T 2 * relaxometry and volume predict semi‐quantitative histological scoring of an ACL bridge‐enhanced primary repair in a porcine model

Magnetic resonance imaging (MRI) variables, such as T 2 * and volume, can predict the healing ligament structural properties. How these MR variables relate to semi‐quantitative histology of the healing ACL is yet unknown. We hypothesized that T 2 * and volume would predict the histological scoring of a healing ACL. Yucatan minipigs underwent ACL transection and received bridge‐enhanced ACL repair or no treatment. The surgical legs were harvested after 52 weeks and imaged using a high resolution 2‐echo sequence. For each ligament, the volume and median T 2 * values were determined. The ACL specimens were then histologically analyzed using the advanced Ligament Maturity Index (LMI). The T 2 * of the healing ligaments significantly predicted the total LMI score as well as the cell, collagen and vessel sub‐scores; R 2  = 0.78, 0.67, 0.65, and 0.60, respectively ( p  ≤ 0.001). The ligament volume also predicted the total LMI score, cell, and collagen sub‐scores; R 2  = 0.39, 0.33, 0.37, and 0.60, respectively ( p  ≤ 0.001). A lower ligament T 2 * or a higher volume was associated with higher histological scores of the healing ligaments. This study provides a critical step in the development of a non‐invasive method to evaluate ligament healing on a microscopic scale. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1180–1187, 2015.