## 1 Introduction  
Nowadays, as the human population ages at an accelerated pace [1; 2], the rise in aging-related diseases is emerging as a significant global concern [2; 3; 4]. However, it is important to note that aging affects not only the human population but also companion animals, including dogs [5; 6], cats [7; 8], and horses [9; 10; 11]. Over recent decades, the role of horses in society has evolved from working animals to performance and companion animals. A survey found that 56.5% of horse owners considered their horse to be a companion animal, with 38.4% even viewing their horse as a family member [9]. Companion animals typically receive better geriatric care compared to working animals, leading to a shift in the demographic structure of the horse population towards an aging one. Consequently, as the number of older horses increases, comprising up to 25-29% of the total horse population [10; 12; 13], there is a growing need to monitor the progression of age-related diseases in equine medicine [9]. Older individuals, including both humans [2] and animals [14; 15], are prone to injuries and degenerative musculoskeletal conditions that significantly impact their quality of life [16; 17]. In humans, sarcopenia [3] and osteoarthritis (OA) [18] are recognized as the most prevalent age-related musculoskeletal disorders. Similarly, in horses, OA is a primary cause of lameness [15], often leading to chronic pain and euthanasia in older equines [19]. The parallel occurrence of OA in elderly humans and horses is supported by similarities in pathogenesis, prompting the widespread use of equine models to study OA-related pathological changes [20]. Additionally, apart from naturally occurring OA [21], anatomical and histological resemblances exist between the human knee and tarsal joints and the equine tarsal joint [22; 23; 24]. In both cases, the thickness and properties of the articular cartilage exhibit striking similarities, enabling the successful modeling of cellular structure and biochemical outcomes [22; 23; 24]. Moreover, due to their larger body size, horses offer ease of tissue and fluid collection, along with a wide array of imaging modalities and clinical examination possibilities [25; 26]. These factors collectively underscore the significance of monitoring OA signs during treatment and rehabilitation in both human [20; 21; 22; 23; 24; 25; 26] and veterinary [23; 26; 27; 28; 29; 30; 31; 32; 33] medicine. Therefore, this study focuses on enhancing diagnostic strategies for equine tarsal joint OA, commonly referred to as bone spavin [34; 35]. The diagnostic approach to equine OA typically begins with assessing clinical symptoms, such as lameness and joint stiffness [28; 29; 33], which often stem from the mild chronic inflammation associated with OA. Consequently, the diagnostic protocol involves a basic clinical examination and a thorough orthopedic assessment, complemented using flexion tests, local anesthesia, and diagnostic imaging [28; 36]. Various imaging modalities may be employed to evaluate the affected joint, including conventional radiography [29; 33; 36; 37], ultrasound [32], computed tomography (CT) [38], magnetic resonance imaging (MRI) [37; 39], and, to a lesser extent, positron emission tomography (PET) [40]. These modalities help identify structural changes within the clinically suspected joint [41], such as subchondral bone lysis or sclerosis, joint space alterations, and osteophyte formation [28; 29; 33; 36; 37]. However, assessments of bone mineral density (BMD) and its correlation with the progressive loss of muscle mass, strength, and function [42] are less frequently conducted. BMD evaluation can be performed using X-ray beam attenuation measurements, including single-energy X-ray computed digital absorptiometry (CDA) [43; 44; 45; 46; 47; 48] or dual-energy X-ray absorptiometry (DXA) [45; 47; 49]. While DXA is the preferred method in human medicine [50; 51; 52], its application in equine clinical settings is limited [46; 47].