Morphometric analysis of maxillary arch dimensions using Cone Beam Computer Tomography (CBCT)

Abstract

Knowledge of morphology of the maxillary arch is important in many spacialities of dentistry. This retrospective cross-sectional study was performed using 109 randomly selected maxillary CBCT images of patients in the age range of 18–60 from the archives in Faculty of Dental Sciences, University of Peradeniya, Sri Lanka. EzDent software was used for measurements. Measurements related to the width were acquired from coronal sections while the measurements related to antero-posterior dimensions were performed on axial sections. A majority had an oval shaped maxillary arch (64.8%) followed by V shaped one.(29.2%). The mean height of the alveolar ridge at the upper canine position was 8.9 mm and the mean height of the alveolar ridge at the first molar level was 20.1 mm. Mann Whitney U test was used to compare measurements of variables between males and females. Difference in the height of alveolar ridge at the junction of soft and hard palate, width of the alveolar ridge at the canine, first molar and second molar and also the antero-posterior distance of the arch at the inter canine distance and junction between the hard and soft palate were statistically significant. Kruskal Wallis test was used to compare the differences between age groups. Chisquare values didnot indicate significant differences of measurements according to the age group of the participants. In conclusion, there is a significant difference in all maxillary arch parameters between the different genders. No significant differences in arch parameters were observed among the different age groups.

1. Introduction

Knowledge of the morphology of the dental arch is of paramount importance in many disciplines of dentistry such as prosthodontics, orthodontics, forensic odontology, oral and maxillofacial surgery and anthropology.¹ Understanding arch morphology both in terms of shape and dimensions is important in many ways in prosthodontics. This knowledge is useful in the appropriate selection of impression trays to the fabrication of removable prosthesis. Overextended trays can lead to tissue distortion and patient discomfort during impression making. On the other hand if the stock trays are under extended or too small, the required area cannot be captured.The shape of the arch influences the expected outcome of a removable prosthesis and helps the clinician to predict the final outcome for theadaptation of suitable techniques during the removable prosthesis fabrication.A narrow V shaped arch form is considered to be most unfavorable while a well-developed U shaped arch form is considered to be favorable in terms of resisting lateral forces.Moreover, information related to ridge dimensions influence the choice in terms of morphology, size and length of osseo-intergrated implants and also positioning of such implants.

In orthodontic treatment the arch form and shape are usually modified to achieve the treatment goals by various forms of wires used in the course of treatment. Relieving of crowding through modifying thearch length by means ofdentalarch expansion is one of the methods used to gain space for orthodontic alignment preventing the necessity of extractions.² The patient's existing arch form appears to be the best guide for the stability of the arch form after treatment.³ Therefore, this information is vital for the appropriate selection of arch wires and to ensure long term occlusal stability in orthodontics. Hence, dental arch morphology has been studied extensively with the hope of defining proper protocolsfor tooth position, optimum aesthetics, function and long-term stability.^4,5 It would also be a useful guide during orthognathic surgery and placement of implants.^6,7

Lee et al., in 2011 have put forward a method to classify dental arch forms.⁸ Further, studies have shown that human dental arch morphology may vary among individuals according to tooth size, tooth position, pattern of craniofacial growth and by several genetic and environmental factors.^7,9,10 Presence of some basic differences in dental arch size and shape among different populations and various racial groups has been reported in several clinical and anthropological studies11, 12, 13 highlighting the need for population specific values and forms.¹⁴

To the best of authors’ knowledge, no previous study has explored the maxillary arch dimensions in the Sri Lankan population. However, a recent study by Arambawatta et al.¹⁵ had assessed the arch forms using study casts. While this is acceptable in order to determine the arch form there are concerns when study casts are used to determine the dimensions. Both the properties of impression material and the cast material would influence the accuracy and the dimensional stability of the obtained cast. In addition, the morphology of the gingival tissue may not reveal true dimensions of the underlying skeletal form. Hence, it can be postulated that CBCT measurements would be more accurate in revealing the skeletal dimensions compared to measurements obtained by the study casts. Therefore, the objective of the study was to ascertain maxillary arch morphology in a group of Sri Lankan adults using CBCT. Our specific objectives were to describe the prevalence of different types of maxillary arch form, dimensions in terms of arch width, antero-posterior length and the ridge height. Further, any significant correlation between different arch forms and gender and age was also to be analyzed.

