Subchondral bone microdamage accumulation in distal metacarpus of Thoroughbred racehorses

Characterization and quantification of in-vitro equine bone resorption in 3D using μCT and deep learning-aided feature segmentation

High cyclic strains induce formation of microcracks in bone, triggering targeted bone remodeling, which entails osteoclastic resorption. Racehorse bone is an ideal model for studying the effects of high-intensity loading, as it is subject to focal formation of microcracks and subsequent bone resorption. The volume of resorption in vitro is considered a direct indicator of osteoclast activity but indirect 2D measurements are used more often. Our objective was to develop an accurate, high-throughput method to quantify equine osteoclast resorption volume in μCT 3D images. Here, equine osteoclasts were cultured on equine bone slices and imaged with μCT pre- and postculture. Individual resorption events were then isolated and analyzed in 3D. Modal volume, maximum depth, and aspect ratio of resorption events were calculated. A convolutional neural network (CNN U-Net-like) was subsequently trained to identify resorption events on post-culture μCT images alone, without the need for pre-culture imaging, using archival bone slices with known resorption areas and paired CTX-I biomarker levels in culture media. 3D resorption volume measurements strongly correlated with both the CTX-I levels (p < 0.001) and area measurements (p < 0.001). Our 3D analysis shows that the shapes of resorption events form a continuous spectrum, rather than previously reported pit and trench categories. With more extensive resorption, shapes of increasing complexity appear, although simpler resorption cavity morphologies (small, rounded) remain most common, in acord with the left-hand limit paradigm. Finally, we show that 2D measurements of in vitro osteoclastic resorption are a robust and reliable proxy.

Subchondral bone fatigue injury in the parasagittal condylar grooves of the third metacarpal bone in thoroughbred racehorses elevates site-specific strain concentration

Condylar stress fracture of the distal end of the third metacarpal/metatarsal (MC3/MT3) bones is a major cause of Thoroughbred racehorse injury and euthanasia worldwide. Functional adaptation to exercise and fatigue damage lead to structural changes in the subchondral bone that include increased modeling (resulting in sclerotic bone tissue) and targeted remodeling repair (resulting in focal resorption spaces in the parasagittal groove). Whether these focal structural changes, as detectable by standing computed tomography (sCT), lead to elevated strain at the common site of condylar stress fracture has not been demonstrated. Therefore, the goal of the present study was to compare full-field three-dimensional (3D) strain on the distopalmar aspect of MC3 bone specimens with and without focal subchondral bone injury (SBI). Thirteen forelimb specimens were collected from racing Thoroughbreds for mechanical testing ex vivo and underwent sCT. Subsequently, full-field displacement and strain at the joint surface were determined using stereo digital image correlation. Strain concentration was observed in the parasagittal groove (PSG) of the loaded condyles, and those with SBI in the PSG showed higher strain rates in this region than control bones. PSG strain rate in condyles with PSG SBI was more sensitive to CT density distribution in comparison with condyles with no sCT-detectable injury. Findings from this study help to interpret structural changes in the subchondral bone due to fatigue damage and to assess risk of incipient stress fracture in a patient-specific manner.

Assessment of subchondral bone microdamage quantification using contrast-enhanced imaging techniques

Bone microdamage is common at subchondral bone (SCB) sites subjected to repeated high rate and magnitude of loading in the limbs of athletic animals and humans. Microdamage can affect the biomechanical behaviour of bone under physiological loading conditions. To understand the effects of microdamage on the mechanical properties of SCB, it is important to be able to quantify it. The extent of SCB microdamage had been previously estimated qualitatively using plain microcomputed tomography (μCT) and a radiocontrast quantification method has been used for trabecular bone but this method may not be directly applicable to SCB due to differences in bone structure. In the current study, SCB microdamage detection using lead uranyl acetate (LUA) and quantification by contrast-enhanced μCT and backscattered scanning electron microscopy (SEM) imaging techniques were assessed to determine the specificity of the labels to microdamage and the accuracy of damaged bone volume metrices. SCB specimens from the metacarpus of racehorses, with the hyaline articular cartilage (HAC) removed, were grouped into two with one group subjected to ex vivo uniaxial compression loading to create experimental bone damage. The other group was not loaded to preserve the pre-existing in vivo propagated bone microdamage. A subset of each group was stained with LUA using an established or a modified protocol to determine label penetration into SCB. The μCT and SEM images of stained specimens showed that penetration of LUA into the SCB was better using the modified protocol, and this protocol was repeated in SCB specimens with intact hyaline articular cartilage. The percentage of total label localised to bone microdamage was determined on SEM images, and the estimated labelled bone volume determined by μCT in SCB groups was compared. Label was present around diffuse and linear microdamage as well as oblique linear microcracks present at the articular surface, except in microcracks with high-density mineral infills. Bone surfaces lining pores with recent mineralisation were also labelled. Labelled bone volume fraction (LV/BV) estimated by μCT was higher in the absence of HAC. At least 50% of total labels were localised to bone microdamage when the bone area fraction (B.Ar/T.Ar) of the SCB was greater than 0.85 but less than 30% when B.Ar/T.Ar of the SCB was less than 0.85. To adjust for LUA labels on bone surfaces, a measure of the LV/BV corrected for bone surface area (LV/BV BS-1) was used to quantify damaged SCB. In conclusion, removal of HAC and using a modified labelling protocol effectively stained damaged SCB of the metacarpus of racehorses and represents a technique useful for quantifying microdamage in SCB. This method can facilitate future investigations of the effects of microdamage on joint physiology.

Preliminary study of proton magnetic resonance spectroscopy to assess bone marrow adiposity in the third metacarpus or metatarsus in Thoroughbred racehorses

BACKGROUND

Magnetic resonance spectroscopy (MRS) has been used to investigate metabolic changes within human bone. It may be possible to use MRS to investigate bone metabolism and fracture risk in the distal third metacarpal/tarsal bone (MC/MTIII) in racehorses.

OBJECTIVES

To determine the feasibility of using MRS as a quantitative imaging technique in equine bone by using the 1H spectra for the MC/MTIII to calculate fat content (FC).

STUDY DESIGN

Observational cross-sectional study.

METHODS

Limbs from Thoroughbred racehorses were collected from horses that died or were subjected to euthanasia on racecourses. Each limb underwent magnetic resonance imaging (MRI) at 3 T followed by single-voxel MRS at three regions of interest (ROI) within MC/MTIII (lateral condyle, medial condyle, proximal bone marrow [PBM]). Percentage FC was calculated at each ROI. Each limb underwent computed tomography (CT) and bone mineral density (BMD) was calculated for the same ROIs. All MR and CT images were graded for sclerosis. Histology slides were graded for sclerosis and proximal marrow space was calculated. Pearson or Spearman correlations were used to assess the relationship between BMD, FC and marrow space. Kruskal-Wallis tests were used to check for differences between sclerosis groups for BMD or FC.

RESULTS

Eighteen limbs from 10 horses were included. A negative correlation was identified for mean BMD and FC for the lateral condyle (correlation coefficient = -0.60, p = 0.01) and PBM (correlation coefficient = -0.5, p = 0.04). There was a significant difference between median BMD for different sclerosis grades in the condyles on both MRI and CT. A significant difference in FC was identified between sclerosis groups in the lateral condyle on MRI and CT.

MAIN LIMITATIONS

Small sample size.

CONCLUSIONS

1H Proton MRS is feasible in the equine MC/MTIII. Further work is required to evaluate the use of this technique to predict fracture risk in racehorses.

Differences in bone turnover markers and injury risks between local and international horses: A Victorian Spring Racing Carnival study

BACKGROUND

Musculoskeletal injuries (MSI) are common in racehorses and have been of increasing concern in horses travelling internationally to compete. Understanding the differences in bone turnover between local horses and international horses following long-distance air transportation may inform MSI prevention strategies.

OBJECTIVES

To understand the differences in bone turnover markers and risk of MSI between local horses and international horses following long-distance air transportation.

STUDY DESIGN

Prospective cohort.

METHODS

The concentrations of bone turnover markers (OCN and CTXI), markers of stress (cortisol), inflammation (serum amyloid A) and circadian rhythm (melatonin), and bisphosphonates were determined in blood samples collected twice (14-17 days apart), from horses following international travel (n = 69), and from local horses (n = 79). The associations between markers, long-distance travel and MSI were determined using multivariable generalised linear regression models.

