Three-dimensional evaluation of hybrid expander appliances: A pilot study

Maxillary constriction is a common orthodontic problem and usually accompanied with unilateral or bilateral posterior crossbite.^1,2  Rapid maxillary expansion (RME) has become a useful treatment method for severe transversal maxillary constriction and posterior crossbite^1,3–5  since its introduction by Angell⁶  in 1860 and its increased popularity in the 1900s. Different types of appliances and treatment protocols have been introduced for a constricted maxilla. The most common RME appliances are tooth-borne.^7–9 

To obtain separation of maxillary midpalatal suture interlocks, heavy force is needed. This heavy force produces mechanical stress on teeth and adverse effect on teeth such as buccal tipping, gingival recession, fenestration of buccal cortex, and root resorption of the posterior teeth.^10–12  To avoid such complications, clinicians have designed RME appliances that attach directly to the palatal surfaces of the maxilla. Use of palatal distractor types of these bone-borne RME appliances has not been become widespread.

To minimize the surgical procedure and cost, mini-screw has begun to be used as a boneanchor.^13,14  Popular types of these appliances are used: (1) two mini-screws on rugae as anterior anchorage unit and first maxillary molars as posterior anchorage unit and (2) four mini-screws for total anchorage unit. We designed an alternative to these methods and produced new bone-borne RME appliances used with two mini-screws and acrylic palatal pad (Figure 1). Since this appliance comprises two mini-screws and palatal pad, it could be named a hybrid expander.

Conventional records, such as cephalometric radiographs, panoramic radiographs, and dental cast models provide limited information because of superimposition of the anatomic structures and difficulties in landmark identification.^15,16 

Three-dimensional volumetric imaging, such as cone-beam computed tomography (CBCT), not only permits the acquisition of an overlay-free image, but also allows minimal image distortion, superimposition, and low radiation dosage that are comparable to a full-mouth series of periapicals.^17–19 

The aim of this pilot study was to assess the clinical applicability of this bone-borne hybrid expander and evaluate the three-dimensional analysis of the movement of teeth, alveolar processes, and skeletal structures caused by this appliance.

Ethical approval for this study was obtained from the Selçuk University Medicine Faculty Research Ethics Committee (2013/51), and informed written consent was obtained from all the study participants. The sample consisted of five boys and four girls, 13.61 ± 0.72 years old undergoing RME with the hybrid expander. Subjects were recruited from the patients at the orthodontic clinic of Selçuk University, Konya. Inclusion criteria were as follows:

• maxillary transverse deficiencies,

• no developmental deformity,

• skeletal Class I malocclusion,

• no cleft and palate,

• 2- to 3-mm overbite, and

• need for nonextraction fixed orthodontic treatment.

Expansion was made minimally invasive by using a hybrid expander. It was designed to be fixed on hard palate with one mini-screw on each side, for a total of two mini-screws. Two self-tapping mini-screws (Excalibur, SIA, Caserta, Italy) (1.6 mm in diameter, 10 mm) were inserted bilaterally between the upper second premolars and first molars on the palate at a 45°. The dental cast was obtained, and we signed mini-screw points on this model. Hyrax screw (G&H Orthodontics, Franklin, Ind) was adapted as deep as possible on the midline of the palatal vault. The hybrid expander appliance was produced, which had acrylic palatal pad not covering any teeth (Figure 1). The hybrid expander gets support only from palate and the mini-screws, not covering teeth. That which makes this appliance more hygienic than tooth-borne palatal expanders. The appliance was cemented to the mini-screws with resin composite (Transbond XT, 3M Unitek, Monrovia, Calif). The appliance activation was made two times a day (2× quarter turn  =  0.5 mm) for the first week to overcome the resistance of the mid-palatal sutures, and then one time a day for 3 weeks. Thirty-five quarter turns of 0.25 mm caused 8.75 mm of maxillary expansion.

Pretreatment and posttreatment CBCT images were obtained by using Kodak equipment (Model CS 9300, Carestream Health Inc, Rochester, NY) set as follows: exposures were made at 8.0 mA and 70 kV for 6.15 seconds, and with an axial slice thickness of 0.18 mm. The patients were asked to put their head in Frankfort horizontal position for the CBCT scans. The Digital Imaging and Communications in Medicine (DICOM) images were imported, and cross-sectional slices were made with the Simplant Software (13.01 Version, Materialise, Leuven, Belgium). By using the Simplant Software program, the three-dimensional image reconstructions were standardized by using the Frankfort horizontal plane (represented by a line on the image) as the x-axis, the transporionic plane as the y-axis, and the midsagittal plane as the z-axis. To establish the standard orientation of the craniofacial structures, three-dimensional reference planes were initially set according to the reference point ELSA (Figure 2).²⁰  The center of the line connecting the geometric centers of each foramen spinosum was identified as ELSA. The horizontal plane was defined as the plane that passed through the auditory external meatus and ELSA. The vertical plane was defined as the plane that was located perpendicularly to the horizontal plane passing through the medial edge of the foramen magnum. The frontal plane was located perpendicular to the horizontal and vertical planes. All transversal linear and angular measurements were recorded for each scan before RME (T1) and after treatment (T2). One investigator performed the measurements. All points measured are listed in Table 1 (Figure 3).

