Optimal Screw Length for Osteochondritis Dissecans Fixation of the Pediatric Distal Femur: A Magnetic Resonance Imaging Comparision Study of Pediatric Epiphyseal Thickness.

Background: Magnetic Resonance Imaging (MRI) allows orthopaedic surgeons to assess Osteochondritis Dissecans (OCD) of the distal femur in children. When surgical fixation is indicated, headless compression screws can be used to stabilize the fragment and allow it to heal. During screw placement it is important to make sure the screw is not too long or it may cross the physis, leading to physeal growth arrest. Hypothesis/Purpose: The purpose of the study was to determine the optimal screw length for osteochondritis dissecans fixation of the pediatric distal femur. A secondary objective was to compare the thickness of the epiphysis of the pediatric distal femur at different locations and screw trajectories. Methods: This retrospective, IRB-approved, single-institution study identified 113 pediatric patients (ages 6-17 years old) with a knee MRI from January 1, 2008 to January 1, 2020. A total of 6 measurements were obtained on each pediatric MRI by two residents. The longitudinal axis of the femur was used as a reference trajectory. Three measurements were obtained from medial femoral condyle and lateral femoral condyle, respectively. The lengths measured were from the anteromedial (AM), posteromedial (PM), and distalmedial (DM) on the medial femoral condyle to the physis. On the lateral femoral condyle, the anterolateral (AL), posterolateral (PL), and distallateral (DL) were measured. 2 sample t-testing, chi-square analysis, and Pearson's correlation coefficient were utilized to find significant associations. Results: Of the 113 pediatric knees identified, 32 did not meet inclusion criteria, yielding 81 patients for final analysis. The average epiphyseal thickness of the bone was 25.5 mm ± 4.2 (CI 24.5 – 26.4, range 14.1 – 33.1). There was a difference in epiphyseal thickness based on screw trajectory () with the average thickness being 23.0, 22.2, 30.5, 29.3, 25.5, and 22.3 mm for the AM, AL, PM, PL, DM and DL respectively. There was a negative correlation between increasing age and epiphyseal cartilage thickness (). There was a positive correlation between increasing age and bone only epiphyseal thickness (). Conclusion: Our study outlines important measurements that orthopedic surgeons must be aware of when operating on OCD lesions in pediatric patients. These results can aid in determining the appropriate screw length that can be implanted into the pediatric distal femur to avoid damaging the distal femur. Our study results show that the optimal screw length to use in pediatric OCD lesion is 18mm in order to avoid physeal disturbance in the majority of pediatric patients. Figure 1. Sagittal MRI of the lateral distal femur showing the projection used to measure the anterolateral (AL), posterolateral(PL), and distallateral (DL) distances to the physis Figure 2. Sagittal MRI of the medial distal femur showing the projection used to measure the anteromedial (AM), posteromedial (PM), and distalmedial (DM) distances to the physis [ABSTRACT FROM AUTHOR]