Biomechanics of Articular Cartilage and Its Response to Loading

OPBSBiomechanics

Must Know

• Cartilage carries joint load with minimal friction and wear.
• Its compliance spreads contact force over a large area, lowering pressure on cartilage and bone.
• Bone remodels in response to stress. Cartilage does not.
• Joint contact forces are several times the external load because muscles have short moment arms.
• Contact pressure equals contact force divided by contact area.
• Pressurised interstitial fluid carries almost all of the contact load.
• Hydrostatic fluid pressure is both the main load-bearing mechanism and the main stimulus to chondrocytes.

Joint Forces

Elbow example	Value
Hand weight and moment arm	10 N at 30 cm, moment 300 N cm
Biceps moment arm	6 cm
Biceps force	50 N, five times the load
Elbow contact force	40 N, four times the load

• Thumb and shoulder can reach 10-fold multiplication of external force.
• Daily activities give contact pressures of 1 to 6 MPa.
• Strenuous sport may reach 12 MPa briefly, on limited evidence.
• Cartilage and subchondral plate may crack above 15 MPa.

More on worked forces

• The joint contact force line passes through the axis of rotation, so it adds no moment.
• A 500 N weight per hand in a biceps curl gives about 2,000 N at each elbow.
• In a squat at 60 degrees, quadriceps force is about 1,600 N, spread over about 650 mm2 of patellofemoral contact.
• One MPa is about 145 pounds per square inch.
• Moment equals force times perpendicular distance from the axis of rotation.
• Lower limb joints carry larger loads and have larger articular surfaces.

Composition

Component	Wet weight	Dry weight
Interstitial fluid	60 to 85 percent	Not applicable
Collagen	15 to 22 percent	50 to 80 percent
Proteoglycan	4 to 7 percent	15 to 30 percent
Chondrocytes	Under 10 percent of volume

• Type II collagen forms fibrils of 20 to 200 nm diameter. Types VI, IX and XI are also present.
• Chondroitin and keratan sulfate carry negative charge, giving fixed charge density.
• Superficial zone has the most collagen and water and very little proteoglycan.
• Collagen and water fall with depth.
• Superficial fibres run parallel to the surface along split lines. Deep fibres are perpendicular to bone.
• Thickness ranges from under 1 mm in small upper limb joints to about 3 mm in large joints.

More on ultrastructure

• Split lines are seen by pricking the surface with a needle and may follow lines of greatest curvature.
• Middle zone fibrils form two families arching from the surface that merge in the deep zone.
• Superficial zone protein may aid lubrication and wear resistance.
• Link protein ties aggrecan to a hyaluronan backbone, forming complexes of millions in molar mass.
• Patella and tibial plateau can reach 7 mm.

Fluid Pressurisation

• Hydraulic permeability (k) is low, so fluid meets high frictional drag through the matrix.
• Interstitial fluid moves about 0.1 µm per second. A chondrocyte is about 16 µm across.
• Fluid loss in daily activity is negligible, under 6 percent of volume after strenuous exercise.
• Fluid pressurisation stiffens cartilage under dynamic load.
• Under sustained load fluid pressure subsides and strain increases.
• Physiological loading strains cartilage by about 10 percent of thickness.

Modulus	Condition	Value
Dynamic (Edyn)	Cyclic loading, fluid pressurised	About 37 MPa (patellofemoral)
Equilibrium (Eeq)	Sustained load, fluid pressure gone	About 0.4 MPa
Eeq after proteoglycan digestion	Proteoglycan removed	About 0.003 MPa

• Edyn is nearly 100 times Eeq.

More on the patellofemoral test

• Cyclic pressure of 3.3 MPa at 1 Hz gave no more than 9 percent cyclic thickness change.
• Edyn is estimated as applied pressure divided by compressive strain.
• Fluid flows from loaded high pressure regions to unloaded low pressure regions.

