science

Synovial fluid

Synovial fluid is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Synovial fluid rather than just read about it. In short: Synovial fluid, also called synovia,[help 1] is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its egg white–like consistency, the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.

Key takeaways

Synovial fluid belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Synovial fluid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Synovial fluid from memory before moving on to harder problems.

Reference excerpt

Synovial fluid, also called synovia,[help 1] is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its egg white–like consistency, the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement. Synovial fluid is a small component of the transcellular fluid component of extracellular fluid.

Structure The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joints. Synovial fluid is an ultrafiltrate from blood, and contains proteins derived from the blood plasma and proteins that are produced by cells within the joint tissues. The fluid contains hyaluronan secreted by fibroblast-like cells in the synovial membrane, lubricin (proteoglycan 4; PRG4) secreted by the surface chondrocytes of the articular cartilage and interstitial fluid filtered from the blood plasma. This fluid forms a thin layer (roughly 50 μm) at the surface of cartilage and also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space. The fluid in articular cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held in the cartilage is squeezed out mechanically to maintain a layer of fluid on the cartilage surface (so-called weeping lubrication). The functions of the synovial fluid include:

reduction of friction — synovial fluid lubricates the articulating joints shock absorption — as a dilatant fluid, that possesses rheopectic properties, 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. 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 in the fluid against the synovial membrane forming a barrier against cells migrating into, or fluid migrating out of, the joint space. This function is dependent on the molecular weight of the hyaluronan.

Composition Synovial tissue is sterile and composed of vascularized connective tissue that lacks a basement membrane. Two cell types (type A and type B) are present: Type A is derived from blood monocytes, and it removes the wear-and-tear debris from the synovial fluid. Type B produces hyaluronan. Synovial fluid is made of hyaluronic acid and lubricin, proteinases, and collagenases. Synovial fluid exhibits non-Newtonian flow characteristics; the viscosity coefficient is not a constant and the fluid is not linearly viscous. Synovial fluid has anti-thixotropic characteristics; under conditions of sudden increase in pressure as in some kind of mechanical shock, its viscosity suddenly increases. Normal synovial fluid contains 3–4 mg/ml hyaluronan (hyaluronic acid), a polymer of disaccharides composed of D-glucuronic acid and D-N-acetylglucosamine joined by alternating beta-1,4 and beta-1,3 glycosidic bonds. Hyaluronan is synthesized by the synovial membrane and secreted into the joint cavity to increase the viscosity and elasticity of articular cartilages and to lubricate the surfaces between synovium and cartilage. Synovial fluid contains lubricin (also known as PRG4) as a second lubricating component, secreted by synovial fibroblasts. Chiefly, it is responsible for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. There also is some evidence that it helps regulate synovial cell growth. 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:

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.

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). 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 monosodium urate crystals, calcium pyrophosphate, hydroxyapatite and corticosteroid crystals. Monosodium urate crystals are seen in gout or gouty arthritis and appear as needle-shaped negatively birefringent crystals varying in length from 2 to 20 μm. With negative birefringence, the crystals appear yellow in parallel light and blue with perpendicular light. Calcium pyrophosphate crystals are seen in pseudogout (also known as calcium pyrophosphate deposition disease or, CPPD). These crystals are rod-shaped or rhomboids varying in length from 2 to 20 μm and with positive birefringence (blue with parallel light, yellow with perpendicular light). 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.

Cracking joints

… excerpt ends here. Continue reading the full article.

Illustrations

Synovial fluid: Synovial joint — Synovial joint

Worked examples

Example 1 — a first encounter with Synovial fluid

Start with the simplest possible case. Write down what Synovial fluid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Synovial fluid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Synovial fluid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Synovial fluid

In research: Synovial fluid appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Synovial fluid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Synovial fluid is common in secondary-school and first-year university syllabi. It links to neighbouring topics Body fluids, Joints, so understanding it makes those chapters shorter.
In everyday life: Look for Synovial fluid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Synovial fluid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Synovial fluid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Synovial fluid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Synovial fluid in simple terms?

Why does Synovial fluid matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Synovial fluid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Synovial fluid.