Joint Mechanics Flashcards
Synovial Joints
- have articular cavity filled with synovial fluid
- hyaline cartilage covers ends of bones
- space surrounded by
- synovial membrane secretes fluid- fibrous membrane structural
Articular capsule made of fibrous capsule and synovial membrane
- fibrous membrane structural
Solid Joints
Type Types:
1. Fibrous: suture, gomphosis, and syndesmosis (interosseous membrane between radius and ulna)
- Cartilagenous: synchondrosis (hyaline cartilage type II) and symphysis (fibrocartilage type I)
Cartilagenous Joints
Number of synchondroses varies with age
Increasing in number through early teens as new centers of ossification develop
Decreasing in late teens early 20’s as growth plates fuse throughout body
Planar, Hinge, Saddle, Condyloid, Ball and Socket, and Pivot Joint Examples
Planar joint: acromioclavicular joint
Hinge: ulna articulation with humerus; only one plane of motion
Saddle: trapezium and 1st metacarpal
Condyloid: metacarpal phalangeal joint in 2nd digit
Saddle and condyloid: allow flexion, extension, abduction, and adduction thus allowing circumduction, but not long axis rotation
Ball and socket: allows all the above motions + long axis
Pivot: atlantoaxial joint
Maintenance of Posture for the Hip, Knee, and Ankle
ACL: all the weight will rest and prevent hyperextending of the knee
Iliofemoral Ligament: primary limiter of hip hyperextension
Soleus: prevents dorsiflexion at the ankle
Articular Capsule
Made of:
1. Fibrous capsule (stratum fibrosum)
Dense CT, attaches/continuous with periosteum, reinforced by ligaments & tendons, may thicken to form capsular (intrinsic) ligaments
Highly innervated, poorly vascularized
- Synovial membrane (synovium; stratum synovium)
Covers all the non-cartilaginous surfaces inside joint capsule
Highly vascularized, poorly innervated
Synovial Membrane
The cruciate ligaments are intraarticular but extrasynovial
“Housemaids knee”= prepatellar bursitis (between patella and skin)
“Clergyman’s knee”= infrapatellar bursitis (superficial and deep in relation to the patellar ligament below the patella)
Suprapatellar bursa is kept taught during leg extension via the articularis genus muscle
Synovial membranes are found in the form of tendon bursae and tendon sheaths
Histology of Synovial Membrane
Intima: synoviocytes; secretes synovial fluid and macrophages keep the fluid clean; CT with stem cells and blood vessels to bring blood to synoviocytes
The deeper subintima contains relatively unspecialized counterparts such as blood vessels, macrophages, and fibroblasts; mostly collagenous; vascularized (nutrients for synovium as well as avascular cartilage)
Synoviocytes
Within the intima layer of synovial membrane
Type A = macrophage-like, debris-removal (phagocytosis of undesirable substances from synovial fluid)
Type B = fibroblast-like; synthesizes:
hyaluronic acid
lubricin (glycoprotein)
Synovial Fluid
Ultrafiltrate of blood plasma by Type B synoviocytes
Content:
- Hyaluronic acid – traps water in joint spaces
- Lubricin – lubricates joint surfaces
Normally: clear, pale yellow; small volumes
Articular Cartilage
Composed of hyaline cartilage
Devoid of nerves, blood supply and lymphatic channels (physiologically isolated)
Permeable (i.e., porous)
Lubricated by synovial fluid
Zones of Articular Cartilage
SMDCB
1. Superficial Tangential Zone – oblong chondrocytes
2. Middle Zone – round, randomly distributed
3. Deep Zone – columnar; perpendicular to tidemark
4. Calcified Cartilage Zone
5. Bone
Tide Mark – border between deep zone and calcified cartilage
Collagen of Articular Cartilage
STZ: collagen tightly woven into sheets; arranged parallel to surface with few cells
MZ: randomly arranged fibrils; less packed to accommodate PG & water
DZ: large radially-arranged bundles; cross tidemark into calcified zone to anchor articular cartilage to subchondral bone
Proteoglycans of Articular Cartilage
Proteoglycan monomer: glycosaminoglycans (GAGs) attached to protein core
GAG: chondroitin sulfate, keratin sulfate
Superficial: resists stretch to prevent cartilage detachment from bone
Aggrecans are covered with negatively charge endings that stick off and act like sponge and attracted to water
Collagen fibrils and PGs interact to form porous composite and fiber-reinforcing organic solid matrix
Swollen with water
Superficial Tangential Zone
Chondrocytes: small, oval or elongate, long axis parallel to articular surface
Collagen Fibrils: dense sheets; oriented parallel to surface
PGs: low PG
Water: most abundant in STZ and decreases linearly to concentration of about 65% in DZ
Middle Zone
Chondrocytes: round, randomly distributed
Collagen Fibrils: lower than STZ; wavier fibrils, randomly oriented
PGs: high PGs
Deep Zone
Chondrocytes: large, round & in vertical columns perpendicular to tidemark; most active in synthesizing organic components of AC
Collagen Fibrils: large bundles; oriented perpendicular to subchondral bone plate
PGs: med-high PG content
Biomechanics of Articular Cartilage
Mechanical response is related to fluid flow through the tissue.
