Flashcards in Spinal Biomechanics Deck (178):
What is biomechanics?
The function of the musculo-skeletal system
What are the 3 measurements in physics?
Length, mass and time
What is the International System of Units?
Adapted from earlier MKS system
Meters (m), kilograms (kg) and seconds (s).
What is mass?
The quantity of matter in an object.
What is the basic unit of mass?
Where is the standard cylindrical piece of platinum-iridium kept?
Sevres in France
What does mass equal?
m = weight / gravity
What is weight?
The force that something exerts on anything which freely supports it.
What is weight expressed in?
Who showed that all bodies, regardless of mass, have the same acceleration when they fall freely under the action of gravity?
How is gravity denoted?
the italic letter "g"
What is the universal gravitational acceleration force?
9.81 m/s squared
How would you calculate the weight of a person who has a mass of 75 kg?
F = m * italic g
F = 75 kg * 9.81 m/s squared
F = 735N
Describe gravitational force.
Pulls all objects towards the center of the earth.
What is Newton's law of universal gravitation?
Any two particles of matter attract one another with a force which is proportional to their masses and inversely proportional to the square of their distance apart.
Usually denotes a push or a pull.
When a body is acted upon by a force it will move.
"That which changes a body's state of rest, or uniform motion in a straight line."
What is a Newton?
One newton is the force that is required to give one kilogram an acceleration of one metre per second.
What is inertia?
The property of a body which causes it to tend to continue in its present state of rest or motion, unless acted upon by a force.
What is Newton's first law of motion?
Every body continues in its state of rest or uniform motion in a straight line unless compelled by some other force to do otherwise.
The product of its mass and velocity.
What is the unit of momentum in the SI system?
1 kilogram multiplied by one metre per second (kg m/s).
Explain momentum in relation to two bodies, a heavy one and a light one when the same force is acted upon them for the same length of time.
The lighter body builds up a higher velocity than the heavier one, however they gain the same momentum.
What is Newton's second law of motion?
The rate of change of momentum of a body is proportional to the applied force and takes place in the direction in which the force acts.
What is Newton's 3rd law of motion?
For every action there is an equal and opposite reaction.
How can Newton's 3rd law be described through the firing of a gun?
Mass of bullet x muzzle velocity = mass of gun x recoil velocity. The sum total of their momenta is zero.
Momentum is a vector quantity. What does vector quantity mean?
It has direction as well as magnitude.
What is the sum of two or more vectors?
What are a number of vectors that are added together called?
The components of the resultant.
What can vector diagrams be applied to?
Force, velocity and acceleration
When is torque created?
Every time we open a door, turn on a tap or tighten up a nut.
What are the two factors important when producing torque?
1st the magnitude of the force applied
2nd the distance of its line of action from its axis or fulcrum about which turning takes place.
When can a large turning effect take place even if the amount of force is small?
If the distance from the fulcrum is large.
What is the moment of a force?
The moment of a force (a) about a point or fulcrum is the product of the force and the perpendicular distance (b) of its line of action from the point.
What is a couple?
A pair of equal and opposite forces acting at a distance about the centre of rotation of the object to which the moment is applied. An example would be wing nuts.
Describe equilibrium (balance)
The condition of being equally balanced.
Both the loads and rotary forces have to be equal.
What is the point around which a lever rotates?
How do you calculate the amount of turning effect or moment?
Multiplying the distance (in meters) from the fulcrum by the load applied (in newtons) to get a quantity called a turning moment and is expressed in newton metres.
What are the 3 orders levers can be divided into?
Describe first order levers
Seesaw is an example of this type of lever.
The fulcrum lies between the applied force and the resistance or load.
Not many examples of this in the body, but one would be the neck muscles.
Describe second order levers
The resistance is located between the applied force and the fulcrum.
An example would be a wheelbarrow.
An example in the body would be in performing plantar flexion when the calf muscles act across a second class lever.
Describe third order levers
A force is applied between the resistance and the fulcrum.
Most common in the body.
Biceps which flex the elbow.
Name given to forces acting within a material which enables it to resist external loading.
How is stress generally classified?
Its type of loading.
How is direct stress measured?
load / cross-sectional area
What is tension stress?
What is compression stress?
What is bending stress?
What is shear stress?
Stresses coming from opposite sides.
What is torsion stress?
Twisting from opposite directions.
What is combined loading?
Compression and combined loading stresses combined.
What is a neutral plane?
In the axial center of a bar there is neither compression or extension when bending stress is applied.
What are contact stresses?
When loads are transferred between articulating surfaces.
