Dental Arch Dimensions in a Matched Pairs Study of Hypodontia Patients and Controls

Dimensions in a Matched Pairs Study of Hypodontia Patients and Controls. Images of the of patients and their matched controls were analysed using a 2D image analysis Measurements of were the arch circumference, arch length, intercanine width, intermolar width and the depth of the palatal vault.


Introduction
Hypodontia is a common variation of tooth number in the population. In the permanent dentition approximately 25% of individuals have 1 or more congenitally missing third molars and some 3.5% to 7% of the population have hypodontia of other permanent teeth [1][2][3][4][5][6]. The condition is more frequent in females and approximately 90% of affected patients have less than 4 permanent teeth, other than third molars, congenitally absent.
The condition can present challenges requiring careful long term treatment planning and care involving general practice, paediatric dentistry, orthodontics and restorative dentistry. Variations in tooth size and shape are well established in patients with congenitally missing teeth and may also occur in other components of the stomatognathic system [7][8][9]. As part of an international collaboration investigating the aetiology and clinical implications of hypodontia, this paper explores dental arch morphology in mild or moderate hypodontia. The dental arches and the dentition are two closely related components of the stomatognathic system, which develop in anatomical proximity over an extended time period from early in utero to early adulthood. The dental arches and the dentition are both complex systems, whose development is determined by multiple interactions between genetic, epigenetic, and environmental factors [10][11][12]. Interactions continue as development progresses through cellular, soft tissue and mineralisation stages to the emergence of the mature phenotype [9]. Hypodontia is an outcome of these complex interactions [13].
Similarly, in addition to genetic factors, the dimensions and shape of the dental arch are influenced by the configuration of the underlying basal bone and the actions of prenatal and postnatal environmental factors [14][15][16][17]. Postnatal environmental factors that have been identified include: the intraoral and circumoral musculature [18,19], sucking habits [20], postural and breathing patterns [21] and early loss of primary teeth [22]. There have been varying results in previous studies of dental arch morphology in patients with hypodontia. Woodsworth, et al. [23] found no significant differences in hypodontia patients compared to controls, Paulino, et al. [24] found greater intercanine and intermolar distances in the permanent dentition of adolescent and young adult men than in women, while Nelson, et al. [7] and Higgins [25] report the upper arch depth and chords were significantly reduced. They found greater differences in severe hypodontia. Sex differences are present in arch dimensions [26] and the degree of change in hypodontia may vary between male and female patients. Moreover, the differences may be greater in the upper arch than the lower [27] and may be influenced by the location of the congenitally missing teeth [7]. The aim of the present study is to investigate dental arch dimensions using a well validated 2D image analysis system [7,28,29] in a sample of hypodontia patients and matched controls to determine if there are any differences and, if so, how these relate to the sex of the patient, the location of the congenitally missing teeth and the upper and lower arches.

Materials and Methods
This study was approved by the Ethics Committee of the

Type of measurement Description
Arch length The distance between the contact points of the central incisors and the line drawn between the most mesial surface points of the first molars.
Intercanine width Distance between the cuspal tips of the canines.

Intermolar width
Distance between the most buccal cervical point (used as landmarks) of the first molars.

Arch circumference
A polyline drawn through the mesial and distal contact points of teeth from the distal side of the first molar on one side of the arch to the distal surface of the opposite first molar.

Depth of the palatal vault
The deepest point of the vault was assessed visually and marked with a black pen, the model was positioned perpendicular on the stand and captured this way. The most palatal cervical points of upper first molars were connected and a perpendicular was drawn on that line to the black point.
Alginate impressions (Ypeen Premium, SpofaDental) were taken for each individual from the upper and the lower arch. Study models were made from dental stone (FujiRock, GC). Images of the study models were taken with a digital camera (Nikon D3100, Nikon Corporation, Japan) and macro lens (Tamron SP AF-S 90 mm f/2.8).
The camera was fixed above the dental cast, on an adjustable stand (Kaiser 5360, Kaiser Fototechnik, Germany) with two fixed led bulbs providing standard lighting conditions. Images of the dental arches were transferred using View NX2 (Nikon Corporation) and processed by the Image Pro Insight 9.3 software (Media Cybernetics, USA). Each image taken included a ten-millimeter scale for calibration and the measurements were made directly on the images. The 2D measurements of the dental arches were the arch circumference, arch length, intercanine width, intermolar width and the depth of the palatal vault. The definitions used for these measurements are given in (Table 1

