13.2 Injection Techniques, Failures & Complications
Key Takeaways
- Infiltration works best where cortical bone is thin (maxilla); IANB targets the mandibular foramen for mandibular molar pulpal anesthesia, often with a separate long buccal injection for molar buccal gingiva.
- PSA anesthetizes maxillary molars but may miss the mesiobuccal root of the first molar and carries pterygoid plexus hematoma risk; mental blocks soft tissue, while incisive technique drives solution into the foramen for pulpal effect.
- Prevent needle breakage by not bending or hubbing needles; retrieve visible fragments immediately and refer for imaging-guided removal if lost in tissue.
- Facial nerve paralysis after IANB usually means LA entered the parotid—reassure, protect the eye, expect recovery with drug offset; manage hematoma and trismus supportively.
- Systemic LA toxicity progresses from CNS prodromes (metallic taste, tinnitus, twitching) to seizures and cardiovascular collapse—prevent with aspiration, slow injection, and MRD limits; manage with oxygen, airway support, benzodiazepines for seizures, and advanced cardiac care.
13.2 Injection Techniques, Failures & Complications
Quick Answer: Match technique to anatomy: infiltration near apex for maxillary (and many mandibular) teeth; IANB for mandibular molars when dense cortical bone blocks infiltration; PSA for maxillary molars (mind the pterygoid plexus); mental/incisive for premolars/anterior mandible. Failures come from anatomy, infection, inadequate volume, or wrong nerve. Complications include needle breakage, hematoma, facial nerve paralysis, trismus, paresthesia, and systemic LA toxicity (CNS excitation → depression → cardiovascular collapse).
Technique questions on the AFK link head-and-neck anatomy to safe injection: where the needle tip should be, which teeth go numb, why a block fails, and how to manage the common adverse events. Review CN V divisions (V2, V3) alongside this section.
Infiltration Anesthesia
Supraperiosteal (infiltration) deposits LA near the apex of the target tooth so the solution diffuses through porous bone to terminal nerve branches.
| Feature | Detail |
|---|---|
| Best sites | Maxillary teeth (thin cortical plate); mandibular incisors; often mandibular premolars; articaine infiltration may anesthetize many mandibular teeth that lidocaine infiltration cannot |
| Technique keys | Stretch mucosa, bevel toward bone, deposit near apex, aspirate when deep, slow injection |
| Advantages | Simple, low complication rate, hemostasis if vaso used, limited soft-tissue numbness outside field |
| Failures | Dense bone, infection/low pH, insufficient volume, deposition too far from apex, accessory innervation |
| Contraindication relative | Acute abscess at injection site—prefer block proximal to infection plus drainage/antibiotics as indicated |
Intraseptal, PDL (intraligamentary), and intraosseous injections are supplemental when conventional infiltration/block fails—small volumes, slow injection, risk of post-op bite trauma or selective high-pressure effects; useful for “hot” teeth and endodontic emergencies.
Inferior Alveolar Nerve Block (IANB / Mandibular Block)
The inferior alveolar nerve (V3) enters the mandibular foramen on the medial ramus; LA near this site anesthetizes mandibular teeth to the midline on that side, plus typically lower lip and chin skin via mental nerve (continuation), with lingual nerve often anesthetized if solution spreads (tongue/floor of mouth).
| Element | Teaching points |
|---|---|
| Landmarks | Coronoid notch (greatest concavity on anterior ramus), pterygomandibular raphe, occlusal plane of mandibular teeth; height of injection ~6–10 mm above mandibular occlusal plane in adults (adjust for child/edentulous) |
| Target | Soft tissue superior to mandibular foramen, before nerve enters bone |
| Needle | Long (≈32 mm) 25- or 27-gauge common; advance until bone gently contacted, withdraw slightly, aspirate, deposit |
| Teeth anesthetized | All mandibular teeth on side (pulpal) if successful; soft tissue: buccal mucosa anterior to mental foramen via mental nerve; buccal nerve separate for molar buccal gingiva |
| Need for long buccal | Yes for surgical procedures on mandibular molars’ buccal soft tissue |
| Onset | Often 3–5+ minutes; lip numbness is a useful (not infallible) sign |
Common IANB failures and fixes
| Cause | Fix / note |
|---|---|
| Deposition too low | Re-inject higher toward mandibular foramen level |
| Deposition too far anterior | Deeper approach toward bone of ramus; check medial direction |
| Accessory innervation (mylohyoid, cervical) | Supplemental infiltration, Gow-Gates, Akinosi-Vazirani, or PDL/intraosseous |
| Bifid IAN / anatomic variation | Alternate block techniques; imaging if chronic failure |
| “Hot” irreversible pulpitis | Volume, wait time, articaine infiltration supplement, intraosseous, sedation adjuncts |
| Cross-innervation of midline incisors | Contralateral infiltration |
Gow-Gates (higher V3 trunk) and Akinosi-Vazirani (closed-mouth) are alternative mandibular blocks for failures or trismus—recognize names and indications more than millimeter recipes on AFK.
