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Year : 2017  |  Volume : 51  |  Issue : 3  |  Page : 160-167

The analysis of three-dimensional effects of nitanium palatal expander 2 and hyrax maxillary expansion appliances on craniofacial structures: Afinite element study

1 Prof, Department of Orthodontics, Al-Badar Dental College and Hospital, Gulbarga, Karnataka, India
2 PG Student, Department of Orthodontics, Al-Badar Dental College and Hospital, Gulbarga, Karnataka, India

Date of Submission28-Oct-2016
Date of Acceptance30-May-2017
Date of Web Publication17-Jul-2017

Correspondence Address:
Avinash Kumar
Department of Orthodontics, Al-Badar Dental College and Hospital, Gulbarga, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jios.jios_213_16

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Objectives: To analyze three-dimensional effects of stress distribution and displacement on the craniofacial structures, following the application of forces from Nitanium Palatal Expander 2 (NPE2) and Hyrax appliance in early mixed dentition period using finite element analysis. Materials and Methods: Three-dimensional finite element models of the young dried human skull, NPE2 and Hyrax were constructed, and the initial activation of the expanders was simulated to carry out the analysis and to evaluate the von misses stresses and displacement on the craniofacial structures. Results: Both the models demonstrated the highest stresses at the mid-palatal suture, with maximum posterior dislocation. The inferior nasal floor showed highest downward displacement and point A showed outward, backward, and upward displacement in both the models. The pattern of stress distribution was almost similar in both the groups, but NPE2 revealed lower magnitude stresses than Hyrax. The cusp of the erupting canine and the mesiobuccal cusp of the second molar showed outward, backward, and downward displacement signifying eruption pattern following maxillary expansion. Conclusions: Nickel titanium palatal expander-2 and Hyrax produced similar stress pattern in early mixed dentition period finite element model. We conclude from this finite element method study that NPE2 is equally effective as Hyrax when used in early mixed dentition period as it exhibits orthopedic nature of expansion with minimal residual stresses in the craniofacial structures.

Keywords: Finite element, Hyrax, maxillary expansion, nickel titanium palatal expander-2

How to cite this article:
Kumar A, Ghafoor H. The analysis of three-dimensional effects of nitanium palatal expander 2 and hyrax maxillary expansion appliances on craniofacial structures: Afinite element study. J Indian Orthod Soc 2017;51:160-7

How to cite this URL:
Kumar A, Ghafoor H. The analysis of three-dimensional effects of nitanium palatal expander 2 and hyrax maxillary expansion appliances on craniofacial structures: Afinite element study. J Indian Orthod Soc [serial online] 2017 [cited 2019 May 25];51:160-7. Available from: http://www.jios.in/text.asp?2017/51/3/160/210908

  Introduction Top

Transverse maxillary deficiency is a problem which is regularly encountered and treated by orthodontists. Early treatment with palatal expansion has been suggested for transverse discrepancies, particularly unilateral crossbite, since it is related with atypical chewing patterns and development of skeletal asymmetries.[1]

Two basic approaches have been developed to expand the maxilla which is generally divided based on the activation intervals and force exerted by the appliances. Rapid maxillary expansion(RME) uses heavier interrupted forces to maximize orthopedic effects, and slow palatal expansion uses lighter continuous forces to move teeth at rates purported to be more physiologic.[2]

Slow maxillary expansion produces more physiologic response at the mid-palatal suture area.[3] Nitanium Palatal Expander 2 (NPE2) (Ortho Organizers, Inc. 1822 Aston Ave. Carlsbad, CA 92008-7306 USA) delivers a uniform slow, continuous force for maxillary expansion, molar rotation, distalization, and arch development. NPE2 delivers a force of 350 g for every 3mm of increment.[4]

RME is a well-established way for the correction of the transverse maxillary deficiency.[5] Tooth borne rapid expansion appliance was introduced by Biederman [6] in 1968. It makes use of a special screw called Hyrax. It is a nonspring loaded jackscrew with a rigid wire frame; analogous expansion appliance was used in this study.

The finite element method(FEM) splits the area of a structure into discrete elements interrelated at nodes.[7] The FEM, which was introduced as one of the numerical analyses, has become a useful technique for stress analysis in the biologic system [8] and so was used for the purpose of this study.

The objective of this study was to elucidate three-dimensional stress and displacement of the craniofacial region on application of forces from NPE2 and Hyrax maxillary expansion appliances. This study aims to check how diverse the force dissemination and transmission are between the two appliances.

  Materials and Methods Top

Computed tomography(CT) data were obtained from a dry young human skull. Skull bone was checked for any flaws in its structure. Three-dimensional CT scanning was performed by SIEMENS SOMATOM Definition 64(kVp120; mAs 290) in the axial direction. CT scan was taken at 0.5mm intermissions serially to duplicate better and comprehensive characteristics of the skull.

