Announcements

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Announcements

• Article Critiques
• See Syllabus for details
• Rubric and writing guide are posted online
• 1st is due Friday, October 9th
• Must be an original research study article from a
  peer reviewed journal
• If you are in doubt about your article ASK!
• KAS Meetings have started up
• Every other Mondaynext is the 19th
• 700-800pm (or so)
• ESC 208

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From last time

• Biomechanics of Bone
• Remodeling
• Injury and healing
• Changes with age

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Biomechanics of Cartilage

• ESS 4361-001
• Lecture 7
• Reading NF Chapter 3

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Overview

• Cartilage Composition and Structure
• Collagen, Proteoglycan (PG), and Water
• Biomechanics of Cartilage
• Viscoelastic response to loading
• Compression
• Creep and Stress-relaxation
• Permeability
• Tension
• Shear
• Swelling
• Lubrication
• Wear, Regeneration and Degeneration

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Part 1Structure Function
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Joint Motion

• Diarthroidal joints allow substantial motion
• Ends of articulating bones is a layer of hyaline
  articular cartilage
• Hyaline Articular Cartilage
• Precisely suited for high loading (magnitude and
  frequency) without failure
• Isolated tissue
• No blood vessels, lymphatic channels, or nerves

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Articular CartilageFunctions

• Distribute joint loads
• Decrease stresses sustained by joint surface
• Allow relative motion of articulating boney
  surfaces while minimizing friction and wear

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Cartilage Composition

• Chondrocytes
• Manufacture, secrete, organize and maintain
  organic component of extracellular matrix
• lt10
• Organic Matrix
• Collagen fibrils (mostly Type II) (15-20)
• Proteoglycan (4-7)
• Water, Salts and other proteins
• 60-85

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Collagen

• Most abundant protein in body
• In Articular Cartilage
• Fibers are highly organized
• Inhomogeneous distribution (layered effect)

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Video

• Articular Cartilage

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Collagen Type

• Type I
• Most abundant
• Intervertebral discs, skin, meniscus, tendon,
  ligament
• Type II
• Thinner fibril than Type I
• Thus maximum dispersion of collagen throughout
  tissue
• Articular cartilage, nasal septum, sternal
  cartilage, intervertebral disc, meniscus
• More types V, VI, IX, XIall found in articular
  cartilage as well.

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Collagen

• Major Function
• Tensile stiffness
• Strength
• Collagen is strong in tension
• Tendon is 80 collagen with stiffness of 103 MPa
• Tendon has tensile strength of 50 MPa
• Steel has stiffness of 220x103 MPa
• Collagen is weak in compression
• Thin fibers buckle under load

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Proteoglycan

• Protein-polysaccharide molecule, many types of
  which are found in cartilage
• Major Function in Cartilage
• Stability
• Binds to surfaces of collagen fibrils and bonds
  holds tight

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Articular Cartilage Changes With Maturation

• Water content progressively decreases
• Carbohydrate/protein ratio progressively
  decreases
• Thus, decreased size, stiffness and resilience

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Water

• Most abundant component of articular cartilage
• Interstitial fluid
• Contains cations Na, K, and Ca
• Effect mechanical and chemical behavior of
  cartilage
• Permits gas, nutrient and waste movement between
  chondrocytes and synovial fluid
• Compression Loading
• 70 of water may move
• Important in mechanical behavior and joint
  lubrication

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Articular Cartilage Response to Compression

• Compressive stress
• Deformation
• External stress causes increase in internal
  pressure
• If internal pressure exceeds swelling (osmotic)
  pressure in interstitial fluid, liquid will flow
  out of tissue
• Fluid outflow causes PG concentration in
  cartilage to increase
• Osmotic pressure increases until internal
  (cartilage) and external (interstitial fluid)
  pressures are at equilibrium
• PG gel inside collagen network then can withstand
  compression

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Articular Cartilage

• Contains cells and extracellular matrix
• 3 kinds of articular cartilage
• Hyaline (most abundant)
• Elastic
• Fibrocartilage
• Characteristics
• Develops from mesenchyme
• Lack of intrinsic blood vessels, nerves or lymph
  vessels
• Thus, necessary to receive nutrients from
  diffusion

