Introduction
Infraocclusion is a condition that occurs when the occlusal surface of a tooth is located below the occlusal plane of the adjacent teeth. Submergence is a synonymous term also used to describe this clinical presentation, which most commonly affects primary molar teeth.1-3 Infraocclusion of primary teeth is caused by ankylosis of the cementum to the surrounding alveolar bone, thus impeding its eruption.1-3 The incidence of ankylosis is approximately 1.3-38.5% in primary molars.1Ankylosis can occur at any time and results in the primary tooth failing to keep pace with the normal vertical alveolar bone height changes in a growing individual. Therefore, ankylosis is the phenomenon which results in the lack of tooth eruption and subsequently leads to the clinical presentation of infraocclusion.
The prevalence of this condition is also affected by age.1-4 Infraocclusion of the first primary molar is reported to be more common in children under the age of nine and shows a general increase in incidence after the age of three.1-3 In comparison, the prevalence of infraoccluded second primary molars increases after the age of five and is more prevalent after the age of nine.1-3
The prevalence of infraocclusion is significantly higher in the primary dentition compared to the permanent dentition and more commonly found in the mandible as opposed to the maxilla, with no significant gender dimorphism reported. The most commonly affected teeth are the mandibular first and second primary molars, with no predilection between the left and right sides. It has been reported that 1.3-8.9% of children are affected by infraocclusion of one or more of their primary teeth.1-3 In addition, it was found that 18-44% of the affected children’s siblings also presented with infraoccluded teeth, which appears to indicate a genetic contribution to this condition.1-4
Infraocclusion can also be categorised as slight, moderate, or severe.1,2 The Facile classification, as described by Brearly5, defines infraocclusion as slight when the occlusal surface of the tooth is one millimetre below that of its neighbours, moderate when the occlusal surface of the tooth is level to or below the contact point of the neighbouring teeth, and severe when the occlusal surface of the tooth is below the interproximal gingival tissue.1 Both the prevalence and severity of infraocclusion increases with chronological age. Therefore, a younger patient who experiences primary tooth ankylosis is likely to demonstrate more severe infraocclusion due to the longer period of vertical alveolar bone height development. Although most ankylosed primary teeth eventually exfoliate with the ongoing eruption of the succedaneous tooth underneath, delayed exfoliation is common. Despite the relative infrequency of unfavourable outcomes, infraoccluded primary molars are known to be associated with other dental anomalies, including hypodontia.6 The eruption path of the successor tooth must be monitored closely and periodically to avoid undesirable permanent tooth impactions. In extreme cases, the ankylosed primary tooth can submerge to the point of complete coverage by the surrounding hard and soft tissues as detailed in this case report.7
Despite the uncertainty, there is a general consensus that infraocclusion is multifactorial in aetiology.1-3 It has been postulated that infraocclusion may be genetic in nature as this condition can be seen in families, however, there are several factors such as altered local metabolism, local trauma and infection that may also potentially result in the infraocclusion of teeth.1-3
Hypodontia, also known as congenitally missing teeth, is another common dental developmental anomaly that indicates that at least one permanent tooth has failed to form and erupt into the oral cavity.4,8-14
The current consensus is that the aetiology of congenitally missing teeth is multifactorial, with genetic and environmental factors potentially both playing a role.4,8-14 Heritability of congenitally missing teeth has been established through familial studies where parents and their offspring and/or siblings have presented with the same or similar congenitally missing teeth.4,6,8-18 Numerous developmental syndromes such as Down, Book and Rieger are associated with congenitally missing teeth, which suggests the influence of genetic factors on the development of the dentition.6,13-15 It has been suggested that missing anterior teeth have a stronger genetic contribution, while the absence of posterior teeth may be more sporadic in nature.13
When reporting on the prevalence of hypodontia, the inclusion or exclusion of the third molars produces a distinct difference in the data.4 This difference is due to the high frequency of congenitally missing third molars across all genetic populations.4,19 Excluding the third molars, the prevalence of congenitally missing teeth in specific teeth appears to be determined by the population demographics.4,19 The prevalence of congenitally missing teeth in the primary dentition has been reported to be less than 1%, however, the prevalence of missing teeth in the permanent dentition ranges from 1.6-36.5%, when the absence of third molars are included. When missing third molars are excluded from the data, the prevalence of congenitally missing permanent teeth is significantly lower and reported to range between 1.6-6.9%.13 It is also reported that congenitally missing teeth occurs bilaterally in the mouth almost two to three times more frequently than unilaterally. Excluding the third molars, the most common congenitally missing teeth in the permanent dentition are the mandibular second premolars followed by the maxillary lateral incisors.13,14