FINITE ELEMENT ANALYSIS OF PERIODONTAL STRESSES IN FIXED PROSTHODONTICS

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1 FINITE ELEMENT ANALYSIS OF PERIODONTAL STRESSES IN FIXED PROSTHODONTICS Valeria Pendefunda, Arina Ciocan Pendefunda, Nicoleta Ioanid, Alina Apostu, Oana Ţănculescu Odontology, Periodontology and Fixed Prosthodontics Department Faculty of Dental Medicine, Grigore T. Popa University of Medicine and Pharmacy Iaşi, Romania *Corresponding author: Valeria Pendefunda, DMD, PhD Odontology, Periodontology and Fixed Prosthodontics Department Faculty of Dental Medicine Gr. T. Popa University of Medicine and Pharmacy ABSTRACT Aim The aim of the study is to determine by finite element analysis the existence of direct correlations between the abutment periodontal tissues condition and the cantilever bridge in the context of functional occlusion. Materials and method In order to reveal the changes occurring in the periodontal ligament, a distal dental cantilever bridge was considered, retained on 34 and 35, subjected to a force of 350N, deemed to be the maximum force developed by the masseter and pterygoid muscles during mastication. The ALGOR 15 FEMPRO software was used to analyze the periodontal ligament stress. Results Finite element analysis was employed to determine the stresses, specific deformations and displacements undergone by the ligaments under survey. The maximum stress was recorded in the periodontal ligament of the first premolar since this load configuration produces a first degree lever effect with a fulcrum located on the premolar 35 root apex. Conclusions The overstress of periodontal ligament under occlusal forces leads implicitly to the tearing of the ligament fibers either in their body, or in its insertion into the dentin and bone. Keywords: finite element, dental cantilever bridge, periodontal stress INTRODUCTION Periodontal biomechanics is a particularly challenging issue to be dealt with by clinicians, as the supraliminal stresses of poor bridgework affects, in time, the neighboring periodontium. Together with the remaining arch, any cantilever bridge will have to bear various stresses of different directions, orientations, fulcra and strengths. The prosthetic environment dental cantilever bridge binomial requires biomechanical stability [3, 4]. A successful prosthetic therapy depends, on the one hand, on the patient s periodontal health status and, on the other hand, on the observance of the biological and biomechanical requirements by the bridge design, by the materials chosen and by the actual manufacture of the prosthesis. Since it is fairly difficult to conduct an in vivo or in vitro assessment of the forces acting on the periodontal ligament, finite element analysis is preferred as, if the modeling is accurate, it may provide very useful information on the stresses, specific deformation and displacements undergone by both the ligament and the bone [1, 6]. This information is particularly useful when assessing the impact of non-physiological 82

2 stresses (such as value, direction or fulcrum), of the materials included in the bridgework and of the bridge design or geometry. The purpose of our research is to determine, by FEA, the direct correlations exiting between the periodontal changes occurred and the type and morphology of the dental bridges, in the context of functional occlusion. MATERIALS AND METHOD A dental cantilever bridge, retained on 3.4 and 3.5, and a distal extension (3.6) were considered. This type of bridgework is still rather common in dental practice, as it fulfills some needs (such as avoiding removable prostheses in terminal edentations, or preparing the second molar when it is healthy but the premolars are not) but at the same time it involves some risks (such as periodontal premolar overstress or extension breaking) [7]. In order to be able to perform the best simulation of the mechanical phenomena occurring in the dental bridge and in the odonto-periodontal support, the geometric model needs to be as realistic as possible. Therefore, we used the AutoCAD 2009 software (Fig. 1) to achieve 3D images of the mandible, of the premolars prepared for 3.4 and 3.5 cover crowns, of the periodontal ligaments and of the cantilever extension. The meshing of the prepared structures was done by means of the Algor 15 FEMPRO software (Fig. 2). The material properties used were: modulus of elasticity, Poisson s ratio and material density. As the aim of our study was to determine the mechanical stresses on the periodontal ligament, we preferred a nickelchrome alloy for the bridge. The values of these parameters are shown in Table 1. Load application considered the maximum force developed by the masseter and pterygoid muscles during mastication: 350N scalar value and 15 0 force deviation from the vertical line (Fig. 3). Fig. 1. The mandible periodontal ligaments abutment teeth dental bridge group Fig. 2. The mandible dental bridge group Table 1. Material characteristics for each component of the analyzed structure Component Modulus of elasticity Density Poisson s ratio (Mpa) (Kg/m 3 ) Bone Dentine

