B. Ravary-Plumioen

True stress and Poisson's ratio of tendons during loading

Excessive axial tension is very likely involved in the aetiology of tendon lesions, and the most appropriate indicator of tendon stress state is the true stress, the ratio of instantaneous load to instantaneous cross-sectional area (CSA). Difficulties to measure tendon CSA during tension often led to approximate true stress by assuming that CSA is constant during loading (i.e. by the engineering stress) or that tendon is incompressible, implying a Poissons ratio of 0.5, although these hypotheses have never been tested. The objective of this study was to measure tendon CSA variation during quasi-static tensile loading, in order to assess the true stress to which the tendon is subjected and its Poissons ratio. Eight equine superficial digital flexor tendons (SDFT, about 30cm long) were tested in tension until failure while the CSA of each tendon was measured in its metacarpal part by means of a linear laser scanner. Axial elongation and load were synchronously recorded during the test. CSA was found to linearly decrease with strain, with a mean decrease at failure of -10.7±2.8% (mean±standard deviation). True stress at failure was 7.1-13.6% higher than engineering stress, while stress estimation under the hypothesis of incompressibility differed from true stress of -6.6 to 2.3%. Average Poissons ratio was 0.55±0.12 and did not significantly vary with load. From these results on equine SDFT it was demonstrated that tendon in axial quasi-static tension can be considered, at first approximation, as an incompressible material.

Reproducibility of a non-invasive ultrasonic technique of tendon force measurement, determined in vitro in equine superficial digital flexor tendons

A non-invasive ultrasonic (US) technique of tendon force measurement has been recently developed. It is based on the relationship demonstrated between the speed of sound (SOS) in a tendon and the traction force applied to it. The objectives of the present study were to evaluate the variability of this non-linear relationship among 7 equine superficial digital flexor (SDF) tendons, and the reproducibility of SOS measurements in these tendons over successive loading cycles and tests. Seven SDF tendons were equipped with an US probe (1MHz), secured in contact with the skin overlying the tendon metacarpal part. The tendons were submitted to a traction test consisting in 5 cycles of loading/unloading between 50 and 4050N. Four tendons out of the 7 were submitted to 5 additional cycles up to 5550N. The SOS-tendon force relationships appeared similar in shape, although large differences in SOS levels were observed among the tendons. Reproducibility between cycles was evaluated from the root mean square of the standard deviations (RMS-SD) of SOS values observed every 100N, and of force values every 2m/s. Reproducibility of SOS measurements revealed high between successive cycles: above 500N the RMS-SD was less than 2% of the corresponding traction force. Reproducibility between tests was lower, partly due to the experimental set-up; above 500N the difference between the two tests stayed nevertheless below 15% of the corresponding mean traction force. The reproducibility of the US technique here demonstrated in vitro has now to be confirmed in vivo.

Axial speed of sound is related to tendon's nonlinear elasticity

Axial speed of sound (SOS) measurements have been successfully applied to noninvasively evaluate tendon load, while preliminary studies showed that this technique also has a potential clinical interest in the follow up of tendon injuries. The ultrasound propagation theory predicts that the SOS is determined by the effective stiffness, mass density and Poissons ratio of the propagating medium. Tendon stiffness characterizes the tissues mechanical quality, but it is often measured in quasi-static condition and for entire tendon segments, so it might not be the same as the effective stiffness which determines the SOS. The objectives of the present study were to investigate the relationship between axial SOS and tendons nonlinear elasticity, measured in standard laboratory conditions, and to evaluate if tendons mass density and cross-sectional area (CSA) affect the SOS level. Axial SOS was measured during in vitro cycling of 9 equine superficial digital tendons. Each tendons stiffness was characterized with a tangent modulus (the continuous derivative of the true stress/true strain curve) and an elastic modulus (the slope of this curves linear region). Tendons SOS was found to linearly vary with the square root of the tangent modulus during loading; tendons SOS level was found correlated to the elastic moduluss square root and inversely correlated to the tendons CSA, but it was not affected by tendons mass density. These results confirm that tendons tangent and elastic moduli, measured in laboratory conditions, are related to axial SOS and they represent one of its primary determinants.

