ORIGINAL ARTICLES DERIVED FROM RESEARCH

Relationship between posterior dentoalveolar vertical dimension and skeletal classification in orthodontic patients treated with and without first premolar extractions. a cephalometric analysis¹

María Piedad González Sauter²; Marggie Grajales Ramírez³; Eliana Midori Tanaka Lozano.⁴

¹ This project was carried out with the financial and methodological support of Fundación Centro de Investigación y Estudios Odontológicos —CIEO—, Bogotá, Colombia. Article presented as partial requirement for the graduate resident to opt to the title of Orthodontics Specialist at Fundación CIEO, Universidad Militar Nueva Granada, Bogotá, Colombia
² Dentist, Universidad Autónoma de Manizales, Resident of the Graduate Program in Orthodontics, Universidad Militar Nueva Granada, Fundación CIEO, Bogotá, Colombia
³ Dentist, Universidad Nacional de Colombia, Resident of the Graduate Program in Orthodontics, Universidad Militar Nueva Granada, Fundación CIEO, Bogotá, Colombia
⁴Dentist, Pontificia Universidad Javeriana, Orthodontics specialist, Associate Professor, Graduate Program in Orthodontics, Universidad Militar Nueva Granada, Fundación CIEO, Bogotá, Colombia. Ph. D. Visiting Researcher, Kanagawa Dental College, Yokosuka, Japan

González MP, Grajales M, Tanaka EM. Relationship between posterior dentoalveolar vertical dimension and skeletal classification in orthodontic patients treated with and without first premolar extractions. A cephalometric analysis. Rev Fac Odontol Univ Antioq 2012; 23(2): 225-239.

ABSTRACT

INTRODUCTION: the purpose of this study was to determine and to cephalometrically compare the variation of posterior dentoalveolar vertical dimension in orthodontic patients treated with and without extraction of first premolars in Class I and Class II malocclusions and to establish its correlation by means of the Anteroposterior Dysplasia Indicator (APDI). Mehods: pre (T1) and post (T2) treatment lateral cephalograms of 76 patients from Fundación CIEO, aged 22 to 45 years, were skeletally classified according to
METHODS: pre (T1) and post (T2) treatment lateral cephalograms of 76 patients from Fundación CIEO, aged 22 to 45 years, were skeletally classified according to APDI and the type of treatment received, regardless of gender, forming four groups: Class I with and without extractions, and Class II with and without extractions. The posterior dentoalveolar vertical dimension was calculated with linear measurements and its variation among the malocclusion groups was statistically analyzed; also, a multiple correlation analysis between dentoalveolar heights and APDI was performed at T1 and T2.
RESULTS:the intragroup analysis showed a significant increase of posterior dentoalveolar vertical dimension at T1 and T2 in both Class I and Class II groups with first premolar extractions at the lower second premolar (5i) and upper second molar (7s), respectively. The intergroup analysis showed a significant increase in posterior dentoalveolar vertical dimension, according to skeletal malocclusion class, in treatments with first premolar extractions, at the upper second premolar (5s), upper first molar (6s), upper second molar (7s), and lower first molar (6i).
CONCLUSIONS:there was an increase of posterior dentoalveolar vertical dimension at T1 and T2 in all the groups, being statistically significant in Class I and Class II patients treated with first premolar extractions without altering the skeletal classification (APDI).

Key words:orthodontics, cephalometric analysis, vertical dimension, malocclusion, premolar extractions.

