Zygomatic Arch

3D models were constructed and superimposed using voxel based superimposition in Maxilim software installed on a windows XP-based workstation (Intel® core™ 2 Duo; 2.9 GHz, 3.25GB, ATI Radeon™ 3450 HD graphics card). The construction of the 3D models was performed by selecting the range of Hounsfield unit (HU) representing the bony tissues on the DICOM images. This was achieved by selecting a lower threshold value between 250–350 HU. Values above this threshold were automatically selected. The superimposition procedure is an automated procedure that compares the grey values in the two DICOM images voxel by voxel. The user is first required to select the volume of interest (registration area), then to roughly align the 3D models. Consequently the software computes the translation and rotation needed to geometrically align the two DICOM images, and subsequently the constructed 3D models, based on the maximization of mutual information. For each pair of CBCT scans the 3D model construction and superimposition procedure was repeated five times with a time interval of three weeks.
The scans were registered twice on the anterior cranial base and twice on the left zygomatic arch (zygomatic bone + zygomatic process of the temporal bone) by the same operator (RN) (Fig. 1). To test the inter-observer reliability, the scans were superimposed for a fifth time by a second observer (HB) registered on the anterior cranial base.

A computer aided design model was created from the CT scan images of the skull of a patient with skeletal class II malocclusion with prognathic maxilla and vertical maxillary excess which were taken at 0.5 mm slice thickness. 3D models of the frontal bone, nasal bone, maxillary bone, zygomatic bone, temporal bone and sphenoid bone were generated individually. Sutures of the craniofacial complex were generated in the model with a width of 0.5 mm [13 ]. Teeth in the maxillary dentition were segmented and modelled individually. The periodontal ligament surrounding the maxillary teeth were modelled with a thickness of 0.2 mm [14 (link)]. DICOM images were generated and converted into STL file format using MIMICS software which were then assembled into a single unit and transferred to ANSYS software (Fig. 1).Fig. 1

Finite element models. a Finite element model with miniplate. b Finite element model with mini-implant

3D model of a Y-type stainless steel miniplate and three mini-screws of dimension 1.5 × 8 mm to be threaded to fix the miniplate to the zygomatic buttress were generated for model 1. For model 2, two separate stainless steel mini-implants of size 1.5 × 8 mm were generated. One was placed in the interradicular space between the premolars at about 3 mm above the cementoenamel junction while the other was placed between the premolar and the molar at about 4 mm from the cementoenamel junction [15 (link)]. The variation in the height of the mini-implants were created in order to deliver a line of force which passes near the center of resistance of the maxillary arch.
Along with the facial bones, a total of five sutures namely the fronto-maxillary suture (FM), zygomatico-maxillary suture (ZM), zygomatico-temporal suture (ZT), zygomatico-frontal suture (ZF) and pterygomaxillary suture (PM) were analysed individually. Apart from the sutures, prime anatomical landmarks such as frontal process, anterior nasal spine, point A, prosthion and maxillary process of zygoma were evaluated separately. The material properties of all structures were assigned as shown in Table 1.Table 1

Material properties

Material	Young’s modulus (MPa)	Poissons’s ratio
Cortical bone	13,700	0.30
Cancellous bone	7930	0.30
Miniplate	103,000	0.33
Miniscrew	10,300	0.33
Suture	68.65	0.40
Tooth	203,000	0.30
Stainless steel	2,059,000	0.30
Periodontal ligament	50.01	0.49

Forces applied were categorized into three levels. (1) 200 g per side, (2) 300 g per side and (3) 500 g per side. The force was applied 45° to the occlusal plane in order to achieve a line of force passing though the centre of resistance of the maxilla which is in the postero-superior aspect of the zygomatico-frontal suture.

In the present study, the adult Omani patients (aged ≥18 years) who had visited the Department of Radiology and Molecular Imaging at the Sultan Qaboos University Hospital, Muscat, Oman, were studied retrospectively using an electronic medical records database (TrakCare Unified Health Information System). All the consecutive patients of either gender aged ≥18 years who had been referred for a CT scan of the brain from 1 January 2019 to 31 March 2019 were included. After applying the inclusion and exclusion criteria, there were 273 Omani patients. Patients with orbital fractures and ocular or facial surgery or deformity were excluded. Additionally, scans with motion artifacts or incomplete coverage of the orbits and those performed for non-Omani patients were also excluded from the study sample.
All the CT scans were performed as per the routine standard protocol for non-enhanced CT of the brain using 64 slice multidetector CT (Siemens Sensation 64, Siemens AG, Munich, Germany) with a kilovoltage peak of 120 kV and tube current modulation. The images and measurements were assessed using the Synapse Radiology PACS, Version 5.7.102 (Synapse^® Enterprise Imaging, Fujifilm Worldwide, Tokyo, Japan).
The measurements were performed using the reconstructed thin slices of 1.2 mm in the bone window. A window width/level of 2000/500 mm was used while screening the images. The following measurements were performed for every subject: the interorbital distance, interzygomatic distance (IZD), horizontal orbital diameter and vertical orbital diameter. First, the orientation of the axial images was adjusted according to the Frankfort horizontal plane, which is defined as the line from the highest point of the opening of the external auditory canal to the lower margin of the orbital rim.16 (link) After adjusting the axial plane, the IOD was measured as the minimal distance between the medial orbital walls. The IZD was determined as the maximum distance between the anterior aspects of the zygomatic arches [Figure 1]. The horizontal distance of orbit (HDO) was measured as the maximum distance from the anterior lacrimal crest to the lateral orbital wall [Figure 2A]. The vertical distance of orbit (VDO) was performed in the sagittal plane after adjusting the angulation of the sagittal image along the long axis of the orbit and measured as the maximum distance between the frontal and the maxillary bones [Figure 2B]. Finally, OI was calculated using the following formula:
Statistical Package for the Social Sciences (SPSS), version 23.0 (IBM Corporation, Armonk, New York, USA) was used to analyse the data. The data were presented as mean and standard deviation. Independent sample t-test was used to determine the associations between the orbital dimensions and gender, while paired t-test was used to determine the laterality difference. The association between the orbital dimensions and age groups were determined using one-way analysis of variance (ANOVA). The differences were considered significant at P value <0.05.
The study was conducted after receiving ethical approval from the Medical Research Ethics Committee at the Sultan Qaboos University Hospital (#SQU-EC/445/2021).

The pre- and postoperative crania were semi-automatically segmented according to Holte et al. [4 (link)] For alignment of the segmented pre- and postoperative crania, the postoperative CBCT scan was registered to the preoperative CBCT scan by VBR using the anterior cranial base, zygomatic arches, and forehead as the volume of interest unaffected by the surgery [19 (link)]. Next, a curve was manually traced engulfing the preoperative glenoid fossa, which was automatically attracted and attached to the surface of the postoperative skull, defining the postoperative glenoid fossa (Figure 1). A three-dimensional assessment of glenoid fossa changes was performed according to Holte et al. [4 (link)]. However, instead of applying the midsagittal and coronal plane for spatial division of the glenoid fossa through its center of gravity [4 (link)], the glenoid fossa was divided analogously to the division of the condylar head into four sub-regions for spatial analysis using the two previously defined cutting-planes. Glenoid fossa surface discrepancies were represented by color-coded distance maps, [14 (link)] and quantified by the root mean square (RMS) surface distance of the total glenoid fossa and the four defined fossa sub-regions (Figure 1) [4 (link)].