Introduction
Palatal expanders have been advocated for spontaneous correction of some Class II malocclusions. However, little research has been done to determine whether correction or improvement actually occurs with expansion. Past research has not shown whether an anterior functional shift is achieved in patients who have Class II improvement. The objective of this study was to determine whether maxillary expansion causes spontaneous correction or improvement of a Class II malocclusion.
Methods
This was a retrospective study of subjects from an orthodontic office in which models are mounted (articulator) in centric occlusion and maximum intercuspation before and after treatment. The sample included Class II patients (n = 13; mean age, 10 years 3 months) who, during the preceding 2 years, had been treated with expansion alone. Study models made before and after expansion were measured to compare the centric occlusion to the maximum intersuspation position. Condyle position indicator paper was also used to determine whether there was a functional shift after expansion.
Results
The only measurements with statistically significant changes from pretreatment to postexpansion were the maxillary intermolar widths. Seven of the 13 patients showed Class II improvement, but none had an anterior functional shift after expansion.
Conclusions
Maxillary expansion does not predictably improve Class II dental relationships.
Many treatment modalities can be used to correct Class II malocclusions. Expansion of the maxillary arch is often necessary because of the relative constriction of the maxilla as the Class II relationship is corrected. Some think that palatal expansion is not appropriate when treating Class II malocclusions, whereas others believe that the associated dental expansion will cause spontaneous correction or improvement of the problem. This does not happen because of enhanced mandibular growth, but, instead, Class II improvement can occur because widening of the maxillary dental arch removes occlusal interferences, allowing the mandible to move forward to a more comfortable position. When the mandible is free to move forward, a condition is created for the mandible to grow to its full extent. Some believe that maxillary expansion is detrimental to a Class II malocclusion, since the maxilla might be displaced downward and forward, and the mandible can rotate open.
McNamara studied whether maxillary expansion causes spontaneous Class II correction and found that, in the untreated Class II tendency group, the molar relationship of 48% of the subjects remained unchanged, 41% improved, and 11% worsened; in the treated Class II tendency group, 35% remained unchanged, 63% improved, and 2% became worse. Timmons reported that 70% showed an “induced change” (Class II improvement) of at least a quarter cusp difference using archwire expansion. Thirty-eight percent continued to change and had up to a half cusp correction, and 6% had a three-fourths cusp to complete correction.
According to Gianelly, the mandibular arch acts as a “narrow foot” that moves forward after the “shoe” is widened. It was postulated that the mandible in initial contact position in centric relation (CR) is in a distal position because the constricted maxilla is holding it back in that position.
To record a reproducible mandibular position, the use of an articulator is warranted. The Glossary of Prosthodontic terms defines CR as “the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior-superior position against the slopes of the articular eminences.” This position is independent of tooth contact. Centric occlusion (CO), on the other hand, is “the occlusion of opposing teeth when the mandible is in CR. This may or may not coincide with the maximum intercuspation position.” Maximum intercuspation (MI) is “the complete intercuspation of the opposing teeth independent of condylar position.” CR has been shown to be a reproducible position when recorded with bimanual manipulation according to the Roth technique.
Studies have been done to determine whether there is correction of Class II malocclusions with maxillary expansion. However, no study considered functional shift. All studies looked at MI only, and the patient could have been shifting forward to correct the Class II malocclusion.
The purpose of this study was to evaluate Class II relationships before (T1) and after (T2) initial treatment with rapid palatal expansion to determine the response of the mandible, based on articulator mountings; this might help to explain possible improvement in the Class II relationship.
Material and methods
This was a retrospective study analyzing T1 and T2 study models mounted in both CO and MI of young orthodontic patients who had their maxilla expanded nonsurgically with a hyrax tooth-borne expander as part of their orthodontic treatment.
The sample search was made by the orthodontist (P.B.) who provided the records for this study. Selection was based on the following criteria: (1) hyrax expansion of the maxilla at the initial stage of comprehensive orthodontic treatment and (2) adolescents with no posterior crossbite and at least 2 mm toward a Class II molar relationship on at least 1 side.
There were 13 patients in the sample, with ages from 8 years 3 months to 14 years 11 months at T1; the mean age was 10 years 3 months (4 subjects were 8 years of age, 2 were 9 years, 1 was 10 years, 4 were 11 years, 1 was 12 years, and 1 was 14 years). The Roth technique was used by the orthodontist in obtaining her mounted models. The Roth technique uses an anterior stop, fabricated in wax in the CO position, and then the hardened anterior stop is used along with a softened posterior piece of wax to seat the condyles by using the patient’s own musculature.
The records included models mounted in the CO position on an articulator (Panadent, Grand Terrace, Calif). A wax bite in MI was also taken, so the mandibular model could be mounted against the maxillary model in MI. The first set of models was taken at T1, and the second set was taken at T2, including approximately 6 months of stabilization.
The average time between the date the expander was placed and its removal was 6 months (range, 4-8 months). Seven of the 13 patients also had brackets on the 4 maxillary incisors. One patient had a Haas expander (acrylic tissue borne) instead of a hyrax, 1 had a mandibular lip bumper, and 1 had a mandibular lingual holding arch. Also, 1 patient did not have maxillary central incisors erupted at T1, so overjet and overbite could not be measured. None of these factors had an impact on the main objectives of this study.
