· elastic portion is slightly curved
· indicates some yielding during loading
· cortical bone is stiffer
· withstands more stress but
· cortical bone can withstand less strain
· cortical bone fractures after 2% strain
· cancellous bone fractures after 75% strain
· cancellous bone has large capacity for energy storage
· bone demonstrates anisotropy
· strongest and stiffest in directions in which loads most commonly imposed (ie longitudinally)
· direction of failure is transverse
· mechanism of failure is debonding at cement lines and pulling out of osteons
· tension fractures usually occur in cancellous bone with muscle attachments
· eg. olecranon, calcaneum
· direction of failure is oblique
· mechanism of failure is oblique cracking of osteons
· most commonly seen in vertebral bodies
· causes internal angular deformation
· most commonly seen in cancellous bone
· eg. tibial plateau
· direction of failure is
· transverse on tension side
· oblique on compression side
· on compression side may produce
· buckle fracture
· ‘butterfly fragment’
· bone first fails in shear
· initial crack parallel to neutral axis
· then fails in tension
· propagation at diagonal to neutral axis
· usually gives spiral fracture
· most fractures due to combination of loads
· gives variation in fracture patterns
· contraction of muscles alters stress distribution
· decreases or eliminates tensile stress by producing compressive stress
· contraction may increase compressive stress on compression side
· bone can withstand compression better than tension
· muscle contraction effectively increases stress that bone can sustain
· behaviour varies with rate of loading
· when loaded quickly, bone is
· stiffer (ie. less deformation)
· stronger (ie. sustains higher load to failure)
· when bone fractures, stored energy released
· at low loading rate, energy dissipated through single crack
· at high loading rate, greater energy leads to comminution
· fatigue related to load and number of repetitions
· bone does not have asymptotic curve
· ie. bone will fail with enough repetitions
· fatigue also related to frequency of repetitions
· failure occurs when remodelling outpaced by fatigue
· failure usually occurs with continued strenuous physical activity
· muscels fatigued
· less able to contract and counteract stresses imposed on bone
· less able to store energy and neutralise stresses imposed on bone
· may lead to stress fracture
· stress fractures on compressive side are oblique
· stress fractures on tensile side are transverse
· bones have high resistance to bending and torsion because bone tissue distributed at distance from neutral axis
Age
· with age, bone density decreases
· partly counteracted by increasing moment of inertia
· endosteal resorption outstrips periosteal resorption
· cortical diameter increased
· distance from neutral axis increased
Fracture
· with fracture healing, resistance to bending and torsion maximised
· external callus increases diameter of bone
· thus increases moment of inertia
· surgical bone defects weaken bone
Stress raiser
· defect whose length is less than diameter of bone
· produced by screw, drill hole or window
· stress becomes concentrated around defect
· weakening maximal to torsion
· initial weakening is 60%
· disappears after 8 weeks due to remodelling
Open section defect
· defect whose length is greater than diameter of bone
· produced by biopsy or window
· stress unable to be distributed around periphery
· stress changes direction when encounters defect
· weakening maximal to torsion
· can be up to 90%
· bone can remodel to meet mechanical demands placed on it
· summarised as Wolff’s law
· bone laid down where needed and resorbed where not needed
Immobilisation
· immobilisation leads to bone resorption
· leads to decreased strength and stiffness
Implant
· plate and screws shares load with bone
· leads to bone resorption under plate
· resorption causes
· decreased bone diameter
· osteoporosis
· hypertrophy may occur at attachment sites
· due to increased load at these sites