INTRODUCTION

Since vertical dimension is a key element for the stomatognathic system balance1 and directly influences occlusal support, it is essential in any dental procedure, especially in orthodontics, to try to maintain it or to reestablish it in order to achieve the system's proper function while keeping orofacial balance. Therefore, procedures that alter it in a negative way must be avoided.²

Extraction of first premolars is one of the most controversial orthodontic alternatives^{3, 4} as losing posterior teeth implies a reduction in vertical dimension, and therefore loss of occlusal support.⁵

According to Saizar, vertical dimension refers to any height measurement of the position of the mandible in relation to the rest of the face.^6,7 When vertical dimension is measured inside the face it is called intermaxillary distance, and when extraorally calculated it is called facial height. It may be determined in several ways: at rest, when the mandible is relaxed in its natural position and the muscles show minimum activity; in centric occlusion, when the occlusal surfaces are in contact; in maximum opening, or in eccentric occlusion.^{6, 8, 9} On the other hand, in 2002, Kato et al¹⁰ described it as the dentoalveolar distance, in millimeters, between the incisal or cuspal borders of the teeth and their corresponding osseous base, it is, the so-called dentoalveolar vertical dimension.

During growth and craniofacial development, a continuous increase in posterior vertical dimension takes place, and it is determined by the level of eruption of the first permanent molars,¹¹ thus producing leveling of the occlusal plane and allowing forward mandibular adaptation. When vertical dimension increase in the posterior zone is not enough to provide appropriate mandibular adaptation, the anterior mandibular rotation is reduced, thus producing skeletal structures Class II.^10-12 According to Sassouni and Nanda, vertical disparities produce anterioposterior dysplasia.13 Therefore, treatment strategies and objectives must be directed towards vertical control in order to correct anterioposterior disharmonies.

Every vertical dimension alteration must consider the interocclusal distance or free-way space, which is defined as the distance between the vertical dimension at rest and in centric occlusion (2 to 3 mm in average).⁶ Tooth loss or attrition tend to decrease the face's vertical height and to increase free-way space, thus creating constant dislocation of the mandible beyond its resting position during the masticatory process and creating improper traction of the muscles, as well as stretching beyond its normal length, a condition that hardly restores and produces structural changes. Similarly, vertical dimension loss shortens the distance between the origin of the muscle and its insertion, thus producing loss of muscle efficiency and tone. Efforts and muscle traction may produce pain, discomfort, and constant tension, as well as alteration in both phonation and adequate food mastication.¹⁴

The ideal vertical dimension must allow enough interocclusal distance between position at rest and centric occlusion, as well as adequate facial height with teeth in centric occlusion. Also, it must have healthy, esthetic, and phonetically correct dental length and cusp height.⁶ This is why vertical dimension improvement occurs as a result of neuromuscular adaptation based on a harmonic relation of the following criteria, which have been obtained by clinical practice: available prosthetic height, anterior occlusal relationships (overbite and overjet), skeletal type and mandibular morphology, neuromuscular and TMJ coordination, esthetics, and proportion of facial heights.^15-17

According to Sassouni and Nanda,^{13, 18} extraction of the first premolars tends to reduce facial vertical dimension, and as stated by Pearson and Fields,^{1, 19} mesial migration of the molars causes anterior mandibular rotation. Other studies suggest that premolar extraction does not reduce lower anterior facial height and that, on the contrary, may increase it.¹⁵ Although few studies have demonstrated anterior and posterior facial height increase, even with extraction of the first premolars without modifying the mandibular plane, these extractions are still considered as the cause of vertical dimension reduction.^{4, 20, 21}

Al-Nimri³ states that first premolar extraction does not produce significant changes in lower facial height, as opposed to the cases without extractions, which do produce its increment because in Class I patients with extractions, most of the space achieved is used to reduce crowding and to retract the anterior sector.

Meral et al¹ state that extraction of first upper premolars produce a lower percentage of mandibular rotation, and suggest that extraction of these teeth may cause a change in maxilla direction and therefore a change in mandibular rotation in posterior direction. Inclination of the mandibular plane is smaller in patients without extractions. Dental arches adaptation depends on the type of mandibular rotation: if it is anterior, the incisors and molars lean forward and extraction of upper premolars causes inhibition of the anterior mandible growth.

In general, orthodontic treatments have failed to focus on adjusting the occlusal plane and hence the posterior vertical dimension but simply concentrate on achieving harmony of the dental arches instead of focusing on the osseous bases,²² which implies treatments of dentoalveolar compensation even in patients with skeletal discrepancies including extractions (especially of premolars) to facilitate the achievement of dental objectives and occlusal harmony.

