Abstract :
[en] In locomotor biomechanics, three groups of constraints are usually encountered : pressure, traction and torsion (Kamina, 2005).
In supra-maximal conditions, all of these contraints would be responsible for some disease of equine locomotor systems (Radin et al., 1972 ; Radin, 1983 ; Smith et al., 2002).
In order to understand better the constraints in the equine locomotor dynamics, some investigations have been carried out. Moreover, some measurement methods, based on the mechanics of Newton, have been performed. This work consists of three different experimental approaches:
The first approach was an ex vivo micro-systemic model of the diarthrodial cartilage-cartilage contact : from osteochondral specimens in contact, dynamic frictional constraints were measured by tribometry using a pin-on-disc tribometer in Coulomb’s regime. The effects of age and load on the frictional response were studied.
The second approach was an ex vivo macro-systemic model of the diarthrodial cartilage-cartilage contact : from whole fetlock joints fixed into an original pendulum tribometer and mobilized, dynamic visco-frictional constraints have been followed in viscous regime. The effect of a lipid solvent on the synovial fluid was studied.
The third approach was an in vivo macro-systemic model of the diarthrodial cartilage-cartilage contact : from distal forelimbs, cinematic, cinetic and radiographic data have been collected, then tendon tensions and joint contact forces components were calculated using an inverse dynamic analysis. From a sensitivity analysis, the effect of raised hell (0°, 6°, 12°) were elucidated.
In the first approach : (i) an original equine joint friction test model using a cartilage-on-cartilage arrangement (pin-on-disc) has been performed, (ii) cartilage ageing was found, under experimental operating conditions, to be responsible for an increase in friction coefficient, (iii) lubrication remained stable when young cartilage was loaded whereas lubrication of older cartilage was affected by increased load.
In the second approach : (i) an original equine joint pendulum tribometer has been designed, (ii) visco-frictionnal data have confirmed the joint pendulum damping can be modelled like a harmonic oscillator fluid damping, (iii) the lipid solvent injection into the fetlock joint increased the visco-frictional parameters of the joint and the damping time decreased, suggesting that the lubrication capacity was compromised.
In the third approach : (i) an original equine distal limb model with tendinous variable pulleys was described, (ii) polynomial relationships of second order between the variation of the dorsal angle of the fetlock joint and the moment arm variation of the deep digital and superficial digital flexors tendons have been established, (iii) the joint contact force components of the coffin joint and of the fetlock joint have been calculated during the phase of the trot, (iv) the effect of raised heel on the tendinous tensions and on the joint contact force components was established, suggesting that the use of heel wedges to reduce the loading of the coffin joint may in fact increase the loading of the fetlock joint.
The two ex vivo equine articular models (tribometry) are some original and complementary mechanical analysis tools: the pin-on-disc tribometer may highlight the mechanical properties of the triplex cartilage-synovial fluid-cartilage that are related with the structural properties (contact surface), and the pendulum tribometer may clarify the mechanical properties of the articular lubricant in a very realistic intra-articular environment (quasi anaerobic). These two tools could be used to characterize the mechanical properties of some new articular prosthesis and lubricants in the future.
Finally, the numerical in vivo model (dynamic inverse analysis), calculating dynamic tendinous tensions and dynamic joint contact force components in the equine distal forelimb, is equally a very interressing and original tool. Indeed, it measures some articular intrinsic informations, in non-invasive manner (no using of intra-articular and intra-tendinous constraint gaugues) and can be perfectly integrated in a classical equine locomotor analysis.
