R. Christopher Whitton

Musculoskeletal injuries (MSI) in Thoroughbred racehorses are a leading cause of death and premature retirement, and are heavily influenced by training practices. Greater distances of high-speed galloping accumulated during racing campaigns are associated with MSI. Bone injury is the most common MSI, and understanding how training practices influence bone damage accumulation is critical for improving both horse welfare and racing outcomes. This study builds on an existing mathematical model of bone adaptation and damage to investigate the impact of different training programmes on bone injury risk. Several training programmes (three progressive, four race-fit, six rest programmes and two with rest replaced by low-intensity training) were constructed to reflect representative practices undertaken by professional trainers in Victoria, Australia. Training programmes varied in training volume, rest frequency and programme duration. Lower-volume training programmes that included high-speed training achieved sufficient bone adaptation with less accumulation of bone damage, and subsequently lower risk of bone failure. In addition, incorporating more frequent rests (minimum of two per year) and/or longer rest periods (minimum of six weeks) reduced bone damage owing to the extended opportunity to remove and repair bone damage. These results provide an in silico mathematical model of the bone's response to training, demonstrating the effects of training programmes on bone adaptation, damage formation and repair. The findings can guide the design of training programmes that balance both bone adaptation and bone health throughout a horse's racing career.

Condylar stress fracture of the third metacarpal bone (MC3) in Thoroughbred racehorses is a common catastrophic injury, and identification of horses at heightened risk remains subjective. Standing computed tomography (sCT) is a practical screening tool that enables patient-specific finite element analysis (FEA) of the distal MC3 and prediction of subchondral bone strain, as a potential objective classifier of racehorses at heightened risk. The goal of this study was to compare two independently developed sCT-based subject-specific FEA pipelines for virtual mechanical testing of the distal MC3. One pipeline models the full three-dimensional (3D) distal MC3 (UWMSN), while the other uses a simpler approach by using single sCT slices (UMELB). Four (n = 4) MC3 condyles from four Thoroughbred racehorses were selected for the study. Models were generated using both pipelines, and the predicted subchondral bone strain was compared. UMELB predicted smaller subchondral strain compared to UWMSN, likely due to more limited modes of deformation. Although the UWMSN pipeline can identify elevated subchondral strain in horses with high fatigue damage, it is more labor-intensive and computationally expensive. With further tuning and validation, the UMELB pipeline could be used as a simpler and faster approach for the prediction of subchondral strain in the distal MC3.

Background: Hoof trimming and shoeing techniques are used to manage and prevent equine limb injuries. However, quantitative studies comparing the effects of different shoeing techniques on equine joint biomechanics over the full gait cycle are lacking. Objectives: To measure and compare joint motion and net torques at the distal forelimb joints when horses walk overground unshod, with a standard flat shoe, and with a rocker shoe. Study Design: In vivo study. Methods: Gait data were recorded from 12 sound horses during walking. Three shoeing conditions were tested: unshod, flat shoe, and rocker shoe. Data were recorded for each shoeing condition immediately after trimming (short hoof condition) and again after 6 weeks of hoof growth (long hoof condition). Three‐dimensional motion capture and retro‐reflective skin markers recorded left forelimb motion, while synchronised force plates measured the corresponding ground reaction force. Inverse dynamics was used to calculate the net torques developed about the distal forelimb joints. Statistical comparisons were performed with multilevel mixed effects generalised linear models. Results: While there were limited effects of trimming and shoeing, the rocker shoe was associated with higher walking speed (by 9.3 ± 9.7%) and reduced stride duration (by 4.9 ± 6.9%) compared with the flat shoe for the short hoof condition ( p < 0.001). Throughout the stride cycle, the fetlock joint was less extended (by 9.0 ± 13.7°) while the distal interphalangeal joint (DIPJ) was more extended (by 10.7 ± 16.6°) for both shoeing types compared to unshod regardless of hoof growth ( p < 0.005). Higher peak torques were generated at the DIPJ for flat shoe compared to unshod (by 0.05 ± 0.27 Nm/kg) in the short hoof condition, and for flat shoe compared to rocker shoe (by 0.03 ± 0.14 Nm/kg) in the long hoof condition ( p < 0.05 for both). Main Limitations: The horses were tested at a low‐speed walking gait. Conclusions: Forelimb joint biomechanics did not differ substantially across the three shoeing and two hoof‐growth conditions. Future studies should test the robustness of these findings at the trot and canter.

