The objective of this prospective clinical study was to evaluate the dentoalveolar and skeletal effects induced by rapid maxillary expansion (RME) therapy in mixed dentition patients with Class II Division 1 malocclusion compared with a matched untreated Class II Division 1 control group.
The treatment sample consisted of cephalometric records of 50 patients with Class II malocclusion (19 boys, 31 girls) treated with an RME protocol including an acrylic splint expander. Some patients also had a removable mandibular Schwarz appliance or maxillary incisor bracketing as part of their treatment protocol. Postexpansion, the patients were stabilized with a removable maintenance plate or a transpalatal arch. The mean age at the start of treatment of the RME group was 8.8 years (T1), with a prephase 2 treatment cephalogram (T2) taken 4.0 years later. The control sample, derived from the records of 3 longitudinal growth studies, consisted of the cephalometric records of 50 Class II subjects (28 boys, 22 girls). The mean age of initial observation for the control group was 8.9 years, and the mean interval of observation was 4.1 years. All subjects in both groups were prepubertal at T1 and showed comparable prevalence rates for prepubertal or postpubertal stages at T2. Independent-sample Student t tests were used to examine between-group differences.
Class II patients treated with the described bonded RME protocol showed statistically significant increases in mandibular length and advancement of pogonion relative to nasion perpendicular. The acrylic splint RME had significant effects on the anteroposterior relationship of the maxilla and the mandible, as shown by the improvements toward Class I in the maxillomandibular differential value, the Wits appraisal value, and the ANB angle. Patients treated with the bonded RME showed the greatest effects of therapy at the occlusal level, specifically highly significant improvement of Class II molar relationship and decrease in overjet. Treatment with the acrylic splint RME had no sustainable effects on the skeletal vertical dimension, maxillary skeletal position, or maxillary dentoalveolar dimensions.
This study suggests that the protocol described including treatment with a bonded rapid maxillary expander used in the early mixed dentition in Class II Division 1 patients can help to improve the Class II malocclusion as a side-effect, both skeletally and dentally. Evidence for this phenomenon was based previously on anecdotal data; the results of this study show that the improvements are far more pervasive than anticipated.
Class II malocclusions are observed commonly in orthodontic patients, comprising up to one-third of the orthodontic population in the United States. Such patients can have several dentoskeletal combinations, with an abnormal skeletal pattern or a normal skeletal pattern with an altered dental arrangement.
Another component of Class II malocclusions that is important to consider during treatment planning, but often is overlooked, is the transverse dimension. Tollaro et al showed an underlying transverse discrepancy of 3 to 5 mm in many dental arches with Class II malocclusions without posterior crossbites in centric occlusion. When these Class II patients were asked to posture their lower jaw forward in a Class I relationship, this discrepancy (ie, maxillary constriction) could be observed clinically. Vargervik similarly reported on the commonality of maxillary constriction of Class II malocclusion patients. She stated that the correction of a Class II molar relationship without the creation of a posterior crossbite requires an increase in maxillary molar width relative to the mandibular molar width of approximately 2 mm for unilateral Class II and 4 mm for bilateral Class II molar relationship.
The transverse discrepancy discussed is not self-correcting during the mixed dentition transition. This discrepancy, often caused by constriction of the maxilla in Class II patients, has been shown to be both dental and skeletal. Rapid maxillary expansion (RME) therapy therefore is indicated in these patients during the mixed dentition.
RME has been in common use by orthodontists for several decades. Although expansion initially was used to correct posterior crossbites and gain arch perimeter, more possible indications for this technique have been proposed. McNamara advocated the use of expansion in many early mixed dentition Class II patients with mild mandibular retrusion and maxillary constriction. A removable mandibular Schwarz appliance can be used initially to upright the mandibular posterior teeth orthodontically, followed by an orthopedic bonded maxillary expander. The occlusion is stabilized by using a maintenance plate in the mixed dentition or a transpalatal arch or full appliances in the permanent dentition. Widening the maxilla to an overexpanded position (maxillary molar lingual cusps approximating the mandibular molar buccal cusps) often leads to spontaneous forward posturing of the mandible during the retention period. In these patients, the expanded maxillary arch appears to have the function of an endogenous functional appliance that solicits the mandible to be postured into a more anterior position.
Lima Filho and Oliveira Ruellas investigated slow and rapid maxillary expansion in Class II skeletal patients, with 1 group treated with Kloehn cervical headgear while gradually expanding the inner bow. The second group consisted of patients treated with cervical headgear and RME. They reported significant changes in profile analysis with respect to mandibular protrusion in Class II skeletal patients treated with RME. There were significant increases of SNB, B-Hor, and Pog-Hor between the treatment intervals for both treated groups.
The aim of this study was to describe the anteroposterior effects induced by RME therapy in Class II Division 1 patients, focusing specifically on changes that contribute positively to the correction or improvement of a Class II molar relationship.
