Keywords
Anomalies - Craniofacial - Dental - Hypodontia - Morphology
Introduction
Hypodontia is defined as the congenital absence of one or more primary or permanent
teeth.[1] It is the most prevalent developmental dental anomaly in humans with an overall
prevalence of 6.4%. The prevalence varies depending on the population studied, with
Africa having the highest prevalence (13.4%) followed by Europe (7%), Asia and Australia
(6.3%), and North America (5%) and Latin America (4.4%) having lowest prevalence.[2] The most commonly affected teeth by hypodontia except the third molars are the mandibular
second premolars followed by the maxillary lateral incisors, maxillary second premolars,
mandibular central incisors, mandibular lateral incisors, maxillary first premolars,
mandibular first premolars, maxillary canines, mandibular second molars, maxillary
second molars, mandibular canines, maxillary first molars, mandibular first molars
and lastly the maxillary central incisors[2] ([Fig. 1]). In general, this type of dental anomaly is more prevalent in females than males[3] which is contrary to the gender distribution in supernumerary teeth.[4]
[5] Hypodontia can be classified as either syndromic[6] or non-syndromic or according to the number of congenitally missing teeth into mild
with 1 to 2 congenitally missing teeth, moderate with 3 to 5 congenitally missing
teeth, and severe with 6 or more congenitally missing teeth.[7] Although the etiological factors of hypodontia are unknown yet, hypodontia is considered
to be multifactorial where both genetic and environmental factors have shown to play
a role in its etiology.[8] Hypodontia may have a negative impact on the affected individual, such as unaesthetic
appearance, speech problems, malocclusion, and periodontal damage.[9] In addition, it can influence the skeletal relationship and reduce the chewing ability.[10]
Fig. 1 A diagram illustrating the frequency of occurrence of congenitally missing teeth
from the most affected (1) to the least affected (10) teeth in the permanent dentition.
Materials and Methods
A comprehensive literature search was undertaken using both PubMed and Google Scholar
until March 2021, using the keywords: hypodontia, tooth agenesis, congenitally missing
teeth, dental anomalies, and craniofacial morphology. Relevant papers addressing the
association between dental anomalies, craniofacial morphology, and hypodontia were
then selected and included in this review to provide an insight into the influence
of hypodontia on the skeletal structures and to identify the dental anomalies that
can occur in association with hypodontia and complicate the management of those patients.
Exclusion and inclusion criteria: Only studies addressing hypodontia of the permanent dentition excluding third molars
were included in the current review. Studies of hypodontia patients with genetic/medical
conditions, such as Down syndrome, ectodermal dysplasia, and cleft lip and palate
were also excluded. For articles published in the non-English language, data were
taken from the English abstract where possible. No restrictions were applied to the
time of publication of articles. Observational, cross-sectional and longitudinal,
prospective, and retrospective studies were included. The following publication types
were excluded: letters, editorials, post-graduate theses, case reports, and randomized
controlled trials.
Results
Skeletal, Incisor Angulations, and Soft Tissue Changes Associated with Hypodontia
It was found by several studies that patients with tooth agenesis had a different
craniofacial pattern when compared with patients with a normal number of teeth,[11]
[12]
[13]
[14]
[15]
[16] but few studies have investigated the effect of the distribution of congenitally
missing teeth on the craniofacial morphology.[12]
[13]
[14]
[15]
[16]
[17]
According to Endo et al,[17] an anterior and posterior hypodontia exerted a similar pattern on the craniofacial
structures. However, skeletal and dental changes were more remarkable in patients
having both anterior and posterior congenitally missing teeth. These changes include
a greater retroclination of the maxillary incisors, mandibular prognathism, and a
greater anticlockwise rotation of the occlusal plane ([Figs. 2],[3],[4]). Comparing an anterior, posterior, and a combination of both anterior-posterior
teeth absence, Ben-Bassat and Brin[12] showed that skeleto-dental pattern was more exacerbated in patients with congenital
absence of the anterior teeth. On the contrary, patients with anterior and a combination
of anterior-posterior teeth absence showed a similar pattern of skeleto-dental changes.[12]
Fig. 2 Study models of a patient with congenitally missing 12.
Fig. 3 Orthopantomography of the same patient in Fig. 2 confirming the congenital absence
of 12.
Fig. 4 Lateral cephalometric radiograph of the same patient in Fig. 2 showing a class III
skeletal pattern with mandibular prognathism, anticlockwise rotation of the occlusal
plane, retroclined lower incisors, and retroclined upper and lower lips.
It was found that as the severity of hypodontia increased there was more tendency
toward developing a skeletal class III malocclusion.[13]
[16]
According to Ogaard et al,[11] skeletal and soft tissue changes associated with hypodontia are directly related
to the severity of the condition, i.e., the number of congenitally missing teeth.
Patients with 10 and more congenitally missing teeth had the most notable retroclined
pattern of the lower and upper incisors and, therefore, an increase in the interincisal
angle. This, in turn, will lead to the loss of support to the upper and lower lips
and a decrease in the upper and lower lips prominence and an obtuse nasolabial angle.[11]
[12]
[14] In addition, patients with the same number of congenitally missing teeth showed
a statistically significant reduction in the SNA (sella, nasion, A point: the deepest
point on the anterior border of the maxilla on a a lateral cephalometric radiograph)
and ANB (A point, nasion, B point: the deepest point on the anterior border of the
mandible on a a lateral cephalometric radiograph) angles and exhibited a retrognathic
maxilla and straight skeletal profile. Furthermore, patients with a severe form of
hypodontia showed a reduction in the Frankfort-mandibular plane angle, lower face
height, and an increase in the facial axis.[11]
[12]
[13] This decrease in the lower face height can be ascribed to the anterior rotation
of the mandible resulting from a smaller number of teeth and, thus, less support.[11]
[15] Some previous studies have reported a reduction in the length of the maxillary and
mandibular alveolar bone in severe hypodontia.[14]
[16] However, these findings contrast other investigations which showed no reduction
in the maxillary and mandibular alveolar bone height in mild or severe forms of hypodontia.[11]
[18]
Anomalies of Tooth Size and Shape
A few studies have investigated the relationship between congenitally missing teeth
and the mesiodistal and buccolingual dimensions of the clinical crowns of the remaining
teeth. A study conducted by Khalaf[19] to investigate tooth crown dimensions in patients with mild, moderate, and severe
hypodontia revealed that patients with congenitally missing teeth had significantly
smaller teeth in the buccolingual and mesiodistal dimensions than controls. The greatest
reduction in tooth measurements was found in patients with severe hypodontia. According
to Khalaf's study, the most affected tooth in terms of tooth size reduction was the
maxillary lateral incisor and the least affected one was the mandibular first molar.
