Human Bites & Fight Bites

Human saliva contains up to 10 billion (10^9) bacteria per mL, with a diverse mix of aerobes and anaerobes. The unique "gliding" mechanism of the extensor tendon over the MCP joint acts as a one-way valve, sealing the inoculum deep within the joint once the fist is relaxed. This creates a closed-loop abscess within the joint capsule that destroys cartilage (chondrolysis) within 48 hours.

The classic patient is a young adult man presenting hours to days after a fight, with a deceptively small (3-5 mm) laceration over the dorsum of the third, fourth or fifth metacarpophalangeal joint. The history is frequently concealed ("I cut it on a fence").

• Early (first 24 hours): Localised tenderness over the MCPJ, minimal swelling, often no erythema. The wound may already be sealed by gliding of the extensor hood. This benign appearance is the trap.
• Established infection (24-72 hours): Increasing dorsal swelling, erythema, throbbing pain, and the cardinal sign of pain on passive movement of the MCP joint, indicating septic arthritis.
• Late / neglected: Spreading dorsal cellulitis, purulent discharge, fixed flexion from joint destruction, and systemic features. Crepitus or pain out of proportion raises the spectre of necrotising infection.

Always assume a wound overlying the MCPJ in this demographic is a clenched-fist injury with joint penetration until surgical exploration proves otherwise.

Screening

• The Story: Often vague ("cut on metal").
• Hand Position: Was the hand a fist?
• Time: Delayed presentation is common (pain increases overnight).
• Tetanus/Vaccination: Status.

Assume any laceration over the MCPJ is a fight bite.

Examination

• Location: Usually 3rd (Middle) or 4th (Ring) MCPJ (prominent heads).
• Wound: Often small (3-5mm). May be sealed.
• ROM: Pain on passive motion (Septic Arthritis).
• Tendons: Assess extensor mechanism stability (sagittal band injury).
• Neurovascular: Digital nerves.

Look for "gas" or crepitus (rare but ominous - necrotizing fasciitis).

A small, infected, painful wound over the dorsum of the hand has several mimics. The clinical task is to decide which patients have a joint or deep-space process that mandates theatre versus those who can be managed with antibiotics and observation.

Infection-Related Complications

Structural Complications

• Tendon Rupture: May occur from initial injury or secondary to infection. Extensor tendons most vulnerable at MCP level.
• Extensor Subluxation: Sagittal band disruption allows tendon to sublux between metacarpal heads. May need surgical repair.
• Joint Stiffness: Common after septic arthritis. Early mobilization crucial once infection controlled.
• Contractures: Collateral ligament shortening if immobilized in extension. Splint in intrinsic plus position.

Worst-Case Scenarios

In severely neglected cases with immunocompromise or significant delay:

• Amputation: May be required for uncontrollable infection or extensive tissue loss
• Hand function loss: Permanent grip weakness and finger stiffness
• Systemic sepsis: Life-threatening if infection spreads

Outcomes by Presentation Time

Injury Pattern and Prognosis

• Simple bite wound: Good prognosis with early antibiotics
• Clinched fist injury (CFI): Worse prognosis due to joint penetration and contamination
• Joint involvement: Even with treatment, 20-30% develop some permanent stiffness

Factors Predicting Poor Outcome

Negative prognostic factors include:

• Delay in presentation: Greater than 24 hours significantly worsens outcomes
• MCP joint penetration: Risk of septic arthritis and cartilage destruction
• Eikenella corrodens: Often resistant to empiric therapy, may be missed
• Patient comorbidities: Diabetes, immunosuppression, alcoholism
• Incomplete debridement: Retained contamination leads to persistent infection

Long-Term Functional Outcomes

Most patients treated appropriately within 24 hours return to full hand function. Those presenting late or with joint involvement may experience:

• Reduced grip strength (20-40%)
• Limited MCP range of motion
• Cold intolerance
• Persistent pain

• Routine arthrotomy vs selective exploration. Some authors advocate formal arthrotomy for every wound overlying the MCP joint; others reserve it for cases with proven capsular breach, air on imaging, established septic arthritis, or failure of antibiotics. The safe exam position remains: if the joint may be involved, explore it.
• Antibiotic duration. Prophylaxis after a clean early bite (commonly 3-5 days) is distinct from treatment of established septic arthritis or osteomyelitis (often weeks, IV then oral). No high-quality trial defines the optimal duration for the established-infection group.
• Imaging in the clean wound. Plain radiographs are universally recommended to exclude fracture, tooth fragment and intra-articular air. The role of early MRI or ultrasound in the equivocal, apparently clean wound is undefined and rarely changes acute management.
• Negative-pressure wound therapy and distraction fixation. Case reports and small series describe NPWT and joint-spanning distraction for severe established infection, but these are adjuncts, not first-line, and lack comparative evidence.
• Bloodborne-virus testing thresholds. Whether to test asymptomatic recipients of blood-free bites is contested; current synthesis favours universal hepatitis B follow-up but selective HIV/HCV testing based on visible-blood exposure.

Global Epidemiology

Human bites are the third most common mammalian bite presenting to emergency departments worldwide, after dog and cat bites. They cluster in young adult males, often around interpersonal violence and alcohol, with most injuries to the hand. In the Talan multicentre series, 70% of those involved were young adult men and 56% of injuries were clenched-fist injuries. Under-reporting is common because patients conceal an assault mechanism; delayed presentation (beyond 24 hours) is consistently associated with higher complication rates across published series from Europe, North America, Africa and Asia.

Guidelines Side by Side

The consistent global message: amoxicillin-clavulanate is first-line, cephalexin and clindamycin monotherapy are inadequate, hand bites are left open, and a wound overlying the MCP joint mandates surgical exploration.

Evidence and Registry Notes

There is no implant registry for bite injuries, but the evidence base is anchored by a Cochrane systematic review (Medeiros and Saconato) confirming antibiotic prophylaxis reduces infection after human and hand bites, the Talan microbiology series defining the organisms, and surgical series (Chadaev) showing outcome tracks with depth of joint and bone involvement.

Bloodborne Virus Assessment (Global Principle)

• Hepatitis B: Assess immunisation status of the person bitten; offer vaccination or immunoglobulin where indicated, as HBV can transmit via mucosal or non-intact-skin exposure.
• HIV / Hepatitis C: Transmission requires visible-blood exposure and is negligible from ordinary (blood-free) saliva; reserve testing and post-exposure prophylaxis discussion for bloody-saliva exposures.
• Consent and follow-up: Source and recipient testing requires informed consent; baseline and repeat serology where risk is identified.

High-Resource vs Limited-Resource Practice

• High-resource: 24-hour theatre access, formal arthrotomy under regional/general anaesthesia, IV co-amoxiclav, hand-therapy-led rehabilitation, and structured bloodborne-virus pathways.
• Limited-resource: Emphasis on meticulous bedside debridement and irrigation, oral amoxicillin-clavulanate (oral shown equivalent to IV for uncomplicated early bites in the Zubowicz trial), early referral of any joint-penetrating injury, and tetanus cover, which remains a priority where immunisation coverage is incomplete.

Documentation and Prevention

These injuries carry medicolegal weight: record the time and mechanism, clinical findings (with photographs where consented), treatment given, and the prognosis discussed. Prevention centres on reducing alcohol-related violence and on public awareness that a punch to the mouth is a high-risk inoculation injury, not a trivial graze.