Structural variation of the distal condyles of the third metacarpal and third metatarsal bones in the horse

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.

Computed Tomographic Evaluation of the Sagittal Ridge of the Third Metacarpal Bone in Young Thoroughbred Racehorses: A Longitudinal Study

Metacarpophalangeal joint region pain is a common cause of lameness in racehorses. Radiological abnormalities in the sagittal ridge (SR) of the third metacarpal bone have been associated with joint effusion, lameness and reduced sales prices. The aims were to describe computed tomographic (CT) appearance of the SR in racehorses, and to document the progression of these findings over three assessments. Forty yearlings were enrolled at the first examination (time 0). Re-examinations were performed twice, approximately six months apart on 31 (time 1) and 23 (time 2) horses, respectively. Computed tomographic examinations of both metacarpophalangeal regions were performed with the horses in a standing position. Computed tomographic reconstructions were analysed subjectively and objectively. The mean Hounsfield Unit values (Hus) of eight radial segments and location, size and shape of hypoattenuating lesions were recorded. Mean Hus at time 1 were higher than at time 0. There was no difference between mean HU at times 1 and 2. The mean HU values of the dorsal half were higher in the right forelimbs and in fillies. Hypoattenuation was identified in 33/80 (41.3%) limbs at time 0, in 22/62 (35.5%) limbs at time 1 and in 14/46 (30.4%) limbs at time 2. All hypoattenuations were located in the dorsodistal aspect of the SR. The most common shapes were hypoattenuating lesions elongated proximodistally and those extending towards trabecular bone. An increase in attenuation of the SR occurred in the first six months of training. Hypoattenuating lesions could decrease in size and could resolve during early training. In this population, these lesions were not associated with lameness.

Identification of a previously unreported site of subchondral bone injury in the dorsodistolateral calcaneus in Thoroughbred racehorses

BACKGROUND

Increasing accessibility and advancements in computed tomographic (CT) imaging improve understanding of the contributors to poor performance in the Thoroughbred racehorse.

OBJECTIVES

To characterise an unreported site of tarsal subchondral bone injury (SBI) in Thoroughbred racehorses.

STUDY DESIGN

Retrospective, cross-sectional analytical study.

METHODS

Tarsal CT scans of 108 horses were reviewed for evidence of SBI in the dorsodistolateral calcaneus (DDLC). Signalment, including age, breed, sex, and discipline of horses, was recorded. The association of DDLC SBI with Thoroughbred racehorse compared to other breeds/disciplines was calculated. Nuclear scintigraphic scans of the hindlimbs of Thoroughbreds between 2007 and 2022 were also reviewed for increased radiopharmaceutical uptake (IRU) suggestive of DDLC SBI.

RESULTS

Tarsal CT scans of 108 horses were analysed and lesion location and type were recorded. DDLC SBI was identified in 8/108 (7.3%) horses. All lesions were found in racing Thoroughbreds (Fischer's exact test p = 0.002) and in 20% (8/40) of racing Thoroughbreds that underwent tarsal CT. DDLC SBI was determined to be the primary cause of lameness in 3/8. Third tarsal bone fracture was considered the primary cause of lameness in 3/8 horses. A total of 1663 nuclear scintigraphic scans of 1603 Thoroughbred horses were reviewed. Increased IRU in the DDLC was present in 13 horses (0.9%).

MAIN LIMITATIONS

Study limitations include the retrospective nature of the study, incomplete medical records, lack of sensitivity/specificity data for lesion identification on various imaging modalities or comparison to histopathology, and limited substantive follow-up information.

CONCLUSIONS

The DDLC is a previously unreported site of SBI in the Thoroughbred racehorse. The lesion can be the primary source of lameness but may also be found in horses with other clinically relevant osseous injuries. This lesion may be underestimated in the Thoroughbred population. Further investigation is required to determine the true prevalence, clinical relevance, and prognosis in Thoroughbred racehorses with this injury.

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.

Novel computed tomographic analysis demonstrates differences in patterns of bone mineral content between fracture configurations in distal condylar fractures of the third metacarpal/metatarsal bones in 97 Thoroughbred racehorses

OBJECTIVE

To evaluate bone mineral content patterns between fracture configurations using novel CT image analysis.

ANIMALS

CT images from 97 Thoroughbred racehorses with third metacarpal/tarsal condyle fractures provide the case population for analysis.

PROCEDURES

Fractures were grouped by radiographic appearance. Image analysis objectively measured area of highly attenuating pixels (aHAP), areal density of highly attenuating pixels (dHAP) utilizing novel convex hull analysis, and subjective assessment of apparent attenuation intensity ranking (AAIR) for each fracture. Differences between fracture configuration groups were evaluated.

RESULTS

Analysis of dHAP identified lower-density regions of highly attenuating pixels in propagating fractures and higher-density regions of highly attenuating pixels in unicortical fractures (P = .028). Complete and incomplete configurations were almost indistinguishable in dHAP (P = 1.000). The ratio of dHAP between fractured and nonfractured condyles revealed higher density gradients between condyles in unicortical (P = .040) and incomplete (P = .031) fractures than propagating fractures.

CLINICAL RELEVANCE

Differences in patterns of bone mineral content were identified between propagating, bicortical (incomplete and complete), and unicortical fractures of third metacarpal/tarsal bone condyles. Computer-assisted geometric measurement of dHAP identified on CT images could help to assess fracture risk in equine athletes. This application may have greater relevance as standing CT screening becomes more available.

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.

In vitro motions of the medial and lateral proximal sesamoid bones under mid-stance load conditions are consistent with racehorse fracture configurations

Proximal sesamoid bone (PSB) fractures in racehorses are likely fatigue fractures that occur due to repetitive loads and stress remodeling. The loading circumstances that may induce damage in the PSBs are not well understood. The goal of this study was to determine in three-dimensions, PSB motions relative to the opposing metacarpal condyle during simulated mid-stance loads. Seven equine cadaveric forelimbs were axially loaded in a material testing system to simulate standing and mid-stance walk, trot, and gallop load conditions (1.8-10.5 kN). Joint angles were determined by tracking the positions of bone-fixed kinematic markers. Internal-external rotation, abduction-adduction, and flexion-extension of each PSB relative to the third metacarpal condyle were compared between loads and between PSBs using an ANOVA with Tukey-Kramer post hoc tests for pairwise comparisons. The medial PSB rotated externally and the lateral PSB apex abducted during limb loading. Medial PSB external rotation was significantly greater at the gallop load condition than at the walk or trot load conditions. The medial and lateral PSB motions observed in this study are consistent with location of fatigue damage and fracture configurations frequently seen in medial and lateral PSBs from Thoroughbred racehorses. Specifically, medial PSB external rotation is consistent with the development of an abaxial subchondral medial PSB lesion that is reported in association with medial PSB transverse fracture and lateral PSB abduction is consistent with axial longitudinal fracture of the lateral PSB.

