Chapter 15

Agenesis of Premolars

15.1 Introduction

Failure of premolars to form is not expected by patients and parents unless there has been a family history of agenesis. There are no symptoms connected with absence of a premolar that act as a warning until such time as the deciduous molar is seen not to exfoliate. In addition, because the deciduous molars are the last deciduous teeth to be lost, delay in exfoliation may not be noticed, even if the tooth has indeed been recognised by the patient or parent to be a deciduous tooth. Moreover, the agenesis of a premolar does not affect the normal course of events in the transition of the teeth mesial to it.

Agenesis of second premolars is more prevalent in the mandible (approximately 4.4%) than in the maxilla (approximately 1.7%). The clinical consequences of agenesis in the mandible are more significant. Proportionately, agenesis of first premolars is rare (mandibular — about 0.1%; maxillary — about 0.2%).¹⁰

This chapter starts with a discussion of diagnosis and treatment possibilities. Following this, mandibular agenesis is dealt with, divided into space conditions in the dental arch, dental arch relations and the profile, possibility for guiding the development of the dentition, and unilateral agenesis. Agenesis of the maxillary second premolar is covered similarly. Standard solutions to the problems are presented in tabular form.

15.2 Diagnosing agenesis of premolars

Agenesis has strong genetic origins. In history taking it is essential to enquire about missing teeth in other family members. The lack of mandibular second premolars is often found in several children in a family. Whenever 6 months after the exfoliation of one deciduous molar the contralateral tooth is still present and not loosening, agenesis of the successor should be suspected. An asymmetric sequence of emergence also is suspect. In the mandible, careful palpation of the alveolar process will provide more information. At this stage of development the mandibular premolars that have not yet emerged can be felt within the process unless they are in a markedly abnormal position. In the maxilla this is usually less evident. Palpation of mandibular premolars is frequently possible 2 years earlier, although of course the unerupted teeth are less easily found then. At that time a plan of action would be formulated if agenesis were suspected, and radiographic confirmation of the situation would then be necessary. With the circumstances fully appraised, it might then be appropriate to design a strategy for guiding the further development of the dentition.

It is a common belief that infraposition of deciduous molars is often associated with agenesis of the successors. However, the literature does not support this proposition with adequate data.²⁰⁴ A satisfactory basis is lacking for accepting ankylosed deciduous molars as a specific symptom of agenesis.

15.3 Therapeutic possibilities

There are three possible options:

1. Preserve the deciduous molar as long as possible

2. Orthodontically close the spaces that are caused by the absence of the premolar and its predecessor by guiding the development of the dentition and/or use of appliances

3. Replace the missing tooth with a prosthesis

The pros and cons of these alternative treatment modes will be considered in the following paragraphs.

15.4 Agenesis of premolars in the mandible

No other teeth are so often absent as the mandibular second premolars. Bilateral agenesis occurs regularly. Mandibular first premolars are very seldom absent; consequently that condition will not be dealt with specifically.

15.4.1 Space conditions in the mandibular dental arch

In situations of crowding in the mandibular dental arch, removing the deciduous molars and using the existing space to relieve the crowding is the obvious solution. This applies especially to bilateral agenesis and severe anterior crowding. The intended orthodontic space closure does not necessarily involve use of appliances. In cases of agenesis of a second premolar, timely extraction of the second deciduous molar can, in favourable conditions, secure spontaneous correction of the situation without active therapy being needed later (see 15.4.3).

Agenesis frequently is associated with relatively smaller mesiodistal crown diameters of the teeth that do form. That being so, crowding occurs less often, with spacing being more common.

In cases of crowding in the mandibular dental arch and no other need for orthodontic therapy, the first indication is to preserve the deciduous molar. A mandibular second deciduous molar that does not become ankylotic can function well for a long period, sometimes until old age.¹²⁵ In particular, if there has been minimal root resorption the prognosis for prolonged retention is good. An intact crown is also important because indirect pulp changes resulting from caries appear to encourage root resorption. Deciduous teeth with good caries resistance may also exhibit less tendency toward root resorption in the absence of successors.

In situations where regardless of agenesis there would be some need for orthodontic treatment, absence of premolars would be an extra indication for treatment. In this regard, orthodontic closure of any spaces would form a part of the total therapy.

15.4.2 Dental arch relation and profile

The sagittal arch relations and the profile also play an important part in making treatment decisions.

Class I anomalies with mandibular crowding often also exhibit maxillary crowding. Agenesis of a mandibular premolar in such cases calls for extraction of two maxillary premolars (and usually one mandibular premolar from the side opposite to where the agenesis is). A contraindication for this approach is a profile showing a sunken mouth, or where there is a danger of such a profile developing after extractions.

Absence of mandibular premolars, whether because of agenesis or extraction, makes treatment of Class II anomalies significantly more complicated. It makes extraction of maxillary teeth almost unavoidable. Subsequently, treatment with fixed appliances is essential, particularly because bodily movement of the teeth bounding the spaces in the arch will be required. The maxillary incisors often have to be moved bodily palatally.

In Class II/1 anomalies with marked crowding in both dental arches the most likely solution involves finishing each quadrant with just one premolar. When there is no crowding this approach contains the considerable risk of undesirable retrusion of the mandibular incisors, because mandibular premolars and canines tend to drift distally if space is present there. This risk especially prevails in Class II/1 anomalies of a full premolar crown width, and even more so when the lower lip is interposed between maxillary and mandibular incisors or if that becomes possible as the mandibular incisors tilt lingually. The lingual tipping is even more likely when thumb or fingers are still being sucked. In such cases, depending on the seriousness of the abnormality, there may be an indication for a relatively early start with treatment. Timely referral to an orthodontist is always desirable.

In Class II/1 anomalies with a normal mandibular arch and a deciduous second molar in good condition, retention of that tooth can be the most suitable action when agenesis of its successor is apparent. This is particularly so when, as a result, simple orthodontic treatment restricted to the maxillary complex can secure the desired improvement. With Class II/2 anomalies it is generally undesirable to reduce the amount of tooth material in the mandible. To conserve a good deciduous second molar for as long as possible is of great importance when no orthodontic treatment is to be instituted. However, if orthodontic treatment is planned, whether or not a second deciduous molar is conserved must be considered in connection with the total, usually complex, treatment plan. Transplantation of a maxillary premolar to the site of the agenesis certainly comes into view.

Class III anomalies are less of a problem when agenesis of a mandibular premolar occurs. Closing diastemata can be associated with correction of the reversed overjet.

15.4.3 Guiding the development of the dentition

Attention was previously drawn to the fact that it usually is not necessary to verify with radiographs any suspected agenesis of a premolar prior to 1 year before the beginning of the transition of the posterior teeth and the emergence of the second permanent molar. Extraction of a second deciduous molar (and/or a first) can, at that stage, still result in good natural space closure in the mandibular arch. The fi/>

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