Results
The intra-operator and inter-operator reproducibility was excellent, with all ICC values being higher than 0.9. (Table 2).In teeth that were missing, the lower the upper arch circumference was. In the lower arch intermolar width values were significantly higher in the hypodontia group than in controls (Table 3). More significant differences were seen in male patients than in female patients in the upper arch, although the interaction between the sources of variation was not significant in every case.    (Table 7). Regarding the anterior hypodontia subgroup, both the upper and lower arches showed significant correlations between the number of congenitally missing teeth and some of the parameters (Table 7).   Table 6: Mean values of measured parameters in lower arch for anterior and posterior hypodontia subgroups and matched controls; †3 patients were excluded from this section as they had both anterior and posterior hypodontia; SD=Standard Deviation.

Discussion
The validity of the study can be examining the nature and structure of the sample, the study design, the pattern of hypodontia in the subjects, the measurement techniques, the reproducibility found and the raw data. The sample is derived from a single ethnic group and is of a Dental Age [30] where the dental arches have developed to maturity in width and length [31,32]. The sample size is satisfactory as determined by power calculations [7] and the controls are matched for age, sex, and ethnicity. The matched pairs design and the pattern of congenitally missing teeth accords with previous studies [1,6,33]. The accuracy and validity of the 2D image analysis system used here has been established over a series of studies [7,12,28,33].The hypodontia patients included in the present study had significantly smaller arch circumference, arch length and intercanine width in the upper arch than controls. This agrees with the findings of Nelson et al. [7] for their mild/moderate hypodontia group; in their severe hypodontia group the differences were greater. Bu, et al. [26] report similar results.In the present study the only significant difference in the lower arch was a larger intermolar width in the hypodontia group. This has previously been reported by Hobkirk, et al. [34], but not by Fekonja [27] and Higgins [25]. These contrasting findings could have arisen from difference in measurement techniques.
A possible explanation for a larger lower intermolar width could be increased tongue pressure in the lower molar region arising from the position of the tongue in response to the narrower upper arch [34]. Moreover, if the lower second premolars are congenitally absent, the lower second primary molars may be retained, preventing the forward movement of the first permanent molars, and holding them back in a wider arch. Arch dimensions in females and males were investigated separately because differences had been suggested by Berwig, et al. [35]. The present study also showed sex differences, with greater reductions in males compared to their control group. In the upper arch the male hypodontia patients had highly significant reductions in arch circumference, arch length, and intercanine width, while females had less difference in these three parameters from their controls. The location of the congenitally missing teeth had significant impact on the dental arch parameters.
In the upper arch, when maxillary lateral incisors were congenitally absent, the arch circumference, arch length and intercanine width were all significantly reduced, suggesting that the presence or absence of these teeth may have a substantial effect during upper arch development.
While the growth of the maxilla is affected by the missing anterior teeth, in the posterior hypodontia group, in which the maxillary second premolars were congenitally absent, no significant differences were found. In the lower arch the only significant difference was an increase in the intermolar width in the posterior hypodontia group in which the lower second premolars were absent. These changes in the dental arches occurred in those hypodontia patients most frequently encountered in clinical practice. A recent study of the orthodontic treatment of similar patients in a Western Australia private practice reported a trend away from space opening and prosthetic replacement to space closure over the years 2000 to 2017/18 [36]. The findings of the present basic science study suggest that within any such general trend, different treatment plans may be appropriate for individual patients. In (Tables 3-5 in the amount of space available, which will also be affected by the extent of reduction in the size and shape of the teeth present.
In conclusion, the evidence from this study in relation to the aim shows that hypodontia does influence the dimensions of the dental arches. Different parameters in hypodontia patients are affected to different degrees: the upper arch is more affected than the lower; males are more affected than females and the location of the congenitally absent teeth is influential.
The changes are evidence of interactions between two complex adaptive systems, the dentition and the dental arches, that are components of the stomatognathic complex. They also interact with a third component, the tongue. The underlying factors in these interactions during development are genetic, epigenetic and environmental [13]. The question remains as to the relative influence of genes and genetic mutations that are common to both the teeth and the arches compared with the environmental effects arising from the congenital absence of teeth in specific locations, resulting in a lack of stimulus to bone growth. This study provides a basis for further investigations of this and other samples to examine this question further.