The Bone Cartilage Interface and Osteoarthritis

This review describes results obtained with tissue from prior studies of equine and human osteoarthritis (OA). The main methods considered are scanning electron microscopy, novel methods in light microscopy and X-ray Micro-tomography. The same samples have been re-utilised in several ways. The tissues described are hyaline articular cartilage (HAC; or substitutes), with its deep layer, articular calcified cartilage (ACC), whose deep surface is resorbed in cutting cone events to allow the deposition of subchondral bone (SCB). Multiple tidemarks are normal. Turnover at the osteochondral (ACC-HAC-SCB) junction is downregulated by overload exercise, conversely, during rest periods. Consequent lack of support predisposes to microfracture of the ACC-SCB plate, in the resorption-related repair phase of which the plate is further undermined to form sink holes. The following characteristics contribute to the OA scenario: penetrating resorption canals and local loss of ACC; cracking of ACC and SCB; sealing of cracks with High-Density Mineral Infill (HDMI); extrusion of HDMI into HAC to form High-Density Mineral Protrusions (HDMP) in HAC which may fragment and contribute to its destruction; SCB marrow space infilling and densification with (at first) woven bone; disruption, fibrillation and loss of HAC; eburnation; repair with abnormal tissues including fibrocartilage and woven bone; attachment of Sharpey fibres to SCB trabeculae and adipocyte-moulded extensions to trabeculae (excrescences).

How does bone strain vary between the third metacarpal and the proximal phalangeal bones of the equine distal limb?

Strain parameters at injury prone sites of the equine third metacarpal (MC3) and proximal phalangeal (P1) bones were investigated with the aim of improving understanding of injury pathogenesis. We hypothesized that dorsal principal and shear strain patterns, magnitudes and directions would differ from proximal-to-distal; and would be similar from medial-to-lateral across each bone. Unilateral limbs from nine equine cadavers were instrumented with rosette strain gauges during limb loading to 10,500 N. Gauges were attached at seven dorsal sites: middle MC3, distal MC3 (medial, middle, lateral) and proximal P1 (medial, middle, lateral). Outcome measures were analysed with repeated measures analysis of variance. Distal MC3 had the greatest, and proximal P1 the smallest magnitude of minimum principal and shear strains. Directions of maximum and minimum principal strain were similar at the middle and distal MC3 sites with a 20-40° direction difference compared to proximal P1. The patterns of strain magnitude and direction were similar from medial-to-lateral on distal MC3 but varied in pattern and magnitude among the P1 sites. Overall, as load reached maximum, direction of minimum principal strain became more axial in orientation, converging from opposite directions between bones, potentially maximising stability of the distal limb. The difference in strain parameters and strain ratio for adjacent anatomic sites on distal MC3 and proximal P1 was not anticipated, in light of the anatomic congruity of the metacarpophalangeal joint. Based on the predominance of shear strain across proximal P1, shear forces are likely the predominant biomechanical contributor to sagittal fractures of P1.

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.

Appraising the Welfare of Thoroughbred Racehorses in Training in Queensland, Australia: The Incidence and Type of Musculoskeletal Injuries Vary between Two-Year-Old and Older Thoroughbred Racehorses

Simple Summary

Musculoskeletal injuries (MSI) remain a concerning cause of racehorse morbidity and mortality with serious ethical and welfare consequences. Previous research examining risk factors for injuries report inconsistent findings. Age is thought to affect the risk of injury, but, to date, there have been no prospective studies specifically comparing injuries between two-year-old versus older horses. We aimed to: (1) determine the incidence of injuries for two-year-old and older horses, and whether this was affected by training track, season, or rainfall, and (2) determine the types of injuries affecting two-year-old and older horses, and whether horses trialled or raced after injury. Data were collected through personal structured weekly interviews with participating trainers over a 13-month period. Data were analysed using Poisson regression. The incidence of MSI in the current study was low (0.6%). The incidence of MSI in two-year-old horses was higher than older horses. Types of MSI varied between two-year-old and older horses and affected whether horses subsequently trialled or raced from 11 to 23 months after injury. A larger proportion of two-year-old horses had dorsal metacarpal disease and traumatic lacerations. A smaller proportion of two-year-old horses had suspensory desmitis, superficial digital flexor tendonitis, proximal sesamoid bone fractures, and fetlock joint injuries than older horses. Training track and rainfall did not affect the incidence of injuries. The season affected the incidence of injuries in two-year-old horses but not in older horses.

Abstract

Musculoskeletal injuries (MSI) remain a concerning cause of racehorse morbidity and mortality with important ethical and welfare consequences. Previous research examining risk factors for MSI report inconsistent findings. Age is thought to affect MSI risk, but, to date, there have been no prospective studies comparing MSI in two-year-old versus older horses. This study aimed to: (1) determine the incidence of MSI for two-year-old and older horses, and whether this was affected by training track, season, or rainfall, and (2) determine the types of MSI affecting two-year-old and older horses, and whether horses trialled or raced after injury. A prospective survey was conducted with data collected through personal structured weekly interviews with participating trainers over a 13-month period. Data were analysed using Poisson regression. The incidence of MSI in the current study was low (0.6%). The incidence of MSI in two-year-old horses was higher than older horses (p < 0.001). Types of MSI varied between two-year-old and older horses (p < 0.001) and affected whether horses subsequently trailed or raced from 11 to 23 months after injury (p < 0.001). A larger proportion of two-year-old horses had dorsal metacarpal disease and traumatic lacerations. A smaller proportion of two-year-old horses had suspensory ligament desmitis, superficial digital flexor tendonitis, proximal sesamoid bone fractures, and fetlock joint injuries than older horses. Training track and rainfall did not affect MSI. The season affected MSI in two-year-old horses (p < 0.001) but not older horses. The major limitation was that trainers in this study were metropolitan (city) and our findings may not be generalisable to racehorses in regional (country) areas. Another significant limitation was the assumption that MSI was the reason for failure to trial or race after injury. In conclusion, the incidence of MSI was low in the current study and the types and the risk factors for MSI are different for two-year-old and older horses.

