A total of 37 volunteer patients were selected from the 420 patients who had been referred to the Tehran Dental Hospital for 23 months. The patients included 19 women and 18 men aged between 21-49 years old and with a body mass index between 18.2-21.9 kg.m-2. 5.5% of male and 10.5% of female patients had a history of tooth caries and tooth fracture for up to 3 teeth that did not result from jaw or mouth injury, and were previously treated and did not pose a particular problem during the study. Also 55.5% of male and 52.6% of female patients had a history of filled teeth for up to 2 teeth. All patients were evaluated first by the use of a questionnaire that identifies the main complaint, pain history and the bruxism. It should be noted that this research was approved by the Ethics Committee of North Tehran Branch, Islamic Azad University (No. 18245/86-2) which was confirmed with the 1964 Helsinki declaration. According to the ethical standard, all samples have provided informed consent in the study. For all patients before and after the occlusal splint therapy, we applied a diagnostic protocol which is standardized equally, by a professional dentist. It concludes interview and a systematic evaluation of dental, cranial, facial, cervical and other oral structures. The initial inclusion criteria includes, no craniofacial surgery, no use of any medication and no reported systemic disease. The basis of choosing the sleep bruxism patients was the below criteria mentioned by American Academy of Sleep Medicine [25]:
A. occurrence of tooth grinding at least 3/7 nights during 6 months, as approved by a sleep partner; B. clinical presence of tooth wear; C. hypertrophy of masseter muscle; and D. occurrence of the fatigue or tenderness of jaw muscle in the morning.
Among the patients referred to the Tehran Dental Hospital, 36 other volunteer patients included 18 women and 18 men aged between 26-52 years old and with a body mass index between 19.6-22.3 kg.m-2 who had no history of disease or symptoms associated with bruxism or TMJ disorders, were selected as control subjects. They were completely healthy in concern with bruxism or TMJ disorders. But they had toothache with cracked and attrition tooth for up to 4 teeth (22.2% males and 16.6% female), tooth fracture and tooth carries for up to 4 teeth (27.8% males and 38.8% female) and 66.7% males and 77.8% female of control subjects need dental implant for up to 4 teeth which during the diagnosed and treating process, according to specialist’s diagnosis, CT scans were taken from them. Also 61.1% of male and 55.5% of female control subjects had a history of filled teeth for up to 3 teeth. It should be noted that all stages of diagnosis, informing about this study and provided informed consent for control subjects went through as the patients. For low cost, unique accessibility and low effective radiation dose, we use Cone-beam CT (CBCT) scanning for preparing the images. We use Newtom VG system (QR, Verona Italy) CBTC scan set. The scan setting includes: 3.6 mAs and 90 KV with radiation time of 15 seconds and field of view 20×19 inch. Furthermore, the position of samples during scanning was standing and Natural Head Position was the condition of their head. Swallow or breathe were prohibited for patients during image preparing. The voxel size and the slice thickness were 0.3×0.3×0.3 mm. and 0.3 mm, respectively. It should be noted that the jaw relation in providing images for all patients and control subjects was in the maximum intercuspation position.
Using the images obtained from the CT scan of the jaw and teeth of samples and importing DICOM files of these images into Mimics software version 13.1 (Materialise, Leuven, Belgium), the point cloud of the maxilla, mandible and teeth of each sample were produced as separate parts (Fig 1, a). In Mimics, the bony parts of the maxilla, mandible and teeth in image file were kept. After modifying the areas containing soft tissue in all image slices and repeating these modifications layer by layer, the spaces between layers were finally modified and differentiated and the point clouds of the teeth and jaw bone were extracted as the software output. Subsequently, the point clouds were transferred to the CATIA software version 5R21 (Dassault Systemes, Waltham, Mass., USA), and a three-dimensional model of the maxilla, mandible and teeth were built (Fig 1, b).
