21.3 Laceration Management, Complex Wounds & Animal Bites

Systematic Wound Evaluation & Pre-Anesthetic Functional Assessment

Minor traumatic soft-tissue lacerations and bite wounds represent common presentations in primary care, urgent care, and emergency departments. Appropriate wound management balances functional preservation, infection avoidance, and optimal cosmetic outcomes.

Step 1: Pre-Anesthetic Neurovascular & Tendon Examination

Infiltrating local anesthetic alters tissue turgor and produces sensory and motor blockade, making subsequent functional testing invalid. Clinicians must perform and document a meticulous distal neurovascular examination PRIOR TO injecting local anesthetics:

1. Digital Sensory Testing (Two-Point Discrimination):
   • Static two-point discrimination testing along the radial and ulnar digital borders using a paperclip or caliper.
   • Normal Threshold: $< 6\text{ mm}$ at the digital pulp.
   • Abnormal Threshold: Discrimination $\ge 10\text{ mm}$ (or comparison to the contralateral uninjured digit demonstrating a $>2\text{ mm}$ disparity) is highly sensitive for digital nerve laceration, mandating microvascular surgical repair.
2. Motor & Tendon Testing Against Resistance:
   • Flexor Digitorum Profundus (FDP): The examiner stabilizes the proximal interphalangeal (PIP) joint in full extension and instructs the patient to flex the distal interphalangeal (DIP) joint against manual resistance.
   • Flexor Digitorum Superficialis (FDS): The examiner holds all adjacent fingers in complete extension to cancel the mass action of the common FDP muscle belly; the patient is then instructed to flex the PIP joint of the injured finger against resistance.
   • Extensor Digitorum Communis (EDC): Assess active extension of the metacarpophalangeal (MCP) and interphalangeal joints against resistance.
   • Clinical Pearl: A tendon that is $90%$ transected can still produce full active range of motion against gravity. Therefore, testing AGAINST RESISTANCE and visualizing the tendon throughout active range of motion is mandatory to detect partial lacerations.
3. Wound Exploration Under Hemostasis:
   • Explore every laceration under a bloodless field (achieved with a sterile finger tourniquet or blood pressure cuff inflated above systolic pressure) through its complete active and passive range of motion. Inspect the full depth of the wound base to exclude foreign bodies (glass, gravel, radiopaque tooth fragments) and joint capsule disruption.

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Pharmacology of Local & Regional Anesthesia

Local anesthetics block voltage-gated sodium channels along neuronal membranes, arresting action potential propagation. They belong to two chemical classes: amides (contain an "i" before the "-caine": lidocaine, bupivacaine, mepivacaine, prilocaine) and esters (procaine, tetracaine, cocaine; metabolized by plasma pseudocholinesterases; associated with allergic reactions to para-aminobenzoic acid [PABA]).

Anesthetic Profiles and Safety Thresholds

The Role of Epinephrine & Digital Block Safety

• Pharmacological Benefits: Adding epinephrine ($1:100,000\text{ to }1:200,000$) produces local $\alpha_1$-adrenergic vasoconstriction. This significantly limits intra-wound bleeding, enhances visualization, delays systemic vascular absorption (increasing maximum allowable doses), and more than doubles the clinical duration of anesthesia.
• Overturning Historical Dogma (The Digital Epinephrine Myth):
  • Historical medical dogma warned that epinephrine in "fingers, toes, penis, and nose" caused end-artery vasospasm and digital gangrene. Massive prospective trials analyzing $>10,000$ digital procedures have completely disproven this myth; historical cases of gangrene were attributable to acidic procaine solutions contaminated with carbolic acid in the early 20th century.
  • Guideline Consensus: Lidocaine with epinephrine is entirely safe for digital blocks, auricular lacerations, and nasal repairs in healthy individuals. It should be avoided only in patients with severe peripheral arterial occlusive disease (Buerger disease, severe Raynaud phenomenon, or compromised microvascular digital replants). If prolonged vasospasm occurs, it can be reversed with local infiltration of phentolamine ($5\text{ to }10\text{ mg}$ diluted in $10\text{ mL}$ saline).