General Properties:
1. Viscoelastic –
Viscous: resists flow and deformation when load is applied.
Elastic: tendency to stretch instantly and return to normal state when load is removed.
Response is time-dependent (i.e., changes with time)
Stiffness influenced by rate of applied load.
- Anisotropic – response to loading is direction- dependent
Biomechanics of Articular Cartilage: Unloaded to Loaded
Negatively charged GAGs repel each other
Interstitial Fluid (Swelling) Pressure
Total ion concentration is higher inside tissue than in the bathing solution
Ion concentration differences creates a positive osmotic pressure causing matrix to swell.
This swelling support 90% of load for a significant period (several hundreds of seconds) following compressive loading.
Swelling reduces the load supported by the collagen-PG matrix
Biomechanics of STZ, MZ, and DZ
STZ collagen resist AC spread (high tensile strength)
STZ & MZ collagen restrain swelling pressure of PG
DZ collagen fibrils resist AC detachment
Compression Tests: Creep and Stress Relaxation Modes
Stiffness: The higher the stiffness when all fluid flow has ceased, the less the tissue deforms under a given load
Permeability: Indicates the resistance to fluid flow through the cartilage matrix. Highest near joint surface, lowest in deep zone. As cartilage is compressed, permeability decreases
Equilibrium displacement: if permeability is high, fluid flows out of AC easily and equilibrium (i.e., no further deformation) is reached quickly
Flow of fluid in and out of AC is critical for waste removal from and nutrition of AC
Creep Mode: Relatively linear change over time and then repulsion because running out of water
Stress Relaxation: Apply same controlling force over time = stops pushing back against force because water is more equally distributed in matrix
Tensile Test: Direction
Loading results in realignment of collagen fibers
Direction Dependency:
When pulled in the direction of the orientation of the collagen fibers, AC is stiff and strong in tension.
AC is weaker when pulled at an angle to the orientation of the collagen fibers (ANISOTROPY). Collagen fibers will attempt to reorient within the matrix to align to the direction of tension
Middle layers with all the fibers in all different directions; tissue fibers will change angles from random to more lined up to resist deformation
Time dependent situation; if you do this rapidly it does a worse job of realignment of fibers, but if slowly the fiber align much better
Tensile Test: Rate
Loading results in realignment of collagen fibers
Rate Dependency:
When pulled at a slow rate, the collagen network alone is responsible for the tensile strength of cartilage.
When the tissue is loaded rapidly, proteoglycans restrain the rotation of the collagen fibers and tensile strength is reduced.
When you apply shear forces (parallel to surface) find there is no fluid flow and basically little resistance due to collagen fibers and proteoglycans
Shear Tests – Volume changes
No fluid flow in shear = No volume changes.
Fluid has low viscosity, therefore provides little resistance to shear.
Resistance to shear is due to solid matrix (i.e., collagen, PGs, cells, lipids)
Boundary Lubrication
Involves a single monolayer of lubricin adherent to each articular surface
High loads
Low speeds or stationary
Prolonged period
Fluid Film Lubrication
Film of lubricant separates bearing surfaces.
Load on bearing surfaces supported by pressure developed in fluid-film.
Dependent on the viscosity of the fluid due to hyaluronic acid
Loads varying in magnitudes
Surfaces moving quickly
Rheumatoid Arthritis
SM becomes hyperplastic and infiltrated with inflammatory cells (sinovitis)
Thickens into a pannus that releases enzymes that compromise cartilage and bone integrity.
3 Types of Wear on Articular Cartilage
- Interfacial wear – adhesion/abrasion, no separation of joint surfaces by lubricant causing AC-AC adhesion or abrasion
- High impact loading – excessive loads, traumatic event that does not allow sufficient time for internal fluid redistribution
- Fatigue wear – cyclically repeated deformation causes accumulation of microscopic damage that exceeds the repair rate; disruption of the collagen-PG solid matrix and PG washout causing the surface to fray and crack, lesions (fibrillation) that go through the full depth, and erosion
PG washout: decreased stiffness of AC and loses its ability to resist compression
Response to Injury
Repair is limited because cartilage is avascular
location of chondrocytes- nearby proliferate and produce more matrix - territorial matrix (i.e., immediately surrounding lacunae) at first
Supplements, e.g. glucosamine (repair) and chondroitin sulfate (elasticity)
Pain relief
Slow degeneration
OA
Degradation of ECM by matrix degrading enzymes (matrix metalloproteanases) proceeds faster than repair resulting in net loss of collagen and PGs
Joint pain and swelling
Cartilage fibrillation followed by entrance of synovial fluid into cartilage; can cause cysts in bone
Erosion or detachment of cartilage & subchondral bone
Eburnation (exposed bone becomes dense, worn and “polished”)
Vascularization, cracks and osteophyte formation
Components of Articular Cartilage
Cellular:
Chondrocytes- manufacture, secrete, and maintain organic components of ECM
ECM: Mostly type II collagen (10-30%) Proteoglycans Water (60-80%) Inorganic salts, matrix proteins, glycoproteins, and lipids
