Although the pathogenesis of injuries involving the distal end of the third metacarpal bone in horses is complex, mechanical overload has been suggested as an important contributing factor.1–3 Mechanical overload may be acute, resulting from a single event that culminates in catastrophic injury, or chronic, resulting from cumulative damage over an extended period.1 However, despite the clear relationship between mechanical loading and the pathogenesis of injuries involving the distal end of the third metacarpal bone, little is known about the specific mechanisms of bone loading. In particular, the loads exerted by the proximal phalanx and proximal sesamoid bones on the distal end of the third metacarpal bone during locomotion are not well understood, even though these loads likely have an important role in the net mechanical loading of this bone.
Several in vitro studies4–6 have investigated the mechanical properties of the third metacarpal bone in horses and have provided information regarding the types of mechanical loads that it may be able to support. In these studies, it was found that the third metacarpal bone in horses could withstand large axial loads4,5 but only moderate bending4,5 and torsional loads,6 suggesting that this bone is loaded mainly in axial compression. Similarly, results of in vivo measurements of surface strain on the third metacarpal bone in horses,7 in conjunction with results from an in vitro study8 in which an end-loaded, beam-column model was used to calculate loading of the third metacarpal bone under a variety of conditions, suggested that the bone is primarily loaded in axial compression during locomotion, with only small loads producing bending in the sagittal and transverse planes.
A separate study9 involving the application of strain gauges to the third metacarpal bone also found that the third metacarpal bone was loaded primarily in axial compression. In contrast, in that same study,9 analysis of data obtained by filming horses when the forelimbs passed over a force plate indicated that the third metacarpal bone was loaded in sagittal bending. The investigators suggested that the discrepancy in metacarpal loading indicated by the 2 methods may have been attributable to the fact that the force plate analysis did not account for the force exerted by the proximal sesamoid bones on the distal end of the third metacarpal bone.
A hypothetical mechanism for axial loading of the third metacarpal bone was described in a study10 that investigated the structure of the bone. It was suggested that the paired proximal sesamoid bones exerted a force on the metacarpal condyles that partially opposed the force exerted by the first phalanx. The vector sum of these 2 forces was suggested to act along the axis of the third metacarpal bone.10 Although this hypothesis was supported by the trabecular structure of the third metacarpal condyles, no analysis was performed to determine that the structures of the distal portion of the limb could produce the necessary balance of forces during locomotion while simultaneously satisfying all other requirements of a normal gait.
Many recent biomechanical studies11–17 of the forelimb in horses have examined tendinous structures, whereas others have examined the role of muscles during locomotion.18–20 A few studies21–24 have evaluated joint reaction forces as they pertain to the distal sesamoid bone. Several studies25–27 have evaluated contact areas and pressures within the metacarpophalangeal joint by analyzing cadaver limbs under artificial loading conditions. However, results of in vivo studies have not been reported, and mechanical loading of the third metacarpal bone in horses is still not clearly understood. Therefore, the purpose of the study reported here was to assess net mechanical load on the distal end of the third metacarpal bone in horses during walking and trotting. We hypothesized that forces exerted by the paired proximal sesamoid bones and the proximal phalanx would produce a resultant force directed along the shaft of the third metacarpal bone that acted to load the bone primarily in compression.
Finite element analysis
Ground reaction force
Merritt JS. Mechanical modelling of the equine distal forelimb. PhD Thesis, Faculty of Veterinary Science, The University of Melbourne, Melbourne, VIC, Australia, 2007.
Micro Measurements, Rayleigh, NC.
CPE Systems, Melbourne, VIC, Australia.
Beltalong, Eurora, VIC, Australia.
XVISION/GX, model TBX-002A, Toshiba Australia, North Ryde, NSW, Australia.
Stichting Blender Foundation, Amsterdam, The Netherlands.
FEAPPV, University of California, Berkeley, Calif.
Scilab, INRIA ENPC, Domaine de Voluceau, France.
Vicon Peak, Centennial, Colo.
Bertec Corp, Columbus, Ohio.
Muir P, Peterson AL, Sample SJ, et alExercise-induced metacarpophalangeal joint adaptation in the Thoroughbred racehorse. J Anat 2008;213:706–717.
Rybicki EF, Mills EJ, Turner AS, et alIn vitro and analytical studies of forces and moments in equine long bones. J Biomech 1977;10:701–705.
Biewener AA, Thomason J, Goodship A, et alBone stress in the horse forelimb during locomotion at different gaits: a comparison of two experimental methods. J Biomech 1983;16:565–576.
