ABSTRACT: Understanding the basis of osteoarthritis (OA) has seen some interesting advancements in recent years. It has been observed that cartilage degeneration is preceded by subchondral bone lesions, suggesting a key role of this mechanism within the pathogenesis and progression of OA, including the formation of ectopic bone and osteophytes. Moreover, low-grade, chronic inflammation of the synovial lining has gained a central role in the definition of OA pathophysiology, and central immunological mechanisms, innate but also adaptive, are now considered crucial in driving inflammation and tissue destruction. In addition, the role of neuroinflammation and central sensitization mechanisms has been characterized as underlying causes of pain chronicity. This has led to a renewed definition of OA, which is now intended as a complex multifactorial joint pathology caused by inflammatory and metabolic factors underlying joint damage. Since this evidence can directly affect the definition of the correct therapeutic approach to OA, an improved understanding of these pathophysiological mechanisms is fundamental. This review provides an overview of the most updated evidence on OA pathogenesis; it presents the most recent insight on the pathophysiology of OA, describing the interplay between immunological and biochemical mechanisms proposed to drive inflammation and tissue destruction, as well as central sensitization mechanisms. Moreover, although the therapeutic implications consequent to the renewed definition of OA are beyond this review scope, some suggestions for intervention have been addressed.

Pain in osteoarthritis (OA) results from erosion of joint cartilage, resulting in bone contacting bone without an intervening cushion. The periosteum, including its nociceptive innervation, ends at the border of the cartilage meaning that there is no extrinsic neuronal pathway between the opposing denuded bone surfaces to carry a bone-on-bone pain signal to the brain. The pain signaling pathway must therefore originate in nociceptive sensory endings within the subchondral bone itself. Selective ablation of this intrinsic nerve pathway, using any of a variety of approaches, is expected to permanently eliminate OA pain.

Articular cartilage healing and repair is one of the biggest clinical challenges in trauma and orthopaedic medicine, leading to high economic loss due to athletic impairment in sport horses. Multipotent Mesenchymal stem/stromal cells are part of novel therapeutic arsenal aimed at tissue regeneration and restoring organ function. These cells release biomolecules and factors in the culture media, producing the so called conditioned medium, which have a pro-regenerative effect through paracrine mechanisms. Considering this, a therapeutic product combining equine synovial membrane derived Mesenchymal stem/stromal cells and equine umbilical cord stroma-derived Mesenchymal stem/stromal cells conditioned medium was employed. In this work the treatment of two articular defects, in the first phalanx and third metacarpus of the left front limb of a five-year-old horse, is described. Following surgical debridement of the lesions, the treatment comprised two intra-articular administrations of the above-mentioned therapeutic combination, 15 days apart. This was followed by a thorough rehabilitation program. A second-round treatment was performed 6 months later. Diagnostic imaging demonstrated significant improvement of the articular defects. The clinical outcome allowed a full return to the athletic performance demonstrating the safety and potential advantages of this approach when compared with other therapeutic options.