar Instability

There are two types of patellar instability.

• Chronic patellar instability: The kneecap moves slightly out of the trochlear groove but not completely. Chronic patellar instability is also called subluxation.
• Patellar dislocation: The kneecap slips completely out of the trochlear groove and is called dislocation. It is different from a knee dislocation, in which all three bones that make up the knee joint (patella, femur and tibia) are thrust out of alignment.

Patellar Instability Symptoms

The symptoms of chronic patellar subluxation and patellar dislocation are similar, but symptoms of a dislocated kneecap can be more severe. If your kneecap is dislocated, you won’t be able to walk or will have a very hard time walking. While it is painful, the pain can decrease as the ligaments surrounding the kneecap become looser.

Common symptoms include:

• Pain
• Swelling
• Stiffness
• Having a hard time walking and putting pressure on the affected leg when the kneecap is not completely in the trochlear groove
• Knee buckles or locks up when walking
• Kneecap is deformed — it could have excessive swelling and an abnormal shape
• Pain when the kneecap moves out of place — when it slides from left to right in full relaxed extension

Causes of Patellar Instability

Patellar instability is often caused by a forceful injury to the kneecap, such as a body-to-body collision or a fall where you land on your knee. Athletes are prone to patellar instability because their activities can involve twisting the knee while the foot is planted in a stationary position on the ground.

Diagnosing Patellar Instability

To diagnose chronic kneecap subluxation or kneecap dislocation, your doctor or clinician will take a thorough medical history and conduct a physical exam. There may also be X-rays to evaluate your bone structure and the position of the kneecap at rest. Your doctor may order an MRI to evaluate the ligaments surrounding the patella for tears and damage.

When to See a Doctor

Seeing an orthopaedic expert when you notice pain and other knee symptoms is important, so you can begin treatment and avoid further injury to your kneecap and surrounding structures. Left untreated, patellar instability can lead to long-term joint instability and arthrofibrosis. If you experience any of the following, it is time to see a doctor.

• A popping sound with pain
• Intense pain
• The knee buckles
• The knee locks
• Sudden swelling of the kneecap
• Bruising at the knee
• You can’t walk
• Kneecap appears to be out of place

Patellar Instability Treatments

Treatment for both types of patellar instability — subluxation and dislocation — are similar. A timely and accurate diagnosis followed by expert treatment leads to the best outcomes for these complicated joint conditions.

Your doctor will start by ordering an X-ray or MRI of your knee to view the position of your kneecap and surrounding structures and gain a better understanding of the extent of your injury. After confirming diagnosis, treatment may include anti-inflammatory drugs and icepacks to reduce pain, rest and elevation, and physical therapy to make the muscles around the patella stronger, preventing the kneecap from slipping out of the trochlear groove. Your physical therapist may tape your knee to support it or may order special braces to support appropriate joint movement.

If physical therapy and bracing don’t bring the kneecap back into alignment, your doctor may choose to gently move it back into position. This can be uncomfortable, and you will receive pain medication, but repositioning the patella doesn’t require anesthesia. For recurring subluxation/dislocation, surgery may be needed to tighten ligaments and strengthen the area around the kneecap.

Recovery from Patellar Instability

Full recovery with treatment usually takes about six weeks. It depends on how lax your ligaments have become and how many times your patella has moved out of position. During recovery, you should avoid activities like sports that may have led to the unstable condition of your patella. You should also take extra care not to fall by wearing supportive shoes and removing obstacles, such throw rugs, in your environment that could make you more prone to falling.

Your doctor may recommend that you wear knee braces or use crutches while you recover, depending on how lax your ligaments are.