Disorders of the musculoskeletal system most often affect the horse’s ability to move. How severely movement is impaired depends on the type and severity of the problem. Skeletal and joint disorders are the most common. In horses, musculoskeletal injuries are a major source of debilitating pain, financial loss, and loss of performance. Joint disease (including arthritis) and tendon injuries are particularly common and costly in performance horses. Other disorders that can cause problems in the musculoskeletal system include diseases of the muscles, neurologic problems, toxins in the body, hormonal abnormalities, metabolic disorders, infectious diseases, blood and vascular disorders, poor nutrition, and birth defects.
Many different systems in the body rely on the muscles. A horse’s ability to see, breathe, urinate, breed, and even chew and swallow may be affected by a muscular condition. With many disorders, however, the musculoskeletal system is simply the location where signs of other underlying conditions show themselves. Veterinarians often trace the real cause of musculoskeletal trouble back to the nervous system.
Disorders that primarily affect the muscle membrane or muscle fibers are called myopathies. Muscle membrane disorders may be hereditary or acquired. Exertional myopathies (such as "tying up") are examples of myopathies involving the actual muscle fiber. Diagnosing a myopathy usually requires blood tests, biopsies, and electronic tests (such as electromyographic studies or thermography).
Tendons stretch very little, so they are prone to injury and may become torn if a large amount of force is applied to them. Such injuries lead to tendinitis, which is inflammation of the tendons. Because tendons and ligaments are relatively poorly supplied with blood, they heal slowly and often imperfectly. Injuries to ligaments and tendons require patience and careful longterm rehabilitation.
Bone diseases are generally present at birth or the result of nutritional deficiencies or injuries. An imbalanced level of minerals in the diet, particularly of trace minerals such as copper, zinc, and magnesium, is a common dietary cause of bone defects. Growing animals that are fed too much protein can also develop nutritional disorders affecting bones. Getting either too much or too little of certain vitamins, particularly vitamins A and D, can influence bone growth and development.
Most bone disorders stem from some sort of trauma, such as fractures or cracks. Bone fractures are classified as simple (the bone is broken into 2 pieces) or compound (there are 3 or more pieces). They are also divided into closed (the skin is not broken) or open (the skin is broken). Infections that cause bone tissue to break down and die can lead to bone disorders. In other situations, diseases of the ligaments or tendons may cause secondary bone troubles.
Movable joints are vulnerable to joint diseases or disorders affecting their membranes, as well as related ligaments, cartilage, and bone. Joint disorders may be caused by trauma to the joint, longterm inflammation, developmental problems, or infections. Traumatic injuries may produce consequences such as dislocation, fracture, or the distortion of a joint, arthritis, or the rupture of nearby ligaments or membranes.
Osteoarthritis is inflammation of the joints and surrounding bone. It causes progressive deterioration of the affected joint(s). Chronic or longterm inflammation is most commonly seen in joints associated with movement. The weight of a horse's body puts a lot of pressure on its joints. Healthy joint cartilage can withstand this pressure, but longterm inflammation damages the cartilage and other joint tissues. The loss of cartilage in arthritic joints results in impaired function. Osteoarthritis is a major cause of pain and decreased performance in horses.
Recent years have seen great advances in techniques for diagnosing and healing musculoskeletal disorders. When detected early, the disorders often can be corrected, allowing the horse a full return to healthy life.