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