Abstract
Introduction
Cruciate retaining knee replacements are only implanted into patients with “healthy” ligaments. However, partial anterior cruciate ligament (ACL) tears are difficult to diagnose with conventional MRI. Variations of signal intensity within the ligament are suggestive of injury but it is not possible to confirm damage or assess the collagen alignment within the ligaments. The potential use of Magic Angle Directional Imaging (MADI) as a collagen contrast mechanism is not new, but has remained a challenge. In theory, ligament tearing or joint degeneration would decrease tissue anisotropy and reduce the magic angle effect. Spontaneous cruciate ligament rupture is relatively common in dogs. This study presents results from ten canine knees.
Methods
Ethical approval was obtained to collect knees from euthanized dogs requiring a postmortem (PM). A Siemens Verio 3T MRI scanner was used to scan a sphere containing the canine knees in 9 directions to the main magnetic field (B0) with an isotropic 3D-T1-FLASH sequence. After imaging, the knees were dissected and photographed. The images were registered and aligned to compare signal intensity variations. Segmentation using a thresholding technique identified voxels containing collagen. For each collagen-rich voxel the orientation vector was computed using Szeverenyi and Bydder's method. Each orientation vector reflects the net effect of all fibers comprised within a voxel. The assembly of all unit vectors represents the fiber orientation map and was visualised in ParaView using streamlines. The Alignment Index (AI) is defined as a ratio of the fraction of orientations within 20° (solid angle) centred in that direction to the same fraction in a random (flat) case. By computing AI for a regular gridded orientation space we can visualise differences in AI on a hemisphere. AI was normalised so that AI=0 indicates isotropic collagen alignment. Increasing AI values indicate increasingly aligned structures: AI=1 indicates that all collagen fibers are orientated within the cone of 20° centred at the selected direction.
Results
Dogs cranial cruciate ligament (CCL) is similar to human ACL. It's composed of an anteromedial (AM) bundle and a posterolateral (PL) bundle. Two knees were damaged with partial CCL tears, the PL bundle was intact but the AM bundle was torn.
Paraview streamlines of the CCL for healthy and damaged knees differ. The healthy knee has continuous fiber tracts with no ligament disruption. In the AM bundle fibers are discontinuous and the PL bundle fibers are continuous as expected in a partially torn CCL.
The AI for healthy (mean AI=0.25) and damaged CCL (mean AI= 0.075) is significantly different (p<0.01). The damaged AM bundle has a more diffuse spread of less aligned fibers compared to the more concentrated and aligned PL fiber bundle.
Conclusion
This study demonstrates the first visualisation of a CCL partial tear using MADI. Combined with AI, our scanning technique offers a tool to visualise and quantify changes in collagen fiber orientation. Thus, MRI can be used to improve the diagnosis and quantification of partial ligament tears in the knee.
