Bundles of fibers torn away from the surface of the tendon have a tendency to curl
up proximally in the navicular bursa. Crevices, splits and fibrillations of the dorsal
border of the DDFT may be accompanied by either small, discrete or large,
circumscribed foci of necrosis. Chondrocyte clusters can be seen to produce
chondroid matrix in areas of degenerative change.
Degenerative vascular changes consisting of thrombosis and occlusion of septal
arteries and veins are also seen in the distal portion of the DDFT. They were initially
described with equal frequency in horses with clinical navicular disease and agematched control horses. Although it was suggested that these vascular changes
could be age-related, larger and later pathological studies have found that vascular
changes in the DDFT were significantly more common in the DDFT of horses with
foot pain. It was therefore proposed that vascular thrombosis and occlusion could
result in matrix changes that predispose horses to injury of the distal portion of the
DDFT. As these changes are predominantly seen in the intratendinous septa, there
is a strong possibility that they predispose to the development of sagittal splits in the
dorsal surface of the tendon along these septal planes. The lack of any histological
evidence of hemorrhage or inflammatory cell infiltration in core lesions, splits and
abrasions of the distal portion of the DDFT adds further support to the notion that
these lesions may be primarily degenerative in nature.
MR signal intensity varies between different echo sequences not only with severity
but also with duration of a tendon lesion. Acute lesions generally have higher T2
signal intensity due to the presence of fluid and increased cellularity, whereas fibrous
scar tissue in chronic lesions produces a more intermediate to low T2 signal
intensity. It may therefore be possible to use T1-to-T2 signal differences to estimate
the age or stage of healing of a tendon lesion. In the chronic stages of healing by
fibroplasia, signal intensity in core lesions generally decreases in T2 and STIR
images but can remain high in T1-weighted sequences. It was suggested that T2weighted imaging may be more sensitive for detecting the transition of blood-rich
immature granulation tissue to fibrous scar tissue.
Navicular Bursa
Villous hypertrophy, hyperplasia of synovial cells and venous congestio n have been
described in the navicular bursa of horses with navicular disease. There was a
positive association between histological abnormalities of the bursa and lesions of
either the dorsal aspect of the DDFT or the flexor aspect of the navicular bone.
Distal Sesamoidean Impar Ligament
Ageing changes have been described in the impar ligament, characterised by
change in fibroblast shape and increased proteoglycan content. Evidence of
inflammation was also seen at the intersection of the DSIL and DDFT in horses with
navicular syndrome. The functional significance of these findings remains unknown.
Using MRI, injury to the impar ligament is generally seen as part of an injury complex
that includes other components of the navicular apparatus. One histopathological
study limited the histological grading of impar ligament abnormality to the presence
and extent of fibrocartilaginous metaplasia, that was more extensive in diseased than
control limbs. In a more recent report, the histological changes in the mid body of the
impar ligament included degeneration of collagen, loss of fibroblasts, fibrocartilage
metaplasia and reduction in vascularity. These changes were well correlated with
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
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