Fig. 1. (a) The point clouds of the maxilla, mandible and teeth. (b) 3D model. (c) Meshed model.
Three-dimensional models of the maxilla, mandible and teeth of all 37 patients and 36 control subjects were assembled in conditions of no contact pressure respect to each other. Six months after insertion of the occlusal splint, the process of preparing CT scan images, creating the point cloud, and building the three-dimensional models of jaws, teeth and splint were repeated for patients with the splint in their mouth. At this stage, the 3D models of used splints of the same patients were constructed from their CT scan images and the 3D splint models were inserted between the upper and lower teeth of patients. It should be noted that according to experienced dentist prescription, all patients were treated using occlusal splint therapy. However, a sleep hygiene measures combined with relaxation techniques was advised and prescribed for all patients. The material of splints was hard colorless acrylic resin which was polymerized using the method of conventional heat-curing. It is worth noting that due to personal limitations of the patients, there was only possible to create the 3D model of jaw, teeth and splint of 10 women and 11 men after 6 months of using splint. Since the analyses based on FEM are among the most familiar methods for biological simulation [26-39], the assembled models were transferred finally to ABAQUS software version 6.14 (Dassault Systemes) for FE analysis. Furthermore, for comparing the results of computer simulations between the patients and control subjects, and also for comparison of these results in patients before and after the occlusal splint therapy, three specific anatomical points in the skull of the samples were used as set points to synchronize procedures and standardize the samples head position. Also, in order to ensure the correct loading distribution for comparing the biomechanical parameters of the models with each other, the 3D models of the samples were standardized in terms of the definition of the x, y, and z axis relative to one same reference.
Table 1 shows the material properties considered for jaw bones, teeth, and splints [20, 40]. One of the most important points in FE analysis is how different parts of the model interact with each other. In this study, these interactions and constraints were defined based on the actual anatomical function of these components in the human body. The constraint considered for the contact between the inserted teeth on the upper and lower jaws with splint was a surface-to-surface constraint with a friction coefficient of 0.5 [20]. The degree of freedom of the upper surface of maxilla was considered to be zero at all three directions of x, y and z, i.e., the surface was considered to be fixed. Other degrees of freedom were considered in accordance with the real performance of TMJ, so that the necessary degrees of freedom for opening and closing movements of jaw (rotational degree of freedom) as well as the translation and lateral displacement of jaws over each other (translational degree of freedom) were considered (Fig 2, a). According to previous study, a first order Ogden hyperelastic model was used for defining the periodontal ligament with poisson's ratio of 0.45 and material parameter MPa [41]. The average amount of force exerted by the medial pterigoid muscle and masseter muscle for both left and right muscles was assumed to be 50 N [20, 40]. It should be noted that, according to previous studies, the force of these muscles should be applied under a particular angle to the model, as shown in Fig 2, a. One of the most important issues in numerical computer simulations is to ensure the mesh independence of responses [42-46], The tetrahedral element was used for meshing the models (Fig 1, c). The results showed that the maximum difference between the stress values in the medium and fine meshes in all three groups of patients, control subjects and patients after 6 months of using the occlusal splint was less than 1.8%. Therefore, the convergence of responses from the grid and time step was ensured (Fig 2, b).
Table 1. Material properties of jaw bone, teeth and occlusal splint [20, 40].
Parameters
|
Elastic modules (MPa)
|
Poison ratio
|
Jaw bone
|
1370
|
0.3
|
Teeth
|
18000
|
0.31
|
Occlusal splint
|
0.027
|
0.35
|
Fig. 2. (a) Degree of freedom, interactions and constraints between parts. (b) Diagram of maximum stress in head of mandible - number of elements for grid independence study.
Statistical analyses
The mean value, standard deviation (SD) and coefficient of variation (CV) for maximum stress and maximum deformity were calculated in all three groups of patients, control subjects and patients after 6 months of using the occlusal splint using SPSS version 22 (IBM Corp., Armonk , New York, USA).