Introduction
‘Hypodontia’ is the term used to describe the developmental absence of one or more primary or secondary teeth, excluding the third molars. It affects about 2.6%–11.3% of the global population. , The most common missing teeth are maxillary and mandibular third molars > mandibular second premolars > maxillary lateral incisors > maxillary second premolars. This prevalence pattern of absent teeth is in keeping with Butler’s field theory, which states that the last tooth in each class is the most likely to be malformed or missing, with the posterior-most class, that is, the molars, being the most affected. , Teeth are often missing bilaterally and, in cases of severe hypodontia, are often associated with malformation of the remaining teeth. Terms like oligodontia, anodontia, and hypodontia are often used interchangeably, but they have distinct meanings. Anodontia refers to the complete absence of all teeth. Hypodontia is characterized by the absence of one to six teeth. Oligodontia is a congenital condition where six or more permanent teeth are missing, excluding wisdom teeth. It is worth noting that oligodontia is sometimes referred to as partial anodontia.
Hypodontia may be associated with other syndromes or as an isolated occurrence of varying degrees of severity. Such teeth may be referred to as congenitally missing. However, this term is a misnomer as many of the teeth that are typically absent form in the mouth after birth. The management of this condition, hence, presents a unique challenge, as it is a condition that needs to be tackled and treated as early as possible but cannot be definitively diagnosed until the child is at least 6–9 years old. Interestingly, deciduous teeth are far less likely to be congenitally missing, but when they are, there is a correlation with the absence of the succedaneous tooth. The most common deciduous teeth to be congenitally missing are maxillary lateral incisors, followed by mandibular lateral incisors and canines.
Effective clinical management requires careful multidisciplinary planning, with inputs from an orthodontist, a restorative dentist, an oral surgeon and, increasingly, a geneticist.
Since hypodontia frequently presents as spacing, orthodontists are often the first specialists to diagnose the condition. Orthodontic considerations for a single missing tooth in the anterior region are simpler in terms of either closing the space or regaining partially lost space and its maintenance for an implant prosthesis; however, in the buccal region, the considerations are very different, especially from the viewpoint of anchorage considerations. Management of partially missing teeth would require thoughtful planning, keeping in mind the patient’s age, remaining growth potential, coexisting malocclusion, status of oral health and caries proneness and prosthodontic and other restorative care requirements. Hypodontia can also be associated with systemic syndromes and other dental anomalies. It is, therefore, important that patients with hypodontia are screened for other dental and systemic abnormalities.
Aetiology and clinical features
The aetiology of hypodontia can be varied and is poorly understood, though it does have a strong genetic component. It has been linked to polygenic mutations that present with an autosomal dominant pattern of inheritance. , Its expression, however, even within the same family, can vary in severity, as seen in the study by Militi on monozygotic twins. The study assessed three couples of monozygotic twins who had mutations in the PAX9 gene, which is responsible for oligodontia. He found that a few of the twins showed hypodontia, while their siblings exhibited oligodontia. This clearly indicated that there are other environmental or epigenetic modifiers affecting the genetic expression of these mutations.
Environmental factors like smoking, nutritional disturbances or infections like syphilis and scarlet fever during pregnancy or infancy can adversely affect the development of tooth buds, leading to their agenesis. Chemotherapy and radiotherapy during pregnancy have also been linked to tooth agenesis. Meanwhile, traumatic extraction of deciduous teeth during childhood can also damage the developing succedaneous tooth germ.
Hypodontia may also present as a part of a larger syndrome, including ectodermal dysplasia, Down’s syndrome, hemifacial microsomia, orofacial clefting and orofacial digital syndromes. , A common association is found with autosomal recessive inheritance of dental or enamel hypoplasia conditions like ectodermal dysplasia, which can often lead to tooth agenesis. Syndromic cleft lip and palate conditions like Pierre Robin sequence and Van der Woude syndrome can present with hypodontia in up to 70% of cases, with the severity of hypodontia being linked to the severity of the cleft. In these conditions, maxillary lateral incisors are most affected.
The aetiology of a single missing tooth is unknown, while the absence of third molars is often attributed to evolution.
