Cross-section imaging is like taking a single “slice” out of the body as if it were bread, allowing practitioners to see inside that loaf of bread. This contrasts with radiology that simply takes a two-dimensional view of a three-dimensional structure, opening the door for overinterpretation and misdiagnosis.
During her presentation at the 2024 National Alliance of Equine Practitioners (NAEP) Saratoga Convention, held in September, Kathryn Bills, VMD, DACVR, DACVR-EDI, from the University of Pennsylvania School of Veterinary Medicine’s New Bolton Center, explained how to incorporate cross-section imaging into everyday practice, using six key examples: subchondral bone injury, occult fracture, osteoarthritis (OA), suspensory origin pathology, navicular degeneration, and deep digital flexor tendon injury.
Cross-sectional imaging can allow for earlier and more specific diagnosis of a horse’s lameness. However, cross-sectional imaging such as computed tomography (CT) and magnetic resonance imaging (MRI) has limited availability in some areas, can be costly, and requires standing patient compliance or anesthesia.
“As a solution, I suggest using targeted radiographic projections and ultrasound to interrogate common sites of injury that are known to us because of how often we diagnose them with cross-sectional imaging modalities,” said Bills. “Thus, practitioners don’t have to use cross-sectional imaging to make a diagnosis, but we know where the lesions are because of cross-sectional imaging of other cases, and horses injure themselves in repeatable locations.”
Subchondral Bone Pathology
“Using variable angle dorsopalmar views, look for large areas of sclerosis and central areas of demineralization,” Bills explained. “For the medial aspect of the metacarpal condyle, a skyline projection highlights the dorsal aspect of the condyle.”
For the palmar metacarpal condyle, large areas of sclerosis and central demineralization are typically present. You can image these using the flexed dorsopalmar view.
Occult Fracture
One of the most common areas for occult fracture in the hind limb is the central tarsal bone. The four standard radiographic projections, however, might not reveal visible fracture.
“Standing CT can identify an area of sclerosis and a fracture line, but if the clinical index of suspicion that the horse has injured this region is high, go back and take additional DMPLO (dorsomedial-palmarolateral oblique) projections to find that fracture. If you still can’t find it, keep going more oblique,” advised Bills. “This will save the owner money on a CT.”
Osteoarthritis
Bills then presented a case of a 7-year-old eventer with grade 3+ out of 5 lameness localized to the tarsal joints. Scintigraphy was abnormal, but no abnormalities were identified on radiographs.
“CT of the dorsal aspect of distal tarsal bones/joints is where arthritic lesions are normally seen, but in horses with an inappropriate degree of lameness and minimal dorsal changes, look for plantar OA and osteolytic OA specifically using additional oblique projections, looking for subchondral demineralization and osteophytes,” recommended Bills.
She added, “One needs to make a specific/targeted effort to evaluate the plantar aspect of the distal tarsal joints. This is an area that practitioners often ignore. But you can see pathology if you know where to look for it.”
Suspensory Origin Pathology
MRI of the origin of the suspensory shows a dark (hypointense) ligament with the fat and muscle being whiter.
“With in-plane (isotropic or conventional) ultrasound you don’t see that fat and muscle distinction from the surrounding ligament,” said Bills. “One of the tricks for ultrasounding is to unweight (i.e., flex) the limb and use off-angle imaging to see lesions in the dorsal aspect of the tendon where most lesions are, between the fat muscle bundle and the bone. A good ultrasound exam can actually make it look very similar to the MRI.”
Practitioners often look for ligament size and more information about the enthesis with the flexed and off-angle image. A scalloped appearance of the enthesis (bone margin) might be appreciated.
Navicular Degeneration and Deep Digital Flexor Tendon (DDFT) Lesions
We, as practitioners, are increasing our ability to diagnose lesions of the navicular apparatus, especially if incorporating appropriate skyline projections, said Bills.
“Angle matters! If you’re trying to rule out navicular pathology, in addition to the conventional angle for the navicular skyline, try taking an additional, shallower image, around 35 degrees,” she said. “This will increase conspicuity of more distal flexor cortical erosions.”
MRI or ultrasound through the heel bulbs best characterize DDFT lesion in the foot.
Bills said the DDFT gets injured in very predictable ways, typically divided into these four areas:
- Pastern level: the DDFT outside the hoof capsule.
- Suprasesamoidean area, which is the most commonly injured area but difficult to characterize via ultrasound.
- Sesamoidean region.
- Infrasesamoidean region, which is the least common and often most painful.
“Horses with certain lesions are likely to have another lesion,” she said. “If you diagnose a core lesion in the pastern, you should also suspect the lesion extends into the hoof capsule, and larger/more extensive lesions have a poorer prognosis. The lesions evolve in a predictable pattern.”
Final Thoughts
To conclude, Bills said, “We have more imaging tools to use in the field based on the knowledge we have accumulated about common lesion locations that historically have only been characterized with cross-sectional imaging.”
Related Reading
- Fetlock Radiographs: No Association Between Lameness, Pathology
- 10 Radiographic Prepurchase Abnormalities in Sport Horses
- Equine Stifle Radiology Tricks and Surprises
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