Rapid palatal expansion (RPE) appliances have routinely been used to remedy transverse deficiencies in the maxilla since their conception in the 1900s. The effects of RPE on the dentoskeletal complex have been studied with dental casts and lateral and posteroanterior cephalograms. Recent advances in cone-beam computerized tomography (CBCT) 3-dimensional imaging enable the capture and reproduction of a real maxillary section in all 3 planes and hence allow measurement of axial inclinations of the dentition, and changes in the transverse dimensions and the magnitude of displacement of the maxillary halves free from distortion, magnification, and superimposition.

RPE treatment contributes to the increase in the maxillary arch by a combination of orthodontic (tipping and translation) and orthopedic (bony separation and remodeling at the suture) effects. Prior investigators noted transverse increases of 3.5 to 9.5 mm, with averages of 6.04 mm from posteroanterior cephalograms and 6.74 mm from casts. The resultant dental tipping ranged from 0° to 24° for both sides, with an average of 3.1° for 1 side.

The objective of this study was to compare the dimensional changes of skeletal and dental structures in a group of growing patients treated for maxillary constriction before and after RPE, with a matched control group, using CBCT 3-dimensional imaging. The study quantified the increase in the transverse dimensions of the maxilla’s postpalatal expansion therapy at various levels. We attempted to show the quantity and direction of alveolar and dental responses to the therapy and compare our findings with those reported in the literature.

Material and methods

Clinical treatment was rendered at the Advanced Orthodontic Clinic at the University of Southern California in Los Angeles. Twenty-five healthy patients (18 girls; mean age, 12.6 years, range, 8.8-15 years; and 7 boys; mean age, 13.2 years; range, 9.1- 15.8 years) who required RPE for unilateral or bilateral crossbite and orthodontic treatment and had CBCT imaging were selected for the study. Twenty-five controls (with no posterior crossbite), selected for similar age and sex (18 girls; mean age, 12.7 years; range, 8.6-14.7 years; and 7 boys; mean age, 13.2 years; range, 9.2-15.7 years), had orthodontic treatment only (no RPE) and also initial and midtreatment CBCT scans ( Table I ).

Table I

Descriptive statistics for the groups

	RPE group	Control group
Age (y)	12.8 ± 1.84	12.8 ± 1.83
Sex	18 male, 7 female	18 male, 7 female

 

All scans were retrieved from the archives of the department’s NewTom 3G Volume Scanner CBCT unit (QR, Verona, Italy). Clinical judgment and the patients’ age were the key factors in determining that all patients were still growing, and that the treatment preceded the fusion of their midpalatal sutures. All patients were in the late transitional or early permanent dentition stage. They had no craniofacial abnormalities, or previous surgical or extraction treatment.

Patients treated with the RPE had a hyrax palatal expander banded on the maxillary first premolars and first molars. They were monitored weekly for appropriate activation of the appliance. They were advised to activate their appliance 1 or 2 times per day until the required expansion—ie, slight overcorrection of the crossbite defined as the palatal cusp of the maxillary first molar occluded with the buccal cusp of the mandibular first molar—was achieved (average time, 4-6 weeks). After this process, they were stabilized. The RPE (or a transpalatal arch) was used for retention for at least 3 months after expansion. Most patients with RPE had no orthodontic treatment until after the fixed retention period, but several had some appliances placed (such as a 2 × 4) during the fixed retention period. The control patients started orthodontic treatment at approximately the same time as the RPE group started expansion therapy. The scans of all patients were taken as part of beginning and progress records at the midpoint in treatment, which ranged from 11.5 to 24.5 months (average, 17.9 months) for the RPE patients and 11.0 to 30.6 months for the controls (average, 17.1 months). To test for differences in the time elapsed between scans, we conducted a paired t test and found no statistically significant difference between matched pairs of patients ( P = 0.441). The patients were asked to put their head in Frankfort horizontal position for the scans.

The scans were imported, and cross-sectional slices were made with InVivo dental software (Anatomage, San Jose, Calif). The slices were coronal bisections through the buccal grooves and the palatal roots of the maxillary first molars. Transverse measurements ( Fig 1 ) and angular measurements ( Fig 2 ) were recorded for each scan.

Transverse measurements in the coronal view: A, external maxillary widths. NF, Maxillary width parallel to the lower border of the hard palate and tangent to the nasal floor at its most superior level; HP , maxillary width parallel to the lower border of the hard palate and tangent to the hard palate; BAC , maxillary width at the level of the buccal alveolar crest; DA.E′ , dental arch external width measured at the most prominent area of the buccal aspect of the posterior teeth; and DA.E , dental arch external width measured at the level of the buccal cusp tips. B, Internal maxillary widths. PA , Width between the tooth apices measured on the palatal root of the posterior teeth; LAC , maxillary width between the lingual alveolar crests; DA.I′ , dental arch internal width measured at the most prominent area of the lingual aspect of the posterior teeth; and DA.I , dental arch internal width measured at the level of the palatal cusp tips.

Angular measurements in the coronal view: A, Alv, the angle between the alveolar bone and a horizontal reference line parallel to the palatal plane; B, Dent, the angle between the most lingual aspect of the palatal root and a horizontal reference line parallel to the hard palate; C, Incl, the angle between the long axis of the palatal root to a horizontal reference line parallel to the patient’s hard palate. The red line represents the horizontal defined as parallel to the palatal plane.

Identification of the dentoskeletal landmarks and subsequent measurements were manually performed by 1 investigator for both groups (A.K.). Twenty patients (10 RPE treated, 10 controls) were randomly selected, and their images were resliced and remeasured by the same examiner after a 60-day interval.