Comparison of planar digital radiography and helical standing computed tomography for assessment of condylar stress fracture risk in Thoroughbred racehorses

Background: Catastrophic injury has a low incidence but leads to the death of many Thoroughbred racehorses.

Objectives: To determine sensitivity, specificity, and reliability for condylar stress fracture risk assessment from fetlock digital radiographs (DR) and standing computed tomography (sCT).

Study design: Controlled ex vivo experiment.

Methods: A blinded set of thoracic limb fetlock DR and sCT images were prepared from 31 Thoroughbreds. Four observers evaluated the condyles and parasagittal grooves (PSG) of the third metacarpal bone for the extent of dense bone and lucency/fissure and assigned a risk assessment grade for condylar stress fracture. Sensitivity and specificity for detection of subchondral structural changes in the condyles and PSG and for risk assessment for condylar stress fracture were determined by comparison with a reference. Agreement between observers and the reference assessment and reliability between observers were determined. Intra-observer repeatability was also assessed.

Results: Sensitivity for detection of structural change was lower than specificity for both imaging methods and all observers. For horses with normal risk, observer assessment often agreed with the reference. Sensitivity for risk assessment was lower than specificity for all observers. For horses with a high risk of injury, observers generally underestimated risk. Diagnostic sensitivity of risk assessment was improved with sCT imaging, particularly for horses with elevated risk of injury. Assessment repeatability and reliability was better with sCT than DR.

Main limitations: The ex vivo study design influenced DR image sets.

Conclusions: Risk assessment through screening with diagnostic imaging is a promising approach to improve injury prevention in racing Thoroughbreds. Knowledge of sensitivity and specificity of fetlock lesion detection provides the critical guidance needed to improve screening programs for racehorses. We found improved detection of MC3 subchondral structural change and risk assessment for condylar stress fracture with sCT ex vivo.

Methods
Distal limb specimens were collected from catastrophically injured racing Thoroughbreds that were humanely euthanatized for reasons unrelated to this study and stored at -20C. Thoracic limb fetlock digital radiographs and standing computed tomography image sets were generated and blinded. Sensitivity and specificity for detection of structural changes in subchondral bone of the distal end of the third metacarpal bone as well as risk assessment for imminent injury from condylar stress fracture were estimated by four blinded observers. Reliability of estimations was also assessed.