2. Methods

This retrospective cross-sectional study was performed using 109 randomly selected maxillary CBCT images (minimum sample size calculated for the study ws 96) belonging to patients in the age range of 18–60 from the archives in Division of Oral Medicine and Radiology, Faculty of Dental Sciences, University of Peradeniya, Sri Lanka. Only CBCT's of fully dentate maxillary arches up to the second molar teeth were included in the study.All cases demonstrating distorted images, retained deciduous teeth, impacted teeth, advanced periodontitis, history of trauma, craniofacial anomalies such as cleft palate, evidence of craniofacial surgery and images of patients who had undergone orthodontic treatment were excluded from the study. This study protocol was approved by the ERC of the Faculty of Dental Sciences, University of Peradeniya. All CBCT scans were performed using CBCT scanner (Vatech Corporation, South Korea) under standard settings and stored, converted into deidentified DICOM file format. Measurements were obtained using the EzDent software with precision values of 0.1 mm. All the CBCT scans selected for this study were interpreted by two calibrated observers in the axial, coronal, sagittal and trans-axial planes using the minimum available slice thickness. Consensus was reached where there was a disagreement on the interpretation. All the measurements were takenat the level of the occlusal plane and they are as follows;

• 1.
  Measurements related to the width were acquired from the coronal sections as the tips of the cusps were used as reference points.

  • a.
    Inter canine width (ICW) – Mesiodistal distance between the tips of the canine cusps (Fig. 1a)
  • b.
    Inter 1st molar width (IM1W) – Mesiodistal distance between the tips of the mesio-buccal cusps of the right and left first molars (Fig. 1b)
  • c.
    Inter 2nd molar width (IM2W) – Mesiodistal distance between the tips of the mesio-buccal cusps of the right and left second molars

Fig. 1.

Coronal section demonstrating (a) Inter canine width (ICW); (b) Inter 1st molar width (IM1W).

Inter mlar width (IMW) is defined as the linear mesiodistal distance between the tips of the mesio-buccal cusps of the right and left molars and this was taken for both at the first molars and for second molars level. Inoder to minimize the changes that may occur due to the changes in a tooth position, IMW ws obtained at the first molar level as well as in the second molar level.

• 2.
  Measurements related to the antero-posterior (AP) dimensions were performed on axial sections.

  • a.
    AP distance from incisors to canines (ICD) – Distance from a line dropping perpendicular to the ICW line from the most labial point of the central incisors
  • b.
    AP distance at 1st molar- Distance from a line dropping perpendicular to the IM1W line from the most labial point of the central incisors
  • c.
    AP distance to hard and soft palate junction- Distance from a line dropping perpendicular to the hard and soft palate junction from the most labial point of the central incisors
• 3.
  Measurements related to the height were obtained from the coronal sections (Fig. 2);

  • a.
    Height at canine level – Vertical distance from the ICW line at the occlusal plane level to the hard palate
  • b.
    Height at 1st molar level - Vertical distance from the IM1W line at the occlusal plane level to the hard palate
  • c.
    Height at hard and soft palate junction - Vertical distance from the hard and soft palate junction to the occlusal plane level
• 4.The shape of the maxillary arch was classified as Oval-shape, V-shape and U-shape¹

Fig. 2.

Axial sections depicting (a) Height at canine level - Vertical distance from the ICW line at the occlusal plane to the hard palate; (b) Height at 1st molar level - Vertical distance from the IM1W line at the occlusal plane level to the hard palate.

