High Yield Overview

SYNOVIAL FLUID AND SYNOVIUM

Synovium Structure | Type A and B Synoviocytes | Hyaluronic Acid | Boundary and Fluid Film Lubrication

0.1-0.3mmNormal synovium thickness (1-3 cell layers)

3-4 MDaHyaluronic acid molecular weight in normal synovial fluid

0.3-3.5mLNormal synovial fluid volume in knee joint

under 200/μLNormal synovial fluid WBC count

SYNOVIUM STRUCTURE

Intima (Lining)

Pattern1-3 cell layers without basement membrane

TreatmentType A and B synoviocytes

Subintima

PatternLoose connective tissue with vessels

TreatmentFibrous, areolar, or adipose

Type A Synoviocyte

PatternMacrophage-like cell

TreatmentPhagocytosis and debris removal

Type B Synoviocyte

PatternFibroblast-like cell

TreatmentHyaluronic acid synthesis

Critical Must-Knows

Synovium lacks basement membrane - unique feature allowing bidirectional diffusion for joint nutrition
Type A synoviocytes are macrophage-like (CD68+) and remove debris via phagocytosis
Type B synoviocytes are fibroblast-like and synthesize hyaluronic acid for synovial fluid viscosity
Hyaluronic acid (MW 3-4 MDa) provides non-Newtonian viscosity for lubrication
Boundary lubrication (lubricin/PRG4) prevents solid-solid contact at low speeds; fluid film lubrication at high speeds

Examiner's Pearls

"
Normal synovial fluid is acellular (under 200 WBC/μL) with no organisms on Gram stain
"
Septic arthritis: over 50,000 WBC/μL with over 75% neutrophils, positive culture
"
Inflammatory arthritis: 2,000-50,000 WBC/μL, rheumatoid factor may be present
"
Lubrication mechanisms differ by speed: boundary (static/low speed) vs fluid film (high speed)

Clinical Imaging

Imaging Gallery

Synovial fluid promotes pericellular hyaluronan-coat formation and is responsible for differences in adhesion to cartilage. (A) Red blood cell (RBC) exclusion assay showing synovial fluid-mesenchymal — Synovial fluid promotes pericellular hyaluronan-coat formation and is responsible for differences in adhesion to cartilage. (A) Red blood cell (RBC) eCredit: Baboolal TG et al. via Ann. Rheum. Dis. via Open-i (NIH) (Open Access (CC BY))

Electrophoretic pattern of HA in duplicate hSF samples from a 28-year-old donor, 50 year-old-donor, and 72-year-old donor. Mr Molecular weight massCredit: Temple-Wong MM et al. via Arthritis Res. Ther. via Open-i (NIH) (Open Access (CC BY))

A single 400 kDa fl-Dextran molecule in synovial fluid imaged over 50 msec.The Brownian movements of the molecules are tracked. By analyzing, the distances the molecules move between single frames its — A single 400 kDa fl-Dextran molecule in synovial fluid imaged over 50 msec.The Brownian movements of the molecules are tracked. By analyzing, the distCredit: Kohlhof H et al. via Sci Rep via Open-i (NIH) (Open Access (CC BY))

In native cartilage, lubricin bound to the surface facilitates HA aggregation near the surface (A). Lubricin likely entraps HA through entanglements, causing a local increase in viscosity near the tis — In native cartilage, lubricin bound to the surface facilitates HA aggregation near the surface (A). Lubricin likely entraps HA through entanglements, Credit: Bonnevie ED et al. via PLoS ONE via Open-i (NIH) (Open Access (CC BY))

Critical Synovial Fluid and Synovium Exam Points

Synovium Structure

Synovium lacks basement membrane - unique feature among body membranes. The intima (lining layer) is 1-3 cells thick with Type A (macrophage-like) and Type B (fibroblast-like) synoviocytes. Subintima is vascular connective tissue.

Hyaluronic Acid Function

Hyaluronic acid (hyaluronan) is a high molecular weight (3-4 MDa) glycosaminoglycan synthesized by Type B synoviocytes. Provides non-Newtonian viscosity: high viscosity at low shear (shock absorption), low viscosity at high shear (ease of movement).

Lubrication Mechanisms

Two lubrication modes: (1) Boundary lubrication by lubricin (PRG4) adsorbed to cartilage surface prevents solid-solid contact at low speeds. (2) Fluid film lubrication by pressurized synovial fluid separates surfaces at high speeds.

