Morphological, structural, and mechanical characteristics of SCB of the distal metacarpal condyles of Thoroughbred racehorses and their relationship with exercise history have been the focus of investigation in recent years.1-5 The anatomically related gradient in bone mineral density was recently described, and structural and mechanical characteristics of the SCB across the metacarpal condyles were determined by use of micro-CT and compressive in vitro mechanical testing.6,7 The distopalmar SCB of the MC3 in racing Thoroughbreds is denser, more robust, and stronger in the condyles than in the sagittal ridge. Sharp gradients of these properties have been found at the level of the condylar grooves and are believed to be implicated in the pathogenesis of condylar fractures.2,3,8,9
Bones are sensitive to their mechanical environment and respond by adapting their mass and their external and internal architecture to the mechanical requirements.10 For example, the pattern of TBB in the human femur and the sheep calcaneus closely follows principal stress and strain trajectories.10–12 Although this so-called trajectorial or Wolff theory has several limitations, the relationships between TBB microstructure and its mechanical usage have been reported.13,14 Instead of being homogeneously distributed and randomly oriented (isotropy) within a whole bone specimen, bone structural elements (trabeculae) are strategically positioned, conferring upon the bone its anisotropic characteristics: trabeculae are oriented along the principal directions of forces within the bone, making the bone stronger in that particular loading direction.11,12 Therefore, by studying the architectural characteristics of SCB in equine metacarpal condyles, it should be possible to infer, at least in part, the nature of the loads acting on the SCB at that location.
The anistropy of the SCB of the distal condylar area of the MC3 in racing Thoroughbreds can be easily appreciated at the macroscopic level.15 The SCB at this location appears highly organized with robust plates running parallel to the sagittal plane and with few mediolateral connections.16 This form of microstructural anisotropy appears to be more pronounced in horses with a longer racing history.17 Although the microstructure of the SCB is believed to give maximum strength and protection in the sagittal plane in which the bone rotates, it offers minimal resistance to proximal propagation of fracture in the sagittal plane.16 There also seems to be a parallel relationship between the anisotropic pattern of trabecular disposition and the characteristic anatomic course of distal condylar fractures of the MC3 in racing Thoroughbreds.16 Limited research evaluating SCB anisotropy at this location has been performed. Boyde et al16 evaluated the 3-D architecture of 2 beam-like samples of SCB and TBB obtained from a mediolateral dorsal slice (frontal plane) collected at the midpoint of the distal aspect of the condyle. The SCB within those beams had a high DA that was plainly evident via visual inspection. However, the distribution of load around the circumference of the condyles varies substantially during exercise and is dependent on the degree of flexion and extension of the joint.15 The palmar aspect of the metacarpal condyles supports most of the load during racing, and this is the location where major SCB changes occur, including SCB disease.1,3 Therefore, greater changes in SCB anisotropy may be expected at the palmar aspect of the metacarpal condyles, which could be relevant in the pathogenesis of clinical conditions such as traumatic osteoarthritis and condylar fractures.
Mean intercept length analysis is a 3-D stereological method that measures the orientation and anisotropy of porous media, such as TBB. The method is based on measuring the lengths of intersections of a regularly spaced parallel array of test lines with the trabecular structure. The array of test lines is realigned over many orientations, from which a fabric ellipsoid (3-D ellipse) is calculated.18,19 Structures with no preferred orientation are characterized by a spherical ellipsoid, while those with preferential alignment in one direction will have the major axis of the ellipsoid aligned with that direction. Stereological analyses are typically applied to 3-D images acquired by tomographic imaging systems. In particular, micro-CT represents the present gold standard for bone imaging and stereological analysis because it provides nondestructive, high-resolution 3-D images of bone architecture.20,21 Softwarea provides an implementation of the MIL method for analyzing digitally reconstructed 3-D images obtained by micro-CT. Micro-CT and software have been used to study the distopalmar SCB of the MC3 condyles.6
The objective of the study reported here was to determine the anisotropic characteristics of SCB micro-structure in the distopalmar aspect of the MC3 condyles in racing Thoroughbreds by applying the MIL method to 3-D micro-CT images. We hypothesized that the principal trabecular orientation and DA vary with anatomic location (condyles vs sagittal ridge and SCBP vs TBB) and with different stages of SCB disease.
Degree of anisotropy
Third metacarpal bone
Mean intercept length
Proximal sesamoidean bone
Region of interest
Subchondral bone plate
MicroView ABA, version 2.1.2, GE Healthcare, London, ON, Canada.