Premolar extraction as an orthodontic procedure for dental compensation of skeletal discrepancies has been controversial because it is related to changes in vertical dimension, as it can positively or negatively influence the orofacial balance.

Therefore, the objective of this study was to determine the variation of posterior dentoalveolar vertical dimension in orthodontic patients with and without first molar extractions in Class I and Class II malocclusions, by applying criteria of linear measurement and by correlating vertical dimension with skeletal classification by means of the Anteroposterior Dysplasia Indicator (APDI).

MATERIALS AND METHODS

This was an analytic and retrospective study with 76 adult patients with lateral head TeleRx pre- and post- orthodontic treatment, taken at different ages (range age: 22-45 years).

The lateral head TeleRx were collected from the clinical records of patients treated by residents and professors at Fundación CIEO and were selected according to the following criteria: full permanent dentition, except for third molars; skeletal types Class I and Class II (APDI); orthodontic treatment performed with or without extraction of the first premolars, and complete fixed orthodontic brackets up to the second molars, regardless of the technique (standard, straight arch, or self-ligation) and the biomechanics used during the space closure phase.

Exclusion criteria: radiographs of patients with craniofacial growth alterations and genetic disorders.

Once the sample had been collected and classified, a single expert operator proceeded to trace the cephalometric landmarks by hand on cephalometric paper.

The patients were classified according to skeletal class (APDI) and the kind of treatment used, regardless of gender, into four groups:

20 skeletal Class I patients without extractions; 16 skeletal Class I patients with extractions, 18 skeletal Class II patients without extractions, and 22 skeletal Class II patients with extractions. Measurements were taken at T1, corresponding to pre-treatment lateral head TeleRx, and at T2, as to post-treatment lateral head TeleRx.

All the TeleRx were taken with a standardize equipment (Lateral Cephalometer W-115, WEHMER^® Corporation, Lombard, Illinois) with 80 Kv (kilovoltage) and 15 mA (miliamperage) and 1 s exposure.

For selection of the operator in charge of the sample measurements, the variation coefficient (9.8%) was considered; it consisted in calculating his error in relation to the average measurements in the pilot test, corresponding to a maximum deviation of 2 mm from the mean length calculated for the vertical dentoalveolar dimensions.

The sample was initially classified into Class I and Class II, based on the APDI (measured in degrees), considering the summation among:

• Posterior angle formed by the Frankfort plane and the facial plane (Nasion-Pogonion of hard tissues).

• Angle formed between the facial plane and the AB plane, so that if A surpasses B the result would be negative.

• Angle formed by the Frankfort plane and the palatal plane ANS-PNS in which the result would be negative if ANS surpasses PNS.

Patients whose summation ranked between 81,37° ± 3,79° were classified as skeletal Class I; values below this rank were considered as skeletal Class II.²³

The posterior dentoalveolar vertical dimension was calculated with linear measurements in millimeters. For the upper maxilla, the palatal plane (ANS-PNS) lines perpendicular to the cusps of 4 and 5 and mesiobuccal 6 and 7 were used, and they were named as follows: 4s (first upper premolar), 5s (second upper premolar), 6s (first upper molar), and 7s (first upper molar).

For the mandible, the perpendiculars to the mandibular plane (Gonion-Menton) of cusps 4 and 5 and mesiobuccal cusps of 6 and 7 were named as follows: 4i (first lower premolar), 5i (second lower premolar), 6i (first lower molar), 7i (first lower molar) (figura 1).

The statistical analysis was run with these programs: Excel, 2007 version; Stat Plus, free 2008 version, and R-2.13, free 2011 version.

For the intragroup analysis, the Geary test was used in order to verify data normality, and afterwards the Student`s T test was applied for equal or different variances.

