Oral Surgery, Extraction of Mandibular Third Molars

Anatomy and Physiology

The mandibular nerve is the third and largest division of the fifth cranial nerve, known as the trigeminal nerve. It innervates the skin of the lower face and lip, the muscles of mastication, the mandibular dentition, gingivae, and the anterior two-thirds of the tongue. It supplies both sensory and motor function.[7]

The inferior alveolar nerve is the largest branch of the mandibular nerve. It supplies sensation to the skin of the lower cheek, chin, lip, tongue, teeth, and gingivae. The IAN is an important consideration in the extraction of lower third molars, as these two structures are often closely situated.[7] As a result, there is an increased risk of damage to the IAN, which may result in a temporary or permanent altered sensation in its area of distribution. The risk of temporary altered sensation is estimated at 1% to 5%, while permanent dysaesthesia is estimated at 0 to 0.9%.[8]

Cone beam computed tomography (CBCT) can provide further information on the exact relationship between a mandibular third molar and the IAN and quantify the risk of iatrogenic nerve damage.

There are several radiographic features on plain film radiographs, such as orthopantomogram (OPG/DPT), which are suggestive of close proximity of a third molar to the IAN. (See figure 1).

These features include:

1. Loss of cortication of the inferior dental canal (IDC) - this is seen as an absence of the white 'tram lines' on plain film radiographs.
2. Darkening of third molar roots - this is seen as a dark 'banding' across the apical area of the lower third molar roots.
3. Deviation of the IDC - the IDC typically deviates away from the roots of the lower third molar.
4. Narrowing of the IDC - this is characterized by localized narrowing of the IDC around the region of the lower third molar root.
5. Dilaceration of roots - This is classified as excess curvature of the lower third molar roots.
6. Narrowing of the third molar roots - This is seen as an abnormal narrowing of the apical area of the lower third molar roots.

The presence of one or more of these radiographic features indicates the need for further imaging by way of CBCT, which can provide further information on the relationship between the lower third molar and the IAN.

Indications

Lower third molars are the most frequently extracted teeth, accounting for 18% of dental extractions.[1][2] This is due to their posterior position intra-orally, coupled with the high likelihood of impaction, often resulting in pericoronitis. Pericoronitis is the inflammation of the soft tissues surrounding the crown of a partially erupted tooth.[9] 

Indications for extraction of a lower third molar include recurrent episodes of pericoronitis, unrestorable caries, or caries extending into the pulp. Furthermore, a horizontal or mesioangular lower third molar causing disto-cervical caries in the lower second molar is indicated for extraction to facilitate restorative treatment in the lower second molar. In addition, extraction is also indicated in third molars with odontogenic cysts or tumors, those impeding surgery (such as in patients undergoing orthognathic surgery), or in the line of a mandibular fracture. [10]

Lower anterior crowding is not an indication of extraction of lower third molars. Crowding of the lower anterior teeth is sometimes observed simultaneously with the eruption of lower wisdom teeth. This has led to the belief that the erupting third molars cause a mesial migration of teeth, consequently moving the incisors, resulting in crowding.[11] Several studies have investigated this hypothesis. Still, most have not found a cause-effect relationship between anterior crowding and the eruption of lower third molars.[11] Consequently, Zawawi & Melis (2014) argue that extraction of lower third molars does not alleviate anterior crowding and therefore does not justify the potential risk of damage to IAN.[11]

Contraindications

Contraindications of extraction of lower third molars teeth are high risk of IAN damage, complex root morphology, hypercementosis, and ankylosis. In addition, third molars would not be extracted in patients with a history of intravenous bisphosphonate use or radiotherapy.[12]

Where CBCT indicates an intimate relationship between the roots of a lower third molar and the IAN, or there are other contraindications for extraction, coronectomy serves as an alternative procedure to surgical removal. Coronectomy involves the removal of the coronal tooth structure, ensuring the elimination of all enamel while the roots are left in situ. The purpose of this procedure is to remove the source of discomfort, for instance, the bulbous crown of a partially erupted lower third molar leading to food packing and subsequent pericoronitis, while the roots are left in situ, thus reducing the risk of damage to the IAN.

Conversely, there are several instances in which coronectomy is contraindicated, including caries extending into the pulp, medical history of autoimmune conditions or reduced healing potential, such as diabetes mellitus, lower third molar impeding surgery such as orthognathic or trauma surgery, and association of the tooth with extensive cysts or neoplasms.[13]

Preparation

In the clinical assessment of mandibular third molars, it is necessary to obtain a thorough pain, medical and social history. Medical problems that may affect treatment should be identified, including bleeding disorders, history of radiotherapy, bisphosphonate use, and immunocompromise. This is to establish a patient's post-operative healing capacity and avoid unnecessary complex surgical interventions in patients at risk of delayed healing, like those with a history of radiotherapy to the jaw who are at risk of osteoradionecrosis. Thorough assessment at this stage will also identify patients in which pre-operative measures need to be taken, like those with hemophilia.

A thorough examination of the lower third molar focuses on the position, accessibility, and degree of impaction to determine procedural complexity. The factors to be assessed are summarised in Table 1. These factors are assessed both clinically and radiographically.

