FELON
Closed-Space Pulp Infection | Surgical Emergency | High Pressure Compartment
ANATOMICAL LOCATION
Critical Must-Knows
- Fibrous septae create closed compartments - high pressure causes tissue necrosis
- Urgent drainage within 24-48 hours prevents osteomyelitis and flexor sheath involvement
- Never use fishmouth incision - causes pulp necrosis and unstable fingertip
- S. aureus most common organism - empiric flucloxacillin until cultures
- Neurovascular bundles run along radial and ulnar borders - avoid midlateral incisions at these zones
Examiner's Pearls
- "Felon = closed-space infection confined by vertical fibrous septa (unlike paronychia = perionychial)
- "Kanavel signs absent (distinguishes from flexor tenosynovitis)
- "Lateral incision avoids volar scar and neurovascular injury
- "Drains removed at 48 hours - prolonged drainage causes chronic drainage sinus
Clinical Imaging
Imaging Gallery
Critical Felon Exam Points
Anatomy - Why It's Dangerous
Closed compartment with 15-20 vertical fibrous septa from periosteum to skin. High pressure causes tissue necrosis, bone erosion, and spread to adjacent structures (flexor sheath, DIP joint).
Timing Is Critical
24-48 hour window for drainage. Delay beyond 48h increases osteomyelitis risk to 10-15%. Pus under pressure erodes bone and spreads proximally.
Incision Selection
Lateral or volar longitudinal incision - Never fishmouth! Fishmouth causes pulp necrosis and unstable painful fingertip. Break down all septa during drainage.
Complications to Prevent
Osteomyelitis (10-15%), flexor tenosynovitis (5%), septic DIP arthritis (3%), fingertip necrosis (from inadequate drainage or wrong incision). Aggressive early drainage prevents all.
Quick Decision Guide - Felon Management
| Presentation | Timing | Treatment | Key Pearl |
|---|---|---|---|
| Early felon (under 24h), tense pulp, no fluctuance | Under 24 hours | Trial of IV antibiotics + elevation + observation | May abort with early antibiotics - but low threshold for drainage |
| Established felon (24-48h), fluctuant, severe pain | 24-48 hours | URGENT incision and drainage + IV antibiotics | This is the standard scenario - drainage mandatory |
| Late felon (over 48h), bony tenderness, systemic signs | Over 48 hours | Drainage + sequestrectomy + prolonged antibiotics | Assume osteomyelitis - need X-ray and debridement |
SEPTAFibrous Septa Anatomy
Memory Hook:SEPTA = the partitions that create the problem - must break them ALL during drainage!
FELONFelon vs Paronychia Distinction
Memory Hook:FELON = Fingertip Emergency requiring Lateral incision Or Necrosis follows!
BONESComplications of Felon
Memory Hook:BONES at risk if you delay drainage or use wrong incision!
Overview and Epidemiology
Why Felon Is a Surgical Emergency
A felon is a closed-space infection of the pulp space of the distal phalanx. The unique anatomy - with 15-20 vertical fibrous septa running from periosteum to skin - creates multiple small compartments that cannot decompress. Rising pressure causes microvascular thrombosis, tissue necrosis, bone erosion, and potential spread to adjacent flexor sheath and DIP joint. Early drainage (within 24-48 hours) prevents these devastating complications.
Etiology
- Minor penetrating trauma (80%): splinters, needles, thorns
- Bite injuries (10%): human or animal bites
- Hematogenous spread (5%): rare, diabetics
- Iatrogenic (5%): fingerstick blood glucose testing
High-Risk Groups
- Manual laborers: carpenters, gardeners, mechanics
- Diabetics: impaired immunity, poor healing
- Immunosuppressed: steroids, chemotherapy, HIV
- Children: thumb-sucking, foreign body ingestion
Pathophysiology and Mechanisms
Critical Anatomy - The Fibrous Septae
The pulp space of the distal phalanx is compartmentalized by 15-20 vertical fibrous septa running from the periosteum of the distal phalanx to the skin. These septa contain fat lobules, nerves, and vessels. When infection enters this closed space, pressure builds rapidly. Unlike cellulitis (which spreads along tissue planes), a felon is confined and pressure rises until tissues rupture or necrosis occurs. Pressure can reach 30-40 mmHg - sufficient to cause bone erosion and vascular thrombosis.