Locomotion on the edge: Structural properties of the third metacarpal in Thoroughbred and Quarter Horse racehorses and feral Assateague Island ponies

The elongated, distally tapered limbs of horses are adapted for high-speed locomotion. Because these traits are artificially selected for in modern racehorses, they operate at a morphological extreme with a high risk of fracture. Racehorses are subject to different training and racing regimes depending on their breed and gait, and are therefore an interesting model to examine bone functional adaptation under variable biomechanically intense conditions. This study compares bone structural properties in the third metacarpal (MCIII) of Thoroughbred (n = 9) and Quarter Horse (n = 11) racehorses, using feral Assateague Island ponies (n = 6) as an untrained/unraced outgroup, to determine whether structural properties reflect variable racing and training regimes. Geometric section properties and bone mineral densities were determined using peripheral quantitative CT at two diaphyseal sites and through the distal epiphysis. Diaphyseal strength of the MCIII in all three breeds does not differ relative to body size, but in the mid-diaphyseal region Thoroughbreds have higher antero-posterior relative to medio-lateral bending strength than Quarter Horses, as well as higher bone mineral densities in left MCIII epiphyses (particularly in the lateral condyle). Interestingly, all breeds have lower bone mineral densities in the lateral versus medial condyle, an inherent structural feature that may influence predisposition to fracture when running around turns. Our results suggest that despite subtle differences in bone structure between different racehorse breeds, basic morphology of the third metacarpus is relatively similar among racing and non-racing horses, possibly reflecting intense selection (natural and artificial) across domestic equids for similar structural features within distal limb elements.

Physiologic effects of long-term immobilization of the equine distal limb

OBJECTIVE

To describe the effects of distal limb immobilization and remobilization in the equine metacarpophalangeal joint.

STUDY DESIGN

Randomized, prospective experimental study.

ANIMALS

Eight healthy, skeletally mature horses.

METHODS

One forelimb of each horse was immobilized in a fiberglass cast for 8 weeks; this was followed by 12 weeks of a treadmill-based training program after the cast had been removed. Clinical examinations, radiography, computed tomography (CT), nuclear scintigraphy, MRI, and histomorphometry were used to examine the third metacarpal (MC3), proximal phalanx, proximal sesamoid bones, and associated soft tissues in each horse. Serum and synovial fluid were collected for biomarker analyses.

RESULTS

Distal limb immobilization resulted in persistent lameness (P < .001), effusion (P = .002), and a decreased range of motion (P = .012) as well as radiographically visible fragments (P = .036) in the cast forelimb. Bone density was decreased (P < .001) in MC3 according to CT, and trabecular bone fluid was increased (P < .001) according to MRI in the cast forelimb. The cast forelimbs had a change (P = .009) in the appearance of the deep digital flexor tendon according to MRI immediately after removal of the cast. Numerous clinical, radiographic, CT, and MR abnormalities were visible at the end of the study period.

CONCLUSION

Eights weeks of cast immobilization induced changes in bone, cartilage, and periarticular soft tissues that were not reversed after 12 weeks of remobilization.

CLINICAL SIGNIFICANCE

Cast application should be used judiciously in horses with musculoskeletal injuries, balancing appropriate stabilization with potential morbidity secondary to cast placement.

Warmblood fragile foal syndrome type 1 mutation (PLOD1 c.2032G>A) is not associated with catastrophic breakdown and has a low allele frequency in the Thoroughbred breed

Background

Catastrophic fractures are among the most common cause of fatalities in racehorses. Several factors, including genetics, likely contribute to increased risk for fatal injuries. A variant in the procollagen‐lysine, 2‐oxoglutarate 5‐dioxygenase1 gene (PLOD1 c.2032G>A) was shown to cause Warmblood fragile foal syndrome type 1 (WFFS), a fatal recessive defect of the connective tissue. Screening of multiple horse breeds identified the presence of the WFFS allele in the Thoroughbred. PLOD1 is involved in cross‐linking of collagen fibrils and thus could potentially increase the risk of catastrophic breakdown.

Objectives

Estimate the frequency of the WFFS allele (PLOD1 c.2032G>A) and determine if it is a risk factor for catastrophic breakdown in the Thoroughbred.

Study design

Case–control genetic study.

Methods

Genomic DNA from hair and/or tissue samples was genotyped for the WFFS allele. Fisher’s Exact tests were performed to compare allele and carrier frequencies between the case cohort (catastrophic breakdown, n = 22) and several cohorts with no record of injury (n = 138 raced/trained at same track and season and n = 185 older than 7 years and raced during same season), nonracers (n = 92), and a random sample without consideration for racing history (n = 279).

Results

The frequency of the PLOD1 c.2032G>A variant in the Thoroughbred breed is low (1.2%). Seventeen of 716 Thoroughbreds tested were carriers (2.4%) and no WFFS homozygotes were detected. Only one catastrophic breakdown case carried the WFFS allele. No statistically significant difference in allele or carrier frequency was identified between case and control cohorts (P>0.05 in all comparisons performed).

Main limitations

This study evaluated cases from one single track.

Conclusions

This study demonstrated that the PLOD1 c.2032G>A associated with WFFS is present at very low frequency in Thoroughbreds and is not a genetic risk factor for catastrophic breakdown.

Association of catastrophic condylar fracture with bony changes of the third metacarpal bone identified by use of standing magnetic resonance imaging in forelimbs from cadavers of Thoroughbred racehorses in the United States

OBJECTIVE To compare bony changes of the third metacarpal bone (MC3) of Thoroughbred racehorse cadavers with (cases) or without (controls) catastrophic condylar fracture by use of standing MRI. SAMPLE 140 forelimbs from 26 case horses (both forelimbs) and 88 control horses (single forelimb). PROCEDURES Bone marrow lesions (BMLs), identified as a decrease in T1-weighted (T1W) signal and increases in T2*-weighted (T2*W) and short tau inversion recovery (STIR) signals, and dense bone volume percentage (DBVP), identified as decreases in T1W, T2*W, and STIR signals, in the distopalmar aspect of MC3 were recorded. Logistic regression was used to compare fractured and nonfractured limbs of cases and fractured limbs of cases with randomly selected limbs of controls. RESULTS Among cases, fractured limbs were significantly more likely to have BMLs (26/26 [100%]) than were nonfractured limbs (7/26 [27%]). Fractured limbs of cases were significantly more likely to have BMLs (26/26 [100%]) than were limbs of controls (6/88 [7%]). Among cases, there was no significant difference in DBVP between fractured and nonfractured limbs in lateral (26% vs 21%, respectively) or medial (25% vs 20%, respectively) condyles. However, DBVP was significantly greater in fractured limbs of cases than in limbs of controls for lateral (26% vs 16%, respectively) and medial (25% vs 18%, respectively) condyles. CONCLUSIONS AND CLINICAL RELEVANCE Standing MRI revealed a significantly greater degree of bone change in racehorses with condylar fracture when comparing fractured and nonfractured limbs of case horses and fractured limbs of case horses with randomly selected limbs of control horses.

