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Considering Tissue Types When Treating Equine Joint Disease
Veterinarians should consider the type of tissue they want to treat when choosing a management strategy. | The Horse StaffVeterinarians regard the equine joint as an organ, composed of articular cartilage, the synovial membrane lining the inside of the joint capsule, the synovial fluid filling the joint space, the subchondral bone underlying the cartilage layer, and the supporting soft tissue structures. Viewing the joint as an integrated organ has important implications for how veterinarians approach diagnosis and treatment.Veterinarians need to consider the origin of joint inflammation and/or cytokine-driven damage to develop more effective and targeted treatment plans for their patients, said Cricket Russillo, DVM, ISELF-Cert, of High Performance Equine, in Northern Virginia and Florida, during her presentation at the 2025 American Association of Equine Practitioners Convention, held Dec. 6-10, in Denver, Colorado.Joint disease causes 60% of equine lameness problems, often leading to substantial economic losses for horse owners, both direct (medical costs) and indirect (loss of performance). When treating joint disease, practitioners should assess the different joint tissues and essentially reverse engineer a targeted treatment plan based on what joint tissues are likely affected, said Russillo.Articular Cartilage and Equine Joint DiseaseArticular cartilage facilitates smooth movement and absorbs shock. It has limited repair ability, and its breakdown leads to pain, inflammation, and impaired joint function.The horses body produces immunoregulatory proteins called cytokines, which coordinate and shape inflammatory and healing responses. In response to injury, articular cartilage releases pro-inflammatory cytokines such as interleukin-1 beta (IL-1b), IL-6, and tumor necrosis factor alpha (TNFa). These cytokines promote cartilage matrix degradation by upregulating matrix metalloproteinases (MMPs) and aggrecanases, which are both enzymes that contribute to cartilage degradation. If we suspect cartilage injury, the goal should be to protect cartilage integrity and arrest the inflammatory cascade and stabilize the matrix, said Russillo.She recommended veterinarians select systemic chondroprotective agents such as hyaluronic acid (HA), polysulphated glycosaminoglycans (PSGAGs), pentosan polysulfate (PPS), and non-steroidal anti-inflammatory drugs (NSAIDs) for milder cases or younger animals. These medications can be combined with a reduced workload for longer-lasting effects.We also want to focus on the root cause of the inflammation and blunt the activity of the pro-inflammatory cytokines, said Russillo. This is where orthobiologics such as hemoderivatives and stem cells come into play. Orthobiologics might also contribute growth factors that can be beneficial for slowing cartilage degradation, she added.Treating the Synovial Membrane in Equine Joint DiseaseThe synovial membrane plays a critical role in the onset and progression of inflammation. Specialized white blood cells called synovial macrophages adopt different phenotypesessentially, functional states cells adopt in response to their environmentbased on signals in the joint. Macrophages in healthy joints express the M2 phenotype, whereas macrophaes in unhealthy joints express the M1 phenotype, which is pro-inflammatory, producing high levels of TNFa and IL-1b.When regulatory M2 macrophage activity can no longer counterbalance pro-inflammatory M1 signaling, inflammation escalates and stimulates MMPs and aggrecanases. In this environment type B synoviocytesspecialized cells that line joints, tendons, and bursaeundergo hyperplasia (an abnormal increase in cell number), resulting in joint stiffness and dysregulating the production of healthy joint fluid.When we suspect inflammation of the synovium is the primary issue, then there are two avenues of therapy: Blunt inflammation or manipulate the interplay of M1 and M2 synovial macrophage polarization, said Russillo.Referring to intra-articular therapies, or those introduced to the joint, she added, Corticosteroids are extremely effective at mitigating inflammation, orthobiologics can decrease the expression of pro-inflammatory cytokines and stimulate the release of growth factors, and the polyacrylamide (hydro)gels are theorized to drive synovial macrophages to the M2 response.Synovial Fluid and Equine Joint DiseaseSynovial fluid plays a crucial role in providing nutrients to cartilage, lubrication, and removing metabolic waste. Joint inflammation results in excess production of abnormal synovial fluid, with high levels of inflammatory cytokines and reactive oxygen species that damage cartilage.Therapies directed at this joint tissue are aimed at manipulating the level of HA, said Russillo. Directly injecting HA is not often considered beneficial as a sole therapy. But intravenous HA does improve synovial fluid and joint health, and systemic administration of PSGAGs may also help improve synovial fluid quality. These therapies are most likely to benefit young patients with minimal joint pathology (disease or damage).Treating Equine Subchondral Bone ProblemsThe dense, structural subchondral bone beneath articular cartilage plays a pivotal role in joint stability and load distribution. It is a highly adaptive tissue that responds to mechanical loading. Excess repetitive stress can contribute to the development and progression of joint disease that is common in equine athletes across disciplines. Subchondral bone disease might precede and drive cartilage degeneration.The main treatment is exercise modification, which may not be a well-received recommendation by clients, said Russillo. Extralabel bisphosphonates may be considered, and intra-articular orthobiologics may have a beneficial effect (in these cases).Take-Home MessageIf we consider the specific tissue involved in our patients joint disease, it then becomes possible to curate a treatment plan for that particular target tissue and to improve outcomes and client satisfaction, said Russillo. Veterinarians might weigh a combination of treatments in addition to modifying the horses workload.
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