Cartilage Flashcards
What are three types of cartilage (3)
Hyaline (shiny)
Elastic/Yellow (found in ears)
Fibrocartilage (TMJ, Pubic symphysis, menisci)
Main constituents of articular cartilage
Water & electrolytes (65-85% wet weight)
Matrix (Collagen II, Aggrecan - a PG)
Chondrocytes
Structure of Aggrecan
Aggrecan is a core protein upon which GAG disaccharides bind to
Aggrecan-GAG complex aggregate on a hyaluronate backbone
Superficial tangential zone structure
Collagen is tightly packed/dense and parallel to the surface Fine collagen fibres Chondrocytes are flattened and dense Low PG density Highest water content
Superficial tangential zone function
Parallel arrangments of collagen fibres confers:
High shear strength
High tensile strength
Middle zone structure
Collagen is randomly organised Collagen fibres are thicker Chondrocytes are spherical and low density Higher PG content Intermediate water content
How do collagen fibres and PGs interact and how does it affect the structure
Collagen forms a scaffold upon which PGs are immobilised
This limits water permeability
PGs attract cations which attract water
This water cannot easily leave –> high compressibility
Deep zone
Collagen fibres are perpendicular to articular surface (they cross the tidemark)
Collagen fibres are thickest
Chondrocytes are spherical and arranged in columns perpendicular to articular surface
Highest PG content
Lowest water content
Cartilage function (3)
Helps with load bearing - distributes load over larger area
Shock absorber
Provide joints with lubrication
Factors affecting mechanical properties: Tension
STZ has densest collagen content, so highest tensile strength
Tensile stiffness and strength depend on collagen content
Upon initial loading there is a toe region where small strains are seen
In the linear region, the collagen fibres re-align the match orientation of the tensile load
Factors affecting mechanical properties: Compression
Compression is affected by proteoglycan/GAG content
Speed of compression will determine fluid movement out of cartilage
Factors affecting mechanical properties: Shear loading
Directly related to collagen concentration
Shear loading is due to movement of the bone articular surfaces against one another (e.g. rotation)
Does not result in fluid flow
High shear could lead to failure at bone-cartilage boundary
Factors affecting mechanical properties: Viscoelasticity
Viscoelastic means cartilage is biphasic
Has elastic/solid (collagen & PG) and viscous/fluid (water) components.
Its stress and strain responses are time-dependent
Factors affecting mechanical properties: Time
When loaded quickly in compression, there is no fluid flow due to this low permeability –> increases stiffness
When loaded slowly, there is greater fluid flow
Factors affecting mechanical properties: Permeability
Collagen fibres acts as a scaffold upon which PGs are immobilised
This decreases permeability as it is hard for water to flow through
Permeability: STZ > Middle > Deep zone
As cartilage is compressed, water is pushed out and the permeability decreases because negative charge density increases
Factors affecting mechanical properties: Hydration
With age and pathological conditions permeability decreases and water content increase
This reduces compressive strength as water leaves more easily
Biophysical factors affected by loading (5)
Physiochemical effect Cell deformation Increased hydrostatic pressure Electromechanical effect Fluid transport
Explain Physiochemical effect
Increased Fixed Charge Density in response to compression – PGs are pushed closer together)
Chondrocytes sensitive to extracellular osmolarity, therefore may be able to detect change in osmolarity due to efflux of cations
Explain cell deformation
Change in chondrocyte volume stimulates biosynthesis
Loads may induce shear stress on chondrocytes – stimulates aggrecan synthesis
Explain Hydrostatic Pressure
Hydrostatic pressures created in the synovial fluid on loading modulates aggrecan biosynthesis via stimulating chondricyte cytokeleton
Explain Electromechanical effect
Deformation of the cartilage matrix leads to non-uniform distribution of charge density & electric potentials
List causes of Primary Osteoarthritis (1)
Age-related wear and tear
List causes of Secondary Osteoarthritis (4)
Post-traumatic injury
Infection (septic arthritis)
Obesity (large loads on joint)
Immbolisation
What joints are affected in Osteoarthritis
Weight bearing joints (Knee, Spine, Hip)
Genetic Risk factors for OA
Genes involved in endochondral ossification can increase susceptibility
Biological risk factors for OA (4)
Obesity
Old age
Gender (female)
High bone density
Biomechanical risk factors for OA (2)
Joint malalignment
Joint injury
Diagnostic tools for OA
X-ray
MRI
Arthroscopy
Treatment/Management of OA (4)
Weight loss
Physiotherapy
Pain management (NSAIDs)
Surgery
Surgical treatment of OA (6)
Arthroscopic washout & debridement (early stage only) Osteotomy Cartilage graft Autologous Chondrocyte injection Microfracture Knee fusion/Arthodesis (last resort)
Explain microfracture
Subchondral bone is fractured/penetrated
Triggers infiltration of Mesencyhmal stem cells which can produce chondrocytes
Usually ends up producing fibrocartilage