CT images which were obtained in DICOM format they were assembled over one another and transformed into a finite element meshed model[Figure1] using the software Materialise's Interactive Medical Image Control System(MIMIC Version18.0). The skull was meshed with Tetrahedron elements. The total number of elements and nodes in the skull were 864,651 and 247,129. ANSYS Design Modeler(Version16; ANSYS Inc. Integrated Design Analysis Consultants) with beam elements were used to model the appliances NPE2 and Hyrax[Figure2]. The total number of elements and nodes in the NPE2 appliance were 421 and 841. The total number of elements and nodes in the Hyrax appliance were 401 and 1089. ANSYS Professional NLS(Version16; ANSYS Inc., Canonsburg, PA, USA) was used to carry out the analysis.
Figure 1: Finite element mesh model of the skull

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Figure 2: Finite element models of (a) NPE2, (b) Hyrax

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Cortical bone was sculpted according to the early mixed dentition tissue volume given by Farnsworth et al.,[9] the model in early mixed dentition would differ from a model in late mixed dentition in the number of teeth erupted, the amount of root formation and porosity of the alveolus. The width of periodontal ligament was 0.2mm similar to Kronfeld,[10] and the width of the mid-palatal sutures was 0.5mm according to Fricke-Zech et al.[11]

The mechanical properties of the, tooth,[12] suture,[12] cortical bone,[12] cancellous bone,[12] periodontal ligament,[12] stainless steel,[13] and nickel titanium [14] in this model were similar according to the experimental data given in previous studies as presented in [Table1].
Table 1: Young's modulus and Poisson's ratio for various materials in this study

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Boundary conditions were laid at the foramen magnum, and it was completely fixed as suggested by Gautam et al.[15] Nodes of the mid-palatal suture were positioned on the symmetrical plane and were left unconstrained. The center of the lingual alveolar ridge of each tooth was used as an orientation mark to estimate bone displacement planes, respectively. Point A was situated at the incisive foramen on the mid-palatal suture, and point D was situated near the palatine bone. Points B and C separated this A-D line into three equivalent parts[Figure3].
Figure 3: Landmarks on the mid-palatal suture: Point A, Point near incisive foramen; Point D, Point near palatine bone, Point B, C divides the A-D line into three equal parts

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Hyrax expander was activated transversely by 0.5mm in X direction to check for two turn activation of screw force in rapid maxillary expansion therapy 350 g force is used in the activation of NPE2 model.[16] The displacements of maxillofacial complex and Von Misses stresses in different parts of the craniofacial complex were studied.

The three-dimensional coordinates were X, transverse plane; Y, sagittal plane; and Z, vertical plane Positive values indicate outward, backward, and downward displacement on the X, Y, and Z

  Results Top

The biomechanical changes were examined as displacements of various structures in all the three planes(X, Y, and Z axis) along with Von Mises stress distribution over the craniofacial complex after the activation of both the appliances.

In the transverse plane, lateral nasal wall and the inferior orbital rim showed inward displacement rest all structures were displaced outward. Highest displacement was seen at maxillary tuberosity followed by pterygoid hamulus in NPE2, whereas it was seen in medial pterygoid plate followed by the lateral pterygoid plate and maxillary tuberosity in Hyrax model. Sagittally cephalometric point A, maxillary tuberosity, posterior nasal spine(PNS), anterior nasal spine(ANS) and the maxillary process of zygomatic bone showed backward displacement rest all the structure showed forward movement in both the NPE2 and Hyrax model. Highest sagittal displacement in Hyrax model was seen at maxillary tuberosity. Vertically, most of the craniofacial structures showed downward displacement whereas maxillary tuberosity and point A showed upward movement. Inferior nasal floor showed highest downward displacement in both the models. It was noted that the PNS moved downwards along with a negligible movement in the ANS in NPE2 group and almost with the same displacement of ANS in the Hyrax group[Table2].
Table 2: Displacement pattern in (mm) after activation of Hyrax and NPE2 on craniofacial structures

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Mid-palatal suture points were displaced horizontally in both the models. The more transverse opening of midpalatal suture was observed in Hyrax group. All the mid-palatal suture points moved forward sagitally, with a steady fall from the anterior to the posterior regions[Table3].
Table 3: Displacement in (mm) after activation of NPE2 and Hyrax on mid-palatal suture

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In all the three planes, more alveolar bone displacement was observed in Hyrax. Observation of an extrusion of the anterior teeth was noted in NPE2 group, whereas there was more extrusion of the posteriors in Hyrax group. Erupting canine cusp tip showed extrusive movement similar to the mesiobuccal cusp of the erupting second molar which also showed increased eruption in both the model groups[Table4].
Table 4: Displacement in (mm) after activation of NPE2 and Hyrax on dento-alveolar structures

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[Table5] shows Von Mises stresses and the highest stress was recorded at the mid-palatal suture point Point A in Hyrax model, whereas point C in NPE2. Highest stress levels around lateral nasal wall were observed in both the models, the magnitude of stress in Hyrax group was approximately twelve times more than NPE2 model next highest stress was recorded at the temporal process of the zygomatic bone. The least stress was experienced at the fronto-maxillary suture. NPE2 revealed lower magnitude stresses than Hyrax; pronounced skeletal effects of both the appliances were exposed[Figure4],[Figure5],[Figure6],[Figure7],[Figure8],[Figure9].
Table 5: Stress distribution (MPa) at the maxillofacial sutures and landmarks after activation of the NPE2 and Hyrax appliance

