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. 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.Introduction
Methods
Interventional MRI provides a novel non-invasive method of in-vivo weight-bearing analysis of the talo-calcaneal joint. Six healthy males (mean 28.8 years) underwent static right foot weight bearing MRI imaging at 0o, 15o inversion, and eversion. Using known radiological markers the motion of the talus and calcaneum were analysed. The calcaneum externally rotates, plantar-flexes and angulates into varus. The talus shows greater plantarflexion with similar varus angulation, with variable axial rotation. Relative talo-calcaneal motion thus involves, 6o relative talar internal rotation, 3.2o flexion and no motion in the frontal plane. Concurrently the talus moves laterally on the calcaneum, by 6.5mm, with variable translations in other planes. The calcaneum plantar-flexes, undergoes valgus angulation, and shows variable rotation in the axial plane. The talus plantar-flexes less, externally rotates, and shifts into varus. Relative motion in the axial and saggital plane reverses rotations seen during inversion. The 8o of relative valgus talo-calcaneal angulation is achieved through considerable varus angulation of the talus, in a direction opposite to the input motion. This phenomenon has not been previously reported. From coronal MRI data, comparative talo-calcaneal motion in inversion is prevented by high bony congruity, whereas during eversion, the taut posterior tibio-talar ligament appears to prevent talar valgus angulation. We have demonstrated that Interventional MRI scanning is a valuable tool in analysing the weight-bearing motion of the talo-calcaneal joint, whilst approaching the diagnostic accuracy of stereophotogammetry. We have also demonstrated consistent unexpected talar motion in the frontal plane. Talo-calcaneal motion is highly complex involving simultaneous rotation and translation, and hence calculations of instantaneous axes of rotation cannot effectively describe talo-calca-neal motion. We would suggest that relating individual and relative motion of the talus / calcaneum better describes subtalar kinematics.
The vertical configuration open MRI Scanner (Signa SPIO, General Electric) has been used to assess the place of interventional MR in the management of developmental dysplasia of the hip over the last four years. Twenty-six patients have been studied. In static mode, coronal and axial T1 – weighted spin echo images are initially obtained to assess the anatomy of the hip, followed by dynamic imaging in near-real time. In all cases, dynamic imaging was very good for assessing and demonstrating stability. The best position for containment can be assessed and a hip spica applied. Scanning in two planes gives more information and allows more accurate positioning than an arthrogram. Confirmation of location of the hip after application of the spica can be easily demonstrated. Adductor tenotomies have been performed within the imaging volume, and in two cases, this enabled planning of femoral osteotomies. All patients have had a satisfactory outcome, but five have required open reduction and a Salter innominate osteotomy. In ten cases, the opportunity has also arisen to alternative perform an arthrogram, either because of the complexity of the cases, or at a later date as an alternative to a repeat MRI, or because of difficulty with access to the machine. The place of interventional MRI in DDH is not yet defined. As machines get better and the definition improves, the amount of information about the nature of dislocation, the relative size of the acetabulum to the femoral head, the state of the limbus, the best position for containment and stability, and the potential for growth of the acetabulum, particularly posteriorly will be increased. It follows that the potential for more accurate definition of each hip and the outcome is better – and safer – than by arthrography, which remains the ‘gold standard’ but involves radiation and is only one-dimensional.
