Skeletal system of the horse

  The skeletal system 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.

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Functions of 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

1. Long Bones: aid in locomotion, store minerals, and act as levers. They are found mainly in the limbs.
2. Short Bones: Absorb concussion. Found in joints such as the knee, hock, and fetlock.
3. Flat Bones: Enclose body cavities containing organs. The ribs are examples of flat bones.
4. Irregular Bones: Protect the central nervous system. The vertebral column consists of irregular bones.
5. Sesamoids: Bones embedded within a tendon. The horse's proximal digital sesamoids are simply called the "sesamoid bones" by horsemen, his distal digital sesamoid is referred to as the navicular bone.

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

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:

• Nuchal and supraspinous ligaments: the nuchal ligament attaches to the dorsal surface of the cervical vertebrae. Its dorsal section extends from the occipital protuberance of the skull (the poll) to the withers, then narrows to become the supraspinous ligament. It also connects the 2-7th cervical vertebrae to the 1-3rd thoracic vertebrae. Its main purpose is to support the head and allow it to be moved upward or downward.
• Intercapital ligaments: lie between the first through eleventh ribs. Help to prevent thoracic disk herniation.

Ligaments of the legs include:

• Suspensory ligament: runs from the back of the cannon bone (between the two spint bones), then splits into two branches and attaches to the sesamoid bones at the bottom of the fetlock. Branches continue downward and attach to the extensor tendons. The main purpose of the suspensory is to support the fetlock joint, preventing it from overextending. Injury to this ligament is an important cause of lameness in performance horses. The suspensory is a modified muscle, the equine equivalent of the interosseous muscle, which contains both tendon fibers and residual muscle fibers.^[1].
• Interosseous ligaments: connect the cannon bone to each splint bone. Injury to this ligament produces the condition known as "splints".
• Proximal and distal check ligaments: The proximal check ligament originates from the radius and attaches to the superficial digital flexor tendon. The distal check originates from the palmar carpal ligament and attaches to the deep digital flexor tendon, approximately 2/3-way down the metacarpus.
• Plantar ligament: in the hind leg, runs down the lateral side of the tarsus, attaches to the fibular, 4th tarsal, and 3rd metatarsal bones. Injury leads to a condition known as "curb".
• Inter-sesamoidean ligaments: supporting ligaments, run between the two sesamoid bones.
• Distal sesamoidean ligaments: run from the sesamoid bones to the two pastern bones. Important in the stay apparatus.
• Impar ligament: runs between the navicular bone and the 3rd phalanx.
• Annular ligament: goes around the back of the fetlock, surrounding the flexor tendons and their tendon sheath, attaching to the sesamoid bones. It helps to support the fetlock, and provides an enclosed "pulley" for the flexor tendons to run through.
• Sacrosciatic ligament: Originates from the sacrum and coccygeal vertebrae, inserts into the pelvis.

Axial skeleton

See also: Back (horse) and Poll (livestock)

The axial skeleton contain the skull, vertebral column, sternum, and ribs. The sternum consists of multiple sternebrae, which fuse to form one bone, attached to the 10 "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, ^[2] 5-6 lumbar, 5 sacral (which fuse together to form the sacrum), and 15-25 coccygeal^[2] 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 coccygeal 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. ^[3]

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 seven major bones

1. Incisive bone (premaxillary): part of the upper jaw; where the incisors attach
2. Nasal bone: covers the nasal cavity
3. Maxillary bone: the upper portion of the jaw
4. Mandible: lower portion of the jaw; largest bone in the skull
5. Orbit: bone located around the eye
6. Frontal bone: creates the forehead of the horse
7. Temporal fossa: found behind the ears near the vertebral column

Appendicular Skeleton

See also: Equine forelimb anatomy

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.

