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The skeletal system of the horse has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses typically have 205 bones. The pelvic limb typically contains 19 bones, while the thoracic limb contains 20 bones.
Bones serve three major functions in the skeletal system; they act as levers, they store minerals, and they are the site of red blood cell formation. Bones can be classified into five categories
Ligaments and tendons hold the skeletal system together. Ligaments hold bones to bones and tendons hold bones to muscles. Synovial membranes are found in joint capsules, where they contain synovial fluid, which lubricates joints. Bones are covered by a tough membrane called periosteum, which covers the entire bone excluding areas of articulation.
Ligaments attach bone to bone, and are vital in stabilizing joints as well as supporting structures. They are made up of fibrous material that is generally quite strong. Due to their relatively poor blood supply, ligament injuries generally take a long time to heal.
Ligaments of the upper body include:
Ligaments of the legs include:
The axial skeleton contains the skull, vertebral column, sternum, and ribs. The sternum consists of multiple sternebrae, which fuse to form one bone, attached to the 8 "true" pairs of ribs, out of a total of 18.
The vertebral column usually contains 54 bones: 7 cervical vertebrae, including the atlas (C1) and axis (C2) which support and help move the skull, 18 (or rarely, 19) thoracic, 5-6 lumbar, 5 sacral (which fuse together to form the sacrum), and 15-25 caudal vertebrae with an average of 18. Differences in number may occur, particularly in certain breeds. For example, some, though not all, Arabians, may have 5 lumbar vertebrae, opposed to the usual 6, 17 thoracic vertebrae (and ribs) instead of 18, and 16 or 17 caudal vertebrae instead of 18. The withers of the horse are made up by the dorsal spinal processes of the thoracic vertebrae numbers 5 to 9.
The skull consists of 34 bones and contains four cavities: the cranial cavity, the orbital cavity, oral, and the nasal cavity. The cranial cavity encloses and protects the brain and it supports several sense organs. The orbital cavitity surrounds and protects the eye. The oral cavity is a passage way into the respiratory and digestive systems. The nasal cavity leads into the respiratory system, and includes extensive paranasal sinuses. The nasal cavity contains turbinate bones that protect the mucous membrane that lines the cavity from warm inspired air. The skull consists of fourteen major bones
The appendicular skeleton contains the fore and hindlimbs. The hindlimb attaches to the vertebral column via the pelvis, while the forelimb does not directly attach to the spine (as a horse does not have a collar bone), and is instead suspended in place by muscles and tendons. This allows great mobility in the front limb, and is partially responsible for the horse's ability to fold his legs up when jumping. Although the hindlimb supports only about 40% of the weight of the animal, it creates most of the forward movement of the horse, and is stabilized through attachments to the spine.
Bones of the lower limb, present in both the front and hind legs include the cannon bone (3rd metacarpal/3rd metatarsal), splint bones (2nd and 4th metacarpal/metatarsal), proximal sesamoid bones, long pastern (proximal or 1st phalanx), short pastern (middle or 2nd phalanx), coffin bone (distal or 3rd phalanx), and navicular bone (distal sesamoid). There are usually slight differences in these bones when comparing the front and the hind. The 3rd metatarsal is about 1/6 longer than the 3rd metacarpal. Similarly, the 2nd and 4th metatarsals are longer in length when compared to their front-end counterpart. In the hindlimb, the 1st phalanx is shorter and the 2nd phalanx is longer than in the frontlimb. In addition, the 2nd and 3rd phalanx are narrower in the hind limb. The angle created by these three bones in the hindleg is steeper by about 5 degrees, therefore making the pastern angle steeper behind than in front.
Performance horses, like human athletes, place a high amount of stress on their bones and joints. This is especially true if the horse jumps, gallops, or performs sudden turns or changes of pace, as can be seen in racehorses, show jumpers, eventers, polo ponies, reiners, and western performance horses. A high percentage of performance horses develop arthritis, especially if they are worked intensely when young or are worked on poor footing.
One of the goals for management for these horses often involve caring for the joints to reduce the progression of arthritis. This usually includes a close monitoring of the animal's schedule to determine how to reduce the amount of joint concussion he experiences. Examples of this tactic include reducing the number of days the horse jumps to the absolute minimum to still keep him at optimum performance, altering the conditioning schedule so that the horse is not worked on extremely hard ground, and conditioning and training the horse in such a way that he "peaks" before a competition rather than keeping him in intense work year-round. This tactic can help increase the useful working life of the horse.