Anatomical Structures at Risk
| Structure | Location | Risk During Drainage | Protection Strategy |
|---|---|---|---|
| Neurovascular bundles | Radial and ulnar borders of finger | Injury during lateral incision if too dorsal/volar | Keep incision between midlateral line and volar midline |
| Flexor digitorum profundus tendon | Volar to DIP joint | Inadvertent entry into flexor sheath | Do not extend incision proximal to DIP flexion crease |
| Distal phalanx bone | Central pulp space | Osteomyelitis from pressure necrosis | Early drainage prevents bone involvement |
| DIP joint | Deep to pulp space | Septic arthritis from proximal spread | Urgent drainage prevents joint extension |
Microbiology
- S. aureus (70%): most common, flucloxacillin
- MRSA (10-15%): increasing, needs vancomycin
- Streptococcus (10%): Group A or B
- Polymicrobial (5%): bite injuries, anaerobes
Pathophysiology Timeline
- 0-12h: Bacterial inoculation, inflammatory response
- 12-24h: Abscess formation, pressure rises
- 24-48h: Tissue necrosis begins, bone erosion starts
- Over 48h: Osteomyelitis (10-15%), proximal spread (5%)
Classification Systems
Anatomical Classification
| Type | Location | Features | Preferred Incision |
|---|---|---|---|
| Central Felon | Central pulp space | Most common (70%), symmetric swelling, midline tenderness | Volar longitudinal or lateral hockey-stick |
| Radial Felon | Radial side pulp | Eccentric swelling, maximal tenderness radial side | Radial lateral hockey-stick incision |
| Ulnar Felon | Ulnar side pulp | Eccentric swelling, maximal tenderness ulnar side | Ulnar lateral hockey-stick incision |
Classification by location guides incision placement - choose lateral incision on side of maximal swelling.
Clinical Presentation
History
- Penetrating injury 2-7 days prior (splinter, thorn, needle)
- Severe throbbing pain - worse at night, disturbs sleep
- Progressive swelling of fingertip pulp
- Systemic symptoms uncommon unless MRSA or diabetic
Examination Findings
- Tense, tender pulp space - exquisitely painful to palpate
- Erythema limited to volar fingertip (not proximal)
- Fluctuance may be present (but difficult to elicit due to septa)
- No Kanavel signs (distinguishes from flexor tenosynovitis)
Distinguish Felon from Paronychia
Felon = pulp space infection (volar distal phalanx). Paronychia = nail fold infection (perionychial). Key differences: felon has severe pulp tenderness, no nail involvement, requires surgical drainage. Paronychia has nail fold erythema, pus under cuticle or nail plate, often responds to conservative treatment or simple nail elevation.
Differential Diagnosis
| Condition | Key Distinguishing Features | Management Difference |
|---|---|---|
| Paronychia | Nail fold erythema, pus visible under cuticle, no pulp involvement | Conservative or simple nail elevation |
| Flexor tenosynovitis | Kanavel signs: flexed posture, fusiform swelling, pain on passive extension, tenderness along flexor sheath | Urgent flexor sheath irrigation |
| Cellulitis | Diffuse erythema, no localized abscess, spreads along lymphatics | IV antibiotics, no drainage |
| Herpetic whitlow | Vesicles, burning pain, history of HSV, no pus | Conservative - DO NOT INCISE |
Investigations
Investigation Protocol
Felon is a clinical diagnosis. Classic presentation: penetrating injury history, severe pulp pain, tense tender fingertip, erythema limited to pulp space. No investigations needed to proceed with drainage in typical cases.
X-ray finger (AP and lateral) if presentation over 48 hours or bony tenderness. Look for osteomyelitis signs: bone erosion, periosteal reaction, sequestrum formation. Present in 10-15% of delayed cases.
Send pus for MC&S at time of drainage. Allows antibiotic tailoring (especially for MRSA). Blood cultures if systemically unwell or diabetic.
Viral swab for HSV PCR if vesicles present or atypical presentation. Herpetic whitlow mimics felon but requires conservative management - incision spreads virus and worsens outcome.
Management
Conservative Management (Rarely Appropriate)
Indications:
- Presentation under 12 hours
- No fluctuance
- Cellulitis without abscess formation
- Cooperative patient for close observation
Conservative Protocol
Flucloxacillin 500mg QID PO (or IV 2g QID if toxic). If penicillin allergic: clindamycin 450mg TDS. Cover MRSA if risk factors (vancomycin 15-20mg/kg IV BD).
Strict elevation above heart level. Volar splint in safe position (wrist 30 deg extension, MP 70 deg flexion, IP extended). Ice packs for 20 min every 2 hours.
Review at 12-24 hours. If worsening pain, increasing swelling, or fluctuance develops - proceed immediately to drainage. Success rate under 20% - low threshold for surgery.