Important bones and joints of the forelimb

• Scapula (shoulder blade): flat bone with a large area of cartilage that partially forms the withers. The shoulder length and angle is very important to horsemen when evaluating conformation.
• Humerus: lies between the scapula and the radius, making an angle of about 55 degrees down and back.
• Radius: extends from the elbow, where it articulates with the humerus, and travels downward to the carpus. It forms the "forearm" of the horse along with the ulna.
• Ulna: caudal to the radius, it is usually partially fused to that bone in an adult horse.
• Shoulder joint (scapulohumeral joint): usually has an angle of 120-130 degrees when the horse is standing, which can extended to 145 degrees, and flexed to 80 degrees (such as when the horse is jumping and obstacle).
• Elbow joint (humeroradial joint): hinge joint that can flex 55-60 degrees.
• Carpus (knee): consists of 7-8 bones placed in 2 rows to form 3 joints. The 1st carpal bone is present only 50% of the time.

Important bones and joints of the hindlimb

• Pelvis: made up of the os coxae, the largest of the flat bones in a horse. It is made up of the ilium, the ischium, and the pubis, which create a cavity called the acetabulum. The pelvic cavity is larger in diameter in the mare than in the stallion, providing more room for the foal during birth.
• Femur: the largest long bone in a horse. Proximally it forms a ball-and-socket joint with the pelvis to form the hip joint, and distally it meets the tibia and patella at the stifle joint. It serves as an attachment point for the deep and middle glueteal muscles, and the accessory and round ligaments.
• Patella
• Tibia: runs from stifle to hock. The proximal end provides attachment for the middle patellar ligaments, the dustal end provides attachment for the collateral ligaments of the hock.
• Fibula: completely fused to the tibia in most horses.
• Hip joint : Ball-and-socket joint made up of the acetabulum of the pelvis and the femur. It is very stable.
• Stifle joint (femoropatellar joint): actually composed of three joints, it corresponds to the 3 joints are the femoropatellar, the medial femorotibial, and lateral femorotibial, which are stabilized by a network of ligaments. The stifle has an articular angle of about 150 degrees.
• Tarsus (hock): consists of 6 bones (of which one is made up of the fused 1st and 2nd tarsal bones) aligned in 3 rows. The largest bone in the hock, the calcaneus or fibular tarsal bone, corresponds to the human heel, and creates the tuber calcis (point of hock). It is to this point that the tendon of the gastrocnemius, portions of the biceps femoris, and portions of the superficial digital flexor attach.

Bones of the lower limb

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.

Joint care

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 substance 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/Sodium hyaluronate

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 allieviate 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 it may now be given intravenously. Before injecting intra-articularily, 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®, Hylovet®

Polysulfated glycosaminoglycans

Polysulfated glycosaminoglycans (PSGAGs) may be injected either intra-muscular or intra-articular, although it should not be used on 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 inflammed joints.

Examples of PSGAGs include Adequan® and Chondroprotec®.

Corticosteroids

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 from use by most equestrian organizations, partially because they can change the animal's temperament.

Nutraceuticals and oral joint supplements

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. One 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 FDA, there are also considerable tremendous variations in quality from one manufacturer to another.

Omega 3 fatty acids

Omega 3 fatty acids have recently become popular for use in horses, as a way to reduce joint inflammation. The are added as oral supplements into the horse's diet. Studies are currently underway.

NSAIDs

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.

Selected Problems associated with the skeletal system

• Fractures
• Arthritis and Degenerative joint disease (DJD): including bone spavin, ringbone, osselets, omarthritis
• Degenerative suspensory ligament desmitis (DSLD), and sprains of the suspensory ligament
• Navicular disease
• Carpitis (sprained knee)
• Locked kneecap (delayed patellar release or intermittent upward fixation of the patella)
• Curb (sprain of the plantar ligament)
• Splints
• Bucked shins
• Sesamoiditis
• Osteochondrosis

References

1. ^ The suspensory ligament
2. ^ ^{a b} King, Christine, BVSc, MACVSc, and Mansmann, Richard, VMD, PhD. "Equine Lameness." Equine Research, Inc. 1997.
3. ^ Riegal, Ronald J. DVM, and Susan E. Hakola RN. Illustrated Atlas of Clinical Equine Anatomy and Common Disorders of the Horse Vol. II. Equistar Publication, Limited. Marysville, OH. Copyright 2000.

• Forney, Barbara C, MS, VMD.Equine Medications, Revised Edition. Blood Horse Publications. Lexington, KY. Copyright 2007.