# Aspect-Related Mechanical Properties of the Cortical Bone in the Third Metacarpal Bone of Mares
## 1 Introduction  
Fractures of the third metacarpal bone (MC III) in horses encompass a wide spectrum, ranging from marginal intra-articular fractures and condylar fractures to complete fractures of the diaphysis [1, 2, 3, 4, 5]. A significant proportion of MC III fractures are fatigue-related, often caused by improper training or overloading of the horse [6]. The first two types of fractures are relatively common, primarily affecting young racehorses [7], and generally do not pose major therapeutic challenges. Marginal fractures are typically treated arthroscopically [8], while simple condylar fractures are usually managed through osteosynthesis with screwsor screws and plates [3]. Both methods yield promising outcomes, often enabling horses to return to full performance following successful treatment [3; 9; 10]. In contrast, complete fractures of the MC III diaphysis are less common but present substantial therapeutic challenges for veterinarians [1; 4; 11; 12]. These fractures are most frequently caused by kicks from other horses or falls [13] and often result in the euthanasia of the affected horses [14; 15]. Consequently, efforts should focus on simulating bone behavior under physiological load and training [16; 17], predicting bone overloading [18; 19], and planning advanced surgical treatments for severe MC III fractures [12; 19; 20; 21; 22; 23]. Progress in equine clinical practice could be significantly advanced through the development and application of constitutive and biomechanical modeling, such as the finite element method (FEM) [21; 23]. FEM modeling can utilize slice geometry derived from computed tomography (CT) images to create accurate models of bone shapes [23]. With the increasing accessibility of CT imaging in equine clinical diagnostics [5], particularly with the advent of standing sedated CT examinations [24; 25], using CT to model individual bone loads in horses represents a vital step forward in modern orthopedics. However, FEM simulations require specific experimental data on the mechanical properties of cortical and trabecular bones, such as elasticity, strength, and anisotropy, which directly influence the accuracy of stress, strain, and deformation predictions in the model. Without accurate experimental data, the simulation of the forces and loads acting on the equine skeleton may not reflect adequate bone behavior and thus theoretical clinical conditions [21]. Mechanical properties of bones are influenced by the direction of applied forces [26; 27], as well as the architecture and composition of the bone, along with various individual and environmental factors [28; 29; 30; 31]. Equine bone [26; 32], as bovine [27; 33; 34] and human [33] bone, exhibits a typical anisotropic structure. Its structure, composition, and shape are affected by individual factors such as age and sex, as well as load-related factors, including body mass and the type and intensity of training [29; 31]. Given the bone structure can exhibit differences related to age, sex, and body mass, this study was designed to investigate a consistent research group of female, mid-age and mid-size horses. The mechanical properties of the entire MC III in horses, including its trabecular and cortical bone components, have been partially described [35; 36; 37; 38]. However, to the best of our knowledge, no studies have specifically examined the elastic modulus of equine cortical bone at the mid-length of the diaphysis, a region prone to complete fractures of the MC III [1; 3; 13]. Given that the thickness of equine cortical bone varies significantly--from approximately 15 mm on the medial aspect of the mid-length diaphysis to about 5 mm on the lateral aspect of the distal metaphysis [23]--we hypothesized that the mechanical properties of cortical bone, and consequently the calculated elastic modulus, also vary depending on the aspect of the MC III.