Special Considerations for Pediatric Patellar Instability

MEDIAL PATELLOFEMORAL LIGAMENT ANATOMY

The medial patellofemoral ligament (MPFL) functions to limit the lateral translation of the patella.¹⁹ The attachment sites on the femur and patella for the MPFL have been studied in children.^20-23 Cadaveric dissections in specimens aged 2 to 11 years have noted the patellar attachment to be an average of 12 mm in length with the midpoint approximately 5 mm superior to the mid-pole of the patella.²² The patellar footprint of the MPFL insertion was a mean 41% of the entire patellar length.²²

It is important to be aware of the characteristic anatomy of the MPFL, as fixation points should mimic the anatomic insertion as best as possible while also avoiding violation of the nearby physis. The MPFL originates between the adductor tubercle and the medial femoral epicondyle just distal to the distal femoral physis and attaches to the superomedial aspect of the patella.^20-25 In relation to the physis in pediatric patients, the midpoint of MPFL insertion has been measured to be 4 mm to 9 mm distal to the femoral physis.^21,24,25 These measurements represent averages as cadaveric studies have reported that some part of MPFL femoral insertion extends proximal to the distal femoral physis.²¹ A recent report of physeal injury to the posterior distal femoral physis during MPFL reconstruction leading to femoral flexion deformity highlights the importance of physeal-respecting surgery.²⁶

TROCHLEA AND ANTERIOR DISTAL FEMORAL PHYSIS

The relationship between the proximal aspect of the trochlea and the anterior distal femoral physis has been recently studied in 175 knees with dysplastic trochlea.²⁷ Based on magnetic resonance imaging evaluation, the lateral aspect of the trochlea extended proximal to the anterior distal femoral physis in 13% of patients and was at the level of the anterior physis in another 13% of patients (Figure 1).²⁷ Hence, a cautious approach is recommended for any surgery to address trochlear dysplasia or trochlear bump in younger patients to prevent iatrogenic injury to anterior distal femoral physis and resultant genu recurvatum. The distance between the trochlea and the physis increased with increasing age.

LIMB ALIGNMENT

Physiologically, the quadriceps angle (Q angle) changes through the course of growth. As children begin standing and walking, they stand with their feet wider apart and in genu varum.²⁸ Physiologic genu varum can reach 15°.²⁸ This degree lessens during the first 1.5 to 2 years of life, transitioning to physiologic valgus of nearly 12° by 3 years of age.²⁸ Genu valgum, thereafter, gradually decreases to reach the adult value of around 7° to 8° by age 7 years.²⁸ Increased genu valgum is a risk factor for patellar instability. In skeletally immature patients, correction of genu valgum through guided growth may be desirable in patients undergoing patellar stabilization surgery (Figures 2A, 2B).²⁹

PATHOPHYSIOLOGY OF PEDIATRIC PATELLAR DISLOCATION

TROCHLEAR DYSPLASIA

Trochlear dysplasia is an abnormal shape and depth of the TG.³⁰ Up to 96% of patients with patellar dislocation have trochlear dysplasia.^30-33 In a study of patellar instability in children, at least 1 of the 3 signs of trochlear dysplasia (the crossing sign, supratrochlear bump, and double contour sign) was present on lateral radiographs.³⁴ In another study on the growth of trochlear dysplasia in children and adolescents, all grades of trochlear dysplasia were present at all ages (ie, the dysplasia was most likely present at birth and did not necessarily worsen with age and growth).³⁵ The linear dimensions of lateral and medial condylar height as well as trochlear bump increased with age but both the sulcus angle and shape of the trochlea did not change significantly.³⁵ Remodeling of a dysplastic trochlea can happen if the patella is stabilized and appropriately located at a younger age, preferably before 10 years of age.^36,37

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