Synovial Joint Structure and Function

• Synovial joints do not have bones rubbing against each other. Fluid and soft tissues separate and align them.
• Synovial joints feature a joint cavity with synovial fluid, a lubricant rich in albumin and hyaluronic acid for viscosity and lubrication. Synovial fluid also contains phagocytes to eliminate tissue debris.
• Bone surfaces are covered in hyaline articular cartilage. In young, healthy joints, this cartilage is about 2 mm thick.
• The cartilage and synovial fluid reduce friction in synovial joints.
• A fibrous joint capsule encloses the cavity and retains the fluid. It connects to the periosteum of adjoining bones and features an inner synovial membrane that secretes fluid.
• In joints like the jaw, sternoclavicular, and knee, the cartilage forms a meniscus, a pad between bones absorbing shock, guiding movement, and preventing dislocation.
• The major accessory structures within a synovial joint are tendons, ligaments, and bursae.
• Tendons are collagenous connective tissue strips or sheets that attach muscles to bones and stabilize joints.
• Ligaments attach one bone to another.
• Bursae are synovial fluid-filled fibrous sacs located between muscles or where a tendon crosses a bone, cushioning muscles and easing tendon movement. Tendon sheaths, elongated bursae wrapped around tendons, are abundant in the hands and feet.

Types of Synovial Joints

• Six types exist, each with distinct motion patterns based on the articular surfaces of the bones.
• Ball-and-socket joints (e.g., shoulder, hip) have a hemispherical head fitting into a cuplike depression, allowing multiaxial movement, including circumduction.
• Hinge joints (e.g., elbow, knee, interphalangeal joints) allow monaxial movement in one plane, akin to a door hinge, with a convex surface fitting into a concave depression.
• Condyloid (ellipsoid) joints (e.g., radiocarpal joint of the wrist, metacarpophalangeal joints) feature an oval convex surface fitting into a depression, enabling biaxial movement in two directions.
• The saddle joint (e.g., trapeziometacarpal joint at the base of the thumb) resembles a saddle shape, allowing more movement than condyloid or hinge joints.
• Pivot joints feature one bone with a projection fitting into a ringlike ligament of another, allowing rotation on a longitudinal axis (e.g., atlantoaxial joint, proximal radioulnar joint).
• Gliding (plane) joints have flat or slightly concave and convex surfaces, allowing limited sliding movement (e.g., between carpal and tarsal bones, articular processes of vertebrae, sternoclavicular joint).

Bio-Tribology

• Explores friction phenomena in the human body, including joints, lumens, muscles, and tendons.
• Since the 1980s, it and related disciplines have rapidly evolved.
• Focuses on the lubrication mechanisms of human and animal joints and artificial joints, with significant advances in joint lubrication and wear behavior research.

Lubrication of Human Joints

• Common characteristics include load-transmitting bones with blunt ends to provide a bearing area. Bone surfaces are covered with articular cartilage. This cartilage is separated by synovial fluid and enclosed by the synovial membrane, which enables lubrication.
• The coefficient of friction in healthy human joints ranges from 0.001 to 0.03.
• Boundary, weeping, and elastohydrodynamic lubrication theories have been proposed. All these mechanisms play a fundamental role in lubricating action. A squeeze film between opposing cartilage surfaces also contributes. The lubricating mechanism combines squeeze-film action with boundary, weeping, and elastohydrodynamic effects, plus a limited hydrodynamic action for replenishment

Properties of Human Joints

• Articular cartilage lines the articulating surfaces of bones. Its functions are aimed at absorbing wear, minimizing joint friction, and transmitting loads.
• The articular cartilage thickness varies from one joint to another and within a single joint surface. The thickness can range from 4-7 mm in large joints and 1-2 mm in smaller joints.
• Cartilage is composed of cells distributed within a collagen fibril network and a ground substance called chondroitin sulphate filled with a liquid component.
• Two physicochemical processes affecting liquid flow within cartilage are consolidation, which involves decreased liquid content under compression, and swelling, which involves an increase in liquid content.
• Porosity is a key feature found within articular cartilage, as the average pore size is estimated to be about 60°A. These pores are active in weeping lubrication.
• Cartilage surface roughness is coarser than that in engineering bearings. Aged subjects have a waviness pattern.
• Synovial fluid is a clear, yellowish, tacky substance that provides lubrication to freely movable joints
• In a healthy state, synovial fluid forms a sponge-like structure. Its chemical composition comes from a dialysate of blood plasma, the addition of hyaluronic acid, and also consists of a small cellular component. The key property lies in its viscosity due to the hyaluronic acid.
• Hyaluronic acid is a slippery GAG that forms the matrix of connective tissues and acts as a lubricant in joints and the vitreous humor of the eyeball.
• Synovial fluid is a non-Newtonian liquid with shear-thinning properties, meaning its viscosity decreases linearly with shear rate.

