Correlation of magnetic resonance imaging and histologic examination of physeal bars in a rabbit model

Background: The formation of a physeal bony bridge, or bar, is frequently observed in pediatric patients after trauma and is visualized using magnetic resonance imaging (MRI). No study to date has validated the indirect MRI bar area measurements with direct measurements. Purpose: To create a physeal bar using a radiofrequency (RF) ablation technique in a rabbit model and to validate an indirect measurement of the bar area from MRIs with direct histologic measurements. Methods: An epiphysiodesis procedure was performed on the proximal tibial growth plates of 15 skeletally immature rabbit knees using radiofrequency ablation. The rabbits were euthanized 6 weeks postoperatively and volumetric ex vivo MRIs of the knees were acquired. The physeal bar area was calculated from a 3-dimensional reconstruction of the segmented MRIs and from matching histologic sections of the tibia. Results: A physeal bar was successfully created in all the rabbits. A strong correlation, r = 0.83 (P = 0.0001), was found between the MRI and histologic surface area measurements. The mean bar area from MRI measurements was 35.0 ± 20.8 mm² (mean ± SD), and the mean bar area from histologic measurements was 29.8 ± 16.1 mm². Conclusions: This study found excellent correlation between the MRI and histologic physeal bar area measurements. The measurement differences that were found may be attributed to histologic specimen preparation and the geometry chosen to model the physis. Clinical Relevance: The results of this study allow for the quantitative evaluation of in vivo human physes in future studies and development of predictive models for limb length discrepancy.