Supraspinous and Interspinous Ligaments

The tough, fibrous SSpL connects the summits of the dorsal spinous processes and is easily visualized using a linear transducer. Its appearance is similar to other ligaments or tendons, with a striated fiber pattern evident when viewed longitudinally. The ligament lies in close apposition to the interspinous ligaments, whose fibers may run at a different angle, and this can cause off-incidence artifacts if care is not taken. The correct technique involves dynamic assessment of the ligament, tilting the probe forward and backward along its long axis to appreciate the slight curve/angle change of the fibers as they insert onto the DSPs (a standoff pad is therefore very useful). In addition, care should be taken that it is the SSpL being imaged, as the strong aponeurosis of the longissimus muscle has a prominent tendon-like appearance just to the left and right of midline (Figure 9.1). Transverse images of the ligament can be obtained simply by rotating the probe and are useful to corroborate potential injury identified longitudinally.

The supraspinous ligament often appears hyperechogenic relative to the interspinous ligament and this can lead to errors in concluding that there is damage to the SSpL, when in fact it is normal interspinous ligament tissue. As well as this, the fibers of the SSpL alter their angle of orientation slightly as they insert upon the oblique dorsal surfaces of the DSPs. This can lead to misinterpretation due to off-incidence artifacts. The SSpL is larger and more fibrous as it progresses caudally; images in the lumbar region are generally more consistent between individuals but there is a wide variation in “normal” appearance. This may be due to operator factors, anatomical idiosyncrasy, or previous but currently insignificant injury.

Interpretation of potential lesions follows the same basic principles as with other ligaments or tendons, with attention paid to size, echogenicity, and fiber pattern particularly (Figure 9.2). A certain degree of caution needs to be exercised when interpreting the appearance of the SSpL: it seems that a degree of dystrophic, possibly age-related change, is evident in some horses and not necessarily indicative of disease. In addition, sites of injury rarely recover a normal appearance and regions of hypoechogenicity may not relate to current pathology. In almost all cases, evidence of injury will need to be confirmed with other techniques, such as diagnostic anesthesia.

It is easy enough to infiltrate local anesthetic around the “injury” and ascertain whether this alters the horse’s movement/behavior, although there can be difficulties in reproducing that behavior consistently enough to allow this. Bear in mind that the ultrasonographic appearance of most SSpL injuries changes little with time and false positives can be a problem. This also makes follow-up of genuine lesions difficult. SSpL abnormalities are common in areas of significant impingement of the dorsal spinous processes.

An assessment of the health of the DSPs is also possible during examination of the SSpL, in relation to their dorsal bony contour (well-spaced or close, smooth or roughened, presence of osteophytes, etc.). However, as only the very superficial part of these structures can be visualized, the technique is only supplementary to radiography and scintigraphy.

It is practical to begin by performing a rapid, general appraisal of the ligament, running from cranial to caudal in the longitudinal plane. After this, move back to the cranial extent of the prepared area and scan longitudinally, identifying and recording areas of interest. If pathology is suspected, transverse scans (Figure 9.3) are used to try to ascertain the significance of changes. Suspicious areas should be marked, most easily by a short clip in the coat, to allow subsequent diagnostic anesthesia.

The Caudal Thoracic and Lumbar Facet Joints

The anatomy of the thoracic and lumbar joints differs makes imaging of the more cranial thoracic joints more difficult due to their proximity to the dorsal spinous processes (Figure 9.4). Fortunately, most pathology occurs in the last thoracic and cranial lumbar joints, which are easily imaged in horses without excessive subcutaneous fat or thick skin. The technique is compromised severely in patients with these last factors and in some horses it is impossible to obtain diagnostic images.

The technique aims to identify the articulation of the cranial articular process of one vertebra as it interdigitates with the caudal (and medial) process of the vertebra ahead of it (Figures 9.5 and 9.6). The first image to acquire is with the probe (a lower frequency curvilinear, ideally) at right angles to and just to the side of midline (Figure 9.7). By counting the ribs and following them up, the operator can get an idea of the vertebrae being imaged. Moving the probe backward (and, therefore, with the haircoat) the joints will appear and disappear from the field of view as the transducer heads caudally. Normal joints appear as the corner of a box, close to the junction of dorsal and transverse spinous processes. In thin-skinned horses and with high-quality equipment, it is common to be able to identify the individual cranial and caudal processes of each joint as well as the joint space. Disease of the intervertebral articulations (IVAs) results, regardless of the inciting cause being a stress fracture or osteoarthritis, in enlargement of the joint and the loss of this definition, with new bone production, which may progress to joint ankylosis (Figure 9.8). The ultrasound appearance is as if someone has stuck a ball onto the corner of the box, which takes on a rounded shape. It is important to compare adjacent joints on the same side of the horse to establish the validity of changes and also to compare the left and right sides.

Longitudinal images, obtained by placing the transducer parallel to and just to the side of midline, can enable adjacent joints to be imaged in the same field of view (Figure 9.9

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