The effect of implant number and position on the stress behavior of mandibular implant retained overdentures: A three-dimensional finite element analysis.

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The effect of implant number and position on the stress behavior of mandibular implant retained overdentures: A three-dimensional finite element analysis Tolga Topkaya a,n, Murat Yavuz Solmaz b a b

Batman University, Engineering and Architecture Faculty, Mechanical Engineering Department, Turkey Fırat University Engineering Faculty, Mechanical Engineering Department, Turkey

art ic l e i nf o

a b s t r a c t

Article history: Accepted 7 March 2015

The present study evaluated the effects of ball anchor abutment attached to implants with a 4.30 mm diameter and 11 mm insert length on stress distribution in a patient without any remaining teeth in the lower jaw. In the study, the stress analysis was performed for five different configurations (2 with 4 implant-supported and 3 with 2 implant-supported) and three different loading types using ANSYS Workbench software. The stresses measured in the 4 implant-supported models were lower compared to the stresses measured in the 2 implant-supported models. The stresses on the implants intensified on the cervical region of the implants. When the effects of the loading sites on the stress were examined, the loading on the first molar tooth produced the highest stresses on the implants. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Dental implant Removable overdenture Finite element method

1. Introduction The struggle and desire to esthetically restore the lost body parts is as old as the history of humanity. Restoring the function and esthetics of the lost tissues has been the main focus of scientists for centuries. It has been reported that stone, wood, and even animal teeth have been used as the supportive structure in the maxilla (upper jaw) and mandible (lower jaw) (ME, 1995). The improvements have been made to this approach in dentistry, and implants have been developed and introduced into the practice of dentistry to restore lost functions. The materials used in dental implants vary greatly, along with the multiplicity of models. Experimental and mathematical stress analyses are required to select the appropriate geometry and material of dental implants. Generally, the finite element method is used in mathematical analysis. Barbier et al. evaluated axial and non-axial forces around intraosseous implant systems using the finite element method, and showed the need for reducing horizontal loading (Barbier et al., 1998). Lin et al. used functionally graded material (FGM) and titanium as the implant material and evaluated the distribution of stress on the cortical and trabecular bone in a two dimensional mathematical model, in which FGM was found to have provided more homogeneous stress distribution. Their study showed

n Correspondence to: Batman University, Engineering and Architecture Faculty, Mechanical Engineering Department 72100 Batman, TURKEY. E-mail address: [email protected] (T. Topkaya).

that functionally graded material provided better fusion of the implant in the jaw bone and the bone tissue, and faster recovery of the bone tissue (Lin. et al., 2010). Bonnet et al. evaluated biomechanical behaviors of the 4 implant-supported prosthesis according to isotropic and non-isotropic bone characteristics using the finite element method. They constructed the mandible without any remaining teeth and the geometry of the prosthesis using computerized tomography (CT) images. Isotropic and non-isotropic models were compared after the insertion of two vertical and two inclined implants into the mandible, and they found significant differences in terms of stress, strain, and the intensity of strain-energy. They showed that the inclined insertion of the implants created high strain forces (Bonnet et al., 2009). Kleis et al. applied two implant-supported prosthesis by individual alignment or using ball anchor implants, and the connectors were compared after one year. They concluded that individual alignment required higher maintenance when compared to ball anchor implants (Kleis et al., 2010). Sadowsky et al. inserted bar-supported overdentures to the lower jaw, and using a photoelastic method, they experimentally evaluated the difference in stress distribution caused by the use of two or three implant supports (Sadowsky and Caputo, 2004). Barao et al. evaluated the effects of different designs in implant-supported overdentures and implant fixed prosthesis on the stress distribution using finite element method (Barao et al., 2013). Liu et al. investigated the effects of the number of implants used in implant-supported overdentures in three different loading conditions using the finite element method (Liu et al., 2013). Daas et al. investigated the effects of fixed or removable

http://dx.doi.org/10.1016/j.jbiomech.2015.03.006 0021-9290/& 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: Topkaya, T., Solmaz, M.Y., The effect of implant number and position on the stress behavior of mandibular implant retained overdentures: A.... Journal of Biomechanics (2015), http://dx.doi.org/10.1016/j.jbiomech.2015.03.006i

T. Topkaya, M.Y. Solmaz / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Fig. 1. Model of implant system.

tomographic (CT) images were constructed using Solidworks 2012 software, and a stress analysis was performed using the ANSYS 14.0 Workbench program.

