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Basic sciences: Synovial joint

  1. Synovium
     
    Type A synovial cells, containing numerous mitochondria in cytoplasm are macrophage like phagocytic cells, primarily concerned with phagocytosis of joint debris.
     
    Type B synovial cells, which resemble fibroblasts and contain endoplasmic reticulum are primarily responsible for the secretion of hyaluronic acid, protein and prostaglandins of synovial fluid.
  2. Synovial Fluid
     
    Hyaluronic acid gives synovial fluid its thixotropic (flow rate dependent) non-Newtonian viscosity (i.e. viscosity changes according to rate of shear), and lubricating property.

Fluid Kinetics

  1. Newtonian Fluid
     
    Newtonian fluid is a fluid whose viscosity is constant in relation to rate of shear changes. Examples water
  2. Non-Newtonian
     
    Non-Newtonian Fluid is a fluid whose viscosity changes as the rate of shear changes
     
    Thioxotropic
     
    Fluids whose viscosity decreases with increased rates of shear. Eg. Synovial fluid
     
    Rheotropic
     
    Fluids whose viscosity increase with increased rates of shear eg gypsum paste

Synovial Fluid

  1. It is an ultradialysate of blood plasma transudated from synovial capillaries to which hyaluronic acid - protein complex (mucin) has been added by synovial B cells
  2. It is clear viscous yellow fluid which does not clot on standing since it contains no fibrinogen.
    1. Viscosity depends on hyaluronic acid & is lowered in ageing, osteoarthritis and following trauma
    2. It contains 96% water and 4% solute and a pH of 7.3 -7.6 which is reduced in osteoarthritis and after trauma.
    3. Laboratory analysis shows

Culture

Sterile

Cell count

<100 / mm3

Cell type

Mostly lymphocytes & monocytes

Specific gravity

1.008-1.015

Protein content Albumin: Globulin

20-200 mg/mL 1.5:1

Glucose

>75% of serum concentration

Crystals

Absent

 

Synovial Fluid Analysis (Arthrocentesis)

 

Group

Appearance

Cell Count/μL

Neutrophils

Typical Diagnosis

Normal

Clear, colorless to straw colored, good string, low volume

<100

<25

None

I (Noninflammatory)

Clear, straw colored,

200-2,000

<25

Degenerative arthritis, internal derangement, trauma, aseptic necrosis

IIA (Mild inflammatory)

Faintly cloudy moderate string

1,000-
10,000

25-50

Systemic lupus erythematosus and other immune complex arthritides, mild rheumatoid arthritis

IIB (Severe inflammatory)

Translucent
but cloudy, yellow, drops like water

2,000-
75,000

50-75

Moderate to severe rheumatoid arthritis gout, pseudogout, tuberculosis

III (Septic)

Creamy. Opaque. Poor string

>50,000

>80

Acute gout, bacterial infection


Articular cartilage is avascular, alymphatic and aneural structure.

 

It contains

  1. Water 65 to 80 %
  2. Collagen type-2. This gives tensile strength to articular cartilage
  3. Proteoglycan. This gives compressive strength to articular cartilage. Main proteoglycan is choindrotin sulfate. Others are keratin sulfate, dermatan sulfate and heparin sulfate.

Changes in articular cartilage with ageing and with Osteoarthritis.

 

Ageing is most common risk factor for osteoarthritis but it is not synonym with osteoarthritis. Ageing is a physiological phenomenon and osteoarthritis is a disease. DIFFERENCEs between Ageing and Osteoarthritis are
 

 

AGEING

OSTEOARTHRITIS

Water content

collagen

Normal

Abnormal

proteoglycan

 

 

synthesis

Enzymatic degradation

Net

Cathepsin B& D,IL-1 induced chondrolysis

No change





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