Patellofemoral instability (PFI), 1 of the most common patellofemoral disorders observed in skeletally immature patients,1-4 has a reported incidence of 43 per 100,000 skeletally immature patients.3 The incidence is even higher in patients 9 to 15 years, with dislocations occurring in 107 per 100,000 individuals.5,6 In recent years, there has been an increasing interest in studying PFI in skeletally immature pediatric patients, who are particularly susceptible to recurrent dislocations. Studies have indicated that children <16 years are at the highest risk for recurrence.7 Lewallen and colleagues8 noted a 69% failure rate and a 3-fold increase in the probability of recurrent dislocation in skeletally immature patients treated nonoperatively.
Anatomic factors that contribute to PFI include ligament laxity, trochlear dysplasia, patella alta, excessive femoral anteversion or tibial torsion, genu valgum, and increased tibial tubercle-to-trochlear groove distance.1,3,8-11 When considering surgical treatment for PFI, all anatomic factors should be considered, with an emphasis on, and understanding of, the role of residual growth and development. One must also consider balancing static soft tissue, dynamic soft tissue, and osseous constraints of the patellofemoral joint to optimize the overall health and balance of the patella (Figure 1).
CHALLENGES IN TREATING PEDIATRIC PFI
A primary challenge in the treatment of PFI in pediatric patients is accounting for the impact of anatomic changes occurring secondary to growth and maturity. From birth to adulthood, the collagen composition of soft tissue changes from an elastic type III collagen to a stiffer type I collagen.12 These physiologic changes may influence the rigidity of the soft tissue restraints around the patellofemoral joint during periods of rapid growth. Longitudinal growth and rotational changes can also occur at the distal femoral and proximal tibial physes. The position of the tibial tubercle and the magnitude of femoral anteversion may also change after growth in adolescents.
Developing effective technical analogs of surgical procedures performed in mature patients with PFI for use in skeletally immature patients has been a second challenge. For example, a varus-producing distal femoral osteotomy to address genu valgum and PFI13 is contraindicated in immature patients, when a hemiepiphysiodesis for guided growth should be considered.14 Similarly, a periosteal sleeve medialization of the insertion of the patellar tendon may be used instead of a tibial tubercle transfer.15
If a medial patellofemoral ligament (MPFL) reconstruction is considered in immature patients, careful consideration of the position of the MPFL in relation to the distal femoral physis is paramount. Shea and colleagues16 originally described the position of the MPFL, based on Schottle’s point17 on lateral radiographs, to be proximal to the distal femoral physis (Figure 2). However, due to the undulation of the physis, the lateral projection may falsely demonstrate Schottle’s point to be on or just proximal to the physis. Other researchers have evaluated the position of the distal femoral physis to be proximal to the origin of the MPFL by a range of 2.9 mm to 8.5 mm on AP radiograph and MRI, respectively.18,19 More recent cadaveric studies have demonstrated the origin of the MPFL in pediatric specimens in relation to the physis to be variable.20 Although we believe that a relative anatomic femoral position of the MPFL origin can be achieved without disrupting the physis using Schottle’s point, some have indicated concerns that this may produce a nonisometric position, which has not been our experience.21
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