What is cross sectional area?
The area over which load acts.
How is contract stress calculated?
Contact stress = load / area and is measured in Pascals.
What is a Pascal?
The pressure produced by force of one newton, applied with uniform distribution, over an area of one square meter.
What happens when someone walks on snowshoes?
It is distributed over the wide surface area of their show-shoes reducing the contact stress, thus making it less likely for them to sink into the snow.
What is hoop or triangle stress?
The hoop stress is created by the internal pressures of the liquid being resisted by the hoops around the barrel, as seen in the diagram below.
What is direct strain?
A measure of deformation of material when in stress.
How is direct strain measured?
Change in length / original length
What is shear strain?
Shear strain causes a change in the angular relationships within a structure. It is expressed in radians.
A material that returns to its original shape when unloaded.
What is plasticity?
A material that after it is deformed does not return to its original shape.
What happens if stress does not exceed the yield point or elastic limit of the material?
It will return to its original shape and dimensions when the stress is removed.
Define plastic deformation.
When a material has been stressed to such an extent that it will not return to its original shape.
What is modulus?
An expression of the elastic stress-strain relationship of a material.
Stress, strain and modulus equations.
Stress = load / cross-sectional area
Strain = change in length / original length
Modulus = stress / strain
If a material has a high Young's modulus, it will only be able to dissipate contact stresses over a small area, thus concentrating them and producing areas of high stress. Think of a ball on a sheet of rubber vs a sheet of glass.
Stiffness related to diameter.
For every 2mm increase in diameter the stiffness increases by 16x
Bone density greater than 1.5 grams per cubic centimeter is called
Bone density less than 1.5 grams per cubic centimeter is called
Changes in mechanical properties of bone are often related to
Bone material may be regarded as what two phases?
Mineral as one phase and collagen as the other.
Stain and stress in bone
Cortical can withstand greater stress
Cancellous can withstand greater strain
What does Wolff's law state?
Bone that is not stressed is lost.
What is osteopenia?
Too much stress or too little stress
What are the two primary functions of articular cartilage in diarthrodial joints?
To distribute joint loads over a wide area, thus decreasing the stresses across the contacting joint surfaces and to allow relative movement of the opposing joint surface with minimal friction and wear.
Describe articular cartilage physiologically.
It is devoid of blood vessels, lymph channels and nerves. It's cellular density is less that that of any other tissue.
What is the makeup of collagen?
10% Chondrocytes - responsible for secretion and management of the matrix
10% - 30% Collagen content
3% - 10% Proteoglycan content
60% - 87% is water, inorganic salts and other proteins.
What is the most abundant protein in the body?
What is the most important mechanical property of collagen?
Its tensile strength. It has little resistance to compression stress
How are bones can cartilage anisotrophic?
Its material properties vary according to the direction of loading.
What is the most abundant component of articular cartilage?
Water and it is most concentrated near the articular surface.
What happens when the tissue is loaded?
70% of the water may be moved
How does articular cartilage handle stresses?
It has a limited capacity for repair and regeneration, and if subjected to an abnormal range of stresses, it can undergo total failure quite quickly.
What is the primary factor in the etiology of osteoarthritis?
Lubrication insufficiency in the cartilage
What are the two contradicting requirements of the spinal column?
Rigidity and plasticity
Like a ship's mast that rests on the pelvis and extends to the head. At all levels ligament and muscles act as stays lending support to the column.
Multiple structures interlinked by ligaments and muscles. The overall shape can be changed in muscle tension while rigidity is maintained.
What are the 3 functions of the intervertebral disc?
Facilitate movement of the spinal column
Maintain vertebral spacing
What are the disc thicknesses?
3mm - cervical
5mm - thoracic
9mm - lumbar
At what level of the vertebral column is a vertebral disc not present?
What are the superior and inferior shapes of the discs in the cervical spine?
Superior - concave
Inferior - convex
Are cervical discs thicker anteriorly or posteriorly?
Anteriorly which contributes to the lordotic cervical curvature.
Where are flexion and extension measured from?
The plane of the bite, a line drawn between the upper and lower teeth.
What is the range of flexion and extension in the cervical column?
130° with 110° being in the lower cervical column.
How do you measure lateral flexion in the cervical spine?
The angle between the upper border of T1 and the mastoid processes.
What is the range of flexion for the cervical spine?
Around 45° with only 8° in the sub occipital.
Where is rotation of the cervical spine measured from?
The neutral rotation plane of T1 and varies from 80° to 90° for the whole column. 12° in the atlanto-occipital and atlanto-axial joints.
How is the disc broken up?
