Clinical Examination

A thorough working knowledge of the anatomy of the equine pelvis is essential for clinical examination to be useful. Because of the large muscle mass over the horse’s hindquarters, only the bony extremities of the pelvis can be palpated. However, it is often possible to gain information about horses with pelvic injuries by studying the position of these bony landmarks. For example, the normal position and angle of the tuber coxae in the racehorse are often disturbed in horses with fracture or sacroiliac joint instability. The position of the tubera coxae can be assessed by viewing the horse from behind, with an assistant placing fingertips on the craniodorsal extremity of the tubera coxae (Figure 49-1). It is important that the horse stands completely level, with both hind feet together, for this test to be meaningful. Some horses, however, show asymmetry viewed in this way that is not linked to lameness. Similarly, careful palpation of the tubera sacrale can give information about the possible involvement of the sacral wing of the ilium or disruption of the sacroiliac joint.

Fig. 49-1 Assessment of the position of the bony pelvis by fingertip levels placed on the dorsal extremity of the tubera coxae on either side.

Ventral displacement of one tuber sacrale is commonly encountered in ilial wing fractures, where the overlap of the fracture fragments seems to allow the tuber sacrale to move ventrally. Often a pain response is associated with palpation of a horse with tuber sacrale displacement, and sometimes movement of the bone itself may be felt if the fracture is complete. Fracture of a tuber coxae is often produced by external trauma, usually after a fall, but can also occur as a stress-related athletic injury. This usually results in a cranioventral displacement of the fracture fragment because of the distractive forces of associated musculature. The tuber coxae in these horses can often be felt situated in the sublumbar fossa, and the remnant fracture bed can be palpated at the original site. Fractures of the ischium can sometimes be felt by manual palpation, although the extensive muscle spasm and protective boarding that are associated with these fractures often preclude this examination. Usually there is hemorrhage and swelling in the acute phase, but a clear loss of muscle mass or even a “hollow” in the caudal contour of the rump may develop with time as inflammation subsides. Finally, muscle tone in the tail and anus should be evaluated because fractures involving the sacrum can involve neural elements that supply these structures and cause flaccid paralysis of the tail, rectum, anal ring, and vulva (in a filly), that is, the cauda equina syndrome (Figure 49-2). Bilateral ilial wing fractures can produce the same neurological appearance associated with severe nerve root damage consequent on movement of the pelvis in relation to the sacrum.

Fig. 49-2 A horse with a sacral fracture. Note the abrupt angle change just behind the tubera sacrale, where the sacrum and coccyx have moved ventrally. The tail has complete flaccid paralysis. This filly had urine and fecal retention.

Rectal examination allows direct manual assessment of the integrity of the pubis, internal surface of the wing of the ilium, and ventral border of the sacroiliac joint. Sometimes an obvious, sharp discontinuity in the bone surface can be felt, particularly in horses with fracture of the pubis. Gentle rocking of the horse by an assistant, while the clinician maintains digital contact with the bone surface of the pelvis per rectum, sometimes allows an appreciation of relative movement of adjacent bones or, more commonly, a sensation of crepitus. In horses with fracture of the ilial wing, a soft asymmetrical swelling can often be felt at the fracture site, representing a subfascial hematoma. The more serious and potentially fatal hemorrhage that occurs when an iliac artery is severed by the sharp dorsal edge of the fractured ilium often cannot be detected per rectum. If not immediately fatal, this free blood often percolates ventrally to cause massive swelling and edema of the thigh musculature.

Some horses have several stress fractures identified scintigraphically, only one of which may have initially collapsed, leading to overt lameness. Progressive collapse of the pelvis may then occur during the convalescent period, as incomplete fractures become complete and displaced because of bone resorption and weakening of the fracture site. The degree of lameness seen in these horses varies enormously, depending on the type and extent of the fracture, and is considered separately in the following discussions of each class of fracture.