Synovial Fluid Analysis

Normal synovial fluid: Clear, yellow, viscous, acellular (under 200 WBC/μL). Septic arthritis: Purulent, over 50,000 WBC/μL, over 75% PMNs, positive culture. Crystal arthropathy: Monosodium urate (gout, negative birefringence), calcium pyrophosphate (pseudogout, positive birefringence).

Mnemonic

TYPE ABTYPE AB - Synoviocyte Functions

Type A = macrophage-like

Phagocytosis and debris removal

cleY up debris

Type A cells clear waste products

Produces hyaluronic acid

Type B fibroblast-like cells synthesize HA

Essentially no basement membrane

Unique synovial feature for diffusion

A cells have CD68

Macrophage marker on Type A

B cells are fibroblasts

Synthesize matrix proteins

Memory Hook:TYPE AB blood types remind you: Type A cleans (macrophage), Type B builds (fibroblast)

Mnemonic

HYALURONICHYALURONIC - Hyaluronic Acid Properties

High molecular weight

3-4 million Daltons (3-4 MDa)

Yellow clear fluid

Normal synovial fluid appearance

Anionic glycosaminoglycan

Negatively charged, attracts water

Lubrication provider

Non-Newtonian viscosity

Unbranched polymer

Linear chain of disaccharides

Repeating units

D-glucuronic acid + N-acetyl-D-glucosamine

Osmotic properties

Attracts water via Donnan effect

Non-Newtonian

Viscosity decreases with shear rate

Inflammation degrades it

Hyaluronidase and free radicals

Concentration 3-4 mg/mL

Normal synovial fluid HA level

Memory Hook:HYALURONIC = HY molecular weight gives LUBRICation

Mnemonic

BOUNDARYBOUNDARY - Lubrication Mechanisms

Boundary lubrication

Lubricin (PRG4) at low speeds

On cartilage surface

Lubricin adsorbs to articular cartilage

Ultralow friction

Coefficient of friction 0.001-0.01

No direct solid contact

Prevents cartilage wear

Different from fluid film

Two distinct mechanisms

At static or low speed

When fluid film cannot form

Resist compression

Maintains separation under load

hYdrophilic brush

Lubricin structure prevents adhesion

Memory Hook:BOUNDARY keeps cartilage surfaces apart at the boundary between static and moving

Overview and Introduction

The synovium is a specialized mesenchymal tissue that lines the non-cartilaginous surfaces of synovial joints. It produces synovial fluid, which provides nutrition to avascular articular cartilage and enables nearly frictionless joint motion through sophisticated lubrication mechanisms.

Synovial fluid is a dialysate of plasma enriched with hyaluronic acid (hyaluronan), a high molecular weight glycosaminoglycan that provides unique viscoelastic properties essential for joint function.

Synovium Anatomy and Histology

Macroscopic Anatomy

Location and distribution:

Lines joint capsule, intra-articular ligaments, and bone surfaces not covered by cartilage
Does NOT cover articular cartilage or menisci
Extends to bone-cartilage junction (tidemark region)
Forms synovial folds (plicae) and villi that increase surface area

Macroscopic appearance:

Thin membrane (0.1-0.3 mm thickness)
Smooth, glistening surface
Pink to pale red color (vascular)
Highly vascular and innervated

Microscopic Structure

The synovium has two distinct layers:

1. Intima (Lining Layer):

Thickness: 1-3 cell layers (20-40 micrometers)
Cell types: Type A and Type B synoviocytes
NO basement membrane - unique feature allowing bidirectional diffusion
Intercellular gaps: Allow molecules up to 10-20 nm to pass freely
Function: Synovial fluid production, nutrition of cartilage, debris removal

2. Subintima (Sublining Layer):

Structure: Loose connective tissue rich in blood vessels, lymphatics, and nerves
Three subtypes based on predominant tissue:
- Fibrous (collagenous) - most common, near capsule
- Areolar (loose connective tissue) - most common, near bone
- Adipose (fat tissue) - near fat pads
Function: Vascular supply, immune surveillance, structural support

Understanding the lack of basement membrane is critical - this allows efficient nutrient exchange for avascular cartilage.

