Synovial fluid

Synovial fluid
A typical joint
Details
Identifiers
Latin	synovia
MeSH	D013582
TA98	A03.0.00.031
TA2	1535
FMA	12277
Anatomical terminology [edit on Wikidata]

Synovial fluid, also called synovia,

The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity.^[3] Synovial fluid is an ultrafiltrate from plasma, and contains proteins derived from the blood plasma and proteins that are produced by cells within the joint tissues.

• reduction of friction — synovial fluid lubricates the articulating joints[7]^{[page needed]}
• shock absorption — as a
  rheopectic properties,^[8] becoming more viscous under applied pressure; the synovial fluid in diarthrotic joints becomes thick the moment shear is applied in order to protect the joint and subsequently, thins to normal viscosity instantaneously to resume its lubricating function between shocks.^{[9] [dubious – discuss}
  ]
• nutrient and waste transportation — the fluid supplies oxygen and nutrients and removes carbon dioxide and metabolic wastes from the
  chondrocytes in the surrounding cartilage
• molecular sieving - pressure within the joint forces
  hyaluronan.^[10]

It also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint.

Clinical significance

Collection

Synovial fluid may be collected by syringe in a procedure termed arthrocentesis, also known as joint aspiration.

Classification

Synovial fluid may be classified into normal, noninflammatory, inflammatory, septic, and hemorrhagic:

Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum.

Synovial fluid viscosity

Normal:

• Normal
• Traumatic arthritis
• Degenerative (Osteo) arthritis
• Pigmented villonodular synovitis

Normal or decreased:

• Systemic lupus erythematosus

Decreased:

• Rheumatic fever
• Rheumatoid arthritis
• Gout
• Pyogenic (Septic) arthritis
• Tubercular arthritis
• Less of lubrication in joints

Pathology

Many synovial fluid types are associated with specific diagnoses:^[18][19]

Analysis

Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum.

The cytological and biochemical analysis of human synovial fluid began around 1940 using cadaver-derived fluid and comparing characteristics to those of, for instance, bovine synovial fluid.^[20]

Chemistry

The mucin clot test is a very old approach to determining if an inflammatory infiltrate is present. In this test, acetic acid is added to the synovial fluid specimen. In a normal specimen, this should lead to a congealing of the hyaluronic acid, forming a 'mucin clot.' If inflammation is present, a mucin clot is not formed (the hyaluronic acid is degraded).^[21]

Lactate is elevated in septic arthritis, usually above 250 mg/dL.

Complement factors are decreased in rheumatoid arthritis and lupus arthritis.

Microscopy

Microscopic analysis of synovial fluid is performed to evaluate for cell count and crystals. Crystals include

Hydroxyapatite crystals are small and negatively birefringent. They are usually only detectable with an Alizarin Red S stain.

Corticosteroid crystals may be seen following therapeutic corticosteroid injection into the joint space. They appear blunt, jagged, and show variable birefringence.^[21]

Cracking joints

When the two articulating surfaces of a synovial joint are separated from one other, the volume within the joint capsule is increased and a negative pressure results. The volume of synovial fluid within the joint is insufficient to fill the expanding volume of the joint and gases dissolved in the synovial fluid (mostly carbon dioxide) are liberated and quickly fill the empty space, leading to the rapid formation of a bubble.^[22] This process is known as cavitation. Cavitation in synovial joints results in a high frequency 'cracking' sound.^[23][24]

Etymology and pronunciation

The term synovia (/sɪˈnoʊviə/) came to English around 1640 (the anglicized form synovial is first recorded in the mid 18th century) from Neo-Latin, where it was coined perhaps by Paracelsus from Greek συν- "with" and Latin ovum "egg" and -ia because it resembles egg white in consistency and external appearance.^{[25][26][27][28][29]}

The term synovium is a much more recent pseudo-Latin coinage for what is less confusingly called the synovial membrane. It is not recorded in general dictionaries, and medical dictionaries only explain its meaning, not its etymology, but it is apparently derived from the term synovia, i.e. the obfuscated etymology of mixed Greek and Latin elements of the singular term synovia was misunderstood and the word was erroneously reinterpreted as the plural of the previously non-existent term synovium (perhaps in analogy to other plural terms for liquids such as "waters" for amniotic fluid). If one insists on using this pseudo-Latin term synovium for the synovial membrane, the non-Latinate plural synoviums is better and less confusing than synovia.

References