Synovial fluid is produced by synoviocytes in synovial joints and has several important functions. It lubricates and reduces friction between cartilage surfaces, acts as a shock absorber, and provides nutrients to cartilage. Synovial fluid composition includes hyaluronic acid, which increases viscosity and lubricates, and lubricin, which is responsible for boundary layer lubrication. Joint lubrication occurs through several mechanisms - fluid film lubrication involves hydrodynamic and squeeze film actions that generate pressure films, boundary lubrication uses an adsorbed lubricant layer, and mixed lubrication is a combination utilizing both fluid film and boundary layers.

1. P R O F ( D R ) B I P U L B O RT H A K U R
D E P T T. O F O RT H O PA E D I C S , S M C H
COMPOSITION OF
SYNOVIAL FLUID AND
MECHANISM OF JOINT
LUBRICATION

2. SYNOVIAL FLUID
Introduction:
 Synovial fluid is a viscous fluid found
in the cavities of synovial joints.
 It is produced by synoviocytes,
which line the inner membrane
of synovial joints, and secretes
the synovial fluid into the
synovial space.

3. Major functions of synovial fluid:
 Lubrication of joint space
 Reduction of friction
 Acts like a shock absorber
 Provides oxygen and nutrients to the articular cartilage
and disc

4. Synovial fluid is an ultrafiltrate of plasma across
synovial membrane which excludes proteins of high
molecular weight.

5. Composition:
• Hyaluronic acid
- It is synthesized by synovial membrane.
- It increases the viscosity and elasticity of
articular cartilages.
- It lubricates the surface
between synovium
and cartilage.

6. • Lubricin
- It is secreted by synovial cells.
- Responsible for boundary layer lubrication,
which reduces friction between opposing
surface of cartilage.
• Proteoglycans
- It is secreted by fibroblasts.

8. MECHANISM OF JOINT LUBRICATION
Lubrication mechanisms are mainly classified as:
• Fluid-film lubrication
- Hydrodynamic lubrication
- Squeeze-film lubrication
• Boundary lubrication
• Mixed lubrication

10. Fluid film lubrication:
• Thin film of lubricant separates the two articular surfaces.
• Pressure developed in fluid film supports the load on the two
articular surfaces.
• Lubrication characteristics are determined by the properties of
the lubricant:
- Rheological properties
- Viscosity and elasticity
- Film geometry
- Shape of gap between surfaces
- Speed of relative motion of two surfaces

11. Hydrodynamic lubrication:
• It occurs when two non-parallel rigid bearing surfaces
move tangentially with respect to each other, the
surfaces being lubricated by a fluid-film.
• Wedge of converging fluid is formed.
• Lifting pressure is generated in the wedge by fluid
viscosity as the motion drags fluid into the gap.

14. Squeeze-film lubrication:
• Occurs when the two weight bearing surfaces move
perpendicular to each other.
• Wedge of converging fluid is formed.
• Pressure in the fluid film results in viscous resistance of
fluid that acts to impede its escape from the gap.
• It is sufficient to carry high loads for short durations.

17. Boundary lubrication:
• Surfaces of cartilages are protected by an adsorbed
layer of boundary lubricant.
- Direct surface-to-surface contact is prevented
- Most of the surface wear is eliminated
- Lubricin (glycoprotein)is responsible for
boundary lubrication.

18. • It is independent of physical properties of lubricant (e.g.
viscosity) and bearing material (e.g. stiffness)
• It primarily depends on chemical properties of the
lubricant.
• It functions under high load at low relative velocities,
preventing direct contact between surfaces.

21. Mixed lubrication:
1. Combination of fluid-film and boundary lubrication
- Temporal co-existence of fluid-film and
boundary lubrication at spatially distinct
locations.
- Joint surface load sustained by fluid-film and
boundary lubrication.
- Maximum friction in boundary lubricated areas ;
maximum load supported by fluid-film.

23. 2. Boosted lubrication
- Shift of fluid-film lubrication to boundary
lubrication with time, over the same location.
- Articular surfaces protected during loading by
ultrafiltration of synovial fluid through the
collagen-proteoglycan matrix.

24. - Solvent component of synovial fluid passes into the
articular cartilage during squeeze-film action yielding a
concentrated gel of hyaluronic acid-protein complex that
coats and lubricates the surfaces.
- As the articular surfaces approach each other, it
becomes difficult for hyaluronic acid macromolecules to
escape from gap between surfaces.

25. 3. Elastohydrodynamic lubrication
- It is associated with deformable articular cartilage.
- Pressure from fluid-film deforms the surfaces.
- Lubricant escapes less rapidly from between the
bearing surfaces.
- Long lasting lubricant film is generated.
- Stress of articulation is lower and more sustainable.
- It greatly increases the load bearing capacity.

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