Table 1 Material properties. Material

Elastic modulus (MPa)

Ti6Al4V (Bonnet et al. 2009, Sevimay et al. 2005, 110,000 Zhu et al. 2007) Prosthesis (Bonnet et al. 2009) 2940 Cortical bone (Sevimay et al. 2005, Kitagawa et al. 13,700 2005) Trabecular bone (Sevimay et al. 2005, Kitagawa 1370 et al. 2005) O-ring (Bonnet et al. 2009) 15 Mucosa (Bonnet et al. 2009, Fatalla et al. 2012, 1 Sevimay et al. 2005]

Poissons ratio 2. Materials and methods 0.35 0.3 0.3 0.3 0.4 0.3

Table 2 Maximum von Mises stress (MPa) in different components of Nobel Active Implant System. Loading angle

Crown Abutment Screw Implant Cortical bone Cancellous bone

(Chang et al., 2013)

Present analysis

0o

30o

0o

30o

7.5512 13.8777 12.1696 23.6874 2.3523 2.5809

84.5432 274.2010 123.0210 254.6440 20.9631 7.9157

7.8326 16.242 11.6205 23.237 2.022 0.15524

86.235 271.065 123.9852 249.02 18.6324 1.8632

connections between the abutment and prosthesis in two implantsupported removable prostheses on the stress distribution using the three-dimensional finite element method (Daas et al., 2008). Many studies have been conducted on the number of implants to be used in implant-supported prostheses. In treatment planning, the number of implants to support the prosthesis is the most important question to be answered. Burns reported that two or four implants were preferred in implant-supported removable prostheses, and the minimum number was two for the implants, and increasing the number of implants shifted the support from mucosal surfaces to the implants (Burns, 2004). It is possible that the prosthesis is supported by the implants or there are models in which remaining teeth and implants are used to support the prosthesis (Dalkiz et al., 2002). The present study evaluated the effects of the number and configuration of the implants in lower jaw overdentures supported by ball anchor connectors on the distribution of stress on the bone-implant system assembly using finite element method. A design model of an overdenture-implant system and lower jaw from computerized

The present study evaluated the effects of the number and configuration of the implants inserted to the lower jaw without any remaining teeth to support lower overdenture on the stress distribution on the lower jaw and implant system assembly using the finite element method. Clinical applications show that osseointegration between the implant and jaw bone takes a period of 3–6 months (Bozkaya and Müftü, 2003). The present study assumed that osseointegration between the implant and the bone has been completed. This assumption facilitates the design of the model and shortens the time to the solution; however, it may cause differences to occur between the analyses and clinical applications. The present study differs from the previous studies since it evaluates not only the effects of the number of configuration of the implants, but also the contribution of loading parts on the stress distribution in each model. The solid structures of the implant and human lower jaw bone constructed by the SolidWorks 2012 program were transferred to the ANSYS Workbench program for analysis with the finite element method. Computerized tomographic images were used for the construction of the mandible and overdenture. Nobel Replace model implants (Nobel Biocare) were used. Implants with different sizes and diameters can be used depending on the characteristics of the jaw (Topkaya et al., 2013). The present study used implants with a 4.30 mm diameter and 11 mm insert length. The implant system and overdenture were placed on the jaw bone model that was created using CT images. Fig. 1 shows a section from the implant insertion site in the incisor teeth in the two implant-supported model. Different configurations can be used for the insertion of implants in implantsupported prostheses. The two or four implant-supported models are the most commonly preferred configurations. The configurations used in the present study are presented below. The materials and fabric used in the study show different mechanical and physical characteristics. These materials were considered isotropic and homogeneous, and the elasticity modulus and Poisson's ratios were acquired from the literature. Elasticity modules and Poisson's ratios of the materials are presented in Table 1.

