IAN, PSA and Infraorbital Nerve Blocks: Technique Guide

Local anaesthesia techniques in dentistry use nerve blocks to numb large areas of the mouth with a single injection. The three workhorse blocks are the inferior alveolar nerve (IAN) block for the lower teeth, the posterior superior alveolar (PSA) block for the upper back teeth, and the infraorbital block for the upper front teeth and lip.

This matters because these blocks let dentists do painless root canals, extractions and surgery across whole quadrants. Knowing exactly where each nerve runs, how much anaesthetic is safe and what can go wrong is what separates a comfortable patient from a failed or risky injection.

Key Concepts

• IAN block: numbs the lower teeth, chin and lower lip on one side by depositing anaesthetic near the mandibular foramen, where the inferior alveolar nerve enters the jaw.
• PSA block: numbs the upper molars (except sometimes the front root of the first molar) by targeting the nerves entering the back of the upper jaw.
• Infraorbital block: numbs the upper premolars, canine, incisors and upper lip via the infraorbital foramen below the eye.
• 2% lidocaine with 1:100,000 epinephrine: the standard dental cartridge; each 1.8 mL cartridge holds 36 mg of lidocaine and 0.018 mg of epinephrine.
• Landmarks: the coronoid notch and pterygomandibular raphe guide the IAN block; the mucobuccal fold above the second molar guides the PSA block; the infraorbital rim guides the infraorbital block.

Formulas

Safe dosing is calculated from body weight with these formulas:

Maximum lidocaine dose = 7 mg per kg of body weight (with epinephrine), up to an absolute maximum of 500 mg

Maximum cartridges = maximum dose in mg ÷ 36 mg per cartridge

• 7 mg/kg: the accepted maximum for 2% lidocaine with vasoconstrictor; without vasoconstrictor the limit is 4.4 mg/kg up to 300 mg.
• 36 mg: lidocaine content of one standard 1.8 mL cartridge of 2% solution (20 mg per mL × 1.8 mL).
• Assumptions: healthy adult patient; lower limits apply for children, the elderly, and patients with liver disease or heart conditions.
• Limit: the formula gives a ceiling, not a target — most routine treatments need only 1 to 3 cartridges.

Worked Example

A healthy 60 kg adult needs multiple lower molar extractions under IAN blocks:

1. Maximum safe dose = 7 mg/kg × 60 kg = 420 mg of lidocaine.
2. Maximum cartridges = 420 ÷ 36 = 11.67, so 11 cartridges is the absolute ceiling (always round down).
3. The planned treatment needs 2 IAN blocks plus one top-up, totalling 3 cartridges = 3 × 36 = 108 mg.
4. 108 mg is well below the 420 mg ceiling, using only about 26 percent of the safe maximum — the plan is comfortably safe.

Common Misconceptions

• "If the lip is numb, the tooth is numb." Lip numbness confirms the nerve is reached, but a badly infected tooth may still need extra injections because inflammation blocks anaesthetic action.
• "Epinephrine is dangerous for everyone." The tiny 0.018 mg per cartridge is safe for most patients and actually makes the anaesthetic last longer and safer by slowing absorption.

Real-World Applications

• Routine dentistry: IAN blocks make painless lower molar root canals and extractions possible daily.
• Oral surgery: PSA and infraorbital blocks provide the anaesthesia for wisdom-tooth and implant surgery.
• Medically complex patients: choosing the right block avoids multiple painful injections in patients with bleeding disorders.

IAN, PSA and infraorbital blocks are the foundation of painless dentistry: one well-placed injection, guided by anatomical landmarks and confirmed by aspiration, numbs a whole region safely. Master the landmarks, respect the dose formula and aspirate every time, and even the most anxious patient can be treated in complete comfort.