Roles of Proteoglycan and Collagen

Component	Mechanical role
Proteoglycan	Donnan swelling pressurises the collagen mesh. Ground matrix stops fibrils buckling
Collagen fibrils	Resist tension. Block transverse expansion when fluid is pressurised
Interstitial fluid	Carries most load and keeps friction low while pressurised

• Fixed negative charges draw in more cations (Na+) than anions (Cl−).
• The higher internal ion concentration draws in water. This is Donnan osmotic swelling.
• Donnan swelling supplies about 60 percent of Eeq.
• Fully digesting proteoglycan cuts Eeq about 100-fold.
• Collagen buckles in compression and carries load only in tension.
• Fluid pressure preserves volume, so collagen restraining sideways bulge keeps height. Hence Edyn greatly exceeds Eeq.
• Models show tensile matrix stress through the full thickness, aligned with collagen.

More on modelling

• Stress cannot be measured directly. Strain can be measured by imaging.

Friction and Wear

• Friction coefficient is frictional force over normal force.
• Cartilage on cartilage µ can be as low as 0.02, in synovial fluid or saline.
• Under constant load µ rises as fluid pressure subsides, from 0.02 to 0.28 over 20,000 s.
• With 80 percent fluid in each layer, collagen meets collagen over only 4 percent of contact.
• Theoretical minimum µ is about 0.005 (0.12 times 0.04).
• Delamination is the main mechanical wear mode. Particulate (abrasive) wear is insignificant.
• Delamination starts between superficial and middle zones, the weakest layer in shear.
• Friction coefficient does not rise as osteoarthritis progresses.
• Loading drives wear progression but need not initiate it. Impact injury can be the trigger.

More on wear theory

• Delamination comes from cyclic subsurface stress starting a crack that spreads sideways and up, lifting a lamina.
• Matrix stresses are similar in size to contact pressure.
• Until the 1960s synovial fluid was thought to be a fluid-film lubricant like engine oil. Models failed to support this.
• Chemical wear is enzymatic degradation from cartilage, synovium, ligaments, menisci or fat pad.

Degeneration

• First mechanical sign is loss of compressive stiffness.
• Then water rises, surface fibrillates and swells, delamination and proteoglycan loss follow, then thinning.
• End stage is bone on bone contact with osteophytes, tidemark advance and eburnation.
• Softer cartilage compresses more, shrinking contact area and raising pressure on cartilage and bone.
• Meniscectomy or osteotomy that reduces congruence raises contact pressure by the same mechanism.
• Pain arises from bone, ligament insertions or synovium, as cartilage is aneural.
• Poor repair reflects absent blood supply and wound healing cascade.

Biological Response to Loading

• Loading stimuli include fluid and ion flow, fixed charge density change, hydrostatic pressure and cell deformation.
• In degeneration fluid pressure falls and load shifts to the solid matrix and cells.
• Static compression lowers proteoglycan and protein synthesis.
• Slow cyclic loading suppresses proteoglycan synthesis. Faster cyclic loading stimulates it.

More on the cell environment

• Loading aids convective nutrient transport from synovial fluid.
• Ion flow through the charged matrix produces streaming potentials.
• Chondrocytes live in a hypertonic environment.

Numbers to Remember

• Contact pressure. 1 to 6 MPa daily, 12 MPa sport, damage above 15 MPa.
• Moduli. Edyn about 37 MPa, Eeq about 0.4 MPa.
• Friction. µ about 0.02, theoretical 0.005.
• Fluid flow. About 0.1 µm per second.
• Donnan share of Eeq. About 60 percent.

Memory Aids

• Fast load, stiff cartilage. Trapped fluid makes Edyn about 100 times Eeq.
• Twenty times twenty is four. 20 percent solid on each side gives 4 percent solid contact.
• Cheap mattress. Soft cartilage lets bone feel the floor, raising bone stress.

Exam Traps

• Collagen in cartilage works in tension even under joint compression.
• Low friction depends on fluid pressurisation. Friction climbs as pressure dissipates.
• Fluid exudation in daily activity is negligible because permeability is low.
• Abrasive particulate wear is minor in cartilage. Delamination dominates.
• Elbow contact force is less than biceps force, as the hand load acts the other way.

Related pages. Articular Cartilage Defects of the Knee, Menisci & Discs, Bone as a biomaterial

Source

Hung CT, Ateshian GA. Biomechanics of Articular Cartilage and Its Response to Loading. In. Aaron RK, editor. Orthopaedic Basic Science. Foundations of Clinical Practice. 5th ed. Philadelphia, Wolters Kluwer, 2021.

Practise Type X questions on this topic

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 17 October 2026.