Synovial Fluid Composition and Properties

Synovial Fluid Components

Normal synovial fluid characteristics:

Volume: 0.3-3.5 mL in knee (varies by joint size)
Color: Clear to pale yellow (straw-colored)
Viscosity: High (due to hyaluronic acid)
Clarity: Transparent (can read newsprint through it)
Cell count: Under 200 WBC/μL (under 25% neutrophils)

Composition:

1. Plasma dialysate (base fluid):

Water: 85%
Electrolytes: Similar to plasma (Na+, K+, Cl-, Ca2+, Mg2+)
Glucose: 90-100% of plasma glucose (50-100 mg/dL)
Small molecules diffuse freely from blood

2. Hyaluronic acid (key component):

Concentration: 3-4 mg/mL (0.3-0.4%)
Molecular weight: 3-4 million Daltons (3-4 MDa)
Source: Synthesized by Type B synoviocytes
Function: Provides viscosity and elasticity

3. Lubricin (PRG4):

Glycoprotein: 227 kDa molecular weight
Source: Synthesized by Type B synoviocytes and chondrocytes
Function: Boundary lubrication, prevents cartilage adhesion
Concentration: 50-400 micrograms/mL

4. Proteins:

Total protein: 1-2 g/dL (one-third of plasma protein concentration)
Albumin: Predominant protein (passes through synovial membrane easily)
Immunoglobulins: IgG, IgM, IgA present in lower concentrations than plasma
Enzymes: Collagenase, hyaluronidase (at low levels normally)

5. Cells:

Total WBC: Under 200/μL (mostly mononuclear)
Cell types: Monocytes, lymphocytes, synoviocytes (shed from intima)
No RBCs in normal fluid

Normal synovial fluid is essentially a plasma ultrafiltrate enriched with hyaluronic acid and lubricin.

Joint Lubrication Mechanisms

Two Primary Lubrication Mechanisms

Joints achieve remarkably low friction (coefficient 0.001-0.01) through two complementary mechanisms that operate under different conditions:

1. Boundary Lubrication:

Active when: Static loading, low sliding speeds, high loads
Mechanism: Lubricin adsorbed to cartilage surface prevents solid-solid contact
Lubricant: Lubricin (PRG4) - glycoprotein
Coefficient of friction: 0.01-0.02

2. Fluid Film Lubrication:

Active when: High sliding speeds, low to moderate loads
Mechanism: Pressurized synovial fluid separates opposing cartilage surfaces
Lubricant: Synovial fluid (water + hyaluronic acid)
Coefficient of friction: 0.001-0.005

3. Additional mechanisms (contribute but less dominant):

Weeping lubrication: Fluid exudes from cartilage matrix under load
Boosted lubrication: HA macromolecules concentrate in gap, increasing viscosity locally
Hydration lubrication: Hydrated phospholipid layers on cartilage surface

Feature	Boundary Lubrication	Fluid Film Lubrication
Active condition	Low speed, high load	High speed, low-moderate load
Lubricant	Lubricin (PRG4)	Synovial fluid (HA + water)
Mechanism	Adsorbed molecular layer	Pressurized fluid film
Friction coefficient	0.01-0.02	0.001-0.005
Example activity	Standing, static loading	Walking, running

The two mechanisms work together to provide lubrication across the full range of joint activities.

Synovial Fluid Analysis - Clinical Pathology

Classification of Synovial Fluid

Parameter	Normal	Non-inflammatory	Inflammatory	Septic
Color	Clear/pale yellow	Yellow	Yellow/cloudy	Purulent/opaque
Clarity	Transparent	Transparent	Translucent/opaque	Opaque
Viscosity	High	High	Low	Very low
WBC count	under 200/μL	200-2,000/μL	2,000-50,000/μL	over 50,000/μL
PMN %	under 25%	under 25%	over 50%	over 75%
Culture	Negative	Negative	Negative	Positive (50-70%)

Key diagnostic thresholds:

Normal: Under 200 WBC/μL
Non-inflammatory (OA, trauma): 200-2,000 WBC/μL
Inflammatory (RA, gout, pseudogout): 2,000-50,000 WBC/μL
Septic: Over 50,000 WBC/μL (classic), but can overlap with inflammatory

Septic Arthritis Diagnosis

Septic arthritis is a surgical emergency. Classic findings: WBC over 50,000/μL, over 75% PMNs, positive Gram stain or culture. However, WBC counts can overlap with inflammatory arthritis. Always correlate with clinical presentation (fever, acute monoarthritis). If suspicion is high, treat as septic until proven otherwise (washout and antibiotics).

Understanding synovial fluid classification guides diagnosis and management.

