Knee pain is one of the most common complaints encountered in clinical medicine, affecting people of all ages — from young athletes and teenagers to the elderly population. The causes of knee pain range from minor trauma to serious autoimmune disorders or infectious diseases. For medical students, clinicians, and even patients looking to understand their symptoms, a structured evaluation is critical.
This article uses the clever and easy-to-remember mnemonic "CITRUS" to break down the common causes and clues associated with knee pain.
Introduction: Why Knee Pain Assessment Needs a Systematic Approach
The knee joint is the largest and most complex synovial joint in the human body. It bears the brunt of our daily movement and is vulnerable to injuries, inflammation, infections, and degenerative changes. A correct diagnosis not only ensures proper treatment but can prevent chronic disability, deformity, and delayed recovery.
The mnemonic CITRUS offers a holistic view by covering mechanical, infectious, inflammatory, autoimmune, and systemic clues associated with knee pain.
The CITRUS Mnemonic: Overview Table
| Letter | Clue/Category | Clinical Indicators |
|---|---|---|
| C | Crackles / Colour change | Crepitus, bruising, hemarthrosis |
| I | Infection | Fever, pus, septic arthritis |
| T | Temperature / Trauma / Tick bite | Warmth, history of fall, Lyme disease |
| R | Rash / Redness / Raynaud's | Autoimmune markers (e.g. SLE, RA), vasculitis |
| U | Ulcers in mouth / Systemic features | Aphthous ulcers, photosensitivity, hair loss (e.g. lupus) |
| S | Swelling / Stiffness / STD Hx | Effusion, gonococcal arthritis, ankylosis |
Let’s now unpack each letter of CITRUS in clinical detail.
C: Crackles on Movement & Colour Change (Bruising)
Crackles on Movement (Crepitus)
Suggests degenerative joint disease, especially:
- Osteoarthritis
- Chondromalacia patellae
Typically worsens with:
- Prolonged activity
- Squatting or climbing stairs
Crepitus is the hallmark of cartilage wear and tear, often described by patients as a grinding or grating sound during movement.
Colour Change: Bruising or Ecchymosis
- Indicates trauma, ligament injury, or hemarthrosis
- Can also follow fractures, meniscal tears, or joint aspiration
- Bruising behind the knee (posterior) may indicate a Baker’s cyst rupture
I: Infection
Infective causes must always be ruled out first, especially in acute, swollen, painful knees.
Common Infectious Conditions:
1. Septic Arthritis
- Sudden, red, swollen, and hot knee
- Often monoarticular
- Fever present
- Medical emergency
2. Tubercular Arthritis
- Chronic, insidious onset
- Weight loss, night sweats
- Seen in endemic regions
3. Gonococcal Arthritis
- Sexually active individuals
- Asymmetric, migratory pattern
- Often affects knees, ankles, wrists
4. Fungal and Lyme Arthritis
- Tick bites → Lyme disease → Borrelia burgdorferi
- History of outdoor exposure, erythema migrans rash
Diagnostic Tools:
- Arthrocentesis (Joint aspiration)
- Gram stain, culture
- CBC, ESR, CRP
T: Temperature, Trauma, Tick Bite
Temperature (Fever or Warmth)
- Local warmth: inflammation or infection
- Systemic fever: infectious or autoimmune arthritis
- Raised temperature also points toward septic arthritis, gout, rheumatic fever
Trauma
- Fractures (patellar, tibial plateau)
- Ligamentous injuries (ACL, MCL, PCL, LCL)
- Meniscal tears
- Patellar dislocation
Key clues:
- Swelling within hours (hemarthrosis)
- Instability or inability to bear weight
Diagnosis:
- X-rays
- MRI for soft tissue injuries
- Anterior drawer test, McMurray’s test
Tick Bite
- Lyme disease often affects the knees
- Can present weeks after the bite
- Late-stage: chronic monoarthritis
- May mimic juvenile idiopathic arthritis in children
R: Redness, Rash, Raynaud's Phenomenon
These are systemic signs pointing toward autoimmune or inflammatory conditions.
Redness and Rash
Seen in:
Systemic Lupus Erythematosus (SLE)
- Malar rash, photosensitivity
- Non-erosive arthritis
Rheumatoid Arthritis (RA)
- Symmetrical, small joints → knees in advanced stages
- Morning stiffness > 30 min
Psoriatic Arthritis
- Silver scaly rash, nail pitting
- Dactylitis (“sausage digit”)
Raynaud's Phenomenon
Triggered by cold or stress
Often associated with:
- SLE
- Systemic Sclerosis
- Mixed Connective Tissue Disease
U: Ulcers in Mouth, Photosensitivity, Hair Loss
This refers to mucocutaneous and systemic features that guide toward connective tissue disorders.