Material and methods
This investigation was a prospective clinical trial that was part of a larger study of RME conducted in a private practice setting. This longitudinal study was designed to evaluate cephalometrically the skeletal and dentoalveolar effects achieved by treatment with RME in Class II patients compared with a matched control group of Class II subjects.
The primary study sample consisted of 1135 consecutive patients who were treated with RME in the mixed dentition. Treatment was performed in 1 practice by orthodontists who followed a standardized protocol and were homogeneous as to clinical skill and expertise.
Cases were not considered if full records were not taken at the time of the second observation just before full braces (phase 2 treatment). Other reasons for initial exclusion from enrollment, or for dropouts from the prospective study, were extracted or congenitally missing teeth, use of a banded expander or additional mechanics (such as a functional appliance or utility arches during the observation period), thus leaving a subsample of 574 subjects.
Additional exclusionary criteria were applied, reducing the sample to include only Class II Division 1 patients who met the dentitional criteria of all deciduous molars and permanent first molars and incisors at the start of treatment (T1) and all premolars fully erupted about 4 years later before full-appliance treatment (T2). Moreover, all subjects enrolled in the final treatment sample had maxillary constriction shown by an initial mean transpalatal width measurement of 30 mm or less. This measurement was determined during the initial examination and was measured clinically from the most lingual aspect of 1 maxillary first permanent molar to its antimere.
To summarize, the rigorous enrollment criteria were Class II tendency malocclusion (end-to-end first permanent molars) or Class II molar relationship (full-cusp) at T1, early mixed dentition with all 8 first and second deciduous molars at T1 and received a bonded maxillary expander, cephalograms available at 2 observation times (T1 and T2), and early permanent dentition at T2 with all 8 premolars erupted fully.
The final sample consisted of 50 patients with Class II Division 1 malocclusion. The mean age at T1 of the treated group was 8.8 years, with the T2 cephalograms taken 4.0 years later on average ( Table I ).
|T1 age (y)||T2 age (y)||T2-T1 (y)|
|RME (19 boys, 31 girls)||8.8||1.1||12.8||1.1||4.0||0.9|
|Control (28 boys, 22 girls)||8.9||0.9||12.9||1.0||4.1||0.5|
From a pool of untreated subjects followed longitudinally throughout growth, a control group of 50 subjects with Class II malocclusion was matched with the treatment subjects. The cephalograms of the untreated subjects were obtained from 3 longitudinal growth studies to obtain the best match possible to the treatment group, based on the same inclusion criteria described above. Twenty-three subjects were from the University of Michigan Elementary and Secondary Growth Study (Ann Arbor), 21 subjects were from the Broadbent-Bolton Collection at the Bolton-Brush Growth Study Center (Cleveland, Ohio), and 6 subjects were used from the Denver Growth Study (Colo). The mean age at the start of observation for the control group was 8.9 years, and the mean time of observation at T2 was 4.1 years ( Table I ).
Significant effort was directed toward matching the control sample to the treatment sample as closely as possible with respect to sex (male:female ratio), dentition (all deciduous molars present at T1 and all permanent premolars fully erupted at T2) to account for the loss of leeway space in both groups, skeletal maturity (as measured by the cervical vertebral maturation (CVM) stage at T1 and T2), chronologic age at T1 and T2, equal numbers of Class II and Class II tendency (end-on) malocclusions, and observation interval.
At T1, all subjects, both treated and untreated, were prepubertal according to the method derived by Baccetti et al in 2005. At T2, there were no significant differences between prevalence rates for prepubertal and postpubertal stages in the treated group compared with the untreated group ( Table II ).
|RME group||Control group|
|(19 boys, 31 girls)||(28 boys, 22 girls)|
|CVM stage||Subjects at T2 (n)||Subjects at T2 (n)|
All patients received a bonded acrylic splint RME after the T1 cephalogram for an average of 6.7 months. The expansion screw was activated until the palatal cusps of the maxillary posterior teeth approximated the lingual cusps of the mandibular posterior teeth. Twenty-nine, or 58%, of the patients wore a removable mandibular Schwarz expansion appliance before the maxillary expansion appliance. Thirty-five patients also had brackets placed temporarily on the maxillary incisors. Forty-eight treated patients received an acrylic palatal plate for retention after removal of the RME; 2 patients were given a transpalatal arch for retention of expansion after RME removal because of the loss of the maxillary second deciduous molars at RME removal. All patients received a transpalatal arch before the cephalogram at T2. Eleven patients received a mandibular lingual arch before the T2 cephalogram.
Both lateral cephalograms of each patient were hand-traced on 0.003-in matte acetate with a 2H lead pencil at a single sitting. Cephalograms were traced by 1 investigator (S.S.G.); landmark location and the accuracy of the anatomic outlines were verified by a second (J.A.Mc.). The functional occlusal plane was included on each tracing. A customized digitization regimen (version 2.5, Dentofacial Planner, Toronto, Ontario, Canada) that included 78 landmarks and 4 fiducial markers was created and used for the cephalometric evaluation. All measurements were corrected for magnification differences between series and standardized to an enlargement of 8%. The Dentofacial Planner program allowed analysis of cephalometric data and superimpositions among serial cephalograms.
