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

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.

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.

Relationship between Thoroughbred workloads in racing and the fatigue life of equine subchondral bone

Fatigue life (FL) is the number of cycles of load sustained by a material before failure, and is dependent on the load magnitude. For athletes, ‘cycles’ translates to number of strides, with load proportional to speed. To improve previous investigations estimating workload from distance, we used speed (m/s, x) per stride collected using 5 Hz GPS/800 Hz accelerometer sensors as a proxy for limb load to investigate factors associated with FL in a Thoroughbred race start model over 25,234 race starts, using a combination of mathematical and regression modelling. Fore-limb vertical force (NKg^-1) was estimated using a published equation: Vertical force = 2.778 + 2.1376x − 0.0535x². Joint load (σ) was estimated based on the vertical force, scaled according to the maximum speed and defined experimental loads for the expected variation in load distribution across a joint surface (54-90 MPa). Percentage FL (%FL) was estimated using a published equation for cycles to failure (N_f) summed across each race start: N_f = 10^{(σ-134.2)/−14.1.} Multivariable mixed-effects linear regression models were generated on %FL, adjusting for horse-level clustering, presented as coefficients; 95%CI. Scaled to the highest joint load, individual starts accrued a mean of 9.34%FL (sd. 1.64). Older age (coef. 0.03; 0.002–0.04), longer race-distances (non-linear power transformed), and firmer track surfaces (ref. Heavy 10: Good 3 coef. 2.37; 2.26–2.48) were associated with greater %FL, and males accrued less than females (p < 0.01). Most variables associated with %FL are reported risk factors for injury. Monitoring strides in racehorses may therefore allow identification of horses at risk, enabling early detection of injury.

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.

Risk-Factors for Soft-Tissue Injuries, Lacerations and Fractures During Racing in Greyhounds in New Zealand

Recognition of injuries in racing animals is essential to identify potential risk factors so actions can be taken to reduce or mitigate the cause of the injury to safeguard the animal. Racing greyhounds are subject to musculoskeletal injuries associated with athletic pursuit, in particular soft-tissue injuries, lacerations, and fractures. The objective of this study was therefore to determine risk factors for soft-tissue injuries, lacerations and fractures occurring during racing, using a cohort of greyhounds racing in New Zealand between 10th September 2014 and 31st July 2020. Dog-level, race-level and track-level risk factors for each outcome were assessed using mixed-effects multivariable logistic regression including trainer as a random effect. Throughout the study period there were 218,700 race starts by 4,914 greyhounds, with a total of 4,385 injuries. Of these, 3,067 (69.94%) were classed as soft-tissue injuries, 641 (14.62%) were reported as lacerations, and 458 (10.44%) were fractures. Greyhounds with a low racing frequency (racing more than 7 days apart) had 1.33 [95% confidence interval (CI): 1.06-1.67] times the odds of fracture compared to those racing more frequently. Older greyhounds had a greater odds of fracture compared with younger greyhounds. Racing every 7 days had a lower odds of soft-tissue injury compared with racing more than once a week. Dogs over 39 months had 1.53 (95% CI: 1.35-1.73) times the odds of sustaining a soft-tissue injury compared to the younger dogs. Greyhounds originating from Australia had a higher odds of fracture and laceration compared with New Zealand dogs. Better performing dogs (higher class) had a greater odds of fracture and laceration whilst maiden dogs had a higher odds of soft-tissue injury. Greyhounds starting from the outside box had a higher odds of fracture. There was considerable variation in the odds of soft-tissue injury at different racetracks. In conclusion, although the incidence of soft-tissue injuries was higher than other injury types, the repercussion of such injuries was less than those for fractures. The results from this study will help to inform intervention strategies aimed at reducing the rate of injuries in racing greyhounds, enhancing racing safety and greyhound welfare.

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.