3/10 width of nucleus
What motion is capable in the thoracic spine?
20° lateral bending
35° axial rotation
What is extension in the thoracic spine limited by?
The articular processes and the spinous processes, which, being sharply inclined inferiorly and posteriorly are normally almost touching.
What is flexion in the thoracic spine limited by?
The interspinous ligament, the ligamentum flava, the capsular ligaments of the joints and the posterior longitudinal ligament.
Where does the nucleus lie in the discs in the lumbar spine?
More posteriorly than the others.
What percent of the trunk are the lumbar discs responsible for holding?
Pertaining to, situated on, or affecting the same side of the body
Pertaining to, situated on, or coordinated with the opposite side.
What are the ranges of motion achievable in the lumbar spine?
20° of lateral flexion
5° of axial rotation
The spine is the equivalent of a joint with what 3 degrees of freedom?
Flexion and extension, lateral flexion, and axial rotation.
What is the total range of flexion and extension in the column?
Flexion is 110°
Extension is 140°
What is the total range of lateral flexion?
75° - 80° from sacrum to cranium in the frontal plane.
What is the axial rotation throughout the spine?
Lumbar - 5°
Thoracic - 35°
Cervical - 45-50°
Total rotation from pelvis to cranium attains 90°
The functional vertebral column is made up of what 3 pillars?
The vertebral bodies and discs, and the two minor pillars are the linked articular processes and the vertebral arches.
Where is the line of gravity in the body?
Anterior to the spinal column.
The anterior column absorbs around 80% of the axial load as what kind of force?
A compression force.
The posterior of the column is loaded primarily by what type of forces?
What is the vertical force in the discs transformed into?
Lateral forces tightening the annular fibres.
What makes up the posterior column of the vertebral column?
Posterior arches and facet joints
Does the posterior column work as a load-bearing column?
No, they work as an articulated chain.
What is the 80 - 20 rule?
80% of the vertical load is handled by the anterior column while 20% of the axial load is handled by the posterior column.
How does the posterior column handle 20% of the axial load?
As a sagittally directed shear force, which is absorbed mostly by the facet joints and the typical articular structure of the pars articularis.
What is the functional link between the anterior and posterior columns?
A first order lever where the articular processes constitute a fulcrum.
The absorption of forces in the articular process is what?
Active and passive at the same time.
What is the function of the posterior ligament?
To limit the amount of movement within the segment.
What is the tension band function provided by?
The muscles, ligaments and facet joints
What ensures equilibrium between the anterior column and the posterior chain?
Posterior compression forces.
Biomaterials are materials of a natural or manmade origin that are used to direct, supplement or replace the functions of living tissue.
What must a material be in order to be classified as a biomaterial?
Biocompatible - must not cause adverse reactions in the body and should it corrode, the products of corrosion must be tolerable.
Biofunctional - materials must perform in amechanically satisfactory way.
Sterilisable - must be able to be sterilised by use of gamma irradiation or Ethylene Oxide (EtO) without compromising.
How are metals used in spine surgery?
Structural, load bearing for fracture, fixation or tumor reconstruction.
Instrumentation for spinal fusion
Partial or total joint replacement
Why is metal popular in spine surgery?
Metals have high elastic moduli and reasonable yield points
When metals exceed their yield points they produce plastic deformation rather than sudden fracture. - meaning worn components can be replace before total failure.
High fatigue strength and can withstand repeated cyclic loading
Can be manufactured into a variety of parts
Excellent resistance to environments encountered in orthopedic practice.
Describe alloy fabrication.
Extraction of metal ores from mineral deposits
Reacted with oxygen, silicon, nitrogen, sulphur and other metals.
The alloy is formed by combining elements and melting them.
The resulting liquid alloy can then be cast.
What are casting alloys?
Molding followed by mechanical finishing.
What are the most common metals used in spinal implants and implantation devices?
Forged stainless steel alloy
Various forged titanium alloys
Carbon fibre compound
What are the main constituents of stainless steel alloys?
What are the constituents of titanium alloys?
What is the most common commercial alloy?
Why are titanium alloys used in orthopedics?
They have excellent compatibility and can directly integrate with bone.
Materials directly integrating with bone tissue.
What is the disadvantage of the finishing technique known as sintering?
The strength of the alloy is reduced as a result of the heating required to bond the surface finish to the implant.
Describe hydoxyaapatite coatings.
Improves biological fixation.
Has a bioactive ceramic coating that contains calcium, phosphate and hydroxyl groups which form the main constituent of bone.
