# Aspect-Related Mechanical Properties of the Cortical Bone in the Third Metacarpal Bone of Mares
## 4 Discussion
However, neither the intermediate thickness of the dorsal aspect of the MC III nor previous studies account for the greater strength of the dorsal aspect observed in this study for bending in the extero-internal direction and compression in both the extero-internal and proximo-distal directions. It is important to note that in the discussed 3-point bending test, cortical bone samples were subjected to force in the extero-internal direction. However, the compression-force-based elastic modulus in the extero-internal direction was higher in the palmar and dorsal aspects compared to the lateral and medial aspects. The observed differences in bending and compression strength, even when the force acts in the same direction, may be attributed to bone anisotropy [26; 27; 32]. Cortical bone is highly anisotropic with a non-homogeneous morphological structure. Therefore, it returns different values in mechanical tests depending on the direction of loading [27; 50]. Different aspect-related values in mechanical tests, depending on the direction of loading, may have clinical implications, suggesting an increased risk of complete fractures of the MC III diaphysis due to kicks from other horses or falls, rather than from training or competition-related overload. The equine MC III bone can withstand the greatest loads in the proximo-distal direction [42], which aligns with the orientation of fibers in the extracellular matrix (ECM) of cortical bone, a factor that significantly influences bone strength parameters [26; 27; 32]. Additionally, the properties of cortical bone differ depending on the type of loading (i.e., bending vs. compression). This highlights the need for further investigation of the cortical bone structure using uCT [30; 51] and bone composition analysis using the computed digital absorptiometry (CDA) [52; 53], dual energy x-ray absorptiometry (DEXA) [52; 54], or dual energy computed tomography (DE CT) [55]. These studies could help explain the variations in bone response to different forces (the sample reaction force and the axial compression force) in the same direction, as well as the similarities in bone response to different forces applied in different directions. In the discussed 3-point bending test, the extero-internal direction of forces acting was chosen to study the bending of the cortical bone because it aligns with the primary cause for the MC III fractures. These include being kicked by another horse or falling during training or competitions like show jumping or eventing [1; 3; 13], where the force typically acts horizontally to the ground, and which are the most common causes of complete fractures of the MC III diaphysis in horses [1; 2; 3; 4; 5; 12; 22]. Comparison of the axial compression force-based elastic modulus across the tested directions revealed that, regardless of the MC III aspect, the cortical bone is more resistant to compression when the force acts in the proximo-distal direction compared to the extero-internal or latero-lateral directions. This finding aligns with the results of [PERSON] et al. [42], which indicated that the equine skeleton is primarily loaded by compression, and with the observation by [PERSON] and [PERSON] [34] that changing the loading direction from proximo-distal to latero-lateral influences the value of the elastic modulus. However, it should be noted that [PERSON] and [PERSON]'s study investigated bovine, not equine, cortical bone [34]. Additionally, [PERSON] and [PERSON] [33] showed that in bovine and human cortical bone, the Young's modulus of elasticity is more than twice as great for proximo-distal loading compared to extero-internal or latero-lateralloading. The translation of these findings from bovine to equine bone, along with the current results, may provide new insights into why an equine healthy MC III bone does not frequently fracture upon landing after jumping over an obstacle [25] but may fracture after being kicked or from a fall [1; 3; 13]. The limitation of this study is the small sample size.