Teeth that fail to develop and form during tooth germ formation are called congenitally missing teeth. They can be classified into several categories according to their severity and whether they are accompanied by systemic symptoms or not, but the classification criteria and names of congenitally missing teeth have been expressed differently in different literatures. Previous literature has mostly categorized congenital edentulism according to the severity of congenital edentulism into: agenesis, which refers to the absence of one to several teeth; oligodontia, which refers to the absence of most of the teeth and is usually accompanied by systemic anomalies; and edentulousness, which refers to the extremely severe absence of teeth, with the entire mouth being edentulous. There are other classifications that are similar to the above, but with different terminology: individual tooth loss, partial anodontia, and total anodontia. Raymond et al. [1] argued that the term "partial anodontia" used in the previous classifications is literally contradictory and unscientific. The term "partial anodontia" used in the classification is literally contradictory and unscientific.
At present, most scholars [2-4] classify congenital edentulism into two categories: hypodontia, which refers to the congenital absence of one or several teeth, usually without systemic symptoms. Most missing teeth (oligodontia), which refers to the congenital absence of six or more teeth (excluding the third molar) and is usually accompanied by systemic anomalies and/or partial manifestations of a syndrome. Congenital edentulousness (anodontia), on the other hand, is considered a severe manifestation of congenital absence of most teeth.
2 Factors Associated with the Occurrence of Congenital Anodontia
Congenital anodontia occurs in the early stages of tooth genesis, i.e., during the period of bud formation. It can arise for different reasons: 1) Physical obstruction to the development of the dental plate or fracture of the plate during the embryonic period. For example, in orofacial-finger (toe) syndrome, the lower incisor region is covered by proliferating ligaments that mechanically interfere with the dental plate. (ii) Restriction of space, which creates pressure and degrades the tooth germ without minimum nutrition, especially in the third molar. (iii) Causes functional defects in the epithelial tissue and/or failure of mesenchymal induction beneath it.
There is a tendency for the human masticatory organs to degenerate and diminish, and thus congenital edentulism is associated with human racial evolution. As early as 1865, Darwin pointed out that the decreasing size of human teeth and congenital absence of teeth are related to the tendency of the maxilla and mandible to become smaller during evolution.
At present, there are many studies have proved that the occurrence of congenital missing teeth is related to genetic factors, but the views of various scholars are inconsistent in terms of their exact mode and mechanism of genetic transmission. One of the main views are: congenital edentulism has ① autosomal dominant inheritance characteristics; ② autosomal recessive inheritance characteristics; ③ polygenic inheritance characteristics. Most scholars tend to favor polygenic inheritance influenced by the environment [4].
In recent years, with the localization of various molecules in tooth development, such as transfer factors, growth factors, receptor molecules, cell surface molecules, intercellular molecules, and the study of their regulatory roles during tooth development, the study of genetic factors of congenital edentulism has become more in-depth, but scholars are not in agreement with each other. For example, Vastardis et al [5] found that when the MXS1 gene locus is mutated, mice can exhibit severe majority edentulism, while humans exhibit congenital absence of the second premolar and third molar. In addition, other scholars have found that epithelial growth factor (EGF), epithelial growth factor receptor (EGFR), and fibroblast growth factor (FGF) have important roles in tooth developmental period have important roles. Thus, scholars have proposed that mutations in the gene loci of EGF, EGFR, FGF and other factors can cause congenital edentulism. Arte et al. [6], however, used chain linkage analysis to investigate the family lines of seven Finnish families with congenital edentulism over three generations, and took the peripheral blood of the patients and analyzed their fingerprints by PCR, but they excluded the gene loci of EGF, EGFR, and FGF-3 as the loci for congenital edentulism gene mutations.Nieminen et al. [7], however, excluded the gene loci of EGF, EGFR, and FGF-3 as the loci for congenital edentulism gene mutations when they investigated the genes of patients with incisors and premolars with congenital edentulism. Nieminen et al [7] did not find any abnormalities in the MXS1 locus when studying the genes of patients with congenital absence of incisors and premolars.
Two genes for congenital edentulism with syndromes have been cloned, the gene for ectodermal dysplasia (EDA), a mutation that causes anhidrotic ectodermal dysplasia, and the gene for Rieger syndrome [8].
3 Incidence of congenital edentulism
3.1 Incidence of congenital absence of permanent and milk teeth
Congenital edentulism rarely occurs in mammals. However, when they occur in humans, they are more common in permanent teeth than in milk teeth. The incidence of congenital edentulism in permanent teeth is 2.3% to 9.6% (excluding the third molar), and 0.1% to 0.7% in milk teeth, and there is a clear racial difference, with higher incidence in Eskimos, Indians, and Orientals, and lower incidence in Negroes, with a male to female ratio of 2:3. Congenital edentulism in the milk teeth is followed by congenital edentulism in permanent teeth at an incidence of 75% to 80% [9].
Most of the reports on the incidence of congenital edentulism in the literature do not categorize the statistics according to the classification of congenital edentulism and do not include the third molar, which may be due to the very low incidence of congenital majority edentulism and edentulousness; X-rays were not taken in the large-scale population-based surveys.
