The aberrant molar

Signs of posterior crowding

The term ‘stacked molars’ refers to the radiographic appearance of the upper molar teeth being stacked almost vertically, one on top of the other, and is indicative of posterior crowding (Figure 3). In the lower arch, posterior crowding can also be seen on a dental panoramic tomogram (DPT) and in this case the lower second molar may appear distally angulated (Figure 4). This posterior crowding is an indication that mid arch extractions may be appropriate, especially if third permanent molars are present. A distally angulated lower second molar can be problematic in the longer term, as it increases the risk of impaction of the third molar and the angulation will make restorative work more challenging.

Mesially angulated second molar

Mesial impaction of the second molar is not always evident at the beginning of treatment and often occurs when the mesial marginal ridge of the second molar gets caught on the distal aspect of the first molar (Figures 5 a−c). The second molar continues to erupt, but sometimes tips further mesially.

Treatment in the case of the mesially impacted second molar

Uprighting of the second molar requires that the tooth is tipped distally. If the marginal ridge can be freed, for example with a separator, then it is possible it will self-correct. If this is not successful, or the impaction is severe, orthodontic uprighting may be required. Provided a tube can be bonded to the buccal surface of the second molar, either a continuous or a sectional archwire can be used to upright the second molar. A sectional arch^10,11 can be used in combination with an existing fixed appliance or it can be used on its own. For example, a single buccal tube can be bonded onto the second molar and a sectional rectangular stainless steel wire bent such that it passes up the buccal surface of the adjacent first permanent molar and onto its occlusal surface (Figure 6). In this case, it is important that the second molar is opposed so that, as it is tipped distally by the sectional wire, the occlusion with the opposing upper arch tooth encourages the roots of the lower second molar to move mesially. Alternatively, tubes can be bonded onto both the first and second molars and a sectional rectangular nickel titanium wire used to upright the first molar (Figure 7). A lockable attachment is used to prevent the wire from slipping either mesially or distally.

Bonding a button onto the occlusal surface of the second molar and using a fixed segmental spring, or a spring on a removable appliance, will also have the effect of tipping the second molar distally, thereby disimpacting it.¹²

A deep impaction of the second molar may necessitate extraction of this tooth. If this is the case, then the presence and position of the third molar should be assessed (Figure 8). If extraction of the second molar occurs at or near the time of formation of the bifurcation of the third molar, there is a high likelihood that the third molar will erupt into the position of the second molar.^13,14,15,16

Uprighting of the second molar is likely to become easier following any midarch extractions and space closure as the increase in available space may allow self-resolution of the impaction. If orthodontic correction is not possible and extraction is not considered a sensible option, surgical repositioning of the tooth may be required. If the procedure is performed before the completion of root formation, there is often good bony infill between the first and second molar.^17,18

Ectopic eruption of the maxillary molar

Ectopic eruption of the upper first permanent molar is a localized disturbance of eruption where the first molar comes into contact with the distal aspect of the second deciduous molar. This occurs in roughly 4% of all orthodontic patients,¹⁹ with a higher incidence reported in siblings.²⁰ There is often resorption of the distal aspect of the second deciduous molar, in which case only the distal portion of the first molar crown is visible intra-orally (Figure 9). If resorption of the second deciduous molar is extensive, it can lead to a direct communication between the mouth and the dental pulp (Figure 10), resulting in symptoms including pain, which will inevitably necessitate extraction of the second deciduous molar.

Treatment in the case of the ectopic maxillary first molar

Two-thirds of ectopic first molars impacted against the distal of the second deciduous molar will spontaneously self-correct.^21,22 Placement of a separator to free the mesial portion of the first molar will often encourage disimpaction of the tooth. However, access is notoriously difficult and separator placement can be uncomfortable. Removal of the deciduous tooth will allow the first molar to erupt, but the inevitable consequence is space loss and future crowding in the premolar regions as the molar moves further mesially (Figure 11).

Single ankylosed mandibular first molar

Very occasionally, the lower first permanent molar becomes ankylosed and can be seen in close relationship with the inferior alveolar nerve and canal. Fortunately, it is a rare occurrence, with an incidence of only 0.05%.²³ The aetiology is unclear, but it may be a variation of primary failure of eruption, associated with cystic change within the follicle or the presence of odontomes.²⁴ A local abnormality in bone resorption overlying the first molar on one quadrant is unusual, but possible.²⁵ Should the second molar erupt before the first molar, space loss and tipping will result in physical obstruction of the first molar (Figure 12). If root formation has progressed, and there is intimate relation with the lower border of the mandible, then the roots may splay mesially or distally, appearing dilacerated.