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Articular CartilageGrowth

• Interstitial Growth
• Occurs in young cartilage (more flexible)
• Chondrocytes divide within the lacunae forming
  cell nests
• Appositional Growth
• Proceeds in the cartilage layers just beneath the
  perichondrium
• Mesenchymal cells develop into new cartilage
  cells
• New cells are laid down between older cells and
  perichondrium, and produce new matrix components

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Hyaline Cartilage

• Characteristics
• Glassy appearance (translucent)
• Fetal cartilage is primarily hyaline cartilage
  initially
• Collagen gives cartilage its strength
• Principal hyaline cartilage is Type II
• Location
• Surfaces of most joints
• Anterior portions of ribs
• Areas of respiratory system (e.g. trachea, nose,
  bronchi)

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Elastic Cartilage

• Characteristics
• Highly flexible
• Matrix contains elastin and collagen
• More yellow in appearance
• Location
• External ear
• Epiglottis
• Portions of larynx
• Eustachian tube

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Fibrocartilage

• Characteristics
• Strong and flexible
• Resilient
• Endogenous collagen fibers
• Contains no perichondrium
• Location
• Stress points where friction might be problematic
• Essentially a filler material between hyaline
  cartilage and other connective tissues
• Near joints, ligaments, tendons and
  intervertebral disks

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Fibrocartilage 1. Interarticular Fibrocartilage

• Flattened plates iterposed between joint surfaces
  and held in position by ligaments and tendons
  (e.g. meniscus)
• Ends are free of connection
• Function
• Prevent friction between moving joints
• Improve joint geometry
• Protect surfaces of underlying articular
  cartilage
• Location
• Joints where frequent movement and potential
  impact occurs (e.g. Wrist, knee,
  temporomandibular joint, sterno-clavicular joint)

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Fibrocartilage2. Connecting Fibrocartilage

• Function
• Allow adjacent surfaces subtle and slight
  relative motion
• Location
• At limited-motion joints (e.g. intervertebral
  disks)

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Fibrocartilage3. Stratiform Fibrocartilage

• Function
• Minimize friction at tendon-bone articulation
• Location
• Forms layers over bone where tendons act
• Occasionally form an intergral portion of the
  tendon surface

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Fibrocartilage4. Circumferential Fibrocartilage

• Circular ring of cartilage (without a center)
  acting as a spacer within a joint capsule
• Function
• Protects only the bony edge of the joints
• Improves bony fit at joint
• Location
• Hip
• Shoulder (e.g. glenoid labrum)

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2 Videos

• 3 Kinds of Cartilage
• Shotgun Histology Hyalin Cartilage

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Part 2Loading and Remodeling
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Articular CartilageLoading

• Viscoelastic
• Fluid phase
• Solid phase
• During Joint Loading
• Joint forces (0 10 times body weight)
• Contact area (vary through ROM)
• Contact Stress
• 20 MPa in hip during chair rise
• 10 MPa in hip during stair climbing

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Viscoelastic Behavior

• Creep
• Constant load
• Rapid initial deformation
• Slow progressive increase in deformation until
  tissue reaches equilibrium
• Stress Relaxation
• Constant deformation
• Rapid, high initial stress
• Slow progressive decrease in magnitude of stress
  required to maintain that deformation
• In cartilage, this behavior is caused by
  interstitial fluid flow and the associated
  friction (drag)
• Interstitial fluid pressurization supports 90 of
  the applied load to the cartilage surface

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Articular Cartilage In vivo Stress-Strain

• Incredibly complex
• Time-dependant nature
• Magnitude of loading
• Alterations in natural state (pre-load)
• Variable biochemical structure
• Variable structural composition
• Thus, most values (stiffness, energy storage,
  etc.) are obtained from models and in-situ
  experiments

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CompressionBiphasic Creep and Stress Relaxation

• Creep Response in Compression
• Fluid flow is initially high
• Progressively slows until equilibrium is reached
  with interstitial fluid pressure
• Stress Relaxation in Compression
• Compression Phase (Stress Rise)
• Stress rises continuously (nearly linearly) until
  constant deformation is reached
• Stress-Relaxation Phase
• Stress decays along a curve (time-dependent)

