Introduction
Rapid maxillary expansion (RME) is a common orthodontic treatment to correct maxillary transverse deficiency; however, the inability to determine the precise timing of fusion of the midpalatal suture creates difficulty for clinicians to prescribe the appropriate treatment, surgical or nonsurgical expansion. The purpose of this study was to assess the predictive power of the midpalatal suture density ratio (MPSD) for a skeletal response to RME.
Methods
Pre- and posttreatment cone-beam computed tomography scans were obtained from 78 orthodontic patients aged from 8 to 18 years treated with RME. MPSDs were calculated from pretreatment scans, and a prediction was made for the amount of skeletal expansion obtained at the level of the palate after comprehensive orthodontic treatment. Predicted values were compared with actual outcomes as assessed from posttreatment scans, followed by regression analyses to investigate correlations between MPSD and skeletal expansion and equivalence testing to analyze the performance of the predicted measurements.
Results
The MPSDs were not statistically significantly ( P >0.05) correlated with the amount of skeletal expansion achieved. In addition, the predicted skeletal expansion using MPSD was not statistically equivalent to the skeletal expansion achieved using an equivalence margin of ±0.05.
Conclusions
The results suggest that the MPSD obtained from pretreatment cone-beam computed tomography scans were not correlated well enough with the amount of skeletal expansion achieved to be an effective predictor of the amount of long-term skeletal expansion after RME.
Highlights
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Pretreatment scans of patients who had rapid maxillary expansion (RME) were assessed.
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Midpalatal suture density (MPSD) ratios were calculated from the scans.
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The predictive power of the MPSD ratio for a skeletal response to RME was assessed.
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MPSD ratios were not correlated with the amount of skeletal expansion.
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MPSD was not an effective predictor of long-term skeletal expansion after RME.
Rapid maxillary expansion (RME) is a common orthodontic treatment that has been used since the 1860s to correct transverse maxillary deficiencies. RME has historically been used for posterior crossbite correction by widening the narrowed maxillary skeletal base and gaining arch perimeter to alleviate dental crowding. Although there are various designs, the appliance is typically fabricated to attach to the dentition, and the heavy forces generated by the expander transmit through the teeth to the halves of the maxilla. In patients with patent skeletal sutures, the forces will open the midpalatal suture and hold the hemimaxillae apart to encourage callus formation and subsequent bone deposition resulting in skeletal expansion. Because a tooth-borne appliance is used, there are also dental responses to this force application, manifesting in dental tipping and bending of the alveolus described as dentoalveolar expansion. It has been reported that 39%-49% of the total expansion is attributed to dental tipping and 6%-13% to alveolar bending. Typically, it is desirable to maximize the skeletal effects and minimize the dental effects, as the dental tipping may lead to loss of alveolar bone and periodontal attachment level, , root resorption, and fenestrations of the buccal cortex.
As a patient matures, the sutures progressively close by bony interdigitation. In the midpalatal suture, the process starts with the formation of bone spicules in many places along the suture, with the number of spicules increasing with maturation. Subsequently, interdigitation increases, and fusion may occur by ossification. The ossification progresses from posterior to anterior and reduces the ability to force open the suture during conventional RME. , Patients with fused palatal sutures typically require surgically-assisted RME to obtain an increase in skeletal width, which increases costs and risks to the patient.
To aid the clinical decision whether maxillary expansion should be attempted with conventional RME or whether surgical assistance would be necessary, researchers have proposed several indicators of midpalatal suture maturation. Chronological age, which has historically been used, has been put into question because of the wide variation in the timing of sutural maturation. For instance, the fusion of the midpalatal suture has been observed in subjects aged from 15 to 19 years, whereas patients aged up to 71 years have been reported to have no signs of fusion of this suture. Other proposed indicators of sutural maturation include skeletal maturity indicators on hand-wrist radiographs, , cervical vertebral maturation, , , and qualitative assessment of the midpalatal suture on occlusal radiographs and, more recently, cone-beam computed tomography (CBCT) scans. , However, each of these proposed indicators have limitations with regard to their predictive abilities as to whether conventional RME would be successful. ,
Currently, the only way to be certain whether conventional RME could be performed on a patient outside of the growth phase is by trial and error, which results in negative side effects when the treatment is unsuccessful. A reliable way to more closely predict a patient’s skeletal and dentoalveolar response to RME before initiating treatment has the potential of increasing treatment success by allowing clinicians to provide a more accurate prognosis for success in RME candidates who are approaching the end of their growth period. An adolescent patient with early closure of the suture could be identified to avoid the potential negative side effects of attempting conventional RME. Conversely, a young adult with late closure of the suture may be able to avoid surgically-assisted RME in favor of conventional RME, which would prevent the cost and risks of the surgical procedure.
Recently, a novel predictor of skeletal response to RME has been developed using CBCT: the midpalatal suture density ratio (MPSD). This measurement is a comparison of the gray density value of the midpalatal suture to the gray density of the lateral hard palate, expressed as a ratio. This allows for a quantitative measure of calcification of the suture to serve as a proxy for bony interdigitation, reducing the error involved with traditional visual qualitative assessments. In the early stages of maturation of the midpalatal suture, the sutural gap between the halves of the maxilla largely consists of uncalcified connective tissue because there is no bony interdigitation. , This gap appears radiolucent on the CBCT scan with a density similar to tissue of the soft palate, so the MPSD would be close to 0. As the suture continues to mature, there is increased bony interdigitation, and the MPSD increases with the amount of calcified osseous tissue within the suture. Ultimately, the suture matures to a degree in which the amount of calcified tissue approaches that of cortical bone, which would result in an MPSD ratio close to 1. In a recent study, there was a statistically significant negative correlation between the pretreatment MPSD and the skeletal width increase measured at the greater palatine foramina. This promises the potential for clinical use of this measurement to aid everyday clinical decision-making. The discovery of this negative correlation elucidates the trend of skeletal response; however, it warrants further investigation as to whether the measurement can accurately predict skeletal expansion within a clinically relevant range.