Background: Condylar stress fracture of the third metacarpal bone (MC3) is a common catastrophic injury in Thoroughbred racehorses and is associated with parasagittal groove (PSG) subchondral osteolysis. Standing computed tomography (sCT) imaging enables sensitive identification of this fatigue‐induced early subchondral bone injury (SBI), but there is no objective method for identifying racehorses at heightened risk of condylar stress fracture. Objectives: To estimate PSG first principal strain in elite Thoroughbred racehorses that have undergone subjective risk assessment using sCT fetlock screening. Study Design: Retrospective clinical study. Methods: We used fetlock sCT images from nine thoracic limbs from seven Thoroughbred racehorses. A tuned, validated, 3D finite element (FE) analysis was used as a virtual mechanical test to estimate PSG first principal strain in the distal MC3 from these joints. Virtual mechanical testing results were compared with a subjective clinical imaging risk assessment using a screening approach by Racing Victoria. Results: MC3 condyles with PSG SBI consistently and significantly displayed increased levels of first principal strain throughout the PSG. We found focal strain concentrations associated with the SBI location compared to condyles with no evidence of PSG SBI. Diagnosis of SBI with PSG focal osteolysis, FE‐predicted strain elevation, and clinical imaging risk assessment were concordant with R ² = 0.62. Main Limitations: The sample size was small, and our virtual mechanical testing protocol does not account for whole‐joint physiology. Conclusions: Risk assessment through sCT screening is an established approach to injury prevention in racing Thoroughbreds. Concordance of a current clinical imaging risk assessment approach by Racing Victoria with objective FE analysis of principal strain in sites of PSG SBI in the present study suggests 3D FE analysis using a validated pipeline has potential as a new approach for routine assessment of risk of MC3 condylar stress fracture in Thoroughbred racehorses once computational pipeline automation yields a clinically relevant analysis time.

Condylar stress fracture of the third metacarpal bone (MC3) in Thoroughbred racehorses is a common catastrophic injury and identification of horses at heightened risk remains subjective. Standing computed tomography (sCT) is a practical screening tool that is sensitive to fatigue-induced structural changes. Data from sCT also allows for patient-specific finite element analysis (FEA) of the distal MC3 and prediction of subchondral bone strain, as a potential objective classifier of racehorses at heightened risk. The goal of this study was to compare two independently developed sCT-based subject-specific FEA pipelines for virtual mechanical testing of the distal MC3. One pipeline models the full 3D distal MC3 (UWMSN), while the other uses a simpler approach by using single sCT slices (UMELB). Four (n=4) MC3 condyles from four Thoroughbred racehorses were selected for the study. Models were generated using both pipelines and the predicted subchondral bone strain was compared. UMELB predicted smaller subchondral strain compared to UWMSN, likely due to more limited modes of deformation. Although the UWMSN pipeline can identify elevated subchondral strain in horses with high fatigue damage, it is more labor intensive and computationally expensive. With further tuning and validation, the UMELB pipeline could be used as a simpler and faster approach for prediction of subchondral strain in the distal MC3.

Background: Computed tomography (CT) is increasingly used for metacarpo/tarsophalangeal or fetlock joint (MC/TPJ) evaluation in Thoroughbred racehorses. CT features and lesion prevalence in competing Thoroughbreds are unknown.Objectives: To identify standing CT features within the MC/TPJ of elite Thoroughbred staying racehorses.Study Design: Retrospective cohort study.Methods: Computed tomography scans of the MC/TPJ were performed as part of pre‐race screening for fracture risk and graded for CT features. Descriptive statistics and univariable logistic regression models were generated to investigate associations with presence of palmar osteochondral disease (POD), parasagittal groove subchondral lysis and fissure.Results: A total of 406 limbs from 86 horses were scanned; 72 horses were scanned once, 12 horses were scanned twice, and two horses scanned three times. POD was common, being present in 74/86 (86.0%, 95% CI 76.9–92.6) horses and in 231/342 (67.5%, 95% CI 62.3–72.5) limbs on first scan. POD was associated with more career race starts (p = 0.05) but fewer placings per start (p = 0.008). Parasagittal groove (PSG) lysis, previously associated with condylar fracture, was observed in 16/86 (18.6%; 95% CI 11.0–28.4) horses and 20/342 (5.8%; 95% CI 3.6–8.9) limbs on the first scan, whereas fissures in this region, which are not associated with fracture, were observed in 41/86 (47.7%; 95% CI 36.8–58.7) horses and 65/342 (19.0%; 95% CI 15.0–23.6) limbs. Osteoarthritis of the fetlock joint was uncommon.Main Limitations: Horses travelling from overseas had undergone prior screening of their fetlock joints, predominantly with MRI, likely reducing the prevalence of pathology in these horses.Conclusions: POD is common in elite staying Thoroughbred racehorses, however, negatively impacts performance. The prevalence of PSG lysis supports the use of CT screening of high‐risk populations of Thoroughbred racehorses in fracture risk management. Further work is required to determine if these findings are applicable more generally to other racing Thoroughbreds.