Table 1 - Determining factors of procedural complexity of lower third molar extractions

Note – Adapted from Gbotolorun et al. Assessment of Factors Associated With Surgical Difficulty in Impacted Mandibular Third Molar Extraction. J Oral Maxillofac Surg 2007.[14]

Winter's Lines (WAR)

A formal index of quantification of surgical complexity of extraction of lower third molars was suggested by Winter in 1926.[15] This method involves the use of three illusory lines. The first of these is a white line that is drawn across the occlusal surfaces of the lower molars and extends posteriorly above the unerupted third molar. The second line, amber, follows the bone level from the distal aspect of the lower third molar, extending to the interseptal bone of the first molar. The third line, red, is a line drawn perpendicular from the white line extending down to the anticipated point of application on the unerupted lower third molar. This is usually the mesial aspect of the CEJ, except for disto-angular third molars, where the application point is distal. (See figure 2.)

This method illustrates the extent of bone removal required to remove the tooth. Howe argues that with each 1 mm increase in the subcrestal length of the red line, surgical complexity increases three-fold.[15]

Technique or Treatment

Flap Design

An oral surgical flap is defined as a section of mucoperiosteal tissue that is surgically detached from the underlying bone to improve surgical access. This flap should have its own blood supply.[16] Several design principles govern the success of a mucoperiosteal flap. Firstly, to ensure adequate tissue perfusion to the entire flap, the base of the flap should be broader than the unattached edge. This prevents ischaemic necrosis, promotes healing, and improves surgical access. The size of a mucoperiosteal flap must be reflective of its purpose. When extracting lower third molars, the flap must be large enough to visualize the lower third and second molars and prevent tension on the flap.

Most importantly, the lower third molar is closely situated to the lingual nerve, which is protected only by thin mucosa on the mesial aspect of the mandible. A two-sided or three-sided flap, also known as Ward's incision, is the ideal design of choice, with the distal relieving incision positioned buccally to avoid damage to the lingual nerve. The distal relieving incision is made 45 degrees to the distobuccal cusp of the lower third molar.

Previously, lingual nerve protection by way of raising a lingual flap and retraction of the lingual tissues with a Howarth's or similar was practiced to prevent damage to the lingual nerve. Blackburn (1989) demonstrates that this increases the risk of lingual nerve damage and, as such, is no longer indicated.[17]

Surgical Removal

Following the creation of a suitable mucoperiosteal flap, the tissues are retracted using an appropriate instrument, such as a Rake retractor or Minnesota retractor. In addition to the retraction of the soft tissue flap, the purpose of the retractor is to protect the surrounding soft tissues, such as the lips and buccal mucosa.

A buccal gutter is then made using a fissure or round bur on a surgical handpiece to create space for movement of the mandibular third molar. In some instances, this is sufficient to facilitate the elevation of the tooth out of the socket. However, in other cases, the tooth will need to be sectioned before extraction. This typically follows the sequence of decoronation of the tooth, where the coronal portion is removed, followed by the division of the roots, which are delivered separately.[18]

Decoronation of the tooth involves using a fissure or round bur perpendicular to the long axis of the tooth at the level of the furcation to divide the crown from the roots. A thin instrument such as a Couplands size one is placed into this groove and twisted to disimpact the coronal portion of the tooth. Once the crown is removed, the roots can now be visualized and should be divided in the same manner, using a round or fissure bur to create a groove into which a thin metal instrument is placed and twisted to separate the roots. The roots should then be delivered separately. 

The socket should be irrigated and carefully debrided to sound bone, avoiding the apical aspect of the socket to prevent damage to the inferior alveolar nerve. Closure of the socket is typically achieved with resorbable sutures.

Complications

Due to the surgical complexity of third molar surgery, post-operative pain, swelling, and trismus often occur as a result. These effects are transient and typically well-managed with simple analgesia. Other complications include alveolar osteitis, also known as dry socket, which is managed with careful irrigation of the extraction socket with chlorhexidine gluconate or saline and dressing the socket with an obtundent, resorbable material.[19]

Furthermore, patients on anti-coagulation medication or with bleeding disorders may present with prolonged post-operative bleeding, which will need to be managed through debridement of the socket, placement of hemostatic agents in the socket, and closure of the bleeding socket with sutures. While these complications can be seen following any dental extraction, mandibular third molars risk damage to the IAN and the lingual nerve and subsequent temporary or permanent altered sensation of the cheek, chin, lip, tongue, and teeth. Typically, paraesthesia is transient and resolves spontaneously in days, weeks, or months. If the paraesthesia is persistent after six months, it is considered permanent.[20]

Clinical Significance

Oral paraesthesia as a result of third molar surgery is preventable in most cases. Thorough assessment and surgical planning are important steps to minimize the possibility of inferior alveolar or lingual nerve damage. Although relatively uncommon, IAN or lingual nerve paresthesia can profoundly affect a patient's quality of life. As of yet, very few cases report complete resolution of permanent paraesthesia. As a result, good knowledge of head and neck anatomy, surgical technique, and pre-operative planning are critical to successful patient outcomes.

Enhancing Healthcare Team Outcomes

Mandibular third molars can be surgically challenging to remove. Their proximity to essential structures, such as the inferior dental and the lingual nerve, is paramount when surgically planning for extraction. It is, therefore, important to consider alternative treatment modalities, such as coronectomy, where indicated. Careful clinical and radiographic assessment is crucial to successful patient outcomes when extracting mandibular third molars. Dental assistants and/or hygienists must take the appropriate radiographic views for the dentist to correctly identify the condition and the appropriate way forward.