RESULTS

Within 3-5 days post-transport, concentrations of cortisol in international horses were higher than those of local horses (main effect, Coef. 0.39; 95% CI 0.24, 0.54; p < 0.001) but they decreased and were not different to those of local horses at the second timepoint (interaction effect, Coef. -0.27; 95% CI -0.46, -0.07; p = 0.007). After adjusting for age and sex, OCN and CTXI were not significantly different between international and local horses; however, OCN was lower in international horses at timepoint 2 (interaction effect, Coef. -0.16; 95% CI -0.31, -0.01; p = 0.043). The prevalence of MSI was higher in the international (26%; 95% CI 16, 38%) compared with local horses (8%; 95% CI 3, 16%; p < 0.001), with all severe MSI sustained by the international horses. At the second timepoint compared with the first timepoint post-transport, cortisol remained high or increased (interaction effect, Coef. 0.43; 95% CI 0.24, 0.61; p < 0.001) and OCN increased (interaction effect, Coef. 0.26; 95% CI 0.08, 0.44; p = 0.006) in the horses that sustained severe MSI.

MAIN LIMITATIONS

Horse population and racing career parameters differed between groups. Bone turnover markers have low sensitivity to detect local bone changes.

CONCLUSIONS

Most horses showed minimal effects of long-distance air transport within 2 weeks relative to local horses as assessed by stress and bone turnover markers. Screening for persistent high cortisol and evidence of net bone formation after long-distance air transportation may help to identify racehorses at high risk of catastrophic MSI.

Mechanical response of local regions of subchondral bone under physiological loading conditions

Most fractures in the third metacarpal bone of equine athletes occur due to repeated cycles of high load magnitudes and are commonly generated during fast-training workouts. These repetitive loads may induce changes in the microstructure and mechanical properties that can develop into subchondral bone (SCB) injuries near the articular surface. In this study, we investigated the fatigue behaviour of local regions in SCB (near the articular surface i.e., 2 mm superficial SCB and the underlying 2 mm deeper SCB) under a simulated fast-training workout of an equine athlete. A fatigue test on SCB specimens was designed to simulate the fast-training workout, which comprised of repeated load cycles with varying load magnitude, representing the varying gait speed during a fast-training workout. The fatigue test was applied three times to each of the five cylindrical SCB specimens harvested from the left and right metacarpal condyles of five thoroughbred racehorses). All specimens completed at least one fatigue test. Three specimens completed all three fatigue tests with no visible cracks identified with Micro-CT scans. The other two specimens failed in the second fatigue test, and cracks were identified with Micro-CT scans in the various local regions. Using Digital Image Correlation (DIC) analysis, we found that in the local regions of all specimens, modulus decreased between load cycles corresponding to 68 and 93 MPa load magnitudes (equivalent to the fastest gallop speed). Wherein specimens that failed exhibited a greater decrease in modulus (in superficial SCB by 45.64 ± 5.66% and in deeper SCB by -36.85 ± 10.47% (n = 2)) than those not failed (in superficial SCB by -7.45 ± 14.62% and in deeper SCB by -5.67 ± 7.32% (n = 3)). This has provided evidence that the loads on SCB at galloping speeds are most likely to produce fatigue damage and that the damage induced is localised. Furthermore, one of the failed specimens exhibited a peak in the tensile strain rather than compressive strain in the superficial region with a rapid decrease in modulus. In addition, the superficial region of all specimens exhibited greater residual tensile strain than that of the deeper region.

Comparison of Reported Fatalities, Falls and Injuries in Thoroughbred Horse Jumps and Flat Races in the 2022 and 2023 Jumps Race Seasons in Victoria, Australia

Jumps racing is a form of Thoroughbred horse racing that involves hurdles and steeples and typically longer distances, and heavier weights compared with flat racing, which does not incorporate obstacles. In Australia, jumps racing is carried out only in Victoria, one of eight states and territories. The continuation of jumps racing is contentious due to the higher risk of fatalities, falls and injuries for horses, compared with flat racing. While measures have been introduced by the industry to improve the safety of riders and horses, the rates of fatalities, falls and injuries in horses participating in jumps races have not been collectively reported in Australia since the 2012 to 2014 race seasons. Although information on individual horse fatalities, falls and injuries is published by Racing Victoria in Stewards' Reports, the data are not aggregated, and so cannot readily be used to assess trends or evaluate the efficacy of safety measures introduced by the industry. The aim of this study was to determine the fatality, fall and injury rates for horses participating in hurdle and steeplechase races in Victoria in the 2022 and 2023 Thoroughbred horse jumps racing seasons compared with horses participating in flat races at the same race meets. Data on horse fatalities, falls and injuries were extracted from the published Racing Victoria race results and Stewards' Reports for the jumps races (n = 150) and corresponding flat races (n = 157) held at the 38 jumps race meets in Victoria in 2022 and 2023. Overall, horse fatalities, falls and injuries occurred at higher rates in jumps races compared with flat races during the study period. The rate of horse fatalities in jumps races was 3.3 per 1000 starts, with no fatalities in flat races. The rate of horse falls in hurdle races was 24 per 1000 starts and 41.6 per 1000 starts in steeplechase races, comparable with rates previously reported in the 2012 to 2014 seasons. There were no falls in flat races. Horse injuries occurred at a rate of 68.9 per 1000 starts in jumps races compared with 18.8 per 1000 starts in flat races. In hurdle and steeplechase races, veterinary clearance being required following horse injury was 5.4 times (OR 5.4, 95% CI 2.8-10.2) and 7.2 times (OR 7.2, 95% CI 3.3-15.6) more likely, respectively, compared with flat races. The risk of trauma was 4 times more likely in hurdle and steeplechase races (OR 4.8, 95% CI 1.7-13.3 and OR 4.1, 95% CI 1.2-13.4, respectively) and the risk of lameness was increased by 2.5 times in hurdles (OR 2.5, 95% CI 1.2-5.2) and 5.1 times in steeplechase races (OR 5.1, 95% CI 2.3-11.5), compared with flat races. These findings support concerns about the welfare of horses involved in jumps racing and of the need for further safety measures to reduce these risks.

Prospective, longitudinal assessment of subchondral bone morphology and pathology using standing, cone-beam computed tomography in fetlock joints of 2-year-old Thoroughbred racehorses in their first year of training

BACKGROUND

Catastrophic injuries of the fetlock joints occur in Thoroughbred racehorses and are preceded by stress-induced bone injury. Early detection of subchondral bone injury is essential to prevent irreversible damage or bone failure.

OBJECTIVES

To investigate the use of standing, robotic cone-beam computed tomography (CBCT) for assessing longitudinal changes in subchondral bone morphology and pathology of the fetlock joints associated with race training in young Thoroughbreds.

STUDY DESIGN

Observational cohort study.

METHODS

Forty-one 2-year-old Thoroughbred racehorses were recruited prior to the start of race training. Standing CBCT and radiographs of all 4 metacarpo-/metatarsophalangeal (MCP/MTP) joints were obtained at 0, 6 and 12 months. Hyperdensity, as an estimate of subchondral bone sclerosis, was measured in the distal third metacarpal (MC3)/metatarsal (MT3) bone and proximal phalanx (P1) at each time point on computed tomography. CBCTs were examined for subchondral bone pathology consisting of areas of hypodensity within regions of hyperdensity.

RESULTS

Subchondral bone sclerosis increased significantly over time in the medial and lateral MC3/MT3 condyles and in the medial and lateral parasagittal grooves of MC3/MT3. The presence of subchondral bone pathology increased significantly over time in the medial and lateral palmar condyles of MC3/MT3, the lateral parasagittal groove, the medial dorsal condyle and the medial and lateral ridges of P1.

MAIN LIMITATIONS

There was attrition of horses due to relocation, change in ownership, and retirement from racing. Husbandry, training regimens and racing schedules were not controlled for in the study.

CONCLUSIONS

Standing CBCT is an efficient and effective screening tool for assessing subchondral bone morphology and identifying pathology of the fetlock joint in young Thoroughbred racehorses. CBCT may facilitate early detection of bone pathology allowing for timely intervention and prevention of more serious injuries.

QCT-based computational bone strength assessment updated with MRI-derived 'hidden' microporosity

Microdamage accumulated through sustained periods of cyclic loading or single overloading events contributes to bone fragility through a reduction in stiffness and strength. Monitoring microdamage in vivo remains unattainable by clinical imaging modalities. As such, there are no established computational methods for clinical fracture risk assessment that account for microdamage that exists in vivo at any specific timepoint. We propose a method that combines multiple clinical imaging modalities to identify an indicative surrogate, which we term 'hidden porosity', that incorporates pre-existing bone microdamage in vivo. To do so, we use the third metacarpal bone of the equine athlete as an exemplary model for fatigue induced microdamage, which coalesces in the subchondral bone. N = 10 metacarpals were scanned by clinical quantitative computed tomography (QCT) and magnetic resonance imaging (MRI). We used a patch-based similarity method to quantify the signal intensity of a fluid sensitive MRI sequence in bone regions where microdamage coalesces. The method generated MRI-derived pseudoCT images which were then used to determine a pre-existing damage (Dpex) variable to quantify the proposed surrogate and which we incorporate into a nonlinear constitutive model for bone tissue. The minimum, median, and maximum detected Dpex of 0.059, 0.209, and 0.353 reduced material stiffness by 5.9%, 20.9%, and 35.3% as well as yield stress by 5.9%, 20.3%, and 35.3%. Limb-specific voxel-based finite element meshes were equipped with the updated material model. Lateral and medial condyles of each metacarpal were loaded to simulate physiological joint loading during gallop. The degree of detected Dpex correlated with a relative reduction in both condylar stiffness (p = 0.001, R2 > 0.74) and strength (p < 0.001, R2 > 0.80). Our results illustrate the complementary value of looking beyond clinical CT, which neglects the inclusion of microdamage due to partial volume effects. As we use clinically available imaging techniques, our results may aid research beyond the equine model on fracture risk assessment in human diseases such as osteoarthritis, bone cancer, or osteoporosis.

