Primary lateral patellar dislocation — defined as a traumatic displacement of a normal and previously uninjured patellar position in the trochlear groove — is among the most common acute knee injuries in skeletally immature children and adolescents. Usually sports related, it results from a direct blow or fall onto the knee or a noncontact injury that occurs with one foot planted and the tibia externally rotated.
Treatment for patellar dislocation has historically been controversial. Many surgical interventions have been tried, often with unsatisfactory results and no clear evidence as to which is superior. Thus, conservative management remains the standard of care for first-time dislocations. No matter what the initial management, failure to return to sport and recurrent instability are relatively common.
Recurrence, in fact, is one of the chief challenges of managing patellar dislocation in young athletes, especially those who are skeletally immature. Recurrent instability is associated with structural abnormalities such as patella alta, increased tibial tubercle-trochlear groove (TT-TG) distance and trochlear dysplasia — development of the femoral trochlear groove in an unusually shallow, flat, convex or deficient configuration.
Some evidence suggests that in very young children, recurrent instability itself may lead to secondary trochlear dysplasia, further increasing the risk of recurrence. Little human research on secondary dysplasia exists, but a study published in The Knee in 2013 found that rabbits with patellar dislocation developed trochlear dysplasia over time.
Similarly, few published studies have looked at recurrent dislocation exclusively in skeletally immature children. Studies that include both skeletally mature and immature patients have found high recurrence rates. In 2009, Finnish researchers writing in The Journal of Bone & Joint Surgery, American Volume, reported a 40 percent risk of recurrence after nonoperative treatment of first-time patellofemoral dislocations.
A larger study, published by Mayo authors in 2013 in The American Journal of Sports Medicine, found a 38 percent recurrence rate after three years of nonoperative treatment. More than half the patients in that study eventually needed surgery to gain stability.
Other studies have shown that a majority of patients who experience patellofemoral dislocation have a chondral or osteochondral injury. Damage to articular cartilage and underlying bone is associated with increased inflammation and degenerative changes. Yet little research exists on the incidence of arthritis among pediatric patients with patellar instability. A few small studies found that 14 to 29 percent of patients develop arthritis a decade or so after the initial injury.
Diane L. Dahm, M.D., an orthopedic surgeon and researcher specializing in knee injuries at Mayo Clinic's campus in Rochester, Minnesota, says the potential for arthritis may be related to the number of instability episodes and that surgery after an initial dislocation may help prevent recurrences and associated post-traumatic arthritis.
"Our research is focused on continuing to define key risk factors for recurrent patellofemoral instability in skeletally immature patients and implementing treatment protocols to reduce the risk," she says.
To aid that effort, Dr. Dahm and colleagues are part of a five-year, multicenter randomized controlled trial that aims to:
- Determine the risk factors for recurrence after first-time patellofemoral dislocation in skeletally immature patients, including patella alta, high TT-TG distances and trochlear dysplasia
- Evaluate medial patellofemoral ligament (MPFL) reconstruction as a surgical intervention after first-time dislocation
- Use MRI to evaluate the development of trochlear dysplasia in patients undergoing either surgery or conservative management for a first-time dislocation
"Strong evidence already exists that skeletally immature patients are at high risk of recurrence, yet the standard of care remains conservative management unless there is evidence of a displaced osteochondral fragment. Otherwise, most patients are treated with a brace and physical therapy," explains Aaron J. Krych, M.D., a sports medicine clinician and researcher at Mayo's campus in Minnesota, who is part of the study team. "That is mainly because in the past, surgery for kneecap dislocations — usually medial reefing or MPFL repairs — had an unacceptably high failure rate. But as our understanding of the biomechanics of the patellofemoral joint has improved, reconstruction of the MPFL has become an accepted treatment for recurrent patellar instability. We think some patients would also benefit from this surgery after first-time dislocation."
The investigators hope to recruit 240 skeletally immature patients with a first-time patellar dislocation who will be randomized to receive either MPFL reconstruction or treatment as usual. They believe the MPFL group will show a significantly lower risk of instability in the five years after the initial injury than will the controls — the study's primary outcome. Secondary outcomes include functional outcome scores, timing of return to sport, status of the patellofemoral cartilage and trochlear morphology on MRI, and time to repeat dislocation.
"We will also look at anatomical risk factors such as trochlear dysplasia that are thought to contribute to recurrence," Dr. Krych says. "A 2015 study by Mayo authors published in The Journal of Knee Surgery found that patients younger than age 25 with trochlear dysplasia had a 60 to 70 percent recurrence risk at five years, for instance. So if a patient has those risk factors, physicians are more likely to consider surgery after a first-time event. If primary surgery is shown to significantly lower the risk of recurrence, the standard of care will change, which may help reduce the rates of recurrent injury, early arthritis and long-term disability in skeletally immature patients."
For more information
Li W, et al. Femoral trochlear dysplasia after patellar dislocation in rabbits. The Knee. 2013;20:485.
Sillanpää PJ, et al. Treatment with and without initial stabilizing surgery for primary traumatic patellar dislocation: A prospective randomized study. The Journal of Bone & Joint Surgery. American Volume. 2009;91:263.
Lewallen LW, et al. Predictors of recurrent instability after acute patellofemoral dislocation in pediatric and adolescent patients. The American Journal of Sports Medicine. 2013;41:575.
Lewallen LW, et al. First-time patellofemoral dislocation: Risk factors for recurrent instability. The Journal of Knee Surgery. 2015;28:303.