Fractures of the distal limb in Thoroughbred racehorses primarily occur because of accumulation of bone microdamage from high-intensity training. Mathematical models of subchondral bone adaptation of the third metacarpal lateral condyles are capable of approximating existing data for Thoroughbred racehorses in training or at rest. To improve upon previous models, we added a dynamic resorption rate and microdamage accumulation and repair processes. Our ordinary differential equation model simulates the coupled processes of bone adaptation and microdamage accumulation, and is calibrated to data on racehorses in training and rest. Sensitivity analyses of our model suggest that joint loads and distances covered per day are among the most significant parameters for predicting microdamage accumulated during training. We also use the model to compare the impact of incrementally increasing training programmes as horses enter training from a period of rest, and maintenance workloads of horses that are race fit on bone adaptation. We find that high-speed training accounts for the majority of damage to the bone. Furthermore, for horses in race training, the estimated rates of bone repair are unable to offset the rate of damage accumulation under a typical Australian racing campaign, highlighting the need for regular rest from training.

In human CT imaging, opportunistic calculation of bone mineral density (BMD) is used to screen for osteoporosis, where BMD is estimated by Hounsfield Units (HU). There is potential to use HU for estimation of BMD in veterinary imaging. However, HU values can vary between different machines and over time. We undertook a serial measurements study to determine (1) the extent of the difference of the mean HU and noise (HU standard deviation) of eight regions of interest (ROI) of an electron density phantom scanned on two CT machines, a Siemens CT (conventional) machine and the Equina by Asto CT (standing) machine; and (2) to determine if the HU values obtained on each machine differed over time. The mean HU from 30 scans of an electron density phantom were generated from each of the two CT machines (N = 60; conventional and standing). Differences between machines were assessed using a linear mixed effects model. The mean HU of each positive value ROI from the conventional machine was decreased compared with the standing machine (P < 0.001) and the difference was greatest for the denser phantom inserts. A calibration curve equation utilising the four bone-equivalent insert plugs of the phantom, with known calcium hydroxyapatite (CaHA) concentration, was generated using linear regression to represent CaHA, and therefore BMD, for the conventional machine (CaHA = -29.58 + 0.98xROI _mean ) and the standing machine (CaHA = -54.53 + 0.95xROI _mean ). For opportunistic calculation of BMD from HU in horses, the differences in HU observed between different CT machines warrants calibration of HU using a bone equivalent phantom.

Background: Condylar stress fracture of the third metacarpal bone is a common catastrophic musculoskeletal injury in Thoroughbred racehorses. Condylar stress fracture is associated with parasagittal groove (PSG) subchondral osteolysis. Standing computed tomography (sCT) imaging allows for sensitive identification of this fatigue-induced structural change, an early form of subchondral bone injury (SBI), but there is not yet an objective method for identifying racehorses at heightened risk of condylar stress fracture. Objectives: To estimate PSG first principal strain in elite Thoroughbred racehorses that have undergone subjective risk assessment using sCT fetlock screening. Study design: Retrospective clinical study. Methods: We used fetlock sCT images from 9 thoracic limbs from 7 Thoroughbred racehorses. A tuned, validated 3D finite element (FE) analysis approach was used as a virtual mechanical test to estimate PSG first principal strain in the distal third metacarpal bone (MC3) from these joints. Virtual mechanical testing results were compared with a subjective clinical risk assessment using an established screening approach by Racing Victoria. Results: MC3 condyles with PSG SBI consistently and significantly displayed increased levels of first principal strain throughout the PSG. We found focal strain concentrations associated with the anatomical location of SBI compared to condyles with no evidence of SBI in the PSG. Clinical diagnosis of SBI with focal osteolysis, FE-predicted strain elevation, and clinical risk assessment were concordant with recall and precision of 0.86. Main limitations: The sample size was small, and our virtual mechanical testing protocol does not account for whole-joint physiology. Conclusions: Risk assessment through screening with sCT is an established approach to injury prevention in racing Thoroughbreds. Concordance of a current clinical risk assessment approach by Racing Victoria with objective FE analysis of principal strain in sites of SBI in the present study suggests 3D FE analysis using a validated pipeline is an important new approach for routine assessment of risk of MC3 condylar stress fracture in Thoroughbred racehorses.

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.