3. Results

A total of 106 subjects were included in the sample of which, 48(45.3%) were male and 58 (54.7%) were female. The sample consisted of images of individuals between 18 years and 60 years.

3.1. Shape of the maxillary arch

A majority of the sample had an oval shaped maxillary arch (64.8%) followed by V shaped maxillary arch.(29.2%).

Since the sample was non-normally distributed, Mann Whitney u test and Kruskal Wallis tests were used to compare the observations between categories.

3.2. Dimensions of the maxillary arch

The following table summarizes the different measurements taken. The mean height of the alveolar ridge at the upper canine position was 8.9 mm and the mean height of the alveolar ridge at the first molar level was 20.1 mm (Table 1).

Table 1.

Descriptive statistics of different maxillary arch measurements in the sample.

Measurement	N	Mean	Std. Deviation	Minimum	Maximum
Height at canine position	108	8.8898	1.97200	4.10	16.20
Height at 1st molar position	108	20.1046	2.41790	14.50	26.20
Height at Soft and Hard palate junction	106	18.5962	3.08341	10.90	29.10
Width at canine position	108	35.3213	2.65020	28.70	41.50
Width at 1st Molar position	108	56.3963	3.40066	48.30	66.80
Width at 2nd Molar position	108	61.3037	3.59329	53.90	74.60
A-P Inter canine distance (ICD)	108	10.1176	2.10615	4.60	15.40
A-P distance to first Molar	108	32.5370	2.58596	24.60	39.50
A-P to Soft and Hard Palate junction	108	57.8833	4.47518	29.30	67.10
Gender	106	1.55	.500	1	2

3.3. Arch dimensions differences between gender

A Mann Whitney U test was used to compare measurements of variables between males and females. Difference in the height of alveolar ridge at the junction of the soft and hard palate, width of the alveolar ridge at the canine, first molar and second molar and also the antero-posterior distance of the arch at the inter canine distance and junction between the hard and soft palate were statistically significant (Table 2).

Table 2.

Comparison of the measurements between males and females with the significances.

Measurement	Mean Rank		P value
	Male	Female
Height at canine level	59.9	48.21	0.051
Height at 1st molar level	59.39	48.63	0.073
Height at Soft and Hard palate junction	62.95	45.68	0.004
Width at canine	64.31	44.55	0.001
Width at 1st Molar	66.34	42.87	0.000
Width at 2nd Molar	68.08	41.43	0.000
A-P Distance to Inter canine distance (ICD)	60.38	47.81	0.036
A-P distance to first Molar	55.05	52.22	0.636
A-P to Soft and Hard Palate junction	62.71	45.88	0.005

3.4. Arch dimension differences between different age groups

Results were analyzed according to age groups of the participants’ records. Kruskal Wallis test was used to compare the differences between age groups.Chisquare values didnot indicate significant differences of measurements according to the age group of the participants (Table 3).

Table 3.

Difference of measurements between groups with the chi-square value.

Measurement	Mean Rank values by age category						P value
	18–20 N = 2	21–23 N = 3	24–26 N = 33	27–29 N = 33	30–32 N = 33	33 and above N = 2
Height at canine level	58.75	26.50	47.94	55.74	56.06	101.25	0.122
Height at 1st molar level	30	27	57.27	56.48	53.3	91	0.241
Height at Soft and Hard palate junction	39.5	40.17	50.7	60.68	49.7	81.25	0.405
Width at canine	60	56.17	51.21	49.33	56.85	94.25	0.439
Width at 1st Molar	56.75	57.5	47.09	59.95	52.53	59.5	0.689
Width at 2nd Molar	71	58.17	47.64	59.88	50.3	73.25	0.477
A-P Distance to Inter canine distance (ICD)	81	38.67	50.24	53.48	55.7	99	0.217
A-P distance to first Molar	86.75	36.83	50.53	57.15	52.98	42.5	0.512
A-P to Soft and Hard Palate junction	97	48.67	51.26	53.8	52.79	61	0.494

4. Discussion

The assessment of the maxillary arch dimensions is important in many specialties of dentistry such as prosthodontics, orthodontics, planning sports related protective oral devices, forensic dentistry and implant dental practice.