The objective of this study was to evaluate the reliability of the MPSD calculation as a predictive measure of the skeletal response to RME. It was hypothesized that there is a statistically significant negative correlation between the MPSD and skeletal response to maxillary expansion at the level of the greater palatine foramen. It was further hypothesized that there is a high degree of equivalence between the predicted skeletal response and actual skeletal response as measured on posttreatment CBCT scans.
Material and methods
This retrospective cohort study was approved by the Institutional Review Board at the University of Minnesota (study no. 00003544). The pre- and posttreatment CBCT scans of 78 patients with maxillary transverse deficiency treated with RME using a hyrax appliance as part of orthodontic treatment were used. Patients who had incomplete treatment records, previous orthodontic treatment, congenital malformations including cleft lip and palate, inappropriate diagnostic quality of CBCT scans, or history of periodontal disease were excluded from the study. The CBCT images were from patients aged from 8 to 18 years to include a population with varying degrees of sutural maturation.
The descriptive information was recorded from each patient’s treatment record, including age, sex, hyrax expander design, amount of prescribed expansion (number of turns of the expander key translated to the amount of expander activation expressed in millimeters), expander retention time after cessation of activation (in weeks), and total treatment time including fixed appliance therapy (in months). The demographic information and separation of the study population into 3 age groups are shown in Table I .
| Variable | Subgroup | Occurrence | Percentage |
|---|---|---|---|
| Sex | Male | 36 | 46.2 |
| Female | 42 | 53.8 | |
| Age, y | 8-11 | 20 | 25.6 |
| 12-13 | 39 | 50.0 | |
| 14-18 | 19 | 24.4 |
The patients were treated with either a 2-banded hyrax expander (n = 38) or a 4-banded hyrax expander (n = 40). The choice of expander design was at the discretion of the treating clinician. The 2-banded expander had bands on only the permanent maxillary first molars with soldered palatal arms extending to the mesial portion of the palatal surface of the first premolar. The 4-banded expander had bands on both the first molars and the first premolars with a soldered connection between the palatal surfaces of the bands ( Fig 1 ). The expander types were considered equivalent as both expander types were designed to disperse force over the first premolars and permanent first molars. In patients who had RME in the mixed dentition, the force was dispersed over the primary molars and the first permanent molars. The protocol for the active expansion period included turn of the expansion screw once daily until the appropriate correction was achieved. After the completion of the active expansion, the appliance was left in place passively. This postexpansion retention time averaged 17.5 ± 14.0 weeks, after which the appliance was removed. Descriptive statistics for the sample are shown in Table II . Treatment of all patients started with RME, and expansion was completed within the first 2 months of treatment. Therefore, the total treatment time also provides some information about the average period between the end of the expansion and the posttreatment CBCT scan.
| Variable | Mean ± SD | Median | Range |
|---|---|---|---|
| Age at T 1 , y | 13.01 ± 1.66 | 12.92 | 8.74-17.82 |
| Amount of RME activation, mm | 8.09 ± 2.40 | 8.00 | 3.25-16.00 |
| Expander retention time, wk | 17.49 ± 13.97 | 14.00 | 1.00-77.00 |
| Total treatment time, mo | 29.16 ± 7.32 | 28.70 | 11.52-48.84 |
All CBCT scans used in the study had been taken on an i-CAT Next Generation CBCT scanner (Imaging Sciences International, Hatfield, Pa). Both pretreatment (T1) and postcomprehensive treatment (T2) scans were taken at these settings: 18.54 mA, 120 kV, exposure time of 8.9 seconds, voxel size of 0.3 mm, and scanning area of 17 × 23 cm. This procedure resulted in a slice thickness of 0.3 mm for all scans.
To quantify the effects of RME, we made linear measurements between bilateral structures on slices from the T1 and T2 images as detailed below. All measurements were performed by a single operator (S.T.) on digital imaging and communications in medicine images using Invivo imaging software (version 6.0; Anatomage Dental, San Jose, Calif). To blind the operator to both patient and time point, we assigned each image an arbitrary numerical identifier using a random number generator. The digital imaging and communications in medicine image was oriented to bisect the palatal plane in 3 dimensions: first oriented from an axial slice through the hard palate and centered through the midpalatal suture, then through the center of the hard palate from a sagittal slice parallel to the palatal plane, and finally through the center of the hard palate from a coronal slice parallel to the palatal plane ( Fig 2 ).
Linear measurements were made to the nearest 0.1 mm and comprised 1 skeletal measurement and 1 dental measurement. The skeletal measurement was the distance between the lateral margins of the greater palatine foramina (GPFd) on axial slices through the center of the hard palate ( Fig 3 ). This measurement was used to quantify the skeletal expansion at the level of the hard palate. The GPFd previously showed a strong correlation with the MPSD. Because the greater palatine foramina as landmarks for the skeletal measurements are located lingual or apical to the dentition, they remained unaffected by treatment with preadjusted edgewise appliances. To account for the variable amount of expansion performed among patients, we presented each distance as a proportion of the prescribed expansion by dividing the difference in distances between T1 and T2 by the amount of prescribed activation. The difference in distance between the T2 and T1 at the level of the greater palatine foramen divided by the prescribed expansion was termed the greater palatine foramina proportion (GPFp).
Greater palatine foramina proportion(GPFp)=T2GPFd−T1GPFd(mm)Prescribed expander activation(mm)
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