Pathophysiology of Synovial Diseases

Inflammatory Arthritis (Rheumatoid Arthritis)

Synovial pathology:

Synovial hypertrophy: Intimal lining proliferates (10-20 cell layers thick vs normal 1-3)
Pannus formation: Invasive synovial tissue (granulation tissue) erodes cartilage and bone
Inflammatory infiltrate: Lymphocytes (T and B cells), plasma cells, macrophages in subintima
Neovascularization: VEGF-driven angiogenesis brings more inflammatory cells
Cytokine production: TNF-alpha, IL-1, IL-6 drive inflammation and joint destruction

Synovial fluid changes:

High WBC count: 5,000-50,000/μL (inflammatory range)
Neutrophil predominance: 50-75% PMNs
Low viscosity: Hyaluronic acid degraded by hyaluronidase and free radicals
Low complement: C3 and C4 consumed by immune complexes
Rheumatoid factor: May be present

Osteoarthritis

Synovial pathology:

Mild synovitis: Low-grade inflammation (not as severe as RA)
Fibrosis: Subintimal fibrosis and thickening
Cartilage fragments: Released into joint, phagocytosed by Type A synoviocytes

Synovial fluid changes:

Low WBC count: 200-2,000/μL (non-inflammatory)
Normal to decreased viscosity: HA molecular weight decreases over time
Cartilage debris: May contain fragments

Septic Arthritis

Synovial pathology:

Acute inflammation: Massive neutrophil infiltration
Purulent exudate: Pus formation
Cartilage destruction: Bacterial enzymes and neutrophil proteases degrade cartilage rapidly
Vascular thrombosis: Can lead to bone infarction

Synovial fluid changes:

Very high WBC count: Over 50,000/μL (often over 100,000)
Neutrophil predominance: Over 75% (often over 90%)
Positive culture: Staphylococcus aureus most common
Low glucose: Bacteria consume glucose
Very low viscosity: Complete HA degradation

Evidence Base

Hyaluronic Acid and Joint Lubrication

Balazs EA, et al. • J Rheumatol (1993)

Key Findings:

Hyaluronic acid (3-4 MDa) provides non-Newtonian viscosity to synovial fluid
Viscosity decreases with increasing shear rate (ideal for joint function)
In osteoarthritis, HA molecular weight and concentration decrease
Degraded HA impairs fluid film lubrication, increasing cartilage wear

Clinical Implication: Hyaluronic acid is essential for synovial fluid viscoelasticity and lubrication. Degradation in arthritis contributes to joint dysfunction. Viscosupplementation aims to restore HA concentration and molecular weight.

Lubricin and Boundary Lubrication

Jay GD, et al. • PNAS (2007)

Key Findings:

Lubricin (PRG4) provides boundary lubrication by adsorbing to cartilage surface
Creates hydrated molecular brush that prevents solid-solid contact
Lubricin-deficient mice develop precocious joint degeneration
PRG4 mutations in humans cause CACP syndrome with early arthropathy

Clinical Implication: Lubricin is essential for boundary lubrication at low speeds and high loads. Loss of lubricin contributes to cartilage wear in osteoarthritis. Recombinant lubricin is being developed as a therapeutic agent.

Synovial Fluid Analysis in Septic Arthritis

Margaretten ME, et al. • JAMA (2007)

Key Findings:

Systematic review of diagnostic accuracy for septic arthritis
Synovial fluid WBC over 50,000/μL: sensitivity 62%, specificity 92%
Over 75% neutrophils: sensitivity 77%, specificity 73%
Positive Gram stain: sensitivity 50%, specificity 99%
No single test is definitive - clinical correlation essential

Clinical Implication: Synovial fluid analysis is critical for diagnosing septic arthritis, but WBC counts overlap with inflammatory arthritis. Clinical suspicion should drive treatment decisions. Delayed treatment increases morbidity.

Basic Science Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Synovium Structure and Synoviocyte Types

EXAMINER

"Describe the structure of the synovium. What are the two types of synoviocytes and what are their functions?"