GE Medical Systems eXplore Locus Micro CT Scanner, GE Medical Systems, London, ON, Canada.
SB3, Gammex RMI, Middleton, Wis.
Norrdin RWKawcak CECapwell BA, et al. Subchondral bone failure in an equine model of overload arthrosis. Bone 1998;22:133–139.
Riggs CMWhitehouse GHBoyde A. Structural variation of the distal condyles of the third metacarpal and third metatarsal bones in the horse. Equine Vet J 1999;31:130–139.
Riggs CMWhitehouse GHBoyde A. Pathology of the distal condyles of the third metacarpal and third metatarsal bones of the horse. Equine Vet J 1999;31:140–148.
Riggs CMBoyde A. Effect of exercise on bone density in distal regions of the equine third metacarpal bone in 2-year-old Thoroughbreds. Equine Vet J Suppl 1999;(30):555–560.
Kawcak CEMcIlwraith CWNorrdin RW, et al. Clinical effects of exercise on subchondral bone of carpal and metacarpophalangeal joints in horses. Am J Vet Res 2000;61:1252–1258.
Rubio-Martinez LMCruz AMGordon K, et al. Structural characterization of the distal aspect of third metacarpal subchondral bones in Thoroughbred racehorses by micro-computed tomography. Am J Vet Res 2008;69:1413–1422.
Rubio-Martinez LMCruz AMGordon K, et al. Mechanical properties of subchondral bone in the distal aspect of third metacarpal bones from Thoroughbred racehorses. Am J Vet Res 2008;69:1423–1433.
Stepnik MWRadtke CLScollay MC, et al. Scanning electron microscopic examination of third metacarpal/third metatarsal bone failure surfaces in thoroughbred racehorses with condylar fracture. Vet Surg 2004;33:2–10.
Radtke CLDanova NAScollay MC, et al. Macroscopic changes in the distal ends of the third metacarpal and metatarsal bones of Thoroughbred racehorses with condylar fractures. Am J Vet Res 2003;64:1110–1116.
Wolff J. Ueber die innere Architektur der Knochen und ihre Bedeuting fuer die Frage von Knochenwachstum. Virchows Archiv Pathol Anat Physiol 1870;50:389–453.
Lanyon LE. Functional strain in bone tissue as an objective, and controlling stimulus for adaptive bone remodelling. J Biomech 1987;20:1083–1093.
Riggs CMVaughan LCEvans GP, et al. Mechanical implications of collagen fibre orientation in cortical bone of the equine radius. Anat Embryol (Berl) 1993;187:239–248.
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.
Boyde AHaroon YJones SJ, et al. Three dimensional structure of the distal condyles of the third metacarpal bone of the horse. Equine Vet J 1999;31:122–129.
Yoshihara TKaneko MOikawa M, et al. An application of the image analyzer to the soft radiogram of the third metacarpus in horses. Jpn J Vet Sci 1989;51:184–186.
Harrigan TBMann RW. Characterization of microstructural anisotropy in orthotropic materials using a second rank tensor. J Mater Sci 1984;19:761–767.
Odgaard A. Three-dimensional methods for quantification of cancellous bone architecture. Bone 1997;20:315–328.
Wachsmuth LEngelke K. High-resolution imaging of osteoarthritis using microcomputed tomography. Methods Mol Med 2004;101:231–248.
Cruz AMHurtig MBGoldie K, et al. Staging an. progression of subchondral bone disease in the fetlock of the equine athlete, in Proceedings. 15th Annu Sci Meet Eur Coll Vet Surg 2006;132–133.
Easton KLKawcak 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.
Norrdin RWStover SM. Subchondral bone failure in overload arthrosis: a scanning electron microscopic study in horses. J Musculoskelet Neuronal Interact 2006;6:251–257.
Davison KSSiminoski KAdachi JD, et al. Bone strength: the whole is greater than the sum of its parts. Semin Arthritis Rheum 2006;36:22–31.
Norrdin RWBay BKDrews MJ, et al. Overload arthrosis: strain patterns in the equine metacarpal condyle. J Musculoskelet Neuronal Interact 2001;1:357–362.
Young BDSamii VFMattoon JS, et al. Subchondral bone density and cartilage degeneration patterns in osteoarthritic metacarpal condyles of horses. Am J Vet Res 2007;68:841–849.
Fischer KJJacobs CRCarter DR. Computational method for determination of bone and joint loads using bone density distributions. J Biomech 1995;28:1127–1135.