APA 3311 - Bone Biomechanics Cartes

Trabecular bone

- interior, spongy bone with a porous, honeycomb structure

- "Cancellous" bone

- resistor of strain

- stores more energy

composition of bone

1) Inorganic material

- minerals (calcium phosphate)

- gives bone hardness, rigidity, and strength

- makes bon resistant to stress

2) Organic matrix

- mostly type 1 collagen

- makes bone flexible and resilient

- makes bone resistant to strain

3) water

- in both types of bone just in different proportions

Distribution of Hematopoietic tissue in Newborns

- Medullary canal/cavity of longbones

- Cancellous bone (aka spongybone

Distribution of Hematopoietic tissue in Adults

- Axial skeleton

- Heads of long bones (femur &humerus)

- Flat bones (cancellous layer)

- Some irregular bones

Bone marrow composition changes with age:

- Child: marrow is almost exclusively red

- Adult: gradual transition to yellow marrow

Osteoblasts

- cells that produce and coordinate mineralization of bone

- Synthesize and secrete collagen fibers and initiate calcification

Osteocytes

- mature osteoblasts surrounded in bone matrix

Osteoclastes

- cells that breakdown bone

- Concentrated in the endosteum

Bone Modification

- Change in use = change in bone density

- due to mechanical loading

- Different bone modifications depending on location

Ex Long bone

Epiphysis (more trabecular bone) - remodels faster

Diaphysis (more cortical bone) - remodels slower

Wolfe's Law

- Bones will adapt to the degree of mechanical loading.

- Increased loading leads to the strengthening of both internal spongy bone and the outer cortical layer.

Mechanical properties of bone

1) Ductile

- slow rupture

- resists changes in tension

- due to collagen

2) Fragile

- brutal breaks

- resists changes in compression

- produced by minerals

3) Viscoelastic

- Response depends on rate and duration of loading

- produced by water

Stress

intensity or magnitude of the load

Mechanical Functions

1) Support

2) Protection

3) Levers for locomotion

Physiological Functions

1) Mineral Homeostasis: storage ofminerals

2) Hematopoiesis: Formation of red bloodcells

Axial Skeleton

1) Bones is skull/head

2) Spine

3) Rib cage

Appendicular Skeleton

1) upper limb bones

2) lower limb bones

3) Shoulder complex

4) Pelvis

Compact Bone

- Hard, dense bone tissue

- "cortical" bone

- resistor of stress

Strain

deformation (change in dimension) that occurs in a structure in response to applied loads

Elastic Region

- the linear portion of any given stress/strain curve

- elasticity: tissue can return to its original form when load is no longer applied

Breaking Point

- Elastic Strain limit

- transition from elastic to plastic behaviour

- permanent damage/microdamage begins after this point

Plastic Region

- Permanent deformation

- damage is irreversible

- external fibres begin to give way (microtears)

Irreversible breaking point

- the bone ultimately fails/fracture

Load-Deformation Curve

- are under the curve = force of material

Modulus of Elasticity (Young's modulus)

- Measure of stiffness of an object

- defined as the amount of strain resulting from applying a given stress.

Hooke's Law

- states that within the elastic region, the amount a material deforms is proportional to the force applied

or

- the more force you apply, the more the material deforms—as long as you haven't passed the elastic limit

Anisotropic Features of Bone

- Behaviour of the bone depends on the orientation of the load being applied.

- Bone is a non-homogenous material

- Can accept more force longitudinally & less force perpendicularly.

- Skeletal system withstands manyforces of varying orientation,magnitude, etc

Anisotropy

- the structural property of non-uniformity in different directions, as opposed to isotropy.

- An anisotropic object or pattern has properties that differ according to measurement direction.

Compression Force

- forces that compress the ends of the bones in the direction of the length of the bone

- shortening and widening

- necessary for bone growth and deposition

Tension Force

- lengthening and narrowing

- often muscle is the source

- development of bone growths

Shear Forces

- on the surface

- angular deformation

- present during compression and tension loading

- ex: spondylolisthesis: vertebra slide anterior on one another

Flexion (bending) forces

- Material deforms convexly from tension and concavely from compression

- Two Types of Flexion Forces

1) 3 points of application

2) 4 points of application deformation

- Risk of injury on convex part of the form (i.e. 'boot top' fracture in skiers)

Torsion Forces

- Twisting about the longitudinal axis

- 2 opposing forces around the axis

- Material deforms convexly from tension and concavely from compression

- Angular deformation: especially on the periphery

- Common injury: spiral fractures

Deformation Rate

- The bone responds differently depending on:

- Loading rate

- Loading time

-Bone failure is dependent on the loading rate

-Better resistance to loads applied quickly

Loading Rates & Fractures

1) Lower loading rate:

- energy has time to dissipate = single fracture

2) Bigger loading rate:

- energy does not have time to dissipate = bone fragments

Bone Strength Limit

- Bone has a maximum load it can tolerate before injury/fracture

- Bone strength is affected by:

- Physical activity/conditioning

- Immobilization

- Skeletal maturity

- Fatigue

- Loading rate

History of Loading

- Repetition + load both determine injury risk

- As the number of repetitions increases, the amount of load needed to cause injury decreases

- Repeated loading can cause microtrauma

- If microtrauma occurs faster than the bone can repair itself → injury/stress fracture can develop

Effect of fatigue

- reduced shock absorption can change how forces are distributed through the body, potentially increasing stress on certain bones.

*** study figure from slide 49

Physical Activity & Bone Remodeling

- Daily stimulation is needed: bone requires mechanical stresses to grow and strengthen.

- Density is influenced by the number of cycles and the magnitude of loading

- Bone mass loss is ~1% per week during immobilization: Decrease in the mechanical properties of the bone

Effect of bone geometry on mechanical properties

Depends on:

1) Cross-sectional shape/area

2) Bone Length

1) Cross-sectional shape/area

- Bone geometry affects its moment of inertia (I) → resistance to bending.

- For tension and compression, resistance depends mainly on the cross-sectional area.

- For bending (flexion), resistance depends on:

1 - Amount of material

2 - How far that material is distributed from the neutral axis

- Material located farther from the neutral axis → greater moment of inertia → greater resistance to bending.

2) Bone Length

- Longer bones experience larger bending (flexion) moments.

- Bending moment depends on force × distance:[ M = Fd ]

- Therefore, increasing the bone's length increases the moment arm.

- Longer bone → greater bending moment → greater stress

Affect of Aging

- Mechanical properties change with age:

1) bone porosity increases

2) Bone density decreases

Relationship between bone mass, age and sex

- after 40 yrs, bone mass decreases

- bone mass in women is slightly les compared to men

Affect of Menopause

- bone mass loss accelerates

- Ultimate compressive force decreases by 15-20% between 20-39 yrs and 60-89 yrs

- Higher rate of fractures in women

- Bone fails more easily to weaker forces in women

Osteoporosis

- Bone resorption exceeds bone deposition

- Decreased bone density

- Decreased bone mass

- Loss of trabecular integrity (spongy bone)

- Loss of rigidity and lower structure

- highest fracture rates

Risk Factors for Osteoporosis

- Low body weight (BMI < 19)

- Low muscle mass

- Inadequate or excessive exercise

- Early menopause

- hormones (menstrual cycles)

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