The association between Thoroughbred racehorse training practices and musculoskeletal injuries in Victoria, Australia

Catastrophic musculoskeletal injuries (CMI) in horses are associated with both too little and too much high-speed exercise. In order to advise trainers on training and management strategies that minimize the risk of musculoskeletal injury (MSI), a better understanding of how training practices affect MSI in racehorses is needed. Data from prospective studies relating training data and MSI are complicated by the gradual development of pathology and the effect of this on the ability of horses to train consistently prior to the identification of an injury. To circumvent this, 66 Australian Thoroughbred trainers were surveyed on their intended training practices, including rest, pre-training, and race-fit practices. Associations between intended training practices and catastrophic and non-catastrophic race day MSI outcomes in two-year-old and mature (≥three-year-old) horses were assessed using multivariable negative binomial regression models. The incidence of two-year-old race day MSI was lower for trainers who preferred shorter times (weeks) to trial, less time in fast work pre-trial (p = 0.003), shorter, more frequent rest periods (p < 0.01) and higher amounts of fast work at 15.5-16.7 m/s once race-fit (p = 0.001). The incidence of mature horse race day MSI was lower for trainers who preferred longer rest periods (p = 0.026) and a high-volume pre-trial training strategy comprising a high volume of slower speed gallop training and longer times to trial compared to fast and light training programs (p = 0.004) for their mature horses, in addition to higher amounts of fast work at 15.5-16.7 m/s for their race-fit two-year-olds (p = 0.012). Race day CMI incidence was lower for trainers who preferred lower volumes of fast gallop work for their race-fit mature horses (p < 0.05). These results suggest that two-year-old training practices could affect MSI risk later in a horse's career, and that age and stage in training (pre-trial, race-fit) are important considerations when developing training practices to minimize the risk of MSI.

Three-Dimensional Imaging and Histopathological Features of Third Metacarpal/Tarsal Parasagittal Groove and Proximal Phalanx Sagittal Groove Fissures in Thoroughbred Horses

Fissure in the third metacarpal/tarsal parasagittal groove and proximal phalanx sagittal groove is a potential prodromal pathology of fracture; therefore, early identification and characterisation of fissures using non-invasive imaging could be of clinical value. Thirty-three equine cadaver limbs underwent standing cone-beam (CB) computed tomography (CT), fan-beam (FB) CT, low-field magnetic resonance imaging (MRI), and macro/histo-pathological examination. Imaging diagnoses of fissures were compared to microscopic examination. Imaging features of fissures were described. Histopathological findings were scored and compared between locations with and without fissures on CT. Microscopic examination identified 114/291 locations with fissures. The diagnostic sensitivity and specificity were 88.5% and 61.3% for CBCT, 84.1% and 72.3% for FBCT, and 43.6% and 85.2% for MRI. Four types of imaging features of fissures were characterised on CT: (1) CBCT/FBCT hypoattenuating linear defects, (2) CBCT/FBCT striated hypoattenuated lines, (3) CBCT/FBCT subchondral irregularity, and (4) CBCT striated hypoattenuating lines and FBCT subchondral irregularity. Fissures on MRI appeared as subchondral bone hypo-/hyperintense defects. Microscopic scores of subchondral bone sclerosis, microcracks, and collapse were significantly higher in locations with CT-identified fissures. All imaging modalities were able to identify fissures. Fissures identified on CT were associated with histopathology of fatigue injuries.

Relationship between CT-Derived Bone Mineral Density and UTE-MR-Derived Porosity Index in Equine Third Metacarpal and Metatarsal Bones

Fatigue-related subchondral bone injuries of the third metacarpal/metatarsal (McIII/MtIII) bones are common causes of wastage, and they are welfare concerns in racehorses. A better understanding of bone health and strength would improve animal welfare and be of benefit for the racing industry. The porosity index (PI) is an indirect measure of osseous pore size and number in bones, and it is therefore an interesting indicator of bone strength. MRI of compact bone using traditional methods, even with short echo times, fail to generate enough signal to assess bone architecture as water protons are tightly bound. Ultra-short echo time (UTE) sequences aim to increase the amount of signal detected in equine McIII/MtIII condyles. Cadaver specimens were imaged using a novel dual-echo UTE MRI technique, and PI was calculated and validated against quantitative CT-derived bone mineral density (BMD) measures. BMD and PI are inversely correlated in equine distal Mc/MtIII bone, with a weak mean r value of -0.29. There is a statistically significant difference in r values between the forelimbs and hindlimbs. Further work is needed to assess how correlation patterns behave in different areas of bone and to evaluate PI in horses with and without clinically relevant stress injuries.

Long-term monitoring of clodronate in equine hair using liquid chromatography-tandem mass spectrometry

Given the potential for long-term inhibition of bone remodeling/healing and detrimental effects to horses in training, bisphosphonates are tightly regulated in horseracing. Hair has proven to be an effective matrix for detection of drug administration to horses and has been particularly effective in detecting drugs for a long period of time post administration. Thus, hair may prove to be a useful matrix for detection of administration of this class of drugs. The objective of the current study was to develop an assay and assess the usefulness of hair as a matrix for long-term detection of clodronate to horses. Seven horses received a single intramuscular administration of 1.8 mg/kg clodronate. Hair samples were collected prior to and up to 6 months post administration. A liquid chromatography-tandem mass spectrometry method was developed and concentrations of clodronate measured in hair samples. The drug was first detected on day 7 in 4/7 horses, and on days 14, 28 and 35 in the remaining three horses. In 4/7 horses, clodronate was still detectable 6 months post administration. Results of this study demonstrate that, although there was significant inter-individual variability in detection times (63 to 180 days) and several intermediate times where the drug could not be detected but was subsequently detected in later timepoints, clodronate administration was detectable in hair for a prolonged period in most of the horses (4/7) studied.

Metacarpophalangeal Joint Pathology and Bone Mineral Density Increase with Exercise but Not with Incidence of Proximal Sesamoid Bone Fracture in Thoroughbred Racehorses

Proximal sesamoid bone (PSB) fracture is the leading cause of fatal musculoskeletal injury in Thoroughbred racehorses in Hong Kong and the US. Efforts are underway to investigate diagnostic modalities that could help identify racehorses at increased risk of fracture; however, features associated with PSB fracture risk are still poorly understood. The objectives of this study were to (1) investigate third metacarpal (MC3) and PSB density and mineral content using dual-energy X-ray absorptiometry (DXA), computed tomography (CT), Raman spectroscopy, and ash fraction measurements, and (2) investigate PSB quality and metacarpophalangeal joint (MCPJ) pathology using Raman spectroscopy and CT. Forelimbs were collected from 29 Thoroughbred racehorse cadavers (n = 14 PSB fracture, n = 15 control) for DXA and CT imaging, and PSBs were sectioned for Raman spectroscopy and ash fraction measurements. Bone mineral density (BMD) was greater in MC3 condyles and PSBs of horses with more high-speed furlongs. MCPJ pathology, including palmar osteochondral disease (POD), MC3 condylar sclerosis, and MC3 subchondral lysis were greater in horses with more high-speed furlongs. There were no differences in BMD or Raman parameters between fracture and control groups; however, Raman spectroscopy and ash fraction measurements revealed regional differences in PSB BMD and tissue composition. Many parameters, including MC3 and PSB bone mineral density, were strongly correlated with total high-speed furlongs.

Effects of in vivo fatigue-induced microdamage on local subchondral bone strains

Biomechanical strain is a major stimulus of subchondral bone (SCB) tissue adaptation in joints but may also lead to initiation and propagation of microcracks, highlighting the importance of quantifying the intratissue strain in subchondral bone. In the present study, we used micro computed tomography (μCT) imaging, mechanical testing, and digital image correlation (DIC) techniques to evaluate the biomechanical strains in equine SCB under impact compression applied through the articular surface. We aimed to investigate the effects of in vivo accumulated microdamage in equine SCB on the distribution of mechanical impact strain through the articular cartilage. Under the applied strain of 2.0 ± 0.1% (mean ± standard deviation, n=15) to the articular surface of cartilage-bone plugs, the overall thickness of the SCB developed eSCBOverall = 0.7 ± 0.2% in all specimens. Contours of high strains in specimens without microdamage (NDmg) aligned parallel to the cartilage-bone interface with peak tensile, ϵt, and compressive, ϵc, strains of 0.5 ± 0.3% and 1.2 ± 0.4%, respectively at the time of peak compression (n=7). In damaged specimens (Dmg), contours of high strains aligned with the cracks in the imaged plane with peak strains of ϵt= 1.2 ± 0.8% and ϵc= 3.5 ± 2.2%, respectively (n=7). Microdamage was the main predictor of the normalised compressive and tensile strains across the SCB thickness. Results of multivariable analyses revealed presence of microdamage, distance from the articular surface and TMD were the main predictors of normalised compressive and tensile strain. Strain was greater in the superficial bone, particularly for specimens with microdamage. In vivo fatigue-induced microdamage is an important predictor of local subchondral bone strains.

