Cannon bone changes seen in imaging of racehorses may be accumulating microdamage – study

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Contrast arthrography CT images of the distal third metacarpal bone in frontal (A) and sagittal (B) planes of the right hind fetlock of a 4-year-old Thoroughbred gelding with left tibial fracture during race training.
Contrast arthrography CT images of the distal third metacarpal bone in frontal (A) and sagittal (B) planes of the right hind fetlock of a 4-year-old Thoroughbred gelding with left tibial fracture during race training. Cartilage surface irregularity, thickening and hypoattenuation are indicated with white arrows. A frontal plane T1W image of the same limb (C) shows how the presence of cartilage defects can complicate the interpretation of the subchondral bone on MRI, potentially leading to a false-positive diagnosis of subchondral bone defects. Image (D) shows the articular cartilage defects in the parasagittal grooves of the distal third metacarpal bone on post-mortem. Lateral or dorsal are to the left in all images. Johnston Ahem et al

Certain changes to the cannon bone of horses, detectable by imaging, may indicate microdamage accumulation and an increased fracture risk, researchers report.

Early detection of racehorses at risk of stress fracture is crucial to reducing the number of horses who suffer catastrophic fractures while racing.

Bone changes are often visible in the limbs of Thoroughbred racehorses in work, particularly in the fetlock region. However, it is unknown whether some of these changes indicate an impending fracture or are a healthy adaptation to high-speed exercise.

Georgina Johnston and her fellow researchers at the veterinary school at the University of Queensland set out to examine changes in a specific area of the cannon bone – the parasagittal grooves – and assess the usefulness of X-ray, computed tomography (CT) and magnetic resonance imaging (MRI) to detect them.

Bone and articular cartilage problems affecting the parasagittal grooves of the distal third metacarpal and metatarsal bones are common in racing Thoroughbreds, with fractures known to occur in the region.

The study team, reporting in the journal Animals, examined all fetlock joints from 20 horses that died during racing or training, including horses with and without fetlock fracture.

Overall, X-ray was found to be poor for detecting changes in the parasagittal grooves.

Some changes seen in this area in CT and MRI scans were common in racehorses and possibly represent normal bone adaptation when seen in clinical cases, they said.

However, some CT and MRI findings were more prevalent in horses with a fracture. Horses with a fetlock fracture were more likely to have asymmetry in their parasagittal grooves arising from an abnormal increase in density and hardening of bone (sclerosis), and/or bone lesions (lysis). The lysis was not readily detected using MRI, they said.

The authors noted that all fetlocks in the study had CT and/or MRI evidence of parasagittal groove cartilage or bone changes.

Subchondral bone defects in the parasagittal grooves were difficult to differentiate from cartilage defects on MRI and were not associated with fractures.

The authors concluded that while some changes in the parasagittal grooves of Thoroughbred racehorses are common, certain findings and combinations of findings are more prevalent in those with catastrophic fractures.

MRI image of STIR hyperintensity in the parasagittal grooves (B) of the right fore fetlock of a 4-year-old Thoroughbred gelding with a left tibial fracture during race training. The only other parasagittal groove abnormality detected on MRI (A) or CT (C) was mild cartilage fibrillation.
MRI image of STIR hyperintensity in the parasagittal grooves (B) of the right fore fetlock of a 4-year-old Thoroughbred gelding with a left tibial fracture during race training. The only other parasagittal groove abnormality detected on MRI (A) or CT (C) was mild cartilage fibrillation. Lateral is left in the images. Johnston Ahem et al

“Rather than indicating a specific risk of fracture at the parasagittal groove, it is possible that some of these changes indicate skeletal overload and bone fatigue, suggesting an overall increased risk of fracture.”

Such microdamage accumulation has been linked to increased fracture risk.

Bilateral advanced imaging is recommended in clinical cases of suspected fetlock pathology, they said.

The study team comprised Johnston, Benjamin Ahern, Chiara Palmieri and Alex Young.

Johnston, G.C.A.; Ahern, B.J.; Palmieri, C.; Young, A.C. Imaging and Gross Pathological Appearance of Changes in the Parasagittal Grooves of Thoroughbred Racehorses. Animals 2021, 11, 3366. https://doi.org/10.3390/ani11123366

The study, published under a Creative Commons License, can be read here.

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