Week 7 Cartilage Tissue Mechanics

articular (hyaline) cartilage

• flexible and resilient

• collagen only fiber

• spherical chondrocytes

• lacuna of cavity in matrix contains chondrocyte

• synovial joints

fibrocartilage (white)

• resists compression and tension

• very dense 

• rows of thick collagen in matrix 

• annulus fibrosis and knee meniscus 

elastic (yellow) cartilage

highly bendable, matrix contains elastic fibers as well as collagen

outer ear, larynx

articular cartilage characteristics 

• covers load bearing surfaces 

• hyaline 

• no perichondrium (surrounding structure, supportive structure)

• milking- synovial fluid is pushed out and pulled in for exchange of nutrients and waste with pressure on cartilage changes 

• large amounts of proteoglycans and H2O

• low coefficient of frictions 

• susceptible to degenerative changes 

composition of articular cartilage

• articular surface to the tidemark level

• 2 main components

  • extracellular matrix of type II collagen and ground substance

  • cells (chondrocytes)

  • exact composition of each varies by joint

4 zones of articular cartilage

• superficial or tangential zone

• middle or transitional zone

• deep zone

• calcified cartilage zone

superficial or tangential zone

highest concentration of fibrils, aligned parallel to the surface

middle or transitional zone 

fibers appear disorganized 

deep zone

fibers are perpendicular to the surface

calcified cartilage zone

fibers perpendicular to the surface, deeper zones have better orientation to resist secondary tensile loads (deepest zone) since this is where attaches to bone and has the most potential weakness

ground substance

• consists of proteoglycan chains

• negatively charged hydrophilic molecules which repels each other and draws water in 

• consists of 65 to 80% water (wet weight of articular cartilage) 

• hgihest concentration fo PGs in middle zones 

• fluid concentration lowest in deepest regions 

proteoglycan chains

made of glycosaminoglycan (GAG) chains, core protein, hyaluronan 

fibrous cartilage features

• mix of dense CT and articular cartilage

• type I collagen- multidirectional

• sparse fibroblasts and chondrocytes

• moderal PG content

• no perichondrium, largely avascular

function of fibrous cartilage

shock absorption and tensile strength, dissipates load

biphasic material

describes the quality of cartilage that shows its ability to have both a solid phase that is succeeded through proteins that give the cartilage shape and hold tensile resistance, and a fluid phase that is succeeded by water, which also gives resistance to compression

viscoelastic

feature of cartilage that characterizes its material as time-dependent, with initial loading supported by the fluid phase (90% of the load), to a transfer to solid phase over the next 2.5 to 6 hours, and the fluid flow is reversed when the load is removed

lubrication

characteristic of cartilage that explains its smooth gliding surface

biphasic model of cartilage

compressive forces, has a fluid phase and solid phase, the combination of the phases is viscoelastic, damage to the solid matrix impacts both phases, breaks down the stability of the solids, which in turn also decreases the drag on the fluid resulting in marked decrease in compression attenuation

fluid phase 

during compression, fluid moves through cartilage which has a high resistance due to low permeability, this drag acts like a piston to slow down the rate of compression, this fluid dampening protects the solid matrix 

solid phase

elastic behavior of the solids, negative charge- proteoglycans, these molecules become closer together, the charges repel increasing stiffness

synovial fluid

responsible for joint lubrication

boundary lubrication

load bearing surface is coated with a thin layer molecules, mechanism during static positions and high load, lubracin is the molecule in synovial joints

fluid-film lubrication

a wedge of pressurized fluid created when non-parallel surfaces glide, the wedges causes a lifting pressure to keep the irregular surface separated, mechanism during movement 

tension in articular cartilage 

• articular cartilage exhibits cree[s amd stress relaxation 

• initial tension take up the slack in the collagen fibers 

• they then enter a linear region where the collagen fibers are stretched 

nutrition of cartilage

• generally avascular and alymphatic (except maybe calcified layer) so uses synovial fluid exchange- nutritional products, respiratory gasses, systemic signaling molecules, waste products

• uses passive diffusion

• compression-induced convection

• reciprocal loading essential for tissue health

metabolism of cartilage

• chondrocytes

• synthesize, repair, remodel extracellular matrix

• activity regulated by chemical factors and mechanotransduction, electrical fields within cartilage (possible)

• PGs are broken down/synthesize at higher rate that collagen

importance of loading and metabolism

• effects of immobilization on cartilage

• research in animal models

• greater degeneration with rigid immobilization

• cartilage does not completely recover after longer periods of immobilization

• vigorous exercise immediately following immobilzation may increase tissue degeneation, moderate exercise may help

affects of aging on cartilage 

• as skeletal maturity occurs, chondral cell proliferation ceases adn the rate of synthetic activity ceases, the total number of chondrocytes reduces 

• changes start appearing at age 30 

• progressive decline in tensile properties 

osteoarthritis

• cartilage has decreased tensile stiffness and compressive properties

• advanced age is a strong risk factor of primary OA

• in the early phase of disease see increased sythesis and turn over of the matrix to keep up with ongoing damage

• in advanced disease see altered distribution of cells and dying off of cells, so load is more on the solid phase, which leads to more damage, and the subchondral bone may become exposed

primary or idiopathic OA

not sure what has caused the condition, may be localized or generalized in three of more joint areas

secondary OA

know what causes it, posttraumatic, congenital, developmental, etc

repair of cartilage 

• chondrocytes are metabolically active so thet can maintain the matrix, and respond to changes in mechanical stimulus, however are often not able to prevent the matrix loss 

factors defining repair in cartilage

• extent of damage, nature of activity following damage

factors causing a lack of repair in cartilage

• lack of blood flow

• lack of inflammatory processes

• isolation from stem cells

• lack of chondrocyte mobility- can’t move around

• ineffective matrix formation across lesion

repaired cartilage 

• not as good as before 

• see fibrocartilage characteristics 

• eventually will breakdown since its not suited for articular cartilage capacities 

medial treatment of cartilage injury

• oral medications

• corticosterid injections for pain

• viscosupplementation

• regenerative medicine- stem cell transplant

• microfracture and grafting procedures

• arthrocentesis, arthroscopy

• arthodesis (fusion, screw the joint together)

• total joint replacement

rehabilitation of cartilage

after prolonged immobilization of injury

• mindful of degradation of tissue

• greater risk of injury in early phase of 2-4 weeks

• graded joint loading program

• motion is lotion! A/PROM, unloaded, cyclic

post-surgical/procedural treatment of cartilage 

• protocol driven 

• surgical repairs typically have WB restriction 

• injections will have minimal restrictions

• stem cell provider dependent, too early to say…