Comparison of micro-CT post-processing methods for evaluating the trabecular bone volume fraction in a rat ACL-transection model.

Trabecular bone volume fraction assessments are likely sensitive to the analysis method and selection of the region of interest. Currently, there are several methods for selecting the region of interest to analyze trabecular bone in animal models of post-traumatic osteoarthritis. The objective of this study was to compare three published methods for determining the trabecular bone volume fraction of the medial tibial epiphyses in ACL transected and contralateral ACL intact knees. Micro-computed tomography images of both knees were obtained five weeks post-operatively and evaluated using three methods: (1) the Whole Compartment Method that captured the entire medial compartment, (2) the centrally located Single Core Method, and (3) the Triplet Core Method that averaged focal locations in the anterior, central, and posterior regions. The Whole Compartment Method detected significant bone loss in the ACL transected knee compared to the ACL intact knee (p<0.001), with a loss of 15.2±3.9%. The Single Core and the Triplet Core Methods detected losses of 7.5±10.5% (p=0.061) and 14.1±13.7%(p=0.01), respectively. Details regarding segmentation methods are important for facilitating comparisons between studies, and for selecting methods to document trabecular bone changes and treatment outcomes. Based on these findings, the Whole Compartment Method is recommended, as it was least variable and more sensitive for detecting differences in the bone volume fraction in the medial compartment.

The uncertainty of predicting intact anterior cruciate ligament degeneration in terms of structural properties using T(2)(*) relaxometry in a human cadaveric model.

The combination of healing anterior cruciate ligament (ACL) volume and the distributions of T2(*) relaxation times within it have been shown to predict the biomechanical failure properties in a porcine model. This MR-based prediction model has not yet been used to assess ligament degeneration in the aging human knee. Using a set of 15 human cadaveric knees of varying ages, we obtained in situ MR measures of volume and T2(*) of the intact ACL and then related these MR variables to biomechanical outcomes (maximum and yield loads, linear stiffness) obtained via ex vivo failure testing. Using volume in conjunction with the median T2(*) value, the multiple linear regression model did not predict maximum failure load for the intact human ACL; R(2)=0.23, p=0.200. Similar insignificant results were found for yield load and linear stiffness. Naturally restricted distributions of the intact ligament volume and T2(*) (demonstrated by the respective Z-scores) in an older cadaveric population were the likely reason for the insignificant results. These restricted distributions may negatively affect the ability to detect a correlation when one exists. Further research is necessary to understand the relationship of MRI variables and ligament degeneration. While this study failed to find a significant prediction of human biomechanical outcome using these MR variables, with further research, an MR-based approach may offer a tool to longitudinally assess changes in cruciate ligament degradation.

Improving the clinical efficiency of T2(*) mapping of ligament integrity.

Current MR methods use T2(*) relaxation time as a surrogate measure of ligament strength. Currently, a multi-echo voxel-wise least squares fit is the gold standard to create T2(*) maps; however, the post-processing is time-intensive and serves as a stopgap for clinical use. The study objective was to determine if an alternative method could improve post-processing time without sacrificing fidelity of T2(*) values for eventual translational use in the clinic. Using a 6 echo FLASH sequence, three different methods were used to determine intact posterior cruciate ligament (PCL) median T2(*) Two of these methods utilized a voxel-wise method to establish T2(*) maps: (1) a current "gold standard" method using a voxel-wise 6 echo least-squares fit (6LS) and (2) a voxel-wise 2 echo point T2(*) determination (2MM). The third method used median ligament signal intensity and a single nonlinear least-squares fit (6LSROI) instead of a voxel-wise basis. The resulting median T2(*) values of the PCL and computational time were compared. The median T2(*) values were 42% higher using the 2MM compared to the 6LS method (p<0.0001). However, a strong correlation was found for the median T2(*) values between the 2MM and 6LS methods (R(2)=0.80). The median T2(*) values were not significantly different between the 6LS and 6LSROI methods (p=0.519). Using the 2MM (which provides a regional map) and the 6LSROI (which efficiently provides the median T2(*) value) methods in tandem would take only minutes of post-processing computational time compared to the 6LS method (~540 min), and hence would facilitate clinical application of T2(*) maps to predict ligament structural properties as a patient outcome measure.