This reduction in tooth crown size was found in both the anterior and posterior segments,
thus involving the whole dentition.[19]
[20] Furthermore, it has been shown that relatives of patients with hypodontia tended
to have larger tooth dimensions than their affected relatives, but smaller tooth crown
dimensions when compared with a control group.[21]
[22] These results indicate that hypodontia and microdontia may form parts of a genetically
determined condition[19] and are compatible with the multifactorial theory suggested by Brook to explain
the etiology of various dental anomalies in humans.[23] Further support to the multifactorial theory of the etiology of hypodontia has been
provided by other studies which showed an association between hypodontia and microdontia.[24]
[25] A few studies, on the contrary, did not show such an association.[26]
[27] The disagreement between the findings of the aforementioned studies can be attributed
to the racial differences between the population studied, different severities of
hypodontia of the included sample, and the use of different methods to measure tooth
dimensions. The impact of hypodontia on the remaining dentition is not limited to
tooth size but also extends to tooth shape.[28]
[29]
[30]
[31] A study conducted by Al-Shahrani et al[28] to compare the shape of the lower left permanent first molar in patients with different
severities of hypodontia to a control group showed that the lower left permanent first
molar in hypodontia group had a shorter clinical crown, less bulbous labial surface,
flatter gingival margin, less prominent buccal cusps tips, flatter occlusal surface,
and a decreased taperness of the proximal surfaces toward the occlusal surface when
compared with a control.[25] In addition, it was found that patients with hypodontia had a less number of cusps
of the upper first permanent molar[29]
[30] and the lower premolars,[29] an alteration in the shape of the maxillary central incisor including round incisal
edge and less tapered proximal surfaces incisally[31] and a peg-shaped maxillary lateral incisor[32] as seen in [Figs. 5] and [6]. Furthermore, it was found that the greater the severity of hypodontia, the more
the alteration in the tooth shape.[28]
Fig. 5 An intraoral photograph of a patient with congenitally missing 32 associated with
a peg-shaped 22.
Fig. 6 An orthopantomography of the same patient in ([Fig. 2]) with congenitally missing 32 associated with a peg-shaped 22.
Anomalies of Root Length and Width
There is a lack of studies investigating root dimensions in patients with hypodontia.
A recent study[33] has compared root length and widths of the permanent teeth in mild hypodontia patients
with controls and showed that patients with one or two congenitally missing teeth
had a shorter root length of the upper central incisors, upper canines, first premolars,
and lower first molars compared with unaffected controls as well as a decrease in
the root width measured at the midpoint of the root length for the upper central incisors,
lower first premolars, upper first molars, and all second premolars, and a similar
pattern of differences was found with regard to the root width at the cervical region.
Alteration in Arch Dimensions
A few studies have reported a reduction in arch width and length measurements of patients
with congenitally missing teeth compared with a control regardless of the severity
of the hypodontia condition.[34]
[35] However, only one study has compared the mandibular and maxillary arch dimensions
including depth, width, and height of the dental arches in patients with mild, moderate,
and severe hypodontia with a control group and revealed smaller arch dimensions in
all forms of hypodontia. The reduction in arch dimensions in the hypodontia group
was directly proportional to the number of congenitally missing teeth with the maxillary
arch depth being the most affected measurement in all hypodontia groups.[36] These results are contrary to the findings of other studies which failed to show
a reduction in arch dimensions in hypodontia patients. Nevertheless, some of these
studies lack unaffected control group[25] and others had a sample of mixed severity with the mild hypodontia being the predominant
form.[25]
[37]
Other Dental Anomalies Associated with Hypodontia
Other dental anomalies that can occur in association with hypodontia include peg-shaped
maxillary lateral incisor, taurodontism of molars, retained deciduous molars, delayed
development of the permanent teeth, distoangulation of the mandibular second premolar,
and infraocclusion of the deciduous molars.[32]
[38]
[39] These anomalies are also observed in relatives of first and second generations of
hypodontia-affected patients.[39] Other less common anomalies which can be associated with hypodontia are supernumerary
teeth, transmigration, transposition, and ectopic eruption of the permanent molars.[32]
[38]
Conclusions
Patients with hypodontia appear to have a different craniofacial morphology as compared
with controls. They tend to have more class III skeletal pattern and retroclined upper
and lower soft tissue lips as a consequence of retroclined upper and lower incisors.
In addition to changes in craniofacial and soft tissue morphology, and various dental
anomalies are commonly associated with hypodontia such as microdontia of the remaining
dentition, anomalies in tooth shape, particularly peg-shaped upper lateral incisors,
smaller root dimensions of some of the permanent teeth, and decreased maxillary and
mandibular dental arch widths and lengths measurements. Craniofacial morphology changes
and dental anomalies in hypodontia patients are more pronounced as the severity of
the condition increases.