Coating is applied by plasma sprayingyou
What type of surface is Hydoxyapatite (HA)
Not an ingrowth but an ongrowth
Formation of strong oxide surface film on aluminum base and titanium base alloys by placing the parts in a conductive (electrolyte) bath leaving them positively charged.
Metal surfaces are converted to nitrides by reaction with gaseous ammonia or molten potassium cyanate at elevated temperatures. Leaves a nice golden color.
All orthopedic alloys must be passivated by acid treatment which converts surface elements, particularly chromium to oxides or hydroxides.
Describe rough and smooth blasting and polishing.
The smoothness and roughness can affect the performance of an implant.
Rough surfaces may provide good interlocking with bone and cement but may be weaker in terms of fatigue strength compared to smooth surfaces.
A smoothing process using aluminum oxide, silicon carbide, or industrial diamond.
What is rough blasting?
Used to achieve a rough surface for cement to adhere to.
What is sand blasting?
Bombardment of a surface using foreign particles.
What is shot peening?
When larger particles such as ceramic beads or steel balls are shot at the surface to work harden it.
Describe wear in relation to material failure.
Friction - force that resists relative motion between two contacting materials.
Lubrication - method by which the frictional forces between two contacting materials are decreased usually with the addition of a third material.
Wear - the process by which material is removed due to interaction with other materials under repetitive motions and loads
Tribology is the study of friction, lubrication and wear.
What are the 3 types of wear?
Adhesive wear, abrasive wear and 3 body wear.
Describe adhesive wear.
The transfer of one material to the surface of another. An example would be chalk on a chalkboard.
Describe abrasive wear.
Material is worn away. Example would be using sandpaper.
Describe 3 body wear.
A form of abrasive wear and is caused when a hard particle or third body becomes trapped between two articulating surfaces.
Describe crevice corrosion.
Unpredictable. Isn't a need for two dissimilar metals to be present to initiate it.
Is due to oxygen depletion.
Example used is a rubber band wrapped around metal submerged in saltwater. Under the band pitting occurs.
The result of small relative motion between two touching surfaces which produce debris. A form of adhesive wear. The particles in turn cause 3 body wear.
Creep is plastic deformation at stresses below the yield point of a material.
Temperature has a profound affect on creep.
When materials are loaded and unloaded repeatedly they experience fatigue.
Like wear, this process results in the loss of material. It is however a chemical process.
Describe galvanic corrosion.
The commonest form of corrosion.
Rust on steel is an example of this, unlike the rust on iron which is the result of oxidation. When two dissimilar metals are touching or in close proximity in a conductive environment or electrolyte, they form a battery and a current flows between them. One of the pair is said to be anodic or electropositive and the other to be cathodic or electronegative. The anodic material is the one which corrodes. Corrosion can occur between two different metals in the body. Stainless steel and cobalt chrome alloys are the most reactive.
A toxin or other foreign substance that induces an immune response in the body, especially the production of antibodies.
What are lymphocytes?
White blood cells produced in the lymph glands, spleen and bone marrow.
What are the 3 types of lymphocyte in the blood?
T Cells - are thymus dependent
B Cells ar ebone marrow derived
NK Cells are natural killer cells
What does the immune system react to?
Antigens or substances of various types which are foreign to the individual.
What do macrophages do?
Engulf and digest foreign bodies.
What is an allergic reaction?
An inappropriate or excessive immune response to antigens.
Describe granulation response.
The formation of minute, rounded, fleshy connective tissue on the surface of a wound, ulcer or inflamed tissue in the process of healing.
Nodular inflammatory lesions that contain compactly grouped mononuclear phagocytes.
Describe a normal response to implants.
Biomaterials must not only function to their specifications and design, but must also be tolerated by the body.
Define adverse host response.
Rejection of an implant.
What is "coupling" with the host?
Implants must couple with the host to produce a host response.
What can the host response be classified as?
Where do host responses occur?
Often near the implant or at a remote site such as regional lymph nodes.
What are 3 cellular events not usually seen in healing wounds that occur in the presence of all implants?
Macrophages may remain in the vicinity of the implant.
Significant numbers of lymphocytes (and even plasma cells) may be seen near the implant.
At later times, a population of multinucleate giant cells, called foreign body giant cells, may appear and remain for long periods of time.
What are all implants surrounded by?
A fibrous capsule. This is a consequence of the classic foreign body response which attempts to degrade all foreign material and failing that, to surround it.
Describe chronic inflammation.
Some patients are allergic to nickel and unable to tolerate stainless steel alloys. As a result the area can become inflamed. Instead they are offered titanium as an alternative.