Exercise history predicts focal differences in bone volume fraction, mineral density and microdamage in the proximal sesamoid bones of Thoroughbred racehorses

Medial proximal sesamoid bones (PSBs) from Thoroughbred racehorses that did (Case) or did not (Control) experience unilateral biaxial PSB fracture were evaluated for bone volume fraction (BVF), apparent mineral density (AMD), tissue mineral density (TMD), and microdamage in Case fractured, Case contralateral limb intact, and Control bones. A majority of Case bones had a subchondral lesion with high microdamage density, and low BVF, AMD, and TMD. Lesion microdamage and densitometric measures were associated with training history by robust linear regression. Exercise intensity was negatively related to BVF (0.07 ≤ R²  ≤ 0.12) and positively related to microcrack areal density (0.21 ≤ R²  ≤ 0.29) in the lesion; however, in an undamaged site, the relationships were opposite in direction. Regardless of location, TMD decreased with event frequency for both Case and Control, suggesting increased bone remodeling with exercise. Measures of how often animals were removed from active training (layups) predicted a decrease in TMD, AMD, BVF, and microdamage at regions away from the lesion site. A steady-state compartment model was used to organize the differences in the correlations between variables within the data set. The overall conclusions are that at the osteopenic lesion site, repair of microdamage by remodeling was not successful (e.g., lower bone mass, increased damage, and lower mineralization) but that in regions away from the lesion remodeling successfully controlled damage (e.g., higher bone mass, less microdamage, and lower mineralization).

Hoof Impact and Foot-Off Accelerations in Galloping Thoroughbred Racehorses Trialling Eight Shoe-Surface Combinations

The athletic performance and safety of racehorses is influenced by hoof−surface interactions. This intervention study assessed the effect of eight horseshoe−surface combinations on hoof acceleration patterns at impact and foot-off in 13 galloping Thoroughbred racehorses retired from racing. Aluminium, barefoot, GluShu (aluminium−rubber composite) and steel shoeing conditions were trialled on turf and artificial (Martin Collins Activ-Track) surfaces. Shod conditions were applied across all four hooves. Tri-axial accelerometers (SlamStickX, range ±500 g, sampling rate 5000 Hz) were attached to the dorsal hoof wall (x: medio-lateral, medial = positive; y: along dorsal hoof wall, proximal = positive; and z: perpendicular to hoof wall, dorsal = positive). Linear mixed models assessed whether surface, shoeing condition or stride time influenced maximum (most positive) or minimum (most negative) accelerations in x, y and z directions, using ≥40,691 strides (significance at p < 0.05). Day and horse−rider pair were included as random factors, and stride time was included as a covariate. Collective mean accelerations across x, y and z axes were 22−98 g at impact and 17−89 g at foot-off. The mean stride time was 0.48 ± 0.07 s (mean ±2 SD). Impact accelerations were larger on turf in all directions for forelimbs and hindlimbs (p ≤ 0.015), with the exception of the forelimb z-minimum, and in absolute terms, maximum values were typically double the minimum values. The surface type affected all foot-off accelerations (p ≤ 0.022), with the exception of the hindlimb x-maximum; for example, there was an average increase of 17% in z-maximum across limbs on the artificial track. The shoeing condition influenced all impact and foot-off accelerations in the forelimb and hindlimb datasets (p ≤ 0.024), with the exception of the hindlimb impact y-maximum. Barefoot hooves generally experienced the lowest accelerations. The stride time affected all impact and foot-off accelerations (p < 0.001). Identifying factors influencing hoof vibrations upon landing and hoof motion during propulsion bears implication for injury risk and racing outcomes.

Biomechanical and Microstructural Properties of Subchondral Bone From Three Metacarpophalangeal Joint Sites in Thoroughbred Racehorses

Fatigue-induced subchondral bone (SCB) injury is common in racehorses. Understanding how subchondral microstructure and microdamage influence mechanical properties is important for developing injury prevention strategies. Mechanical properties of the disto-palmar third metacarpal condyle (MCIII) correlate poorly with microstructure, and it is unknown whether the properties of other sites within the metacarpophalangeal (fetlock) joint are similarly complex. We aimed to investigate the mechanical and structural properties of equine SCB from specimens with minimal evidence of macroscopic disease. Three sites within the metacarpophalangeal joint were examined: the disto-palmar MCIII, disto-dorsal MCIII, and proximal sesamoid bone. Two regions of interest within the SCB were compared, a 2 mm superficial and an underlying 2 mm deep layer. Cartilage-bone specimens underwent micro-computed tomography, then cyclic compression for 100 cycles at 2 Hz. Disto-dorsal MCIII specimens were loaded to 30 MPa (n = 10), while disto-palmar MCIII (n = 10) and proximal sesamoid (n = 10) specimens were loaded to 40 MPa. Digital image correlation determined local strains. Specimens were stained with lead-uranyl acetate for volumetric microdamage quantification. The dorsal MCIII SCB had lower bone volume fraction (BVTV), bone mineral density (BMD), and stiffness compared to the palmar MCIII and sesamoid bone (p < 0.05). Superficial SCB had higher BVTV and lower BMD than deeper SCB (p < 0.05), except at the palmar MCIII site where there was no difference in BVTV between depths (p = 0.419). At all sites, the deep bone was stiffer (p < 0.001), although the superficial to deep gradient was smaller in the dorsal MCIII. Hysteresis (energy loss) was greater superficially in palmar MCIII and sesamoid (p < 0.001), but not dorsal MCIII specimens (p = 0.118). The stiffness increased with cyclic loading in total cartilage-bone specimens (p < 0.001), but not in superficial and deep layers of the bone, whereas hysteresis decreased with the cycle for all sites and layers (p < 0.001). Superficial equine SCB is uniformly less stiff than deeper bone despite non-uniform differences in bone density and damage levels. The more compliant superficial layer has an important role in energy dissipation, but whether this is a specific adaptation or a result of microdamage accumulation is not clear.

Proximal sesamoid bone microdamage is localized to articular subchondral regions in Thoroughbred racehorses, with similar fracture toughness between fracture and controls

OBJECTIVE

To determine whether proximal sesamoid bone (PSB) microdamage and fracture toughness differ between Thoroughbred racehorses sustaining PSB fracture and controls.

STUDY DESIGN

Cadaveric case-control.

ANIMALS

Twenty-four Thoroughbred racehorses (n = 12 PSB fracture, n = 12 control).

METHODS

Proximal sesamoid bones were dissected, and gross pathological changes and morphological measurements were documented. High-speed exercise history data were evaluated. Microdamage was assessed in fracture, fracture-contralateral limb (FXCL) and control PSBs using whole bone lead uranyl acetate (LUA) staining with micro-CT imaging or basic fuchsin histological analysis. Fracture toughness mechanical testing was carried out in 3-point-bending of microbeams created from PSB flexor cortices. Data were analyzed using ordinal logistic and linear regression models.

RESULTS

Microdamage was detected most commonly in the articular subchondral region of PSBs via LUA micro-CT and basic fuchsin histology. There were no differences in microdamage between FXCL and control PSBs. Fracture toughness values were similar for FXCL (1.31 MPa√m) and control (1.35 MPa√m) PSBs. Exercise histories were similar except that horses sustaining fracture spent a greater percentage of their careers in rest weeks.

CONCLUSION

Microdamage was detected in the articular region of PSBs but was not greater in horses sustaining catastrophic PSB fracture. Fracture toughness of PSB flexor cortices did not differ between FXCL and control PSBs.

CLINICAL SIGNIFICANCE

Although uncommon, microdamage is localized to the articular region of Thoroughbred racehorse PSBs. Catastrophic PSB failure is not associated with lower PSB flexor cortex fracture toughness.

An in vitro model for discovery of osteoclast specific biomarkers towards identification of racehorses at risk for catastrophic fractures

BACKGROUND

Focal bone microcracks with osteoclast recruitment and bone lysis, may reduce fracture resistance in racehorses. As current imaging does not detect all horses at risk for fracture, the discovery of novel serum biomarkers of bone resorption or osteoclast activity could potentially address this unmet clinical need. The biology of equine osteoclasts on their natural substrate, equine bone, has never been studied in vitro and may permit identification of specific biomarkers of their activity.

