A Simple Method of Measuring the Distance From the Schöttle Point to the Medial Distal Femoral Physis With MRI

BACKGROUND: Medial patellofemoral ligament (MPFL) reconstruction is the treatment of choice for recurrent patellar instability in the skeletally immature patient. Avoiding the open physes during anatomic MPFL reconstruction is a challenge in this population. PURPOSE: To describe a novel method using magnetic resonance imaging (MRI) to determine the distance from the Schöttle point to the medial distal femoral physis among skeletally immature individuals with patellar instability. STUDY DESIGN: Descriptive laboratory study. METHODS: Preoperative MRI scans were analyzed from 34 patients with open distal femoral physes and lateral patellar instability. With the multiplanar reconstruction mode on a picture archiving and communication system (PACS), the location of the Schöttle point was determined according to previously reported distances from the posterior femoral cortical line and the posterior origin of the medial femoral condyle. This location was then extrapolated to the most medial sagittal slice on MRI showing the medial distal femoral physis. The distance was measured from this point to the most distal aspect of the physis. RESULTS: The mean age of the study cohort was 13.6 years (range, 10.6-15.7 years); there were 13 males and 21 females. The mean distance from the medial distal femoral physis to the Schöttle point was 7.27 ± 1.78 mm. The Schöttle point was distal to the medial distal femoral physis in all cases. There was no significant correlation between age and mean distance in either the overall study population (r = 0.046, P = .798) or when stratified by sex (females, P = .629; males, P = .089). The distance between the Schöttle point and the medial distal femoral physis was shorter for females than for males (6.51 vs 7.71 mm, P = .043). After adjustment for age, females on average were 1.31 mm closer to the Schöttle point than were males (B = –1.31, P = .041). CONCLUSION: This technique can be used to determine the distance between the medial distal femoral physis and the Schöttle point. The Schöttle point was distal to the physis in all patients, and it was closer to the physis in skeletally immature females compared with age-matched males. CLINICAL RELEVANCE: The long-term repercussions of improperly placed MPFL reconstruction include recurrent patellar instability, increased patellofemoral contact pressures and overtensioning of the ligament, and possibly patellofemoral arthritis. The current technique can be used preoperatively to determine the appropriate safe distance for drilling a socket distal to the physis.