A dental student’s guide to…mandibular third molars

In this month’s guide, Hannah Hook looks at the pre-operative assessment of mandibular third molars.

Extraction of mandibular third molars (M3Ms) has progressively become an increasingly common procedure.

However, despite this, it carries various associated risks. And whilst some of which are considered relatively minor and infrequent, the severity and incidence can vary significantly (Al Ali and Jaber, 2020).

In 2000, NICE guidance for the extraction of wisdom teeth was released. But this is now over 20 years old and more research in this field has since been undertaken.

More recently, in 2020, the Royal College of Surgeons of England (RCSEng) released updated guidance on the assessment of M3Ms. This was produced in response to evidence suggesting there is an increasing level of patient harm due to the retention of M3Ms.

Initial assessment

The assessment of M3Ms starts with a thorough assessment of the patient’s medical and dental history. This aids in triaging the patients based on urgency.

Clinicians should ask patients if they have had any symptoms associated with their M3Ms; such as pain (number of episodes and severity), swelling and limited mouth opening.

You should also complete an extraoral assessment, assessing for signs of infection, such as oedema, erythema, or lymphadenopathy.

M3M assessment

Following this comes clinical examination of the M3Ms:

Eruption status – unerupted, fully or partially erupted
Their position in relation to the mandibular second molar
If they are functional
Their place in the occlusion
Associated periodontal disease
Opposing maxillary third molar
Oedema, erythema.

Radiographic assessment

Radiographs allow gathering of further information on the size, shape, and position of the M3M. Along with the presence or absence of disease within the tooth or surrounding area (Royal College of Surgeons of England, 2020).

An orthopantomogram (OPT) is the standard imaging technique for the assessment of M3Ms. It enables assessment of the M3M in relation to the inferior alveolar nerve (IAN). The radiological signs that a M3M is close the IAN (Figure 1) include (Al Ali and Jaber, 2020):

Darkening of the root
Deflection of the root
Narrowing of the root
Dark and bifid apex of root
Interruption of the white cortical line
Diversion of the IAN canal
Narrowing of the canal.

The 2020 guidance by the RCSEng suggests that cone beam computed tomography (CBCT) is of use when conventional radiographic imaging shows a close relationship between the M3M and the IAN.

CBCT can help show if there is direct contact between the IAN canal and the M3M. Or whether there is bony separation between them (Royal College of Surgeons of England, 2020).

Figure 1: Radiological signs that a M3M is close the IAN (Al Ali and Jaber, 2020)

Indications for extraction

Guidance from the RCSEng (2020) suggests that the ‘Practice of prophylactic removal of third molars should be discontinued in the NHS’.

Therefore, we should restrict surgical removal of M3Ms to patients with evidence of pathology, such as the following (Royal College of Surgeons of England, 2020):

Unrestorable caries
Periodontal disease of M3M or second molar
Non-treatable pulpal and/or periapical pathology
Cellulitis
Abscess
Osteomyelitis
Internal or external resorption of the tooth or adjacent teeth
Fracture of tooth
Disease of follicle including cyst or tumour
M3M is in the surgical field.

A patient who has only experienced one episode of pericoronitis (unless extremely severe), should not be considered as an indication for surgery (Royal College of Surgeons of England, 2020).

Winters lines

Winters lines classification provides an assessment of the degree of difficulty of extraction of a M3M. It comprises of three lines, red, amber, and white. These are superimposed onto a radiograph (Figure 2).

White line

Drawn across the occlusal surface of the mandibular molars extending posteriorly over the M3M
The white line signifies the difference in the occlusal level of the first, second and third molars.

Amber line

Represents the height of the bone level
Shows the amount of bone overlying the M3M
Indicates how much bone removal is required.

Red line

Drawn perpendicular from the amber line to an expected point of application of an elevator
If mesially impacted, then this is usually the cemento-enamel junction (CEJ) on the mesial aspect of the M3M. If distally impacted, then the CEJ on the distal aspect
The red line highlights how much bone will need removing to allow elevation of the tooth
As the red line increases by 1mm in length, the difficulty of extraction is three times more
If the line is less than 5mm in length then removal under local anaesthetic is appropriate. Anything above 5mm may require sedation or a general anaesthetic.

Figure 2: Displaying Winters lines

Factors increasing the difficulty of extraction

Disto-angular positioning
Long thin roots
Divergent roots
Narrow periodontal ligament space
Close relation to the IAN
Close relation to the mandibular second molar
Dense bone.

Coronectomy

If M3Ms are in very close proximity to the IAN and therefore extraction poses a high risk of nerve damage, an alternative method for management is coronectomy.

Another indication for coronectomy is if there is hypercementosis of the M3M (Royal College of Surgeons of England, 2020). However, the criteria for patient selection is strict. This is due to the risk of possible infection, pain and requirement for future removal of the roots. Contraindications include (Royal College of Surgeons of England, 2020):

Non-vital M3M
Caries with a high risk of pulpal involvement
Mobile M3M
M3M with associated apical infection.

Key points

Carry out a thorough initial assessment of the patient, including their medical and dental history
Radiographic assessment with an OPT allows visualisation of the IAN and assessment of the M3M proximity to the canal
There are several signs that a M3M is close to the IAN
If a M3M is deemed to be in close proximity to the IAN then a CBCT is useful in determining whether there is a direct connection between the IAN canal and the M3M
Prophylactic removal of M3Ms should be discontinued
Winters lines provide a method for assessing the difficulty of extraction of M3M
Coronectomy is an option for M3Ms where extraction is very difficult (hypercementosis) or poses a very high risk of nerve damage.

References

Al Ali S and Jaber M (2020) Correlation of panoramic high-risk markers with the cone beam CT findings in the preoperative assessment of the mandibular third molars. J Dent Sci 15: 75-83

Royal College of Surgeons of England (2020) Parameters of care for patients undergoing mandibular third molar surgery

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