The following measurements (in millimeters) were taken from the models mounted in CO and MI at T1 and T2. The molar relationship was measured as the horizontal distance between the mesiobuccal cusp of the maxillary first molar to the buccal groove on the mandibular first molar on both sides. A positive value indicated a Class II direction, and, since all subjects began with a Class II malocclusion, all values were positive. Overjet was measured as the horizontal distance between the maxillary and mandibular anterior teeth. Overbite was measured as the vertical distance between the maxillary and mandibular anterior teeth. Midlines were measured as the distances between the maxillary and mandibular midlines. A negative number indicated that the mandibular midline was to the left of the maxillary line, and a positive number indicated that the mandibular midline was to the right of the maxillary line. Intermolar distance was measured between the lingual groove gingival margins on the maxillary left first molar and the maxillary right first molar.
The model measurements showed whether there was a Class II correction. To determine whether the patient was functioning forward to correct the Class II malocclusion, CO and MI bites were taken. Condyle position indicator (CPI) recordings of CO and MI were made with the articulator on adhesive grid paper fixed to the sliding center platform of the CPI. The device measures the 3-dimensional changes of the articulator condyles between CO and MI. The horizontal and vertical changes in the sagittal plane at the articulator condyle correspond to the x-axis (anteroposterior displacement) and z-axis (superoinferior displacement) changes, and the transverse change corresponded to the y-axis (lateral displacement) change.
If MI changes toward a Class I relationship relative to CO, it will show that the mandibular teeth are relatively forward with a functional shift of the mandible, rather than a true correction, supporting the hypothesis. If MI relative to CO remains unchanged, and there is no change in the Class II relationship, it will show that rapid palatal expansion does not correct the Class II malocclusion. If the Class II relationship improves and there is no change in CO to MI, it means either that the patient grew or the condyles and temporomandibular joint remodeled.
All CO vs MI model and CPI measurements at T1 were taken twice (for a reproducibility study), averaged, and reported by using means and standard deviations. The averaged number was then compared with the single measurements taken at T2.
Paired samples t tests were used on the models, and CPI measurements were taken twice at T1 to ensure reproducibility. They were also used to compare the T1 and T2 measurements.
Results
Reproducibility of the model and CPI measurements was evaluated by statistically analyzing the pretreatment measurements, repeated twice for all 13 subjects. T1-1 means the first T1 measurements, and T1-2 means the second T1 measurements taken to ensure reproducibility. All model measurements were reproducible, with the exception of maxillary and mandibular intermolar widths. There was no significant difference between the 2 measurements taken on the same CPI indexes at different times, indicating that the CPI measurements are reproducible.
Model measurements were taken of each subject. T1 is the average of the reproducibility measurement of T1-1 and T1-2, and T2 is the postexpansion measurement. The statistics for the model measurements at T1 and T2 are given in Table I . Table II shows the changes for model measurements between T1 and T2. The mean changes in MI toward Class II correction were 0.2 mm on the right and 0.19 mm on the left (SD, 1.36 on the right and 0.85 on the left). There was therefore great variability in Class II correction. Only maxillary intermolar width showed statistically significant changes from T1 to T2; this was expected because of the use of a palatal expander.
| Variable | Mean | SD | Low value | High value |
|---|---|---|---|---|
| MI right T1 | 3.200 | 1.655 | 2.5 | 7.8 |
| MI right T2 | 3.000 | 1.716 | 1.5 | 8.0 |
| MI left T1 | 3.529 | 1.803 | 0.0 | 6.3 |
| MI left T2 | 3.335 | 1.570 | 0.0 | 5.6 |
| CO right T1 | 4.692 | 1.327 | 0.0 | 6.6 |
| CO right T2 | 4.192 | 1.846 | 1.0 | 6.5 |
| CO left T1 | 5.235 | 1.388 | 2.2 | 8.5 |
| CO left T2 | 4.788 | 1.649 | 2.0 | 8.5 |
| Overjet T1 | 5.792 | 2.540 | 2.0 | 10.0 |
| Overjet T2 | 4.583 | 2.875 | 2.0 | 10.0 |
| Overbite T1 | 3.635 | 1.910 | 0.0 | 6.0 |
| Overbite T2 | 3.750 | 1.645 | 1.0 | 6.0 |
| Maxillary intermolar width T1 | 31.981 | 3.177 | 26.0 | 38.3 |
| Maxillary intermolar width T2 | 37.785 | 3.411 | 33.4 | 46.4 |
| Mandibular intermolar width T1 | 30.889 | 2.756 | 26.9 | 35.3 |
| Mandibular intermolar width T2 | 31.485 | 2.835 | 26.2 | 36.0 |
| Midlines MI T1 | 0.479 | 1.740 | –3.0 | 3.0 |
| Midlines MI T2 | 0.167 | 0.718 | –1.0 | 2.0 |
| Midlines CO T1 | 0.167 | 1.212 | –2.0 | 3.0 |
| Midlines CO T2 | 0.130 | 0.678 | 0.1 | 2.0 |
| Variable | Mean | SD | t | P value |
|---|---|---|---|---|
| MI right | 0.200 | 1.357 | 0.531 | 0.605 |
| MI left | 0.194 | 0.847 | 0.827 | 0.424 |
| CO right | 0.500 | 1.327 | 1.359 | 0.199 |
| CO left | 0.446 | 0.995 | 1.618 | 0.132 |
| Overjet | 1.208 | 2.019 | 2.074 | 0.062 |
| Overbite | –0.115 | 1.263 | –0.314 | 0.759 |
| Maxillary intermolar width | –5.804 | 1.238 | –16.910 | 0.000 ∗ |
| Mandibular intermolar width | –0.596 | 1.243 | –1.729 | 0.109 |
| Midlines MI | 0.313 | 1.411 | 0.767 | 0.459 |
| Midlines CO | 0.042 | 0.656 | 0.220 | 0.830 |
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