Ultrasonography is an extremely useful tool in the diagnosis and management of a number of conditions affecting the pelvis of horses, augmenting and, in some instances, replacing the established techniques of radiography and scintigraphy. The pelvis can be imaged percutaneously from the dorsal aspect, but also per rectum. Ultrasonographic assessment is useful in a wide variety of investigations, from subtle performance-limiting problems in sports horses to severe lameness in racehorses.

Equipment

Clipping is often necessary, although not in fine-coated horses. A high-frequency linear probe is the most useful for superficial structures but imaging the pelvis also requires a lower-frequency curvilinear or sector probe. Imaging can be difficult in patients with significant subcutaneous fat or thick skin. A rectal probe is necessary for internal examinations.

Indications

Reasons for performing an ultrasound assessment of the pelvis are varied but the commonest indication in non-Thoroughbred practice is evaluation of reduced performance. As for examination of the back and neck, these investigations can be time consuming and frustrating, but the use of ultrasonography enables a greater number of differential diagnoses to be considered. Other indications include lameness suspected to arise from the upper limb that has eluded diagnosis by distal anesthesia or is known to have been associated with a fall or other trauma. In Thoroughbred racehorses, indications also include evaluation of stress fractures known to occur in the pelvis.

The main areas of interest in the pelvis are:

• ilial wings and shafts;
  
• tubera sacrale, tubera ischii, third trochanters, and tubera coxae;
  
• hip joints;
  
• the lumbosacral intercentral and intertransverse joints and sacroiliac joints;
  
• internal bony structures (per rectum).

The Ilium

The most important reason for imaging the ilium is the detection of fractures. In racehorses, these occur most frequently as fatigue injuries (stress fractures) caused by the accumulation of damage with cyclical loading and the vast majority originate on the caudal aspect of the ilial wing, close to the sacroiliac joint and the junction of the wing and shaft of the ilium. As they are caused by cumulative damage, horses often show extensive signs of injury on ultrasonography by the time they are presented for lameness examination or poor performance. However, while ultrasonography is very useful in the detection of these injuries, a normal ultrasound scan does not rule out injury to this region – some horses have “hotspots” on a bone scan that have no abnormalities ultrasonographically. In other types of horse, the most common reason for imaging the ilium is because of lameness thought to be caused by trauma to the pelvis – displacement of the tuber coxa or “knocked down hip” being the most common.

Figure 8.1 shows the technique described by Shepherd and Pilsworth to map the ilial wing by imagining four lines superimposed on the pelvis. The transducer is placed just off midline, holding the probe at 90° to the midline on the side of interest, and moved towards the tuber coxa slowly maintaining the image of the bone surface as a smooth, continuous echogenic line (Figure 8.2). It appears as a sweeping curve running from the tuber sacrale to the tuber coxa. It is important that the examination should cover the entire surface of the ilial wing, as stress fractures may produce pathology in a localized region – typically the caudal aspect – so that a single image may miss these injuries. The operator must ensure that, particularly at the cranial and caudal sites (lines 1 and 3 in the illustration), the edges of the bone are imaged. This can be achieved by tilting the probe forwards and backwards so that the bone surface temporarily disappears from view at these sites.

By following a systematic approach, incomplete fractures will not be missed. Be aware of edge artifacts produced by the many large vessels and fascial planes between the muscles – these can give the impression that there is discontinuity in the surface of the bone. Subtle injuries may present with only minor changes to the contour of the bone surface (Figure 8.3) and in some cases it is possible to identify pathology of different ages – recent, sharply defined fracture lines cranially and more longstanding callus caudally (Figure 8.4). Immature callus will appear irregular and allow some of the ultrasound beam to pass through. Mature callus is smooth and of equal echogenicity to the adjacent, normal bone. As fractures heal the development of callus can be followed, as its contour changes from convex to concave and it becomes more echogenic with maturation. This is useful as fractures are monitored during recuperation.

Rotating the probe to image the ilial wing in a para-sagittal plane is sometimes interesting, but its main use is to identify the ilial shaft easily – the smooth caudal edge of the wing becomes the long smooth surface of the shaft and can be followed down to the hip joint. Ilial shaft fractures can be difficult to diagnose unless they are complete – unlike ilial wing fractures, early changes to the bone that precede complete failure are rarely detected. When complete, the normal linear contour to the bone surface that is visualized by running the transducer from cranial to caudal is interrupted, often with a step in the surface evident. This is frequently accompanied by significant hemorrhage. Young animals can suffer “green stick” fractures of the ilial shaft.

Tubera Sacrale, Ischii, and Coxae

Imaging the bony prominences of the pelvis is best done with a combination of linear high frequency and lower frequency curvilinear transducers.

The tubera sacrale can be identified, along with the dorsal parts of the dorsal sacroiliac ligaments, in transverse and longitudinal planes. Asymmetry of these structures is quite common and does not correlate well with clinical disease.

Serving as the attachment for the large biceps femoris, semimembranosus, and semitendinosus muscles (all of which can be imaged), the tuber ischium can be injured as a result of direct trauma (frequently a fall) or as an avulsion injury in athletic horses. The bony prominences are easily palpated either side of the tail head and can be imaged horizontally and vertically. Although they have a certain degree of roughening in normal horses, they present a continuous surface readily amenable to ultrasound imaging. Careful comparison with the opposite side is useful – most injuries seem to involve displacement of bone, so roughening is rarely the only sign. Injury usually results in significant disruption to the normal bone contour and in some cases clear fragmentation (Figure 8.5).

Fractures to the tuber coxa also occur as a result of direct trauma or as an exercise-induced avulsion injury. They result in the typical “knocked down hip” appearance, and the sharp edge of the parent bone may, rarely, erode through the skin. Although not usually a challenge to diagnose clinically, information regarding fracture configuration can be obtained through ultrasonography (Figures 8.6 and 8.7). It is also useful to scan from the fracture towards midline and attempt to reconstruct a mental map of the fracture configuration, particularly with regard as to whether the injury affects the shaft of the ilium. Some racehorses will suffer severe lameness caused by avulsion injury of the tuber coxa, but the typical clinical appearance associated with displacement of the fragment may not become evident for several days after the onset of lameness.

Third Trochanter

A much more common site of injury than is widely recognized, the third trochanter serves as the insertion for the superficial gluteal and its tendon which is easily visualized attaching to this bony prominence in the upper femur. Injuries typically occur in a fall but sometimes present as an avulsion. The area is easily palpated and amenable to percutaneous ultrasonography so do not forget to include this when assessing sudden-onset upper limb lameness (Figure 8.8

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