Page 450 - Adams and Stashak's Lameness in Horses, 7th Edition

416 Chapter 3

MAGNETIC RESONANCE IMAGING OF to the proximal plantar metatarsal region with diagnos­
tic anesthesia illustrates the concern that local anesthetic
THE METACARPAL AND METATARSAL REGIONS
VetBooks.ir Introduction techniques lack specificity for accurate localization of
6,93
pain causing lameness in this area.
Lameness caused by pain associated with the proxi­
mal palmar metacarpal and plantar metatarsal regions
is common in equine athletes. Some of the causes of MRI Abnormalities in the Proximal Metacarpal/
lameness in this region are difficult to diagnose accu­ Metatarsal Regions
rately because affected horses frequently have no or Proximal Suspensory Desmopathy
equivocal radiological or ultrasonographic signs of
disease. It may thus not always be possible to distin­ The normal MRI anatomy of the fore and hind suspen­
guish between normal anatomy, imaging artifacts, and sory ligaments has been described in detail. 14,59,94,124,127 The
structural abnormalities of the suspensory ligament normal suspensory ligament is a low signal intensity
using ultrasonography. The tomographic nature of structure of hypointense dense collagen fibers, inter­
52
MRI allows assessment of size and signal changes spersed with two centrally located high signal intensity
within the complex anatomy of this region and results tissue bundles, that each contain sparse muscle fibers
in good soft tissue contrast and detail without surrounded by a layer of loose and adipose connective
superimposition. tissue and neurovascular structures. Normal cross‐sec­
There have been several reports with widely varying tional areas have been defined for the proximal part of
results on the incidence of injuries diagnosed with both the suspensory ligament in the forelimb and vary from
low‐ and high‐field MRI systems in the metacarpal and 1.61 to 1.90 cm in one study and from 1.94 to 2.32 in
2
5
metatarsal regions (Tables 3.5 and 3.6). 5,6,28,93,128 In another. There is considerable variation between
127
general, transverse MRI slices that are aligned perpen­ horses in the MRI appearance of the proximal portion
124
dicular to the long axis of the cannon bone (parallel of the forelimb suspensory ligament, due to the varia­
with the carpometacarpal/tarsometatarsal joint) are most tion in the amount, shape, and signal intensity of the
127
useful for diagnosis. muscle and fat tissue bundles. Within these bundles,
In the forelimb (Table 3.5), the reported distribution high signal intensity areas correspond to adipose tissue
of MRI abnormalities appears to depend on field and intermediate signal intensity areas to muscle tissue.
strength of the system used. A very low incidence of soft The large variability in the MRI appearance of the prox­
tissue injury in the proximal metacarpal region has been imal portion of the forelimb suspensory ligament in
reported in a low‐field study, while high‐field studies non‐lame horses should be borne in mind when inter­
128
127
consistently indicate much higher numbers of injuries of preting MR images of lame horses. Therefore, com­
the accessory ligament of the DDFT and the suspensory parison with the contralateral limb or with published
ligament. 5,28 This may be explained by easier recognition reference images may be useful to help with interpreta­
of bone signal change than soft tissue change in standing tion of suspected MRI changes. 124
low‐field MR systems, because motion artifact in stand­ In hindlimbs, the suspensory ligament starts as a nar­
ing patients can lead to poor recognition of anatomical row triangular band, evolves into a right‐angled triangle
detail as well as of signs of injury in soft tissue struc­ that is wider on the lateral aspect than medially, and
tures. Motion artifact results in blurring of signal and contains a dorsal central cleft dividing it in two lobes. At
loss of contour definition of the muscle and adipose tis­ 4 cm distal to the tarsometatarsal joint, it becomes oval
sue bundles within the suspensory ligament. Inability to to bean shaped and separates away from the plantar
accurately define these muscle and adipose tissue bun­ cortex of the third metatarsal bone to continue distally,
dles hampers the ability to distinguish these bundles while both lobes merge into one oval ligament.
161
from areas of fiber damage in the collagenous part of Normal cross‐sectional areas have been defined for the
the suspensory ligament. Lesion occurrence in forelimbs proximal part of the suspensory ligament and have var­
2 161
is also heavily influenced by the horse’s discipline and ied from 1.12 to 2.23 cm . Injuries of the proximal
age. A recent high‐field study showed a much higher part of the suspensory ligament may result in enlarge­
prevalence of osseous abnormalities of the third meta­ ment, contour changes, focal and diffuse signal increase,
carpal bone in young Quarter horses used for cutting and periligamentous fibrosis.
than previously reported in studies of mixed breed sport Abnormalities are best seen in transverse PD images.
horse populations. 5,28 Lesions can be graded as mild, moderate, or severe,
As the only retrospective MRI studies of the metatar­ depending on the abnormal signal intensity and the pro­
sal region available have been performed with high‐field portion of total cross‐sectional area of the ligament
magnets, 6,28,93 a comparison between low‐ and high‐field affected.
injury prevalence is not possible for the hindlimb Enlargement of the cross‐sectional area of the suspen­
(Table 3.6). However, given that motion artifact tends to sory ligament may vary from mild to severe, but because
be more pronounced in standing MRI of the hindlimb of the variation in size within limbs and between horses,
than the forelimb, the same concerns apply. By far the this feature should be interpreted with caution especially
most common injury in the proximal metatarsal region when the enlargement is mild. 5,93,127,161 Even though
is desmopathy of the proximal part of the suspensory marked enlargement may result in a decrease of the
ligament with or without an enthesis injury of the third space between the dorsal margin of the suspensory liga­
metatarsal bone at the attachment of the suspensory ment and the palmar or plantar surface of the bone,
ligament. The high occurrence of MR abnormalities of periligamentous fibrosis and adhesion formation are
the distal tarsal region in horses with lameness localized uncommon features of proximal suspensory desmopathy.