A Linear Laser Scanner to Measure Cross-Sectional Shape and Area of Biological Specimens During Mechanical Testing

Measure of the cross-sectional area (CSA) of biological specimens is a primary concern for many biomechanical tests. Different procedures are presented in literature but besides the fact that noncontact techniques are required during mechanical testing, most of these procedures lack accuracy or speed. Moreover, they often require a precise positioning of the specimen, which is not always feasible, and do not enable the measure of the same section during tension. The objective of this study was to design a noncontact, fast, and accurate device capable of acquiring CSA of specimens mounted on a testing machine. A system based on the horizontal linear displacement of two charge-coupled device reflectance laser devices next to the specimen, one for each side, was chosen. The whole measuring block is mounted on a vertical linear guide to allow following the measured zone during sample tension (or compression). The device was validated by measuring the CSA of metallic rods machined with geometrical shapes (circular, hexagonal, semicircular, and triangular) as well as an equine superficial digital flexor tendon (SDFT) in static condition. We also performed measurements during mechanical testing of three SDFTs, obtaining the CSA variations until tendon rupture. The system was revealed to be very fast with acquisition times in the order of 0.1 s and interacquisition time of about 1.5 s. Measurements of the geometrical shapes yielded mean errors lower than 1.4% (n=20 for each shape) while the tendon CSA at rest was 90.29 ± 1.69 mm(2) (n=20). As for the tendons that underwent tension, a mean of 60 measures were performed for each test, which lasted about 2 min until rupture (at 20 mm/min), finding CSA variations linear with stress (R(2)>0.85). The proposed device was revealed to be accurate and repeatable. It is easy to assemble and operate and capable of moving to follow a defined zone on the specimen during testing. The system does not need precise centering of the sample and can perform noncontact measures during mechanical testing; therefore, it can be used to measure variations of the specimen CSA during a tension (or compression) test in order to determine, for instance, the true stress and transverse deformations.

Discrimination of two equine racing surfaces based on forelimb dynamic and hoof kinematic variables at the canter

The type and condition of sport surfaces affect performance and can also be a risk factor for injury. Combining the use a 3-dimensional dynamometric horseshoe (DHS), an accelerometer and high-speed cameras, variables reflecting hoof-ground interaction and maximal limb loading can be measured. The aim of the present study was to compare the effects of two racing surfaces, turf and all-weather waxed (AWW), on the forelimbs of five horses at the canter. Vertical hoof velocity before impact was higher on AWW. Maximal deceleration at impact (vertical impact shock) was not significantly different between the two surfaces, whereas the corresponding vertical force peak at impact measured by the DHS was higher on turf. Low frequency (0-200 Hz) vibration energy was also higher on turf; however high frequency (>400 Hz) vibration energy tended to be higher on AWW. The maximal longitudinal force during braking and the maximal vertical force at mid-stance were lower on AWW and their times of occurrence were delayed. AWW was also characterised by larger slip distances and sink distances, both during braking and at maximal sink. On a given surface, no systematic association was found between maximal vertical force at mid-stance and either sink distance or vertical impact shock. This study confirms the damping properties of AWW, which appear to be more efficient for low frequency events. Given the biomechanical changes induced by equestrian surfaces, combining dynamic and kinematic approaches is strongly recommended for a reliable assessment of hoof-ground interaction and maximal limb loading.

Comparison of superficial digital flexor tendon loading on asphalt and sand in horses at the walk and trot

The incidence of superficial digital flexor tendon (SDFT) injuries is one of the highest of all equine musculoskeletal conditions. Horses with SDFT injuries commonly show no improvement of lameness on soft ground, unlike those suffering from distal bone or joint lesions. The aim of this study was to compare the SDFT loading in five horses at the walk and trot on asphalt and sand using a non-invasive ultrasonic tendon force measurement device. Three horses were equipped with the ultrasonic device, whereas the other two horses were equipped with the ultrasonic device and a dynamometric horseshoe (DHS); the DHS was used to calibrate the measured values of tendon speed of sound (SOS) converted to tendon force, while a previously established ground reaction force pattern was used to calibrate SOS measurements for the other three horses. Although the horses tended to be slower on S, maximal tendon force was higher on sand than on asphalt at the trot (+6%); there was no significant difference between the two surfaces at the walk. The duration of tendon loading was longer on S (+5%) and the area under the tendon force-time curve was larger on S (+10%) at both walk and trot. SDFT loading is significantly affected by the ground surface and the observed increase in SDFT loading on sand compared with asphalt is consistent with clinical observations in horses with SDFT injuries.

