# Aspect-Related Mechanical Properties of the Cortical Bone in the Third Metacarpal Bone of Mares
## 4 Discussion  
The current study was designed to investigate the aspect-related mechanical properties of the cortical bone in MC III in a consistent research group. Therefore, only mares were included, as bone structure can exhibit sex-related differences [29,31]. [PERSON] et al. [29] reported sex-related variability in bone remodeling in racehorses, with males exhibiting higher levels of serum biomarkers for bone formation and resorption compared to females. [PERSON] et al. [31] found that the fractal dimension of the lateral aspect of the proximal phalanx was lower in males than in females, when they assessed bone radiographic texture. [PERSON] et al. [31] also highlighted age-related characteristics of equine trabecular bone in the proximal phalanx and MC III, showing an increase in bone fraction with age and training. However, to date, there are no reports on differences in the occurrence of complete fractures of the MC III diaphysis between mares, geldings, and stallions [13]. Therefore, the same generalization was ultimately applied when concluding equine predispositions to these fractures. A comparison of the cross-sectional diaphysis of the MC III revealed that the cortical bone is thickest on the medial aspect and thinnest on the palmar aspect. The increased thickness of the cortical bone on the medial aspect of the forelimb is an adaptation to the natural loading of the limb during movement [16,17]. The forces acting on MC III along the long axis of the limb are greater than those acting in other directions due to the need to support a high body weight [42]. However, the resultant force on the MC III is a combination of forces acting along the limb's long axis and in the transverse and oblique forces. Non-vertical component forces are generated during limb abduction and are influenced by the horse's posture [43]. Non-vertical component forces subject the bone to bending in the dorsal and sagittal planes, particularly in the externo-internal and latero-lateral directions [44, 45, 46, 47]. This effect is especially relevant since many horses exhibit a toe-out and addicted thoracic posture rather than a completely straight posture [43,48]. Although non-vertical component forces are much smaller than the vertical forces acting in the proximal-distal direction [44, 45, 46, 47], they may contribute to the remodeling of the equine cortical bone [49]. The current results support the hypothesis that the mechanical properties of cortical bone vary depending on the aspect of the MC III, confirming the aspect-related specificity of the elastic modulus. Therefore, a single elastic modulus should not be generalized to the entire MC III diaphysis, and aspect-specific cortical bone properties should be incorporated into future FEM models. This improvement may enhance the modeling of MC III [21] for various clinical applications, such as training optimization [16,17], overload prediction [18,19], and surgical planning--particularly for advanced surgical treatments of complete MC III fractures [12, 19, 20, 21, 22, 23]. Notably, the thinner cortical bone on the palmar aspect of the MC III coincides with a lower sample reaction force-based elastic modulus and a lower axial compression force-based elastic modulus in the proximo-distal direction. Interestingly, no differences in relative bone density were observed between the studied aspects, suggesting that the reduced bending and compression strength on the palmar aspect of the MC III may be attributed to the thinner cortical bone. However, this hypothesis requires further investigation involving a larger cohort of horses. The current results partially align with those reported by [PERSON] et al. [50], who documented the highest values of [PERSON]'s modulus of elasticity for the lateral and medial aspects of the equine MC III, followed by the dorsal and palmar aspects. It is worth noting that their study only involved a compression test of cylindrical samples loaded in the proximo-distal direction [50].