The Importance of Subchondral Bone in the Pathophysiology of Osteoarthritis

Subchondral bone plays a critical role in the pathogenesis of osteochondral disease across veterinary species. The subchondral bone is highly adaptable, with the ability to model and remodel in response to loading stresses experienced by the joint. Repetitive stress injuries within the joint can result in primary or secondary pathologic lesions within the subchondral bone, which have been recognized to contribute to the development and progression of osteoarthritis. Recent advances in diagnostic imaging, particularly volumetric imaging modalities have facilitated earlier identification of subchondral bone disease. Despite these advancements, limitations in our knowledge about subchondral bone makes treatment and prevention of these conditions challenging. The purpose of this report is to review our current understanding of subchondral bone and its relationship to osteoarthritis across veterinary species, with a specific focus in the research that has been performed in horses. It can be concluded that our current understanding of subchondral bone is advancing, and future experimental, clinical and pathologic studies will provide additional insight about subchondral bone and its relationship to joint disease.

Subchondral bone morphology in the metacarpus of racehorses in training changes with distance from the articular surface but not with age

The repetitive large loads generated during high-speed training and racing commonly cause subchondral bone injuries in the metacarpal condyles of racehorses. Adaptive bone modelling leads to focal sclerosis at the site of highest loading in the palmar aspect of the metacarpal condyles. Information on whether and how adaptive modelling of subchondral bone changes during the career of a racehorse is sparse. The aim of this cross-sectional study was to describe the changes in subchondral bone micromorphology in the area of highest loading in the palmar aspect of the metacarpal condyle in thoroughbred racehorses as a function of age and training. Bone morphology parameters derived from micro-CT images were evaluated using principal component analysis and mixed-effects linear regression models. The largest differences in micromorphology were observed in untrained horses between the age of 16 and 20 months. Age and duration of a training period had no influence on tissue mineral density, bone volume fraction or number and area of closed pores to a depth of 5.1 mm from the articular surface in 2- to 4-year-old racehorses in training. Horses with subchondral bone injuries had more pores in cross-section compared with horses without subchondral bone injuries. Differences in bone volume fraction were due to the volume of less mineralised bone. Tissue mineral density increased and bone volume fraction decreased with increasing distance from the articular surface up to 5.1 mm from the articular surface. Further research is required to elucidate the biomechanical and pathophysiological consequences of these gradients of micromorphological parameters in the subchondral bone.

Increased sclerostin associated with stress fracture of the third metacarpal bone in the Thoroughbred racehorse

Objectives

The exact aetiology and pathogenesis of microdamage-induced long bone fractures remain unknown. These fractures are likely to be the result of inadequate bone remodelling in response to damage. This study aims to identify an association of osteocyte apoptosis, the presence of osteocytic osteolysis, and any alterations in sclerostin expression with a fracture of the third metacarpal (Mc-III) bone of Thoroughbred racehorses.

Methods

A total of 30 Mc-III bones were obtained; ten bones were fractured during racing, ten were from the contralateral limb, and ten were from control horses. Each Mc-III bone was divided into a fracture site, condyle, condylar groove, and sagittal ridge. Microcracks and diffuse microdamage were quantified. Apoptotic osteocytes were measured using TUNEL staining. Cathepsin K, matrix metalloproteinase-13 (MMP-13), HtrA1, and sclerostin expression were analyzed.

Results

In the fracture group, microdamage was elevated 38.9% (sd 2.6) compared with controls. There was no difference in the osteocyte number and the percentage of apoptotic cells between contralateral limb and unraced control; however, there were significantly fewer apoptotic cells in fractured samples (p < 0.02). Immunohistochemistry showed that in deep zones of the fractured samples, sclerostin expression was significantly higher (p < 0.03) than the total number of osteocytes. No increase in cathepsin K, MMP-13, or HtrA1 was present.

Conclusion

There is increased microdamage in Mc-III bones that have fractured during racing. In this study, this is not associated with osteocyte apoptosis or osteocytic osteolysis. The finding of increased sclerostin in the region of the fracture suggests that this protein may be playing a key role in the regulation of bone microdamage during stress adaptation.

Cite this article: N. Hopper, E. Singer, F. Henson. Increased sclerostin associated with stress fracture of the third metacarpal bone in the Thoroughbred racehorse. Bone Joint Res 2018;7:94–102. DOI: 10.1302/2046-3758.71.BJR-2016-0202.R4.

Mathematical modelling of bone adaptation of the metacarpal subchondral bone in racehorses

In Thoroughbred racehorses, fractures of the distal limb are commonly catastrophic. Most of these fractures occur due to the accumulation of fatigue damage from repetitive loading, as evidenced by microdamage at the predilection sites for fracture. Adaptation of the bone in response to training loads is important for fatigue resistance. In order to better understand the mechanism of subchondral bone adaptation to its loading environment, we utilised a square root function defining the relationship between bone volume fraction [Formula: see text] and specific surface [Formula: see text] of the subchondral bone of the lateral condyles of the third metacarpal bone (MCIII) of the racehorse, and using this equation, developed a mathematical model of subchondral bone that adapts to loading conditions observed in vivo. The model is expressed as an ordinary differential equation incorporating a formation rate that is dependent on strain energy density. The loading conditions applied to a selected subchondral region, i.e. volume of interest, were estimated based on joint contact forces sustained by racehorses in training. For each of the initial conditions of [Formula: see text] we found no difference between subsequent homoeostatic [Formula: see text] at any given loading condition, but the time to reach equilibrium differed by initial [Formula: see text] and loading condition. We found that the observed values for [Formula: see text] from the mathematical model output were a good approximation to the existing data for racehorses in training or at rest. This model provides the basis for understanding the effect of changes to training strategies that may reduce the risk of racehorse injury.

