The Impacted Third Molar Tooth

Incidence of impaction in third molars was reported to be as high as 75% in the European population [1]. Impaction of third molars are a common condition with which there are peri-operative risks including transient and/or permanent iatrogenic nerve injury [1]. Although some impacted third molar cases may often warrant specialist surgical intervention, this lesson will aim to provide an overview and a step by step guide to the radiological examination of impacted teeth to help gage the difficulty of extraction. Some of the radiologic principles when examining the radiograph will be revised from the special investigations lessons.

The third molar is located at the distal end of the body of the mandible, in proximity with the relatively thin ramus. It is good to note that this is an area of weakness that is prone to fracture if excessive force is used during elevation of the impacted tooth usually occurring when adequate preliminary bone removal hasn't been carried out [1]. The lingual alveolar bone in the region is also thinner than the buccal. A literature review about impacted third molars suggests the roots of third molars are in close proximity with the mandibular canal, in most cases. In some cases the roots can contact or penetrate into the mandibular canal or they can be deflected. More importantly, proximity of the molar roots with the canal has been associated with an increased risk of iatrogenic nerve damage peri-operatively [1]. In order to minimise the risk of peri-operative complications during 3^rd molar surgery, several classifications were developed to assess the difficulty of the surgical procedure to allow adequate planning. For the purpose of this lesson we will discuss winters classification for 3^rd molars and Parallax when assessing impacted canines.  

What does the radiograph need to include?

When taking a radiograph of the impacted third molar the areas needed to carry out a thorough assessment are:

• Location and configuration of the 3^rd molar tooth
• Surrounding bone
• Mandibular canal/ surrounding structures
• The 2^nd Molar

Choosing the type of Radiograph

PAs

Periapical radiographs using a long cone paralleling technique have the advantage of reduction of radiation dose, reduction of magnification and a true relationship between bone height and adjacent teeth is demonstrated. The drawbacks of using only a PA radiograph is poor processing due to the flexibility of the film leading to a poor quality image. However, the biggest problem is that the mandibular canal cannot be clearly identified in the third molar region. Furthermore, the angulation of the PA film can affect the perceived location of the mandibular canal with the bone crest. 

Panoramic

Panoramic radiographs are extra-oral radiographs which can provide broad coverage for the oral structures and transmits only 1/10 the dosage of full mouth radiographs [1]. The shortcomings of panoramic radiographs include: high distortion, phantom images, and a lower image resolution. This can consequently hide some important vital structures, e.g. cervical spine images overlapping the anterior aspect of the mandible. Furthermore it does not accurately project the buccolingual relation of the 3^rdmolar to the inferior alveolar canal [1].

CBCT

Cone Beam Computed Tomography (CBCT) can provide a 3D image of the 3^rd molar and adjacent anatomical structure, this can be indicated in cases where the 2D radiograph demonstrates the mandibular 3^rd molar being in close proximity with the mandibular canal [1]. CBCT can optimise risk assessment and aid surgical diagnosis and treatment planning. This was evident in a prospective study that evaluated the role of CBCT in patients considered to be at increased risk of alveolar nerve injury from the panoramic radiograph. However, after reviewing the CBCT images, a significant number of patients were reclassified to a lower risk for iatrogenic inferior alveolar nerve injury (IAN), and this change in risk assessment also resulted in a different surgical approach. Despite all the advantages of CBCT and all the detail it provides, it transmits significantly more radiation to the patient and therefore, only used when patients are perceived to be at an increased risk for IAN.  This is in accordance to IRMER policies to keep the dose as low as reasonably possible (ALARP).

3^rd molar classifications and risk factor identification

The main features that should be considered are [2, 3]:

• Angulation of the 3^rd molar crown, this can be: Mesioangular, Distoangular, Horizontal, Vertical, Transverse or Inverted.

• 2^nd and 3^rd molar crown: Size, shape, presence/extent of caries and/or resorption.

• 3^rd molar roots: Number of roots, shape curvature/divergence (favourable and unfavourable), the level of development.

• Proximity/ relationship of 3^rd molar apices with ID canal. Note: the apices of the third molar often appear close to the mandibular canal. This appearance is due to the structures being superimposed [3]. However, the structures can actually be in close proximity.

• The normal radiographic appearance of the ID canal typically is that of two thin, parallel radiopaque lines (so called Tram lines) [3] and a radiographic appearance that may indicate a close proximity of the roots and ID nerve are
  • Loss of Tramlines
  • Narrowing of the Tramlines
  • A sudden change in the direction of tramlines or root
  • And/or a radiolucent band that appears across the root. 

Methods of assessing third molar depth/ complexity

Winter’s lines

This method involves the use of three imaginary lines drawn on a geometrically accurate PA radiograph.

1. The white line is drawn along the occlusal surfaces of the 1^st and 2^nd molars.

2. The amber line is drawn along the crest of the interproximal bone of the 1^st and 2^nd molar, extending distally along the internal oblique ridge, this indicates the margin of peri-coronal bone surrounding the 3^rd molar.

3. The red line is perpendicular to the white line (1^stline) and extends to the part of the crown where the elevator would be applied. If the length of the red line is greater than or equal to 5mm in length, the extraction is considered sufficiently difficult and sedation or GA may be indicated. Note: This measurement is taken from the point at which the red line crosses the amber line, and ends at the point of application.

