Where should I inject local anesthesia and what wound prep is recommended for debriding an infected wound on the knee?

Local Anesthesia Administration and Wound Preparation for Infected Knee Wound Debridement

Inject local anesthesia into the healthy tissue surrounding the infected wound margins, avoiding direct injection into infected tissue, and prepare the wound with sterile normal saline irrigation only—no iodine or antibiotic solutions. 1

Where to Inject the Local Anesthesia

Injection Technique for Infected Wounds

• Infiltrate the anesthetic into the intact skin and subcutaneous tissue around the wound perimeter, creating a field block that encircles the infected area 1
• Never inject directly into infected tissue, as this can spread bacteria systemically and reduces anesthetic efficacy due to the acidic pH of infected tissue 1
• Use a systematic approach: Start at one edge of the wound and advance circumferentially, injecting as you slowly withdraw the needle to create a continuous ring of anesthesia 1, 2
• Aspirate before each injection to avoid intravascular administration, which is critical for preventing local anesthetic systemic toxicity 1, 3

Specific Dosing for Knee Wound Debridement

• For adults, use no more than 4.5 mg/kg of lidocaine or 7.0 mg/kg of lidocaine with epinephrine in a single treatment session 1
• Lidocaine with epinephrine (1:100,000) is preferred for most wound debridement as it provides vasoconstriction, prolongs anesthetic duration, and reduces systemic absorption 1, 4
• Use incremental injections rather than bolus administration to minimize toxicity risk 1
• For a knee wound, typically 20-40 mL of 1% lidocaine with epinephrine is sufficient for adequate field block anesthesia 4, 2

Pain Reduction Strategies During Injection

• Buffer the lidocaine with sodium bicarbonate (mix 8.4% sodium bicarbonate with 1% lidocaine with epinephrine in a 1:9 or 1:10 ratio) to reduce injection pain by 20-40% 1
• Inject slowly to minimize discomfort 1
• Warm the anesthetic solution to 40°C before injection 1
• Use a small-gauge needle (27-30 gauge) when possible 2

Wound Preparation Protocol

Pre-Debridement Cleansing

• Cleanse the wound with copious sterile normal saline irrigation only—this is the gold standard for infected wound preparation 1
• Do NOT use iodine-containing solutions or antibiotic-containing solutions for wound cleansing, as these have not been shown to improve outcomes and may impair healing 1
• Remove only superficial debris during initial cleansing; deeper debridement should be performed after adequate anesthesia is achieved 1

Sharp Debridement Technique

• Perform sharp debridement with scalpel, scissors, or tissue nippers as this is the preferred method over autolytic or enzymatic debridement for infected wounds 1
• Remove all necrotic tissue, slough, foreign material, and surrounding callus/hyperkeratosis to eliminate the bacterial reservoir and expose the true wound extent 1
• Warn the patient that bleeding is expected and the wound will appear larger after debridement when its full extent is exposed 1
• Collect tissue specimens for culture during debridement to guide antibiotic therapy 1
• Examine for deep tissue involvement, particularly bone exposure or involvement, which would indicate osteomyelitis 1

Post-Debridement Management

• Do NOT close infected wounds primarily—they must heal by secondary intention or delayed primary closure after infection resolves 1
• Apply moist wound healing dressings after debridement 1
• Elevate the knee to reduce swelling and accelerate healing 1
• Plan for repeat debridement sessions if necrotic tissue continues to form 1

Critical Safety Considerations

Monitoring for Local Anesthetic Toxicity

• Continuously communicate with the patient during and after injection to monitor for early toxicity signs 1, 3
• Watch for perioral numbness, metallic taste, tinnitus, confusion, or seizures—these indicate systemic toxicity requiring immediate intervention 3
• Use the lowest effective dose to minimize toxicity risk 1, 3

Common Pitfalls to Avoid

• Do not inject into the infected wound bed itself—this spreads infection and provides inadequate anesthesia 1
• Do not attempt complete debridement in one session if the patient experiences excessive pain—stop and schedule additional sessions over several days 1
• Do not debride primarily ischemic wounds without vascular surgery consultation, as this may worsen tissue loss 1
• Do not exceed maximum lidocaine dosing, especially in elderly or smaller patients 1

Alternative Anesthesia Options

• For patients with true lidocaine allergy, use ester-type local anesthetics, bacteriostatic normal saline, or 1% diphenhydramine as alternatives 1
• For extensive debridement requiring deeper tissue removal, consider performing the procedure in an operative suite under procedural sedation or general anesthesia 1