Common lesions of the distal end of the third metacarpal/metatarsal bone in racehorse catastrophic breakdown injuries

Equine catastrophic skeletal breakdown injury is a serious issue within the racing industry, given the impact on equine and human health. The metacarpo- and metatarso-phalangeal (fetlock) joints are common sites of catastrophic injury. However, lesions involving articular cartilage, subchondral bone, and synovium are commonly identified within the fetlock of the contralateral limb; hence, it is imperative that lesions in both limbs are evaluated and characterized during postmortem examination. Bone and articular cartilage changes typically occur in specific locations, related to cyclic fetlock load and overextension during high-speed exercise. Associations between preexisting degenerative fetlock lesions and catastrophic injury are a focus of continued research. These lesions often occur because of adaptive failure related to cumulative damage. Further investigation of these lesions is imperative to determine their impact on equine performance or injury. Ultimately, consistent documentation of catastrophic versus non-catastrophic osteochondral lesions provided by pathologists, in the context of training history, diagnostic imaging, and the presence or absence of catastrophic injury, will contribute to further understanding of skeletal responses associated with catastrophic failure.

Effect of track surface firmness on the development of musculoskeletal injuries in French Trotters during four months of harness race training

OBJECTIVE To evaluate the effect of track surface firmness on the development of musculoskeletal injuries in French Trotters during 4 months of race training. ANIMALS 12 healthy 3-year-old French Trotters. PROCEDURES Horses were paired on the basis of sex and body mass. Horses within each pair were randomly assigned to either a hard-track or soft-track group. The counterclockwise training protocol was the same for both groups. Surface firmness of each track was monitored throughout the training period. Radiography, ultrasonography, MRI, and scintigraphy were performed on all 4 limbs of each horse before and after 2 and 4 months of training. Lesions were described, and lesion severity was classified with a 5-point system, where 0 = no lesions and 4 = severe lesion. RESULTS 86 lesions were identified, of which 46 (53.5%) were classified as potentially clinically relevant (grade, ≥ 2). Of the 18 moderate and severe lesions, 15 were identified in horses of the hard-track group, and 10 of those were in forelimbs. Moderate to severe tendinopathy of the superficial digital flexor tendon of the forelimb developed in 3 of the 6 horses of the hard-track group but none of the horses of the soft-track group. Metatarsal condyle injuries were more frequent in horses of the hard-track group than horses of the soft-track group. Severe lesions were identified only in left limbs. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that track surface firmness is a risk factor for musculoskeletal injuries in horses trained for harness racing.

Epidemiology of racing injuries in Thoroughbred racehorses with special reference to bone fractures: Japanese experience from the 1980s to 2000s

This report describes the descriptive epidemiology of racing fractures that occurred from the 1980s to 2000s on racetracks of the Japan Racing Association (JRA). The incidence of racehorse fractures during flat racing was approximately 1–2%. Fractures occurring during a race are more likely to occur in a forelimb. Fractures mostly occur at the third and fourth corners of oval tracks and on the home stretch. They also occur more frequently at the time of changing the leading limb. Comparison of the incidence of racing fracture between before and after reconstruction of the geometrical configuration of a racetrack revealed that there was an outstanding reduction in the number of serious fractures in the year before and after reconstruction. It was postulated that the improvement in racing time, possibly influenced by reconstructing the geometrical configuration of the racetrack, was connected to the reduction in the number of fractures. Of non-biological race- and course-related factors, type of course (dirt or turf), track surface condition, differences between racecourses, and racing distance significantly influence racing time. By using an instrumented shoe, vertical ground reaction forces (VGRFs) on the forelimb during galloping and the relationships between a rough dirt and woodchip track surface and a smooth dirt and woodchip surface were measured. Relating the incidence of racing fractures with track conditions in general showed that track surface has significant effects on the incidence of fracture, with the incidence of fractures increasing as track conditions on dirt worsen and a tendency for the incidence of fractures to decrease as track conditions on turf worsen. It seems probable that track condition in general may affect the incidence of fracture. The incidence of fracture in horses during both racing and training decreased as the years progressed.

Does subchondral bone of the equine proximal phalanx adapt to race training?

Sagittal fractures of the first phalanx are a common, potentially catastrophic injury in racehorses. These fractures are often linked to an acute, one time, biomechanical event; however, recent evidence implies that chronic exposure to stress can lead to the accumulation of bony changes that affect the structural integrity of the bone and increase the likelihood of fracture. The aim of the study was to compare variations of two common metrics of bone adaptation - subchondral bone density and thickness across the proximal articular surface of the first phalanx in Thoroughbred horses that (1) raced but never experienced a first phalanx fracture (Raced Control); (2) raced and had experienced fracture of the contralateral first phalanx (Contralateral to Fracture); (3) had never raced or experienced a first phalanx fracture (Unraced Control). A total of 22 first phalangeal bones were sampled post-mortem and imaged using micro-computed tomography calibrated for mineral density measures. Measurements of volumetric subchondral bone mineral density and thickness were taken from images at five sites from medial to lateral, in three coronal planes (25, 50 and 75% dorsal-palmar). At each of the 15 sites, measurements were repeated and averaged across 10 adjacent micro-computed tomography slices of bone, spanning 0.75 mm. The magnitude and variance of these measurements were compared between sites and between cohorts with non-parametric statistical tests. Across the proximal osteochondral surface of the first phalanx, the pattern of subchondral bone volumetric bone mineral density and thickness varied with each coronal section studied. The subchondral bone thickness was greater for the central and dorsal coronal sections, compared with the palmar section. For the race-fit groups (Raced Control and Contralateral to Fracture), the highest volumetric bone mineral density was in the central sagittal groove. The volumetric bone mineral density was significantly greater in the sagittal groove in the central coronal section in the raced than the unraced group. The Contralateral to Fracture group demonstrated significantly greater variance of volumetric bone mineral density compared with the Raced Control and Unraced Control (P < 0.0001), with no difference in variance noted between the Raced Control and Unraced Control groups. There was a small (R rank = 0.3) but significant correlation between subchondral bone volumetric bone mineral density and thickness in the Contralateral to Fracture group (P = 0.005). The findings demonstrate that differences exist in subchondral bone volumetric bone mineral density and thickness across the proximal osteochondral surface of the equine first phalanx in horses with different training histories. The findings also demonstrate that the subchondral bone of the sagittal groove of the equine first phalanx adapts to race-training in the race-fit groups (Raced Control and Contralateral to Fracture) with an increase in volumetric bone mineral density relative to unraced controls. Within the race-trained groups, the Contralateral to Fracture bones had a greater variance of volumetric bone mineral density, suggesting that stress-induced bone adaptation had become more erratic, potentially contributing to the aetiology of sagittal fractures of the first phalanx in the Thoroughbred racehorse.