Statistical Package for the Social Sciences (SPSS 17.0, Chicago, Ill) was used for data analysis, and a P value < .05 was considered statistically significant. Numerical data were given as mean ± SD. Images of five patients were randomly selected, and the initial measurements of each side were repeated by the same investigator 2 weeks after the first measurements. Intraclass and interclass correlation coefficients were performed to assess the reliability of the measurements as described by Houston²¹  in same images, and the coefficients of reliability for the measurements were above 0.903 (0.903–0.987) (Table 2). In addition, the statistical difference between the two tracings was tested with a Wilcoxon test; no significant difference was found (P < .05), confirming the intraobserver reliability of the measurements. Because the size of the study group was only 9, the statistical evaluations of these data were performed using nonparametric tests. Statistically significant differences between the T1 and T2 sides were determined using the Wilcoxon test. Statistical significance was set at P < .05.

All 18 mini-screws were still stable when the expansion finished. A V-shaped opening of the suture and the dentition was observed with the greatest amount of opening anteriorly and inferiorly directed in the transverse dimension. The amount of transmitted screw expansion (8.75 mm) on teeth and skeletal structures is shown in Table 3. There was significant expansion on all teeth and alveolar crest parameters (P < .05). In skeletal structures, only anterior nasal spine and A point significantly expanded (P < .05). The maximum transverse increase (8.05  =  92%) was found in the maxillary first premolar region on teeth. The minimum transverse increase (4.38  =  50%) was found on the apex of maxillary central incisors region on teeth. Approximately the same amount of transverse increase was obtained on maxillary first premolar region on teeth with the amount of screw expansion. On the skeletal structure, the maximum transverse increase (7.09  =  81%) was found in point A, the minimum transverse increase (0  =  0%) was found in lower margins of orbita, zygomaxillare, and point B.

The amount of expansion caused tipping of teeth as shown in Table 4. Tipping of posterior teeth was significantly increased by the impact of expansion. The most increased tipping was found on the maxillary left first premolar tooth (3.24°), and the least increased tipping was found on the maxillary right first molar tooth (2.88°).

The objective of this study was to evaluate the effects of bone-borne hybrid expander with two mini-screws on maxillary teeth and maxillary bone. In clinical experience, the transversal deficiencies of all patients were successfully treated.

Posterior insertion of two mini-screws between the second premolar and first molar, located approximately 5–6 mm from tooth, seems to be preferable and harmless for teeth. This area has been described as a safe place for mini-screws.²²  Using the hybrid expander appliance is a minimally invasive method compared with the bone-borne RME appliance method and minimally harmless for teeth compared with the conventional RME and hybrid hyrax appliance method.^13,23 

Our data showed significant increases in oral cavity width at the levels of the maxillary premolars and molars (mean  =  8.05 mm; 92%, mean  =  7.79 mm; 89% of appliances expansion) after the expansion. Bone segment were expanded (between 95% and 99%) more effectively than dental segment in alveolar crest. Our results are compatible with the results of Tausche et al.,⁵  who reported that the transverse increase at the level of dental segment was maximum in premolar and molar region and that alveolar crest regions were expanded between 99% and 107% according to the appliance expansion. They used surgical approach in young adults, and RME was obtained with Dresden distractor hybrid hyrax.

The V-shaped opening was obtained with hybrid expander in midpalatal suture on horizontal and coronal planes. Anterior nasal spine was affected 55%; however, the posterior nasal spine was affected 18% from the expansion. This result was different from the results of Habersack et al.²⁴  and Christie et al.,²⁵  who described parallel opening on the midpalatal suture. On the other hand, Wertz and Dreskin,²⁶  Silva Filho et al.,²⁷  and Tausche et al.⁵  demonstrate that the opening of midpalatal suture in the area of ANS was wider than in PNS after RME. Perhaps, the nature of the parallel opening on midpalatal suture could be young age.²⁵ 

In conventional RME, Christie et al.²⁵  reported that the amount of buccal tipping was 6.22° for the right molar and 5.50° for the left molar. Similarly, Kilic et al.²⁸  found that the amount of buccal tipping was 7.01° for the right molar and 6.79° for the left molar in the hyrax group when they compared the dentoalveolar inclination of patients treated with either a hyrax expander or an acrylic expander. In our study, we determined that the mean amount of buccal tipping was 2.88° for the right and 3.02° for the left molar. Tausche et al.⁵  evaluated the dentoalveolar effect of bone-borne Dresden distractor and found 3.5° buccal tipping for the right molar and 2.5° for the left molar, similar to us. The bone-borne expander caused molar buccal tipping of approximately half that of the conventional Hass and Hyrax expander as inferred in the literature. This greater tipping could be regarded as an expansion force transmission to bone via the teeth. On the other hand, in the bone-borne expander, expansion force transmission is to bone directly.

Hybrid expander appliance with only two mini-screws, no tooth support, and with exchangeable abutments is effective for RME. This appliance makes RME more hygienic for teeth and gingival tissue. Fixed orthodontic treatment could be begun immediately and an expansion, retention period is unnecessary. Other appliances could be used for retention such as a transpalatal arch or Nance.

Further studies to evaluate the dentoalveolar and maxillary three-dimensional effects of the hybrid expander should be designed by comparing tooth-borne and tooth-and-tissue–borne appliances including a control group. Another limitation of this study is the small sample size. To overcome this limitation, the same author performed all measurements, and the high accuracy (a coefficient of more than 0.903) of the quantitative measurements supports the reliability of the results. Future studies with a large sample size are needed for the further evaluation of this appliance.

• RME could be obtained and supported with two mini-screws and palatal bone without the support of teeth.

• RME with hybrid expander appliance showed V-shaped opening of suture on horizontal and coronal planes.

• Hybrid expander appliance caused less buccal tipping on posterior teeth than conventional RME appliances when literature results were compared.

• Hybrid expander appliance could prevent teeth from root resorption, dehiscence, and fenestration because of no tooth support.