Diagnostic Ultrasonography

Ultrasonography is useful for diagnosing pelvic fractures and has proved especially useful in demonstrating fractures of the ilial wing (Figure 49-3), ilial shaft, tuber coxae, and ischium. Ultrasonography is quick, easy, and within the capability of clinicians with a suitable ultrasound machine. Ultrasonography may eliminate the requirement for a horse to travel to a referral center for diagnosis, and the risk associated with radiography under general anesthesia can be avoided. Ultrasonography has obvious limitations. For example, adequate imaging of the sacral wing, sacroiliac joint, and the femoral head is not possible. Fractures with minimal displacement or poorly developed callus are also difficult to image, as are incomplete fractures involving the ventral surface of the ilium. For this reason, ultrasonography should not be regarded as a standalone imaging modality for identifying a fracture of the pelvis but should be used with a thorough clinical examination and, if available, scintigraphy. In many horses, the exact site and extent of the fracture can be determined, which allow improved prognostic and management advice to be given. The healing process can be monitored by serial examinations, allowing the management program to be tailored to the individual horse (see Figure 49-3, B and C). A longitudinal- or sector-array ultrasound transducer can be used, provided it has a deep enough penetration to see the bone surface (i.e., a 3.5- or 5-MHz transducer). The muscle mass lying above the bone structures acts as a natural “standoff,” bringing the bone surfaces into the focal zone of the ultrasound beam. A separate standoff may be required to evaluate the position of the tubera sacrale and to detect any displacement. In thin-coated horses, no clipping is required, provided adequate saturation of the coat is achieved by degreasing with a detergent solution (chlorhexidine) or by soaking in surgical spirit, followed by application of a coupling gel before scanning. Horses with thicker hair coats must be clipped to obtain images of adequate quality. Images can be difficult to produce in horses with large amounts of subcutaneous fat because of the attenuating properties of this tissue. Numerous blood vessels running through the musculature can create acoustic shadows, which may be confused with a discontinuity of the bone surface. Identifying the bony landmarks such as the tubera sacrale, tubera coxae, cranial and caudal margins of the ilial wing, and greater trochanters of the femur allow anatomical orientation. A dry pelvic specimen is also useful in orientation. Both sides of the pelvis should be evaluated because the normal side can be used for comparison. However, keep in mind that bilateral ilial wing stress fractures occur and both sides may be abnormal. For recording and reference purposes, the area of the ilial wing imaged is referred to as line A, B, or C, and the distance from the tuber sacrale is measured. Scans aligned longitudinally along the ilial shaft are referred to as line D. This simple system is useful, especially in follow-up examinations. A systematic method for recording ultrasonographic findings has been published elsewhere.⁸

Fig. 49-3 A, Ultrasonographic image of the right ilial wing obtained using a 3- to 5-MHz transducer. There is a stress fracture of the ilial wing (arrow). The fracture appears as a discontinuity in the pelvis surface, with some comminution evident. Adjacent to the fracture is irregular echogenic material (callus), and just dorsal to the fracture is an anechogenic defect in the muscle (hemorrhage). Healing can be monitored by serial ultrasonographic examination, here at 63 days (B) and 109 days (C) following injury.

Radiography

Radiography of the pelvis is easily carried out in a foal when it can be performed under heavy sedation, for example, with a combination of detomidine and butorphanol, with the foal allowed to stand again shortly after the procedure. Standard projections include a ventrodorsal image with the foal lying on its back and a lateral image with the foal lying on its side on top of the cassette. A grid is mandatory to reduce scattered radiation produced by the large mass of soft tissue covering the pelvis. Radiography in adult horses is considerably more difficult. General anesthesia is required for thorough radiographic examination of the pelvis for numerous reasons, including the safety of the horse, personnel, and radiographic equipment. Administering general anesthesia to a horse suspected of having a pelvic fracture is contraindicated because of the possibility of the horse displacing the fracture on recovery. Fracture displacement can lead to fatal hemorrhage or worsening of clinical signs, which is particularly true in horses with an incomplete fracture of the wing and shaft of the ilium. It is also difficult to get high-quality radiographs of the pelvic bones without extensive previous starvation of the horse to allow emptying of the gastrointestinal tract. Small, incomplete, pelvic stress fractures are extremely difficult to see radiologically. Three techniques for standing radiography in the horse have been described.^9-11 However, in my experience this has proved difficult, presents real risks of damage to the equipment, and is a substantial radiation hazard to attending personnel. The standing examination is also limited to examination of the acetabulum and ilial shaft, and images of the ilial wing are obscured by the sublumbar and overlying dorsal lumbar musculature. Image quality obtained in this way is also often poor; however, it may permit detection of substantially displaced fractures. Although radiography used to be the imaging technique of choice for assessing horses with pelvic fractures, it has been replaced by combined ultrasonographic and scintigraphic evaluation in many hospitals.

Scintigraphy

Scintigraphy is considered the most sensitive method of assessing acute bone damage in the horse (see Chapter 19).^3,12-17