Changes in the midpalatal and pterygopalatine sutures induced by micro-implant-supported skeletal expander, analyzed with a novel 3D method based on CBCT imaging

Abstract

Background: Mini-implant-assisted rapid palatal expansion (MARPE) appliances have been developed with the aim to enhance the orthopedic effect induced by rapid maxillary expansion (RME). Maxillary Skeletal Expander (MSE) is a particular type of MARPE appliance characterized by the presence of four mini-implants positioned in the posterior part of the palate with bi-cortical engagement. The aim of the present study is to evaluate the MSE effects on the midpalatal and pterygopalatine sutures in late adolescents, using high-resolution CBCT. Specific aims are to define the magnitude and sagittal parallelism of midpalatal suture opening, to measure the extent of transverse asymmetry of split, and to illustrate the possibility of splitting the pterygopalatine suture.

Methods: Fifteen subjects (mean age of 17.2 years; range, 13.9-26.2 years) were treated with MSE. Pre- and post-treatment CBCT exams were taken and superimposed. A novel methodology based on three new reference planes was utilized to analyze the sutural changes. Parameters were compared from pre- to post-treatment and between genders non-parametrically using the Wilcoxon sign rank test. For the frequency of openings in the lower part of the pterygopalatine suture, the Fisher's exact test was used.

Results: Regarding the magnitude of midpalatal suture opening, the split at anterior nasal spine (ANS) and at posterior nasal spine (PNS) was 4.8 and 4.3 mm, respectively. The amount of split at PNS was 90% of that at ANS, showing that the opening of the midpalatal suture was almost perfectly parallel antero-posteriorly. On average, one half of the anterior nasal spine (ANS) moved more than the contralateral one by 1.1 mm. Openings between the lateral and medial plates of the pterygoid process were detectable in 53% of the sutures (P < 0.05). No significant differences were found in the magnitude and frequency of suture opening between males and females. Correlation between age and suture opening was negligible (R 2 range, 0.3-4.2%).

Conclusions: Midpalatal suture was successfully split by MSE in late adolescents, and the opening was almost perfectly parallel in a sagittal direction. Regarding the extent of transverse asymmetry of the split, on average one half of ANS moved more than the contralateral one by 1.1 mm. Pterygopalatine suture was split in its lower region by MSE, as the pyramidal process was pulled out from the pterygoid process. Patient gender and age had a negligible influence on suture opening for the age group considered in the study.

Keywords: Cone beam computed tomography (CBCT); Midpalatal suture; Palatal expansion; Pterygopalatine suture.

Conflict of interest statement

Authors’ information

The micro-implant-supported skeletal expander used in the present study has been developed and used since 2003. Nowadays, it is widely used at UCLA Orthodontic Clinic where the study was performed.

Ethics approval and consent to participate

The present retrospective study received approval from the Institutional Review Board at University of California, Los Angeles (UCLA), IRB number 16-001662.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

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Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Fig. 1

Measurement of maxillary (a) and mandibular width (b) with a digital caliper on the stone models. The frontal view of maxillary (blue line) and mandibular width (red line) is shown in (c). In this patient, the maxillary width is 55.0 mm, the mandibular width is 59.6 mm, and the maxillary transverse deficiency is 4.6 mm (59.6 − 55.0). The amount of maxillary skeletal expansion required for the patient is equal to the maxillary transverse deficiency (4.6 mm)

Fig. 2

Maxillary Skeletal Expander (MSE). a Appliance positioned in the posterior part of the palate. b Measurement of the distance between the two halves of the expansion jackscrew on the post-expansion CBCT; the opening of the midpalatal suture can also be seen in the figure

Fig. 3

Schematic representation of the three main reference planes: maxillary sagittal plane (MSP), axial palatal plane (APP), and V-coronal plane (VCP). Reference planes are localized in the pre-expansion CBCT and become the reference lines to measure the displacement of skeletal structures in the post-expansion CBCT

Fig. 4

Sections used to localize the MSP and APP. a Axial section. b Sagittal section. c Coronal section

Fig. 5

Axial palatal section (APS), lower nasal section (LNS), and upper nasal section (UNS) utilized to analyze the lateral and sagittal displacement of the maxilla and pterygoid plates and the modifications along the entire length of the pterygopalatine suture. a Superimposition of pre-expansion and post-expansion 3D model on anterior cranial base. b APS, superimposed image. c LNS, superimposed image. d UNS, superimposed image. e Anterior cranial base, superimposed image. It can be noticed how bones of the maxillo-facial complex are displaced in the APS, LNS, and UNS, while no displacement takes place on anterior cranial base. For this particular patient, skeletal changes are larger on the right side of the skull rather than on the left side, in all three axial sections (APS, LNS, UNS). Facial soft tissue modifications can also be detected on the APS, LNS, and UNS. Blue lines, MSP and VCP

Fig. 6

Landmarks identified in the axial palatal section in the post-expansion CBCT. 1 Right anterior nasal spine (Rt ANS), 2 left anterior nasal spine (Lt ANS), 3 right posterior nasal spine (Rt PNS), 4 left posterior nasal spine (Lt PNS), 5 most medial point of the lateral plate of the right pterygoid process (Rt Lat Pter), 6 most lateral point of the medial plate of the right pterygoid process (Rt Med Pter), 7 most lateral point of the medial plate of the left pterygoid process (Lt Med Pter), 8 most medial point of the lateral plate of the left pterygoid process (Lt Lat Pter)

Fig. 7

Measurements on axial palatal section on post-expansion CBCT. Blue line, maxillary sagittal plane. a Patient with a complete disengagement of the pyramidal process from the pterygoid plates. b Patient with a partial disengagement of the pyramidal process from the pterygoid plates in the left pterygopalatine suture; the opening is present between the pyramidal process and the medial pterygoid plate

Fig. 8

Relation between the APS with palatine bone and pterygoid plates. a Posterior view of the left palatine bone. b Anterior view of the left pterygoid process of the sphenoid bone

Fig. 9

Reference lines used in conventional 2D postero-anterior cephalometric analysis: a pre-expansion and b post-expansion; ANS is located on the left side of the vertical reference line. Maxillary sagittal plane (MSP) as novel reference plane: c pre-expansion and d post-expansion; ANS is located in the MSP

Fig. 10

Sagittal parallelism of midpalatal suture opening obtained with MSE. a Borders of the midpalatal suture (yellow lines) moving almost perfectly parallel to each other; amount of split at PNS (4.3 mm) was 90% of that at ANS (4.8 mm). b Position of MSE in the posterior part of the palate, providing an expansion force (red arrows) in line with the zygomatic buttress bones, which represent a major resistance to the movement of maxillary halves

Fig. 11

Schematic representation of transverse asymmetry of split of the midpalatal suture. a Example where the lateral movement of right maxilla (x) is larger than that of left maxilla (y). b Example where the lateral movement of left maxilla (y) is larger than that of right maxilla (x). Blue line, MSP