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CT scans could be used to gauge leg fracture risk in racehorses, say researchers

Subchondral fatigue crack dimensions were measured in transverse (A) and reconstructed frontal plane (B) computed tomography (CT) images. In addition, the area of the crack array was measured in the parasagittal plane (C).

Subchondral fatigue crack dimensions were measured in transverse (A) and reconstructed frontal plane (B) computed tomography (CT) images. In addition, the area of the crack array was measured in the parasagittal plane (C).

American researchers believe CT imaging could be used to quantify fatigue-induced cracks in leg bones in thoroughbred racehorses, identifying those at greatest risk of catastrophic fractures.

The researchers, who carried out their work at the University of Wisconsin-Madison, based their findings on a series of tests involving leg bones from thoroughbreds that had died as a result of catastrophic leg fractures.

Peter Muir and his colleagues undertook x-ray imaging, CT scans and mechanical testing of bones taken from 40 horses and reported their findings in the open-access journal, PLoS ONE.

The researchers said stress fractures arising from the lower end of the third metacarpal bone (MC3) was a common serious injury in thoroughbreds.

There was currently no method for predicting fracture risk clinically, they said.

Aside from the imaging work, the researchers developed a mechanical model which enabled them to test the bone under compressive loading that modeled high-speed running.

Using this model, they were able to measure the tiny amount of motion – micromotion – in the fine bone cracks that they had measured using CT scans.

They found that bones with parasagittal crack areas of more than 30 square millimetres may have a high risk of suffering condylar fractures. These fractures, they said, would arise because of crack micromotion, leading to failure in the bone.

A custom-made jig held the bone to allow load testing.

A custom-made jig held the bone to allow load testing.

“Based on our results, it is likely that fatigue crack arrays exceeding these dimensions are exposed to micromotion during high-speed running and are, therefore, vulnerable to catastrophic crack propagation,” they wrote.

They said their results suggested that CT scans could be used in thoroughbred racehorses to quantify fatigue crack dimensions, with those showing parasagittal cracks of more than 30 square millimitres at high risk for development of condylar fracture.

The researchers said results from their mechanical testing of the bones suggest that it is likely that some degree of fatigue crack micromotion is present in a large proportion of thoroughbred racehorses during training and racing.

“Although subchondral crack micromotion may not be the only determinant of a stress fracture, microcrack propagation, as opposed to initiation, is likely an important component of stress fracture development,” they said.

Dubois M-S, Morello S, Rayment K, Markel MD, Vanderby R Jr, et al. (2014) Computed Tomographic Imaging of Subchondral Fatigue Cracks in the Distal End of the Third Metacarpal Bone in the Thoroughbred Racehorse Can Predict Crack Micromotion in an Ex-Vivo Model. PLoS ONE 9(7): e101230. doi:10.1371/journal.pone.0101230

The full study can be read here

Representative images of the distal end of the third metacarpal bone illustrating articular saucer stress fracture formation in a racing Thoroughbred horse with severe palmar osteochondral disease. (A) In reconstructed computed tomography images, a circular articular saucer fracture is present in both condyles. A halo of reduced bone density adjacent to the saucer fracture is caused by the reparative remodeling response in the subchondral plate. Extensive adaptive subchondral sclerosis is also evident. (B) Hyaline cartilage overlying the stress fracture has been replaced by fibrocartilage. (C) Extensive fatigue damage to the underlying subchondral plate is also evident, including parasagittal cracks in the condylar grooves (arrows). (D) Propagation of the fracture line at the interface of the vascular remodeling response to the fatigue injury in the subchondral plate is evident (arrows) on a microradiograph of an oblique frontal bone section.

Representative images of the distal end of the third metacarpal bone illustrating articular saucer stress fracture formation in a racing Thoroughbred horse with severe palmar osteochondral disease. (A) In reconstructed computed tomography images, a circular articular saucer fracture is present in both condyles. A halo of reduced bone density adjacent to the saucer fracture is caused by the reparative remodeling response in the subchondral plate. Extensive adaptive subchondral sclerosis is also evident. (B) Hyaline cartilage overlying the stress fracture has been replaced by fibrocartilage. (C) Extensive fatigue damage to the underlying subchondral plate is also evident, including parasagittal cracks in the condylar grooves (arrows). (D) Propagation of the fracture line at the interface of the vascular remodeling response to the fatigue injury in the subchondral plate is evident (arrows) on a microradiograph of an oblique frontal bone section.

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