OBJECTIVES

1) Establish osteoclast cultures on equine bone, 2) Measure biomarkers (tartrate resistant acid phosphatase isoform 5b (TRACP-5b) and C-terminal telopeptide of type I collagen (CTX-I)) in vitro and 3) Study the effects of inflammation.

STUDY DESIGN

In vitro experiments.

METHODS

Haematopoietic stem cells, from 5 equine sternal bone marrow aspirates, were differentiated into osteoclasts and cultured either alone or on equine bone slices, with or without pro-inflammatory stimulus (IL-1β or LPS). CTX-I and TRACP-5b were immunoassayed in the media. Osteoclast numbers and bone resorption area were assessed.

RESULTS

TRACP-5b increased over time without bone (p < 0.0001) and correlated with osteoclast number (r = 0.63, p < 0.001). CTX-I and TRACP-5b increased with time for cultures with bone (p = 0.002; p = 0.02 respectively), correlated with each other (r = 0.64, p < 0.002) and correlated with bone resorption (r = 0.85, p < 0.001; r = 0.82, p < 0.001 respectively). Inflammation had no measurable effects.

MAIN LIMITATIONS

Specimen numbers limited.

CONCLUSIONS

Equine osteoclasts were successfully cultured on equine bone slices and their bone resorption quantified. TRACP-5b was shown to be a biomarker of equine osteoclast number and bone resorption for the first time; CTX-I was also confirmed to be a biomarker of equine bone resorption in vitro. This robust equine specific in vitro assay will help the study of osteoclast biology.

Morphological analysis of third metacarpus cartilage and subchondral bone in Thoroughbred racehorses: An ex vivo study

Racehorses are exposed to repetitive overload during training and competition, causing joint hyperextension, tissue fatigue, and ultimately skeletal failure. Some degree of bone changes, such as sclerosis, are expected in equine athletes, as adaptation to the biomechanical rigors of training and racing. Understanding the imaging characteristics of the equine joint surface and subchondral bone would allow earlier detection of injuries or adaptation, improving prognosis and training programs. This study sought to describe the joint surface structural patterns and the periarticular structures of the third metacarpal bone (MC3). Both forelimbs of eight horses engaged in daily training programs, aged 3 to 5 years, which were euthanized for reasons unrelated to the metacarpophalangeal (MCP) joints, were collected. Specimens were evaluated through macroscopic inspection, radiography, ultrasonography, and microscopic examinations, such as optical microscopy and microtomography. Analysis of the microtomography images showed that 50% of the samples had higher trabecular thickness in the lateral condyle. Comparison of each imaging examination revealed that ultrasound images were most closely related to the histological examination (p = 0.29) in terms of sensitivity, while macroscopic and radiographic examinations differed most between evaluators. Finally, the irregularities and modifications observed in the articular cartilage surface and subchondral bone were normal adaptations of the anatomical structures of trained racehorses, which should be considered during clinical examination.

Race-Level Reporting of Incidents during Two Seasons (2015/16 to 2016/17) of Harness Racing in New Zealand

The objective of this study was to describe the incident and non-incident reporting of harness racing in New Zealand, the primary injury and reporting outcomes, and to examine horse- and race-level variables associated with the odds of these outcomes. Retrospective stipendiary stewards’ reports of race day events during the 2015/16 to 2016/17 racing seasons were examined. The number of incident and non-incident events and binomial exact 95% confidence intervals (CI) were calculated per 1000 horse starts. Most reports were for non-incidents and an examination was requested for poor performance (11.06 per 1000 starts (95% CI = 10.23–11.89). Races with more than eight participants were 1.9 (95% CI = 1.13–3.4) times more likely to have an incident than races with eight or less participants. The low incidence of significant injuries such as fractures (0.13 per 1000 starts (95% CI = 0.03–0.23) reflects the lower risk of injury in harness racing compared to Thoroughbred racing. The high incidence of poor performance reports highlights the steward’s role in maintaining animal welfare to a high standard.

Imaging and Gross Pathological Appearance of Changes in the Parasagittal Grooves of Thoroughbred Racehorses

Simple Summary

Early detection of racehorses at risk of stress fracture is key to reducing the number of horses with 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 currently unknown whether some of these changes indicate an impending fracture or are a healthy adaptation to high-speed exercise. This study looks at imaging and gross changes in a specific area (parasagittal grooves (PSGs) of the cannon bone) and the utility of X-ray, computed tomography (CT) and magnetic resonance imaging (MRI) to detect the changes. All fetlock joints were assessed from twenty horses that died during racing or training, including horses with and without fetlock fracture. Overall, X-ray was poor for detecting PSG changes. Some PSG changes on CT and MRI were common in Thoroughbred racehorses and possibly represent normal bone adaptation when seen in clinical cases. However, certain CT and MRI findings were more prevalent in horses with a fracture, possibly indicating microdamage accumulation and increased risk of fracture. Bilateral advanced imaging is recommended in clinical cases of suspected fetlock pathology.

Abstract

(1) Background: Parasagittal groove (PSG) changes are often present on advanced imaging of racing Thoroughbred fetlocks and have been suggested to indicate increased fracture risk. Currently, there is limited evidence differentiating the imaging appearance of prodromal changes in horses at risk of fracture from horses with normal adaptive modelling in response to galloping. This study aims to investigate imaging and gross PSG findings in racing Thoroughbreds and the comparative utility of different imaging modalities to detect PSG changes. (2) Methods: Cadaver limbs were collected from twenty deceased racing/training Thoroughbreds. All fetlocks of each horse were examined with radiography, low-field magnetic resonance imaging (MRI), computed tomography (CT), contrast arthrography and gross pathology. (3) Results: Horses with fetlock fracture were more likely to have lateromedial PSG sclerosis asymmetry and/or lateral PSG lysis. PSG lysis was not readily detected using MRI. PSG subchondral bone defects were difficult to differentiate from cartilage defects on MRI and were not associated with fractures. The clinical relevance of PSG STIR hyperintensity remains unclear. Overall, radiography was poor for detecting PSG changes. (4) Conclusions: Some PSG changes in Thoroughbred racehorses are common; however, certain findings are more prevalent in horses with fractures, possibly indicating microdamage accumulation. Bilateral advanced imaging is recommended in racehorses with suspected fetlock pathology.

Dentinal microcracks and cemental tears related to chemo-mechanical root canal instrumentation: a micro-CT Cadaver Study

BACKGROUND

The prevalence of radicular defects after root canal instrumentation is unresolved. This study used micro-CT to assess the relationship between the formation of radicular defects and chemo-mechanical instrumentation in a cadaver model.

METHODS

Maxillary and mandibular molars (n = 24) were sectioned from cadaver specimens as a tissue block containing the teeth, alveolar bone and attached mucogingival tissues. After a baseline micro-CT scan (13.45 μm), the specimens were distributed into 3 groups (n = 8 molars): Reciproc^® , ProTaper Next™ and Mtwo^® . Micro-CT scans of each specimen were obtained after access, glide path and preparation with each instrument. The pre-operative and final post-operative micro-CT cross-sectional images of the roots were screened by two blinded examiners to identify any pre-existing and new radicular defects. Pre-existing and new radicular defects were examined histologically.

RESULTS

Overall, 16 pre-existing radicular defects were identified in 12 of the 24 molars (50%). Most of these were cemental tears (87.5%), and not true dentinal microcracks. New dentinal microcracks were observed in the post-operative micro-CT scans of only 3 canals (3.9%; 3/77). However, only one of these defects was found to be present histologically.

CONCLUSIONS

Within the limitations of the study, chemo-mechanical instrumentation did not routinely promote the formation of radicular defects.

Determination of correlation of proximal sesamoid bone osteoarthritis with high-speed furlong exercise and catastrophic sesamoid bone fracture in Thoroughbred racehorses

OBJECTIVE

To examine whether proximal sesamoid bone (PSB) articular cartilage and bone osteoarthritic changes or palmar osteochondral disease (POD) scores were associated with exercise history and catastrophic PSB fracture in Thoroughbred racehorses.

SAMPLES

PSBs from 16 Thoroughbred racehorses (8 with and 8 without PSB fracture).

PROCEDURES

Exercise history was collected, and total career high-speed furlongs was used as the measure of total exercise per horse. At necropsy, medial and lateral condyles of the third metacarpus from each forelimb were assigned a POD score, followed by imaging with micro-CT for evaluation of osteophyte size. Three investigators that were blinded to the type of PSB (fracture or no fracture) used the Osteoarthritis Research Society International (OARSI) scoring system to evaluate acellularity, chondrocyte necrosis, cartilage fibrillation, chondrone formation, safranin O stain uptake, and tidemark advancement of 1 central sagittal tissue section/PSB (4 PSBs/horse). Cartilage thickness and bone necrosis were scored on the basis of histologic examination.

