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Patellar/Knee Cap Instability

Kneecap Stability

The patella (kneecap) fits in a groove at the end of the femur (thigh bone) and slides up and down allowing the knee to bend and straighten.  The primary stabilizer of the kneecap is the medial patellofemoral ligament (MPFL). The MPFL connects the kneecap to the inner thigh and holds the patella in its groove.

The quadriceps tendon connects the quadriceps muscle to the patella and the patellar tendon connects the patella to the shin bone (tibia) below the knee. However, sometimes the kneecap can dislocate, slip out of its groove and cause pain, swelling, stiffness, and joint instability. When the patella slips out of its groove it is called “patellofemoral instability” or PFI.

Illustration of patella dislocation

Acute Kneecap Instability

Acute kneecap dislocation 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 (increased q angle, rotational deformities) of the kneecap and tibia (shin bone), among other abnormalities.  Women and girls have naturally loose ligaments which predispose 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.

The patella may return to its normal position on its own without treatment. If not, an orthopedic surgeon may need to reposition the patella (putting the patella back in its groove).  Damage to a ligament on the inner side of the knee (the MPFL) is the most common injury when the kneecap is dislocated or subluxated.  In addition, the risk of a second dislocation is high after an initial injury, dislocation or subluxation (partial dislocation).

Am I at risk for a recurrent patella dislocation?

Approximately, 15 to 44% of patients who suffer a traumatic dislocation will have recurrent dislocations. Two or more dislocations increases the recurrence rate to 49%. Risk factors for recurrent instability include young age, sports-related injuries, patella alta (high patella), and trochlear dysplasia (shallow groove).

Repeated dislocations are not the result of trauma. Instead, they are caused by a shallow groove (trochlear dysplasia), a kneecap that sits too high (patella alta), bone abnormalities, lax muscles, ligament injuries including a torn or stretched MPFL, weak quadriceps muscles, and tight hip flexors.

Illustration of shallow trochlea

What are the symptoms of chronic kneecap instability?

Chronic patellar (knee cap) instability is very painful and can significantly limit participation in physical activities due to fear of recurrent dislocations. Fortunately, there are treatments that can stabilize the patella, so your knee works normally again.

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

How is patellar instability diagnosed?

Dr. Chahla is a knee expert based in Chicago, IL. He will review your medical history including symptoms and history of dislocations and subluxations. He will perform a physical examination and observe how your knee cap moves through a range of motion.

X-rays and other imaging studies will be performed to confirm that your symptoms are being caused by kneecap instability and not by another issue like a fracture. An MRI will be ordered to assess soft tissue damage including a torn MPFL and look for loose bodies and bone bruising resulting from contact between the inside part of the knee cap (medial patellar facet) and the outside of the thigh bone (lateral femoral condyle) that occurs when the knee cap slips out of place. Sometimes a CT scan can be used to determine if the bones are not aligned properly.

How is patellar instability treated?

In acute cases of instability, including the first injury to the MPFL, conservative treatment may be recommended. This includes physical therapy to restore strength and mobility, and the temporary use of a knee brace. Conservative treatment with a period of immobilization followed by physical therapy can be beneficial in some patients depending on their underlying predisposing factors, but can sometimes lead to recurrent dislocations. Even without recurrent dislocations, persistent problems, disability, and arthritis can be present. Thus, MPFL reconstruction may be offered early to prevent these consequences when a patient has recurrent dislocations.

Treatment for chronic knee instability is focused on correcting the underlying cause. The goal is to restore stability. Surgical treatment options are based on the cause of the chronic instability.

When instability is caused by complex issues like a ligament tear or laxity, surgical reconstruction of the MPFL will be recommended to enable the kneecap to track properly and remain in its groove.

When there is a misalignment of the bones, a surgical procedure called an osteotomy may be recommended to realign the bones and prevent future dislocations. When there is cartilage damage, surgery to reposition the kneecap and give it more space to move with respect to the femur (thigh bone) can improve the pain. If needed, knee arthroscopy and cartilage restoration procedures can help by stabilizing fragments of cartilage that might be loose.

After surgery, most patients will be on crutches for a few weeks, and physical therapy will help restore joint function and strengthen the muscles to support the joint. Most patients can return to play in 4-7 months.

Patellar instability is just one possible cause of chronic knee pain. Dr. Jorge Chahla is a renowned orthopedic surgeon in Chicago, Illinois. Contact us today to receive the correct diagnosis and learn about all your options to restore your function and allow you to get back to enjoying the activities you love.

At a Glance

Dr. Jorge Chahla

  • Triple fellowship-trained sports medicine surgeon
  • Performs over 500 surgeries per year
  • Assistant professor of orthopedic surgery at Rush University
  • Learn more

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