Skeletal System

• most common cause for horses to fail to achieve full athletic potential

– damage to bones, ligament or tendons

• structural failure

– mismatch between load applied and strength of the structures

• functions

– mechanical levers for muscles

– supporting framework

– protection for vital organs

– blood formation

– mineral homeostasis

Types of Bones

• long bones

– levers, support of weight, and locomotion

• short

– absorb concussion (joints)

• flat

– enclose cavities with vital organs

• irregular

– spinal column (protect CNS)

• epiphysis - either extremity of bone

– proximal or distal

• metaphysis - separates epiphysis and diaphysis

– epiphyseal cartilage (growth)

– articular cartilage - cartilage covering articular surface

• diaphysis - shaft of a long bone

– periosteum - fibrous membrane that covers surface of bone, except at articular cartilage

• functions– increase diameter of bone

– healing of fractures - splints, spavins, ringbone

– endosteum - fibrous membrane that lines the marrow cavity

• bone growth - addition/deletion of tissue at one of the existing surfaces

• bone changes itself, conforming to stress

– increase or decrease in mass

• osteoblasts - cells form new bone

– new bone produced in weakest areas

– can generate rather than repair and scar

• osteoclasts - resorption of bone cells

• “remodeling” - active process

– abundant blood vessel supply nutrients, oxygen and mineralization

• “modeling” - new growth

• storage reservoir for calcium and phosphorous

– physical stress on bone

– hormonal influences

– nutrition

• bone strength indicators

• mature - growth plate closure

– cross sectional area

– bone mineral content

• immature bone - minerals deposits approx. 65% of space

• mature bone - mineral 95% of bone

• cannon measurement

– 7 inches for 1000 lbs

• nutritional manipulation or exercise regime cannot hasten growth or maturity

– genetics and breed determine skeletal maturity, time

– conditioning improves bone strength

• mechanical forces stimulate change in weight-bearing bones

• exercise

– increases bone strength

– increases mineral deposition

– increases bone mass

– quantity will increase with exercise not necessarily the quality

• mineralization

– rate of mechanical force applied influences degree of development

– consistent running with gradual increase in distance will increase bone mineral content

– occasional exercise has little effect on BMC

• Training

– threshold time of 20-30 minutes to stimulate bone as much as lengthy exercise

– warm-up 10-15 minutes

– gradual incremental exercise

• increase ligament, tendons and muscles

– interval training - build but don’t overload

• Inactivity

– demineralization - skeleton weightless

– cast of disuse for extended period of time

– excessive use of corticosteroids or stimulous of natural corticosteroids

Forces that Stress Bone

• axial compression (squeezing together directly down the bone)

• axial tension (pulling apart)

• torsion (twisting)

• mineral content - compressive strength

• collagen (connective tissue of bone) - tensile strength (stretch)

• torsion - 1/3 of compressive strength

– abnormal conformation

– angular limb deformities

– rutted ground

– inconsistent composition of the ground surface

– lack of uniformity in cushioning of ground

– caulks or toe grabs

Maintenance of bone tissue.1) Pressure will stimulate appositional bone growth.2) Increased weight-bearing will increase bone thickness and density of shaft.

Training EffectsIncreases bone mass and alters the distribution of the mass, so as to provide more bone where high strains occur.

Biochemical Stress

Stress - force per unit areaStrain - degree to which a bone deforms to the stress

Forces that Stress Bones

Compression and tensionTorsion (twisting)

• shape of bone will vary

– along shaft

– cortical wall thickness

• pull of tendons, ligaments and muscles

• effect of weight bearing and support of bodymass

• force created from impact of ground

Representative example of increasing bone strength with age and work.

Decrease in bone strength with sore shins. Decreased again after the animals were put back to work.

• 3 months - 1 year

– increase in compressive strength due to increased BMC

• stabilizes at this level until 3 years

• compressive strength improves again until 7 years

• bone failure in young

– don’t want excessive exercise

• 70% 3 year old race horses develop bucked shins

• eventual overload - stress fracture

Increased Risk for Bone Failure

• weight-bearing stress

• longing- even in big circles

– twisting bones and immature joints

• jumping young

– radically overloads limbs

• long distance work - esp at high speeds

Early Training

• do not pen or stall foals

• pony foals

• 1-3 line driving, driving in light cart

• 3-4 weight-bearing, big circles, increase duration and distance, speed training after foundation is set