Condylar stress fracture of the third metacarpal/metatarsal bone (MC3/MT3) in Thoroughbred racehorses is a common catastrophic injury, putting both the racehorse and the jockey at risk. Microdamage forms in the distal MC3 and may result in strain elevation and higher risk of stress fracture, directly and through focal osteolysis resulting from the associated damage repair by remodelling in the condylar parasagittal grooves (PSG). Standing computed tomography (sCT) is a practical screening tool for detecting fatigue-induced structural changes, but clinical interpretation remains subjective. The goal of this study was to develop and validate an sCT-based, subject-specific finite element analysis pipeline for virtual mechanical testing of the distal MC3. Twelve (n = 12) MC3 condyles with available ex vivo strain were selected. Half of the specimens were used for tuning Young’s modulus in fatigue sites, and the other half were used to evaluate the validity of the pipeline in predicting PSG strain. The tuned model improved the prediction of strain and predicted higher strain in bones with PSG lysis over the untuned model, which is an essential feature for a diagnostic screening tool. The presented pipeline can assist clinicians with the interpretation of sCT-detectable structural changes and, ultimately, with the assessment of condylar fracture risk in racing Thoroughbreds. The standing computed tomography (sCT) images used in the linked manuscript can be found here. Specimens are labeled with unique IDs, which can be found in condyleID.txt. Essential DICOM tags are embedded in all of the image sets.

A library of 12 MC3 condyles (n = 12) from 11 thoracic limb specimens, with available surface strain data under quasi-static loading of these condyles from previous ex vivo mechanical testing, was used for this study. Specimens were collected from Thoroughbred racehorses that were euthanized because of catastrophic racetrack injuries, including contralateral MC3 fractures through the PSG. Age, sex, and training history of the specimens were not available. These condyles included specimens with (n = 4, PSG-SBI group) and without (n = 8, CTRL group) sCT-detectable focal lysis in the PSG of the distal end of the MC3 bone. The existence of fatigue damage in the PSG of the specimens in the PSG-SBI group was previously confirmed following soft tissue dissection. The sCT scans of the frozen limbs were acquired using an Equina^® scanner (Asto CT Inc., Middleton, WI, USA) at exposure of 160 kVp and 8 mA, with a slice thickness of 0.55 mm.