Hypodontia can therefore be considered a multifactorial condition that includes genetic influences. Research on conditions associated with severe hypodontia or anodontia in ectodermal dysplasia has linked these conditions to specific genes. However, the variable expression of the trait suggests a polygenic mode of inheritance influenced by epistatic genes and environmental factors interacting. Newer developments in molecular genetics have established the role of the muscle-specific homeobox genes ( MSX1 and MSX2 ) in dental development.
Clinically, patients with hypodontia present with a spaced dentition, over-retained deciduous teeth and often a deviated midline. They present with the chief complaint of an aesthetic concern. They tend to have mildly underdeveloped jaws and a reduced lower anterior facial height. Dentally, patients may have microdontia with enamel hypoplasia and talon cusp or taurodontic molars.
Classification
Hypodontia is classified under the following categories:
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Based on the number of congenitally missing teeth excluding third molars :
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Mild hypodontia—1–3 teeth missing
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Moderate hypodontia—3–5 teeth missing
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Severe hypodontia/oligodontia—more than 6 teeth missing
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Anodontia—all teeth missing
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Based on aetiology:
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Sporadic isolated hypodontia
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Familial isolated hypodontia
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Syndromic hypodontia
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Diagnostic tools and treatment approach
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Radiographs: Orthopantomograms and cone beam computed tomography (CBCT). It is important to note that radiography is helpful only after the age of 9 years for a definitive diagnosis of hypodontia. This is to allow the minimum calcification of all the tooth buds (except third molars) required for radiographic examination while accounting for children who might have delayed development of permanent teeth.
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Clinical assessment for symptoms of associated syndromes
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Familial history
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Consult with a clinical geneticist for familial and severe hypodontia
Treatment approach
Treatment of hypodontia needs to address the following :
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Patient’s appearance
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Speech
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Masticatory efficiency
To achieve these goals, treatment for hypodontia requires an interdisciplinary approach with a team comprising of paediatric dentists, orthodontists and prosthodontists with additional input from general dentists and oral surgeons.
The initial planning of the treatment is crucial so that every dental team member can work with a clear end goal in mind and provide the best possible outcome for patients.
Role of a paediatric dentist
Hypodontia treatment must start as early as possible, even before a definitive diagnosis can be reached. This starts with paediatric dentists treating children in the mixed dentition phase. In the initial phases of treatment, parents need to be counselled about the importance of oral hygiene instructions; dietary advice; the use of fluorides, both locally and systemically; fissure sealants and regular review appointments to maintain the remaining teeth in sound health. These become even more important when children are given temporary removable partial dentures or overdentures. Paediatric dentists may also need to extract infraoccluded deciduous molars or canines if permanent canines show signs of improper eruption. In patients with midline diastema due to high frenal attachment, frenectomy may be indicated with minor orthodontic correction.
Role of a prosthodontist
The role of the prosthodontist begins in the early permanent dentition phase. Removable retainers or provisional resin-bonded fixed partial dentures can be given to adolescents depending on the spacing between teeth. These are extremely important as they help maintain space for the final prosthesis. Additionally, they can help boost the child’s confidence as it can significantly improve their smile during the growing years. Any permanent implant prosthesis needs to be avoided at this stage, as there are significant growth changes taking place; with vertical growth at the mandibular condyle and ramus, there is a compensatory eruption of the maxillary and mandibular molars to maintain the occlusion, which cannot be replicated by implants. If the final prosthesis is placed too early, there can be significant functional and aesthetic problems.
Once patients reach adulthood and their facial growth is complete, they can be given dental implants, fixed partial dentures or final overdentures, as required. Restoration of malformed teeth is also done by prosthodontists. For this, orthodontic space creation or space distribution may be required.
Role of an orthodontist
The orthodontist often works in tandem with the prosthodontist and should be brought in during the early permanent dentition phase. They are involved in space planning and even space closure, as required. As orthodontists, it might be tempting to jump to a substitution treatment plan or conventional orthodontic space closure; however, it is important to remember that hypodontia requires a multidisciplinary approach, and the treatment plan should be congruous with the needs of the patient and other dental specialists. An orthodontist can contribute to the treatment of hypodontia by the following procedures:
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Space closure with/without substitution of missing teeth
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Opening or maintaining space for restoration or prosthesis
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Autotransplantation
I. Space closure with/without substitution of missing teeth
Space closure is indicated in patients with severe crowding or when there is an Angle’s Class II Division 1 or Class III malocclusion, which necessitates single arch extraction in the arch with missing teeth. In this scenario, the space provided by the missing teeth functions as the extraction space that warrants closure. Studies have shown that the popularity of space closure has increased over prosthetic rehabilitation. This may be due to the drive for improved periodontal health or the growing predictability of orthodontic treatment with the use of temporary anchorage devices.
