Introduction
The objectives of this study were to evaluate the effects of bicortical engagement by microimplants with maxillary skeletal expanders on pterygopalatine sutures opening and to analyze the postexpansion skeletal changes associated with it.
Methods
Eighteen subjects treated with maxillary skeletal expanders were examined for pterygopalatine suture openings. Eight subjects who showed no evidence of the suture opening were assigned to the nonsplit group (NG), whereas 10 subjects with opened sutures were assigned to the split group (SG). Preexpansion and postexpansion cone-beam computed tomography images were superimposed for each group, and the changes in the 2 groups were compared. Finally, cone-beam computed tomography volumes were reoriented along the axis of each microimplant to check the bicortical engagement of the 4 microimplants.
Results
There was a significant correlation between the bicortical engagement of the orthodontic microimplants and the pterygopalatine suture opening ( P = 0.0003). In the NG, the average amount of transverse expansion measured at the center of resistance of the maxillary first molars, anterior nasal spine, and posterior nasal spine (PNS) was 4.33 mm, 2.22 mm, and 1.58 mm, respectively, whereas the transverse expansion in the SG was 5.29 mm, 2.21 mm, and 2.46 mm, respectively. The magnitude of transverse expansion at PNS was significantly higher in the SG than in the NG ( P = 0.036). The PNS also showed a significant anterior displacement in the SG (0.89 mm) compared with the NG (0.06 mm) ( P = 0.033).
Conclusions
Bicortical microimplant anchorage is essential for pterygopalatine suture opening in microimplant-assisted maxillary skeletal expansion, which may result in further skeletal expansion and forward movement in the posterior part of the palatomaxillary complex.
Highlights
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Midpalatal expansion increased after the resistance of the pterygopalatine suture was overcome.
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Posterior nasal spine showed significant anterior displacement after the suture opening.
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Bicortical microimplant anchorage correlated with pterygopalatine suture opening.
Since the advent of the Hass-type of rapid palatal expander, various tooth-borne appliances have been introduced in an attempt to correct maxillary constriction. However, their tooth-borne design has led to many detrimental effects such as dentoalveolar tipping, root resorption, buccal bone dehiscence, gingival recession, reduced skeletal effects of the expansion, and loss of long-term stability. Furthermore, the use of these appliances is limited to prepubertal patients because the midpalatal suture does not have heavy interdigitation in younger patients. For postpubertal patients, surgically assisted rapid palatal expansion is recommended because of the greater amount of interdigitation of the suture. , Microimplant-assisted rapid palatal expanders were suggested as an alternative to surgically assisted rapid palatal expansion, and a successful expansion was reported in a young adult. Maxillary skeletal expanders (MSEs) are a specific type of microimplant-assisted rapid palatal expander that features 4 microimplants located more posteriorly to maximize the likelihood of bicortical engagement into the palatal bone and nasal floor. This design enables the orthopedic force to be delivered more directly to high-resistance areas such as the pterygopalatine suture and zygomatic buttress. ,
Although the midpalatal suture is the primary target to be separated for maxillary transverse expansion, higher resistance against the opening of this suture is found in the structures surrounding the maxillae, such as the sphenoid bone (posterior to the maxilla) or zygomatic bone (lateral to the maxilla). The pterygopalatine suture is reportedly a major source of resistance during palatal expansion, which could not be disarticulated with tooth-borne palatal expanders because of excessive interdigitation. Expansion by tooth-borne appliances generates a V-shaped expansion of the palate with a larger opening at the anterior nasal spine (ANS) and gradually less split toward the posterior nasal spine (PNS) as a result of the rigid resistance from the pterygopalatine suture. , , The amount of split at PNS (1.15 mm) was reported to be 40% of that at ANS (3.01 mm) with tooth-borne expanders.
The pterygoid notch is an anatomic space on the inferior portion of the pterygoid processes between the medial and lateral plates, in which the pyramidal process of the palatine bone is fitted. Under normal circumstances, there is no visible space between the plates in the presence of the pyramidal process. However, with the expansion force from MSE, the pyramidal process can be pulled away from the pterygoid plates once the rigid pterygopalatine suture that connects them is split. These changes can generate a space between the plates that is detectable on cone-beam computed tomography (CBCT) images. In a previous study, MSE was shown to disarticulate the pterygopalatine suture in almost half of the patients (53%), thus exhibiting more parallel expansion patterns, with the opening at PNS (4.33 mm) being 90% of that at ANS (4.75 mm). However, no study has investigated the factors affecting the suture opening.
