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INTRODUCTION
Congenital absence of teeth, hypodontia,1 is the most prevalent craniofacial malformation and dental anomaly.2 Its reported prevalence varies across studies, continents, racial groups, dentitions, sexes, and jaws. The prevalence ranges widely among Caucasians (3.9% to 11.3%)3 and is higher among African populations (13.4%), followed by European (7%), Asian (6.3%), and Australian (6.3%) populations.4 Female subjects are more likely to show hypodontia than male.3-6 The occurrence of this condition can be classified as common, less common, and rare.3 Canine agenesis refers to failure of canine formation, which are considered to be the most stable teeth; agenesis of maxillary canines is less common, while that of mandibular canines is rarely observed.7 Nevertheless, the absence of canines complicates orthodontic treatment planning because of their esthetic and functional importance.8
Information regarding the global and regional distribution of canine agenesis is of paramount importance since it can elucidate the treatment need, complexity of treatment, and the resources required to manage these cases. Early detection may facilitate interventions to ameliorate the disease process, such as early primary tooth removal to enhance space closure or maintenance of the predecessor to ensure adequate alveolar bone for future replacement.9 Some degree of multidisciplinary combined management may be required, especially in cases of unilateral agenesis. Furthermore, the assessment of agenesis prevalence by continents can reflect the comparative frequency of missing teeth in different regions of the world. There is a paucity of studies on the prevalence of canine agenesis, with very few studies reporting the prevalence of agenesis exclusively, and most only superficially referring to individual studies without analyzing the combined prevalence, and instead only focused on the prevalence of hypodontia in general.
The aim of the current review was to summarize the available worldwide data on canine agenesis. The primary objective was to systematically evaluate the available evidence related to its prevalence in different general populations and continents. The secondary objectives were to report the prevalence by jaw (maxilla and mandible), sex (male and female), and location (unilateral or bilateral), and to report the associated dental anomalies. Identifying the overall prevalence and pattern can enhance management and better treatment planning of this condition.
MATERIALS AND METHODS
Protocol and registration
This systematic review was conducted and reported in accordance with the Cochrane Handbook for Systematic Reviews of Interventions and Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).10 The review protocol was registered in the National Institute of Health Research database (https://www.crd.york.ac.uk/prospero/; protocol no: CRD42019120204; registration Date: March 14, 2019). Ethical approval was unnecessary since we retrieved data from previously published studies in which informed consent had been obtained by the primary investigators.
Eligibility criteria
Inclusion criteria
Participants: Male and female subjects with no age restriction; sample size of 50 participants or more
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Outcome measures
Study design: Observational studies (population-based studies, hospital/clinic-based studies, and cross-sectional studies), studies supported by radiographic imaging of the teeth or relevant history and records
Published English studies with no publication-year restriction
Exclusion criteria
Studies on syndromic patients (e.g., patients with a cleft involving the alveolus or those with Down’s syndrome)
Case reports, case series, systematic reviews, or meta-analyses
Studies that reported canine agenesis in specific samples of patients with teeth agenesis that cannot be generalized to the general population, e.g., canine agenesis in hypodontia patients with no relevance to the general population.
Information sources, search strategy, and study selection
Comprehensive electronic database searches without publication-year restrictions were conducted for literature published until May 4, 2019 (Table 1 and Figure 1). Only articles in English were included from relevant databases such as PubMed, Web of Science, Scopus, OpenGrey, and Science Direct. In addition, hand searches of relevant journals, such as those listed in relevant systematic reviews, was performed. Articles and reference lists of the included studies were individually screened for additional relevant studies. The corresponding authors were contacted for obtaining clarifications or additional information when necessary.
The search strategy was implemented using a combination of Medical Subject Headings (MeSH) and free-text words for PubMed and optimized for each database (Table 1). Literature search, study inclusion, methodology quality assessment, and data extraction were carried out independently and in duplicate by two pairs of reviewers (S.S. & M.C.W. and S.A.M. & J.J.) who were not blinded to the authors, and the results were revised by the fifth author (M.M.S.F.).
Eligible articles were assessed in two phases. In the first phase, only titles and abstracts were screened. Full-text assessment was then conducted in the second phase to determine final eligibility. Articles were excluded when they did not meet one or more of the inclusion criteria. Any disagreements were resolved by discussion and consultation with the fifth author (M.M.S.F.) for consensus.
Data items
A standardized data extraction sheet was designed for data extraction by the two pairs of independent reviewers in duplicate (S.S. & M.C.W. and S.A.M. & J.J.). Data extraction included general information (the names of the authors, the year of publication, and the study setting), data pertaining to methods (study design), participant data (sample size, age, sex, country, region, race, and population) and outcome data (primary and secondary outcomes mentioned). Race referred to a group of people who shared similar physical characteristics.
