Special Considerations for Pediatric Patellar Instability

MPFL TEAR PATTERN

The MPFL serves as an important constraint to the patella to prevent lateral dislocation, primarily during the first 20° to 30° of knee flexion.^55,56 Injury to the MPFL is noted in over 90% of patients who suffer a patellar dislocation.⁵⁷ The location of MPFL tears in pediatric patients is variably reported at the patellar attachment (10%-61%), femoral attachment (12%-73%), both (12%-35%) or mid-substance (2.5%-15%).^25,57 The most common tear patterns in pediatric patients are tears at the patellar attachment.^25,57 This tear pattern may be accompanied by an avulsion fracture of the medial rim of the patella, though this fracture, being extra-articular, seldom needs treatment.

CLASSIFICATION

While several authors have established extensive classification systems of patellar dislocation based on both clinical and radiographic presentation and reviews of the literature, a single classification system has not been recognized as the gold standard. In this section, in addition to presenting our preferred methods of classification, we will review some of the more recent and extensive classification systems for patellar dislocation and patellar instability.

Dejour and colleagues³¹ initially used both the presence of patellofemoral anatomic abnormalities and pain to define 3 types of patellar instability: major, objective, and potential patellar instability. Major patellar instability indicates that the patient has experienced more than 1 documented dislocation, objective instability involves one dislocation in addition to an associated anatomic abnormality, and potential patellar instability refers to cases in which the patient has radiographic abnormalities and patellar pain.³¹ Garin and colleagues⁵⁸ more simplistically divided patellar dislocation patients into 2 groups: major (permanent or habitual) dislocation of the patella and recurrent dislocation. Sillanpaa⁵⁹ stressed the distinction between first-time dislocation and recurrent dislocation specifically in the context of acute injuries. These classification systems were formulated with adults as the most relevant population; however, classifications targeted specifically to pediatric patients have recently been presented in the literature.

Historically, pediatric patella dislocations were simply categorized as traumatic or congenital.⁶⁰ In 2014, Chotel and colleagues⁶¹ focused on classifying patellar dislocation by extensively reviewing anatomic, biomechanical, pathophysiological, and clinical patterns seen most commonly in children. They included 5 categories: congenital dislocation, permanent dislocation, habitual dislocation during knee flexion, habitual dislocation during knee extension, and recurrent dislocation; however, they did not address traumatic dislocations.⁶¹ Congenital dislocation is a rare condition, typically presenting at birth, which produces a pattern of functional genu valgum.⁶² Permanent dislocation typically presents after the child has started walking, but before the age of 5 years.⁶¹ The 2 variations of habitual dislocation typically present between ages 5 and 8 years.⁶¹ The final category is the most common and typically occurs during pre-adolescence or adolescence as a result of an atraumatic or trivial traumatic event or sports injury.¹ Using more specific terminology, Hiemstra and colleagues⁶³ modeled a classification system based on the traumatic, unilateral, bankart lesion, surgery (TUBS)/atraumatic, multidirectional, bilateral, rehabilitation, inferior shift (AMBRI) for shoulder dislocation classifications. The patellar dislocation system is used to identify 2 distinct subsets of patients in the patellofemoral instability population. One subset is defined by the acronym WARPS (weak, atraumatic, risky anatomy, pain, and subluxation), the other is STAID (strong, traumatic, anatomy normal, instability, and dislocation).⁶⁴ Patients categorized by the WARPS acronym tend to experience atraumatic onsets of patellofemoral instability and demonstrate anatomic issues that increase this instability. These underlying anatomic issues include valgus alignment, ligamentous laxity, rotational abnormalities, shallow and short TG, and patella alta. On the other hand, STAID patients describe a traumatic dislocation event and do not have underlying anatomic abnormalities that predispose them to instability.⁶⁴

Taking into account these previous classifications, Frosch and colleagues⁶⁵ added specific pathologies including “instability,” “maltracking,” and “loss of patellar tracking,” in addition to both clinical and radiographic factors to define 5 types of patellar dislocation and their specific treatment recommendations.⁶⁵ Type 1 involves simple dislocation with neither maltracking nor instability and a low risk of redislocation.⁶⁵ Type 2 is defined as primary dislocation followed by subsequent high risk of dislocation and no maltracking.⁶⁵ Type 3 is divided into 5 subcategories of instability and maltracking issues involving soft tissue contracture, patella alta, pathological tibial tuberosity, and TG distance.⁶⁵ Type 4 is defined as the highly unstable “floating patella,” and type 5 involves patellar maltracking without instability ⁶⁵. In terms of treatment, conservative rehabilitation is recommended for type 1 whereas MPFL reconstruction tends to show positive outcomes for both types 2 and 3.^66-70

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