In addition, most owners usually augment this good management with proper nutrition, joint supplements, and joint injections. Commonly used substances are listed below. It is important for owners to check with their various equestrian organizations to be sure that the substances they use are not banned from competition, as they will have to plan their administration schedule around the animal's competitive career.
Hyaluronic acid (HA) naturally occurs, and is identical, in all animals. It is made by chondrocytes and the synovial lining of a joint, and being an unsulfonated glycosaminoglycan, it is a type of proteoglycan (chemicals that make up cartilage, the synovial lining, and joint fluid). HA helps to alleviate arthritis by reducing inflammation within the joint, allowing it to produce better synovial fluid, and may help to heal the joint by supporting the connective tissue. HA was originally injected into the joint of the horse, but one product is labeled for intravenous use. Before injecting intra-articularly, some veterinarians drain some of the excess joint fluid.
Although HA may be expensive, it is considered very safe to use and it has proven to be a very successful drug.
Brand names include Legend, Hyalovet
Polysulfated glycosaminoglycans (PSGAGs) may be injected either intra-muscularly or intra-articularly, although they should not be used within infected joints. PSGAGs are chemically similar to the main component of joint fluid and cartilage—glycosaminoglycans (GAGs)—and are also thought to reduce the inflammation in the joint, as well as protect the cartilage. This occurs because PSGAGs inhibit prostaglandin as well as the enzymes which destroy cartilage, and also have anti-inflammatory effects. In addition, polysulfated glycosaminoglycan restores the synovial fluid, which acts as a lubricant to enhance joint movement.
After IM injection, PSGAGs take 2 hours to move into all tissues and are at their highest levels 48 hours following IM injection, but last for 72 hours. Although they move into all tissues within the horse, they are found in highest concentrations in inflamed joints.
Corticosteroids (glucocorticoids) are naturally produced by the adrenal gland to control metabolism, as well as reduce inflammation. The latter effect is the main reason they are used in the joints: intra-articular injections of corticosteroids decrease the inflammation associated with arthritis, partially by reducing the production of prostaglandin. Additionally, it is thought that they may increase the production of glucose, although this is still debated by veterinarians.
Corticosteroids can have very bad side effects if given chronically. Additionally, injections within the joint will still "leak" corticosteroids into the surrounding tissues and circulate throughout the body. It is therefore best to perform radiographs of the intended joint beforehand, and have a qualified veterinarian assist in determining the proper dosages for the horse. Possible side effects include a decrease of immune response within the injected joint (which makes it more likely to infection), laminitis, and suppression of natural production of corticosteroids by the horse. Additionally, they should not be used in horses with Cushing's disease.
Corticosteroids are banned in certain circumstances from use by many equestrian organizations, partially because they can change the animal's temperament.
Many owners supplement their joint health program by feeding supplements containing various nutraceutical substances which are thought to benefit the joint, including oral glycosaminoglycans (glucosamine and chondroitin sulfate), methylsulfonylmethane (MSM), and, most recently, hyaluronic acid (HA). Effectiveness of these substances in oral form is hotly debated. On one side, a number of consumer-based tests suggest they can be effective. Many anecdotal reports testify to the effectiveness of various formulas, particularly those with high dosages of glucosamine. On the other side, organizations such as the American Association of Equine Practitioners (AAEP) point out that very few independent studies using a truly scientific model have been performed on oral joint supplements, and thus, the extent of their positive effects is debatable. Because nutraceuticals are not regulated by the U.S. Food and Drug Administration, there are also considerable tremendous variations in quality from one manufacturer to another. e
Omega 3 fatty acids, used in the past to improve hair coat shine, have recently become popular for use in horses as a way to reduce joint inflammation. They are added as oral supplements into the horse's diet, most commonly as ground flaxseed, or commercially-prepared products based on flaxseed and containing other ingredients, particularly rice bran. Studies are currently underway to determine effectiveness.
Non-steroidal anti-inflammatory drugs (NSAIDs) help to reduce inflammation by inhibiting prostaglandins and other chemicals, and also provide pain relief. However, they do not cure the underlying problem or speed the rate of recovery, and can therefore mask potential problems. Owners should not keep their horses on prolonged NSAID use without the permission of their vet, due to serious side-effects. It is therefore not to be used as a "daily supplement."
Phenylbutazone ("bute") is one of the most common NSAIDs given for joint pain.