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What do you think when you hear the word biomechanics?. What are some subdisciplines of bionechanics?. Advanced Biomechanics of Physical Activity (KIN 831). Lecture 1 Biomechanics of Bone. Single Joint System*. Dr. Eugene W. Brown Department of Kinesiology Michigan State University
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Advanced Biomechanics of Physical Activity (KIN 831) Lecture 1 Biomechanics of Bone
Single Joint System* Dr. Eugene W. Brown Department of Kinesiology Michigan State University * Material included in this presentation is derived primarily from two sources: Enoka, R. M. (1994). Neuromechanical basis of kinesiology. (2nd ed.). Champaign, Il: Human Kinetics. Nordin, M. & Frankel, V. H. (1989). Basic Biomechanics of the Musculoskeletal System. (2nd ed.). Philadelphia: Lea & Febiger.
Components of a Single Joint System • Rigid Link (Bone, Tendon, Ligament) • Joint • Muscle • Neuron • Sensory Receptor
Some Purposes of Bone • Provides mechanical support • Produces red blood cells • Protects internal organs • Provides rigid mechanical links and muscle attachment sites • Facilitates muscle action and body movement • Serves as active ion reservoir for calcium and phosphorus
Wolff’s Law “Every change in the form and function of a bone or of their function alone is followed by certain definitive secondary alteration in their external conformation, in accordance with mathematical laws”.
Composition and Structure of Bone • Consists of cells and an organic extracellular matrix of fibers and ground substance • High content of inorganic materials (mineral salts combined with organic matrix) • Organic component flexible and resiliant • Inorganic component hard and rigid • Mineral portion of bone primarily calcium and phosphate (minerals 65-70% of dry weight) • Bone is reservoir for essential minerals (e.g., calcium)
Composition and Structure of Bone • Collagen • Mineral salts embedded in variously oriented protein collagen (strength in various directions) in extracellular matrix • Tough and pliable, resists stretching • 95% of extracellular matrix (25-30%) of dry weight of bone
Schematic illustration of section of the shaft of long bone without inner marrow Concentric layers of mineralized matrix that surround a central canal containing blood vessels and nerves
Haversian canal – small canal at center of each osteon containing blood vessels and nerve cells • Lamellae - concentric layers of mineralized matrix surrounding haversian canal • Lacunae – small cavities at boundaries of each lamella containing one bone cell or osteocyte • Canaliculi – small channels that radiate from lacuna connecting lacunae of adjacent lamellae and reaching havesrian canal
Cement line • -limit of canaliculi • -collagen fibers in bone matrix do not cross cement line • -weakest portion of bone’s microstructure
Microscopic-macroscopic structure of bone. Data form Rho et al., 1998.
Two Types of Bone • compact (or cortical) bone – outer shell, dense structure, surrounds cancellous bone • Cancellous (or trabicular) bone • Does not contain haversion canals • contains red bone marrow in spaces -------------------------------------------------------- • Biomechanical properties are similar; differ in porosity and density (see figure) • Quantity of compact and cancellous tissue in bone differs by function
Periosteum • Dense fibrous membrane that surrounds bone; outer layer permeated by blood vessels and nerve fibers that pass into cortex via Volkmann’s canals • Inner osteogenic layer contains osteocytes (generate new bone) and osteoblasts (bone repair)
Endosteum • Lines medullary cavityof long bones, filled with yellow fatty marrow • Contains osteoblasts and osteoclasts (resorption of bone)
Biphasic Behavior of Bone • Minerals hard and rigid • Collagen and ground substance resilient -------------------------------------------------------- Combination stronger than either alone
Load Deformation Curve • B – max. load before deformation • D’ – deformation before structural change • Area under curve is force x distance = work= energy
Load Deformation Curve • Slope of elastic region defines stiffness • Area under curve defines energy that can be stored • Elastic region – return to original configuration once load is removed • Plastic region – deformation of material • Load deformation curve is usefull when determining comparative characteristics of whole structures (e.g., bone, tendon, cartilage, ligaments)
What is the function of normalization? • Independent of geometry of material • Permits comparison of different materials (e.g., bone, tendons, cartilage, ligaments)
Normalizing Load • Stress – force/area • Strain – length change/initial length (unitless value) • Two types of strain • Linear – causes change in length • Shear – causes change in angular relations (radians)
Stress-Strain Relationships • Similar to load deformation curve
Stress-Strain Relationships Elastic modulus (Young’s modulus) – slope of the stress-strain curve in the elastic region (measure of stiffness) Plastic modulus – slope of the stress-strain curve in the plastic region Area under stress strain curve is measure of energy absorbed
Relationships of Age to Stress-Strain Characteristics of Bone • indirect relation between age and energy absorption
Cortical vs. Cancellous Bone • Cortical bone stiffer, withstand greater stress but less strain before failure • Cancellous bone fractures when strain exceeds 75% • Cortical bone fractures when strain exceeds 2% • Cancellous bone has larger capacity to store energy
Properties of Stiffness and Brittle/Ductile Interpretation?
Properties of Stiffness and Brittle/Ductile • Metal – large plastic region • Virtually no plastic region in glass • Stress-strain curve of bone not linear • Yielding of bone tested in tension caused by debonding of osteons at cement lines and microfractures
Ductile and Brittle Fracture • Young bone more ductile • Bone more brittle at higher loading rates
Typical Response of Long Bone to Loads • greatest resistance to compression • weakest response to shear loads • intermediate strength for tension
Safety Factor • Safety factor - bones are 2 to 5 times stronger than forces they commonly encounter in activities of daily living; bone strength and stiffness are greatest in the direction in which loads are most commonly imposed (see figure)
Remodeling of Bone • Wolff’s Law • Remodeling – balance between bone absorption of osteoclasts and bone formation by osteoblasts • osteoporosis –increase porosity of bone, decrease in density and strength, increase in vulnerability to fractures • piezoelectric effect – electric potential created when collagen fibers in bone slip relative to one another, facilitates bone growth • use of electric and magnetic stimulation to facilitate bone healing
Factors Influencing the Dynamic Response of Bone • Mechanical properties of bone • Geometry • Loading mode • Rate of loading • Frequency of loading
Factors Influencing the Dynamic Response of Bone • Result of loading of bone in transverse and longitudinal directions dissimilar (anisotrophy) • Bone tends to be strongest in directions most commonly loaded
Behavior of bone under tension, compression, bending, shear,torsion, and combined loading
Behavior of Bone Under Tension • under tensile loading structure lengthens and narrows • equal and opposite loads applied outward • maximum tensile stress occurs on a plane perpendicular to the applied load (see figure)