Positive aspiration on IANB is relatively common (inferior alveolar vessels)—redirect and re-aspirate; never inject if blood aspirated into cartridge without repositioning.
Posterior Superior Alveolar (PSA) Nerve Block
The PSA nerves supply maxillary molars (often except mesiobuccal root of first molar, which may need MSA or infiltration).
| Feature | Detail |
|---|---|
| Indication | Multiple maxillary molars; when infiltration inadequate |
| Needle path | Mucobuccal fold over second molar; upward, inward, backward (~45° each plane teaching) along posterior maxilla |
| Depth | Short needle often preferred to reduce over-insertion risk; ~16 mm typical adult teaching depth—do not hub |
| Major risk | Hematoma from pterygoid plexus of veins (± maxillary artery) → rapid cheek swelling |
| Aspiration | Critical; high vascularity |
| MB root of #16/26 | Often needs additional infiltration or MSA block |
Management of PSA hematoma: stop injection, apply firm pressure, ice, reassure (self-limiting discoloration over days), postpone further deep injections in area, document, analgesics as needed; rare airway threat only if massive—monitor.
Mental / Incisive Nerve Block and Other Maxillary Blocks
| Block | Target / use |
|---|---|
| Mental nerve block | Mental foramen (usually between apices of mandibular premolars); soft tissue of lip/chin/buccal gingiva anteriorly—not reliable pulpal for teeth unless solution enters foramen as incisive block |
| Incisive nerve block | Mental block + digital pressure to drive LA into canal → pulpal anesthesia of premolars ± anterior teeth on that side |
| MSA | Premolars and MB root of maxillary first molar when present |
| ASA / infraorbital | Maxillary anterior teeth and supporting tissues |
| Greater palatine / nasopalatine | Palatal soft tissue for surgery; very painful if not topped up carefully—topical + slow pressure |
| Maxillary (V2) block | Entire maxillary quadrant; advanced technique (high tuberosity or greater palatine canal approaches)—specialist comfort zone |
Needle Breakage
Breakage is rare with modern disposable needles but still tested.
| Risk factor | Prevention / response |
|---|---|
| Bending needle before insertion | Never intentionally bend; change direction by withdrawing almost fully |
| Hubbing the needle (insertion to hub) | Leave a visible portion outside tissue for retrieval if fracture |
| Sudden patient movement | Stable hand rests; warn patient; good communication |
| Using 30-gauge long needles for deep blocks | Prefer appropriate gauge/length for depth (25/27 long for IANB) |
| If needle breaks and is visible | Retrieve with hemostat immediately if end protrudes |
| If needle lost in tissue | Do not dig blindly; calm patient, map location, refer for surgical removal (often imaging-guided); document thoroughly |
Hematoma, Trismus, Facial Nerve Paralysis, Paresthesia
Hematoma
Extravasation of blood into tissues after vessel nicking. PSA and IANB/infraorbital regions classic. Swelling may be rapid. Manage with pressure, ice first 4–6 hours, then heat after 24–48 h in many protocols, analgesics, warn about ecchymosis lasting 7–14 days, infection rare.
Trismus
Limited opening from spasm/inflammation of medial pterygoid (IANB path) or hematoma/infection. Manage with heat, analgesics/NSAIDs, soft diet, gentle physiotherapy/opening exercises; rule out infection if delayed worsening with fever.
