HOW-TO SESSION: PRE-PURCHASE EXAM FOR THE WESTERN HORSE


with right forelimb lameness being more common than left forelimb lameness, and left hind lameness being more common than right hindlimb lameness.2 In addition, barrel racing horses have increased carpal disease and back pain as well as suspensory ligament injury and bruised feet.2,3 Similar to bar- rel horses, team roping horses have specific injury patterns.4 In heading horses, the right forelimb is most frequently affected, and bilateral forelimb lameness is more prevalent than in heeling horses, which have a higher incidence of bilateral hindlimb lameness. Associated with these specific injury patterns, heading horses have increased soft tissue injury in the pastern region, whereas heeling horses are more likely to have metatarsophalangeal joint disease.4 These discipline-specific patterns of in- jury should be considered when selecting and eval- uating imaging in a pre-purchase examination.


3. Radiographic Examination


There are differing opinions on the standard radio- graphic views used in the pre-purchase examination of the western performance horse. Traditionally, the most common views obtained were feet, hocks, and stifles. However, more commonly, a full series of radiographs is obtained, including the front and hind fetlocks and the cervical spine, in addi- tion to the feet, hocks, and stifles. The back is also being imaged with increasing frequency. The west- ern discipline being examined and its specific predilection sites of injury will factor into the selec- tion of radiographic views. Furthermore, the iden- tification of radiographic abnormalities may then necessitate additional radiographic views. The na- vicular bone is of particular interest in the western performance horse. Although radiographic evalua- tion of the navicular bone is by no means a new concept, radiograph evaluation and interpretation of the navicular bone bear further discussion. The navicular bone is evaluated on the standard


foot study. However, additional views may be help- ful in evaluating the flexor surface, an area of par- ticular interest due to the significance of injury in this region. The skyline view of the navicular bone in the foot study is typically obtained at 55 degrees. A detailed assessment of this view should be per- formed with evaluation of the flexor surface margin and density, the density of the trabecular bone (cor- ticomedullary junction), and the size of the synovial invaginations, as well as other anatomic features of the navicular bone. However, a more shallow sky- line view may actually highlight the area of the flexor surface that is most often affected with bone loss. A 30-degree skyline view has been shown to increase the visibility of density changes in the flexor surface at the mid to distal aspect of the navicular bone.5 The two, differently angled, sky- line views allow a more thorough assessment of the navicular bone flexor surface. However, be- cause of the increased obliquity of the 30-degree skyline view relative to the corticomedullary junc-


462 2019  Vol. 65  AAEP PROCEEDINGS


Fig. 1. Skyline navicular views (A–D) with increasing visibility of the distal interphalangeal joint space between the dorsal aspect of the navicular bone and the palmar aspect of the middle pha- lanx. The joint space is not visible on A. The joint space is partially visible on B and C, indicating the x-ray beam was aligned with the proximal or distal aspect of the joint. This can be identified by separation of the proximal and distal margins (arrows on B) of the navicular bone flexor surface and superimposition of a single margin over the palmar margin of the middle phalanx. However, in D the joint space is completely visible, which provides the most accurate representation of the trabecular bone density.


tion, the 55-degree skyline view remains important for evaluating the density of the trabecular bone and identifying sclerosis, which can be an indication of pathologic change. In clinical practice, navicular bones are often reported to be sclerotic radiograph- ically. However, corresponding magnetic reso- nance imaging (MRI) examinations frequently do not demonstrate increased density in the navicular bone. This discrepancy seems to be the result of radiographic obliquity and technique. The visibil- ity of the articulation between the middle phalanx and the dorsal margin of the navicular bone is one criterion used to evaluate the quality of the skyline view positioning. With good visibility of the joint space, the radiograph should accurately represent the anatomy. However, in many cases, the joint space appears visible, indicating good positioning of the skyline view, but on closer examination it be- comes evident that the x-ray beam was only angled through the joint space at one level. This can be determined by identifying two dorsal navicular bone margins, one proximal and one distal. In the cases with less than ideal positioning, one dorsal navicu- lar bone margin will be superimposed over the pal- mar aspect of the middle phalanx, which can result in the false appearance of increased trabecular bone density within the navicular bone (Fig. 1). Recogni- tion of this particular feature will aid in better po- sitioning of the skyline view and ideally lead to the production of a radiograph that will more accurately represent the trabecular bone density. When evaluating the navicular bone, an assess-


ment of the trabecular bone density should be per- formed on all available views. An assessment of


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