COMPARING MRI RESULTS TO PATHOLOGY
Michael Schramme
VetAgro Sup, Campus Vétérinaire de Lyon, Marcy L’Etoile, Rhône-Alpes, France
Magnetic resonance (MR) imaging is now widely used in the diagnosis of equine foot
lameness. It has been able to provide diagnostic information not available from other
imaging techniques. Understanding of the significance of alterations in signal
intensity and patterns relies on the study of pathological changes that occur in the
various disease processes that affect different tissues in the foot. The purpose of this
paper is to review the current histopathological knowledge on various anatomical
structures of the foot and the suitability of MR imaging for their identification.
Navicular Bone
The most common type of MR signal abnormality seen in the navicular bones of
horses with lameness associated with navicular disease is STIR signal
hyperintensity in the medullary cavity of the navicular bone with or without additional
areas of T2 and PD signal hypointensity. Medullary STIR hyperintensity may be focal
near the distal border of the navicular bone, or extend from the distal border in a
vertical band along the palmar cortex to the proximal border of the bone, or spread
diffusely throughout the medullary cavity. In horses with chronic navicular disease,
abnormal signal hyperintensities at the level of the palmar surface of the navicular
bone are equally common as those in the medullary cavity. These can be areas of
subtle, focal increase, caused by synovial fluid pooling at a site of early fibrocartilage
loss, or more extensive signal increase extending deeper within the cortical bone of
the flexor cortex.
Degenerative changes in the palmar fibrocartilage of the navicular bone occur in the
distal half of the palmar aspect of the navicular bone, especially centered around the
sagittal ridge, and may extend into the subchondral bone. Loss of fibrocartilage in
this location is the most common lesion significantly associated with navicular
disease and most likely represents the earliest pathology of classic navicular
disease. Fibrocartilage loss from this location remains difficult to identify in vivo, even
with the use of MRI. Progression of fibrocartilage loss may result in cortical bone
erosion in the flexor cortex, and even in osteonecrosis and fibroplasia extending into
the spongiosa. Degenerative change of the spongiosa is generally only seen dorsal
to extensive fibrocartilage damage. There may also be edema, congestion and
fibrosis of the marrow stroma within the medullary bone. Clinical experience with
MRI in horses with foot pain from classic navicular disease provides support for the
progression of lesions as outlined above.
However, there is a small number of horses with diffuse abnormalities of the medulla
characterized by ‘bone edema’ signal (increased signal in fat suppressed images)
but no detectable abnormalities of the flexor fibrocartilage or cortex. Post mortem
examination has revealed evidence of necrosis of medullary fat cells and active
remodeling of medullary trabeculae, with both osteoclastic and osteoblastic activity
along trabecular surfaces. In other horses with ‘abnormal medullary fluid signal’,
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
218