Buffering Local Anesthesia with Sodium Bicarbonate

• Commercial lidocaine solutions are packaged at an acidic pH ($5.0\text{ to }6.5$; or $3.5\text{ to }4.5$ with epinephrine) to prevent spontaneous oxidative degradation of epinephrine, which causes severe burning and stinging pain upon tissue infiltration.
• Buffering Protocol: Mix $8.4%$ Sodium Bicarbonate with $1%$ Lidocaine in a 1:9 volume ratio ($1\text{ mL}$ of $8.4%$ bicarbonate added to $9\text{ mL}$ of lidocaine).
• Mechanism: Neutralizing the solution converts charged cationic lidocaine into uncharged, lipid-soluble free-base lidocaine. This dramatically decreases injection pain and accelerates lipid membrane penetration, speeding the onset of surgical anesthesia.

Topical Anesthetics in Pediatric Wound Care

• LET Gel (Lidocaine $4%$, Epinephrine $0.1%$, Tetracaine $0.5%$): Applied directly onto the open wound base under an occlusive dressing for 20 to 30 minutes prior to repair. Achieves complete dermal anesthesia for facial and scalp lacerations in young children, avoiding needle infiltration anxiety.

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Wound Cleansing, High-Pressure Irrigation & Debridement

Copious wound irrigation is the single most effective intervention to prevent post-repair wound infections, reducing bacterial load below the critical infection threshold ($<10^5\text{ organisms/gram of tissue}$).

High-Pressure Irrigation Mechanics

• Required Pressure: Effective mechanical removal of bacterial bioburden, micro-particulates, and organic debris requires an irrigation pressure of $7\text{ to }8\text{ pounds per square inch (psi)}$.
• Equipment: Achieved by attaching an 18- or 19-gauge angiocatheter (or plastic splash guard) to a $30\text{ to }60\text{ mL}$ syringe. Gravity bulb syringes or IV tubing bags deliver $<1\text{ to }2\text{ psi}$ and are completely ineffective at dislodging bacteria adherent to tissue.
• Irrigation Volume: Deliver $50\text{ to }100\text{ mL}$ of fluid per centimeter of laceration length (minimum $250\text{ to }500\text{ mL}$ for clean wounds; $\ge 1,000\text{ mL}$ for heavily contaminated, crushed, or bite wounds).

Selection of Irrigation Fluid

• Tap Water vs. Sterile Normal Saline: Multiple randomized clinical trials and Cochrane reviews confirm that clean, potable municipal tap water is non-inferior to sterile $0.9%$ normal saline in post-closure infection rates. Clean tap water or sterile saline may be used interchangeably.

[!CAUTION] STRICT CONTRAINDICATION: CYTOTOXIC ANTISEPTICS IN OPEN WOUNDS: Hydrogen peroxide, povidone-iodine surgical scrub (Betadine scrub), chlorhexidine, and isopropyl alcohol are STRICTLY CONTRAINDICATED inside open traumatic wounds. These chemical detergents are non-selective cellular toxins that lyse host fibroblasts, keratinocytes, and tissue macrophages, impairing innate defense mechanisms, provoking tissue necrosis, and significantly increasing post-closure infection rates while worsening scar formation. Non-detergent povidone-iodine ($10%$) or chlorhexidine is indicated solely for prepping the intact skin surrounding the wound edges, never for instillation into the open wound base.

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Wound Closure Timing, Suture Selection & Removal

Timing of Wound Closure

1. Primary Closure (Primary Intention): Wound margins are approximated immediately with sutures, staples, or adhesive. Indicated for clean, fresh traumatic lacerations within the "Golden Period":
   • Extremities, Trunk, and Back: Within 12 to 18 hours of injury.
   • Face and Scalp: Up to 24 hours after injury, due to exceptionally rich facial microvascular perfusion, which resists infection and facilitates primary healing.
2. Delayed Primary Closure (Tertiary Intention): Indicated for heavily contaminated lacerations, mammalian bite wounds presenting $>12\text{ to }24\text{ hours}$ old, or high-velocity blast injuries. The wound is anesthetized, copiously irrigated, conservatively debrided, and packed open with sterile saline-moistened gauze. If no clinical infection develops after 3 to 5 days, the wound edges are closed primarily.
3. Secondary Intention: The wound is left open to heal spontaneously via granulation tissue formation and myofibroblast-driven wound contraction. Indicated for deep puncture wounds, heavily infected wounds, and cat bite punctures.

Suture Materials & Suture Removal Intervals

• Percutaneous Skin Closure: Monofilament non-absorbable sutures (Nylon [Ethilon] or Polypropylene [Prolene]) are the gold standard. Their smooth surface minimizes bacterial adherence and tissue reactivity compared to braided silk or braided absorbable sutures.
• Deep Dermal Closure: Absorbable polyglactin 910 (Vicryl) or poliglecaprone 25 (Monocryl) sutures eliminate dead space and relieve tension on the epidermal layer, preventing wound spreading and depressed scars.
• Suture Removal Intervals: Sutures must remain in place long enough to achieve adequate tensile strength, but must be removed before epithelialization extends along the suture tracts, producing permanent cross-hatch ("railroad track") scars.