Risk of bias across studies
Critical appraisal of the study was performed using a modified version of Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist11 consisting of seven items related to (1) study design, (2) study setting, (3) participant criteria, (4) sample size, (5) variable description, (6) outcome measurements, and (7) statistical analysis. The quality of the studies was categorized as weak (3 and less), moderate (4 or 5), or high (6 or more) by two pairs of independent reviewers in duplicate (S.S. & M.C.W. and S.A.M. & J.J.). Any disagreements were resolved by discussion and consultation with the fifth author for consensus (M.M.S.F.).
RESULTS
Study selection
Initial database and hand searches yielded a total of 6,770 studies, of which 2,490 duplicates were excluded (Figure 1). Subsequent exclusion by title (3,157) and abstract (1,071) yielded 52 remaining articles, which were considered for this review. This included 18 articles identified from database searches and 34 articles identified with hand searches.
Study characteristics
Full texts of all 52 articles were assessed for eligibility. Two studies12,13 were excluded since they did not report actual prevalence data or data that can be used to calculate the prevalence for their populations, and another study excluded samples with canine agenesis in both arches,14 which affected the accuracy of the overall prevalence estimation.
Risk of bias within studies
Critical appraisal of the remaining 49 articles was performed using the modified STROBE checklist (Table 4). After excluding 14 studies (28.6%) that were of moderate quality (modified STROBE score of 4 or 5), 35 (71.4%) high-quality studies5,15-48 (modified STROBE score of 6 or 7) were included in this systematic review.
Results of individual studies
Description of the studies
A total of 35 studies were included in this review. The demographic data are summarized in Table 5. Slightly less than two-thirds15-34 (57.1%) of the selected studies were retrospective, about a third35-46 (34.3%) were cross-sectional, while two studies29,47 were both retrospective and cross-sectional. Almost half (48.6%)17-21,24,29-35,42,45-47 of the studies were published after the year 2010. The sample size varied from 85 to 100,577, with a median of 1,622 participants. The study participants, aged 5–46 years, were recruited equally from orthodontic (36.1%) and dental clinics (36.1%), followed by the pediatric clinic (13.9%). One study45 recruited patients from both orthodontic and pediatric clinics, another44 recruited them from dental and orthodontic clinics, while another37 recruited participants from the Australian defense force. Only two studies39,43 were epidemiological surveys. Almost an equal number of studies were from Asia (n = 14, 40.0%) and Europe (n = 12, 34.3%), followed by South America (n = 4, 11.4%), North America (n = 2, 5.7%), Australia (n = 2, 5.7%), and Africa (n = 1, 2.9%). The global reported population prevalence of canine agenesis ranged from 0.0% to 4.7% (Table 6), with high variations among studies, and the median prevalence was 0.29%. The term population refers to the people living in a particular country. The prevalence of canine agenesis in the maxilla (between 0.03% and 4.7%) was higher than that in the mandible (between 0% and 1.12%).
Synthesis of results
Primary outcomes
Due to the high variation, the overall prevalence was calculated from studies that reported canine agenesis data either by individual (population prevalence based on the number of assessed individuals) or by the number of missing teeth (prevalence of canine agenesis based on the number of missing teeth). The overall prevalence of canine agenesis based on the total number of missing teeth (2.99%) (Table 3) was higher than the population prevalence based on the number of assessed individuals (0.30%) (Table 2). The prevalence in the general population excluding the orthodontic population was higher (0.38%) than that investigated among the orthodontic population (0.10%). The overall population prevalence was the highest in Asia (0.54%), followed by Africa (0.33%),23 and was the least in Europe and South America (0.19% in both continents). Similarly, the prevalence of agenesis as a percentage of missing teeth was the highest in Asia (7.40%), followed by Oceania (Australia) (7.14%), South America (4.51%), Africa (3.80%), and Europe (2.55%), with the lowest prevalence in North America (1.85%) (Table 6).
Secondary outcomes
Pooled prevalence based on studies that reported the prevalence in the maxilla and mandible using data for individuals revealed the same findings, with the maxilla-only prevalence being the highest (88.57%), followed by the prevalence in both the maxilla and mandible (8.57%), and the mandible-only prevalence being the least (2.86%) (Table 7). Similarly, the overall prevalence by teeth was greater in the maxilla (73.73%) than the mandible (26.27%). By continent, the prevalence in the maxilla was the highest in Asia (0.78%), followed by Africa (0.33%) based on one study,25 South America (0.19%) based on one study,14 and the least in Europe (0.13%) based on five studies.16,20,34,36,41 Meanwhile, the prevalence of agenesis in the mandible only and in both maxilla and mandible was only reported in Europe (0.01% and 0.03%, respectively; Table 7). Among all forms of canine agenesis, the overall prevalence of missing maxillary permanent canines was almost similar (35.60% and 39.63%, respectively). Likewise, in the mandible, the prevalence of missing mandibular permanent canines was almost similar (12.07% and 12.69%, respectively).