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CompressionLoading Behavior

• Excess stress in cartilage is difficult, since
  stress relaxation readily attenuates the stress
  within the tissue
• Thus, rapid spreading of the contact area in the
  joint during articulation

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Articular Cartilage Permeability

• Highly porous (80) with interconnected pores
  (thus permeable)
• Measure of the ease with which fluid can flow
  through the material
• Inversely proportional to frictional drag exerted
  by the fluid flowing through the material
• Articular Cartilage
• Very low permeability
• Thus very high frictional resistive force

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TensionLoading Behavior

• Highly complex
• Load causes a volumetric change (fluid flow)
• Anisotropic
• Stiffer and stronger parallel to fibers
• Inhomogeneous
• Stiffer and stronger in superficial regions
• Viscoelastic
• Due to internal friction and flow of interstitial
  fluid
• Thus, utilize a tangent line of stress-strain
  curve to estimate stiffness
• Failure
• Collagen fibers in specimen are ruptured

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Loading BehaviorShear

• Pure Shear
• No fluid flow, thus maintains volume
• But, this is with small dynamic loads
• Dynamic Shear Modulus (d)
• Total resistance
• Total frictional energy dissipation
• For pure elastic material d is 0º
• Pure viscous fluid, d is 90º
• Shear stiffness of articular cartilage is derived
  from its collagen, rather than the PG
• Thus increased collagen concentration will
  increase ultimate strength of the cartilage

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Swelling

• Significant contribution to load-bearing capacity
  of articular cartilage
• Contribution of swelling decreases with load
  magnitude
• Multifaceted Phenomenon
• Electrical gradients
• Chemical gradients
• Temperature
• Other factors

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Lubrication

• Boundary
• Single layer of lubricant absorbed on each
  surface
• Prevents direct surface-to-surface contact
• Eliminates most wear on surface
• Higher coefficients of friction than fluid-film
• Fluid-film
• Thin film increases surface-surface separation
• Likely the primary lubricant of synovial joints
• Intact synovial joints have extremely low
  coefficient of friction (0.02)
• Cartilage is nearly frictionless

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Lubrication

• Likely dual actions in synovial joints
• Boundary
• Operates in high load areas to keep articulating
  surfaces from coming together
• Fluid-Film
• Operates under less severe conditions
• Loads are low or oscillating (cycling)
• Contact surfaces are moving at high relative
  speeds
• Cartilage generates lubrication as based on
  experienced loading
• Interstitial fluid pressure may even play a role
  in lubrication

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Wear

• Unwanted removal of material from solid surfaces
  by mechanical action.
• Interfacial Wear
• Interaction of bearing surfaces without
  lubrication (rare)
• Adhesive Surface fragments adhere to each other
  and are torn off
• Abrasive Soft material is scraped by a harder
  one (more likely)
• Fatigue Wear
• Bearing deformation under load
• Accumulation of microscopic damage under
  repetitive stressing
• Repeated high loads over short period
• Repeated low loads over long or extended period
• Can occur even in well-lubricated joints
• Synovial Joint Impact Loading
• Rapid application of a high load
• Load is applied too quick for internal fluid to
  redistribute

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Wear
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Regeneration

• Limited capacity for repair and regeneration
• Total failure can quickly occur with sufficient
  or abnormal stress

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Regeneration
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Degeneration
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Joint Cartilage Videos

• Anatomy of the Shoulder
• Osteoarthritis

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Cartilage Injury Videos

• Meniscus Tears by Dr. Allen Mishra
• Meniscus Transplantation
• Meniscus Transplant

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Summary

• Cartilage Composition and Structure
• Collagen, Proteoglycan (PG), and Water
• Biomechanics of Cartilage
• Viscoelastic response to loading
• Compression
• Creep and Stress-relaxation
• Permeability
• Tension
• Shear
• Swelling
• Lubrication
• Wear, Regeneration and Degeneration

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For Next Time

• Biomechanics of Tendon and Ligament
• Structure, Function and Loading
• Chapter 4