7.2 Assessment & Surgical Management of Impacted Third Molars

7.2 Assessment & Surgical Management of Impacted Third Molars

Surgical management of impacted third molars represents a major component of oral surgery practice in Australia. The Australian Dental Council (ADC) Written Examination places strong emphasis on radiographic diagnostic classifications, nerve injury risk stratification on Orthopantomograms (OPG) and Cone-Beam Computed Tomography (CBCT), evidence-based indications for removal versus retention (ADA and SIGN/NICE guidelines), surgical sectioning mechanics, and coronectomy protocols.

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1. Diagnostic Classification Systems

Precise preoperative classification of impacted mandibular third molars predicts surgical difficulty, guides flap design, and dictates tooth sectioning strategies.

A. Winter's Angulation Classification

Categorizes the spatial inclination of the impacted third molar relative to the long axis of the second molar:

1. Mesioangular (43%): Crown angled anteriorly toward second molar. Most common type; easy to moderate surgical difficulty.
2. Horizontal (3%): Long axis of third molar perpendicular to second molar. Requires crown-root sectioning at CEJ.
3. Vertical (38%): Long axis parallel to second molar. Entrapped beneath anterior border of ramus or distal second molar contour.
4. Distoangular (6%): Crown angled posteriorly toward ascending ramus. Most difficult surgical removal in the mandible because the withdrawal path is directed backward into solid ramus bone, requiring extensive distal bone guttering.

B. Pell & Gregory Classification System

Surgical Difficulty Summary: A Class III, Class C, Distoangular impaction represents the highest level of surgical difficulty and risk of complication.

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2. Radiographic Risk Assessment & IAN Proximity Signs

Evaluation of the Orthopantomogram (OPG) is mandatory to assess the relationship between mandibular third molar roots and the Inferior Alveolar Nerve (IAN) canal.

A. The 7 Classic Rood & Shehab Signs of IAN Proximity

1. Darkening of the Root Tip: Radiolucency across the root apex where the canal crosses. Highly predictive of intimate contact or nerve grooving.
2. Deflection / Dilaceration of Roots: Root apex curves abruptly when encountering the nerve canal wall.
3. Narrowing of the Root: Root diameter contracts where crossed by the canal.
4. Dark and Bifid Root Apex: Double root tip appearance created by nerve canal indentation.
5. Interruption of Canal Radiopaque Borders (Tramlines): Disruption of the white cortical lines defining the IAN canal boundaries.
6. Diversion of the Canal: The IAN canal alters its direction as it crosses the root.
7. Narrowing of the Canal: Diameter of the IAN canal reduces as it passes the root.

B. Role of CBCT 3D Imaging

If one or more high-risk signs are identified on OPG, Cone-Beam Computed Tomography (CBCT) is indicated to establish:

• Spatial location of IAN canal (buccal, lingual, interradicular, or inferior to roots).
• Presence of cortical loss surrounding the nerve canal.
• Inter-radicular encirclement of the nerve trunk.

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3. Indications & Contraindications for Third Molar Removal

In accordance with Australian Dental Association (ADA) guidelines and international consensus (SIGN / NICE guidelines), prophylactic removal of asymptomatic, pathology-free impacted third molars is NOT recommended.

A. Clear Evidence-Based Indications for Extraction

1. Recurrent Pericoronitis: Two or more distinct episodes of pericoronitis, or a single severe episode accompanied by fascial space cellulitis/abscess.
2. Unrestorable Caries: Extensive caries in the third molar or on the distal surface of the adjacent second molar caused by food impaction.
3. Pulpal / Periapical Pathology: Non-treatable pulpal necrosis or apical periodontitis.
4. Root Resorption: Internal or external root resorption of the adjacent second molar.
5. Associated Cystic / Neoplastic Pathology: Dentigerous cyst, keratocyst, or ameloblastoma associated with the impacted tooth follicle.
6. Surgical Facilitation: Prior to orthognathic surgery or radiation therapy involving the posterior jaw.

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4. Surgical Sectioning Mechanics & Lingual Nerve Protection

A. Mucoperiosteal Flap & Retraction

• An envelope flap is initiated along the anterior border of the ramus, extending down to the distal of the second molar and through the buccal sulcus to the first molar.
• Lingual Protection: The Lingual Nerve lies in close proximity to the lingual cortical plate (and above the crest in 15–20% of patients). Do not elevate the lingual mucoperiosteum or place lingual split-lines. If lingual retraction is required, use a broad, blunt retractor (e.g., Howarth retractor) held strictly subperiosteally.

B. Bone Removal & Sectioning Protocols

• Mesioangular Impaction: Perform buccal guttering to CEJ level. Section the distal half of the crown or mesial cusp to create space, then elevate remainder.
• Horizontal Impaction: Gutter buccal bone to expose CEJ. Section the entire crown from the roots at the CEJ using a fissure bur. Elevate and remove crown first, then elevate mesial and distal roots sequentially into the created space using Cryer elevators.
• Distoangular Impaction: Requires generous distal bone removal along the ascending ramus before crown sectioning.

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5. Coronectomy Protocol & Long-Term Management

Coronectomy (intentional partial root retention) is an evidence-based surgical procedure designed to prevent IAN injury in high-risk cases where third molar roots intimately contact or encircle the IAN canal.

A. Surgical Steps for Coronectomy

1. Elevate a full-thickness mucoperiosteal flap and perform conservative buccal bone guttering to expose the CEJ.
2. Section the crown horizontally at or 1–2 mm below the CEJ using a fissure bur, cutting through 75% of the tooth width.
3. Fracture off the crown gently using a straight elevator. DO NOT rotate elevators between roots, as this mobilizes root fragments.
4. Reduce remaining root faces 3 to 5 mm below the top of the alveolar bone crest using a round bur.
5. Inspect root surfaces: ensure zero root mobility, smooth sharp enamel/dentine spurs, copiously irrigate with sterile saline, and achieve primary mucosa-to-mucosa tension-free closure.

B. Absolute Contraindications to Coronectomy

• Root mobility following crown fracture.
• Caries extending into the root pulp chamber or pulpal necrosis.
• Periapical radiolucency or root-associated cystic pathology.
• Horizontally impacted teeth where crown sectioning cannot be performed without root mobilization.