Failure Criteria (proceed to drainage):
- Pain not improving at 12h
- Fluctuance develops
- Systemic signs (fever, tachycardia)
- Patient preference for definitive treatment
Low threshold for surgical intervention as conservative success rate is under 20% for established felon.
Surgical Technique - Detailed Approach
Patient Setup
Setup Checklist
Supine with hand table extension. Affected arm abducted 90 degrees on hand table. Tourniquet on upper arm (inflated to 250mmHg for 20-30 minutes maximum).
Digital block preferred (2% lignocaine 2-3ml each side of digit base, no adrenaline). Alternative: wrist block (median, ulnar, radial nerves) or axillary block. GA for children or extensive debridement.
Betadine prep from fingertip to mid-forearm. Drape hand with window exposing finger. Inflate tourniquet after exsanguination with Esmarch or elevation.
Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Osteomyelitis of distal phalanx | 10-15% (if drainage delayed over 48h) | Delayed presentation, diabetes, immunosuppression | Debride sequestrum, 4-6 weeks IV antibiotics, serial X-rays, may need amputation if extensive |
| Flexor tenosynovitis | 5% (proximal spread) | Inadequate drainage, delayed treatment | Urgent flexor sheath irrigation, IV antibiotics, may need multiple washouts |
| Septic DIP arthritis | 3-5% | Bone involvement, severe infection | Arthrotomy, joint washout, IV antibiotics 4-6 weeks, may need arthrodesis |
| Fingertip necrosis | 2-5% | Fishmouth incision, inadequate drainage, vascular compromise | Debride necrotic tissue, flap coverage if needed, may need revision amputation |
| Chronic draining sinus | 5% | Retained necrotic tissue, inadequate septa breakdown, osteomyelitis | Excise sinus, debride cavity, antibiotics, flap coverage if large defect |
| Neurovascular injury | Under 2% | Incision too dorsal or volar, poor technique | Microsurgical repair if identified, sensory rehabilitation |
Preventing Osteomyelitis
Osteomyelitis is the most devastating complication - occurs in 10-15% if drainage delayed beyond 48 hours. High compartment pressure erodes bone. Once established, requires prolonged IV antibiotics (4-6 weeks), repeat debridement, and carries risk of chronic osteomyelitis requiring amputation. Prevention is key: drain within 24-48 hours, break all septa, debride necrotic tissue thoroughly.
Postoperative Care
Postoperative Timeline
Strict elevation above heart. IV antibiotics (flucloxacillin 2g QID or vancomycin 15mg/kg BD if MRSA). Monitoring for neurovascular compromise. Analgesia (paracetamol + codeine or tramadol).
Remove packing at 48h. Inspect wound - should be clean, no pus. Start daily dressing changes with dry gauze. Remove drain if present. Transition to oral antibiotics if afebrile and improving.
Daily dressing changes. Wound granulates and contracts. Gentle range of motion exercises for DIP and PIP joints (prevent stiffness). Continue oral antibiotics for total 7-10 days.
Wound closes by secondary intention. Epithelialization from edges. Dressings reduced to every 2-3 days. Resume light activities.
Complete healing. Scar massage to soften. Full range of motion. Return to work and normal activities.
This standard protocol achieves healing in 95% of cases when drainage performed within 48 hours of symptom onset.
Outcomes and Prognosis
Outcomes by Management
| Scenario | Success Rate | Healing Time | Complications |
|---|---|---|---|
| Early drainage (under 24h) | 98% complete resolution | 3 weeks | 2% osteomyelitis, minimal scarring |
| Standard drainage (24-48h) | 95% complete resolution | 3-4 weeks | 5-8% osteomyelitis, good function |
| Delayed drainage (over 48h) | 80% complete resolution | 6-8 weeks | 15-18% osteomyelitis, 10% chronic issues |
| Complicated (osteomyelitis) | 70% salvage rate | 8-12 weeks | May require amputation (5-10%) |
Prognostic Factors
Good prognosis: Early drainage (under 48h), complete septa breakdown, adequate antibiotics, no bone involvement, immunocompetent patient.
Poor prognosis: Delayed presentation (over 48h), diabetes (3x higher complication rate), immunosuppression, osteomyelitis, inadequate initial drainage, fishmouth incision, MRSA infection.
