Data sources

PubMed provided the primary data source and was supplemented by a manual search of reference lists from relevant papers and books.

Study selection

Studies were included if they met the following inclusion criteria: all participants were Caucasian; only agenesis in the permanent dentition was considered; congenital absence was diagnosed clinically and radiographically; study participants had been selected randomly; study participants were >3 years old; and sample size was >1000.

Data extraction and synthesis

Data were extracted to calculate the overall prevalence of agenesis (no. children with >1 missing tooth/total no. in sample), the prevalence in boys versus girls, and the proportion of agenesis by tooth type. This information was then plotted chronologically.

Results

In total, 19 articles from the literature search met the inclusion criteria. The earliest published paper (1936) revealed a prevalence of 3.4%, whereas the most recent paper (1993) revealed a prevalence of 6.5%. Prevalence of agenesis varied from a low of 0.1% (1943) to a high of 10.1% (1973). Where gender differences were considered, girls were reported to have more agenic teeth than boys.

Conclusions

Although the time period is short and the data limited it would appear that hypodontia has been diagnosed more often in more recent studies.

Commentary

This article evaluated changes in the prevalence of hypodontia during the twentieth century, including in addition a descriptive analysis of this prevalence based on gender. From the literature search, a total of 19 publications were finally included, based on well-defined selection criteria.

A few comments regarding the methodology of this so-called meta-analysis are warranted to understand the article's potential and limitations. A meta-analysis is a statistical tool that allows a single estimate to be produced from the results of several independent studies inside a systematic review:¹ it should not be considered a publication type. The current "meta-analysis" is not supported by a well-structured systematic review. The authors attempted a systematic search of the available literature using only one electronic database search (PubMed) and a hand search of reference lists. This use of only one database to search for available literature risks missing key articles. Several other databases should be included to guarantee to readers that every effort was made to find the available evidence.^{2, 3} No indication was given here about the search terms used, which prevents us completing the search by ourselves.^{4, 5}

In addition, the authors indicate only that every study was assigned a similar weight, but no further indication is found in the text as to the actual statistical approach used for the meta-analysis. There should be a clear indication of the statistical technique used because several options exist and not all apply to a given situation.⁶

All the data presented in this publication are of a descriptive nature. Why then does this review present itself as a meta-analysis, if it does not present any meta-analytical data?

Despite these limitations, the authors made a significant effort to summarise the available evidence of a secular trend in hypodontia. A good review is always welcomed when contradictory reports seem to exist. Additional searches in other electronic databases may have added a few extra publications, but the impact of these "missing" publications probably would not affect the magnitude or direction of the reported findings.

Practice point

• Limited available evidence suggests a trend of increased hypodontia through the twentieth century, although the timeframe considered is not adequate to define a secular trend.

References

1. Chalmers I, Altman D. Systematic Reviews, London: BMJ Publishing Group; 1995.
2. Moher D, Pham B, Klassen TP, Schulz KF, Berlin JA, Jadad AR, et al. What contributions do languages other than English make on the results of meta-analyses? J Clin Epidemiol 2000; 53:964–972. | Article | PubMed | ISI | ChemPort |
3. Egger M, Juni P, Bartlett C, Holenstein F, Sterne J. How important are comprehensive literature searches and the assessment of trial quality in systematic reviews? Empirical study. Health Technol Assess 2003; 7:1–76. | ChemPort |
4. Glenny AM, Esposito M, Coulthard P, Worthington HV. The assessment of systematic reviews in dentistry. Eur J Oral Sci 2003; 111:85–92. | Article | PubMed | ISI | ChemPort |
5. Higgins JPT, Green S. Locating and selecting studies. Section 5. In Cochrane Handbook for Systematic Reviews of Interventions 4.2.5 (May 2005 update). The Cochrane Library. Edited by JPT Higgins, S Green. Chichester: John Wiley; 2005, Issue 3.
6. Sutton AJ, Abrams KR, Jones DR, Sheldon TA, Song F. Methods for meta-analysis in medical research, Chichester: John Wiley; 2000.