Chapter 3: Tissue Mechanics

what are the components of bone?

rigid mineral component, flexible collagen component

how can the strength of a bone be evaluated?

by evaluating the relationship between the load (external force) and the amount of deformation (internal reaction)

load deformation curve

what is the measure of the ultimate strength of bone?

the deformation and load at failure, energy absorption

what is the anisotropic behavior of bone?

the behavior depends on the load imposed

bone is strongest when loaded longitudinally and weakest when loaded across a surface

strongest because bone is habitually loaded in the longitudinal direction

what is the viscoelastic behavior of bone?

bone responds differently depending on the rate of loading

at higher rates of loading, bone withstands greater load

at lower rates of loading, bone fractures at about ½ the load (ex: pushing straw into juice box)

what is the elastic response of bone?

when load is applied, bone will change shape

what is the elastic region of bone?

the amount the bone will change shape and return to it’s original shape or length when loaded or unloaded

what is the yield point of bone?

the point at which microtears and debonding of bone occur

what is the plastic region of bone?

bone begins to permanently deform and will eventually fracture if loading continues

what is the stiffness of bone?

the slope of the curve during the elastic phase

load/deformation

represents the resistance to loads as structure deforms

stiff materials will deform little with increased loads

less stiff materials will deform and elongate significantly before failure

what is compliance of bone?

deformation/load

how much deformation occurs per unit of load

usually refers to soft tissue behavior

what is stress for bone?

the load normalized to tissue cross sectional area

internal force divided by cross sectional area

force/area

what is the strain on a bone?

the length normalized for tissue length (percentage)

change in length as a function of normal length (how much deformation vs. original tissue shape)

deformation of the material

what is the safety factor of bone?

the absolute load it takes to fail

bone as a safety factor of between 2 and 5 - this means that bone will fail at between 2 and 5 times the forces it normally encounters

what is Wolffe’s Law?

bone strength increases and decreases as the functional forces on the bone increase and decrease

what are characteristics of cortical bone?

it is stiffer

withstands more stress but fractures shorter length

has 2% strain

what are characteristics of cancellous bone?

it fractures at 75% strain, much softer

what are compression forces?

bone ends are pressed together, bone becomes shorter and wider

oblique cracking of osteons

common in vertebrae (most common in cervical spine)

what are tension forces?

bone ends are pulled apart, bone tends to lengthen and narrow

source of tension is usually muscle

debonding of osteons occurs

typically failure occurs first in cancellous bone (tibial tuberosity and 5th metatarsal - at the styloid process where the fibularis brevis attaches - Jones fracture)

what is shear force?

force applied in parallel to the surface

produces internal angular deformation

always present with compressive and tensile loads

fractures are most common in cancellous bone

what is bending force?

compression and tension on opposite sides

concave side has compression, convex side has tension

forces increase with greater distance from the axis (greater torque applied)

bone usually fails on the tensile side because it can withstand greater compressive forces

what are point bending fracture?

three point fractures at the middle force and the fracture starts on the tension side

in four point fractures, the magnitude of bending is equal throughout the bone and it fractures at the weakest point

what are torsion forces?

twisting force that causes shear stress throughout the bone

increases in magnitude as distance from axis increases

occur parallel and perpendicular to axis

tension/compressive forces occur in a plane diagonal to the axis

results in spiral fractures (ex: ringing out a towel)

inert failure is a result of shear (produces crack parallel to the axis)

second failure is a result of tension (produces oblique crack)

what is the geometry of a spiral fracture?

torsion forces are proportional to the distance from the axis (moment arm for rotation increases)

bones with a larger diameter are more resistance to torsional loads

bones will tend to fracture at narrower points because the diameter is smaller

what role do muscles play in stress on bones?

muscles can aid in decreasing the stress on the bone (ex: skiing)

the gastrocnemius decreases tensile forces on the concave side of the bone

how is failure due to loading dependent on rate of loading?

faster rates of loading produce higher failure loads, allow more energy storage, and do not affect deformation at failure

what are fatigue fracture?

stress fractures due to repetitive loading over time

higher loads require fewer repetitions and are also frequency dependent

what is the structural difference between tendons and ligaments?

the collagen arrangement

what is the structure of tendons?

they are arranged in parallel, very stiff, least deformation vs. tensile forces, and most susceptible to compression and shear

what is the structure of ligaments?

collagen arranged in near parallel, resistance to tensile forces

act as a joint stabilizer, provides multidirectional stability and accommodates shear and compressive forces well

what is creep?

the length of tissue increases when force (load) is held constant

this is the principle when we try to stretch tissue

increases flexibility

what is load relaxation?

when length of tissue is held constant, force declines

the amount of load tissue can take before failing does not change

the length/stretch of tissue will change because of creep, takes less load to maintain this stretch

what is an injury to a muscle called?

a strain

what are the 2 ways to strain a muscle?

passive tension strain and active tension strain

what is a passive tension strain?

failure occurs at the musculotendinous junction

leaves the muscle attached at the tendon

what is an active tension strain?

at the MT junction

mechanical strain at failure is similar for tetanus, submaximal activation, and no contraction

stress at failure is only 15% greater for active muscle

100% increase in energy absorption

both the active and passive elements absorb energy

with fatigue or weakness, muscle is more susceptible to injury (cannot contract as hard and absorb as much energy)

most non-contact soft tissue injuries occur around 2/3-3/4 through the activity as muscles fatigue and central drive drops

what is the healing process for partial tears?

inflammation starts in 1-2 days, and by the 7th day fibrous tissue replaces inflammation

strength production is

70% immediately after injury

50% within 2 hours

90% by the 7th day

tensile strength at day 7 is 77% of normal, but strength is 90% of normal

therefore, active muscle contraction produces more tension than tissue is capable of handling

the muscle is susceptible to reinjury

how can partial tears be prevented?

cyclic stretch produces increased length to failure with no change in load to failure or energy storage

a warm up may increase strain at failure and may increase force production (i.e. energy storage)

what is the effect of fatigue on muscle?

decreases force (stress) at failure

no change in strain (change in shape) at failure

decreased energy absorption