Page 454 - Adams and Stashak's Lameness in Horses, 7th Edition

420 Chapter 3

MRI evidence of traumatic bone injury is not only lim­ absence of ultrasonographic abnormalities. 41,93,150 MRI
ited to the proximal palmar/plantar aspect of the third is not used routinely for the diagnosis of injuries of the
VetBooks.ir tered in the dorsal third metacarpal/metatarsal bone, as done with ultrasonography. However, MRI may be more
flexor tendons in the metacarpal region as this is mainly
metacarpal/metatarsal bones but also has been encoun­
well as in the proximal aspect of the splint bones.
sensitive than ultrasonography for monitoring tendon
29,93
repair. 86,88,160 The T2 versus T1 signal differences in heal­
ing tendon may prove to be helpful in planning the reha­
Desmopathy of the Accessory Ligament of the Deep Digital bilitation of horses with tendon injuries more
Flexor Tendon
accurately. 160,190
The accessory ligament of the DDFT often has het­ Osseous and soft tissue abnormalities of the second
erogeneous signal intensity when examined in either a and fourth metacarpal bones (exostosis, synostosis,
125
high‐field or a low‐field magnet. In a cadaver MR osteitis) and their interosseous ligaments (syndesmop­
imaging study, the majority of accessory ligaments had athy) have been reported as very common in two stud­
low to intermediate signal intensity but contained ies of proximal metacarpal MRI. 5,128 These include
oblique bands of high signal intensity crisscrossing irregularities of the periosteal and endosteal aspects of
through the ligament. This was thought to be caused by the cortex, osseous fluid, and/or sclerosis, increased
the presence of thick collagen bundles arranged in large intraligamentous STIR signal and enthesophyte forma­
crimps, crossing obliquely and randomly through this tion at the origin, and insertion of the interosseous
ligament, thereby causing extensive magic angle ligaments. Synostosis may lead to osseous fusion
125
effects. Moreover, the cross‐sectional area of the between the second or fourth and the third metacarpal
accessory ligament varied widely ranging from 68.1 to bones. These abnormalities are common in young
2
299 mm in its most proximal 7 cm. In addition, the horses, especially endurance and race horses, and their
authors cautioned that the presence of normally attached clinical significance as a cause of lameness is not
fibrous bands and the large variability in signal intensity always clear.
and size of the accessory ligament in non‐lame horses
should be borne in mind when interpreting MR images
of lame horses. 125
MRI signs of desmitis were size increase and focal to
diffuse intraligamentous signal hyperintensity. Lesions MAGNETIC RESONANCE IMAGING
28
generally extend from 1 to 4 cm distal to the carpometa­ OF THE CARPAL REGION
carpal joint. Comparison with the contralateral limb
was considered essential to allow detection of subtle sig­ Introduction
nal and size changes. In this study, desmitis of the acces­ Practical problems may arise when scanning limb
sory ligament was the most common MRI finding in areas proximal to the third metacarpal and metatarsal
horses with proximal metacarpal region lameness. regions with both high‐field magnets or low‐field stand­
However, this observation has not been supported by ing systems. It can be difficult to position the carpus or
others, 5,128 who never mentioned desmitis of the accessory tarsus in the isocenter of most long, closed‐bore, high‐
ligament when discussing injuries in the proximal meta­ field magnets. When MR images are obtained on
carpal region. standing sedated horses with low‐field open magnets,
Abnormalities of the DDFT or the suspensory liga­ increased movement at these proximal levels can render
ment may accompany desmitis of the accessory liga­ diagnostic quality imaging difficult, in spite of advances
ment. In horses with chronic desmitis, irregularity of in motion correction software. This problem does not
142
the borders of the ligament and loss of the distinct high appear to occur with the horse’s limb positioned under
signal border between the accessory ligament and the general anesthesia in an open low‐field magnet, pro­
DDFT may occur. Low signal scar tissue bridging the vided that the magnet opening is sufficiently wide to
hyperintense space between the ligament and the flexor accommodate the area of interest.
tendons may be indicative of adhesion formation but Most carpal injuries can be sufficiently evaluated
must be distinguished from the normal fibrous bands on using radiography, ultrasonography, and scintigraphy.
the lateral aspect of the ligament. 125 Consequently, few clinical reports of in vivo MRI results
in horses are available, 128,146,175 and cadaveric studies
have been used to describe the normal low‐ and high‐
Other Abnormalities
field MRI anatomy of and signal distribution in the car­
Other injuries have been reported in horses with pus. 71,116,120,126 MRI was able to identify several ligaments
lameness localized to the proximal palmar/plantar meta­ that cannot be shown with other imaging techniques
carpal/metatarsal region with diagnostic analgesia with (except CT arthrography), including the transverse
a much lower incidence than those listed above intercarpal ligaments, the radiocarpal ligament, the
(Tables 3.6 and 3.7). Mostly these findings involve the short palmar carpal ligaments, and the carpometacarpal
distal tarsal and carpal joints (see “MRI Abnormalities ligaments. 71,126 Transverse MRI images are typically
of the Carpal Region and MRI Abnormalities of the most useful to evaluate the soft tissues of the carpal
Tarsal Region”) and indicate the lack of specificity of region.
diagnostic analgesia of the proximal metacarpal and Even though diagnosis of carpal lameness can usually
metatarsal regions. 6,93 be made with conventional imaging modalities, MRI of
Occasionally focal fiber disruption without thicken­ the carpus may be useful when no imaging abnormali­
ing is recognized with MRI in the SDFT or DDFT in the ties are present, when the degree of lameness does not