Using the 2^nd molar as a guide

1. The roots of the adjacent second molar are divided horizontally into thirds.

2. A horizontal line from the point of application of an elevator to the 2^nd molar is drawn.

3. If the point of application lies opposite to the coronal, middle or apical 3^rd of the root the extraction is assessed as being easy, moderate or difficult respectively.

Note:

• The buccal or lingual inclination of the molars.
• A lower oblique occlusal radiograph OR/ 90 degrees lower occlusal taken on the side of interest, can help determine the tooth position in the horizontal plane, e.g. crown pointing out towards the cheek (buccally).
• The second molar should also be assessed to gage the long term prognosis and whether it should be extracted with/ instead of the third molar. 

The Impacted Canine

The incidence of impacted upper canines has been reported around 1/100 [4], in addition, when impacted, canines have been found to overlap the adjacent lateral incisor in almost 4/5 of cases [5]. The upper canines should be present on buccal palpation at 9 years old, if not present this should be identify early as the child may need an orthodontic referral. The purpose of the radiographic assessment is to determine the location, size, shape, and any related pathology to the maxillary canine, as well as identifying any resorption of the adjacent lateral incisor. Impacted canines will require specialist intervention such as an orthodontist alongside the oral surgery team to plan whether intervention is necessary and discuss the risks and benefits with the patient.

Assessment of the canine 

Note: Canines have a long path of eruption, in addition to being the last tooth in the upper arch to come through (other than the 8); therefore, crowding can be a common occurrence preventing the canine from entering the arch. It is also important to remember our anatomy of the hard palate, as the greater palatine vessels and nerves are present; therefore, careful flap design and planning is needed to preserve these structures.

Common Radiographs used:

• PAs
• Panoramic radiograph
• Upper standard occlusal

These radiographs may be used together to help ascertain the bucco-lingual location of the canine. This is of paramount importance when treatment planning, as it will influence the path of access, flap design and bone removal used.

Radiographic Interpretation 

Features that need to be examined include:

• The crown: size and shape this will help ascertain if there is enough space for the canine in the arch.

• Presence and severity of resorption: consider if there resorption involved and how many teeth are affected.

• Presence of any abnormalities/disease e.g. Odontomes, supernumerary’s or dentigerous cyst is more commonly associated with impacted teeth and can impede eruption.

• The root: Size, shape and stage of development

• Surrounding structures: 
  • This could be the deciduous canine, note the root length and degree of resorption.
  • Condition of surrounding bone
  • Is the deciduous tooth still present? and what condition is the C in?

Orthodontic Prognosis/ Complexity of the Ectopic Canine

As for the wisdom tooth, the oral surgeon will look at the complexity of the extraction using the Winter's lines. When discussing the images of the canine and planning, the orthodontic needs to know the likelihood of the treatment success before they communicate to the surgeons to carry out the chosen procedure. Methods of doing this are:

• Horizontal Overlap: Of the adjacent teeth
  • None = Good
  • Half = Average/ Fair
  • Complete = Poor

• Vertical Hight: Taken from the CEJ. Essentially, the more apical the poorer the prognosis
  • >50% Average
  • Above the apex = Poor

• Angulation: To the midline
  • 0-15 degree = good
  • 16-30 degrees = aAverage
  • 31+ degrees = Poor

• Root Apex postion: Horizontal plane
  • Normal postion = good
  • Above the first premolar = average
  • Above the second premolar = Poor

Localising the canine

Parallax

Parallax is defined as: the apparent displacement of an object because of different positions of the observer [3]. This means if two objects are viewed, in two separate planes, are viewed from two different positions, the objects will appear to have moved in a certain direction.

A useful acronym to remember for parallax is the SLOB rule: Two radiographs are used and the question to answer is: Does the unerupted tooth move with the second position of the radiograph or opposite to the second radiograph? This can be carried out in the horizontal or vertical direction.

Same = Lingual / Opposite= Buccal.

Horizontal Parallax:

This is the horizontal movement of the tube head of the X-ray.

This involves either:

Taking two PAs, e.g. one centred on the central incisor and the other at the impacted canine region.

OR/

A PA centred on the canine region with an upper standard occlusal centred in the midline. Note: the upper occlusal shows both sides of the arch.

• Palatally (think lingual in the slob rule) positioned canines will appear to have moved in the same direction as the tube head.
• Bucally positioned canines will appear to have moved in the opposite direction of the tube head.
• Unerupted canines in the same plane as the teeth will appear not to have moved.

Vertical Parallax 

This is the vertical movement of the tube head of the X-ray.

This involves a combination of one upper standard occlusal which is angled at 65-70 degrees, and a panoramic radiograph taken at 8 degrees.  

Important: To make things a little more complex: We can look at the tooth as a whole, and decide which way it moves; however, using the methods above, the tooth could actually move in two directions.

Example:

• The crown could move in the opposite direction to the x-ray beam and the root could move in same direction. Therefore, when applying the SLOB rule, we can determine that the tooth has a buccally placed crown with its apex of the root facing palatally (lingual); meaning the tooth is lying across the hard palate.

CBCT

CBCT allows the visualisation of the unerupted canine in different planes thus there is no need to use the principles of Parallax. However, this method transmits a higher dose of radiation to an often younger patient, so only used when necessary.  

Other:

• Stereoscopic views
• Cross-section Tomography