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Patellar Instability

By: Daniel Curtis, MD

What is the patella?

The patella, also known as the kneecap, is a small bone located in the front of the knee. The patella connects the quadriceps muscles to the patellar tendon and, ultimately, the shinbone (tibia). The quadriceps muscles are some of the strongest muscles in the body, allowing you to walk, run, and jump. As these muscles contract and relax, the patella glides back and forth through a groove in the knee, called the trochlea.

What is patellar instability?

Occasionally, the patella can slide or pop out of the groove (trochlea) that it normally glides in. This can result in a partial or full dislocation, which is usually very painful. When a patient has one or more of these dislocations, it is called patellar instability or an unstable kneecap.

What are the causes of patellar instability?

Patellar instability can occur due to a direct blow to the knee (e.g. a football player’s helmet hitting the kneecap) or due to abnormal positioning of the knee that may occur in sports, dance, or a fall. Usually, patients with patellar instability have specific risk factors that predispose them to a dislocation, including abnormal alignment of the legs or a shallow groove.

Symptoms of patellar instability:

Patients who experience a patellar dislocation will usually have severe pain in the front of the knee. Their knee will usually swell and it will be difficult for them to bend the knee without discomfort. The kneecap may or may not go back to its normal position within the groove after dislocating – sometimes the patient or a healthcare provider must put it back in place.

What structures are injured in patellar instability?

Normally, the kneecap is held in place within the groove by a band of tissue called the medial patellofemoral ligament (MPFL). This tissue acts as a rope or checkrein to prevent the kneecap from moving out of the groove. When the kneecap (patella) dislocates, the MPFL is damaged in almost all cases.

Another part of the knee that can be injured when the patella dislocates is the cartilage. Cartilage is a smooth material that helps all of our joints to glide and bend easily, including the kneecap (patella) and its groove (trochlea). When the patella dislocates, these cartilage surfaces can be damaged, causing a piece of cartilage or a piece of bone and cartilage, to break off. When a piece of cartilage and/or bone breaks off, it is called a “loose body” and can float around the knee. A “loose body” can cause the knee to lock or catch and often needs to be removed.

If I dislocate my patella, will it happen again?

In general, a patient who has had a prior patellar dislocation is at a higher risk of having another; and a patient who has had multiple patellar dislocations is at an even higher risk. Patients who are younger and have shallow grooves are at the highest risk of repeat dislocations.


The history and physical examination often provide evidence that a patellar dislocation has occurred. Your doctor will usually get x-rays of the knee to view the position of the kneecap, as well as the depth of the groove. When a patellar dislocation occurs, an MRI is also usually ordered. The MRI helps to look to for evidence of damaged structures (the MPFL or the cartilage) and to see if there is a “loose body” within the knee. Occasionally, a CT scan may be needed to evaluate the bony structures of the knee.


When a patellar dislocation occurs, the first step is to make sure the patella is back in place within the groove. This often happens on its own, but occasionally requires the patient, a friend, or a healthcare provider to put it back in place (called a reduction). When a patient presents to clinic after a recent dislocation and/or reduction, multiple treatment options exist. The treatment path that your surgeon recommends will depend on the results of your x-rays and MRI, as well as the number of dislocations you have had in the past.

Conservative Treatment Options:

Conservative treatment is recommended for most patients after their first dislocation who do not have a “loose body” within the knee. Conservative treatment consists of:

  • Rest: avoiding athletic or contact activities for a period of time. This will allow the swelling and inflammation in the knee to subside.
  • Ice: this will help to decrease swelling and inflammation, leading to less pain.
  • Elevation: this will help to decrease swelling and pain in the knee.
  • Brace or sleeve: a brace or sleeve may help to alleviate swelling and keep the kneecap in place.
  • Physical therapy: to help strengthen the quadriceps and other muscles in the legs and core to prevent future dislocations.

Surgical Options:

Surgical treatment is indicated in patients with a history of multiple dislocations, in patients with a “loose body”, or rarely in patients after their first dislocation who are at high risk for another. The goal of surgery is to prevent future dislocations.

There are several different surgical options for patellar instability that may be recommended depending on a patient’s specific situation. These include:

  • Arthroscopy: a minimally invasive technique in which small incisions are made to insert a camera and small instruments into the knee joint. These instruments can be used to remove loose bodies and view the cartilage surfaces of the knee.
  • Open surgery: open surgery through incisions is often required to treat patellar instability. Your doctor will talk to you about the type of surgery recommended, but it may include reconstruction of the damaged ligament (MPFL) or shifting a part of the shinbone (tibial tubercle) that the patella and quadriceps tendon are connected to. Rarely, the cartilage is badly damaged and a procedure is required to repair it.

Following Surgery:

These surgeries are all performed on an outpatient basis, meaning you will go home the same day. A numbing medication is typically injected during surgery and a prescription for pain medication will be given to help with pain after surgery.

Typically, you will use a brace after surgery that keeps your knee straight and the amount of weight you put on your leg (weightbearing) may be restricted. The type of surgery you have will determine how long you need a brace or to keep the weight off your leg. A physical therapy program, specific to your surgery, will also be provided to guide you through restoring motion, followed by strength and function to your knee. The total recovery time depends on your specific surgery, but usually a minimum of 4-6 months after surgery are required.

Risk and Complications:

Complications from patellar stabilization surgery are relatively rare but depend on the type of surgery your surgeon recommends. These risks may include bleeding, infection, blood clots, prominent screws, damage to nerves or blood vessels, persistent pain, or the need for further surgery.

At Reno Orthopedic Center, your sports medicine surgeons are experienced in the treatment of patellar instability. As a ROC patient, we will provide you with the best and most up to date techniques available.