Surface strains around the midshaft of the third metacarpal bone during turning

REASONS FOR PERFORMING STUDY: Bone strains quantify skeletal effects of specific exercise and hence assist in designing training programmes to avoid bone injury. OBJECTIVE: To test whether compressive strains increase on the lateral surface of the inside third metacarpal bone (McIII) and the medial surface of the outside McIII in a turn. METHODS: Rosette strain gauges on dorsal, medial and lateral surfaces of the midshaft of the left McIII in 2 Thoroughbred geldings were recorded simultaneously during turning at the walk on a bitumen surface. RESULTS: Medial surface: Compression peaks were larger in the outside limb. Tension peaks were larger in the inside limb and in a tighter turn. On the lateral surface compression and tension peaks were larger on the inside limb, which showed the largest recorded strains (compression of -1400 microstrains). Dorsal compression strains were larger on the outside limb and on a larger circle. Tensile strains were similar in both directions and larger on a larger circle. CONCLUSIONS: Compressive strains increased on the lateral surface of the inside McIII and medial surface of the outside McIII in a turn. POTENTIAL RELEVANCE: Slow-speed turning exercise may be sufficient to maintain bone mechanical characteristics in the inside limb lateral McIII cortex. Further work is needed to confirm these findings and to determine whether faster gaits and/or tighter turns are sufficient to cause bone modelling levels of strain in the medial and lateral McIII cortex.     

Third metacarpal bone laterality asymmetry and midshaft dimensions in Thoroughbred racehorses

OBJECTIVE: The aims of this study were to test whether longer third metacarpal (MC3) bones had thicker dorsal cortices in a group of racehorses that were exercising at similar maximum speeds, and to establish if horses with larger differences in length between their right and left MC3 bones showed larger differences in the dorsal cortical thickness between the two limbs. DESIGN: An observational study. PROCEDURE: Forty Thoroughbred racehorses aged between 2 and 6 years and in training at racing speed at two racing stables were used. Two sets of radiographs of each left and right metacarpus of each horse were measured for bone length and dorsal cortical width according to standardised methods. RESULTS: The dorsal cortex thickness showed a linear relationship with bone length for the range of lengths between 25 and 30 cm for both the right MC3 (R2 = 0.30, P = 0.0003) and the left (R2 = 0.23, P = 0.002). The longer bones had thicker dorsal cortices. When results from the two limbs were combined to test if the difference in length between the right and left MC3 in an individual horse was associated with a thicker dorsal cortex in the longer MC3 there was no consistent relationship (R2 = 0.008, P = 0.58). CONCLUSION: In this sample of racehorses longer MC3 bones were likely to have been exposed to a greater dorsopalmar bending moment at the mid shaft that was reflected in a thicker dorsal cortex. The lack of a relationship between midshaft thickness and bone length within individual horses suggests that direct mechanical effects of conformation and environment were less important than the individual's level of skill (or the degree of laterality in their movements) developed before their exposure to fast exercise. It is likely that racehorses with longer right MC3 bones were more able to control the loading of the right MC3 than the left during fast exercise.     

Computed tomography of proximal metatarsal and concurrent third tarsal bone fractures in a Thoroughbred racehorse

A 2‐year‐old Thoroughbred filly presented to the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California at Davis for whole body nuclear scintigraphic evaluation after an acute onset of nonweightbearing right hindlimb lameness post race. The use of computed tomography (CT) enabled visualisation of additional pathology that was not initially appreciable using combined imaging modalities of nuclear scintigraphy and digital radiography, which ultimately altered the selected course of treatment. Computed tomography in horses with fractures of the cuboidal bones of the hock can provide valuable additional information regarding fracture configuration and radiographically occult pathology, which may help to guide clinical decisions about treatment.     

Gait and speed as exercise components of risk factors associated with onset of fatigue injury of the third metacarpal bone in 2-year-old Thoroughbred racehorses

OBJECTIVE: To determine the degree to which components of the training program of 2-year-old Thoroughbred racehorses influence their susceptibility to fatigue injury of the third metacarpal bone (bucked shins). ANIMALS: 226 two-year-old Thoroughbred racehorses. PROCEDURE: Daily training information and health reports on 2-year-old Thoroughbreds were compiled from records provided from 5 commercial stables. For each horse, data (exercise variables) were collected that comprised distance jogged (approx speed of 5 m/s), galloped (approx 11 m/s), and breezed (approx 15 to 16 m/s) until a single instance of bucked shins was reported. Data were coded for analysis using cross-tabulation, graphic, and survival techniques. RESULTS: Of 226 horses, 56 had bucked shins, 9 completed the observation period without bucked shins, and 161 were lost to follow-up. Distinct training strategies were used at stables resulting in significantly different survival profiles among stables. Mean (+/- SD) allocation of exercise to breezing was 0.15 +/- 0.13 miles/wk (maximum, 0.64 miles/wk), to galloping was 4.47 +/- 1.52 miles/wk (maximum, 9.56 miles/wk), and to jogging was 2.34 +/- 1.70 miles/wk (maximum, 8.53 miles/wk). Survival (ie, lack of bucked shins during 1 year of monitoring) was found to be significantly reduced by exercise allocation to breezing, significantly increased by exercise allocation to galloping, and uninfluenced by exercise allocation to jogging. The log of the hazard ratio was reduced by 4.2 +/- 1.5/mile breezed and increased by 0.3 +/- 0.1/mile galloped. CONCLUSIONS AND CLINICAL RELEVANCE: Relationships between different gaits and speeds in the training regimen influence the incidence of bucked shins. To reduce the incidence of bucked shins, trainers should consider allocating more training effort to regular short-distance breezing and less to long-distance galloping.     

