Tissue loading
In lumbar flexion which area of the intervertebral disc will undergo the greatest compression
anterior portion of disc should be in compression posterior portion will be in tension
Wolff's Law
A bone grows or remodels in response to forces or demands placed upon it bone (strength +mineral content) changes based on the stresses applied to the bone
Hookes Law
E= modulus of elasticity = stress/strain proportional elastic region: -stress is directly proportional to strain (linear) -modulus of elasticity defines the slope of the linear
Deformation
Elastic materials: deform instantaneously when they are subjected to externally applied loads and resume their shape when load is removed - stress is a function of strain only, no time dependent behavior Plastic materials: deform instantaneously when they are subjected to externally applied loads and do not fully recover their shape when load is removed * materials may exhibit elastic behaviors up to a certain loads beyond which they will exhibit plastic behavior
Compressive load example
F= F load * cos (theta) = 1200 x cos (35) = 982.98 N compressive stress= F/A =982.98/ 0.0009 m^2 = 1092202 Pa
Shear load example
F= F load * sin (theta) = 1200 x sin (35) = 688.29 N shear stress= F/A 688.29/0.0009 m ^2 = 764,766 Pa
Pressure
Force per unit area P= F/A Increase force, increase pressure decrease force, decrease pressure increase area, decrease force decrease area, increase force
Stress-Strain Curve
The relationship between the stress and strain that a particular material displays look at lecture slide
Strain
amount of deformation w respect to the structure normal: ratio of the change in length strain= change in length/ length =(l-L)/L L= original length l= final length can be compressive or tensil
Acute loading
application of a single force of sufficient magnitude to cause injury to a biological tissue macrotrauma ex: ligament tear or bone fracture
Bending
asymmetric loading that produces tension on one side of a body's longitudinal axis and compression on the other side failure on the tension side concave side in compression convex side in tension more likely to fracture on convex side because harder to withstand tensile stress
Osteoporosis
bone becomes weak and may break from minor fall or even from sneezing/bumping into furniture lifestyle disease that is dependent on habits no clear onset peak bone mass during childhood is important predictor weight bearing exercise in pre-puberty years may help dietary calcium (absorption of vitamin D)
If you laterally flexed your lumbar spine to the left, where would tension and where would compression be in your intervertebral discs
compression on left tension on right
Mechanical stress load on the body
compressive tensile shear torsion bending
porous
containing pores/cavities low porosity= 5-30% non-mineralized tissue high porosity= 30-90 +% non-mineralized tissue
epiphyseal fracture
damage to the growth plate area of younger bone <18 years old if hyaline cartilage is disrupted bone growth may end can be difficult injury to treat, often w long term effects
bone atrophy
decrease in bone mass w disuse
stress/strain curve w different bones
different materials have different moduli of elasticity and stress strain curves cortical bone reaches higher stress without a whole lot of strain (greater level of stress before fracture) trabecular strains fast/readily under low loads (stress) trabecular bone can undergo a lot more strain before fracture
osteocytes
direct bone remodeling activity (regulate osteoblasts and osteoclasts)
Stress
distribution of force within a body, quantified as force divided by the area over which the force acts internal pressure stress= F/A normal= 10-20 N/cm^2
deformation
ductile: -able to yield at normal temperatures -large plastic deformation prior to failure -ex: steel brittle: -rupture occurs during elastic deformation -failure w/o undergoing plastic deformation -ex: cast iron, glass, stone * look at graph in slides
Bone loss in the jaw
due to gum disease, dentures, loss of teeth may lead to: -loss of teeth -non retentive dentures -mandibular fractures -inability to get implants -jaw joint issues due to abnormal loading
Spinal loading
easy to view many of these loading types in the spinal column and consider the effects of such loads on the intervertebral disc
Buccal exostosis
excess bone deposition due to clenching (excess loading)
Anisiotropic
exhibiting different mechanical properties in response to loads from different directions compression has more stress to fracture tension has 2nd most stress to fracture shear has least amount of stress to fracture
axial/normal stress
force resultant is perpendicular to the plane compressive stress: stresses caused by forces that tend to shrink materials tensile stress: stresses caused by forces that tend to stretch materials
spondylolisthesis
forward slipping of one vertebra over another
longitudinal bone growth
grows at epiphyseal plate (cartilaginous disc) most fuse by age 18 ending longitudinal growth
greenstick fracture