2.1. Method validation For validating the method Chang et al. model was solved for same material properties and boundary conditions (Chang et al., 2013). Results show that used model has good agreement with reference investigation. Maximum von Mises stress values of reference model and current analysis given in Table 2. Stress distributions of Nobel Active implant for two different loading conditions is given in Fig. 2.

2.2. Boundary conditions and loading During mastication, the jaw muscles exert different pressures on different regions (Daas et al., 2008). While defining the boundary conditions, varying loading pressures were applied to three different loading sites. The loading sites and loading pressures were obtained from the study of Daas et al. The jawbone was considered motionless and the cortical bone was immobilized. The loading sites and load levels are given below. (Figs. 3 and 4)



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Fig. 2. Stress distributions of dental implant according to Chang et al. boundary conditions (a) vertical loading (b) oblique loading.

3. Results The stresses on the cortical bone for the constructed models in three different loading conditions are presented in Fig. 5. The figures show that the stresses are higher in the cortical bone where the implant was inserted and maximum stress was observed in this location. In models where loading was applied to the first molar teeth, the highest von Mises stress was measured as 131.23 MPa in the 4CPM model, and the lowest von Mises stress was measured as 41.891 in the 2I model. In models where loading was applied to the left canine tooth, maximum stress was measured as 12.898 MPa in the 2C model, and minimum stress was measured as 4.185 MPa in the 4IPM model. In models where loading was applied to the incisor teeth, maximum stress was measured as 13.618 MPa in the 2PM model, and minimum stress was measured as 3.4894 MPa in the 2I model. Maximum von Mises stress distribution on the cortical bone is presented in Fig. 5 for the three different loading conditions. Table 3 indicates stress distribution in the cortical and trabecular bones in three different loading conditions for each model. The stress distributions around the implants are presented in Fig. 6 for models showing the maximum and minimum stresses. When the results for all models are evaluated in tandem, the lowest stress was measured as 0.10722 MPa in the 2C model on the left implant where the loading was applied to the first molar tooth. The highest stress was measured as 399.68 MPa in the 4CPM model below the second premolar tooth where the loading was applied to the first molar tooth. Fig. 6 indicates how the loading side affected the stress around the implants in each model. As seen in the figure, the stresses measured in all models were close to each other around all implants in a given model when the loading was applied on the incisor teeth. The loading on the canine teeth and first molar teeth produced higher stress around the load-bearing implants. The loading on the first molar teeth produced the highest stresses around the implants in the left side. (Fig. 7).

4. Discussion The primary goal of the present study was to determine the optimum number and configuration of the implants to support

