30.3 Third Molar Surgery and NICE Guidance

3. Impacted Mandibular Third Molars: NICE Guidelines & Radiographic Assessment

Mandibular third molar surgery represents one of the most common minor oral surgical procedures in the UK, governed strictly by national clinical guidelines.

NICE Third Molar Removal Indications (Technology Appraisal 1)
  │
  ├── PROHIBITED: Routine Prophylactic Removal of Pathology-Free Third Molars
  │
  └── MANDATED CLINICAL INDICATIONS FOR SURGICAL REMOVAL:
        ├── Recurrent Pericoronitis (≥ 2 significant episodes with pain/trismus)
        ├── Unrestorable Dental Caries (In third molar or distal of second molar)
        ├── Non-treatable Pulpal or Periapical Pathology
        ├── Dentigerous Cyst or Other Odontogenic Pathology
        ├── External Root Resorption of Adjacent Second Molar
        └── Cellulitis, Fascial Abscess, or Osteomyelitis

NICE Guidelines on Third Molar Removal (TA1, 2000)

In 2000, the National Institute for Health and Care Excellence (NICE) issued Technology Appraisal Guidance No. 1, which revolutionized UK dental practice:

• Prophylactic Removal Banned: The routine, prophylactic removal of pathology-free impacted third molars is strictly not recommended under NHS regulations.
• Approved Pathological Indications for Removal:
  1. Recurrent Pericoronitis: Defined explicitly as two or more significant episodes associated with pain, swelling, purulence, trismus, or systemic malaise.
  2. Unrestorable Dental Caries: In the third molar itself or on the distal cervical surface of the adjacent second permanent molar.
  3. Non-treatable Pulpal / Periapical Pathology: Irreversible pulpitis or periapical periodontitis not amenable to endodontic therapy.
  4. Cellulitis, Abscess, or Osteomyelitis: Spreading infection into fascial spaces.
  5. Follicular Disease: Dentigerous cyst, keratocystic odontogenic tumour, or ameloblastoma.
  6. External Root Resorption: Active resorption of the distal root of the second molar caused by the impacted crown.
  7. Surgical Access: Tooth situated directly in the line of orthognathic osteotomies or tumour resections.

Surgical Classifications of Impaction

• Winter's Classification (Angulation relative to the long axis of the second molar):
  • Mesioangular (45%): Crown tilted mesially towards the second molar; most common impaction.
  • Horizontal (10%): Long axis is horizontal ($90^\circ$ to second molar).
  • Vertical (40%): Long axis parallel to second molar.
  • Distoangular (5%): Crown tilted distally into the ascending ramus. Most difficult mandibular impaction to remove because its path of withdrawal requires substantial bone removal from the ramus and the roots curve towards the inferior alveolar nerve.
• Pell & Gregory Classification:
  • Relationship to Anterior Border of Ramus:
    • Class I: Sufficient space between anterior border of ramus and distal surface of second molar to accommodate third molar crown diameter.
    • Class II: Space is less than the mesiodistal diameter of the crown.
    • Class III: Crown is located completely within the bone of the ascending ramus.
  • Depth Relative to Occlusal Plane:
    • Position A: Highest point of crown is level with or above the occlusal plane of the second molar.
    • Position B: Highest point of crown is between the occlusal plane and cervical line of the second molar.
    • Position C: Highest point of crown is completely below the cervical line of the second molar (deepest impaction).

Radiographic Risk Assessment for Inferior Alveolar Nerve (IAN) Injury

Evaluation of the proximity between mandibular third molar roots and the inferior alveolar nerve canal on a Dental Panoramic Tomogram (DPT / OPG) is based on the Rood and Shehab (1990) Criteria:

Rood & Shehab High-Risk Radiographic Signs on DPT
  ┌───────────────────────────────────────────────┬───────────────────────────────────────────────┐
  │ High-Risk Signs Relating to Tooth Roots       │ High-Risk Signs Relating to Mandibular Canal  │
  ├───────────────────────────────────────────────┼───────────────────────────────────────────────┤
  │ 1. **Darkening of the Root** (Highest risk)   │ 4. **Interruption of Cortical Tramlines**     │
  │    Root appears radiolucent where canal crosses│    White radiopaque canal border lost         │
  │ 2. **Deflection / Deviation of Roots**        │ 5. **Diversion of Canal**                     │
  │    Root bends abruptly around the nerve canal │    Canal path deviates sharply at root apex   │
  │ 3. **Narrowing of the Root**                  │ 6. **Narrowing of Canal**                     │
  │    Root diameter constricts across the canal  │    Canal diameter constricts across root      │
  │ 7. **Dark and Bifid Root Apex**               │                                               │
  └───────────────────────────────────────────────┴───────────────────────────────────────────────┘

• Highest Predictive Value: The three signs most strongly correlated with true anatomical contact and risk of post-operative IAN injury are:
  1. Darkening of the root (caused by grooving of the root surface by the nerve).
  2. Interruption of the radiopaque cortical line ("tramline") of the canal.
  3. Diversion (change in direction) of the canal.
• 3D Imaging (CBCT): When one or more of these high-risk signs are present on a DPT, Cone Beam Computed Tomography (CBCT) is indicated to confirm whether the canal runs buccal, lingual, or interradicular, and whether cortical bone separation is preserved.

Coronectomy (Intentional Partial Odontectomy)

Coronectomy is an evidence-based surgical alternative to complete extraction for high-risk mandibular third molars intimately associated with the IAN:

• Surgical Objective: Remove the full clinical crown while intentionally leaving the vital root fragments undisturbed in the bone, eliminating IAN traction or compression.
• Surgical Protocol:
  1. Raise a full-thickness mucoperiosteal flap and perform conservative buccal bone guttering.
  2. Section the crown horizontally at the amelocemental junction using a fissure bur, cutting through approximately three-quarters of the tooth thickness, and gently fracture off the crown with an elevator (avoids bur perforation into the lingual plate and protects the lingual nerve).
  3. Trim the remaining root fragments with a round bur so that they sit 3 to 4 mm below the crest of alveolar bone.
  4. Copious saline irrigation to remove all enamel and dentine debris; do NOT mobilize the roots.
  5. Primary soft tissue closure over the root stump.
• Absolute Contraindications to Coronectomy:
  • Pre-existing mobility of the third molar (disrupted PDL will act as an infected foreign body).
  • Dental caries extending into the pulp chamber or root canal system.
  • Pre-existing pulpal necrosis or periapical pathology.
  • Medically compromised patients at high risk of osteonecrosis (e.g. intravenous bisphosphonates, active chemotherapy, head/neck radiotherapy).