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Figure 4: Transverse (x) displacement of Maxillary complex post-activation of the expansion device (a) NPE2, (b) Hyrax

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Figure 5: Distribution of stress after activation of the device (a) NPE2, (b) Hyrax in frontal view

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Figure 6: Distribution of stress after activation of the device (a) NPE2, (b) Hyrax in the sagittal view

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Figure 7: Distribution of stress after activation of the device (a) NPE2, (b) Hyrax in the lateral view

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Figure 8: Distribution of stress after activation of the device (a) Nitanium Palatal Expander 2 (NPE2), (b) Hyrax in the occlusal view

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Figure 9: Distribution of stress after activation of the device (a) NPE2, (b) Hyrax in the cross-sectional view at the first deciduous molar (a and b), second deciduous molar (c and d), and first permanent molar (e and f)

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  Discussion Top

The maxillary expansion is commonly used in growing children to eliminate the transverse discrepancy between the dental arches due to maxillary constriction.[17] Works of Krebs,[18] Akkaya et al.,[19] and Skeiller [20] affirmed that rapid maxillary expansion appliances disclosed the finest example of true orthopedic nature in such that variations are produced primarily in the underlying skeletal structures similar to what was observed in our study.

An alternative to traditional rapid palatal expansion, slow expansion uses lower orthopedic forces and takes months instead of weeks to accomplish the same amount of expansion.[21] McAndrew demonstrated that the application of light continuous forces in areas of periosteal growth allows normal arch dimensions to develop at any age without undue tipping of abutment teeth.[22] Proffit [23] suggested that approximately 0.5mm of expansion per week is the maximum rate at which the tissues of the mid-palatal suture can adapt.

In this study, we found increase in the maxillary width attained through a separation of two maxillary processes and buccal tipping of the teeth and alveolar processes, similar finding was observed in a study by Da Silva et al.[24] We noticed that there was a horizontal displacement of the mid-palatal suture indicating the transverse separation of two maxillary halves in both the models. Assessment of the dentoalveolar displacement in both the models also showed an increase in the maxillary transverse dimensions, indicating both skeletal effects of the expansion and lateral bending of the alveolar process.

Buccal tipping of the posterior teeth is the most important side effect of maxillary expansion.[17] A systematic review conducted by Lione et al.[25] concluded that heavy forces produce an increased buccal inclination of anchored teeth at the end of the expansion. In our study, we observed that there was a tipping of the first permanent molar along with some extrusion observed in both the models, however, the tipping observed in model activated by Hyrax was higher than NPE2. Alarge amount of force generated and suddenly directed to the crowns of the first molars may be the cause of this greater inclination of the teeth.[26]

In this study, in the early mixed dentition period, we noticed both orthopedic and orthodontic effects in both the models which can be supported by the fact that in the early mixed dentition group, the palate expanded, the dental inclination also occurs along with the palatal expansion due to increased interdigitations of the two palatal halves.[27]

In agreement with our results, the opening of the mid-palatal suture in both the models showed that the expansion in the posterior area was more than that in the anterior region which was comparable to that noted in the study.[28],[29]

It has been testified that the decay following activation of a jackscrew is rapid and that the rate of decay rapidly decreases, whereas the displacements seem to remain constant. The nearly constant presence of residual forces and the apparent cumulative nature of these forces suggest the possibility that the forces produced by the third and fourth activation are probably greater than the forces present later in treatment. This phenomenon which is mechanically determined as “relaxation” plays a key role in the response of the craniofacial complex.[30] NPE2 whereas produces a uniform, slow, continuous force for maxillary expansion, molar rotation and distalization, and arch development. The force application of NPE2 is preprogrammed and it is self-limiting.[31] It is observed in this study that NPE2 showed orthopedic changes when used in mixed dentition, which is reinforced by the findings of studies that NPE2 even though being an orthodontic appliance showed orthopedic changes.[32]

This study provides an explanation about the reaction of bony structures and teeth to the transverse expansion forces which are the tissue responses when clinically the dynamic process of maxillary expansion is carried out.

Limitations of the study

One of the drawbacks of this study is that a clinical environment cannot be created in the computer constructed model similar to patients head movements and mastication forces hence the outcomes of this study can be used for highlighting underlying skeletal and dental response in human tissues generated by the activation of appliances.

  Conclusion Top

In this study, we can mark out that the nature of expansion in early mixed dentition is a combination of skeletal and dental expansion. The greatest changes were observed in transverse dimensions, whereas the changes in vertical and anteroposterior directions were minimal. The nature of stress distribution of NPE2 was similar to Hyrax; however, NPE2 revealed the lower magnitude of stresses than Hyrax. We conclude from this FEM study that NPE2 is equally effective as Hyrax when used in early mixed dentition period as it exhibits orthopedic nature of expansion with minimal residual stresses in the craniofacial structures. However, these effects should be evaluated further with a clinical study.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure1], [Figure2], [Figure3], [Figure4], [Figure5], [Figure6], [Figure7], [Figure8], [Figure9]

  [Table1], [Table2], [Table3], [Table4], [Table5]


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