Thomason JJ. The relationship of structure to mechanical function in the third metacarpal bone of the horse, Equus caballus. Can J Zool 1985;63:1420–1428.
Lochner FK, Milne DW, Mills EJ, et alIn vivo and in vitro measurement of tendon strain in the horse. Am J Vet Res 1980;41:1929–1937.
Jansen MO, van Buiten A, van den Bogert AJ, et alStrain of the musculus interosseous medius and its rami extensorii in the horse, deduced from in vivo kinematics. Acta Anat (Basel) 1993;147:118–124.
Jansen MO, van den Bogert AJ, Riemersma DJ, et alIn vivo tendon forces in the forelimb of ponies at the walk, validated by ground reaction force measurements. Acta Anat (Basel) 1993;146:162–167.
Riemersma DJ, van den Bogert AJ, Jansen MO, et alInfluence of shoeing on ground reaction forces and tendon strains in the forelimbs of ponies. Equine Vet J 1996;28:126–132.
Meershoek LS, van den Bogert A, Schamhardt HC. Model formulation and determination of in vitro parameters of a noninvasive method to calculate flexor tendon forces in the equine forelimb. Am J Vet Res 2001;62:1585–1593.
Meershoek LS, Lanovaz JL. Sensitivity analysis and application to trotting of a noninvasive method to calculate flexor tendon forces in the equine forelimb. Am J Vet Res 2001;62:1594–1598.
Meershoek LS, Lanovaz JL, Schamhardt HC, et alCalculated forelimb flexor tendon forces in horses with experimentally induced superficial digital flexor tendinitis and the effects of application of heel wedges. Am J Vet Res 2002;63:432–437.
Swanstrom MD, Zarucco L, Hubbard M, et alMusculoskeletal modelling and dynamic simulation of the Thoroughbred equine forelimb during stance phase of gallop. J Biomech Eng 2005;127:318–328.
Willemen MA, Savelberg HHCM, Barneveld A. The effect of orthopaedic shoeing on the force exerted by the deep digital flexor tendon on the navicular bone in horses. Equine Vet J 1999;31:25–30.
Wilson AM, McGuigan MP, Fouracre L, et alThe force and contact stress on the navicular bone during trot locomotion in sound horses and horses with navicular disease. Equine Vet J 2001;33:159–165.
McGuigan MP, Wilson AM. The effect of bilateral palmar digital nerve analgesia on the compressive force experienced by the navicular bone in horses with navicular disease. Equine Vet J 2001;33:166–171.
Eliashar E, McGuigan MP, Wilson AM. Relationship of foot conformation and force applied to the navicular bone of sound horses at the trot. Equine Vet J 2004;36:431–435.
Colahan P, Turner TA, Poulos P, et alMechanical functions and sources of injury in the fetlock and carpus, in Proceedings. 33rd Annu Meet Am Assoc Equine Pract 1987;33:689–699.
Brama PAJ, Karssenberg D, Barneveld A, et alContact areas and pressure distribution on the proximal articular surface of the proximal phalanx under sagittal plane loading. Equine Vet J 2001;33:26–32.
Easton KL, Kawcak CE. Evaluation of increased subchondral bone density in areas of contact in the metacarpophalangeal joint during joint loading in horses. Am J Vet Res 2007;68:816–821.
Merritt JS, Davies HMS, Burvill CR, et alInfluence of muscle-tendon wrapping on calculations of joint reaction forces in the equine distal forelimb. J Biomed Biotechnol 2008;2008:165730.
Merritt JS, Burvill CR, Pandy MG, et alDetermination of mechanical loading components of the equine metacarpus from measurements of strain during walking. Equine Vet J Suppl 2006;(36):440–444.
Les CM, Stover SM, Keyak JH, et alThe distribution of material properties in the equine third metacarpal bone serves to enhance sagittal bending. J Biomech 1997;30:355–361.
Weinans H, Blankevoort L. Reconstruction of bone loading conditions from in vivo strain measurements. J Biomech 1995;28:739–744.
Le Jeune SS, Macdonald MH, Stover SM, et alBiomechanical investigation of the association between suspensory ligament injury and lateral condylar fracture in thoroughbred racehorses. Vet Surg 2003;32:585–597.
Parente EJ, Richardson DW, Spencer P. Basal sesamoidean fractures in horses: 57 cases (1980–1991). J Am Vet Med Assoc 1993;202:1293–1297.