The results of palatal expansion have long been evaluated with study models and 2-dimensional radiography such as lateral and posteroanterior cephalograms, , but the previous studies had fundamental limitations because of their 2-dimensional perspective. The development of 3-dimensional (3D) radiographs such as CBCT images enabled researchers and clinicians to study living subjects more reliably. , Because of the progressive improvements in CBCT resolution and the introduction of multiplanar 3D reconstruction software, it is now possible to accurately measure and analyze the 3D nature of craniofacial bones and sutures. ,
The specific aims of this study were to evaluate the effects of bicortical engagement of microimplants on the pterygopalatine suture opening and to compare skeletal changes of the split group (SG) with those of the nonsplit group (NG) at the key landmarks around the midpalatal suture using a 3D reconstruction software.
Material and methods
This retrospective study was approved by the Institutional Review Board of Wonkwang University Dental Hospital (approval no. WKUDHIRB201907-01). A total of 18 patients aged 9-27 years (mean age, 19.8 ± 4.8 years; 6 males, 12 females) treated in Wonkwang University Dental Hospital were included in the study. CBCT images were reoriented and examined using OnDemand 3D software (Cybermed, Seoul, South Korea) to verify whether or not the patient showed evidence of splits in the lower part of the pterygopalatine suture, as suggested in a recent study. Eight of the 18 subjects showed no evidence of suture opening and were assigned to the NG, whereas 10 subjects showed evidence of disarticulation of the suture in either right, left, or both sides and were assigned to the SG. The suture was considered open when the crescent continuity of the pterygoid plates was lost in the area adjacent to the pyramidal process, and the distance from the most lateral point of the medial pterygoid plate to the most medial end of the lateral plates was measurable ( Fig 1 ).
The inclusion criteria were as follows: (1) patients who presented with either posterior crossbite or airway problems because of maxillary constriction, (2) use of MSE as the first step of treatment, (3) successful opening of the midpalatal suture, and (4) having CBCT images taken before (T0) and within a month after expansion (T1). The exclusion criteria were as follows: (1) history of trauma in the craniofacial area, (2) patients with the craniofacial syndrome, (3) those having had previous orthognathic surgery or (4) previous orthodontic and orthopedic treatment, (5) concomitant facemask therapy, and (6) >6 mm of difference between maxillary and mandibular intermolar width.
MSE II appliances with four 1.5 × 11.0-mm microimplants (MSE; BioMaterials Korea, Seoul, South Korea) were used in this study ( Fig 2 ). The activation rate was 1 turn per day for early teens (aged <15 years), 2 turns per day for late teens (aged ≥15 years), and 4 turns per day for adult patients (aged >20 years) before the midpalatal suture opening. Once a diastema was observed, the activation rate was set at 2 turns per day regardless of age and was maintained until proper expansion was achieved.
Before taking CBCT images, all patients were instructed to be seated in an upright position such that the imaginary Frankfort horizontal plane of the patients was parallel to the floor. Their heads were fixed with a chincup and ear rods to secure their position during the CBCT scan. The CBCT scanner (Alphard-3030; ASAHI Roentgen IND, Kyoto, Japan) was set to 80 kVp and 7.0 mA for adults and 80 kVp and 3.0 mA for adolescents and children. Scan time was 17 seconds with a voxel size of 0.39 mm and a field of view of 19.97 cm using the cranial mode.