RESULTS

POD score, osteophyte size score, percentage of bone necrosis, tidemark advancement, chondrone formation, and total OARSI score were greater in horses with more accrued total career high-speed furlongs. Scores for POD, osteophyte size, fibrillation, acellularity, chondrone formation, and total OARSI were greater for horses with PSB fracture.

CONCLUSIONS AND CLINICAL REVELANCE

OARSI scoring revealed that more advanced osteoarthritic changes strongly correlated with total career high-speed furlongs and PSB fracture. However, the effect of exercise was dominant, suggesting that exercise history will be important to include in future models that aim to assess risk factors for catastrophic PSB fracture.

Use of Omics Data in Fracture Prediction; a Scoping and Systematic Review in Horses and Humans

Simple Summary

Despite many recent advances in imaging and epidemiological data analysis, musculoskeletal injuries continue to be a welfare issue in racehorses. Omics studies describe the study of protein, genetic material (both DNA and RNA, including microRNAs—small non-coding ribonucleic acids) and metabolites that may provide insights into the pathophysiology of disease or opportunities to monitor response to treatment when measured in bodily fluids. As these fields of study are scientifically complex and highly specialised, it is timely to perform a review of the current literature to allow for the design of robust studies that allow for repeatable work. Systematic reviews have been introduced into the medical literature and are a methodological way of searching for relevant papers followed by critical review of the content and a detection of biases. The objectives of the current systematic review were to identify and critically appraise the literature pertaining to microRNA (miRNA) and their target genes that are correlated with stress fractures in racehorses and humans. The object was to define a panel of miRNAs and their target genes as potential biomarkers in either horses or human subjects. The online scientific databases were searched and a reviewed was performed according to preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. MicroRNA profiling studies in horses continue to emerge, but as of yet, no miRNA profile can reliably predict the occurrence of fractures. It is very important that future studies are well designed to mitigate the effects of variation in sample size, exercise and normalisation methods.

Abstract

Despite many recent advances in imaging and epidemiological data analysis, musculoskeletal injuries continue to be a welfare issue in racehorses. Peptide biomarker studies have failed to consistently predict bone injury. Molecular profiling studies provide an opportunity to study equine musculoskeletal disease. A systematic review of the literature was performed using preferred reporting items for systematic reviews and meta-analyses protocols (PRISMA-P) guidelines to assess the use of miRNA profiling studies in equine and human musculoskeletal injuries. Data were extracted from 40 papers between 2008 and 2020. Three miRNA studies profiling equine musculoskeletal disease were identified, none of which related to equine stress fractures. Eleven papers studied miRNA profiles in osteoporotic human patients with fractures, but differentially expressed miRNAs were not consistent between studies. MicroRNA target prediction programmes also produced conflicting results between studies. Exercise affected miRNA profiles in both horse and human studies (e.g., miR-21 was upregulated by endurance exercise and miR-125b was downregulated by exercise). MicroRNA profiling studies in horses continue to emerge, but as yet, no miRNA profile can reliably predict the occurrence of fractures. It is very important that future studies are well designed to mitigate the effects of variation in sample size, exercise and normalisation methods.

A Prospective Study of Training Methods for Two-Year-Old Thoroughbred Racehorses in Queensland, Australia, and Analysis of the Differences in Training Methods between Trainers of Varying Stable Sizes

Simple Summary

Musculoskeletal injuries present significant concerns for the global Thoroughbred racing industry. The development of training and management strategies to reduce injuries has been challenging due to conflicting findings about the risk factors for injury, and limited understanding of the role of different training methodologies. There is little published data on patterns of rest periods and exercise data and how these vary between trainers and between different racing jurisdictions. We describe training methodologies for 535 two-year-old horses providing 1258 training preparations and 7512 weeks of exercise. We investigated the variation in training methodologies between trainers from small, medium and large stable sizes. Significant differences were observed between trainers, with horses from larger stables accumulating a greater high-speed exercise volume, attaining training milestones more frequently and taking less time to reach their training milestones. We also highlight similarities and differences in training methods used in Queensland, Australia, and those previously reported from other geographic locations. A better understanding of training methods is an essential step towards reducing the impact of injuries.

Abstract

Worldwide, musculoskeletal injuries remain a major problem for the Thoroughbred racing industry. There is a strong interest in developing training and management strategies to reduce the impact of musculoskeletal injuries, however, progress has been limited by studies reporting conflicting findings, and a limited understanding of the role of different training methods in preventing injury. There is little data on patterns of rest periods and exercise data and how these vary between trainers. This prospective study of two-year-old racehorses was conducted in Queensland, Australia and involved weekly personal structured interviews with 26 trainers over 56 weeks. Detailed daily exercise data for 535 horses providing 1258 training preparations and 7512 weeks at risk were collected. Trainers were categorised into three groups by the mean number of two-year-old horses that they had in work each week over the study duration: (1) Small stables with five or less, (2) Medium stables with 6 to 15 and (3) Large stables with greater than 15 horses in training. Differences between trainers with small, medium and large stable sizes were evaluated using linear regression, Kruskal–Wallis equality-of-populations rank test if linear models were mis-specified or Chi-squared tests for categorical variables. Significant differences were observed between trainers, with horses from larger stables accumulating a greater high-speed exercise volume (p < 0.001), attaining training milestones more frequently (p = 0.01) and taking less time to reach their training milestones (p = 0.001). This study provides detailed data to which training practices from other locations can be compared. Presenting actual training data rather than trainers’ estimation of a typical program provides a more accurate assessment of training practices. Understanding how training practices vary between regions improves comparability of studies investigating risk factors and is an important step towards reducing the impact of musculoskeletal injuries.

The Risk Factors for Musculoskeletal Injuries in Thoroughbred Racehorses in Queensland, Australia: How These Vary for Two-Year-Old and Older Horses and with Type of Injury

Simple Summary

Musculoskeletal injuries (MSI) continue to affect Thoroughbred racehorses internationally, despite over thirty years of research into this problem. Studies of risk factors for musculoskeletal injuries report inconsistent findings. Consequently, developing training strategies to mitigate the risk of MSI is difficult. We identified factors associated with particularly high odds of injury in this population of racehorses. Two-year-old horses from primiparous mares (first foals born) are at increased odds of MSI, particularly dorsal metacarpal disease (“shinsoreness”). Two-year-old horses that have had a total preparation length of between 10 and 14 weeks also have increased odds of injury. Horses of all ages that travelled a total distance of 2.4–3.8 km (12–19 furlongs) at a gallop (faster than 15 m/s; 13 s/furlong; 900 m/min; 55 km/h) in the last four weeks and horses three years and older that travelled 3.0–4.8 km (15–24 furlongs) at three-quarter pace and above (faster than 13 m/s; 15 s/furlong; 800 m/min; 48 km/h) also have increased odds of injury. We recommend that these horses should be monitored closely for impending signs of injury. We also observed a non-linear relationship between high-speed exercise and musculoskeletal injuries. This highlights the importance of high-speed exercise to enable tissue adaptation to training. Finally, in some situations, increasing the number of days worked at a slow pace may be more effective for preventing MSI, if horses are perceived at a higher risk, than resting the horse altogether. Early identification of horses at increased risk of injury and appropriate intervention could substantially reduce the impact of musculoskeletal injuries in Thoroughbred racehorses.

Abstract

Musculoskeletal injuries (MSI) continue to affect Thoroughbred racehorses internationally. There is a strong interest in developing training and management strategies to reduce their impact, however, studies of risk factors report inconsistent findings. Furthermore, many injuries and fatalities occur during training rather than during racing, yet most studies report racing data only. By combining racing and training data a larger exposure to risk factors and a larger number of musculoskeletal injuries are captured and the true effect of risk factors may be more accurately represented. Furthermore, modifications to reduce the impact of MSI are more readily implemented at the training level. Our study aimed to: (1) determine the risk factors for musculoskeletal injuries and whether these are different for two-year-old and older horses and (2) determine whether risk factors vary with type of injury. This was performed by repeating analyses by age category and injury type. Data from 202 cases and 202 matched controls were collected through weekly interviews with trainers and analysed using conditional logistic regression. Increasing dam parity significantly reduced the odds of injury in horses of all age groups because of the effect in two-year-old horses (odds ratio (OR) 0.08; 95% confidence interval (CI) 0.02, 0.36; p < 0.001). Increasing total preparation length is associated with higher odds of injury in horses of all ages (OR 5.56; 95% CI 1.59, 19.46; p = 0.01), but particularly in two-year-old horses (OR 8.05; 95% CI 1.92, 33.76; p = 0.004). Increasing number of days exercised at a slow pace decreased the odds of injury in horses of all ages (OR 0.09; 95% CI 0.03, 0.28; p < 0.001). The distance travelled at three-quarter pace and above (faster than 13 m/s; 15 s/furlong; 800 m/min; 48 km/h) and the total distance travelled at a gallop (faster than 15 m/s; 13 s/furlong; 900 m/min; 55 km/h) in the past four weeks significantly affected the odds of injury. There was a non-linear association between high-speed exercise and injury whereby the odds of injury initially increased and subsequently decreased as accumulated high-speed exercise distance increased. None of the racing career and performance indices affected the odds of injury. We identified horses in this population that have particularly high odds of injury. Two-year-old horses from primiparous mares are at increased odds of injury, particularly dorsal metacarpal disease. Two-year-old horses that have had a total preparation length of between 10 and 14 weeks also have increased odds of injury. Horses of all ages that travelled a total distance of 2.4–3.8 km (12–19 furlongs) at a gallop in the last four weeks and horses three years and older that travelled 3.0–4.8 km (15–24 furlongs) at three-quarter pace and above also have increased odds of injury. We recommend that these horses should be monitored closely for impending signs of injury. Increasing the number of days worked at a slow pace may be more effective for preventing injury, if horses are perceived at a higher risk, than resting the horse altogether. Early identification of horses at increased risk and appropriate intervention could substantially reduce the impact of musculoskeletal injuries in Thoroughbred racehorses.