Effect of hoof angle on joint contact area in the equine metacarpophalangeal joint following simulated impact loading ex vivo

REASONS FOR PERFORMING STUDY

To add to the existing data on impact loading of the metacarpophalangeal (MCP) joint as a precursor to assessing the potential role of impact in joint disease.

OBJECTIVES

To examine the effect of impact loading on contact areas of the first phalanx (P1) and proximal sesamoids (PS) with the third metacarpal (McIII) under 3 hoof-strike conditions (toe-first, flat, heel-first).

STUDY DESIGN

Randomised, repeated controlled experiment using cadaver material.

METHODS

Eight cadaver limbs were subjected to randomised, repeated controlled trials where the hoof was struck by a pendulum impact machine (impact velocity 3.55 m/s) under 3 strike conditions. Data from pressure sensitive film placed over medial and lateral McIII condyles and lateromedially across the dorsal aspect of McIII were quantified: total areas of P1 and PS contact (cm(2) ) at maximum recorded pressure; centroid locations of contact areas relative to the sagittal ridge (cm) and transverse ridge (cm) and dispersion of pixels (cm(4) ) for each McIII condyle (medial/lateral). The effect of the strike conditions on each variable were statistically tested using repeated-measures ANOVA (α = 0.05).

RESULTS

Contact area between P1 and McIII condyles fell in well-defined areas bounded by the sagittal and transverse ridge, contact areas from PS were smaller and widely dispersed across McIII palmar border. Ratio of contact area of P1 to PS was 2.83 (P<0001). Hoof strike had no significant effect on contact area (P>0.54) CONCLUSIONS: Contact at impact (primarily from P1 and distally situated on McIII), contrasts with contact areas at midstance from both P1 and PS, symmetrically placed. Under impact, the greatest contact area was on the dorsal aspect of the medial condyle and coincides with the area subjected to the greatest increase in subchondral bone stiffening in joint disease.

Computed tomographic imaging of subchondral fatigue cracks in the distal end of the third metacarpal bone in the thoroughbred racehorse can predict crack micromotion in an ex-vivo model

Articular stress fracture arising from the distal end of the third metacarpal bone (MC3) is a common serious injury in Thoroughbred racehorses. Currently, there is no method for predicting fracture risk clinically. We describe an ex-vivo biomechanical model in which we measured subchondral crack micromotion under compressive loading that modeled high speed running. Using this model, we determined the relationship between subchondral crack dimensions measured using computed tomography (CT) and crack micromotion. Thoracic limbs from 40 Thoroughbred racehorses that had sustained a catastrophic injury were studied. Limbs were radiographed and examined using CT. Parasagittal subchondral fatigue crack dimensions were measured on CT images using image analysis software. MC3 bones with fatigue cracks were tested using five cycles of compressive loading at -7,500N (38 condyles, 18 horses). Crack motion was recorded using an extensometer. Mechanical testing was validated using bones with 3 mm and 5 mm deep parasagittal subchondral slots that modeled naturally occurring fatigue cracks. After testing, subchondral crack density was determined histologically. Creation of parasagittal subchondral slots induced significant micromotion during loading (p<0.001). In our biomechanical model, we found a significant positive correlation between extensometer micromotion and parasagittal crack area derived from reconstructed CT images (SR = 0.32, p<0.05). Correlations with transverse and frontal plane crack lengths were not significant. Histologic fatigue damage was not significantly correlated with crack dimensions determined by CT or extensometer micromotion. Bones with parasagittal crack area measurements above 30 mm2 may have a high risk of crack propagation and condylar fracture in vivo because of crack micromotion. In conclusion, our results suggest that CT could be used to quantify subchondral fatigue crack dimensions in racing Thoroughbred horses in-vivo to assess risk of condylar fracture. Horses with parasagittal crack arrays that exceed 30 mm2 may have a high risk for development of condylar fracture.

Compressive fatigue life of subchondral bone of the metacarpal condyle in thoroughbred racehorses

In racehorses, fatigue related subchondral bone injury leads to overt fracture or articular surface collapse and subsequent articular cartilage degeneration. We hypothesised that the fatigue behaviour of equine subchondral bone in compression follows a power law function similar to that observed in cortical and trabecular bone. We determined the fatigue life of equine metacarpal subchondral bone in-vitro and investigated the factors influencing initial bone stiffness. Subchondral bone specimens were loaded cyclically in compression [54MPa (n=6), 66MPa (n=6), 78MPa (n=5), and 90MPa (n=6)] until failure. The fatigue life curve was determined by linear regression from log transformed number of cycles to failure and load. A general linear model was used to investigate the influence of the following variables on initial Young's Modulus: age (4-8years), specimen storage time (31-864days), time in training since most recent rest period (6-32weeks), limb, actual density (1.6873-1.8684g/cm(3)), subchondral bone injury grade (0-3), and cause of death (fatigue injury vs. other). Number of cycles to failure was (median, range) 223,603, 78,316-806,792 at 54MPa; 69,908, 146-149,855 at 66MPa; 13204, 614-16,425 at 78MPa (n=3); and 4001, 152-11,568 at 90MPa. The fatigue life curve was σ=112.2-9.6 log10Nf, (R(2)=0.52, P<0.001), where Nf is number of cycles to failure and σ is load. Removal of the three horses with the highest SCBI grade resulted in: σ=134.2-14.1 log10Nf, (R(2)=0.72, P<0.001). Initial Young's Modulus (mean±SD) was 2500±494MPa (n=22). Actual density (ρ) was the only variable retained in the model to describe initial Young's Modulus (E): E=-8196.7+5880.6ρ, (R(2)=0.34, P=0.0044). The fatigue behaviour of equine subchondral bone in compression is similar to that of cortical and trabecular bone. These data can be used to model the development of SCBI to optimize training regimes.

Vascular perfusion of the dorsal and palmar condyles of the equine third metacarpal bone

REASONS FOR PERFORMING STUDY

Palmar osteochondral disease (POD) is an overload arthrosis that commonly affects fetlock joints of racing Thoroughbreds (TB) but the aetiopathogenesis of the disease has not been well defined.

OBJECTIVES

The aim of this study was to compare India ink perfusion in the dorsal and palmar condyles of the third metacarpal bone (McIII) in both passively flexed and maximally extended fetlock joints from paired equine cadaver limbs.

STUDY DESIGN

Descriptive cadaver study comparing perfusion of condyles of McIII in paired cadaver limbs in flexion (control group) and maximal extension (intervention group).

