Tissue Mechanics Flashcards
Osseous Tissue
Dense connective tissue, provides structural support to body, takes majority of loading
soft tissues
tendons, ligaments, articular cartilage, intervertebral disk and muscles
made of extracellular collagen fibers
Connective tissue proper
- provide support and stability to musculoskeletal system
- provide connection between bones (ligaments)
- joint lubrication (articular cartilage)
- can withstand large load and strain
- time dependent behavior
- passive structures
Tendons
- connect muscle-bone
- transmit tensile loads, produce joint motion, maintain posture
- collagen fibrils align in direction of tendon
- less elastin = stonger/stiffer
Ligaments
- connect bone-bone
- guide motion, joint stability, static constraint
- collagen fibrils oriented in different direction
- more elastin = larger strain
Extracellular matrix
- 80% of tendon and ligament tissue
- composed mostly of water (70%) type 1 collagen, elastin and proteoglycans
Proteoglycans
- complex macromolecules that consist of central long protein core
- 20% of ground substance
Elastin
Elastic protein that gives tendons and ligaments elastic properties
Collagen fibre
- long, straight, unbranched
- three fibrous proteins stranded together
- provides strength and resilience - act like spring
Tendons and Ligaments Structure
- collagen fibrils connected to each other by the proteoglycans
- collagen fibrils within fascicles parallel
- fascicles in tendons aligned with orientation of tendon
- fascicles in ligaments do not have to be aligned
Load and deformation for tendons and ligaments
- initial nonlinear toe region: displays large strain with only small stresses as wavy collagen straightened out
- Quasilinear region: displays relatively elastic behavior. Fibers straightened out and the stiffness increased rapidly
- Yield and failure region: progressive failures of collagen fibers take place at yield and then small force reduction occurs subsequently the tangent modulus decreases
Articular (hyaline) cartilage
- distribute joint loads, low friction and wear
- matrix, 25% wet weight
- water 75% wet weight
- triphasic material: porous matrix, water and ions
- 4 layers: superficial tangential, middle, deep and calcified
- resist compression loads: charged density (proteoglycans) and viscoelastic
- resistance to compression is primarily due to repulsive negative charges of trapped proteoglycan
- ions, recall repulsive negative charge is responsible for Ha
Intervertebral disc
- anulus fibrosis, nucleous pulposus and cartilaginous endplates
- inflated tire model: pressurized fluid confined by fibers
- aging weakens the strength and structure
- disc is highly viscoelastic, exhibiting creep response and showing hysteresis in strain recovery
- pressure P causes 3 stresses (axial, hoop and radial)
chondrocytes
- predominant cell in cartilage
- live in small chamber (lacunae)
fibrous cartilage
- resists compression
- prevents bone-bone contact
- limits relative movement
- pads within knee joint, b/w pubic bones of pelvis, intervertebral discs
elastic cartilage
- provides support, but tolerates distortion within damage and returns to original shape
- auricle of external ear, epiglottis, auditory canal, cuneiform cartilages of larynx
Hyaline cartilage
- provides stiff but flexible support
- reduces friction between bony surfaces
- between ribs, synovial joints, trachea, bronchi, nasal septum
4 layers of hyaline/articular cartilage
superficial tangential
middle zone
deep zone
calcified cartilage zone
why is cartilage slow healing
due to being poorly vascularized
steps of cartilage growth
- cells in the cellular layer of the perichondrium differentiate into chondroblasts
- these immature chondroblasts secrete new matrix
- as the matrix enlarges, more chondroblasts are incorporated, they are replaced by divisions of stem cells in the perichondrium
- chondrocyte undergoes division within a lacuna surrounded by cartilage matrix
- as the daughter cells secrete additional matrix, they move apart, expanding the cartilage from within
Avascular
lack of blood vessels
three types of cartilage
- hyaline
- elastic
- fibrous
Articular cartilage
- type of hyaline cartilage
- distribute loads over wide area to decrease stress
- allow relative movement of the opposing joint surfaces w minimal friction/wear
- 4 layers
superficial tangential zone
- collagen fibers tend to run tangent to the AC surface
- collagen content is highest and PGC content is lowest
- 10-20% of the thickness of AC
Middle zone
- collagen fibers are distributed randomly
- 40-60% of the thickness of the AC
Deep zone
- about 30% of the thickness of the AC
- collagen fibers are thickest and perpendicular to the surface
- contains highest concentration of the PGCs and the lowest water content
calcified cartilage zone
contains a combination of the cartilage and mineral
Glycosaminoglycan GAG chains
long polymers that are negatively charged under physiological conditions
Water in articular cartilage
- essential to the health of this avascular tissue
- nutrient delivery, waste removal
- most abundant component
- concentrated at the articular surface (80%)
- concentration decreases linearly with increasing depth
swelling pressure
- swelling pressure associated with the repelling forces of tightly packed fixed negatively charged GAG chains
- cause the aggregate to extend and occupy a large solution domain
- increased resistance even in the absence of compressive forces
charge density
- external stress applied on articular cartilage causes internal pressure to exceed swelling pressure, liquid then flows out of tissue
- as fluid flows out, PG concentration increases
-will then lead to increased charged density and increased resistance to compression
aggregate compressive modulus
Ha= stress comp equ / strain comp equ
as cartilage is compressed and fluid is forced out…
- permeability decreases and prevents further fluid loss
- increase of dynamic stiffness
increase of PG content…
leads to higher repulsive negative charges and such higher Ha
tensile equilibrium modulus
not affected by water or PG content, but increases with increasing collagen content
biomechanically, articular cartilage should be viewed as..
a multiphasic material
biphasic material
AC is composed of a porous, permeable collagen PG solid matrix (approx 25% by wet weight) filled by the freely moveable interstitial fluid (approx 75% by wet weight)
Triphasic medium
AC has another phase, ion phase
intervertebral discs
- serves a hydrostatic function in the motion segment, storing energy and distributing loads, this function is reduced with disc degeneration
- exhibit viscoelastic properties (creep, relaxation) and hystersis
functions of CT
establishing structural framework for body
transporting fluids and dissolved materials
provides protection for delicate organs
storing energy reserves
transmit tensile load from muscle to bone
maintain body posture
CT
fat tissue
ligaments
tendons
fixed cells
stationary
responsible for maintenance, repair and energy storage
wandering cells
responsible for the defence and repair of damaged tissues
ligament functions
increase mechanical stability of joint
guide joint motion
acts as static constraint
ligaments connect ……..
bone to bone
Ligaments lose up to 50% of………… from immobility in a few…………….
strength
weeks
restoration of ligaments happends ……………..
slower can take many months
chondroblasts
predominant cell in growing cartilage
produces new matrix until end skeletal stops growing
cartilaginous endplates
thin layer of hyaline cartilage
acts as interface b/w bone and annulus fibrosis and nucleus pulposus
nucleus pulposus
fluid like viscous gel
located at center of disk
consists of water, PG and collagen
anulus fibrosis
set of concentric rings of collagen type 1 sheets
resists internal swelling
intimately connected to the nucleus
internal pressure of the nucleus will maintain………..
net tension
with aging… discs
nucleus dehydrates into viscous gel
unable to develop large hydrostatic forces
annulus thickens and may become mineralized
leads to non-uniform stress distribution