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Mammalian & Human Bites: Microbiology & Clinical Profiles

                         MAMMALIAN BITE CLINICAL SPECTRUM

   DOG BITES (~80-90%)          CAT BITES (~10-15%)          HUMAN BITES ('FIGHT BITES')
  ┌─────────────────────┐      ┌─────────────────────┐      ┌─────────────────────────┐
  │ • Crushing / tears  │      │ • Deep puncture     │      │ • Clenched fist over    │
  │ • S. aureus, Strep  │      │ • Pasteurella (>75%)│      │   3rd/4th MCP joint     │
  │ • Capnocytophaga:   │      │ • High infection    │      │ • Eikenella corrodens   │
  │   Fulminant sepsis  │      │   rate (>50-80%)    │      │ • Tooth foreign body    │
  │   in asplenia       │      │ • NEVER close       │      │ • Tenosynovitis &       │
  │ • Augmentin 1st-line│      │   primarily         │      │   septic arthritis      │
  └─────────────────────┘      └─────────────────────┘      └─────────────────────────┘

1. Dog Bites (Canine Trauma)

• Represent $80%\text{ to }90%$ of all reported mammalian bite wounds.
• Mechanism: Large canine jaws deliver crushing forces up to $200\text{ to }400\text{ psi}$, producing extensive contusion, tearing, tissue devitalization, and avulsion lacerations.
• Microbiology: Polymicrobial mix of canine oral flora and human cutaneous pathogens: Pasteurella canis, Staphylococcus aureus, Streptococcus pyogenes, and anaerobes (Fusobacterium, Bacteroides).
• Capnocytophaga canimorsus: A fastidious Gram-negative rod present in normal canine saliva. In patients with asplenia, functional hyposplenism, cirrhosis, chronic alcoholism, or severe immunosuppression, C. canimorsus can produce catastrophic, fulminant bacteremia, overwhelming septic shock, disseminated intravascular coagulation (DIC), purpura fulminans, and peripheral symmetrical gangrene, with a mortality rate exceeding $30%$.

2. Cat Bites (Feline Puncture Wounds)

• Represent $10%\text{ to }15%$ of mammalian bites, but carry an infection rate exceeding $50%\text{ to }80%$ (more than double that of dog bites).
• Mechanism: Long, sharp, slender feline teeth act as natural hypodermic needles, inoculating bacterial pathogens deep through subcutaneous tissue planes into avascular flexor tendon sheaths, joint capsules, and periosteum.
• Pathogen: Pasteurella multocida is isolated in $>75%\text{ to }90%$ of cat bites. Characterized by an exceptionally rapid incubation period: intense local erythema, warmth, pain, and seropurulent drainage typically develop within 6 to 24 hours of injury.
• Complications: High incidence of closed-space hand infections, suppurative flexor tenosynovitis, septic arthritis, and osteomyelitis.
• Mandatory Rule: Cat bite puncture wounds should NEVER be closed primarily. They must be left open to heal by secondary intention.

3. Human Bites & Clenched-Fist "Fight Bites"

• Mechanism: Human bite injuries occur either via direct occlusional bites or, more ominously, as a Clenched-Fist Injury ("Fight Bite") sustained when a patient punches another individual in the mouth, striking teeth. The impact lacerates the thin dorsal skin overlying the 3rd or 4th metacarpophalangeal (MCP) joint.
• Patho-anatomy: During the clenched-fist impact, the extensor tendon and joint capsule are stretched tightly over the metacarpal head and penetrated by teeth. When the hand is subsequently relaxed into an open position, the extensor tendon slides proximally beneath the skin, carrying virulent human oral organisms deep into the subaponeurotic space and MCP joint cavity, where they proliferate under anaerobic conditions.
• Microbiology: Polymicrobial infection dominated by Eikenella corrodens (a fastidious facultative anaerobic Gram-negative rod unique to human oral flora), viridans group streptococci, Staphylococcus aureus, and oral anaerobes (Fusobacterium nucleatum, Prevotella species).
• Clinical Diagnostic Rules:
  • Any laceration overlying the dorsal MCP joints must be presumed a clenched-fist injury until proven otherwise, regardless of patient denial.
  • Mandatory Radiography: Obtain plain radiographs of the hand in multiple views to rule out metacarpal head fractures ("boxer's fracture"), cortical violation, and radiopaque tooth fragments retained within the joint space.
  • Kanavel's Four Cardinal Signs of Suppurative Flexor Tenosynovitis:
    1. Symmetrical, uniform sausage-like swelling along the entire digit;
    2. The involved digit held in slight flexion at rest;
    3. Exquisite tenderness along the entire anatomical course of the flexor tendon sheath;
    4. Severe, agonizing pain elicited upon PASSIVE EXTENSION of the digit (the most sensitive early sign).
  • Management: Clenched-fist wounds must NEVER be closed primarily. They require formal surgical exploration through both flexion and extension, copious high-pressure irrigation, splinting in the position of function, urgent hand surgical consultation, and immediate broad-spectrum intravenous or oral antimicrobial coverage.