METHODS

Pairs of forelimbs were acquired from 5 TB horses subjected to euthanasia for reasons unrelated to lameness. Limb pairs were perfused intra-arterially with India ink and then randomly assigned to passive flexion or maximal extension of the fetlock joint. Limbs were sectioned sagittally in 3 mm sections through the fetlock and 12 sections per limb processed using a modified tissue-clearing technique. Sections were subsequently digitally imaged and bone perfusion evaluated with image analysis software.

RESULTS

Greater perfusion of the dorsal condyle than of palmar condyle was observed in 78% of sections from limbs in passive flexion and 92% of maximally extended sections. Perfusion to the palmar aspect of the condyle was significantly decreased (P < 0.0001) when the limbs were placed in maximal extension compared to passive flexion.

CONCLUSIONS

The palmar condyle of McIII had less perfusion than the dorsal condyle when the fetlock joint was in passive flexion and this difference was exacerbated by maximal extension. Based on the anatomical location of POD lesions, perfusion differences between the dorsal and palmar condyles of McIII may be associated with development of these lesions.

Evaluation of experimental impact injury for inducing post-traumatic osteoarthritis in the metacarpophalangeal joints of horses

OBJECTIVE

To determine whether a single contusive impact injury to the palmar aspect of the metacarpus would progress to post-traumatic osteoarthritis or palmar osteochondral disease in horses.

ANIMALS

12 horses.

PROCEDURES

In each horse, an impact injury was created on the palmar aspect of the medial metacarpal condyle of 1 randomly chosen limb with an impactor device under arthroscopic and fluoroscopic guidance. The opposite limb was sham operated as a control. A low to moderate amount of forced exercise was instituted, and horses were evaluated clinically via lameness examinations weekly for 5 months, then biweekly until endpoint, with synovial fluid analysis performed at 0, 1, 2, 3, 4, 6, 8, and 10 months and radiography at baseline and endpoint. Macroscopic examination, micro-CT, and sample collection for cartilage viability and sulfated glycosaminoglycan content, histologic evaluation, immunohistochemical analysis, and fluorochrome analysis were performed following euthanasia at 1 (3 horses), 4 (4), and 8 to 10 (5) months after surgery.

RESULTS

There was variability in impact lesion location, depth, and area on macroscopic inspection, but on histologic evaluation, cartilage defects were less variable. Mean sulfated glycosaminoglycan concentration from cartilage at the impact site was significantly lower than that at a similar site in control limbs. Higher concentrations of cartilage oligomeric matrix protein were observed in synovial fluid from impact-injured joints.

CONCLUSIONS AND CLINICAL RELEVANCE

The impact injury method caused mild focal osteoarthritic lesions in the metacarpophalangeal joint, but did not progress to palmar osteochondral disease at this site. Repeated injury is probably required for the development of palmar osteochondral disease.

Comprehensive surface-based morphometry reveals the association of fracture risk and bone geometry

Surface-based morphometry method is advantageous in its objectivity and increased capability in detecting focal morphological changes, but has not been applied in bone-related research. Orthopedics research in human has confirmed the association of the bone geometry in proximal femur and its fracture. In this study, surface-based morphometry is used to test the hypothesis that there is relationship between bone geometry and fracture risk of the proximal sesamoid bone (PSB) in forelimbs of Thoroughbred racehorses. The PSB surfaces were extracted from CT images of nonfractured forelegs (i.e., right foreleg in this study) of 6 racehorses with fractures in the contralateral (i.e., left) foreleg, and the right forelegs of 6 matched controls. Significant differences were detected at the abaxial margin of the medial PSB base which was found to be up to 3.5 mm more prominent in the fracture-group compared to the control-group. This study demonstrated a successful application of computational morphometry in bone. The detected anatomical differences may lead to a larger moment arm generated via the medial branch of the suspensory apparatus, increasing pressure on the sesamoid surface, and thus potentially predisposing to fracture. Findings from this pilot study not only increase the likelihood of accurate PSB fracture risk assessment, but also shed light on investigating the influence of sports and exercise on human athletes.

Investigation into the morphology of the third metacarpal bone in the horse

AIMS

To describe key morphological attributes of the third metacarpal bone (Mc3) of horses and to determine whether or not the symmetry of the Mc3 varied significantly between limbs of the same horse.

METHODS

Ten pairs of metacarpi were collected from slaughter facilities. The age and breed of the horses were recorded. Fixed points and axes that could be easily reproduced between bones were identified on high-quality photographic images of each bone. Using image analysis, three angles were measured. Angle gamma measured the rotation around the long axis of the diaphysis of Mc3, angle delta the angle between the dorsal long axis of the cannon bone and the surface of the condyle of Mc3, and angle theta the angle between the surface of the condyle and the long axis of the sagittal ridge of the condyle of Mc3. These angles represent some of the characteristic morphologic relationships within the equine Mc3.

RESULTS

The coefficient of variation for angles gamma, delta and theta and were 1.2%, 0.2% and 0.5%, respectively. Angle gamma was larger in the left compared with the right limb (p=0.041). Angles delta and theta were larger in the right compared with the left limb (p=0.001 and p=0.003, respectively). There was a single outlier in a left limb for angle gamma. When this was excluded from the analysis, angle gamma in the left limb was still larger than in the right limb. Angle delta was consistently greater than 90° in 19/20 metacarpi.

CONCLUSIONS

There were significant morphological differences in the Mc3 between the left and right limbs of the 10 horses examined. These findings provide some reliable reference data for future investigation. Further work is required to document these differences in a larger population of horses and to determine whether the morphology of the Mc3 is influenced by age or other factors such as use of the animal.

Statistical analysis of bone mineral density using voxel-based morphometry-an application on proximal sesamoid bones in racehorses

Voxel-based morphometry (VBM) is a computational technique that has been used to analyze statistical differences between groups of MR brain images. This study outlines a new VBM pipeline, designed for determining statistical variation in bone mineral density (BMD). CT images of proximal sesamoid bone (PSB) specimens from the right forelimb of six racehorses that had suffered PSB fractures were compared with six age-matched control specimens. Following segmentation, masked gray-scale images were co-aligned to a statistical template generated with all 12 CT datasets iteratively. Student t-tests were performed voxel-by-voxel on spatially aligned 3D images to reveal significant differences in the spatial variation of bone density between the fracture and control groups. Overall density and densities from the axial and abaxial areas of PSBs were compared between groups. The BMD in abaxial regions of the medial and lateral PSBs of the fracture group were 12.7% (p = 0.044) and 13.5% (p = 0.047) higher, respectively, than controls. The overall mean density of paired PSB and the medial and lateral PSBs separately were higher in the fracture group. The VBM pipeline facilitates detailed comparison of density variation between bone groups at the voxel level.