By sex, the overall ratio of canine agenesis was higher in females than in males, with a female:male ratio of 1.23. However, this was only true in Europe and South America, wherein females were twice as much affected than males. In Asia, the ratio was higher among males (ratio = 0.88), while the prevalence was the same for both sexes (ratio = 1) in Africa (Table 8). The overall bilateral:unilateral agenesis ratio was 1.13. The worldwide prevalence of unilateral agenesis was almost similar to that of the bilateral form (50.0% and 46.7%, respectively). However, in Asia, the prevalence of unilateral agenesis was almost double that of bilateral agenesis (66.7% and 33.3%, respectively). In Europe, the prevalence of bilateral (58.8%) agenesis was higher than that of unilateral agenesis (35.3%) (Table 9).
DISCUSSION
This systematic review attempted to evaluate the global distribution of canine agenesis in isolation.3,49 We presented the population prevalence of canine agenesis in terms of individuals, which better reflected the actual treatment need, unlike another review4 that reported the prevalence in terms of the number of missing teeth. Reports based on individual prevalence without considering the population sample can be biased, since the bigger quantity of smaller-sized studies may overwhelm the smaller quantity of bigger-sized studies and distort the final summary. Therefore, we recalculated the prevalence in every included study to generate an overall prevalence.
In this review, an almost equal proportion of the included studies were conducted in Asia (37.1%) and Europe (37.1%); this was in contrast to the review on the overall prevalence of hypodontia, in which most studies were conducted in European countries (43.0%), followed by the Asian region (32.0%).4 The global distribution in this review ranged from 0.0% to 4.7%, with a pooled overall prevalence of 0.30%, which is much lower than that of hypodontia (6.4%).4 Polder et al.3 reported that canines are one of the rarely missing teeth after the first and the second molars.
In this review, the prevalence of canine agenesis was higher in the Asian region than in the African, European, and South American regions. In contrast, the overall prevalence of hypodontia was the highest in Africa (13.4%, 95% confidence interval [CI]: 9.7, 18.0), followed by Europe (7% CI: 6.0, 8.0%) and Asia (6.3% CI: 4.4, 9.1).4 This suggests that canine agenesis per se is more common in the Asian region than in the European region, possibly due to the racial differences between the two continents.
Our findings showing that canine agenesis was more common in the maxilla than the mandible are in agreement with the general pattern of hypodontia reported in two other systematic reviews addressing hypodontia, both of which reported marked differences between the jaws in relation to the frequency of agenesis of various tooth types.3,4 Similarly, the greater prevalence in females is in agreement with the findings of these two systematic reviews.3,4 However, in Asia, the higher prevalence in males may indicate a genetic inheritance of this trait among males. Bilateral agenesis was more prevalent than unilateral agenesis in Europe, similar to the general pattern of hypodontia except for the maxillary lateral incisors.3 However, in Asia, the prevalence of unilateral agenesis was double that of the bilateral form, indicating a genetic inheritance pattern among Asians. The overall information presented in this review could provide valuable guidance to clinicians for treatment planning and managing patients with canine agenesis.
Since we aimed to report the prevalence based on the number of individuals with missing canines, we could not include studies reporting the number of missing canines instead of the number of individuals with missing canines; this limited our ability to present the data in terms of combined prevalence by number of teeth. Most studies assessed either orthodontic patients,5,14,17,19,22-25,29,31,32,43 pediatric patients,21,26,27,33 both orthodontic and pediatric patients,15 both orthodontic and dental patients,44 or dental patients.16,18,20,28,30,35,39-41,45-47 Three studies were epidemiological surveys of school children,34,37,42 one enrolled defense force recruits,36 while only one study was a truly epidemiological study on a general population.38
CONCLUSION
The global distribution of canine agenesis ranged from 0.0% to 4.7%, with a pooled overall population prevalence of 0.30%.
The population prevalence of canine agenesis was the highest in Asia (0.54%), followed by Africa (0.33%); the least prevalence was observed in Europe and South America (0.19% for both continents).
The highest prevalence was of the maxilla-only form (88.57%), followed by the presentation in both maxilla and mandible (8.57%), while the mandible-only form showed the lowest prevalence (2.86%).
Canine agenesis was more common in females, with an overall female:male ratio of 1.23, except in Asia (0.88) and Africa (1).
In Asia, the prevalence of unilateral agenesis was almost double that of bilateral agenesis, but in Europe, bilateral agenesis was more common.
With a clearer picture of the occurrence of canine agenesis and its accompanying predilection, management of the condition can be better predicted and planned. Future research on prevalence is suggested to report both in terms of missing teeth and individuals, also moving forward, research linked to its aetiology and genetic-based treatment can be considered.
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