Ulcers in Mouth
- SLE: Painless, recurrent ulcers
- Behçet’s disease: Oral + genital ulcers + arthritis
Photosensitivity
- SLE hallmark
- Worsening of rash/joint pain with sun exposure
Hair Loss
- Telogen effluvium in systemic inflammation
- SLE-induced alopecia
S: Swelling, Stiffness, STD History
Swelling
- Effusion: Synovitis, trauma, crystal arthritis
- Baker’s cyst
- May be boggy (RA) or tense and warm (septic)
Knee joint tap analysis:
| Type | Appearance | WBC Count | Common Cause |
|---|---|---|---|
| Normal | Clear | <200 | Physiologic |
| Non-inflammatory | Straw-colored | 200–2,000 | OA, trauma |
| Inflammatory | Cloudy | 2,000–50,000 | RA, gout |
| Septic | Purulent | >50,000 | Bacterial arthritis |
Stiffness
- Morning stiffness >30 mins = Inflammatory (RA, SLE)
- Stiffness after inactivity = Osteoarthritis
STD History
Sexually transmitted diseases like gonorrhea or chlamydia can lead to:
Reactive arthritis (Reiter’s syndrome)
- Conjunctivitis, urethritis, arthritis triad
- HLA-B27 association
- Common in young, sexually active
- Migratory pattern
Differential Diagnosis Based on CITRUS
| Mnemonic Letter | Example Diagnosis |
|---|---|
| C | Osteoarthritis, Hemarthrosis |
| I | Septic arthritis, TB arthritis |
| T | ACL tear, Lyme disease |
| R | SLE, Psoriatic arthritis, RA |
| U | Behçet’s disease, SLE |
| S | Reactive arthritis, Gonococcal arthritis |
Red Flag Signs in Knee Pain
- Hot, red, swollen joint with systemic fever → Septic arthritis
- Sudden inability to bear weight post trauma → Ligament tear/fracture
- Night pain with swelling in young patients → Bone tumors
- Recurrent joint pain with rash or ulcers → Autoimmune disease
- Migratory arthritis with STD history → Gonococcal arthritis
Knee Pain Diagnostic Toolkit
| Test | Purpose |
|---|---|
| X-ray Knee | Bony changes, osteoarthritis, fracture |
| MRI Knee | Ligament tears, meniscus injury, soft tissue |
| Arthrocentesis | Synovial fluid analysis |
| CBC, ESR, CRP | Inflammatory markers |
| ANA, RF, Anti-CCP | Autoimmune screen |
| HLA-B27 | Seronegative spondyloarthropathies |
| Urethral swab / STD Panel | Reactive or gonococcal arthritis |
Treatment Strategies Based on Cause
| Cause | Treatment |
|---|---|
| OA | NSAIDs, weight reduction, physiotherapy |
| RA/SLE | DMARDs (methotrexate, hydroxychloroquine), steroids |
| Septic arthritis | IV antibiotics, joint drainage |
| Trauma | RICE protocol, orthopedics referral |
| Gout | Colchicine, NSAIDs, allopurinol for prophylaxis |
| Lyme disease | Doxycycline or amoxicillin |
| Psoriatic arthritis | Methotrexate, biologics |
Frequently Asked Questions (FAQs)
Q1. What does crepitus in the knee indicate?
It often points to cartilage degeneration, commonly seen in osteoarthritis.
Q2. Can a sexually transmitted infection cause knee pain?
Yes. Gonorrhea and chlamydia can lead to gonococcal arthritis or reactive arthritis.
Q3. When should I suspect autoimmune disease in knee pain?
If joint pain is accompanied by rash, mouth ulcers, morning stiffness, hair loss, or Raynaud’s phenomenon, think SLE or RA.
Q4. What is the urgency of treating septic arthritis?
It is a medical emergency. Delayed treatment can lead to joint destruction or sepsis.
Q5. How do I distinguish between osteoarthritis and rheumatoid arthritis of the knee?
OA affects older individuals with activity-related pain and asymmetric joints, whereas RA shows morning stiffness, symmetrical involvement, and systemic signs.
Conclusion: CITRUS Brings Clarity to Knee Pain Diagnosis
The CITRUS mnemonic is more than a memory aid; it is a clinical lens through which knee pain can be evaluated with diagnostic precision. Whether it’s a trauma-related injury, autoimmune flare, infection, or degeneration, the clues hidden in CITRUS can steer clinicians toward the right investigation and treatment plan.
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