Third metacarpal condylar fatigue fractures in equine athletes occur within previously modelled subchondral bone

Bone modelling and remodelling reduce the risk of fatigue fractures; the former by adapting bone to its loading circumstances, the latter by replacing fatigued bone. Remodelling transiently increases porosity because of the normal delay in onset of the formation phase of the remodelling sequence. Protracted intense loading suppresses remodelling leaving modelling as the only means of maintaining bone strength. We therefore hypothesized that race horses with fatigue fractures of the distal third metacarpal bone (MC3) will have reduced porosity associated with suppressed remodelling while continued adaptive modelling will result in higher volume fraction (BV/TV) at this site. Using high resolution peripheral quantitative computed tomography (HR-pQCT), we measured the distal aspect of the MC3 obtained at postmortem from 13 thoroughbred race horses with condylar fractures of the MC3 (cases), 8 horses without fractures (training controls), 14 horses with a fracture at another site (fractured controls) and 9 horses resting from training (resting controls). Porosity of the subchondral bone of MC3 was lower in cases than resting controls (12±1.4% vs. 18±1.6%, P=0.017) although areas of focal porosity were observed adjacent to fractures in 6/13 horses. BV/TV of the distal metacarpal epiphysis tended to be higher in horses with condylar fractures (0.79±0.015) than training controls (0.74±0.019, P=0.070), but also higher in controls with a fracture elsewhere (0.79±0.014) than the training controls (0.74±0.019, P=0.040). BV/TV was higher in horses over three years of age than those aged two or three years (0.79±0.01 vs. 0.74±0.01, P=0.016). All metacarpal condylar fractures occurred within focal areas of high BV/TV. We infer that intense training in equine athletes suppresses remodelling of third metacarpal subchondral bone limiting damage repair while modelling increases regional bone volume in an attempt to minimise local stresses but may fail to offset bone fragility.

Correlation of signal attenuation-based quantitative magnetic resonance imaging with quantitative computed tomographic measurements of subchondral bone mineral density in metacarpophalangeal joints of horses

OBJECTIVE

To evaluate the ability of signal attenuation-based quantitative magnetic resonance imaging (QMRI) to estimate subchondral bone mineral density (BMD) as assessed via quantitative computed tomography (QCT) in osteoarthritic joints of horses.

SAMPLE POPULATION

20 metacarpophalangeal joints from 10 horse cadavers.

PROCEDURES

Magnetic resonance (MR) images (dorsal and transverse T1-weighted gradient recalled echo [GRE] and dorsal T2*-weighted GRE fast imaging employing steady-state acquisition [T2*-FIESTA]) and transverse single-slice computed tomographic (CT) images of the joints were acquired. Magnetic resonance signal intensity (SI) and CT attenuation were quantified in 6 regions of interest (ROIs) in the subchondral bone of third metacarpal condyles. Separate ROIs were established in the air close to the joint and used to generate corrected ratios and SIs. Computed tomographic attenuation was corrected by use of a calibration phantom to obtain a K(2)HPO(4)-equivalent density of bone. Correlations between QMRI performed with different MR imaging sequences and QCT measurements were evaluated. The intraobserver repeatability of ROI measurements was tested for each modality.

RESULTS

Measurement repeatability was excellent for QCT (R(2) = 98.3%) and QMRI (R(2) = 98.8%). Transverse (R(2) = 77%) or dorsal (R(2) = 77%) T1-weighted GRE and QCT BMD measurements were negatively correlated, as were dorsal T2*-FIESTA and QCT (R(2) = 80%) measurements. Decreased bone SI during MR imaging linearly reflected increased BMD.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of this ex vivo study suggested that signal attenuation-based QMRI was a reliable, clinically applicable method for indirect estimation of subchondral BMD in osteoarthritic metacarpophalangeal joints of horses.

Comparative rheumatology: what can be learnt from naturally occurring musculoskeletal disorders in domestic animals?

Examples of naturally occurring musculoskeletal disorders are extremely common in veterinary species and provide a valuable comparative research resource, which can provide compelling comparative data on the aetiopathogenesis and treatment of many common human musculoskeletal diseases. In particular, orthopaedic diseases are a common morbidity in both dogs and horses. In this review, we give an overview of the common musculoskeletal diseases encountered in these species: for instance, tendon and ligament injuries, arthropathies and stress fractures, as well as an insight into the basic biology of these conditions. In doing so, we aim to demonstrate the similarities and differences between these disorders and similar conditions in man.

Risk factors for fatal lateral condylar fracture of the third metacarpus/metatarsus in UK racing

REASONS FOR PERFORMING STUDY

Lateral condylar fractures of the third metacarpus/metatarsus are the most common cause of equine fatality on UK racecourses. Identification of risk factors for such injuries and the subsequent implementation of intervention strategies could significantly reduce the total number of racecourse fatalities.

OBJECTIVES

To identify horse-, race- and course-level risk factors for fatal lateral condylar fracture in Thoroughbreds on UK racecourses.

METHODS

Case-control study designs were used. Case horses were defined as those that were subjected to euthanasia having sustained an irreparable lateral condylar fracture while racing at any of the 59 UK racecourses. Case races were defined as those in which one or more horses sustained a fatal lateral condylar fracture. Three controls for each case horse were selected at random from the race in which the case was running. Three controls for each case race were selected at random from all races of the same type held in the same year. Ninety-eight cases were included in the study. Conditional logistic regression was used to identify the relationship between a number of independent variables and the likelihood of fracture.

RESULTS

Horses doing no gallop work during training and those in their first year of racing were at significantly increased risk of fracture on the racecourse. Case horses were also more likely to have started racing as 3- or 4-year-olds. Fractures were found to be more likely in longer races with a larger number of runners, races in which professional jockeys were not permitted to ride and races in which the going was described as firm or hard.

CONCLUSIONS AND POTENTIAL RELEVANCE

Modifications to training schedules, specifically within the first year of racing, may have a large impact on the risk of fatal lateral condylar fracture on the racecourse. Horses should do some gallop work in training and our results suggest that the minimum distance galloped should be between 201 m (1 furlong) and 1609 m (8 furlongs) per week. The association with age at first race requires further investigation for flat and National Hunt racing separately. A reduction in the number of races taking place on very firm going could have an impact on the number of lateral condylar fractures.