41. Fracture biology_biomechanics Flashcards
T/Fbone is stronger when loaded rapidly than when loaded slowly
TRUEbone is stronger when loaded rapidly than when loaded slowly (viscoelastic)
bone mechanical property of anisotropy
mechanical properties of bone strongly depend on the direction of loadingfor example, long bones are stronger and better resist forces placed along the long axis than those across the axis.
stress vs strain
stress (y axis): force per unit area (N/meter ^2)strain (x axis): local deformation (units of length/length) or the change in unit length over original length (usually a percentage)
strength vs stiffness of a material
strength: ultimate load a material can withstand before ultimate failurestiffness: RATE at which material deforms when the load is applied; slope of the ascending linear portion of the stress/strain curve; aka elastic modulus
stress/strain curve vs load/displacement curve
stress/strain curve—-materialsload/displacement—-structures/fractures
yield point
Ypoint at which strain exceeds the materials ability to recoverrender the material permanently deformed
plastic vs elastic deformation
elastic deformation–reversible deformation before yield point is reached, material can recover to original formplastic deformation–irreversible deformation past the yield point where strain exceeds the materials ability to recover
failure point
Umaterial can no longer withstand strain and fails
cortical bone material properties (3)
anisotropic–stronger when loaded longitudinallybrittle–plastic deformation phase is steep/short viscoelastic –more rapid it is loaded, the stiffer it becomespizoelastic–as bone deforms it produces an electrical charge; compression is NEG (osteoblasts), tension is POS (osteoclasts)
cancellous bone material properties (
porosity–75-95% make it weaker and more compliantless stiff and lower yield point than cortical boneshort elastic phase, long plastic phase (from collapse of trabeculae)
what happens to stress when the surface area is decreased by one half
stress = force/meters squared (N/m^2)when area is decreased by 1/2, stress doubles
how is toughness of a material defined
AREA under the stress vs strain curvearea under the curve represent the total energy absorbed during the loading process
forces acting on bone
axial–parallel to long axis–made up of tensile (lengthen) and compressive (shorten) forcessheartorsional bending (4 types)
4 types of bending (moments) and the anticipated fracture site associated with them
- pure bending: apply equal and opposite forces at each end of the bone (uniform bending); fracture can occur anywhere; RARE2. cantilever bending: fixed on one end, and load applied transversely on other end; fracture occurs near site of fixation3. 3 point bending: equal load on both ends and opposite load applied somewhere in the middle; fracture occurs at the site of load application4. 4 point bending: equal load on both ends and 2 opposite loads applied in the middle; fracture occurs btwn two inner loads; RARE
compressive forces on bone result in what type of fracture configuration
OBLIQUE–due to bone being dramatically weaker in shear than compressionwhen loaded in compression, bone fails along the lines of highest shear (usually at 30-45 degrees the to direction of compressive force
tensile forces on the bone result in what type of fracture configuration
TRANSVERSE or perpendicular to the direction of tensile forcerarely occurs as a sole entity
bending forces on bone result in what type of fracture configuration
TRANSVERSE during bending, the side opposite the load (convex) is under tension and under the load (concave) is compressionbc bone is weaker in tension, fracture starts on convex or tension surface
bending and compression forces on the bone result in what type of fracture configuration
COMBINATIONmay start transverse from bending force on tensile side and propagate , split and deviate creating a butterfly fragment
torsional forces on the bone result in what type of fracture configuration
SPIRAL
causes of pathologic fractures
- neoplasia2. osteopenia from immobilization3. osteomyelitis4. iatrogenic bone damage (ie. THR)5. post op implant removalLOW energy fractures–> simple fracture patternsusually at site (stress riser) btwn healthy and dz bone
fatigue fractures
repetitive loading causes bone damage at a rate faster than it can be repaireddecreases tensile strength of bone more rapidly than compressive strengthRARE (maybe FCP, avascular necrosis)
fracture gap size and strain associated
small fracture gap HIGH STRAINlarge fracture gap LOW STRAINfor the same amount of instability present
strain necessary for bone formation
LOW strain environment needed for bone formations< 2 % strain
bone healing mechanisms in place to DECREASE strain and aid in bone formation/healing
- fracture resorption (increasing the denominator)2. callus formation (decreasing the numerator)strain = change in length/original length
types of bone healing
- primary—contact vs gap2. secondary: inflammatory (hematoma), reparative (angiogenesis; GT), remodeling (Wolffs law; piezoelectric properties)
stages of secondary bone healing
- inflammatory (hematoma–no stability)2. reparative (angiogenesis; GT–some stability, tolerates high strain to make fibrous connective tissue soft callus) –interfragmentary strain decreases to form fibrous hard callusfibrous hard callus–>cartilage–>(endochondral ossification) mineralization–>woven bone3. remodeling (Wolffs law; piezoelectric takes –>woven bone–>lamellar bone/Haversian system cortical bone under low strain environment)
what are the two physical requirements for primary bone healing to take place
- interfragmentary strain < 2%2. < 1mm gap
difference btwn contact and gap repair for primary bone healing
Both do not form callus; Both are PRIMARY healing; need stable fracture conditionsBUT contact–direct Haversion remodeling occurs (cutting cones, longitudinal lamellar bone); bone union and remodeling occur in ONE phasegap–two stages: layers of bone laid perpendicular and THEN replaced by longitudinal lamellar bone through Haversian remodeling; bone union and remodeling take place in two stages
concept of distraction osteogenesis
Gavril Ilizarovmechanically induced bone formation via intramembranous ossification1. within gap is fibrous inter zone (radiolucent, central)2. either side of zone = osteoblasts, vascular that deposit osteoid3. with distraction the fibrous inter zone mineralizes
T/FChondroblasts are formed under LOW oxygen tension
TRUEinitial low oxygen tension is a signal for formation of capillary buds and chondroblasts
tension side of femur vs tibia
femur= craniolateraltibia=craniolateral
tension side of humerus
humerus = craniolateral
tension side of radius and ulna
radius= cranialulna = caudal
granulation tissue can withstand ___________% deformation before failure
100% strain
fibrous tissue can withstand ___________% deformation before failure
10-15 % strain
bone can withstand ___________% deformation before failure
2% strain