overdenture in different loading conditions in patients without any remaining teeth. The analyses showed that the loading site was the most important parameter affecting the stress distribution in the whole system. It was seen that the loading on incisor teeth and canine tooth produced lower stress on the cortical bone in the 4 implant-supported models (4IPM, 4CPM) compared to the 2 implantsupported models (2I, 2C, 2PM), whereas loading on the first molar teeth produced the highest stress in 4 implant-supported models. In the evaluation of the stresses on the dental implants, the highest stresses were observed on the models in which loading was applied to the first molar tooth. Increasing the number of implants enabled more even stress distribution on the implants. Ogawa et al., Şahin et al. and Duyck et al. found similar results in their investigations (Ogawa et al., 2010; Sahin et al., 2002; Duyck et al., 2000). In literature researchers reported that increasing implant number decreases stress values on implant as reported present study. Posterior region is most affected region from chewing forces on Stomagnetic System and molar teeth are most important teeth for chewing. Rangert et al. investigated Nobel Replace implant supported prothesis and they reported that fracture rate of implant is 14%. Their investigation showed importance of molar teeth on chewing (Rangert et al., 1995). The stresses around the implants were found to be higher than the stresses measured on the jawbone. This could be explained by the elasticity module of the Ti6Al4V implant material being 8-fold higher than the cortical bone layer and 80-fold higher than the trabecular bone layer (Merdji et al., 2010; Esmail et al., 2010). As seen in Table 2, the evaluation of the maximum von Mises stresses on the cortical and trabecular bone layers revealed that maximum von Mises stresses on the cortical bone layers were much higher than the von Mises stresses on the trabecular bone layer. This finding indicates that cortical bone layer provides much higher support to the implant than the trabecular layer provides. Because of the contact of close friend to the cortical interface of bone and implant, the loading applied to the implant is directly transmitted to the cortical bone, which explains the clinical marginal bone loss around the implants (Kurniawan et al., 2012; Merdji et al., 2010.; Bilhan et al., 2015). In all models, the stress on the dental implant was higher on the cervical region of the implant. This finding is related to the implant being supported by the cortical bone layer and the cervical region of



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Fig. 3. Images of the mandible, prosthesis placement after insertion of the implants, (a)- The model with the insertion of two implants in lateral incisor site (2I), (b)- The model with the insertion of two implants in the canine teeth site (2C), (c)- The model with the insertion of two implants in the first premolar site (2PM), (d)- The model with the insertion of four implants in the lateral incisor and first premolar sites (4IPM), (e)- The model with the insertion of four implants in the canine teeth and second premolar site (4CPM).

the implant being seated in the cortical bone layer. Esmail et al. explained this finding in a study conducted in 2010.

5. Conclusion The present study evaluated stress distributions on the dental implant-supported removable prostheses in 2 or 4 implant-supported models in different configurations. The results of the present study are discussed below.

1. In all models, loading on the first molar tooth produced the highest stress on the implant. 2. The stresses in 4 implant-supported models were lower than the stresses in the 2 implant-supported models in all loading conditions. This can be explained by the fact that the stresses are shared by the four implants. 3. In the 2 implant-supported models, the stresses in the 2PM model in which the implants were inserted in the first molar sites, were lower compared to the other 2 implant-supported models. 4. The stresses on the implants intensified on the cervical region of the implants.



Fig. 4. Boundary conditions (a) 40 N to incisors, (b) 55 N to canine, (c) 100 N to first molar (Daas et al. 2008).

Fig. 5. Maximum von Mises stresses for three different loading type (a-2PM, b-2C and c-4CPM).


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Table 3 Maximum von Mises stresses that occur on bone (MPa). Model

2PM 2C 2I 4IPM 4CPM

Force to ıncisors

Force to canine

Force to molar

Cortical bone

Cancellous bone

Cortical bone

Cancellous bone

Cortical bone

Cancellous bone

10.54 12.898 11.683 4.185 9.134

0.12238 0.083616 0.14562 0.042721 0.099374

13.618 9.3435 3.4894 3.6631 6.974

0.094826 0.096931 0.038917 0.031462 0.11733

50.191 69.346 41.891 75.273 131.23

0.51996 0.77637 0.54929 0.39265 0.41277

Fig. 6. von Mises stress distribution of implants (a. 4CPM and b. 2C).

5. The highest stresses were observed on the cortical bone layer. 6. The most critical loading site in 2 and 4 implant-supported models was first molar tooth. This finding was explained by the fact that the loading stress is primarily counteracted by the load-bearing

implant and transferred to the other implants after attenuation. The stress values observed in the implants support this notion. 7. This work shows that for moderate levels of the compact bone is not overloaded by any one of the implant systems investigated here.



7

Fig. 7. The stress values of implants (a: two implant used models and b: four implant used models).

Conflict of interest statement All authors declare no financial and personal conflict of interest in this study.

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