In Situ, noninvasive, T2*-weighted MRI-derived parameters predict ex vivo structural properties of an anterior cruciate ligament reconstruction or bioenhanced primary repair in a porcine model

Abstract

Background: Magnetic resonance imaging (MRI) is a noninvasive technology that can quantitatively assess anterior cruciate ligament (ACL) graft size and signal intensity. However, how those properties relate to reconstructed or repaired ligament strength during the healing process is yet unknown.

Hypothesis: Magnetic resonance imaging-derived measures of graft volume and signal intensity are significant predictors of the structural properties of an ACL or ACL graft after 15 weeks and 52 weeks of healing.

Study design: Controlled laboratory study.

Methods: The current data were gathered from 2 experiments evaluating ACL reconstruction and repair techniques. In the first experiment, pigs underwent unilateral ACL transection and received (1) ACL reconstruction, (2) ACL reconstruction with collagen-platelet composite (CPC), or (3) no treatment. The surgical legs were harvested after 15 weeks of healing. In the second experiment, pigs underwent ACL transection and received (1) ACL reconstruction, (2) ACL reconstruction with CPC, (3) bioenhanced ACL primary repair with CPC, or (4) no treatment. The surgical legs were harvested after 52 weeks. The harvested knees were imaged using a T2*-weighted 3-dimensional constructive interference in steady state (CISS) sequence. Each ligament was segmented from the scans, and the intra-articular volume and the median grayscale values were determined. Mechanical testing was performed to establish the ligament structural properties.

Results: Volume significantly predicted the structural properties (maximum load, yield load, and linear stiffness) of the ligaments and grafts (R (2) = 0.56, 0.56, and 0.49, respectively; P ≤ .001). Likewise, the median grayscale values (ie, signal intensity) significantly predicted the structural properties of the ligaments and grafts (R (2) = 0.42, 0.37, and 0.40, respectively; P < .001). The combination of these 2 parameters in a multiple regression model improved the predictions (R (2) = 0.73, 0.72, and 0.68, respectively; P ≤ .001).

Conclusion: Volume and grayscale values from high-resolution T2*-weighted MRI scans are predictive of structural properties of the healing ligament or graft in a porcine model.

Clinical relevance: This study provides a critical step in the development of a noninvasive method to predict the structural properties of the healing ACL graft or repair. This technique may prove beneficial as a surrogate outcome measure in preclinical animal and clinical studies.

Fig. 1

3-D segmentation process illustrated on one sagittal slice of the image stack. A) 2D Graft location; B) Graft segmented; C) 3D model of graft.

Fig. 2

The graft structural properties for the 15 and 52 week specimens (A- Maximum Load, B-Yield Load, and C- Linear Stiffness) as a function of ligament volume (A1, A2 and A3) and the median grayscale value (B1, B2 and B3) in the linear regression models. Dashed lines represent 95% confidence interval.

Fig. 3

The reconstructed, repaired and transected ligament prediction plane for maximum load as a function of volume and of median grayscale value. The ligaments at 52 weeks (black circles) had a significantly lower median grayscale value than ligaments at the 15 week time point (gray circles) (p≤0.001). Similar plots were found for the yield load and the linear stiffness prediction models.

Fig. 4

Actual maximum load versus predicted Maximum Load for both 15 and 52 week time points, calculated using volume and of median grayscale value.