4. Puncture Wounds Through Rubber-Soled Shoes

• Puncture wounds to the plantar foot sustained while wearing athletic shoes drive rubber and foam shoe debris into deep plantar fascial compartments.
• Foam shoe linings harbor Pseudomonas aeruginosa. Deep inoculation into the calcaneus or metatarsals carries a high risk of Pseudomonas osteochondritis and osteomyelitis.
• Management: High-pressure irrigation, probing for retained foam/rubber foreign bodies, avoidance of primary closure, and prophylactic treatment with an oral fluoroquinolone covering Pseudomonas (Ciprofloxacin $500\text{ to }750\text{ mg}$ PO BID for 7 to 10 days).

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Antimicrobial Prophylaxis for Bite Wounds

Indications for Prophylactic Antibiotics (3 to 5 Days)

Routine antibiotic prophylaxis is not necessary for trivial, superficial dog scratches in healthy individuals presenting within a few hours. Prophylactic antibiotics are strictly indicated for:

1. All cat bite wounds (due to the $>50%$ baseline infection rate);
2. All human bite wounds (including all clenched-fist injuries);
3. All puncture wounds and deep crush wounds;
4. Any bite wound involving high-risk anatomical structures: the hands, feet, face, or overlying major joints;
5. Wounds presenting $>8\text{ to }12\text{ hours}$ after injury;
6. Any mammalian bite in an immunocompromised host (asplenia, cirrhosis, diabetes mellitus, active chemotherapy, chronic steroid use).

First-Line Antimicrobial Regimen

• Amoxicillin-Clavulanate (Augmentin): $875/125\text{ mg}$ PO twice daily for 3 to 5 days (prophylaxis) or 7 to 10 days (early established cellulitis).
• Coverage: Delivers optimal bactericidal coverage against Pasteurella multocida, Eikenella corrodens, methicillin-susceptible Staphylococcus aureus, Streptococcus, and oral anaerobes.

Penicillin-Allergic Alternative Regimens

• Option 1: Doxycycline ($100\text{ mg}$ PO BID): Excellent coverage against Pasteurella multocida, Staphylococcus, and Streptococcus. (Preferred oral alternative in non-pregnant adults).
• Option 2: Fluoroquinolone PLUS Anaerobic Coverage: Ciprofloxacin ($500\text{ mg}$ PO BID) PLUS Metronidazole ($500\text{ mg}$ PO TID).
• Option 3: Trimethoprim-Sulfamethoxazole (TMP-SMX 1 to 2 DS tabs PO BID) PLUS Clindamycin or Metronidazole.

[!CAUTION] THE INEFFECTIVE ANTIBIOTIC TRAP: Cephalexin, dicloxacillin, clindamycin, erythromycin, and azithromycin have POOR or NO activity against Pasteurella multocida and Eikenella corrodens. Prescribing cephalexin monotherapy for a cat bite or clenched-fist human bite is a classic board examination error that leads to treatment failure, suppurative tenosynovitis, and permanent joint disability.

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CDC & ACIP Rabies Post-Exposure Prophylaxis (PEP)

Rabies is caused by a neurotropic RNA lyssavirus transmitted through the saliva of infected mammals. Once clinical encephalitic or paralytic rabies symptoms develop, the disease carries a virtually $100%$ case fatality rate. Timely post-exposure prophylaxis is $100%$ effective when administered correctly before viral entry into peripheral nerve endings.