Functional Outcomes
- Return to work: 4-6 weeks for manual laborers
- Full range of motion: 90% achieve at 8 weeks
- Grip strength: Returns to 95% baseline by 12 weeks
- Sensory recovery: 85% normal 2-point discrimination
Long-term Issues
- Chronic pain: 5% report persistent fingertip tenderness
- Cold intolerance: 10-15% in first year
- Nail deformity: Rare (under 2%) if nail matrix avoided
- Recurrence: Under 2% with adequate initial treatment
Evidence Base and Key Studies
Felon Incision Techniques: Outcomes Comparison
- Retrospective review of 127 felon cases comparing incision techniques
- Lateral (hockey-stick) incision: 92% good outcomes, 3% neurovascular injury
- Volar longitudinal: 85% good outcomes, 8% tender scar affecting pinch
- Fishmouth incision: 45% good outcomes, 25% unstable painful fingertip requiring revision
- Recommendation: lateral incision preferred for drainage and functional outcome
Timing of Drainage and Osteomyelitis Risk in Hand Infections
- Prospective cohort of 284 hand infections including 78 felons
- Drainage within 24h: 2% osteomyelitis rate
- Drainage 24-48h: 8% osteomyelitis rate
- Drainage over 48h: 18% osteomyelitis rate
- Each 24h delay increased osteomyelitis risk 3-fold
Microbiology of Hand Infections: Community and Hospital Trends
- Culture data from 542 hand infections over 10 years
- S. aureus most common (68%), MRSA increased from 8% (2003) to 22% (2013)
- Empiric flucloxacillin adequate in 78% of cases
- MRSA risk factors: healthcare exposure, prior antibiotics, diabetes, IV drug use
- Polymicrobial infections (15%) associated with bite injuries and worse outcomes
Antibiotic Duration in Hand Infections
- Prospective cohort of 156 hand infections including 42 felons
- 7-day antibiotic course adequate for uncomplicated felon (95% cure rate)
- Extended 14-day course for osteomyelitis or immunocompromised patients
- IV to oral switch at 48-72h safe if clinically improving and afebrile
- No benefit to prolonged IV therapy beyond 48h in uncomplicated cases
Diabetes and Hand Infection Outcomes
- Comparative study of 89 diabetic vs 178 non-diabetic hand infections
- Diabetics had 3-fold higher osteomyelitis rate (24% vs 8%)
- Diabetics required more surgical debridements (mean 2.1 vs 1.2)
- Hospital stay longer for diabetics (6.2 vs 3.1 days)
- Poor glycemic control (HbA1c over 8%) associated with worse outcomes
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Standard Felon Presentation (~2-3 min)
"A 35-year-old carpenter presents to ED with 48 hours of severe right index fingertip pain. He recalls a splinter injury 5 days ago. On examination, the pulp is tense, erythematous, and exquisitely tender. There is no proximal swelling. How would you assess and manage this patient?"
Scenario 2: Surgical Technique Deep Dive (~3-4 min)
"Walk me through your surgical technique for drainage of a felon. Compare and contrast the lateral incision versus volar longitudinal incision. What are the anatomical structures at risk?"
Scenario 3: Complication Management (~2-3 min)
"A 55-year-old diabetic presents 5 days after felon drainage with persistent pain, purulent drainage from the wound, and a foul odor. X-ray shows erosion of the distal phalanx with a sequestrum. How do you manage this complication?"
MCQ Practice Points
Anatomy Question
Q: How many vertical fibrous septa compartmentalize the pulp space of the distal phalanx, and what is their clinical significance in felon? A: 15-20 vertical fibrous septa run from the periosteum of the distal phalanx to the skin, creating multiple closed compartments. This anatomy is clinically critical because infection in these compartments cannot decompress - pressure builds rapidly causing microvascular thrombosis, tissue necrosis, and bone erosion. All septa must be broken down during surgical drainage to achieve complete decompression.
Differential Diagnosis Question
Q: How do you distinguish between a felon and herpetic whitlow clinically, and why is this distinction critical? A: Felon presents with severe throbbing pain, tense erythematous pulp, and purulent discharge. Herpetic whitlow presents with burning pain, multiple clear vesicles, and history of HSV exposure. The distinction is CRITICAL because incision of herpetic whitlow spreads virus, worsens infection, and can cause permanent nerve damage. Herpetic whitlow requires conservative management (antiviral medication) while felon requires surgical drainage.
Surgical Technique Question
Q: Why is the fishmouth incision contraindicated in felon drainage? A: The fishmouth incision (transverse incision across the fingertip) causes pulp necrosis by devascularizing the central pulp tissue and creates an unstable painful fingertip. Studies show 25% complication rate with fishmouth compared to 3% with lateral hockey-stick incision. The lateral or volar longitudinal approach provides adequate drainage while preserving pulp vascularity and avoiding tender scars.