Regional superimpositions on the cranial base, maxilla, and mandible were accomplished by hand, as advocated previously by Ricketts and McNamara. Cranial base superimpositions detected changes in maxillary and mandibular skeletal position. Films were oriented along the basion-nasion line and registered at the most posterosuperior aspect of the pterygomaxillary fissure, with the contour of the cranium posterior to the foramen magnum used to verify the accuracy of the superimposition. Maxillary regional superimpositions identified movements of the maxillary dentition relative to the maxillary basal bone. The maxilla was superimposed along the palatal plane by registering on bony internal details of the maxilla superior to the incisors, and the superior and inferior surfaces of the hard palate. Mandibular regional superimpositions characterized movements of the mandibular dentition relative to the mandibular basal bone. Mandibular superimpositions were performed posteriorly on the outline of the inferior alveolar nerve canal and any tooth germs (before root formation) and anteriorly on the internal structures of the mandibular symphysis.
Descriptive statistics, including means and standard deviations, were calculated for age, duration of treatment, values at T1 and T2, and changes between T1 and T2 of all cephalometric measures for the 2 groups. The data were analyzed with a Windows-based statistical software package (version 12.0, SPSS, Chicago, Ill). Statistical significance was tested at P <0.05, P <0.01, and P <0.001.
After the assessment of normal distribution of the data (Shapiro Wilks test), independent-sample Student t tests were used to examine between-group differences of the means of the cephalometric measures of the starting forms of the 2 groups, as assessed on the basis of a series of diagnostic variables. Comparison of T2 to T1 changes over time between the treated and untreated groups also was accomplished with independent-sample Student t tests.
The power of the study also was calculated by the SPSS software. The statistical power of a study indicates the probability that a significant difference between groups can be detected when one truly exists. The power of a statistical test is influenced by the variance of the means, the common standard deviation of sample means, the level of significance at which the test is run, and the sample size. On the basis of the expected differences in the changes of molar relationship (main target variable) between the 2 groups, given the sample size (n = 50) and a P value of 0.05, the power of this study was 100%. This means that there is no probability that one does not see the differences described by chance alone (false negatives).
As mentioned previously, assembling a well-matched control group of untreated Class II subjects was the highest priority in designing this study. Exploratory chi-square statistical tests were performed for sex distribution and skeletal maturity levels measured by the CVM stage at T2 (all subjects in both groups were prepubertal at T1). There was no significant difference for sex distribution of the 2 samples (chi-square = 2.57; P = 0.109). There also was no significant difference among the 2 groups at T2 for the prepubertal or postpubertal CVM stages (chi-square test = 2.60; P = 0.107); this emphasizes the adequacy of the matched control group.
Thirty-two patients in the treated group were categorized as Class II tendency at T1, and 18 had full-cusp Class II malocclusions initially. Similarly, 30 subjects in the control group had a Class II tendency molar relationship at T1, and 20 had full-cusp Class II malocclusions. Differences between the cephalometric T1-T2 changes for the Class II tendency subgroup of patients and the Class II subgroup of patients in both groups were tested statistically (Mann-Whitney U tests, P <0.05). Differences in outcomes among subgroups of subjects treated with different protocols (Schwarz appliance, maxillary bracketing, maintenance plate, and lingual arch) were tested with Mann-Whitney U tests ( P <0.05). The use of nonparametric statistics for these comparisons was due to the limited number of subjects in each group.
Descriptive data and statistical comparison for starting forms and cephalometric changes in the 2 groups from T1 to T2 are given in Tables III and IV , respectively.
n = 50
|RME vs control|
|Cephalometric measurement||Mean||SD||Mean||SD||Mean difference||P value †|
|Maxillary A-P skeletal|
|Mandibular A-P skeletal|
n = 50
n = 50
|RME vs Control|
|Cephalometric measurement||Mean||SD||Mean||SD||Mean difference||P value §|
|Maxillary A-P skeletal|
|Pt A-Na perp (mm)||0.1||1.4||0.1||1.3||0.0||0.936||NS|
|Co-Pt A (mm)||5.1||2.0||5.4||2.1||−0.3||0.591||NS|
|Mandibular A-P skeletal|
|Pg-Na perp (mm)||1.9||1.9||0.8||2.1||1.1||0.005||†|
|Mx/mn diff (mm)||4.0||2.2||2.4||2.3||1.6||0.001||†|
|Gonial angle (°)||−1.7||2.4||−1.8||2.7||0.1||0.825||NS|
|U1-Pt A vert (mm)||0.7||1.2||0.6||1.4||0.1||0.515||NS|
|UL to E-plane (mm)||−2.3||1.7||−1.4||3.1||−0.9||0.062||NS|
|LL to E-plane (mm)||−1.4||1.9||−1.1||3.0||−0.3||0.460||NS|
|Nasolabial angle (°)||−0.4||10.1||4.1||13.1||−4.5||0.060||NS|