3.2 Incidence of congenitally missing teeth in different tooth positions
There are many reports on the preferred tooth positions of congenitally missing teeth, and there are obvious racial differences, with some literature reporting that congenitally missing teeth are mostly found in mandibular second premolar in Europeans, in maxillary lateral incisors in North Americans, and in mandibular incisors in Japanese [1]. The prevalence of congenitally missing third molars varies widely among ethnic groups, ranging from 1.0% in black Africans to 30% in Japanese. Most data show that the highest incidence of congenitally missing teeth in the permanent dentition is the third molar, which is about 30%, followed by the mandibular second premolar (0.8% to 6.4%), and again by the maxillary lateral incisors (1.1% to 3.2%). The incidence of deciduous teeth is in descending order: mandibular deciduous incisors, maxillary deciduous incisors, and deciduous cuspids. From this anthropologists have proposed that the human dentition is in a transitional period and that in the future the dentition may be unilateral with: one incisor, one cuspid, one premolar, and two molars [1, 9].
3.3 Occurrence of Congenital Majority Missing Teeth and Edentulousness
The incidence of congenital majority missing teeth is 0.3% in the permanent dentition and even less in the deciduous dentition [10]. Congenital edentulousness is very rare and is seen only in patients with severe ectodontia. According to 125 cases of congenital concomitant maxillary and mandibular edentulism reported in the literature, both the milk and permanent tooth rows occurred simultaneously or only the permanent tooth row alone [1].
4 Clinical manifestations of dental anomalies associated with congenital edentulism
Congenital edentulism is characterized by congenital loss of teeth, which can occur alone but is more commonly associated with other dental anomalies. It has been reported that congenitally missing teeth and too small teeth (microdontia) tend to occur in the same dentition, and thus some scholars believe that too small teeth, conical teeth (cone-shaped tooth) is also a manifestation of congenital missing teeth [11].
Congenital edentulism is accompanied by other dental anomalies such as congenital absence of other teeth, small teeth, conical teeth, cow-shaped teeth (taurodontism), delayed development of the teeth, eruption sequence abnormalities, molar sinking, misalignment of permanent cuspids, ectopic eruption of the first permanent molar, wider gaps in the dentition, tooth crowding, and insufficient development of alveolar bone [9 to 12]. Symons et al [11] counted the percentage of various malocclusions in 176 patients congenitally missing one or more second premolar teeth. The results were: congenital absence of other teeth accounted for 63.6%, permanent tooth ambulation accounted for 13.1%, molar subsidence accounted for 9.7%, dental crowding accounted for 6.8%, midline deviation (maxillary) accounted for 6.8%, widening of the dental gap accounted for 3.6%, ectopic eruption accounted for 2.8%, and late eruption accounted for 2.8%.
Congenital absence of teeth accompanied by other dental abnormalities, the most common is the congenital absence of other teeth. In patients with congenital absence of the third molar, the incidence of congenital absence of other teeth was higher than in the normal population as follows: mandibular second premolar was 32%, maxillary lateral incisor was 21%, maxillary second premolar was 15%, mandibular lateral incisor was 12%, maxillary cuspids were 7%, mandibular mesial incisors were 4%, mandibular first premolar and cuspids, and maxillary first premolar was 1% [9].
5 Principles of treatment of congenitally missing teeth
The choice of treatment plan for congenitally missing teeth must be decided after doing a comprehensive examination, based on the side view of the child, the location and number of missing teeth, and the arrangement of teeth [1, 9]. Generally, patients with congenitally missing anterior teeth should first consider aesthetic problems, and patients with congenitally missing posterior teeth should focus on function. Currently commonly used orthodontic methods are to close the gap, centralize the gap or maintain the gap to implant or denture repair, for the alveolar bone development is insufficient for most of the missing teeth and edentulous patients can be surgical bone grafting, postoperative implant or denture repair.
6 Systemic systemic diseases associated with congenital edentulism
Congenital edentulism can occur alone or as a systemic abnormality and/or as a partial manifestation of a syndrome. Congenital edentulism has been described in more than 120 syndromes. The following systemic anomalies or syndromes are usually accompanied by congenital absence of most teeth [1, 9, 12].
6.1 Ectodermal dysplasia (Ectoderrmal dysplasia)
Some scholars have categorized Ectoderrmal dysplasia into 117 types, but there is no uniformity in the definition and classification of Ectodermal dysplasia. The clinical manifestations of patients are lack or scarcity of hair and body hair, dry skin, lack of some or all sweat glands, prominent forehead, saddle nose, and the most prominent manifestations in the oral cavity are congenital majority of missing teeth, widening of the gap between the remaining teeth, dwarfed conical teeth, and malformations such as dysplasia of the labial tie.
6.2 Oro-facial-digital Ⅰ and Ⅱ syndromes (Oro-facial-digital Ⅰ and Ⅱ syndromes)
The patient's lower incisors are underdeveloped, the teeth are sparse, the fingers (toes) are concurrent with the fingers (toes), and the fingers (toes) are too short or curved.
6.3 Rieger syndrome (Rieger syndrome)
Partial loss of the oval bone, underdevelopment of the premaxilla, congenital absence of the maxillary anterior teeth, and hypoplasia of the cornea and iris.
6.4 Craniomandibular dermatodysostosis (Craniomandibular dermatodysostosis)
It is a very rare condition in which the patient has hypoplasia of the central part of the maxilla and mandible, and congenital loss of the mandibular central incisor. ■
Author: Qing Deng (Institute of Pediatric Dentistry, Railway University, Shanghai 200072)