Due to the close association of lameness, spinal movement and back pain, gait asymmetries should not be overlooked and both should be investigated and treated accordingly.
Diagnostic imaging modalities for the back include radiography, ultrasonography and scintigraphy (bone scan). Standard imaging modalities allow the assessment of the most relevant areas of the back. Gamma scintigraphy might be useful to further assess radiographic and ultrasonographic suspicions or findings. Furthermore, bone scan allows the assessment of bigger breeds, where standard imaging modalities are insufficient due to poor or non-diagnostic image quality. Ultrasonography of the articular process joints of the back is frequently compromised in horses with significant soft tissue coverage. It is important to carefully interpret imaging findings and confirm their clinical significance with diagnostic local anaesthesia or trial medications, where possible.
• Radiography allows the visualisation of the vertebral column, intervertebral articulations (IVAs/’facet joints’) and vertebral bodies to a varying degree, depending on location, equipment and patient size. Latero-lateral views as well as oblique projections can be obtained. Standard radiographs ideally include three to five overlapping latero-lateral radiographs using radiopaque markers, such as coins, for orientation. Collimated views of specific regions of interest can help to increase image quality by reducing scatter. Main limitations of the technique include attenuation and inability to visualise lumbar vertebral bodies. Care should be taken when interpreting or measuring interspinous spaces (ISS). Geometric distortion markedly affects the appearance of the spaces, depending on the shape and the distance of the dorsal spinous processes to the focal spot emitting X-ray beams (Djernæs et al 2017). Furthermore, the head position influences the distance between thoracic dorsal spinous processes and it is therefore advised to acquire back radiographs in a neutral position with the muzzle at shoulder level (Berner et al 2012). To further assess intervertebral articulations, oblique projections are obtained with the beam angled 20˚ from ventral to dorsal and centred 15-20cm ventral to dorsal midline (Butler et al. 2000; Girodroux et al. 2009). The X-ray plate should be aligned perpendicular to the beam and as close as possible to the horse. The highly-curved shape of the ribs is causing superimpositions with to the thoracic IVAs. In the lumbar spine there is superimposition of the transverse processes and abdominal structures.
• Ultrasonography of the back should be combined with the radiographic examination and is particularly useful to examine superficial soft-tissue structures and the intervertebral articulations. To assess all structures of interest, both linear and convex transducers are required.
For the supra- and interspinous ligaments (SSL and ISL) longitudinal views are most useful. The images are best acquired with a high-frequency linear transducer using a stand-off pad. The head should be kept in a neutral position as high head carriage may cause relaxation of the SSL and therefore a hypoechogenic appearance. Superficial fibres of the SSL have a horizontal orientation whereas deep fibres of the ISL run in caudoventral direction appearing hypoechogenic in comparison. Care should be taken to position the probe exactly midline at the level of the dorsal spinous processes. The aponeuroses of the epaxial musculature can be imaged off-midline with a parallel fibre pattern similar to the SSL.
In the thoracic (saddle) region the SSL is thin and wide, whereas further caudal in the lumbar region the ligament is thick and narrow with a more echogenic, homogenous appearance. The transverse view of the SSL is less useful due to being more prone to artifacts. In order to achieve the best image quality, the transducer should be tilted cranially and caudally. Published scientific data about the ultrasonographic appearance of the SSL in normal horses and horses with back pain offering information ab