For the intergroup analysis, the Shapiro-Wilk test was used first in order to verify data normality. The ANOVA test was applied in two ways in the data with normal distribution, and the Kruskal-Wallis test was later applied for those data that did not yield normal distribution. In order to determine the relation between posterior dentoalveolar vertical dimension and skeletal classification, the Pearson's multiple correlation analysis was performed between the dentoalveolar heights and the APDI, before (T1) and after (T2) orthodontic treatment.

RESULTS

After applying the statistical test (Student's T test) for the intragroup analysis no significant differences were observed when comparing groups Class I and Class II without extractions (table 1). On the contrary, significant differences were found (p < 0,05) between Class I with extractions for 5i and Class II with extractions for 7s (table 2).

In the intergroup analysis, once the Shapiro-Wilk test was applied, significant differences (p < 0.05) were observed at 5s (figure 2), 7s (figure 3), and 6i (figure 4) when compared according to the skeletal classification in the treatments that included premolar extraction (Class I with extractions and Class II with extractions); however, they did not show statistical significance when compared with the times of measurement (T1 and T2).

The Kruskal-Wallis test, applied to the data that did not show normal distribution, only reported significant difference (p < 0.05) at 6s (figure 5) when compared according to the skeletal classification in treatments which included extraction of first premolars (Class I with extractions and Class II with extractions); this was observed when comparing T1 and T2 (figure 6).

After comparing dentoalveolar heights between T1 and T2, statistical significance (p < 0.05) was found for all the measured teeth. The correlation (ρ ≥ 80%) occurred at 7s, 4i, 5i, 6i and 7i in the Class I group without extractions and in the Class II group without extractions at 5s, 6s, 7s, 6i and 7i, as well as in the Class I group with extractions at 5i and 7i, and at 6i for the Class II group with extractions. No correlation was found among the APDI measured at T1 and T2 for the four groups.

DISCUSSION

Vertical dimension in orthodontics is usually established by means of angular measurement,^{1, 18, 21, 24} linear projections of cephalometric osseous landmarks, ^{1, 3, 15, 18} or clinical estimation on soft tissues.^{8, 25}

Nevertheless, this study used the measurement criteria reported by Kato et al,¹⁰ which allowed evaluating the posterior vertical dimension of the teeth up to their osseous base, with the intention of determining and comparing posterior dentoalveolar vertical dimension in orthodontic patients with and without extraction of premolars in Class I and Class II malocclusions.

The results of this study showed that extracting first premolars in patients with Class I mal-occlusion produced a statistically significant difference in posterior dentoalveolar vertical dimension at T1 and T2 for 5i, while in patients with Class II malocclusion the statistical significance was obtained for 7s. In these two cases, it may be assumed that there is some influence of the orthodontic mechanics used during the space closure phase, which involves both bracket position and the type of biomechanics and anchorage used in order to achieve adequate occlusion and to maintain or modify stability of the stomatognathic system. This is in agreement with the study by Meral et al in 2004,¹ who demonstrated that vertical and sagittal growth of the mandible is a key factor in facial profile and depends on the nasomaxillary vertical complex, including the posterior dentoalveolar region, and suggested that most of the studies on the effects of treatment with extractions present variations due to the mechanics used

Conversely, in 2006, Hans et al²¹ found no significant difference between treatments with and without extraction applying a specific mechanics (Tweed). For this reason, the orthodontic mechanics cannot be isolated from the practice of extracting premolars—a fact that was not considered for this study.

This study did not consider elements such as orthodontic technique, biomechanics, the use of intra and extraoral anchorage accessories, or previous or simultaneous use of orthopedic appliances. It is important to note, however, that the patients were treated with standard orthodontic systems, straight archwire and self-ligation. Contrariwise, the study by Kim et al, which evaluated the effects of premolar extractions on vertical dimension in Class I patients, did take into account, as an exclusion criteria, the use of intra- and extraoral anchorage orthopedic appliances and demonstrated increase in the vertical dimension lines after treatment, thus concluding that such increments could have been a consequence of residual growth, as the study was performed with a group of teenagers. Therefore, premolar extraction does not influence growing patterns.¹⁸.