Microstructural properties of the proximal sesamoid bones of Thoroughbred racehorses in training

BACKGROUND

Proximal sesamoid bone fractures are common catastrophic injuries in racehorses. Understanding the response of proximal sesamoid bones to race training can inform fracture prevention strategies.

OBJECTIVES

To describe proximal sesamoid bone microstructure of racehorses and to investigate the associations between microstructure and racing histories.

STUDY DESIGN

Cross-sectional.

METHODS

Proximal sesamoid bones from 63 Thoroughbred racehorses were imaged using micro-computed tomography. Bone volume fraction (BVTV) and bone material density (BMD) of the whole bone and four regions (apical, midbody dorsal, midbody palmar and basilar) were determined. Generalised linear regression models were used to identify the associations between bone parameters and race histories of the horses.

RESULTS

The mean sesamoid BVTV was 0.79 ± 0.08 and BMD was 806.02 ± 24.66 mg HA/ccm. BVTV was greater in medial sesamoids compared with lateral sesamoids (0.80 ± 0.07 vs 0.79 ± 0.08; P < .001) predominantly due to differences in the apical region (medial-0.76 ± 0.08 vs lateral-0.72 ± 0.07; P < .001). BVTV in the midbody dorsal region (0.86 ± 0.06) was greater than other regions (midbody palmar-0.79 ± 0.07, basilar-0.78 ± 0.06 and apical-0.74 ± 0.08; P < .001). BVTV was greater in sesamoids with more microcracks on their articular surface (Coef. 0.005; 95% CI 0.001, 0.009; P = .01), greater extent of bone resorption on their abaxial surface (Grade 2-0.82 ± 0.05 vs Grade 1-0.80 ± 0.05 or Grade 0-0.79 ± 0.06; P = .006), in horses with a low (0.82 ± 0.07) or mid handicap rating (0.78 ± 0.08) compared with high rating (0.76 ± 0.07; P < .001), in 2- to 5-year-old horses (0.81 ± 0.07) compared with younger (0.68 ± 0.08) or older horses (0.77 ± 0.08; P < .001) and in horses that commenced their racing career at less than 4 years of age (0.79 ± 0.08 vs 0.77 ± 0.77; P < .001). BMD was greater in the midbody dorsal (828.6 ± 19.6 mg HA/ccm) compared with other regions (apical-805.8 ± 21.8, midbody palmar-804.7 ± 18.4 and basilar-785.0 ± 17.1; P < .001), in horses with a handicap rating (low-812.1 ± 20.0, mid-821.8 ± 21.3 and high-814.6 ± 19.4) compared with those with no rating (791.08 ± 24.4, P < .001), in females (806.7 ± 22.0) and geldings (812.2 ± 22.4) compared with entires (792.7 ± 26.2; P = .02) and in older horses (<2-year-old-763.7 ± 24.8 vs 2- to 5-year-old-802.7 ± 23.4, and 6- to 12-year-old-817.8 ± 20.0; P = .002).

MAIN LIMITATIONS

Data were cross-sectional.

CONCLUSIONS

Densification of the proximal sesamoid bones is associated with the commencement of racing in younger horses and the presence of bone fatigue-related pathology. Lower sesamoid BVTV was associated with longevity and better performance.

The Effect of Combined Training and Racing High-Speed Exercise History on Musculoskeletal Injuries in Thoroughbred Racehorses: A Systematic Review and Meta-Analysis of the Current Literature

Simple Summary

Despite extensive international research, musculoskeletal injuries remain a key problem for the Thoroughbred racing industry, with multiple welfare and ethical consequences. High-speed exercise (HSE) history is an important risk factor for injury. However, studies report conflicting findings concerning the effect of HSE on the risk of injury. Although most injuries and fatalities occur during training rather than racing, many research studies only evaluate injuries reported on race day, which likely underrepresents the injuries acquired by these horses. This study aimed to determine the effect of combined training and racing HSE on injuries in racehorses. We performed a systematic search of the relevant literature and evaluated the effect of seven measures of HSE on injury through meta-analyses. The total career HSE distance significantly affected the odds of musculoskeletal injuries (MSI). With every 5-furlong (1 km) increase in career HSE distance, the odds of MSI increased by 2%. The average HSE distance per day also affected the odds of MSI. With every additional furlong (200 m) of average HSE per day, the odds of MSI increased by 73%. However, the strength of the available evidence is restricted by methodological limitations, which will need to be addressed in the future, to explore the level of risk and mechanisms for injury in more detail.

Abstract

Despite over three decades of active research, musculoskeletal injuries (MSI) remain a global problem for the Thoroughbred (TB) racing industry. High-speed exercise history (HSEH) has been identified as an important risk factor for MSI. However, the nature of this relationship remains unclear, with an apparent protective effect of HSE against injury, before it becomes potentially harmful. Many MSI cases and fatalities occur during training rather than during racing, resulting in an underestimation of injury from studies focused on race day. The objective of this study was to examine the current evidence of the effect of combined training and racing HSEH on MSI in TB flat racehorses, through a systematic review and meta-analysis. A systematic search of the relevant literature was performed using PubMed^®, Scopus^®, Web of Science^®, and Embase^® online databases and the gray literature using sites containing “.edu” or “.edu.au”. Studies included in the review had explored seven different measures of HSE, including total career HSE distance, cumulative HSE distance in the 30 and 60 days before MSI, average HSE distance per day, per event and per 30 days, and the total number of HSE events. The total cumulative career HSE distance significantly affected the odds of MSI, with every 5-furlong increase, the odds of MSI increased by 2% (OR = 1.02; 95% CI 1.01, 1.03; p = 0.004). The average HSE distance per day also affected the odds of MSI, with every additional furlong increasing the odds of MSI by 73% (OR = 1.73; 95% CI 1.29, 2.31; p < 0.001). Other measures of HSE were not found to be consistently associated with risk of MSI, but these results should be interpreted with caution. Significant methodological limitations were identified and influence the comparability of studies. Standardizing the measures of HSE in studies of MSI, and describing training conditions in more detail, would support a more thorough investigation of the relationship between HSE and MSI. An improved understanding of this relationship is critical to mitigating the impact of MSI in the Thoroughbred racehorse.

Fatigue behavior of subchondral bone under simulated physiological loads of equine athletic training

Fatigue-induced subchondral bone (SCB) injuries are prevalent among athletes due to the repetitive application of high magnitude loads on joints during intense physical training. Existing fatigue studies on bone utilize a standard fatigue test approach by applying loads of a constant magnitude and frequency even though physiological/realistic loading is a combination of various load magnitudes and frequencies. Metal materials in implant and aerospace applications have been studied for fatigue behavior under physiological or realistic loading, however, no such study has been conducted on biological materials like bones. In this study, we investigated fatigue behavior of SCB under the range of loads likely to occur during a fast-workout of an equine athlete in training. A loading protocol was developed by simulating physiological loads occurring during a fast-workout of a racehorse in training, which consisted of a sequence of compression-compression load cycles, including a warm-up (32, 54, 61 MPa) and cool-down (61, 54, 32 MPa) before and after the slow/fast/slow gallop phase of training, also referred to as a training loop. This loading protocol/training loop was applied at room temperature in load-control mode to cylindrical SCB specimens (n = 12) harvested from third metacarpal medial condyles (MCIII) of twelve thoroughbred racehorses and repeated until fatigue failure. The mean ± standard deviation for total time-to-failure (TTF) was 76,393 ± 64,243 s (equivalent to 18.3 ± 15.7 training workouts) for n = 12 specimens. We observed the highest relative energy loss (REL, hysteresis loss normalized to energy absorbed in a load cycle) under loads equivalent to gallop speeds and all specimens failed under these gallop loads. This demonstrates the importance of the gallop speeds in the development of SCB injury, consistent with observations made in live racehorses. Moreover, specimens with higher mean REL and lower mean stiffness during the first loop had a shorter fatigue life which further confirms the detrimental effect of high energy loss in SCB. Further studies are required to reconcile our results with fatigue injuries among equine athletes and understand the influence of different training programs on the fatigue behavior of subchondral bone.

