suppressed images in the spongiosa, linear fluid signal intensity between the
attachments of the CSL and DSIL; or OCLLs in the distal half of the bone. In addition,
there are abnormalities restricted to the distal border including marked increase in
number and size of synovial invaginations, entheseous new bone, and distal border
fragments. Potentially reversible lesions consistent with acute trauma of the navicular
bone may also occur.
Radiographically, the most common lesions of the spongiosa involving an increase in
fluid-like signal in fat-suppressed MR images, remain invisible.
Scintigraphic studies have indicated that there is evidence of increased bone
turnover in association with some forms of navicular disease, even in the absence of
radiological abnormalities of the bone. Increased RU predominantly reflects
increased osteoblastic activity but may also reflect a functional adaptation to foot
conformation and the biomechanical forces on the navicular bone or preclinical
disease.
Comparison between scintigraphy and MRI has demonstrated that many horses with
focal moderate or intense IRU have abnormalities of the navicular bone detectable
using MRI (Dyson and Murray 2007). However, scintigraphy can also produce falsenegative results, indicating that pathological abnormalities of the navicular bone are
not always associated with increased osteoblastic activity. If increased signal
intensity in the spongiosa was the principle pathological change in horses with foot
pain, without IRU, it was speculated that treatment with the bisphosphonate,
tiludronate, may be of benefit, particularly in light of osteoclasts identified in the
current study, but negligible improvement was seen in 12 of 12 horses
Distal border fragments
There is considerable controversy concerning the potential clinical significance of
distal border fragments of the navicular bone. In a radiological study comparing 55
sound horses and 377 lame horses, fragments were observed in 3.7% and 8.7% of
sound and lame horses, respectively. In lame horses, distal border fragments were
present in 24.1% of horses with a diagnosis of primary navicular disease and in
12.9% of horses with navicular pathology and other associated lesions (Biggi et al.
2012). There was an association between fragments and the overall navicular bone
grade (P= 0.0013), radiolucent areas at the angles of the distal border of the
navicular bone (P < 0.001), and the number and size of the synovial invaginations
along the distal border of the navicular bone (P < 0.001). It was concluded that
fragments may be part of navicular disease.
The correlation between the presence of distal border fragments detected using
high-field MRI and their radiological detection was evaluated in 427 horses (Biggi et
al. 2010). Medium and large fragments were most likely to be detected radiologically
especially if grade 4 or 5, but up to 43% of large fragments were missed. There were
significant associations between the presence of a fragment on radiography and the
total MRI grade of the navicular bone. There was also an association between an
increased number and size of the synovial invaginations along the distal border on
radiographs and the presence of both a distal border fragment on MR images and
OCLLs on MR images.
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
227