The objectives of this study were to determine the prevalence of tooth agenesis (excluding third molars) and its patterns in a Dutch population of patients with ns-RS and ns-CP and to test the hypothesis that tooth agenesis in ns-RS is mainly related to the developmental disturbances that also produced the palatal cleft seen in the majority of patients. The present study included 115 patients with ns-RS and observed tooth agenesis in 47.8% of patients. This number corresponds roughly to the prevalence rates for tooth agenesis in ns-RS previously found in Canada [7] (32.9%) and in Norway [6] (42.3%), as well as to the rates observed for patient samples including syndromic cases in Sweden [15] (35.7%) and Finland (50%) [16]. Contrary to the aforementioned Canadian and Norwegian studies, we found a significantly greater prevalence of hypodontia in female patients. In the Swedish study by Larsson et al. [15] and the Finnish study by Rintala et al. [16], no distinction was made between male and female patients. This finding matches the result of a large meta-analysis of tooth agenesis in Caucasian populations, which showed that females are 1.37 times more susceptible to tooth agenesis than males [5]. The influence of sex in the pathophysiology of tooth agenesis remains unclear though.
In the comparison group of 191 patients with ns-CP, the prevalence of tooth agenesis was lower with 29.8%, which lies within the range of 21.0–36.8% described for ns-CP in other patient populations [17–19]. Again, tooth agenesis demonstrated a (non-significant) tendency to a greater prevalence in the mandibula than the maxilla, matching the findings of Aizenbud et al. [20] in an Israeli patient population. However, a maxillary predominance in ns-CP was observed in Finland by Ranta and Tulensalo [21] and in New York by Shapira et al. [22], though the latter study only included 9 patients with hypodontia. In both ns-CP and ns-RS, the most commonly missing teeth were the second premolars in both dental arches and the maxillary lateral incisors, which matches the findings of earlier studies [7, 20–23]. Although the second premolars and lateral incisors are also the most frequently absent teeth after the third molars in the general population [5], the prevalence of tooth agenesis is substantially higher in the patient groups studied.
With regards to the patterns of hypodontia, this study showed that in line with the literature [6, 7, 20], tooth agenesis occurred more often in the lower dental arch than in the upper arch. Furthermore, in ns-RS, tooth agenesis was shown to present itself bilaterally in two-thirds of patients, analogous to the findings of Andersson et al. [6] for Norway and Antonarakis and Suri [7] for Canada. As in the latter study, a completely symmetrical pattern was seen in around 45% of ns-RS patients with tooth agenesis. Moreover, similar to the Norwegian and Canadian studies, the teeth most frequently absent were the second premolars in both arches followed by the maxillary lateral incisors. Although mandibular tooth agenesis was observed more frequently in ns-RS, the occurrence of bilateral and symmetrical patterns of agenesis was not significantly different in ns-CP. Interestingly, nearly all patterns of tooth agenesis per quadrant observed in ns-CP were also seen in ns-RS, though in the latter group, additional patterns were recorded.
This study also examined the relationship between the extent of the palatal cleft and the degree of hypodontia. Developmental anomalies of the dentition are increasingly considered to be a subphenotype of the cleft population since both teeth and lip/palate are derived from the branchial arch precursors and influenced by related morphogenetic patterning signals [4]. Indeed, several candidate genes have been associated with both dental anomalies and clefting: IRF6, MSX1, PAX9, and TGFB3 [4, 9]. Moreover, a small number of earlier studies found a positive correlation (of unknown strength) between the cleft extent and the severity of dental agenesis in patients with ns-RS [6] and ns-CP [21]. However, the present study found no evidence for any association in either group of patients.
In summary, this study showed a greater prevalence of tooth agenesis in ns-RS relative to ns-CP, a more pronounced mandibular agenesis in ns-RS, and the absence of a direct relationship between the extent of the palatal cleft and hypodontia in ns-RS. Consequently, additional (or even different) developmental disturbances are probably involved in tooth agenesis in ns-RS, with disturbances related to mandibular hypoplasia, the most likely candidates. The failure of mandibular outgrowth in ns-RS is hypothesized to result from intrauterine constraints on the mandible [24], defects in both the generation and growth of Meckel’s cartilage (the first mandibular skeletal element) [25], as well muscular defects with failure of tongue descent [4, 26, 27]. Mandibular hypoplasia could lead to hypodontia through spatial constraints with agenesis of the last of a class of teeth to develop, such as the mandibular second premolars. The prevalence of agenesis of the maxillary over the mandibular lateral incisors could similarly be explained by spatial constraints caused by the development of the canine and the later calcification of the maxillary lateral incisors [28]. Alternatively, the developmental vulnerability of the maxillary lateral incisor may be attributed to the complex origin of its germ at the site of fusion between the medial nasal and maxillary facial outgrowths [29, 30]. However, mandibular hypodontia in ns-RS could also have a more direct etiology in defects in the genes regulating odontogenesis, particularly in patients with agenesis of deciduous precursors. Unfortunately, the genetic background of RS remains poorly understood with only mutations in the SOX9 gene implicated in the non-syndromic form of the condition [4]. Subphenotypes of ns-RS with mandibular hypodontia have diminished mandibular dimensions and a different facial morphology compared to those without hypodontia [31, 32]. Therefore, future research efforts into the genetic traits of these patient populations could allow for a more tailored planning of orthodontic treatment and/or orthognathic surgery.
The strength of this study is the inclusion of a comparison group of patients with ns-CP and the use of the TAC system which precisely elucidated the differences in the prevalences and patterns of tooth agenesis between ns-RS and ns-CP. In addition, this study examined the relationship between the degree of hypodontia and the extent of the palatal cleft using formal statistical analysis. Despite these qualities, the study is subject to several limitations. First, it has a retrospective character without a strict case-control design. Second, the study did not assess tooth agenesis in the deciduous dentition.