EXCEPTIONAL ANSWER

The synovium is a specialized connective tissue that lines synovial joints, consisting of two layers: intima and subintima. The intima is the lining layer, only 1-3 cell layers thick (20-40 micrometers), and uniquely lacks a basement membrane. This allows bidirectional diffusion of nutrients and waste between blood and synovial fluid. The intima contains two cell types: Type A and Type B synoviocytes. Type A synoviocytes are macrophage-like cells derived from bone marrow monocytes. They express macrophage markers like CD68 and have numerous lysosomes for phagocytosis. Their primary function is to remove debris, dead cells, and immune complexes from the joint. They also present antigens and secrete inflammatory cytokines. Type B synoviocytes are fibroblast-like cells of mesenchymal origin. They express fibroblast markers like vimentin and have extensive rough endoplasmic reticulum for protein synthesis. Their primary function is to synthesize hyaluronic acid, which provides synovial fluid viscosity, and lubricin (PRG4) for boundary lubrication. Type B cells also produce collagen for synovial structure. Type B cells are more numerous, making up 70-80% of intimal cells, while Type A cells constitute 20-30%. The subintima is the deeper layer consisting of loose connective tissue rich in blood vessels, lymphatics, and nerves. It provides vascular supply and structural support. The combination of Type A debris removal and Type B lubricant synthesis creates an optimal joint environment.

KEY POINTS TO SCORE

Synovium has two layers: intima (lining, 1-3 cells thick) and subintima (vascular connective tissue)

Intima uniquely lacks basement membrane - allows bidirectional diffusion

Type A synoviocytes: macrophage-like, bone marrow origin, CD68+

Type A function: phagocytosis, debris removal, antigen presentation

Type B synoviocytes: fibroblast-like, mesenchymal origin, vimentin+

Type B function: hyaluronic acid synthesis (viscosity), lubricin synthesis (lubrication)

Ratio: Type B (70-80%) more numerous than Type A (20-30%)

Subintima: vascular, provides nutrients and structural support

COMMON TRAPS

✗Not mentioning lack of basement membrane (critical feature)

✗Confusing Type A and Type B functions

✗Missing the ratio of Type A to Type B cells

✗Not explaining how synovium provides nutrition to avascular cartilage

LIKELY FOLLOW-UPS

"What happens to the synovium in rheumatoid arthritis?"

"Why is the lack of basement membrane important?"

"What markers distinguish Type A from Type B cells?"

VIVA SCENARIOChallenging

Scenario 2: Joint Lubrication Mechanisms

EXAMINER

"Explain the mechanisms of joint lubrication. How do boundary lubrication and fluid film lubrication differ?"

EXCEPTIONAL ANSWER

Synovial joints achieve remarkably low friction (coefficient 0.001-0.01) through two primary mechanisms: boundary lubrication and fluid film lubrication. Boundary lubrication operates at low speeds and high loads. The key molecule is lubricin, also called proteoglycan 4 or PRG4, a 227 kDa glycoprotein synthesized by Type B synoviocytes and superficial zone chondrocytes. Lubricin adsorbs to the articular cartilage surface via its terminal globular domains. The central mucin domain, which is heavily glycosylated, extends perpendicular from the surface creating a molecular brush structure. This brush is highly hydrated. When opposing cartilage surfaces approach under load, the hydrated brushes create steric and hydration repulsion, preventing direct solid-solid contact. During sliding motion, the hydrated layers slide past each other with very low friction (coefficient 0.01-0.02). Boundary lubrication is critical during static loading, slow movements like getting up from a chair, and high loads that squeeze out fluid films. Lubricin deficiency, such as in CACP syndrome due to PRG4 mutations, leads to precocious joint failure. Fluid film lubrication operates at high speeds and moderate loads. It has two subtypes: hydrodynamic and squeeze film. In hydrodynamic lubrication, sliding motion creates a converging wedge that drags synovial fluid into the narrowing gap, generating pressure that lifts the surfaces apart. In squeeze film lubrication, rapid loading squeezes surfaces together but the viscous fluid resists rapid extrusion, temporarily supporting the load. Hyaluronic acid is critical for fluid film lubrication because of its non-Newtonian viscosity - high viscosity at low shear rates provides shock absorption, while low viscosity at high shear rates reduces drag during movement. Normal synovial fluid contains 3-4 MDa hyaluronic acid at 3-4 mg/mL concentration. In osteoarthritis and inflammatory arthritis, hyaluronic acid is degraded by hyaluronidase and free radicals, reducing both molecular weight and concentration. This impairs fluid film lubrication, increasing cartilage contact and wear during walking and running. The two mechanisms are complementary: boundary lubrication protects during low-speed, high-load conditions, while fluid film lubrication provides ultra-low friction during normal activities. Together they enable the remarkable durability of synovial joints.