Landmarks around the midpalatal suture were traced on the pretreatment and posttreatment CBCT images of each subject using OnDemand 3D software. Three-dimensional coordinates (x, y, and z) were automatically given to each traced landmark ( Fig 3 ). The landmarks used for measurement were as follows: ANS, PNS, and center of resistance of the maxillary first molar (CoRM6). Landmarks such as nasion (N), sella (S), porion (Po), orbitale (Or), and basion (Ba) were used as reference points for the exact orientation of the CBCT images ( Table I ). The T1 scan was then superimposed on the T0 scan using stable anatomic structures and anterior cranial base as a reference for adult patients and anterior cranial fossae for growing patients. They were then precisely superimposed again on the basis of the voxel gray scale pattern. The superimposition of the CBCT images is a semi-automatic process that has been validated for accuracy in a recent study. After superimposition, the coordinates of N and S in preexpansion and postexpansion images were checked to see whether they were within the 0.05 mm error limit. The changes in the 3D coordinates of all landmarks at T0 and T1 were measured. Finally, CBCT volumes were reoriented along the axis of each microimplant to check the bicortical engagement of the 4 microimplants ( Fig 4 ). During the entire process, any patient information that might direct the group orientation was concealed to avoid bias.
| Landmarks | Definitions |
|---|---|
| ANS | A pointed projection at the anterior extremity of the intermaxillary suture |
| ANSR | The right half of ANS after midpalatal suture is separated |
| ANSL | The left half of ANS after midpalatal suture is separated |
| PNS | Medial end of the posterior border of the horizontal plate of palatine bone |
| PNSR | The right half of PNS after midpalatal suture is separated |
| PNSL | The left half of PNS after midpalatal suture is separated |
| CoRM6 | The center of resistance of the maxillary first molar; the most superior and buccal point of the furcation area of the mesiobuccal and distobuccal roots of the maxillary first molar |
| Nasion | The most medial and superior point of the frontonasal suture |
| Sella | The center of hypophyseal fossa |
| Porion | The most superior point of external the auditory meatus |
| Orbitale | The lowest point on the inferior orbital margin |
| Basion | The anterior margin of the foramen magnum |
Statistical analysis
In a preliminary study, the sample size was determined on the basis of the difference in the amount of transverse expansion and anterior movement at PNS between NG and SG randomly selected from 10 patients (5 NG and 5 SG; mean difference, 0.89; standard deviation, 0.58 mm). The measurements at PNS were adopted because of its proximity to the pterygopalatine suture, which might be closely related to the suture opening. At least 8 subjects were required per group to provide a power of 0.80 with a 2-tailed α value of 0.05. The sample size was also considered enough to detect the correlation between the bicortical engagement of the microimplant and the pterygopalatine suture opening because the number of microimplants is 4 times more than the number of the patients included in this study. Finally, 18 subjects were selected because of the possibility that the final groups (NG and SG) would have an unequal sample size, which might decrease the power level. For the entire computation, G∗Power was used (version 3.1.9.2; Franz Faul, Christan-Albrechts Universitat, Kirel, Germany).
The Shapiro-Wilk test was used to check the normal distribution of the samples. To compare the mean difference between the 2 groups, either the independent sample t test or the Mann-Whitney U Test was used on the basis of the normality of the samples. Fisher exact test and binary logistic regression analysis were used to evaluate the correlation between the bicortical engagement of the microimplants and suture opening. The landmarks for measurement (ANS, PNS, and CoRM6) were traced twice by the same rater at different times to evaluate the intraclass correlation coefficient. The intraclass correlation coefficient ranged from 0.842 to 0.928, which was considered very reliable. Patients were assigned to either of the 2 groups on the basis of the evidence of their pterygopalatine suture split. The evaluation of the suture split was performed again by another rater to check interclass reliability with Cohen’s kappa. Cohen’s kappa value was 1.00, which indicated a perfect match between the raters. SPSS software was used for the statistical analyses (version 26.0; IBM Corp, Armonk, NY).
Results
In the NG, the average amount of transverse expansion measured at the CoRM6, ANS, and PNS was 4.33 mm, 2.22 mm, and 1.58 mm, respectively, whereas the transverse expansion in the SG was 5.29 mm, 2.21 mm, and 2.46 mm, respectively. A significant difference was found in the amount of transverse expansion at PNS between the 2 groups ( P = 0.036), whereas it was insignificant at CoRM6 and ANS ( P >0.05) ( Table II ). The amount of expansion at PNS was 71% of that at ANS in the NG, whereas the expansion at PNS was 111% of that at ANS in the SG ( Fig 5 ). PNS also showed significant anterior displacement in the SG (0.89 ± 0.49 mm) compared with the NG (0.06 ± 0.64 mm) ( P = 0.033) ( Table III ).