In complete denture prosthodontics, provision of an acceptable complete denture relies heavily on the maxillary arch dimensions which existed prior to the loss of teeth. This is especially important in the selection of impression trays through to the construction of the denture to be harmonious with the patient's facial form and appearance. In selection of impression trays and maintaining an inventory of trays the maxillary arch dimension is of great significance which may depend on the population and other variables. Frequent adjustment and modifications of stock trays are not easily achieved and if adjusted would make especially the metal trays unusable. In a study carried out on young Korean adults the arch length was on average 44.13 mm and width 64.12 mm. The predominant form was concluded to be of the ovoid morphology. However in the present study the arch length was 62.71 mm in males and 45.88 mm in females which is considerably different to the Korean population. Even when the width at the 2nd molar region is compared the Sri Lankan study reveled that the width is larger compared to the Korean population.¹⁶ Even when compared to a study done on adults of a Saudi Arabian population, the current study revealed larger dimensions. The present study showed significant differences among the genders with reference to arch length and width at all points of measurement. This would imply that in the manufacture or ordering of stock impression trays these gender and ethnic differences should be taken into consideration for selection of the appropriate sizes.^17,18

In addition, arch dimensions play an important role in planning orthodontic tooth movement and determine the post treatment stability of the aligned dentition. When considering the arch type the predominant form was ovoid which was similar to the Korean and Saudi population.¹⁶ Arch dimensions play a decisive role in the planning of orthodontics and its ultimate outcomes. Arch wires are manufactured considering the arch width, length and form. Thus, depending on the population of study and the variations in dimensions of the maxillary arch, orthodontic components such as arch wires and other inter maxillary devices have to be customized.¹⁷ As revealed in this study, a significant difference was seen between the arch length and inter molar width among the genders appropriate selection of these devices would be necessary. In addition, when arch expansion is planned these results would be beneficial in determining the limits of the treatment outcomes and also to decide on the feasibility of such modalities.¹⁹

Sports related protective devices such as mouth guards are commonly prescribed. Most of these devices are available off the shelf in sports shops. In planning inventory of these items, the prevalence of the different arch forms whether oval, U or V shaped and the length and width need careful assessment. In addition, as stated above the significant gender variations of these parameters should be paid due consideration. Thus, in the Sri Lankan population such ready-made mouth-guards should conform to the predominant oval shape with at least 65 mm arch length and width to conform to the male population and a small dimension for the female population.²⁰

The outcomes of this study would help in defining the norms of the Sri Lankan population in terms of maxillary dimensions and such information would be also invaluable in forensic dentistry for determining the approximate race of forensic material in un-identified remains.²¹ As gender variations also exist this would help determining the gender and also aid in forensic recreations.²²

Implant dentistry is a popular means of replacing missing teeth. In this context the arch height and width play a significant role in selection of the size and length of implants. The mean height of the alveolar ridge in the canine region was 8.9 mm and 20.1 mm in the first molar region. Thus, appropriate implant lengths should be selected and planned to obtain optimum results during implantation of teeth especially when carrying out immediate implants.^23,24 This may also be an important aspect when planning inventories of implant stocks.The study conducted for Sri Lankan population could be considered as a limitation as the results cannot be generalized for the global population.

5. Conclusion

Difference in the height of alveolar ridge at the junction of the soft and hard palate, width of the alveolar ridge at the canine, first molar and second molar and also the antero-posterior distance of the arch at the inter canine distance and junction between the hard and soft palate were statistically significant between the genders. No significant differences in arch parameters were observed among the different age groups.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.