Tooth movement

Sir, studies on overeruption of unopposed posterior teeth are scarce and therefore we read the article by Craddock and Youngson¹ with great interest. The elegant method using 3D scanning of models and a great number of patients promised valuable results. However, there are some shortcomings that reduce the value of their findings. More specifically, there is a lack of information about the age when the antagonist teeth were lost and how long the studied teeth had been unopposed.

After referring to a study reporting significant overeruption of lower second molars after orthodontic extraction of upper second molars in children², the authors wrote “there is no reason to suppose that this movement does not also occur in unopposed adult sites”. This is an unsupported extrapolation conflicting with evidence of greater potential of dentofacial adaptation in children than in adults. In a Swedish study³, molars that lost their antagonists when the persons were above age 26 showed less severe overeruption than those for whom the age at loss of the antagonist tooth was unknown. This study³ reported no overeruption in 18% and ≥ 2 mm in 24% of molars that had been unopposed for more than 10 years. The present study¹ had fairly similar results: 17% no overeruption, 32% > 2 mm. The interpretation of the results differed, however. The British study¹ emphasised the high prevalence of overeruption of unopposed posterior teeth and its negative clinical significance. In the Swedish study³ it was concluded that not all unopposed teeth overerupted. It was also suggested that the finding that 3/4 of the unopposed teeth had overerupted < 2mm did not give strong support for a general prophylactic replacement of lost antagonists. Support for this opinion can also be found in the literature on the shortened dental arch concept – loss of molar support does not in general lead to impairment of the health of the masticatory system. A retrospective study by Shugars et al⁴ (not mentioned in the present article) found that after a mean of 6.9 years the extrusion of an unopposed tooth was <1 mm in 99% of the cases. The authors concluded that the great majority of their patients did not exhibit the expected adverse consequences generally assumed to be associated with non–replacement of a single posterior tooth.

Based on recent results, it would seem justified to discuss prosthodontic decision – making for unopposed teeth on an individual level rather than continue the general textbook warning for adverse consequences of not replacing lost posterior teeth.

Finally, a comment regarding “incidence” used both in the title and repeatedly in the text in the paper¹. Incidence and prevalence may be considered synonyms in many dictionaries, but they are not in epidemiology and medicine. Incidence means the rate at which a certain event occurs, e.g. the number of new cases of a specific disease occurring during a certain period in a population at risk (Dorland's Medical Dictionary, 2000).

The authors of the paper HL Craddock and CC Youngson respond: We thank the corresponding authors for their interest in our paper and their already important contributions to research in this area. They raise some interesting points and provide (embarrassingly!) accurate guidance to the correct use of the English language.

However, some comments in their letter may have arisen from a misconception of the purpose of the article. The aim of the study, as stated in the title and text, was to determine the proportion of individuals in the study population having overeruption and occlusal interferences at that time. We did not try to determine any factors implicated in the aetiology of overeruption. Age of tooth loss may indeed be a factor but, in designing our study, we recognised that we would not be able to accurately determine the age at which each tooth was extracted and so this was (intentionally) excluded. We consider that only a prospective study could do this with accuracy.

A second point that seems to concern the correspondents is our statement, following a discussion of tooth movement in children, that “there is no reason to suppose that this movement does not also occur in unopposed adult sites” and they make the observation that “This is an unsupported extrapolation ....” We acknowledge this. However, although there is greater potential for dentofacial adaptation in children we are not aware of evidence that suggests that it is completely absent in adults. We are currently investigating this aspect as part of another, larger, study of adults which may provide further evidence one way or the other.

The correspondents note that we did not include the article by Shugar et al. (2000). That study was not included within our article as their methodology was radiograph–based and observations were made of static relationships between teeth (given the limitations of assessing tilting or rotation of teeth from radiographs). Our study was quite specifically looking at occlusal interferences which are dynamic in nature. As interferences could not be assessed from radiographs we considered that Shugar et al's study was not entirely relevant to our paper.

We agree that many prosthetic textbooks suggest replacement of missing posterior teeth based upon traditional beliefs rather than scientific evidence and certainly do not advocate the mechanistic replacement of every tooth. However, we have noted that there are consequences of leaving an edentulous space with an antagonist tooth. If a prosthesis is being considered we hope that our study would encourage clinicians to examine for RCP and protrusive interferences before undertaking its provision.

Our study aimed, with its recognised limitations, to add to the limited evidence base on the prevalence of overeruption of teeth in adults in the UK and the role of these in occlusal interferences and has formed an early part of an ongoing investigation. We are grateful to the corresponding authors for raising the matters for discussion.