The aforementioned studies were performed on adolescents and did not find significant differences, contrary to the present study that was conducted on adult patients (ages 22 to 45 years) who have already finished active growth.

The results obtained with statistical significance allowed concluding that variation in the posterior dentoalveolar vertical dimension increased among the skeletal classes and the times of measurement (T1 and T2), which disagrees with the study published in 2006 by Al-Nimri, who suggested that premolar extractions do not produce significant facial height variations, but extractions do increase facial height. The vertical changes with premolar extraction do not differ from the changes produced by treatments without extractions, as in most cases the space left by the extraction is used to reduce crowding and to retract the anterior sector, especially in Class I.³ It is important to note that the measurement criteria used in this study were different (linear).

Furthermore, in 1999 Kocadereli¹⁵ pointed out that there were not enough publications supporting the fact of molars mesialization implying reduction of the vertical dimension, as shown in the present study, in which some posterior dentoalveolar vertical dimension increase occurred in patients with extractions.

In general, it is stated that extractions are performed in patients with vertical pattern as they help controlling vertical dimension, and that it is necessary to avoid them in brachiocephalic patients in order to prevent excessive vertical closure. In 2008, Sivakumar and Valiathan reported that closure of the extraction space may cause side extrusive effects with a tendency to vertical dimension increase. Although few studies have demonstrated increments of the anterior and posterior facial height values, even with premolar extractions without changes in the mandibular plane, these extractions are still considered to be the cause of reduced vertical dimension. Similarly, they pointed out that vertical dimension increases in groups with or without extractions, being greater in groups with extractions. The mesial movement of upper and lower posterior teeth coincided with extrusions that increased vertical dimension.⁴

It is important to bear in mind that these comparisons were made in a short period of time, so it was not enough to assess the real effects on posterior dentoalveolar vertical dimension as a consequence of extractions and the orthodontic mechanics used. It would be therefore necessary to make a long-term follow-up of such treatments in order to observe the behavior of the supporting dental tissue as the occlusal function changes after treatment, as well as the possible effects on posterior dentoalveolar vertical dimension, and therefore on lower anterior facial height.

Although the present study found out a difference between T1 and T2 as an indication of posterior dentoalveolar vertical dimension increase in all the groups (1-2 mm average), such difference was statistically significant only at 5i and 7s of Class I and Class II groups with extractions, respectively.

Since there is no significant correlation between APDI before and after treatment, this vertical dimension increase cannot be related to changes in skeletal classification, although this could clinically imply changes in terms of facial height and therefore in facial profile.

Considering the limitations of a retrospective study, it is recommendable for future studies to assess the variation of vertical dimension in the medium and long terms, as well as to perform studies comparing different variables such as dentoalveolar vertical dimension, basal, mandibular rotation, lower facial height and skeletal classification in different orthodontic techniques, or to design an orthogonal system that allows obtaining greater accuracy and reliability of the linear measurements, just to name a few objectives.

CONCLUSIONS

• An increase of posterior dentoalveolar vertical dimension was observed in all the groups at T1 and T2, being statistically significant in the groups with extraction of first premolars in Class I at 5i, and in Class II at 7s.

• An increase of dentoalveolar vertical dimension was observed in all the groups with extraction of first premolars by comparing Class I and Class II, and this was statistically significant for 5s, 7s, 6s, and 6i, being greater in Class II.

• The dentoalveolar height of all the measured teeth increased after performing orthodontic treatment, being significant for some of the teeth, and this could have clinical implications but without altering skeletal classification.

ACKNOWLEDGEMENTS

To the Research Committee of Fundación Centro de Investigación y Estudios Odontológicos, CIEO, for its methodological and statistical assistance in this study.

CORRESPONDING AUTHOR

Eliana Midori Tanaka Lozano
Research Committee
Orthodontics Graduate Program
Universidad Militar Nueva Granada
Fundación CIEO Bogotá, Colombia
Email addresses:emtanaka@gmail.com
em.tanaka@cieo.edu.c

REFERENCES