Finite Element Modelling

Fig. 4.1

Main steps involved in the reconstruction of 3D model of human craniofacial as shown in the right sagittal view (a) computed tomography image of craniofacial (b) mask layer (green) creation (c) edited mask layer and (d) 3D model of craniofacial
Similar steps were repeated to reconstruct the 3D models of framework representing a partial prosthesis and mucosa soft tissues based on similar CT image datasets. The model of upper jaw denture was reconstructed carefully to maintain its design and geometry as closely as possible to the original patient’s complete denture. Subsequently, a partial framework with flange was modelled with 1.52–3.46 mm in thickness (t), 12.45–19.06 mm in width (w) and 15.41–18.37 mm in height (h). There were two different designs of framework produced in which the design for the intrasinus approach was bulkier than the one for the extramaxillary approach due to the expected emergence of implant heads slightly in the palatal area (Fig. 4.2). The gap existed along the maxillary arch between the palatal surface of bone and the inside surface of complete framework was used to develop a soft tissue model (Fig. 4.3). As a result, the soft tissue model had a thickness ranging from 2.16 (edentulous ridge) to 5.58 mm (hard palate) that lie within the ranges of the in vivo measurement done by Uchida et al., which is from 2.06 to 5.77 mm for male group [1].

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Fig. 4.2

(a) Three-dimensional model of partial framework design used in the (b) intrasinus and (c) extramaxillary approaches. The prosthesis was modelled as one part only (considered as “framework”) where the outer acrylic layer was ignored
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Fig. 4.3

The reconstruction of 3D model of soft tissue. (a) Gap existed along the maxillary arch between bone and framework. (b) Final model of soft tissue shown in the isometric and cross-sectional views
All generated 3D models of craniofacial, framework and soft tissue were then visualised into their original position to determine the reference of coordinate system. It is important to make sure that the reconstructed models were in a standard position prior to the analysis for an accurate models representation. The models were probably not in the standard position or orientation as desired after scanning that could be due to the nature of the scan. In this text, it was clearly shown that the craniofacial model was not in a standard position where the model was slightly inclined in a certain angle as viewed from coronal and sagittal planes (Fig. 4.4).

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Fig. 4.4

Visualisation of reconstructed 3D model of craniofacial in the original position as shown in the (a) coronal and (b) left sagittal view
To position the craniofacial model into a standard orientation, the model has to be repositioned referring to the Frankfort horizontal plane. The Frankfort horizontal plane can be defined as a plane established by the lowest point on the margin of the right or left bony orbit and the highest point in the margin of the left or right auditory meatus. In addition, the occlusal plane was also determined at the incisal point and tips of the distobuccal cusps of the second lower molars. This plane forms an angle of 15–20° respected to the Frankfort horizontal plane. Since the direction of simulated masticatory force has to be perpendicular to the occlusal plane in the analysis, the occlusal plane was positioned to lie on the x-axis and y-axis or the xy plane. The z-axis or implant axis is perpendicular to the occlusal plane. The models were then exported into a FEA software, MSC/MARC 2007 for the repositioning. As a result, the model was rotated about 16.5 and 4.4º along the x-axis and y-axis, respectively as depicted Fig. 4.5.

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Fig. 4.5

Repositioning of craniofacial model from (a) original to (b) standard position based on the Frankfort horizontal (yellow) and occlusal planes (red)
The selected region of interest was in the maxilla and the zygomatic bone on both sides, which also covered the infrazygomatic crest, anterior nasal spine, zygomatic process, temporal process, frontal process and the orbital floor surface (Fig. 4.6). The model dimensions were 111.9 mm in length, 46.5 mm in height and 52.4 mm in width. Both maxilla and zygomatic bone were reconstructed by consisting of two bone layers, cortical and cancellous bone as depicted in Fig. 4.7. The cortical layer of the maxilla had a thickness ranging from 0.50 to 1.17 mm covering the alveolar ridge and infrazygomatic crest regions with a thicker layer was obtained towards the maxillonasal and maxillozygomatic trajectories. Determination of the region of interest size is important so that the distribution of stresses to the end of bone segment will not impinge on the future results. According to Teixeira et al., a minimum bone length of 4.2 mm between implant body and segment end was relevant to ignore the stress variations around implant body [2]. The measurement on the models showed that the lengths were 11.28 and 20.61 mm for the intrasinus and extramaxillary approaches, respectively.