In vitro comparison of Thoroughbred and Standardbred racehorses with regard to local fatigue failure of the third metacarpal bone

In vitro local fatigue testing of the third metacarpal bone from Thoroughbred and Standardbred racehorses was performed, using fully reversed cyclic bending. Strain number of cycle data were accumulated and indicated that significant difference did not exist between the 2 breeds. It was concluded that the marked difference in the incidence of fatigue failure in the third metacarpal bone (bucked shins, dorsal metacarpal disease) seen in Thoroughbreds and Standardbreds was not related to any differences between the 2 breeds in mechanical properties of the bone.     

Effects of treadmill exercise on cortical bone in the third metacarpus of young horses.

The effects of exercise and relative inactivity on cortical bone were compared in young horses. Two groups were used; one was given a 14-week programme of exercise (n = 6) and the other kept as unexercised controls (n = 6). The first nine weeks of exercise involved trotting and cantering (2 to 4 km d-1 at speeds up to 12 m s-1) on a treadmill set at an incline of 3 degrees. Over the next five weeks the horses were trained at near maximal speeds (that is, up to 14.5 m s-1) with no incline of the treadmill. At the end of the programme marked differences in cortical porosity and distribution of subperiosteal osteogenesis at the mid-shaft of the third metacarpal bone were found between the groups. Histomorphometrical examination of the dorsal cortex showed minimal bone remodelling in the exercised horses, but extensive modelling as evidenced by the large amount of subperiosteal bone formation. In contrast, the unexercised horses had significantly more bone remodelling and less formation of subperiosteal bone. The histomorphometric and microradiographic findings provided an explanation for changes in the non-invasive bone measurements that occurred during training. Bone mineral content of the mid-metacarpus was found to increase more in the exercised than the unexercised horses despite a lower overall growth in bodyweight. In those horses that completed the full training programme, ultrasound speed increased significantly by the end of the training programme. It remained unchanged in the horse that did not complete the full exercise programme and decreased slightly in the unexercised horses. The difference in ultrasound speed between the groups was considered to reflect differences in intracortical bone porosity, endosteal bone formation and alterations in skin thickness. The stiffness of cortical bone increased significantly in the exercised horses but remained unaltered in the unexercised horses.     

Reliability of cardiorespiratory measurements with a new ergospirometer during intense treadmill exercise in Thoroughbred horses

This study investigated the reliability of measurements with a new equine ergospirometer (Quadflow). Heart rate and blood lactate responses during exercise in horses wearing the Quadflow and an open flow mask were also compared. The mean percentage error of the oxygen uptake measurements was 8.2% (range 2.1-12.5%). Percent error for peak expiratory flow rates ranged from 6.1% to 9.4 %, and for minute ventilation from 2.5% to 7.4%. The coefficients of variation of the means of four measurements in two horses exercising continuously at 9.0 m/s were <5% for variables related to pulmonary ventilation, and was 7.7% for oxygen uptake. The Quadflow mask resulted in small increases in blood lactate concentration and relative heart rate during submaximal exercise. It was concluded that between- and within-test reliability statistics for important measurements in equine clinical exercise testing were acceptable for routine use in a veterinary practice or research laboratory.     

Galloping exercise induces regional changes in bone density within the third and radial carpal bones of Thoroughbred horses

This study was performed to test the hypothesis that a localised bone hypertrophy could occur within the subchondral cancellous architecture of the third and radial carpal bones. Using 2 levels of controlled and defined exercise, it was observed that a high intensity treadmill exercise protocol resulted in functional adaptation of the carpal bones. The increase in trabecular thickening and density was seen to be localised to those regions underlying common sites of cartilage degradation, the interface of the thickened trabeculae with the normal architecture in the third carpal bone was coincident with a common site of clinical fractures. The bone changes were determined both qualitatively on examination of slab radiographs and quantified by dual energy x-ray absorptiometry. The findings from this study are relevant to mechanical factors involved in the pathophysiology of joint degeneration. The potential clinical implications of this study are in relation to changes in the type and duration of exercise regimens used in training of equine athletes. The rapid response of bone to mechanical stimulation has implications in the longer term for localised cartilage degradation. Imaging techniques could be developed to monitor these early bone changes in the specific areas identified in this study and thus allow appropriate changes in training intensity to minimise subsequent damage to the articular surface.     