Abundant osteoclasts in the subchondral bone of the juvenile Thoroughbred metacarpus suggest an important role in joint maturation

BACKGROUND

The administration of bisphosphonate medications, which target osteoclastic-bone remodelling, to juvenile and adult racehorses is a matter of debate owing to concerns that these molecules remain bound to the bone-mineralised matrix and may interfere with subsequent bone growth, adaptation to exercise and healing of bone microdamage in equine athletes. Osteoclasts participate in endochondral ossification, subchondral bone remodelling and bone repair. There is a knowledge gap on the role of equine osteoclast biology in the growth and maturation of joint surfaces and this information is important to inform judicious bisphosphonate use.

OBJECTIVES

Measure and compare the osteoclast density in the subchondral bone of Thoroughbred (TB) distal third metacarpi (McIII) at different sites, varying depths from the articular surface and with age (0-84 months).

STUDY DESIGN

Ex vivo cadaveric study.

METHODS

McIIIs from foals, yearlings and adults were collected, fixed in formaldehyde and stored at 4°C. Sections were cut from the lateral hemi-metacarpus, stained and scored for cartilage degeneration. Osteoclasts were counted on immunohistochemically (Cathepsin K) stained sections. Osteoclast density was compared in regions of interest (ROIs-the sagittal ridge, axial and abaxial condyle) and also at two depths (0-3 mm and 3-6 mm) into the subchondral bone below the osteochondral junction.

RESULTS

The osteoclast density was consistently highest in the subchondral cortical bone plate (0-3 mm) when compared with the deeper trabecular bone in all age groups. Furthermore, the osteoclast density was significantly higher in juvenile Thoroughbreds (foals and yearlings) within both sites in the subchondral bone when compared with adults.

MAIN LIMITATIONS

The number of specimens available for study was restricted.

CONCLUSIONS

Osteoclasts are important in normal McIII epiphyseal and articular surface maturation and have a propensity to localise at the osteochondral junction and subchondral cortical bone plate zone in juvenile Thoroughbreds.

Insertional characteristics of three types of transfixation pin taps in third metacarpal bones from equine cadavers

OBJECTIVE

To compare heat generation and mechanical bone damage achieved with 2 tapered and 1 cylindrical transfixation pin taps in third metacarpal bones from equine cadavers.

SAMPLE

18 pairs (36 specimens) of third metacarpal bones from euthanized horses with no known metacarpal disease.

PROCEDURES

In each bone, an investigator drilled 3 holes for placement of a 6.3-mm cylindrical transfixation pin, a 6.3-mm tapered pin using a prototype tapered tap, and a 6.3-mm tapered pin using a revised tapered tap. One bone of each pair was tapped by hand and the other with an electric drill. Temperatures of the drill bits, reamers, and taps were measured and used to compare heat generation among tap groups and tapping methods (hand vs power tapping). Macrodamage (all bone pairs) and microdamage (6 bone pairs) were assessed.

RESULTS

The revised tapered tap resulted in less heat generation and less total thread microdamage, compared with the prototype tapered and cylindrical taps. Power tapping created less bone damage but higher temperatures than did hand tapping for all bone groups.

CONCLUSIONS AND CLINICAL RELEVANCE

The revised tap design for tapered pin insertion was superior to the prototype tap design and yielded similar or less bone damage than achieved with cylindrical pin insertion in equine third metacarpal bone specimens. We recommend careful hand tapping for tapered pin insertion rather than power tapping, which generated greater heat. The revised tapered tap could be expected to perform better than a cylindrical pin tap in terms of thermal and mechanical microdamage and should be used for insertion of tapered transfixation pins.

Effects of in vivo fatigue-induced subchondral bone microdamage on the mechanical response of cartilage-bone under a single impact compression

Subchondral bone (SCB) microdamage is prevalent in the joints of human athletes and animals subjected to high rate and magnitude cyclic loading of the articular surface. Quantifying the effect of such focal in vivo fatigue-induced microdamage on the mechanical response of the tissue is critical for the understanding of joint surface injury and the development of osteoarthritis. Thus, we aimed to quantify the mechanical properties of cartilage-bone from equine third metacarpal (MC3) condyles, which is a common area of accumulated microdamage due to repetitive impact loading. We chose a non-destructive technique, i.e. high-resolution microcomputed tomography (µCT) imaging, to identify various degrees of in vivo microdamage in SCB prior to mechanical testing; because µCT imaging can only identify a proportion of accumulated microdamage, we aimed to identify racing and training history variables that provide additional information on the prior loading history of the samples. We then performed unconfined high-rate compression of approximately 2% strain at 45%/s strain rate to simulate a cycle of gallop and used real-time strain measurements using digital image correlation (DIC) techniques to find the stiffness and shock absorbing ability (relative energy loss) of the cartilage-bone unit, and those associated with cartilage and SCB. Results indicated that stiffness of cartilage-bone and those associated with the SCB decreased with increasing grade of damage. Whole specimen stiffness also increased, and relative energy loss decreased with higher TMD, whereas bone volume fraction of the SCB was only associated negatively with the stiffness of the bone. Overall, the degree of subchondral bone damage observed with µCT was the main predictor of stiffness and relative energy loss of the articular surface of the third metacarpal bone of Thoroughbred racehorses under impact loading.

The relationship between microstructure, stiffness and compressive fatigue life of equine subchondral bone

Subchondral bone injuries often precede articular cartilage damage in osteoarthritis and are common in thoroughbred racehorses due to the accumulation of fatigue damage from high speed racing and training. Thus, racehorses provide a model to investigate the role of subchondral bone in joint disease. We assessed the association of horse and racing related factors and micro-CT based micromorphology of three separate subchondral bone layers with the initial stiffness and compressive fatigue life of bone plugs. Furthermore, we investigated three different definitions of fatigue failure of subchondral bone during compressive fatigue testing. Initial stiffness was 2,362 ± 443 MPa (mean ± standard deviation). Median compressive fatigue life during cyclic loading to -78 MPa was 16,879 (range 210 to 57,064). Subchondral bone stiffness increased over a median of 24% (range 3%-42%) of fatigue life to a maximum of 3,614 ± 635 MPa. Compressive fatigue life was positively associated with bone volume fraction in the deeper layers of subchondral bone, maximal stiffness, and the number of cycles to maximal stiffness. Initial stiffness was positively associated with tissue mineral density in the deeper layers and bone volume fraction in the superficial layer. Most specimens with a fatigue life of less than 5,500 cycles fractured grossly before reaching 30% reduction of maximal stiffness. Cycles to 10% reduction of maximal stiffness correlated strongly with cycles to lowest recorded stiffness at gross fracture and thus is a valid alternative failure definition for compressive fatigue testing of subchondral bone. Our results show that subchondral bone sclerosis as a result of high speed exercise and measured as bone volume fraction is positively associated with compressive fatigue life and thus has a protective effect on subchondral bone. Further research is required to reconcile this finding with the common collocation of fatigue damage in sclerotic subchondral bone of racehorses.

Investigation of changes in bone density and chemical composition associated with bone marrow oedema-type appearances in magnetic resonance images of the equine forelimb

Background

The aetiology of bone marrow oedema-like abnormalities (BMOA) seen on magnetic resonance imaging (MRI) is as yet not fully understood. The current study aimed to investigate the potential of projection radiography and Raman microspectroscopy to provide information regarding the underlying physiological changes associated with BMOA in equine bone samples.

Methods

MRI was used to assess 65 limbs from 43 horses. A subset of 13 limbs provided 25 samples, 8 with BMOA present and 17 as controls; these were examined with projection radiography to assess bone mineral density and Raman spectroscopy to assess bone composition. Statistical analysis was conducted using SPSS, the relationship between BMOA and age was tested using binary logistic regression, other outcome measures via unpaired t-tests.

Results

Overall BMOA was found to be associated with locally increased bone density (p = 0.011), suggesting increased bone formation; however, no measurable changes relating to bone remodelling were found, and there were no detectable changes in the chemical composition of bone.

Conclusions

BMOA is associated with locally increased bone density, without an associated change in the chemical composition of bone, suggesting this is not linked to BMOA. The presence of increased bone density associated with BMOA does appear to suggest that an increased amount of bone formation is occurring in these regions, but as Raman microspectroscopy data do not demonstrate any significant changes in bone chemical composition associated with BMOA, it would appear that the increased bone volume is due to a greater amount of bone being formed rather than an imbalance in relation to bone remodelling.

The study provides a proof of principle for the use of Raman microspectroscopy and projection radiography in in vitro studies of BMOA.

Electronic supplementary material

The online version of this article (10.1186/s12891-019-2693-y) contains supplementary material, which is available to authorized users.