46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
43
Nowhere has this problem, the detection of lesions of uncertain significance, been
more marked than distal border fragmentation of the navicular bone. This finding is
clearly visible on MRI scans, and it has been shown that horses with distal border
fragments are more likely to be lame than horses without. The presence of distal
border fragments can be predicted by radiographic lucency of the lateral or medial
angles of the distal border of the bone. However, it also clearly documented that
sound horses can have distal border fragmentation [18]. The significance of this
finding, in the absence of abnormalities of the navicular bone or impar ligament,
remains difficult to interpret.
MRI has proved a very useful imaging modality for examination of the fetlock joint,
particularly in the racehorse. In this location the movement artefact is limited and
can be controlled, while higher up the limb, typically in the origin of the suspensory
ligament, it can be very difficult to obtain diagnostic images. MRI shows detail of the
bone structure, which are not apparent radiographs. Fluid accumulation, “bruising”
or bone oedema can be detected on fat suppressed sequences. Thus there has been
considerable interest in the assessment of the fetlock of the racehorse, possibly to
predict the development of a fracture before it becomes complete and
radiographically visible. Incomplete “fissure” fractures of the metacarpal/tarsal
condyles can be easily detected. However, the significance of these fissures is not as
obvious as might initially be thought. Post mortem studies of racing fatalities have
shown that these fissures are present in 9% of “normal” limbs, those where horses
were destroyed for reasons other than cannon bone fractures [19]. The same study
showed a non-significant reduction in focal palmar osteonecrosis lesions, adding
some anecdotal support to the view that palmar osteochondral disease is a separate
entity, which does not result in fracture.
Proximal suspensory desmitis has proved a similar enigma to foot lameness. It is a
common condition, relatively easily diagnosed with nerve blocks, but lacking clear
and definitive imaging abnormalities. Thus there has been considerable interest in
MRI of this area. Some conditions have been established. Bone oedema of the
proximal metacarpus is identified in racehorses, and is probably a distinct condition
[20]. In a series of 50 lame horses with proximal metacarpal lameness, sclerosis of
the third carpal bone was identified as the commonest abnormality [21]. The
suspensory ligament has proved to be similar on MRI to ultrasound. Namely, a very
heterogenous structure, with complex and variable appearance [22]. Currently, MRI
of the proximal susp ensory ligament is largely for the detection of concurrent bone
injury, and this is much more likely in the racehorse.
The other key issue with new imaging modalities, that of hitherto unsuspected image
artefacts, has also been true with MRI. At 54.7º to the magnetic field (B0), the
magnetic dipole forces (between two magnets, the attraction of the north pole for
the south pole, balanced with the repulsion of the north pole for the north pole) fall
to zero. This results in a prolongation of the T2 time – the time taken for the
protons to spin out of phase with each other. This artefact results in an artefactual
hyperintensity of ligaments and tendons when they are between 30º and 75º to B0,
with increasing hyperintensity closer to 55º. This artefact was first identified in the
horse in the insertion of the deep digital flexor tendon on scans obtained in “high
field” magnets under general anaesthesia. The presence of the artefact has been
clearly documented in the lateral collateral ligament of the distal inter-phalangeal
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