Animal Risk Stratification in North America

• High-Risk Vectors: Bats, raccoons, skunks, and foxes. In the United States, bats are the leading source of human rabies infections.
  • The Bat Bedroom Rule: Any direct physical contact with a bat, OR finding a bat inside a room with an unattended infant, a sleeping person, an intoxicated individual, or a cognitively impaired adult constitutes an exposure mandating immediate Rabies PEP, unless the bat is captured intact and tests negative for rabies by public health direct fluorescent antibody testing.
• Domestic Dogs, Cats, and Ferrets:
  • A healthy, domesticated dog, cat, or ferret that bites a human can be quarantined and observed for 10 days by local animal control.
  • The rabies virus is excreted in saliva only during the immediate pre-symptomatic period (a few days prior to death). If the animal remains healthy, alert, and asymptomatic throughout the 10-day quarantine, it could not have transmitted rabies at the time of the bite, and PEP is not required.
  • If the animal develops symptoms, dies, or is a stray that escaped quarantine, initiate PEP immediately.
• Low-Risk Animals: Small rodents (mice, rats, squirrels, chipmunks, guinea pigs) and lagomorphs (rabbits, hares) almost never survive encounters with rabid predators and have never been documented to transmit rabies to humans in North America. PEP is not routinely indicated.

The ACIP Rabies PEP Protocol

                         ACIP RABIES POST-EXPOSURE PROPHYLAXIS (PEP)

                                  Suspected Rabies Exposure
                                              │
                               Documented Prior Rabies Series?
                                              │
                     ┌────────────────────────┴────────────────────────┐
                     ▼                                                 ▼
           PREVIOUSLY UNVACCINATED                           PREVIOUSLY VACCINATED
                     │                                                 │
     ┌───────────────┴───────────────┐                                 ▼
     ▼                               ▼                       RABIES VACCINE ONLY
  HUMAN RABIES                    RABIES                       (1.0 mL IM in Deltoid)
  IMMUNE GLOBULIN (HRIG)         VACCINE                      • Day 0 and Day 3
  • 20 IU/kg body weight         (1.0 mL IM in Deltoid)       • NO HRIG indicated!
  • Infiltrate full volume       • Day 0, 3, 7, and 14
    into and around wound margins  (4 doses total)
  • Remaining volume IM distant  • (5th dose Day 28
    from vaccine site              if immunocompromised)

1. For Previously Unvaccinated Individuals:
   • Human Rabies Immune Globulin (HRIG):
     • Dose: $20\text{ IU/kg}$ actual body weight administered on Day 0 (can be administered up to Day 7 if not immediately available).
     • Administration: Infiltrate as much of the full calculated volume as anatomically feasible directly into and around the wound margins. Any remaining volume is injected intramuscularly at an anatomical site distant from the vaccine (e.g., anterolateral thigh).
     • Rationale: Delivers immediate passive neutralizing antibodies while the patient mounts active humoral immunity.
   • Rabies Vaccine (HDCV or PCECV):
     • Dose: $1.0\text{ mL}$ IM administered on Days 0, 3, 7, and 14 (4 doses total).
     • Site: Administer strictly in the deltoid muscle in adults (or anterolateral thigh in small children). NEVER administer the rabies vaccine in the gluteal muscle, as gluteal adipose tissue leads to significantly lower neutralizing antibody seroconversion rates.
     • Immunocompromised Patients: Receive a 5-dose vaccine series (Days 0, 3, 7, 14, and 28) followed by serological titer verification.
2. For Previously Vaccinated Individuals:
   • Administer 2 doses of rabies vaccine ($1.0\text{ mL}$ IM in deltoid) on Days 0 and 3.
   • Do NOT administer HRIG, as circulating memory cells mount a rapid anamnestic antibody surge, and exogenous immune globulin blunts this response.

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CDC & ACIP Tetanus Wound Prophylaxis Guidelines

Clostridium tetani is a ubiquitous, anaerobic, spore-forming soil bacillus that elaborates tetanospasmin, an exotoxin that blocks inhibitory neurotransmitter (GABA and glycine) release at spinal motor interneurons, producing severe muscle rigidity and painful autonomic spasms. Tetanus prophylaxis is stratified strictly by wound classification and documented immunization history:

Tetanus Prophylaxis Decision Matrix

• Vaccine Formulations: Tdap (Tetanus, Reduced Diphtheria, Acellular Pertussis) is preferred over Td for any patient $\ge 11$ years of age who has not previously received Tdap.
• Tetanus Immune Globulin (TIG): Administered as $250\text{ units}$ IM using a separate syringe at an anatomical site distant from the vaccine. TIG is indicated solely for contaminated wounds in patients who have received fewer than 3 lifetime doses of tetanus toxoid or whose vaccination status is unknown.