Space closure can be achieved by substituting the missing teeth or moving the adjacent teeth into the existing space. Another space closure method can be a premolar rotation followed by buildup. This can be used in cases of missing canines. However, the rotation is likely to relapse, which can compromise occlusion in the long term.
A major advantage of space closure with substitution using natural tooth/teeth over implants is that unlike implants, teeth maintain the periodontium and retain sensory and proprioceptive function. Moreover, unlike implants, space closure can be done in growing adolescents with good long-term stability of the occlusion. However, it has a more extended treatment duration and requires good patient compliance. Patient burnout can be a concern.
II. Opening or maintaining space for restoration or prosthesis
When space closure with substitution is not advisable for a patient, either because there is excessive space in the arch or the patient’s profile is not amenable to retraction of teeth, the patient needs to be prepared for a prosthesis for the missing tooth. Since tooth agenesis leaves excessive space, ectopic eruption of the adjacent teeth into the space of the missing tooth is often seen, which reduces the space required for ideal prosthetic rehabilitation. In such cases, orthodontic space opening is required not only to provide space for the prosthesis but also to redistribute the spaces more favourably.
Dental implants cannot be placed until facial growth is complete; it may be necessary to maintain space for the implant prosthesis with open-coil springs and riding pontics during the treatment phase and later with provisional resin-bonded fixed partial dentures. However, a concern during space maintenance is the width of the alveolar ridge.
The important considerations for orthodontic management of missing maxillary lateral incisors and mandibular second premolars with these options are discussed later.
III. Autotransplantation
Autotransplantation can be suggested in adolescents as an alternative to implants. Autotransplanted teeth have shown a high success rate and are a viable alternative in the maxillary anterior region. Teeth are transplanted with their root lengths ranging between two-thirds and four-fifths of the total root length formation. This technique is advantageous in cases where premolar extraction is indicated. Thus, the extracted premolars are used to replace the missing tooth.
Autotransplantation is done after initial alignment and expansion of the recipient site space. The graft is carefully removed with the follicle intact. When replacing maxillary central incisors, premolars are placed with a 90-degree rotation to match the mesiodistal with of the incisor. No direct force, including occlusal forces, is applied for 3 months, and no orthodontic force for 6–9 months before the alignment of the transplanted tooth can be completed. Also see chapter 77 .
Orthodontic considerations in management of missing maxillary lateral incisors
When maxillary lateral incisors or other anterior teeth are missing, aesthetics become the major concern. It has often been seen that for unilaterally missing lateral incisors, space opening and prosthetic replacement is preferred, while in cases of bilateral missing laterals, space closure with substitution is the preferred treatment modality. Whether space closure with canine substitution ( Fig. 78.1 ), space opening/distribution ( Fig. 78.2 ) for single implants or tooth-supported restorations is undertaken as the treatment of choice, orthodontics plays a significant role.
Case Study 1: Bilateral canine substitution for missing maxillary lateral incisors and malocclusion requiring extraction of all first premolars. Pre-treatment records of an 18-year-old female patient presenting with congenitally missing maxillary lateral incisors, spacing between the maxillary anterior teeth, a class II buccal segment relationship bilaterally, crowding in the lower arch and deep bite. She had a mild class II skeletal base relationship, a decreased mandibular plane angle and relatively normally inclined maxillary and mandibular incisors. Treatment involved bilateral closure of the missing lateral incisor spaces with canine substitution, and extraction of first bicuspids in the mandibular arch to provide space for relief of crowding. Sliding space closure mechanics was employed using a 0.022-in. slot pre-adjusted edgewise appliance. Care was taken to establish torque on the maxillary anterior teeth before space consolidation.
