CorpusHuman Anatomy
SkeletalFluid

Synovial fluid

Synovia

Also known as: joint fluid, synovia

A small volume of clear, viscous fluid in synovial joint cavities that lubricates the articular surfaces and nourishes the avascular articular cartilage.

Synovial Joint — Structure

Overview

Normal volume (knee)≈1–4 mL
Normal white cell count< 200 cells/µL
Inflammatory effusion≈2000–50 000 cells/µL
Septic effusionTypically > 50 000 cells/µL, mostly neutrophils

Synovial fluid is a dialysate (ultrafiltrate) of plasma to which synoviocytes add hyaluronan (giving it its high viscosity and "egg-white" stringiness — hence syn-ovia, "with egg") and lubricin. Even the knee normally contains only ≈1–4 mL. It is clear and pale yellow with < 200 white cells/µL (mostly mononuclear) and a glucose concentration similar to plasma. Its viscosity falls as shear rate increases (shear-thinning), suiting it to both slow, heavily loaded movement and fast motion.

Functions include lubrication (boundary lubrication by lubricin and hyaluronan, fluid-film and "weeping" lubrication from compressed cartilage), nutrition of chondrocytes by diffusion, and shock absorption. Joint aspiration (arthrocentesis) and analysis of appearance, cell count, Gram stain, culture and crystals is the key test in an acutely swollen joint.

Location

Cavities of synovial joints, bursae and tendon sheaths.

Function

  • Lubricates articular surfaces (very low friction)
  • Nourishes articular cartilage and removes waste
  • Absorbs and distributes loads
  • Contains phagocytic cells that remove debris

Clinical significance

Synovial fluid analysis

Normal/non-inflammatory (OA): clear, viscous, < 2000 WBC/µL. Inflammatory (RA, crystal, reactive): cloudy, low viscosity, 2000–50 000 WBC/µL. Septic: purulent, often > 50 000 WBC/µL, low glucose, positive Gram stain/culture. Polarised microscopy identifies urate (gout) and CPPD (pseudogout) crystals.

Test yourself

Which joint is classified as a saddle (sellar) synovial joint?
  1. Carpometacarpal joint of the thumb
  2. Median atlanto-axial joint
  3. Radiocarpal joint
  4. Acromioclavicular joint

Answer: Carpometacarpal joint of the thumb

The first carpometacarpal joint (trapezium–1st metacarpal) is the classic saddle joint, allowing opposition. The median atlanto-axial is a pivot, the radiocarpal a condyloid (ellipsoid) and the acromioclavicular a plane joint.

More skeletal system questions

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.