Short communicationThe use of magnetic resonance imaging to predict ACL graft structural properties
Introduction
The injured ACL is commonly replaced by a tendon graft during reconstruction surgery. Because ACL reconstruction does not restore normal kinematics or reduce the risk for premature osteoarthritis, there is an ongoing need to improve treatment options and to verify that new treatment options improve healing. Direct biomechanical measurements of the graft structural properties are frequently used to assess graft healing in research studies. However, these destructive measurements require post-mortem testing, do not permit longitudinal assessments within subjects, and are not suitable for clinical trials. A non-invasive method, such as MRI, that predicts the graft structural properties in vivo is needed to longitudinally evaluate ACL graft healing in vivo within a subject.
MRI has been used to assess ACL graft maturation (Howell et al., 1991b, Howell et al., 1995, Orrego et al., 2008, Stockle et al., 1998, Weiler et al., 2001). In an effort to establish the relationship to the biomechanical properties of a healing graft, Weiler et al. (2001) found a correlation between the MR signal-to-noise quotient to the failure load, linear stiffness, and tensile strength of the graft in the sheep. However, evaluation of the signal-to-noise quotient is limited since it is dependent on acquisition and scanner characteristics. Parameters that are independent of the acquisition characteristics are needed. It seems reasonable to assume that geometric MRI measurements, such as ACL graft volume, would reflect the graft structural properties. T2 mapping may also provide insight because T2 relaxation times are affected by proteoglycan, collagen, and water content (Li et al., 2011, Mosher et al., 2000, Regatte et al., 2002, Wayne et al., 2003, White et al., 2006), factors associated with ligament/graft healing (Frank, 2004).
The objective of this study was to determine if an MRI-based method, which utilizes both graft volume and T2-mapping, is predictive of the structural properties of the graft and AP knee laxity at 30° and 60° knee flexion. We hypothesized that there are significant correlations between graft volume, T2 relaxation time, and graft volume normalized by T2 relaxation time with the structural properties of an ACL autograft and AP knee laxity when measured 6 weeks after ACL reconstruction in the goat model.
Section snippets
Model of ACL reconstruction
Nine 4-year-old male Nubian-cross goats underwent unilateral ACL reconstruction using a bone-patellar tendon-bone autograft under an approved IACUC protocol as previously reported (Spindler et al., 2009). Five goats had their graft enhanced with a collagen–platelet composite, while four were enhanced with a collagen sponge only. One surgeon performed the procedures to minimize surgical variations. The differences in structural properties and AP knee laxity between the two treatment groups for
Results
All graft failures occurred in the midsubstance. There was a significant correlation between graft volume and the failure load (r2=0.502; p=0.049). When the volume was normalized by the T2 relaxation parameter, the variability was reduced and the predictability was improved (r2=0.687; p=0.011; Fig. 1). When the T2 parameter alone was correlated to failure load, the relationship was not significant (r2=0.191; p=0.278).
There was a significant correlation between graft volume and the linear
Discussion
This study demonstrates significant correlations between ACL graft volume via MRI and the structural properties of the ACL autograft and AP knee laxity after 6 weeks of healing. T2 relaxation time alone was not significant. T2 relaxation time is affected by several compositional factors, including water, proteoglycan, and collagen content and collagen orientation, the interactions of which may increase variability and reduce the correlation coefficient. However, it was interesting to note that
Conflict of interest statement
The authors have no financial or personal relationships that could bias this work.
Acknowledgments
Funding for this study was received from NIH; RO1-AR049199 (BCF) and RO1-AR052772 (MMM), the Vanderbilt’s Kenneth D. Schermerhorn Endowed Professorship and the RIH Orthopaedic Foundation. The authors would like to thank Alison Biercevicz for her help with the creation of the figures.
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