Clinical Success in Impacted Third Molar Extraction (eBook)

Extraction of third molars is one of the most common procedures in dentistry. Studies aimed at obtaining a better understanding of the reasons leading to the extraction of one or several third molars have been reported in the English and Scandinavian literature.

The National Institute of Health (NIH) has published the conclusions of the consensus development conference held in 1979 and, while waiting for an overall consensus on the indications for extractions, made the following recommendations:

Despite the accumulated experience in dealing with diagnosis and treatment of developmental anomalies concerning the third molars, extraction still remains controversial when patients are asymptomatic. Indeed, the proportion of prophylactic extractions is increasing and represents from 18% to 40% of the overall third molar extractions carried out in developed countries (Liedholm et al; Lysel and Rohlin). The majority of third molar extractions take place between the ages of 20 and 29 years (Liedholm et al). The main reasons for deciding on this course of action are:

The quality of the outcome of this procedure is determined by a series of factors:

• The relevance of the indication for the prophylactic extraction of one or several third molars

• The stage at which this decision is made, the type of surgical procedure selected, and the skill of the surgeon and the surgical team

However, as yet it has not been possible to establish universal guidelines concerning the relevant indications for prophylactic extractions because of the widely varying criteria used in different countries by different practitioners and different scientific communities (Worrall et al).

Furthermore, even from a review of the current literature, it is not possible to establish a significant risk-benefit ratio. In addition, the decision to proceed with an extraction is often made during a single consultation using only one radiographic examination in young patients who have completed their growth phase.

In order to establish a diagnosis it is essential to fully understand and be able to estimate the stage of eruption of the third molar. The latter is related to the evaluation of the prognosis in terms of impaction, partial retention or enclavement, tooth and periodontal lesions on the second and/or third molar, and the risk of anterior tooth crowding, which should always be avoided.

The English clinical practice consensus committees have suggested three factors that should be considered in the decision-making process for extraction of third molars (Liedholm et al):

• The age of the patient

• The angle formed between the great axis of the tooth and the occlusal plane, as well as the uprighting dynamics of this axis•

• The eruptive position

Some additional factors should also be taken into account:

• Oral hygiene

• Carious and periodontal indexes

The angulation of the axis of the third molar can be classified as follows, depending on the degree of distortion (Liedholm et al; Winter; see chapter 4):

• Mesially inclined

• Horizontal

• Vertical

• Distally inclined

Additionally, the eruptive stage can be specified as follows (Liedholm et al; see chapter 2):

• Totally erupted

• Partially covered with soft tissue

• Totally covered with soft tissue

• Totally covered with bone

The NIH has established that (NIH; Worrall et al):

In the context of orthodontic treatment, the extraction indication ratios for so-called prophylactic extractions are continually increasing, which naturally leads us to question this indication.

Embryology and eruption of the third molars

Like all human molars, the third molar is an accessional tooth, as distinct from the other teeth, which are known as replacement or successional teeth. The third molar originates from the primitive dental lamina and the bud only becomes evident at around the age of 4 or 5 years. Calcification occurs between 9 and 10 years of age, with full completion of the crown taking place between the ages of 12 and 15 years. As the eruptive movement begins, the tooth establishes an upright axis. The space available for its eruption depends on the growth of the posterior region of the arch. Emergence into the oral cavity occurs between 17 and 21 years of age. The tooth drifts along the distal aspect of the second molar in order to reach the level of the occlusal plane. Root formation is completed between the ages of 18 and 25 years.

The third molar encounters some difficulty in correcting its eruptive curve to the upright position because the direction of its growth often brings it under the cervical line of the second molar, thus causing it to become partially retained within the tissue.

As a general rule, the third mandibular molar usually fills the space formed by the retromolar triangle distal to the second molar.

In the maxilla, the complete absence of any bone obstruction allows the third molar to erupt in either the correct position or a labial position. The only obstacle to the smooth eruption of the third maxillary molar is the muscle-tendon-aponeurotic component (pterygoids, buccinator, or ligaments).

Developmental prognosis and eruption axis of the third molar

Most studies have shown that the eruption of the third molar is a multifactorial mechanism. None of these studies has established any significant correlation between the angulation of mandibular third molar and its impaction.

In the early stages of calcification, the mandibular third molar presents a physiologic mesial and lingual orientation. The change in angulation that leads to the vertical positioning of the tooth should occur between the ages of 14 and 16 years (Richardson, 1978). Establishing this vertical axis may occur during differential growth events in the crown surface and the mesial root in relation to the distal aspect.

According to Richardson, if growth occurs predominantly in the mesial area of the crown, vertical positioning of the tooth will result; but if growth of the distal root predominates, then the tooth bud will develop in a horizontal position (Fig 1-1).

In summary, it appears that the developmental prognosis for the third molar can be established from the age of 16 years.

Björk correlates the lack of appropriate space for the eruption of the third molar with three factors:

• The direction of vertical condylar growth

• Reduced mandibular length

• Backward orientation of tooth eruption

Silling notes that Class II skeletal relationships (ie, mandible positioned in an excessively posterior sagittal position in relation to the maxilla) with a short mandible and a closed mandibular angle are associated with impaction. Conversely, the percentage of impaction decreases in the case of a hyperdeveloped mandible (skeletal Class III).

Indications requiring the use of orthodontic techniques

Some techniques indicate the need for extraction before treatment is initiated.

Importance of posterior crowding

In the Tweed-Merrifield orthodontic technique, the arch is divided into three regions; the second and third molars are located in the posterior region. Crowding is measured by comparing the available space with that required to accommodate the second and third mandibular molars. The available space is the distance between the anterior border of the ramus and the distal aspect of the first molar, parallel to the occlusal plane. The required space is the sum of the respective mesiodistal diameters...