Third Molars

Question 1

In which jaw is agenesis of the third molar more common?

Answer 1

maxilla

Question 2

What gene contributes to agenesis of third molars?

Answer 2

PAX9 gene

Question 3

What is a likely radiographic sign of third molars not developing?

Answer 3

Missing at age 14 on radiograph

Question 4

When do third molars erupt?

Answer 4

18-24 years old

Question 5

What is the most common reason for third molars failing to erupt?

Answer 5

Impacted third molars

Question 6

What are the consequences of impacted third molars?

Answer 6

caries, pericoronitis, cyst formation

Question 7

What nerves are at risk with an impacted mandibular third molar? And which nerves are we most concerned about?

Answer 7

** inferior alveolar n.
* lingual n.
n. to mylohyoid
long buccal n.

Question 8

What does the inferior alveolar n. supply?

Answer 8

sensory supply to all mandibular teeth on that side;
mucosa and skin of lower lip and chin on that side

also transmits branchial motor fibres to the mylohyoid and the anterior digastric muscles.

Question 9

What is the pathway of the inferior alveolar n.?

Answer 9

the mandibular division of the trigeminal n. (CNV3)

runs between the medial pterygoid muscle and the mandible, passing into the ramus of the mandible at the mandibular foramen.
it continues through the mandible anteriorly till it emerges at the mental foramen as the mental n.

Question 10

What does the lingual n. supply?

Answer 10

sensory supply to mucosa of anterior 2/3 of tongue, floor of mouth, lingual gingiva

special taste sensation from anterior 2/3 of tongue back to the chord tympani

parasympathetic innervation from the chords tympani to the submandibular glands

Question 11

What is the pathway of the lingual n.?

Answer 11

branch of the mandibular division of the trigeminal n. (CNV3)

Question 12

What is the relationship of the lingual n. to the mandible?

Answer 12

the lingual n. lies close to the lingual plate in the mandibular and retromolar area

lies about 0 - 3.5mm medial to the mandible

lies at or above level lingual plate in 15-18% of cases

Question 13

What are the indications for extracting mandibular third molars?

Answer 13

1. Therapeutic indications:
   - infection (most common)
   - cysts (20-50 years old, most commonly dentigerous cyst)
   - tumours
   - external resorption of 7 or 8
2. Surgical indications:
   - orthognathic
   - fractured mandible
   - resection of disease tissue
3. High risk of disease
4. Medical indications:
   - awaiting cardiac surgery
   - immunosuppressed or to prevent osteonecrosis
5. Accessibility - limited access
6. Patient age - complications and recovery time increases with age
7. Autotransplantation
8. GA - for removal of one third molar then consider the removal of opposing or opposite or contralateral third molar to avoid future GA

Question 14

What is pericoronitis?

Answer 14

Inflammation of the gingiva around the crown of a partially erupted tooth

Question 15

What are the signs and symptoms of pericoronitis?

Answer 15

pain (commonly throbbing)
swelling (intra or extra oral)
bad taste
pus discharge
occlusal trauma to operculum
ulceration of operculum
evidence of cheek biting
halitosis
limited mouth opening dysphagia
pyrexia
malaise
regional lymphadenopathy

Question 16

What is the management of pericoronitis?

Answer 16

usually transient and self-limiting, but if acutely symptomatic:

• incision of localised pericoronal abscess
  with or without LA (IDB)
• irrigation with warm saline or CHX MW (some authorities may not recommend CHX due to cases of anaphylaxis)
• upper third molar XLA if traumatising operculum
• instruct pt to frequent warm saline or CHX W
• advice pt pain relief
• instruct pt to keep fluid levels up and keep eating (soft/liquid diet if necessary)
• do not prescribe AB unless severe pericoronitis, systemically unwell, EP swelling, immunocompromised
• if large EO swelling, systematically unwell, trismus, dysphagia: refer to maxillofacial unit or A&E

Question 17

What are the predisposing factors of pericoronitis?

Answer 17

• partial eruption and vertical or distoangular impaction
• opposing maxillary M3M or M2M causing mechanical trauma contributing to recurrent infection
• URTIs + stress + fatigue pericoronitis
• poor OH
• insufficient space between the ascending ramus of the lower jaw and the distal aspect of M2M
• white race
• full dentition

Question 18

What are the radiographic signs associated with a significantly increased risk of nerve injury (inferior alveolar n.) during M3M surgery?

Answer 18

1. diversion of the inferior dental canal
2. darkening of the roots where crossed by the canal
3. interruption of the white lines of the canal

Question 19

How can we determine the depth of impaction?

Answer 19

From OPT.
Superficial: crown of 8 is at same height as crown of 7
Moderate: in between
Deep: crown of 8 is at same level of roots of 7

This gives an indication of amount of bone removal required.

Question 20

What is the most to least common angulation of impacted M3Ms?

Answer 20

40% mesially impacted
30-38% vertically impacted
6-15% distally impacted
3-15% horizontally impacted
transverse or aberrant (less common)

Question 21

What are the treatment options for impacted M3Ms?

Answer 21

• referral
• clinical review
• removal of M3M
• extraction of maxillary third molar
• coronectomy

less commonly:
- operculectomy
- surgical exposure
- pre-surgical orthodontics
- surgical reimplantation/autotransplantation

Question 22

What guidelines are available for third molars?

Answer 22

NICE Guidance on Extraction of Wisdom Teeth 2000

SIGN43 Management of Unerupted and Impacted Third Molar Teeth 2000

FDS, RCS 2020 Parameters of Care for patients undergoing mandibular third molar surgery