IN-DEPTH: THE RELATIONSHIP BETWEEN IMAGING FINDINGS AND LAMENESS


triangular in shape. Quarter Horses normally have smaller synovial invaginations than some other breeds such as Warmbloods and should be assessed as such. Even within the breed there is a range of normal and therefore comparison between limbs is helpful to identify some changes. The sig- nificance of dilated synovial invaginations remains a source of debate and depends on the degree of change. Recently it has been suggested that syno- vial invagination dilation may be associated with synovitis of the distal interphalangeal joint rather than primary navicular disease, although this con- cept needs further exploration.2 In the author’s opinion, mild-to-moderate synovial invagination di- lation in the absence of other abnormalities in the bone can be incidental and may not change over time. Dilation of the synovial invaginations be- comes a concern when the enlargement results in cyst-like dilation or extends to the palmar endos- teum of the flexor cortex. Enlarged synovial in- vaginations that extend to the palmar aspect can cause pressure resorption of the flexor cortex, result- ing in a flexor cortical defect, which is a clinically relevant finding.


Distal Border Fragments


Quarter Horses are a small component of the popu- lation in the reported studies regarding the signifi- cance of distal border fragments. The literature is mixed on the significance of these findings; the study with the greatest number of Quarter Horses did not show a significant relationship between distal bor- der fragments identified on magnetic resonance im- aging (MRI) and lameness.3 On MRI, fragments can be seen on the distal border of otherwise com- pletely normal navicular bones or can also be a com- ponent of other navicular apparatus changes. The significance is likely a continuum of severity with other changes, and in isolation, these fragments are not likely to be a primary source of lameness.


Proximal Vascular Channels


Dilation of the proximal vascular supply of the na- vicular bones can be difficult to identify radiograph- ically until advanced changes are present, but observing the shape of the navicular bone on the lateral view is helpful to identify an increased concave margin to the proximal border of the na- vicular bone. This change is more readily identi- fied with MRI. Dilation of the proximal vascular channels is rarely found incidentally, in sound limbs, or in isolation and is an indication of navic- ular disease that is clinically significant. In- creased size and number of the proximal vascular channels has been shown to be associated with lesions of the flexor cortex and deep digital flexor tendon.4


Sclerosis


Trabecular and endosteal sclerosis of the navicular bone are significant indicators of navicular disease


40 2020  Vol. 66  AAEP PROCEEDINGS


Fig. 1. Navicular skyline view of an 8-year-old Quarter Horse used for roping. Radiographic changes include marked sclerosis of the spongiosa, decreased corticomedually distinction and lu- cencies consistent with flexor cortical lysis medial and lateral of the sagittal ridge.


and often accompany palmar fibrocartilage damage and/or flexor cortical bone loss (Fig. 1). This can be a challenge for image interpretation as positional artifacts can mimic sclerosis. As this is a clinically significant finding when real, it is important to dis- tinguish artifact from true change. The finding should be repeatable on both the lateral and skyline radiograph; if seen on only one view, it is more likely artifact.


Flexor Cortical Lysis


Flexor cortical erosions are a clinically significant finding that are routinely associated with lameness. While small flexor cortex lesions can be found in the contralateral non or less lame leg on a bilateral MRI or computed tomography (CT) foot study, it is rare to find these lesions incidentally in horses that are in full work and completely sound. While some horses with these lesions can be serviceably sound and manageable for a period of time, generally these lesions progress and often have other comorbidities such as damage to the deep digital flexor tendon and navicular bursitis.


Deep Digital Flexor Tendinopathy


Damage to the deep digital flexor tendon can occur in conjunction with navicular disease or in isolation. While horses can manage for a while with some degree of tendon damage, lesions often progress or worsen with continued work. Dorsal border fray- ing is most frequently found at the level of the prox- imal recess of the navicular bursa and has been reported to have improved likelihood for return to work than other lesions types.5 Deep digital flexor tendon damage is much more likely to be found in lame horses than sound horses, indicating that it is rarely an incidental finding.6 An important prac- tice note is that deep digital flexor tendon core le-


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