Patellofemoral instability (PFI) is a complex disorder that most commonly occurs in children and adolescents. Patellar dislocation can result from isolated trauma, but in some cases, anatomic abnormalities, including patella alta, trochlear dysplasia, or laxity of the soft tissue restraints of the knees, predispose certain patients to more frequent patellofemoral dislocation. We typically categorize pediatric patellar dislocation into four types, which include traumatic dislocation, fixed patellar dislocation, obligatory patellar dislocation, and syndromic types such as cases associated with Ehlers-Danlos syndrome and Marfan syndrome. This review focuses on traumatic dislocation.

Incidence of patellar dislocation is reported to be 43 in 100,000 individuals.¹⁹ The peak age of dislocation is 15 years, and females tend to dislocate more than males. The rate of recurrent instability and/or recurrent dislocation has been widely studied, but the reported rates vary widely from 14% to 71%²⁰,²¹,²²,²³,²⁴,²⁵ (Table 17.1).

Risk factors such as family history and younger age at first dislocation have been associated with recurrent instability. As expected, abnormal anatomy including patellofemoral malalignment as measured by increased Q angle, trochlear dysplasia, and abnormal patellar configuration are also associated with recurrent dislocations or instability.²⁰,²¹,²⁴

The patellofemoral joint is stabilized against lateral instability by both osseous and soft tissue structures. In patients with normal trochlea anatomy, the patella enters the osseous confines of the trochlea at 30 degrees of knee flexion which provides static stability, whereas in extension, the medial patellofemoral ligament (MPFL) is the most important stabilizer of the patellofemoral joint. As opposed to the cruciate ligaments, the MPFL has relatively low tension throughout flexion and extension, between 2 N and 10 N.¹ Furthermore, the MPFL remains isometric throughout flexion from 0 to 110 degrees and then becomes slack past 110 degrees of flexion.² This biomechanical data suggests that the MPFL acts as a “checkrein” for the patella in that it is a main stabilizer against lateral patellar dislocation when an outside force is exerted on the knee in extension. When put to the test in this action of forced lateral subluxation, the native MPFL can withstand a tension of approximately 200 N.³ Studies have shown that when the patella dislocates from trauma, the MPFL is torn in approximately 90% of patients.⁴ The zone of injury of the MPFL tear, whether it tears off of the patella or its femoral insertion, has been the topic of a number of radiographic studies.⁴,⁵,⁶,⁷,⁸,⁹,¹⁰ The studies isolated to adolescent patients⁵,⁶,⁷ have found that the MPFL more commonly is torn near its patellar insertion. In a study of 43 children with acute patellar dislocation, as confirmed by magnetic resonance imaging (MRI), 61% had tears isolated to the patellar insertion.⁷

The MPFL lies in the second layer of the knee as described by Warren and Marshall.¹¹ The patellar attachment is a fan-like attachment at the junction between the proximal and middle thirds of the superomedial border of the patella.¹² A recent anatomic study investigated the patellar insertion of the MPFL in eight cadavers and found the proximal fibers of the MPFL were mainly attached to the vastus intermedius tendon, whereas its distal fibers were inserted partially on the medial retinaculum.¹³ The authors proposed that this anatomic relationship allows the MPFL to pull both the patella and the patellar tendon, albeit indirectly, medially to keep the patella engaged in the trochlea throughout flexion and extension. The femoral attachment of the MPFL is bounded by the adductor tubercle proximally and the femoral attachment of the medial collateral ligament (MCL) distally.¹² In the anterior to posterior dimension, it lies approximately 1.3 mm anterior to a line drawn along the posterior cortex of the femoral diaphysis.¹⁴

Surgically addressing the MPFL has become a mainstay of treating PFI given both the biomechanical importance of the MPFL as a restraint to lateral dislocation as well as the high frequency of MPFL injury following traumatic patellar dislocation. Similar to the experience with anterior cruciate ligament repair, repairing the remaining ends of a torn MPFL has not been a successful surgical option.¹⁵,¹⁶ Dozens of different techniques of MPFL reconstruction have been reported in the literature, and there is still no consensus supporting one technique over another.¹⁷ Although MPFL reconstruction in the skeletally mature patient is well described in the literature, there are fewer studies that focus on patients with open growth plates.¹⁷,¹⁸