What is patellofemoral or patellar (kneecap) instability?
Patients who have a tracking problem in the patellofemoral joint, where the patella (kneecap) does not stay in the groove on the femur, are vulnerable to a spectrum of knee conditions. These include:
- Subluxations: in which the patella slips partially but not completely comes out of the trochlea. These partial dislocations can result in pain, swelling, a popping or cracking sensation, or stiffness.
- Dislocations: a traumatic injury in which soft tissues are damaged as the patella completely “jumps” the track and then comes forcibly back into place. Because the bone always dislocates outward, the ligament on the inside – the medial patellofemoral ligament (MPFL) – tears or stretches.
Some patients who experience kneecap instability also have a malformation of the femur and tibia which places excessive stress on the medial patellofemoral ligament (MPFL). This condition, known as complex patellofemoral instability usually requires an interdisciplinary medical approach to treat.
What are the signs and symptoms of patellofemoral instability?
Physical signs of dislocation include significant swelling of the knee and an “apprehension sign," an anxious response to the orthopedic specialist guiding the patella outward and attempting to mimic the dislocation. An MRI after a kneecap dislocation reveals damage to the ligament, and bruises on the bone inside of the patella and on the outside of the femur that occur when the kneecap “re-locates” back in place. The MRI is also helpful in evaluating the knee for evidence of cartilage injury which is very common after dislocations.
What causes patellar instability?
People who experience a first-time patella dislocation frequently do so while participating in sports, causing the knee to suddenly buckle and fall. Because ACL tears often happen in the same way, and because they are much more common than patella dislocations, it is important to rule out an ACL tear. In rare cases, a patient comes in with an ACL tear and is found to have had a patella dislocation as well.
Who is at risk for kneecap dislocation and subluxation?
People at risk for dislocation and subluxation include both young women who are loose-jointed, as well as female athletes who may experience a more traumatic dislocation while playing their sport. Subluxation and dislocations do occur in men and boys but much less frequently. Individuals in these groups share common risk factors, including
- a shallow (or even absent) groove on the trochlea or femur
- an abnormal insertion of the patellar tendon on the tibia (shin)
- knock knees
- high riding kneecap
In the case of a shallow groove (track), the patella is not as well-controlled as it is by the deep groove that is present in a normal patellofemoral joint. As a result, less energy is required to force the patella from its track.
Patients with malalignment that results from a knock-kneed posture are subject to a greater than normal force on the patella, which pulls the bone outward, out of the trochlear groove, and toward the outside of the knee. In a normal knee, the tendon that connects the patella to the tibia maintains a force that is in line with the patella, (tracking in alignment with the trochlear groove). Orthopedists use an index called the TT-TG (tibial tuberosity trochlear groove) to measure the degree of malalignment present and guide treatment recommendations.
Individuals with patella alta, a patella or kneecap that is located higher up on the femur than normal are also at increased risk of dislocation, as the patella must travel a greater distance during flexion of the knee before engaging fully in the groove or track of the femur. The joint is particularly vulnerable to instability during this period.
Although dislocation is very painful, after the knee quiets down and returns to baseline, there may be little to no pain in between the instability episodes. Pain and instability don’t always go hand-in-hand. This can be problematic since pain-free patients may delay seeking treatment despite recurrent episodes of instability while cartilage damage continues to progress with each dislocation or subluxation.
What are treatments for kneecap instability?
The standard of care for first-time dislocations is non-operative treatment, where the torn ligaments can heal on their own. However, an MRI is important to evaluate the degree of damage that has occurred even after one dislocation. Surgery is necessary if a piece of bone or a piece of cartilage is a loose body (free floating in the knee) - as these can cause locking, buckling, or additional pain in the knee if left untreated. This type of injury may also be referred to as an osteochondral fracture. In these patients, surgery is performed to both remove the loose body or repair it and stabilize the patella at the same time.
Nonsurgical Treatments
In patients who do not require surgery, the knee is commonly immobilized in a splint or brace for a few days to weeks to allow the knee to calm down and for swelling and pain to subside. The orthopedist may also drain fluid from the knee to reduce discomfort at the first office visit if there is considerable swelling. Physical therapy is the primary course of treatment after a first-time dislocation and is started within the first 1-2 weeks after the dislocation, to achieve normal range of motion and strength. Physical therapy after a dislocation usually continues for between 2-3 months and it can take as long as 4 -5 months for some athletes to return to their pre-injury level of play.
Once a patient has dislocated his or her patella or knee cap, he or she is at an increased risk of it happening again whether it is in the form of subluxation or a full dislocation. Although injured ligaments do “fill in” and heal during recovery, these structures are generally stretched from the injury and are less able to control the patella - further contributing to the risk of another instability episode.
Statistics show that this risk of having another dislocation or subluxation after a first-time dislocation is somewhere between 20-40%; and that after the second dislocation, the risk of recurrence goes up to greater than 50%. Younger patients (under the age of 25) are at even greater risk, especially those in whom the growth plates (a site at the ends of the bone where new tissue is produced and bone growth continues until skeletal maturity) are still open with redislocation rates reaching 70%.
Precision Pain Care and Rehabilitation has two convenient locations in Richmond Hill – Queens, and New Hyde Park – Long Island. Call the Queens office at (718) 215-1888 or (516) 419-4480 for the Long Island office to arrange an appointment with our Interventional Pain Management Specialists, Dr. Jeffrey Chacko or Dr. Sonny Ahluwalia.
