T2* 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 T2* 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 T2* 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 T2* values were determined. The ACL specimens were then histologically analyzed using the advanced Ligament Maturity Index (LMI). The T2* 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 T2* 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.