Imaging the navicular region in the lame horse

Over the past decades, imaging has seen tremendous technologic advances that have transformed the practice of veterinary medicine with the arrival on the veterinary market of new technologies enabling to image the equine patient with unprecedented detail. When assessing horses’ feet, low-field standing magnetic resonance imaging systems have now become available in many equine veterinary clinics in Europe and through the world and have provided large benefits to assess the navicular region. Prior to the arrival of magnetic resonance imaging as a diagnostic tool in equine orthopaedics, the imaging approach to the navicular region was to assess the foot starting with radiology. Then either based on clinical data and radiographic examination rest or treatment was established, or, in a minority of cases and depending of the skills of the available ultrasonographer, ultrasonography was proposed as a complement. Today, radiology still remains the first modality applied on the equine foot to image the navicular region. Radiology is able to detect relatively late osseous changes and some of the previously considered significant radiographic findings have become questionable since magnetic resonance imaging demonstrated that lame horses presenting these abnormalities also have other major changes in surrounding structures. Radiographs are also very useful to assess foot conformation that can be of paramount importance in the management of horses with abnormalities located in the navicular region.

In contrary to other region of the horse’s limb, ultrasonography has not reached the same widespread use to assess the navicular region. This is mainly due to the technical difficulty of the application of this technique in such a region where the soft tissue structures are partially embedded into the hoof capsule and to the need of a second specific ultrasound probe (a microconvex) not really used for any other application in equine practice but absolutely essential for the assessment of the foot. Ultrasonographic examination of the foot requires approaching the anatomical structure contained in the hoof capsule using multiple approaches. The assessment of the palmar aspect of the foot by an approach through the distal pastern requires the use of a high frequency microconvex probe with a relatively small head but rarely the navicular bone is reached using this approach. The transcuneal approach that uses the frog as an acoustic window performed using either a 7.5MHz linear or a 5-7MHz convex transducer is used to assess the palmar navicular surface. Careful preparation of the foot is necessary to obtain good quality images: the frog has to be trimmed flat and has to soak in warm water for at least 15 minutes before ultrasonography. Despite technical limitation, ultrasonographic examination has a place in navicular imaging. Most suprasesamoidean deep digital flexor tendon lesions and the prodotrochlear bursitis giving an increase in size of the proximal palmar recesses of the bursa will be routinely visualized at ultrasonography and their detection will give useful information about their accessibility for ultrasound-guidance for treatment injections. Moreover ultrasound is very sensitive for flexor surface irregularity and may reduce the need of or clarify a sky-line view. [...]