The aim of this study was to determine the sonographic appearance especially of the short parts of the tibiotarsal collateral ligament in both extended and flexed limbs. Attention was given as to whether examination in either extension or flexion would change the appearance of and/or aid identification and assessment of the individual ligaments.
Tarsocrural collateral ligament (functional) anatomy and the appearance of collateral ligament injury as detected by different imaging modalities have been described in the literature.
Seven pairs of tarsi of horses that had been euthanized for reasons other than hind limb lameness were isolated and prepared for sonography. Ultrasonography was performed using a linear high-frequency transducer. The long and short parts of the medial and lateral collateral ligaments of each specimen were examined in extension and flexion. Criteria for evaluation were visibility of origin, body and insertion of each individual ligament according to visibility of organized fibers and clear margins. Images were recorded of the origin, the body, and the insertion of each ligament and analyzed.
All the long collateral ligaments (n = 28) were imaged in all joints left and right, medially and laterally, and in extension and flexion. Overall, the three parts of the short collateral ligament were visualized more easily on the medial aspect of the limb, both in extension 76.6% and in flexion 87.8%. On the lateral aspect, 59.6% of the short collateral ligament was visualized in extension and only 54.4% in flexion. The middle part of the short collateral ligament was most consistently entirely imaged; in flexion 90% medially and 92% laterally and in extension 94.6% medially and 83.6% laterally. The profound part of the short collateral ligament was most difficult to visualize, its mid-part was imaged in 83.5% on the medial aspect in flexion. The mid-part of the short collateral ligament showed mild torsion of its fibers in extension (when it is relaxed) and parallel fiber arrangement in flexion. Imaging was hampered in some cases by subcutaneous and intrafascial gas accumulation due to the preparation of the specimens.
Ultrasonographic examination allows identification of the individual subsections of the short tarsocrural collateral ligament. The long collateral ligament is comfortably identified and completely imaged. Flexion of the tarsus helps to identify the individual parts of the short collateral ligaments, especially on the medial aspect of the tarsus. It may be beneficial to scan the tarsocrural collateral ligaments in flexion especially if injury to the short part is suspected. Whether this applies to the live equine patient and to injured collateral ligaments too is currently under investigation.