Week 7 Tissue: Mechanics of Bone

diaphysis

long shaft of bone

epiphysis

ends of bone

epiphyseal line/plate

growth plate, seals in females in teens, for males in mid 20s, a weak spot

metaphysis

between the epiphysis and diaphysis

articular cartilage

covers the epiphysis on ends of the bone

periosteum 

sheath-like bone covering attached by Sharpey’s fibers and contains nociceptors (pain)

medullary cavity

hollow chamber in bone, red marrow produces blood cells, yellow marrow is adipose

endosteum

thin layer lining the medullary cavity, important for blood transfer back and forth

cortical bone

aka compact, hard and dense, for protection adn stretgth, withstand force, example is mid femoral shaft

cancellous bone 

spongy and trabecular, less dense with series of holes, femoral head is example, absorb force from different locations in femoral head

bone

specialized connective tissue, features osteon (haversian system) that secretes collagen and mineralized ground substance in concentric spirals, good blood supply that enables repair and remodeling

osteon

aka haversian system, subunit of bone that secretes collagen and mineralized ground substance in concentric spirals known as lamellae

blood supply of bones

nutrient arteries, periosteal arteries, epiphysial arteries, and metaphysial arteries

nerve supply of bones

run with the arteries and innervate bone, peristeal nerve is superficial on the periosteum, vaso-motor nerves go into the bones, not sensory

wolfe’s law (mechanotransduction)

physical characteristics of bone are based on what we do with them! bone is laid down in areas of high stress and reabsorbed in areas of low stress, mechanical properties vary and be based on function and structure, shows us the importance of loading the bone after injury to heal correctly (rehab implication)

structures of bones

hollow tube to resist compression, flat bone to serve at attachment point or protection

force distribution on the bone

can vary with the lines of force that act upon it, ie can be stiff in one direction and more flexible in others, cortical bone haversian systems align along lines of force, and similarly trabeculae align along lines of force

ultimate stress that cortical bone can withstand

• compressive stress: 200 MPa

• Tensile stress 125 MPa

• Shear stress 50-75 MPa

ultimate stress that trabecular bone can withstand

• compression 2.6 MPa

• Tension 2.4 MPa

force bone is better at resisting

better at resisting compression than tension (and it’s brittle)

interpretation of the stress strain curves (young’s modulus is the slope)

the steeper the line, the stiffer the tissue, flatter line means more elastic

fracture toughness 

measure of a bone’s ability to resist crack growth if a crack has been initiatied, rang efrom 3.3-6.4 MPa/m², low value indicated boen is brittle (like ceramics)

ductile

what a high fracture toughness would tell us about th ematerial, resists fracture well

bone will fail before metal

implications for metal implants in bone since metal is much more ductile

bone strain rate

stiffness increases with increase speed of loading, allows body to be protective against trauma, falls, jumps, etc

bone changes with activity and inactivity 

• increasing loading/activity- denser bone 

• decrease loading/activity- thinner bone, less remodeling 

activities for increasing bone density (least to most)

• swim, cycle

• brisk walk

• run, jog

• jump, strength train

bone changes with age

• decreased stiffness, fracture toughness, ultimate strength

• due to changes in mineral composition, inactivity, and other things maybe

• can lead to osteopenia and eventually osteoporosis

factors contributing to osteopenia 

activity, level, hormonal changes, diet

osteopenia

precursor to osteoporosis, starting to lose bone density, can avoid progression if make the right changes

factors that bone fractures are classified by

• position of the bone ends after fracture

• completeness of the break

• the orientation of the bone to the long axis

• whether or not the bone ends penetrate the skin

types of bone fractures 

transverse, linear, oblique, spiral, greenstick, comminuted, avulsion, impacted, open/compound

bone healing requirements

• viable fragments with intact blood supply

• mechanical rest (external mobilization like cast)

• absence of infection

factors that promote recovery

• stop smoking, don’t drink too much

• balanced diet with sufficient calcium

• adhere to activity restrictions

• maintain strength and ROM of other joints and body regions

• maintain fitness level with modified activity

bone healing- inflammatory phase 

• first few days to week 

• local cell death due to ischemia 

• local swelling and warmth 

• inflammatory cells invade and release lysosomal enxymes 

• osteoblastic/osteoclastic activity is stimulated 

• hematoma develops 

bone healing- early reparative phase

• 2-6 weeks

• resorption of necrotic bone

• characterized by differentiation of cells

• fracture hematoma (invaded by chondroblasts and fibroblasts which lay down the matrix for the callus)

• early soft callus composed of mainly fibrous tissue and cartilage with a very small amount of bone

• increased vascularity

bone healing- Late reparative phase

• 8-12 weeks

• osteoblasts minerlize the soft callus and from a hard callus of woven bone, increases stability

• still immature and weak, bridging of the fracture

• clinical and radiological healing during this phase

• phase is complete when fracture is stable

bone healing- remodeling phase

• months to years

• bone is removed in tiny increments and then replaced by new bone

• osteoblastic and osteoclastic activity

• immature bone replaces bu mature bone

• stability increases

• adult skeleton continuously replaces itself at rate of 10- 18% per year (accelerated during fracture repair)

• responds to loading characteristics according to wolfe’s law

factors that affect bone healing

• severity of fracture

• location of fracture

• type of bone involved

• soft tissue damage

• type of fixation

• extent of overall trauma

• age

• co-morbitities

• etc

general timeline of bone healing for children

4-6 weeks (though depends on location and other factors)

general timeline of bone healing for adolescent

6-8 weeks (though depends on location and other factors)

general timeline of bone healing for adults

10-18 weeks (though depends on location and other factors)

bone implications for practice

goal during immobilization-

• preserve function with ADs, slings, modifiications of activities, compensatory mechanisms for function

• maintain strength and ROM of surrounding joints with home exercise program

• goal after cast removal is to return to previous function

• address factors that may have predisposed to injury

things to do after cast removal

• strengthening and ROM

• scar mobilization

• functional training

• education of timeline and return to prior functional level

• home exercise program

percentage decrease of bone measures per 10 years after about 30 yo