KEY POINTS TO SCORE

Two primary mechanisms: boundary lubrication and fluid film lubrication

Boundary lubrication: lubricin (PRG4) adsorbed to cartilage surface

Lubricin creates hydrated molecular brush with steric and hydration repulsion

Boundary active at: low speed, high load, static conditions (coefficient 0.01-0.02)

Lubricin deficiency (PRG4 mutations) causes CACP syndrome and early joint failure

Fluid film lubrication: pressurized synovial fluid separates surfaces

Two subtypes: hydrodynamic (wedge) and squeeze film

Hyaluronic acid provides non-Newtonian viscosity (3-4 MDa, 3-4 mg/mL)

Fluid film active at: high speed, moderate load (coefficient 0.001-0.005)

In arthritis: HA degradation impairs fluid film, increases wear

COMMON TRAPS

✗Not distinguishing boundary from fluid film mechanisms

✗Missing lubricin as the key boundary lubricant

✗Not explaining non-Newtonian viscosity of hyaluronic acid

✗Forgetting to mention when each mechanism operates

LIKELY FOLLOW-UPS

"What is the structure of lubricin and how does it work?"

"Why is hyaluronic acid non-Newtonian and why does that matter?"

"What happens to lubrication in osteoarthritis?"

VIVA SCENARIOStandard

Scenario 3: Synovial Fluid Analysis

EXAMINER

"You aspirate synovial fluid from an acutely swollen knee. Describe how you would analyze the fluid and interpret the results."

EXCEPTIONAL ANSWER

Synovial fluid analysis is critical for diagnosing joint pathology. I would assess the fluid systematically. First, gross appearance: normal synovial fluid is clear to pale yellow, transparent, and highly viscous. Cloudy or purulent fluid suggests infection or inflammation. Bloody fluid indicates trauma or hemarthrosis. Second, I would perform a cell count and differential. Normal synovial fluid has under 200 white blood cells per microliter with under 25% neutrophils. Non-inflammatory conditions like osteoarthritis have 200-2,000 WBC/μL. Inflammatory arthritis like rheumatoid arthritis or gout has 2,000-50,000 WBC/μL with over 50% neutrophils. Septic arthritis typically has over 50,000 WBC/μL with over 75% neutrophils, though there is overlap with inflammatory conditions. Third, I would send for Gram stain and culture. Gram stain is positive in 50-70% of bacterial arthritis cases. Culture should be on blood culture agar and chocolate agar with immediate inoculation. For tuberculous arthritis, Lowenstein-Jensen medium is needed, and lymphocytes predominate rather than neutrophils. Fourth, crystal analysis using polarized microscopy is essential. Monosodium urate crystals in gout are needle-shaped with negative birefringence - yellow when parallel to the compensator axis. Calcium pyrophosphate crystals in pseudogout are rhomboid with positive birefringence - blue when parallel to the compensator. Crystals may be intracellular within neutrophils or extracellular. Fifth, additional tests include glucose (under 50% of serum glucose suggests septic arthritis), lactate (over 10 mmol/L suggests septic), and protein (increased in inflammation). In rheumatoid arthritis, complement levels (C3, C4) may be low due to immune complex consumption. The most critical distinction is septic versus non-septic arthritis. If clinical suspicion is high for septic arthritis based on acute presentation, fever, and elevated inflammatory markers, I would treat as septic until proven otherwise with urgent washout and antibiotics, even if WBC count is in the inflammatory range. Delayed treatment of septic arthritis leads to rapid cartilage destruction and poor outcomes.

KEY POINTS TO SCORE

Gross appearance: clear/yellow (normal), cloudy (inflammatory), purulent (septic)

Cell count: under 200 (normal), 200-2,000 (non-inflammatory), 2,000-50,000 (inflammatory), over 50,000 (septic)

Differential: over 75% PMNs suggests septic arthritis

Gram stain (50-70% sensitive) and culture (blood culture agar, immediate inoculation)

Crystal analysis: MSU (gout, needle, negative birefringence), CPP (pseudogout, rhomboid, positive birefringence)

Glucose: under 50% of serum in septic arthritis

Lactate: over 10 mmol/L suggests septic

TB arthritis: lymphocyte predominance, culture on Lowenstein-Jensen

Septic arthritis is surgical emergency - treat if suspicion high despite WBC overlap

COMMON TRAPS

✗Not mentioning crystal analysis under polarized microscopy

✗Confusing positive and negative birefringence

✗Missing that septic arthritis can have WBC overlap with inflammatory

✗Not emphasizing urgency of septic arthritis treatment

LIKELY FOLLOW-UPS

"How do you differentiate monosodium urate from calcium pyrophosphate crystals?"

"What is the sensitivity of Gram stain for septic arthritis?"

"When would you suspect tuberculous arthritis?"

Management Algorithm