Comparative kinematic analysis of the leading and trailing forelimbs of horses cantering on a turf and a synthetic surface

REASONS FOR PERFORMING STUDY The relationship between track surface properties and limb kinematics is poorly understood. Hoof orientation within the track surface has never been quantified under training conditions. Previously described kinematic and dynamic differences between leading and trailing forelimbs at the canter poorly correlate with epidemiological data regarding injuries. OBJECTIVES To compare joint kinematics and hoof orientation in the leading and trailing forelimbs of horses cantering on turf and on a synthetic surface. STUDY DESIGN Noninvasive experimental study. METHODS The right forelimb of 5 horses was equipped with markers facing the main joints while markers and a dynamometric horseshoe were placed on the hoof. The horses were filmed with 2 high-speed cameras (1000 Hz) while cantering (30 km/h). Recordings were repeated at each lead and alternated on turf and on a synthetic surface. Joint angles and angles of the hoof and limb to the track were measured from the 2-dimensional coordinates of the markers. RESULTS Elbow, carpus and fetlock were more maximally flexed during swing and had a larger range of motion throughout the stride in the leading forelimb. Maximal carpal extension during stance was also larger on this limb, which had a more toe-up orientation. Comparing surfaces, the limb was more oblique at landing, the range of motion of the hoof into the surface was larger, most kinematic events were delayed and fetlock and carpus extension velocities were smaller on the synthetic surface. CONCLUSIONS The differences between limbs were more prominent than those between surfaces and the more toe-up orientation on the hoof of the leading forelimb suggests a different loading of that limbs joints and tendons. Differences between limbs may be important in the interpretation of lead changes in lame horses. While the synthetic surface appears to be less strenuous for the joints in the forelimbs, it was associated with changes in timing of the kinematic events of the stride.

Use of a 3D dynamometric horseshoe for the measurement of grip parameters in a horse cantering on right and left circles on two surfaces

The notion of grip, or traction, between a sport surface and the athlete’s shoes is a critical parameter as it conditions performance but can also be a factor of injury. In human sport, grip has been investigated in order to improve the design of shoes and surfaces through characteristics such as dynamic traction coefficient and dynamic vertical torque. The same kind of improvement could be obtained for equine competition surfaces on which the horses have to perform sharp turns. In this context, biomechanical parameters related to grip must be defined and measured under sport conditions, which require the measurement of the triaxial hoof reaction force (HRF). The purpose of this study was to comparatively assess the grip of two surfaces, one designed for competition and the other for instruction, while measuring the HRF in a horse cantering in circles.

Achilles tendon force and axial speed of sound: a calibration method under clinical conditions.

Achilles tendon force (ATF) is a variable of the locomotion which could bring much insight on movement analysis and muscle function in clinical conditions, for instance in the evaluation of treatment efficacy following cardiovascular or neurological events altering the locomotion. ATF has been measured with invasive devices, such as the buckle transducer (Komi 2000) or the optic fibre technique (Finni et al. 1998), but their invasive nature precluded their systematic application in clinical environment. Axial speed of sound (SOS) measurement is a relatively novel technique (Pourcelot, Defontaine, et al. 2005) which is currently being utilised to non-invasively evaluate equine tendonforceduring locomotion (Crevier-Denoixetal. 2009). A preliminary feasibility study was carried out on humans (Pourcelot, Defontaine, et al. 2005; Pourcelot, Van den Bogert, et al. 2005), in which SOS was measured in Achilles tendon at walk and during step climbing. However, a proper calibration method to transform SOS measurements in force values is still lacking. In this work, we propose a simple calibration method for the Achilles tendon, which could be utilised in clinical conditions. The calibration was then tested by evaluating ATF at walk.

External loads on the leading and trailing forelimbs of a jumping horse at landing measured with a dynamometric horseshoe

with a dynamometric horseshoe N. Crevier-Denoix*, M. Camus, S. Falala, B. Ravary-Plumioen, L. Douilly-Holden, D. Robin, J.-M. Denoix, H. Chateau and P. Pourcelot Université Paris Est, Ecole Nationale Vétérinaire d’Alfort, USC 957 BPLC, F-94700 Maisons-Alfort, France; INRA, USC 957 BPLC, F-94700 Maisons-Alfort, France; Université Paris Est, Ecole Nationale Vétérinaire d’Alfort, CIRALE, F-14430 Goustranville, France