Facial nerve paralysis
If LA is deposited into the parotid gland (too far posterior on IANB, missing bone contact), CN VII branches may be anesthetized → unilateral facial droop, inability to close eye.
| Action | Detail |
|---|---|
| Reassure | Temporary—duration of soft-tissue anesthesia hours |
| Eye care | Tape eye closed or use lubricant if blink lost; protect cornea |
| Document | Explain transient motor block vs stroke (no other neuro deficits; recent dental injection context) |
| Prevention | Always contact bone on IANB before depositing full volume; correct depth/angulation |
Paresthesia / dysesthesia
Prolonged altered sensation after LA (especially reported with some 4% solutions near lingual/IAN—association debated). Most resolve over weeks–months; persistent cases need specialist evaluation, mapping, and possible microsurgical referral. Always document pre-existing neuropathy.
Other local complications
- Soft-tissue injury (lip/tongue chewing) especially children after long-acting blocks—warn caregivers
- Infection rare with sterile technique; avoid injecting through infected tissue when possible
- Epithelial desquamation / sterile abscess from topical or high concentration
- Ocular signs (diplopia, temporary blindness rare) from intra-arterial injection or diffusion—stop, monitor, ophthalmology if persistent
Local Anesthetic Systemic Toxicity (LAST)
Toxicity follows intravascular injection or absolute overdose (especially children). Severity scales with plasma concentration and rate of rise.
Classic progression (CNS then CV)
| Stage | Clinical features |
|---|---|
| Early CNS excitation | Circumoral numbness, metallic taste, tinnitus, lightheadedness, visual/auditory disturbance, anxiety, muscle twitching |
| Later CNS | Seizures, then CNS depression: drowsiness, unconsciousness, respiratory arrest |
| Cardiovascular | Initially hypertension/tachycardia possible; then myocardial depression, bradycardia, vasodilation, hypotension, ventricular arrhythmias, asystole—bupivacaine especially cardiotoxic |
| Biphasic teaching | Excitation → depression; not every patient shows a neat sequence |
Prevention
- Know and respect MRD (Section 13.1)
- Aspirate in two planes when indicated; inject slowly
- Use lowest effective dose and concentration
- Fractionate doses; communicate with patient during injection
- Extra caution: small children, elderly, heart failure, liver disease, hypoxia, acidosis (lower seizure threshold)
Management principles (AFK-level)
- Stop injection; call for help / activate emergency response as needed
- ABC: airway, breathing, circulation; 100% oxygen; ventilatory support
- Position supine; monitor vitals
- Seizures: benzodiazepines per emergency protocol (e.g., midazolam/diazepam—know office emergency kit contents)
- Cardiovascular collapse: ACLS-based care; lipid emulsion therapy is a modern antidote concept for severe LAST (know existence; hospital-level)
- Avoid excess vasopressin/local further LA; follow current emergency algorithms
Differentiate from epinephrine reaction: epi causes brief anxiety, tachycardia, palpitations, ↑ BP soon after injection, usually self-limited; toxicity has sensory prodromes, seizures, progressive CV collapse. Allergy/anaphylaxis has urticaria, bronchospasm, hypotension with distributive shock pattern—epinephrine IM is treatment, opposite of withholding catecholamines for pure LAST.
Rapid review list
- Infiltration: maxilla + porous bone; blocks when cortex dense or multi-tooth
- IANB: mandibular foramen landmarks; long buccal separate for molar gingiva
- PSA: molars ± miss MB root of first molar; pterygoid plexus hematoma
- Mental = soft tissue; incisive = drive into foramen for pulpal
- Never hub needles; retrieve visible broken needle; refer if lost
- Facial palsy after IANB → parotid injection; protect eye; temporary
- LAST: tinnitus/metallic taste → seizures → coma/CV collapse; O₂, benzos, ACLS, lipids
Section 13.3 covers systemic analgesics, nitrous oxide, oral sedation principles, and ASA physical status for safe case selection.
Which statement about the posterior superior alveolar (PSA) nerve block is most accurate?
A patient develops unilateral facial paralysis within minutes after an inferior alveolar nerve block. The most likely mechanism and immediate management emphasis are:
Early systemic local anesthetic toxicity is most likely to present with which constellation?
Which practice best reduces the risk of needle breakage and facilitates retrieval if breakage occurs?