Lubrication of Joints

• Articular surfaces in synovial joints experience minimum wear due to lubrication processes.
• Hydrodynamics or fluid-film lubrication involves a viscous fluid film compressed between two surfaces, generating pressure to support the load and prevent touching.
• Clinical trials have indicated that reduced viscosity can cause arthritis.
• Synovial fluid samples are elastic. Under high shearing, molecules align and recoil when shearing stops. In hydrodynamic lubrication, the friction coefficient changes slightly with the change in viscosity.
• Boundary lubrication occurs when surfaces are close, with surface interactions and lubricant molecule prevalence. It is found in natural and artificial joints, and depends on the adsorption of lubricating components on articulating surfaces.
• Potential boundary lubricants such as found in synovial fluid bind to cartilage and provide a protective boundary.
• The physical properties of boundary lubricants minimize wear. Synovial fluid does not separate articulating surfaces in metal-on-metal bearing, leading to wear.
• In synovial joints, glycoprotein is the adsorbed molecule. Its layer is between 1 and 100 nanometers.
• Under pressure, a gel forms on the cartilage surface, and the synovial fluid trapped in depressions maintains boundary lubrication.

Elastohydrodynamic Lubrication

• In this lubrication, or EHL, regime, elastic deformation of solid surfaces plays a role. Synovial joints operate
• Research shows that cartilage roughness can be smoothed out by micro-elastohydrodynamic action. EHL is commonly found in highly loaded oscillating bearing surfaces like ball bearings and artificial hip joints.

Mixed Lubrication

• A regime combining boundary, hydrodynamic, and elastohydrodynamic lubrication. Solid contact occurs frequently and studies on dog ankle joints indicated presence of mixed lubrication.

Synovial Fluid Filtration by Cartilage

• This involves a biphasic mixture theory, looking at compressive situations and the effect of fluid flow. The mechanism directs fluid into cartilage boosting lubrication.

Weeping Lubrication

• Sokoloff explains this by linking cartilage as permeable and soaked in synovial fluid. Applying a load to the soaked cartilage pressurizes the liquid, causing its flow to the surface carrying the load and dropping the friction coefficient.
• This also lowers the pressure of the fluid at the surface. Cartilage is constantly replenished by synovial fluid, and this lubrication bears most of the load, leaving lubricated contact.

Squeeze Film Lubrication

• Unsworth reports increased film thickness and this film can carry loads imposed on joints when walking.
• Steady state loading yields constant friction coefficients, and sudden loading reveals lower initial friction with time.
• These increasing friction coefficient characteristics can be explained in terms of the above squeeze film mechanism. With high loading, the friction increases as its film thins.

Fluid Film Lubrication

• It completely separates articulating surfaces that make up a metal on metal bearing, resulting in low friction and wear. A few forms are as follows:
• Hydrodynamic lubrication occurs when rigid surfaces are not parallel and are separated by a fluid film in relation to each other. The viscosity within the forms between the gaps results in pressure between the surfaces.
• Squeeze film lubrication happens when bearing surfaces move perpendicularly. Viscosity forces out the lubricant, carrying high loads for its period.

Mechanism of Lubrication

• The mechanism in human joints is complex, and hydrodynamic action is negligible for loaded joints.
• For lightly loaded joints, surfaces separate by hydrodynamic pressure generation which replenishes squeeze-film action.
• Boundary lubrication occurs when fluid film thickness is similar to the thickness on articular surfaces. Load is sustained by fluid film lubrication.

Other Types of Lubrication Specific to Synovial Joints:

• Elastohydrodynamic Lubrication: Occurs in articulations where bearing surfaces are not rigid.
• It hypothesised that only the solvent part of the lubricant fluid filters into articular cartilage, thus having boosted lubricaiton

Lubrication Mechanisms in Joints

• Intact synovial joints have a low friction coefficient pointing toward using fluid film lubrication
• Hip joints during walking may exhibit elastohydrodynamic and squeeze film lubrication
• During prolonged standing, fluid film diminishes due to a squeeze film mechanism, but an adsorbed layer helps protection of the joint surfaces.

Biotribology of Total Hip Replacement

• Extreme wear of hip joint cartilage is disabling and requires treatment, which may require hip replacement,
• Particles can provoke inflammatory tissue reactions, resulting in bone loss, and possibly requiring surgical revision.
• Metal-on-metal bearings are rising in popularity as it suits younger and active patients.
• Lubirication is a key factor in wear and friction for hip implants.

Wear and Types of Friction

• The Stribeck curve shows different modes of friction such as boundary, mixed film lubrication, elastohydrodynamic (continuous unbroken EHL) and hydrodynamic(thick unbroken films).
• In the setting of a narrowing gap, the sequence of modes are as follows: boundary, mixed film, EHL and then hydrodynamic.