Ultrasonograpy of the Equine Foot
soaking, but it is generally advisable to soak the foot in advance whenever possible; likewise, a few conformational types are not amenable to obtaining a diagnostic quality image. If one is able to depress the frog slightly with digital palpation then it should be amenable to sonographic imaging. If it feels rock hard or has unusually irregular conformation, imaging may be difficult to impossible. Preparation of both feet is recommended for purposes of contralateral limb comparison, bearing in mind however that many foot lamenesses are bilateral and considerable variation in foot conformation may exist, making comparison to a contralateral “control” more difficult.
Care should be taken with probe orientation to facilitate anatomical recognition. The structures are most recognizable when imaged in a sagittal plane initially, with the marker thus oriented toward the toe or the heel region. It can then be slid medially and laterally (parasagittal planes) as much as the frog allows. Transverse views should also be attempted. The probe should be slid dorsally and caudally along the frog, or otherwise manipulated as necessary, in order to center over the specific structure (navicular, impar ligament, DDFT insertion) being evaluated and minimize incidence artifacts.
The digital cushion is the most palmar structure, about 1-2 cm depth. It may be fairly uniformly mildly echoic or may present as having an echoic outer rim surrounding a more hypoechoic central region. The next most prominent structures are typically the navicular bone and P3. The navicular bone appears as the short, hyperechoic bony echo which is located superficially (palmarly) to the bony echo of P3. Extending from the shadow of the navicular bone will be the bony echo of P3, which will course superficially to a very shallow depth towards the toe.
The DDFT will be visible as a linear, hypoechoic structure deep to the digital cushion and overlying the navicular bone. It can be seen coursing over the navicular bone to its insertion on P3. Distinct fiber pattern is more visible towards its insertion on P3. Playing with the probe angle over the DDFT (to minimize a poor incidence angle) aids in identifying fiber pattern. In my experience there is some variation in the normally detected fiber pattern of the DDFT amongst horses, thus evaluation of the contralateral limb is advisable in most cases. Likewise, the palmar margin of the DDFT can appear hypoechoic in some horses, and contralateral limb evaluation is also advisable in these cases. Immediately over the navicular bone, the DDFT is more fibro cartilaginous and thus tends to appear more hypoechoic with less discernable fiber pattern. Combined thickness of the DDFT and distal digital annular ligament in clinically normal feet are reported to be 4.26 +/- .19 cm (Grewal and McClure, 2004). Width of the DDFT proximal to insertion has been reported by Sage and Turner, 2002 as .36 +/-.05 cm.
The impar ligament is visualized deep to the DDFT, in the roughly triangular region between the navicular bone and P3. If an increased volume of effusion is present in the navicular bursa, it is easier to identify. It should demonstrate a fiber pattern similar that of other ligaments. Thicknesses have been reported to be 2-3 mm (Busoni and Denoix, 2001) and 3.19 +/-.23 mm (Grewal and McClure, 2004) in a sagittal plane. The area has been reported as .22cm squared (Sage and Turner, 2002). Anechoic fluid dorsal to the deep flexor tendon represents effusion in the navicular bursa.