3 Periodontal ligament Ni-Cr Alloy Fig. 3. Application of the resulting forces corresponding to the masseter and pterygoid muscles Fig. 4. Support to: the molar 36 area (top); the premolar 3.5 area (middle), the contact in premolar 3.4 area (bottom) Fig. 5. Stress distribution throughout the whole group The action of the forces developed by the manducatory muscles produces reaction forces in two areas, during mastication, namely in the temporomandibular joint and in the area where the dental bridge touches the food. The following options were considered for the dental bridge (Fig. 4): contact on the area corresponding to the missing molar 3.6; contact on the area corresponding to premolar 3.5; contact in the premolar 3.4 area. RESULTS AND DISCUSSIONS The analysis results were shown Fig. 6. Stress distribution when the force is applied on 36 (distal extension) depending on the areas on the bridge where the stresses were applied. Support on the distal extension (3.6) The forces applied on the distal extension revealed stresses, specific deformations and displacements within the whole boneligaments-premolars-bridge group (Fig. 5). Further to the analysis of the forces acting on the bridge, we noted certain stress concentrators, especially between molar 36 and premolar 3.5, which indicate the highest stress area, i.e. the area with the highest breaking risk (Fig. 6). The analysis of the stresses on the periodontal ligaments showed that the maximum stress values were lower than the 84

4 ones acting on the dental bridge - 1,5 Mpa (Fig. 7). This may be accounted for by the low modulus of elasticity of the periodontal ligament as compared to the dental bridge material. Specific deformations occur especially in the periodontal ligament of premolar 3.4, which is normal since its modulus of elasticity is three orders of magnitude lower than that of the surrounding structures (dentine, alveolar bone). Also, specific deformation distribution is similar to that of the stresses, since the proportionality factor between the two is shown by the modulus of elasticity of the periodontal ligament. When one isolates the two ligaments, one may notice that the displacements are predominantly horizontal, as shown in figure 8, which also illustrates the position of the non-displaced ligament. More stress was applied on the ligament of premolar 34, since we may say that this load configuration produces a first degree lever effect with a fulcrum located on the premolar 35 root apex. This type of bridge is not recommendable, since the loads that cause considerable asymmetries in the stress and displacement distribution may lead to the tearing of the ligament body or of the ligament-bone junction and, hence, they may impair on the dental implant and bridgework. Support on premolar 35 When the same maximum stress value, i.e. 45Mpa, was applied, we noted that this time the stresses were lower in the bridge area due mainly to the absence of the tipping movement that characterized the previous case. The ligament stress distribution value was also low, just like the specific deformations, the values of which were lower than in the previous case (Fig. 9). The deformations (displacements) were also low, almost undetectable, symmetric, and similar to those occurring in normally loaded premolars. Fig. 7. Stress distribution (left) and specific deformation distribution (right) on the periodontal ligaments of premolars 3.4 and 3.5, when the support is on 3.6 Fig. 8. Deformations of the periodontal ligaments of premolars 3.4 and 3.5 when the support is on the distal extension Fig. 9. Stress distribution (left) and specific deformation distribution (right) on the periodontal ligaments of premolars 3.4 and Fig. 10. Stress distribution (left) and specific deformation distribution (right) on the periodontal ligaments of premolars 34 85

5 3.5, when the support is on PM 3.5. and 35, when the support is on PM 3.4. Fig. 11. Deformations of the two periodontal ligaments Support on premolar 34 Stress distribution in this case was also lower than in the first one, although this time the stress applied to the bridge was higher in the support area. Just like in the previous cases, the stress on the ligaments had a high value, but the stress and specific deformation distribution was pretty similar to that of the first case (Fig. 10). Although the stresses and specific deformations were somewhat similar to the first case, when the support was on the distal extension, the deformations or displacements showed considerable differences. On the one hand, as it may be seen in figure 11, there was compression of the ligament of premolar 34 towards the canine and expansion from premolar 3.5. Also, the ligament of premolar 35 suffered compression towards premolar 3.4 and expansion from the molar 3.6 area. Just like in the first case, the ligament deformation values are rather high and, associated with the existing stresses, they may lead to periodontal ligament tearing in the upper area, especially in the premolar 3.4 area. The loss of this ligament results in overstraining the ligament of premolar 3.5 and finally in the jeopardizing of the whole dental implantation and of the bridgework stability [8, 9]. CONCLUSIONS Finite element analysis was employed to determine the stresses, specific deformations and displacements undergone by the periodontal ligaments considered. The overstress of periodontal ligament under occlusal forces leads implicitly to the tearing of the ligament fibers either in their body, or in its insertion into the dentin and bone. The dental cantilever bridge supported by the abutment premolars 34 and 35 and the cantilever extension for 36 overstrain the ligament of premolar 34, especially when the force acts on the distal extension (3.6), since we may say that this load configuration produces a first degree lever effect with a fulcrum located on the premolar 3.5 root apex. In order to have the same characteristics and functionality of the substitute item that it replaces, a dental cantilever bridge must show very good congruence and stability in the prosthetic environment. 86

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