Ultrasonograpy of the Equine Foot
Ultrasound can provide important diagnostic information on the soft tissue and bony structures of the foot, despite some inherent limitations of field of view and variations in image quality. A thorough history, physical examination and lameness examination should be performed prior to the ultrasound examination (or any diagnostic imaging modality) and serves as an aid to defining a region of interest and in interpretation of findings.
COFFIN JOINT AND COLLATERALS
The collateral ligaments of the coffin joint can be visualized with a 7.5 MHz or higher linear transducer. In particularly large horses or those with unusual conformation or swelling, a lower frequency and/or curvilinear probe is often necessary. The dorsomedial and dorsolateral aspects of the coronary band should be clipped from the level of the coronary band to about 2-3 centimeters in a proximal direction.
The collateral ligaments of the coffin joint arise proximally from the depressions on either side (medial, lateral) of P2 and insert deep to the hoof capsule in the depressions on either side of the extensor process of P3. Thus, only the proximal portion (1/2 to 2/3rds of the ligament) is accessible to sonographic visualization. Nonetheless, lesions can be identified and routine ultrasound of the collaterals is recommended when evaluating lameness referable to the foot. Visualization through the coronary band is possible. The collaterals are fairly dorsally located on the foot, at about 11 and 1 on a clock face, with dorsal being 12 o’clock. They can and should be evaluated in both short and long axis, and followed as far distal as the hoof capsule allows. They appear as dense, echoic structures which are round to oval in shape in short axis and have a typical ligamentous fiber alignment in long axis. Due to difficulties in maintaining a perpendicular angle of incidence, a central hypoechoic artifact can rather easily be created in the short axis view. Careful technique and use of the sagittal view should help establish whether the region represents a lesion or an artifact. Some normal variation in shape/symmetry may occur in without representing a pathological process. Critique technique and use of contralateral limb comparison (when contralateral limb is not clinical) may aid in determining whether an active lesion is present. Cross sectional area of the medial and lateral collateral ligaments has been reported to be .63+/-.05 cm2 and .62+/-.04 cm2 respectively (Sage and Turner, 2002).
The synovial outpouching of the coffin joint itself appears as a slightly inhomogeneous hypoechoic region at the level of the coronary band. It is much more prominent and easy to identify when effusion is present. It is typically best seen dorsally, dorsomedially and dorsolaterally rather than palmar medial or lateral. Coffin joint effusion, however, can be appreciated from the palmar window, as will be discussed in the next section.
It is worth noting that Dyson (2004) reported that in 62 horses with desmitis of the collateral ligament of the DIP diagnosed on MRI, only 20 of those (32%) had lesions detectable on ultrasound, indicating a high rate of false negatives for evaluation of this structure. This should be kept in mind during evaluation of the collateral