incomplete fracture caused by the bending of the bone the convex side ruptures due to tensile stress
bone hypertrophy
increase in bone mass w stress
circumferential bone growth
increases bone diameter (inner wall is eaten away at same time so dont get this dramatic increase) occurs throughout most of lifespan periosteum builds concentric layers of bone bone is simultaneously resorbed on the medullary side
Anterior pelvic tilt during squat
increases lumbar extension (lordosis) increases shear load component and decreases axial load component
Trabecular/Cancellous/Spongy Bone
less compact mineralized CT high porosity found in vertebrae and ends of long bones
Maintain upright posture
listed in order of decreasing internal pressure: 1.) sitting slouched 2.) standing leaning forward 3.) sitting erect 4.) standing erect 5.) lying flat
torsion
load producing twisting of a body around its longitudinal axis causes shear stress in the material
Problems related to atrophy
loss of bone mineral density reduces bone strength (increases liklihood of injury) space flight, bed ridden patients, aging osteoporosis
Mitigating the risk for injury
maintain upright posture maintain a neutral spine sustain intra-abdominal pressure during lifting reduce moment arm of the load during lifting avoid twisting while lifting
Shear stress
measure of the intensity of internal forces acting parallel to a plane
Composition of bone tissues
minerals: calcium carbonate, calcium phosphate (60-70% of bone weight) -stiffness= compression strength collagen: protein, cable-like (25-30% dry weight) and adds to tensile strength -flexibility= tensile strength water: carries nutrients to and waste away (25% of total weight) and adds to compressive strength
Bone remodeling
occurs continuously throughout life density and shape of bone change w load fatigued damaged older bone is resorbed formation of new bone in response about 25% of trabecular bone is remodeled yearly
stress fractures
overuse, low load conditions functional ability often retained could be warning of other nutritional or pathological conditions
Center of Pressure (COP)
point that represents the place of application of the sum of all forces acting on a surface doesnt have to be in point of contact
Compression
pressing/squeezing force directed axially through the body
Tension
pulling or stretching force directed axially through a body
Force
push or pull load that tends to produce an acceleration of a body in the direction of its application forces may deform an object, change its state of motion, or both F=m*a F= mass x acceleration
repetitive loading
repeated application of a sub-acute load that is usually of relatively low magnitude microtrauma ex: shin splints, stress fractures, tendonitis
Combined loading
simultaneous action of more than one of the pure forms of loading
Ansiotropy
some materials act better under different directions of load because of the microstructure of the material wolff's law= body will adapt to loads under which it is placed bones are much better at withstanding axial loading than a load applied at an oblique angle loading angle matter more axial the load the greater ability to withstand the load
Elasticity (of strain)
the ability of a material to resume its original size and shape linear elastic material= stress is linearly proportional to strain
Bone deformation
ultimate strength (MPa) and ultimate strain (%) of cortical bone from the human femur as a function of age * we need both mineral and collagen stress increased with ages from 10-40 and then decreases from 40-90 strain decreases with age (less ductile with age)
Spondylolysis
unilateral or bilateral "scotty dog" fractures of the pars interarticularis (isthmus)
Shear
force directed parallel to a surface
Biomechanical properties of bone
* depends on bone type cortical bone: stiffer, withstand more stress but less strain before fracture trabecular bone: high strain but low stress before fracture
loading in the spine (complex and likelihood of injury depends on # of factors)
1.) Load magnitude + direction - greater loads especially shear loads increase the risk for slippage and disc herniation 2.) Muscle tension - poor lifting form and increase load torque can cause tension which also contribute to excess loading of the spine 3.) Time dependent - under sustained load intervertebral discs lose water and weight and more load is transmitted through the facets of the vertebrae
Curvature in spine
Cervical - concave/lordotic Thoracic - convex/kyphotic Lumbar - concave/lordotic Sacral - convex/kyphotic
modulus of elasticity example: given the stress-strain graph shown here, determine the modulus of elasticity for this bone. how much stress is needed to cause 0.003 strain? -stress= 17 MPa -strain= 0.0008
E= 17 MPa/ 0.008= 2125 MPa stress= 0.003 x 2125= 6.375 Mpa
Biomechanical properties
age: young ductile---> old brittle rate of loading: low rate ductile----> high rate brittle
Cortical (compact) bone
compact mineralized connective tissue low porosity found in shafts of long bones
Osteoblasts
deposit new bone
Spiral fracture
oblique break due to torsional loading
osteoclasts
resorb bone