Wedge-shaped conformation of the dorsolateral aspect of the third tarsal bone in the Thoroughbred racehorse is associated with development of slab fractures in this site.

Anecdotal evidence suggested that many cases of third tarsal bone (T3) fracture encountered clinically were associated with an abnormal shape to this bone. The radiographs of 10 normal horses and 10 horses affected with slab fracture of T3 were therefore examined to ascertain if any pre-existent radiological abnormality was present in cases of fracture. Measurement of the maximum and minimum width between the proximal and distal articular surfaces of the dorsolateral aspect of this bone was carried out on a standardised dorso-50 degrees medial-palmarolateral radiographic projection of the tarsus of each horse. To avoid artefacts produced by possible image magnification, ratios of these values were used for comparison between horses. The results showed that wedge shaped conformation of T3, in which the articular surfaces of the bone converge and then diverge again on the dorsolateral aspect, was over-represented in the population of horses sustaining T3 fracture when compared to controls. This information may be important in making judgements on the suitability of horses during prepurchase examination for racing.     

The efficacy of dantrolene sodium in controlling exertional rhabdomyolysis in the Thoroughbred racehorse

REASONS FOR PERFORMING STUDY: Dantrolene sodium (Dantrium) has been used extensively for the treatment of myopathies in man and anecdotal evidence suggests it is of clinical benefit in the control of exercise-induced rhabdomyolysis (ER) in racehorses, although data to support this are currently lacking. OBJECTIVES: To investigate the efficacy of oral dantrolene sodium in controlling ER in a randomised, double-blind, placebo-controlled crossover trial involving 77 Thoroughbred racehorses in Newmarket, UK. METHODS: Horses were treated on 2 occasions 1 week apart, with treatment days coinciding with a return to exercise following 2 days box rest on each occasion. For the first treatment, each horse was randomly selected to receive either 800 mg dantrolene sodium or a colour-matched placebo administered orally 1 h before exercise. This was followed by crossover to the other treatment on the second occasion, with each horse thereby acting as its own control. Degree of ER was assessed using rising serum creatine kinase (CK) levels, by subtracting pre-exercise blood CK levels from those measured in 6 h post exercise blood samples. For each horse, the difference in change between pre- and post exercise CK values between placebo and dantrolene treatments was calculated, with positive values indicating a greater rise with placebo than with dantrolene sodium treatment. RESULTS: The overall mean difference for all horses was +104.8 iu/l and the null hypothesis, that there was no true difference in non-normally distributed post exercise rises in CK values between placebo and dantrolene treatments, was rejected (P = 0.0013) using the nonparametric Wilcoxon signed rank test. Additionally, no horses given dantrolene sodium showed clinical signs of ER, whereas 3 horses given the placebo developed ER following exercise. The incidence of ER in the study was 4% (3/77). CONCLUSIONS: The results confirmed that oral administration of dantrolene sodium, 1 h before exercise, had a statistically significant effect on reducing the difference between pre- and post exercise plasma CK levels compared with a placebo in the same animals, and preventing clinical ER in susceptible individuals. POTENTIAL RELEVANCE: This study suggested that dantrolene sodium is of use in controlling ER in the Thoroughbred racehorse. Further investigation into pre- and post exercise myoplasmic calcium levels and the repeat of the study late in the season when horses receive a much higher energy ration and more strenuous exercise would appear to be warranted.     

Fibro‐osseous tumour of the second metacarpal bone in a Thoroughbred filly

A 7‐month‐old Thoroughbred filly was evaluated because of an enlarging mass present on the proximomedial aspect of the left metacarpus. The mass was first noticed at age 2 months and had continued to increase in size. The mass was firm, nonpainful to palpation, covered with normal haired skin and had never been associated with lameness. Radiographs obtained at ages 2 and 7 months revealed an enlarging and partially mineralised mass associated with the second metacarpal bone. Based on the clinical and radiographic progression, as well as the location of the mass, a poor prognosis for future athletic soundness was given and the filly was subjected to euthanasia. The mass had histological features of both an ossifying fibroma and osteoma.     

Bone mineral content of the third metacarpal bone in Quarter Horse foals from birth to one year of age

Dual photon absorptiometry (DPA) is a valid method for bone mineral content (BMC) evaluation, but there are no reports on the use of this technology in in vivo studies in the horse. The aim of the current study was to evaluate the changes of BMC of the third metacarpal bone (MC3) in vivo in foals during their first year of life using DPA. The BMC of the MC3 of 41 Quarter Horse foals was evaluated every 4 weeks during their first year of life by DPA using a bienergetic Gadolinium-153 (¹⁵³Gd) source. To the authors' knowledge, this is the first report of an in vivo study of BMC in foals using DPA by means of a bienergetic ¹⁵³Gd source. The results demonstrated a positive correlation of age, heart girth, and withers height with BMC of the MC3 independent of gender and an exponential behavior of the BMC increment along the first year of life. There was an effect of weaning associated with change in diet and stall confinement on BMC in foals weaned at age 17 weeks, but not in foals weaned at age between 19 and 24 weeks.