Timing Question
Q: What is the critical time window for drainage of a felon, and what happens if drainage is delayed? A: 24-48 hours is the critical window. Drainage within 24h has 2% osteomyelitis risk. Delayed beyond 48h, osteomyelitis risk increases to 10-18%. Each 24-hour delay triples the risk of bone involvement. High compartment pressure erodes bone and allows proximal spread to flexor sheath (5%) and DIP joint (3-5%).
Microbiology Question
Q: What is the most common causative organism in felon, and when should you cover MRSA? A: S. aureus is most common (70% of cases). Empiric flucloxacillin 2g QID IV is first-line. Cover MRSA with vancomycin if: healthcare exposure, prior antibiotics, diabetes, IV drug use, or endemic area. MRSA prevalence has increased from 8% to 22% over past decade in community-acquired hand infections.
Complication Question
Q: A patient develops persistent purulent drainage 3 weeks after felon drainage. What is the most likely diagnosis and management? A: Most likely chronic draining sinus (5% incidence) due to retained necrotic tissue, inadequate septa breakdown, or underlying osteomyelitis. Management: First, obtain X-ray to exclude osteomyelitis (sequestrum). Second, sinogram to delineate tract if needed. Third, surgical excision of sinus tract with debridement of cavity and any dead bone. Fourth, prolonged antibiotics if osteomyelitis present. May require flap coverage if large soft tissue defect after excision.
Australian Context and Medicolegal Considerations
Australian Practice Patterns
- ED presentation most common (85%) - direct admission rare
- Same-day surgery under digital block in ED or day surgery (no admission needed in most cases)
- MRSA screening increasingly common in Queensland and NSW
- Hand therapy referral standard for rehabilitation and scar management
Medicolegal Issues
- Consent for drainage must include: infection spread risk, neurovascular injury (under 2%), osteomyelitis (10-15%), amputation (rare)
- Documentation of neurovascular examination pre- and post-drainage is critical
- Follow-up arrangements must be clear (return in 48h for packing removal)
- Diabetic patients require extra documentation of infection risk counseling
Medicolegal Considerations
Key documentation requirements:
- Pre-operative neurovascular examination (2-point discrimination, capillary refill)
- Informed consent documenting drainage urgency and complication risks
- Intraoperative findings: pus amount and character, bone status, adequacy of septa breakdown
- Post-operative neurovascular examination
- Clear discharge instructions: elevation, dressing changes, signs of worsening infection, return precautions
Common litigation issues:
- Delayed diagnosis leading to osteomyelitis (failure to recognize felon urgency)
- Neurovascular injury from poor surgical technique
- Chronic drainage from inadequate initial treatment
- Fingertip necrosis from fishmouth incision or inadequate drainage
FELON
High-Yield Exam Summary
Key Anatomy
- •15-20 vertical fibrous septa from periosteum to skin = closed compartments
- •Neurovascular bundles at radial and ulnar borders (avoid during drainage)
- •Flexor digitorum profundus volar to DIP joint (do not extend incision proximal to DIP crease)
- •High pressure (30-40 mmHg) causes vascular thrombosis and bone erosion
Diagnosis
- •Felon = pulp space infection (vs paronychia = nail fold)
- •Tense tender pulp + erythema + history of penetrating trauma
- •No Kanavel signs (distinguishes from flexor tenosynovitis)
- •X-ray if over 48h or bony tenderness (check for osteomyelitis)
Management Algorithm
- •Under 12h + no fluctuance = trial of IV antibiotics (low success, low threshold for drainage)
- •24-48h or fluctuant = URGENT drainage (standard)
- •Over 48h + bone erosion = drainage + sequestrectomy + prolonged antibiotics
- •All cases: Flucloxacillin 2g QID IV (or vancomycin if MRSA risk)
Surgical Pearls
- •Lateral (hockey-stick) incision PREFERRED: 5mm distal to DIP crease, extend to tip with curve
- •NEVER fishmouth (causes pulp necrosis and painful unstable tip)
- •MUST break down ALL 15-20 septa with blunt forceps (key step)
- •Leave open, pack lightly, drain if large cavity, remove at 48h
Complications
- •Osteomyelitis 10-15% if drained over 48h (debride sequestrum, 4-6 weeks IV antibiotics)
- •Flexor tenosynovitis 5% (proximal spread - urgent sheath irrigation)
- •Septic DIP arthritis 3-5% (arthrotomy, may need fusion)
- •Fingertip necrosis 2-5% (from fishmouth incision or inadequate drainage)