Surgical and Procedural Dermatology: Margins, Anesthesia, and Technique
Anatomy and margins
Surgical questions test anatomy, margin logic, and complication prevention. Know the danger zones: temporal region (frontal branch of the facial nerve), glabella and nasolabial fold (vascular supply), and postauricular sulcus. For excisions, margins are lesion-specific: low-risk basal cell carcinoma 4 mm; squamous cell carcinoma 4–6 mm; melanoma in situ 5 mm; invasive melanoma 1–2 cm depending on Breslow thickness. Mohs micrographic surgery is indicated for recurrent, ill-defined, or cosmetically sensitive tumors, and for melanoma in situ on the face.
Anesthesia: lidocaine 1% is the workhorse. Maximum dose is approximately 4.5 mg/kg plain and 7 mg/kg with epinephrine. For a 70 kg patient, that is 315 mg plain (31.5 mL of 1%) or 490 mg with epinephrine (49 mL). Trap: epinephrine is safe in digits and ears; the old prohibition is a myth. Another trap is failing to account for lidocaine in tumescent solution.
Procedures: shave biopsy for exophytic lesions, punch for deep dermal or pannicular processes, and excisional biopsy for suspected melanoma. Suture choice favors buried absorbable sutures for tension and fine nonabsorbable for epidermal approximation. Hemostasis with electrocoagulation or aluminum chloride; avoid aggressive cautery near cartilage. Laser physics questions favor selective photothermolysis - matching wavelength to chromophore and pulse duration to thermal relaxation time.
Rule of thumb: before cutting, state the diagnosis, margin, and closure plan. Worked example: a 70 kg patient needs a 3 cm excision; calculate the maximum lidocaine with epinephrine (490 mg, 49 mL of 1%) before infiltrating, and document the dose.