A comparison of three-dimensional stress distribution and displacement of naso-maxillary complex on application of forces using quad-helix and nickel titanium palatal expander 2 (NPE2): a FEM study
In young patients, slow maxillary expansion is said to provide the maximum rate at which the mid-face sutures can adapt, with minimum tearing and haemorrhaging compared with rapid maxillary expansion [14–16]. Animal and histological studies indicate that slow maxillary expansion improves conservation of the suture and can produce a more stable result than rapid maxillary expansion [14, 15] Some clinical studies also suggest that slow maxillary expansion is more stable than rapid maxillary expansion [16]. Changes in craniofacial skeleton arising from orthodontic treatment are more complex than envisaged from two-dimensional cephalometric assessments, and so we decided to do a three-dimensional study of the craniofacial skeleton using finite element analysis.
In the present study, we witnessed skeletal changes following slow maxillary expansion, similar to those Hicks [17] had reported; according to him, substantial skeletal changes with slow maxillary expansion can be observed especially in younger children. The theory is that the main resistance to the opening of the mid-palatal suture is not the suture itself but the surrounding tissues such as the circum-maxillary structures and mid-face sutures [13]. This observation lends support to studies that noted the buttressing effect of the zygomatic processes against forces of expansion [18] supporting to evidence what we noticed in our study.
Orthopaedic force distribution after the activation of both the appliances quad-helix and NPE2 were observed to be analogous to what Chaconas and Caputo [19] stated that there was the buttressing of the maxillary tuberosity with the pterygoid plates; the sphenoid bone allowed the forces to then radiate to the base of the medial pterygoid plate from this region the forces then branched superiorly toward the malar and zygomatic bones. Specifically, the areas of the zygomatico-maxillary and zygomatico-temporal sutures were affected. The forces then radiated supero-medially toward the medial wall of the orbit and concentrated at the junction of the nasal and lachrymal bones. From our study, it is evident that the maxillary buttresses are the main areas of resistance with the forces on the maxillary molars; the stress radiates to the three main buttresses of the mid-face cranial complex: the naso-maxillary, the zygomatico-maxillary, and the pterygo-maxillary [20].
Sandikcioglu et al. [21] in his study achieved more posterior expansion of the palate; however, both the models in the present study exhibited similar results showing greater posterior dislocation of the mid-palatal suture than in the anterior region.
Our study exhibits downward and forward displacement of maxilla similar to the displacement observed by Jafari et al. [22] who studied stress distribution and displacement of various craniofacial structures following transverse orthopaedic forces. In the present study, we found backward movement of the point A which can be supported by Wertz [23] and Sandikcioglu et al. [21] who reported that the point A moved slightly backward and the ANB mostly showed high values. According to Wertz research, if it is assumed that point A does not move forward during rapid maxillary expansion, the change in the ANB angle could be a result of posterior rotation of point B [23].
During expansion, not all changes are caused by alveolar bending but are partly due to the tipping of teeth in the alveolar bone; this tipping is usually accompanied by some extrusion [24]. Similar outcomes were seen in the present study where tipping and slight extrusion of the molars were seen. Herold [25] presented greater buccal tipping in a sample treated with quad-helix. Shetty et al. [26] demonstrated tipping and extrusion of teeth following the use of NPE2.
It is witnessed in the present study that when correctly employed, the quad-helix can produce results similar to the rapid maxillary expansion and also correct all the transverse problems in the growing patients [14]. In the same way, NPE2 showed orthopaedic changes when used in mixed dentition, which is reinforced by the findings of studies that NPE2 even though being an orthodontic appliance showed orthopaedic changes [26].
The cusp of the erupting canine and the mesiobuccal cusp of the erupting second molar showed outward, backward and downward displacement indicating the speeding up for eruption which was similar to study [27] where rapid maxillary expansion was effective in treating patients in the late mixed dentition with palatally displaced canines. Baccetti et al. [28] stated that maxillary expansion is effective as an interceptive procedure to prevent final impaction of maxillary canines with palatal displacement in the early mixed dentition.
Quad-helix model showed high stress levels around pterygo-maxillary suture, whereas the stress levels were very minimal in NPE2 model which supports the findings of Donohue et al. [29] who identified that both the quad-helix and NPE2 were equally efficacious maxillary expanders; however, according to them, quad-helix appliance produced more controlled differential expansion between the first molars than NPE2 in their clinical comparison between the two and so they stated quad-helix more individually predictable in expansion.
The results of the present study using three-dimensional finite element model of a young skull provided explanation about the response of appliance activation within the bony tissues; however, finite element method has certain limitations like the results being applicable to the generated model and may not necessary apply to all individuals; moreover, clinical environment cannot be created in the model like mastication forces and patient movements; therefore, the results are for qualitative purpose for emphasizing skeletal responses of the appliances in human tissues.