From Volume 10, Issue 3, July 2017 | Pages 96-101
Second premolars are one of the last successional teeth to erupt in the maxillary and mandibular arches. Early loss of primary teeth or first permanent molars can lead to disrupted eruption of these teeth. This article gives an overview of the possible aetiology and treatment of the aberrant second premolar.
Second premolar teeth are one of the final successional teeth to erupt into the occlusion. Although they often erupt without incidence, this is not always the case. There is potential for unerupted second premolars to cause damage to adjacent teeth or migrate from their expected position. This paper presents a summary of second premolar formation and eruption and possible consequences of impaction.
The maxillary and mandibular second premolars begin to calcify between 24 and 30 months post-partum. They are often the last successional teeth to erupt, usually between the ages of 10 and 12 years in both the mandibular and maxillary arches, and are preceded by the second primary molars. Following eruption of the premolars, which are narrower than their predecessors, there is a slight decrease in arch length as the ‘E’ space is closed by physiological mesial drift.
The premolar tooth germs develop apically to the roots of the primary second molars and are in close proximity to either the maxillary sinus or the mental foramen. As the teeth develop, the crowns often become positioned between the roots of the primary tooth (Figure 1). There is subsequent resorption of the primary tooth roots under the eruptive influence of the permanent tooth.
Excluding third molars, the mandibular second premolar is the most common tooth to be developmentally absent, whilst the maxillary second premolar is the third most commonly missing.1 Literature specific to premolar teeth is limited, but the overall prevalence of missing second premolars is reported as approximately 2%.2
Treatment when the second premolar is absent will depend on the severity of the malocclusion, the presence of crowding, the condition and long-term prognosis of the retained second deciduous molar(s) and the wishes of the patient and the patient's family. Treatment planning must therefore be performed on a case-by-case basis. In some instances it may be advisable to maintain the primary tooth for as many years as possible, whereas in other cases it may be prudent to extract the primary tooth and either close the resultant space using orthodontics, or provide the patient with some form of prosthetic replacement. If the decision is made to retain the primary tooth, it is possible for it to last into the fifth decade of life, even if there is some evidence of root resorption.3,4
The lower second premolars are the third most commonly impacted tooth after the third molar and the maxillary permanent canine.5 The most frequent cause for the impaction is premature loss of the primary predecessor and subsequent space loss by mesial movement of the first molar (Figure 2). In the lower arch the second premolar may either remain impacted within the line of the arch, or it may erupt lingually. In the upper arch the same is true, but in this case the upper second premolar often erupts palatally.
Where there has been space loss following extraction of primary teeth, it is generally expected that the second premolar will erupt spontaneously on the lingual or palatal aspect of the arch. Where appropriate, it can be removed with ease under local anaesthesia or, alternatively, space may be created orthodontically for its inclusion in the arch. Careful analysis and determination of the position and inclination of the tooth must be made radiographically, in order to assess whether the crown has indeed deflected out of the line of the arch. If space is created, the tooth will often erupt into a reasonable position, but, very occasionally, it will need to be exposed, bonded and then traction applied to bring it into the line of the arch (Figure 3).
If the premolar remains impacted in the line of the arch, there are potential risks associated with leaving it in situ, principally a risk of resorption of the roots of the adjacent teeth.
It is well known that unerupted maxillary canine teeth have the potential to resorb upper incisors in a large proportion of cases, should the crowns and roots be in close proximity.6,7,8 This same risk applies to the unerupted second premolar. Should the tooth continue to erupt into the line of the arch, there is a significant risk of resorption of the first premolar, the first permanent molar, or both. Howard described three cases of impacted, unerupted mandibular second premolars that had caused resorption of adjacent teeth. In all cases, the impacted tooth erupted when the resorbed erupted tooth was extracted.9 Tracey and Lee also reported on three cases of unerupted premolars causing extensive resorption of the adjacent first permanent molars, all of which subsequently required extraction.10 They highlighted the importance of determining the bucco-lingual position of the unerupted second premolar in order to assess whether the tooth is likely to erupt or will remain impacted, in which case there is an increased risk of resorption of the roots of the adjacent teeth. Similarly in the upper arch, an unerupted maxillary second premolar can also lead to resorption of the roots of the first molar11 and/or first premolar. In the case of the patient illustrated in Figures 4–6, early loss of primary teeth due to caries had led to space loss and subsequent buccal crowding of the upper permanent canines and impaction of the upper second premolars. The radiographs taken at the time show extensive root resorption of the upper left first permanent molar and first premolar (Figure 5). Extraction of these resorbing teeth permitted the second premolar to erupt into the line of the arch (Figure 6).
Early loss of the mandibular first permanent molar may result in distal migration of the mandibular second premolar, if the latter tooth escapes from between the roots of the second primary molar (Figures 7a and b). In such cases, the distal impaction may self-correct, with the second premolar erupting mesially to the second molar without orthodontic intervention.12 However, it can prevent mesial movement of the second permanent molar into the first molar extraction space so that, when the second premolar erupts, a large amount of space remains between the first and second premolars. Such a space can subsequently be difficult to manage (Figure 8).
Although an ectopic path of migration can follow the early loss of primary predecessors or a first permanent molar, as outlined, sometimes it occurs for no apparent reason other than perhaps an ectopically positioned second premolar tooth crypt. The degree of migration and the potential effects can be marked. In some instances, the second premolar migrates sufficiently apical to the adjacent teeth that they escape resorption entirely (Figure 9). However, as with impacted canines, migrating second premolars can also lead to significant resorption of the roots of adjacent teeth. Figure 10 shows a periapical radiograph and a cone beam computed tomograph (CBCT) image of an unerupted upper second premolar migrating distally, leading to resorption of the upper first permanent molar.
When a mandibular second premolar migrates distally, it will either self-correct when it meets the mesial aspect of the second molar, or it may continue migrating distally. The depth of the crown is an important factor in determining whether the second premolar will cause resorption of the adjacent tooth root or, if positioned apical to the root tip, continue its eruptive path along the body of the mandible.9 Sutton reviewed 62 cases of distal migration of the unerupted second premolar and reported a case of a tooth migrating to the coronoid process.13 Orton and McDonald described a lower right mandibular second premolar continuing to migrate along the body of the mandible and into the ramus.14 The tooth might have continued to migrate into the coronoid process, but it was removed due to concerns that it might perforate the cortex.14 Infante-Cossio et al described a case where a patient attended with pain in the left angle of her mandible associated with an unerupted mandibular second premolar that had migrated and perforated the cortical bone, requiring a sagittal osteotomy to facilitate its removal.15 Second premolars may migrate completely to the condyle or coronoid process.16,17 Removal of these migrated teeth is indicated to protect bony anatomical structures, despite the complexity and risk of surgical intervention.
When a migrating second premolar is exposed and bonded, orthodontic treatment to bring an almost horizontal tooth into the arch can be a challenge, as considerable apical root movement is required. Figure 11 illustrates the pre-treatment position of an almost horizontal upper second premolar which, following exposure and bonding and the application of traction, erupted almost in the same horizontal position. Considerable effort was required to enable it to be uprighted (Figures 12a and b) and brought into the line of the arch.
This paper presents a summary of second premolar eruption, and highlights some of the potential clinical outcomes when this eruptive process is disturbed. It reinforces to clinicians the vigilance that should be taken when carrying out clinical and radiographic examination of young patients, with the aim of identifying potential aberrant second premolars at an early stage so that appropriate treatment decisions can be made.