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Fig. 4.6

Three-dimensional model of craniofacial with region of interest (blue colour) in the (a) isometric (b) coronal (c) left sagittal and (d) bottom axial view
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Fig. 4.7

Distribution of cancellous bone layer (dark green colour) shown in the (a) isometric and (b) cross-sectional views from the midsagittal and posterior planes

4.2 Pre-surgical Planning of Implants Fixation

The height (h) and width (w) of the atrophic maxilla to be treated were measured to determine a suitable approach for treatment, either through the use of zygomatic implants alone or in conjunction with conventional implants [3]. The measurement was performed by taking several readings based on the 2D CT images at posterior and anterior regions of the maxilla as depicted in Fig. 4.8. The exact dimension was obtained by taking the average value of the readings as presented in Table 4.1. Based on the measurements, the average height of the anterior, left and right posterior maxilla sections were 8.07, 5.50 and 2.61 mm, respectively. The width of the alveolar ridge in the molar region was 9.70 mm. These dimensions fulfilled edentulous jaw classification, described by Cawood and Howell, being Class III and Class V for the anterior and posterior maxillae, respectively [4]. Therefore, the patient could be treated with a zygomatic implant placed bilaterally in conjunction with two conventional dental implants in the anterior region.

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Fig. 4.8

(a) Anterior maxilla measurement (sagittal view) and (b) left posterior maxilla measurement (coronal view)
Table 4.1

Measurement data of the maxillary height
Location
     
Dimensions (mm)
       
 
1
2
3
4
5
6
7
8
Average
Anterior
8.18
8.14
8.14
8.14
8.14
8.18
8.18
7.42
8.07
Posterior (Left)
5.55
5.59
6.28
5.60
5.59
5.59
4.90
4.88
5.50
Posterior (Right)
2.70
2.86
2.77
2.80
2.09
2.07
2.78
2.80
2.61
To determine the length of zygomatic implants to be used, a measurement on bone at the respective areas (zygoma and maxilla) was done by using Mimics/Magics software. The procedure was started by defining the midsagittal plane (Md) that passing through three points; the midpoint of the superior margin of the nasal bone (N), subspinale and the incisive foramen (INF) [5]. A plane was created through the bilateral infraorbital foramen (IF), perpendicular to the Md plane, and it was indicated as PTBIF. A point denoted as point A was shifted 5 mm towards the palatal region from the most inferior point of the alveolar process that crosses a line passing through the infraorbital foramen parallel to the Md plane [5]. This point being the starting point of the zygomatic implant insertion and the end point was determined by the jugale (Ju) point that located at the most depressed point of the transitional region from the lateral margin of the zygomaticofrontal process to the upper margin of the zygomaticotemporal process [5]. All landmarks and measurements are shown in Fig. 4.9. The distance between the crest of the maxillary alveolar process near the palate and the zygoma (A-Ju) was measured to determine the length of implant to be used. The angulation of the zygomatic implant was determined between the A-Ju distance and the PTBIF [5]. Results from the measurement were 48.9 mm for the length and 45.8° for the angle. The values recorded lie within the range of measurement done on cadavers by Uchida et al. where the range of values were 44.3–51.3 mm and 43.8–50.6° for the length and the angle, respectively [5]. Zygomatic implants are available in lengths from 31.5 to 51.5 mm with 5 mm steps. Accordingly, a 46.5 mm zygomatic implant is appropriate for this patient. The selection of longer implant (51.5 mm) is not practical since the apical portion could emerge in more dorsal direction towards the infratemporal fossa, which is should be avoided.

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Fig. 4.9

Landmarks and measurements on bones for the pre-surgical planning of implant fixation shown in the (a) frontal and (b) axial views
In the anterior region, conventional implants with a height of 10.0 mm were used as an optimal selection for D4-type bone. Also, the use of straight abutment is not practical in this case especially for the outcome of prosthesis position due to the morphology of maxilla or insufficient bone volume in the anterior region (Class III). The use of angulated abutment (in particular degree of inclination) can avoid improper prosthesis angulation that can lead to discomfort for patients [3]. Moreover, it is a suitable restorative option to be considered when implants are not in the axial position. Patients could face difficulty in performing oral hygiene as well as affecting their smile by having inadequate prosthesis configuration [3].

4.3 Three-Dimensional Implant Models Construction

A 3D CAD software, SolidWorks 2009 (SolidWorks Corp., Concord, Massachusetts, USA) was utilised to develop the implant models [6]. The construction of implant model required a matched abutment to connect the implant body to the prosthesis. The implants were placed in the maxillary arch in spread-out configuration. Two conventional implants were placed in the lateral incisor region and one zygomatic implant was located per side in the first molar or second premolar region nonsymmetrically relevant to the respective surgical approaches investigated. In the analysis, three different implant configurations in terms of design geometry, diameter and length were modelled. Two 46.5 mm zygomatic implants with 45º of angulated head, different diameter and thread distribution and two straight multi-unit abutments from Brånemark System® (Nobel Biocare AB, Gotebörg, Sweden) were used [7]. The height of abutment was 3.5 mm. For the conventional dental implant, two 4.0 × 10.0 mm implants with an angled multi-unit abutment 30º for each implant was chosen from the same manufacturer with a height of 3.5 mm. Figs. 4.10 and 4.11 show the 3D solid model of implant bodies and matched abutments used in FEA. The abutment body and screw were modelled as one part.

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Fig. 4.10

Three-dimensional solid models of zygomatic implant body used in (a

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Nov 10, 2015 | Posted by in General Dentistry | Comments Off on Finite Element Modelling
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