ABSTRACT INTRODUCTION

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1 ABSTRACT Purpose: Evaluation of stress distribution maxilla under parafunctional loading on short dental implant using 3-dimensional (3D) finite element models. Material and method: 3D maxillary and mandibular bone segments were modeled with Bicon Max 2.5 short dental implant (diameter: 4mm; height: 5mm; Bicon, Boston, Mass) were placed in maxilla. Maxillary bone segment consisting of cortical bone thickness = 0.75 mm and type IV cancellous bone. Crown to implant ratio were 2:1 and the forces were at 25 degrees in a bucco-palatinal direction to the implant long axis. 800 N amount of force was applied as a para- functional loading and 300 N for the normal loading. Results: Maximum and minimum principal and Von Mises stresses were calculated. Stress concentration in the maxillary cortical bone under parafunctional loading was almost four times more than under normal loading. Cancellous bone stress concentration was higher by 2 times in the maxilla under parafunctional loading. Von Mises stresses in implants were increased by 2 folds under parafunctional loading. Conclusion: higher risk of dental implant failure under parafunctional loading due to the possibility of bone resorption INTRODUCTION Dental implants are widely used for tooth loss treatment due to the positive long term results which have been represented with osseointegrated titanium units placed in alveolar bone(1). Still implants are prone to failure due to several factors such as excessive trauma, early loading, bone quality, peri-implantitis and overloading (2-5).Overloading such as Bruxism, affects dental implants in two ways; biological and biomechanical. Biological complications are represented in a pathological bone loss around the cervix of dental implant, while biomechanical complications are represented in failure of one or more components of the dental implant itself such as fracture of the implant or abutment (7). It is expected that bruxism, therefore, considered to not being indicated for dental implant treatment even though it's still debatable whether or not bruxism has a direct relation with implant failure (7). Three-dimensional (3D) finite element analysis (FEA) is a numerical method of examining stresses in continuous structures; such as the peri-implant region and dental implant components as it simulates complex geometric shapes and material properties. FEA has been

2 broadly applied to implant dentistry to assess the deformities of alveolar bone as a result of overloading of dental implants, stress, and strain distribution (8). This study aimed to evaluate the stress distribution of cortical and cancellous bone, short dental implant and it s components in the posterior maxilla in 3D finite element models. MATERIALS AND METHODS 3D maxillary bone segments were modeled using the (MSC PATRAN/NASTRAN 2016) The mathematic model consisted of an average of elements and nodes. Young s moduli of cortical bone, cancellous bone, titanium and ceramic were 14.8, 1.10, 105 and 68.9 GPa, respectively. Poisson s ratio of the bone was assumed to be 0.3 for bone, 0.33 for titanium and 0.28 for ceramic. Maxillary alveolar bone segments were 6 mm in length mesiodistally, 12 mm in width buccopalatally, and 8 mm in height vertically. The cortical bone was simulated with thickness of 0.75 mm (Figure 1a). Two forces were applied on the model at 25 degrees in a bucco-palatinal direction of the long axis of the dental implant. First force magnitude was equal to 300N representing a normal biting force, second force was equal to 800N representing a parafunctional loading force (Figure 1b). Bicon Max 2.5 dental implants (diameter: 4 mm; height: 5 mm; Bicon, Boston, Mass) were placed at the center of the segment and assumed to be osseointegrated. According to the manufacturer s recommendations, the implants were inserted 0.5 mm deeper than the crestal level. Crown restoration was simulated to be cemented on the abutment. The von Mises stress distributions of the implant and abutment and maximum (maximum tension) and minimum (maximum compression) principal stresses values for the cortical and cancellous bone were obtained and evaluated.

3 (a) (b) Figure 1: (a) The dimensions of the simulated alveolar bone segments. (b) An oblique occlusal bite force of 300 N and 800 N applied at a 25 degrees inclination to the buccopalatinal axis of the implant RESULTS Von Mises stresses value for the maxillary dental implant under normal loading forces was MPa and under paranormal loading forces was MPa (the Table; Figures 2 and 3). Von mises stresses value for the maxillary abutment under normal loading forces was MPa and under paranormal loading forces was MPa (the Table, Figures 4 and 5) Maximum principal stress (tensile strength) in the cortical bone of maxilla under normal loading value was MPa, whereas under paranormal loading was MPa. While Minimum principal stress (compressive force) value of cortical bone under normal loading was MPa, and MPa under paranormal loading forces. (the Table, Figures 6-8) In the cancellous bone maximum principal stress were 3.81 MPa under normal loading, whereas under paranormal loading was 8.15 MPa, whereas the minimum principal stress value was MPa under normal loading, and MPa under paranormal loading forces. (the Table, Figures 9-11)

4 Von Mises Max. Tension Max. Compression 300 N Abutment Implant Cortical bone Cancellous bone N Abutment Implant Cortical bone Cancellous bone Von Mises stress Normal loading Paranormal loading Figure 2. Von Mises stress values under normal loading vs. paranormal loading for maxillary dental implant (a) (b) Figure 3.Mesial view of Von Mises stress distribution under normal loading (a) vs. paranormal loading (b) for maxillary dental implant

5 Von Mises stress Normal loading Paranormal loading Figur 4. Von Mises stress values under normal loading vs. paranormal loading for maxillary dental implant abutment (a) (b) Figure 5. Mesial view of Von Mises stresses values under normal loading (a) and paranormal loading (b) for maxillary dental implant abutment Normal loading max. compression max. tensioion paranormal loading Figure 6. Maximum Compression and Maximum Tension values under normal loading and paranormal loading for maxillary cortical bone

6 (a) (b) Figure 7. Buccal view of Maximum Tension values under normal loading (a) and paranormal loading (b) for maxillary cortical bone segment (a) (b) Figure 8. Buccal view of Maximum Compression values under normal loading (a) and paranormal loading (b) for maxillary cortical bone segment Normal loading 8.15 max. compression max. tensioion paranormal loading Figure 9. Maximum Compression and Maximum Tension values under normal loading and paranormal loading for maxillary cancellous bone

7 (a) (b) Figure 10. Buccal view of Maximum Tension values under normal loading (a) paranormal loading (b) for maxillary cancellous bone segment (a) (b) Figure 11. Buccal view of Maximum Compression values under normal loading (a) and paranormal loading (b) for maxillary cancellous bone segment DISCUSSION The simulation and evaluation of complex geometric structures such as dental implants and alveolar bone have made the three-dimensional finite element analysis a famous and regularly performed technique for the stress distribution analysis (8-10). In this study, a 3D FEA method was used as it offers different loading conditions which are required for biomechanical evaluation of dental implants. The aim was to compare the stress distribution on short dental implants surfaces and the surrounding alveolar cortical and cancellous bone under two different loading conditions and whether it is reasonable to place dental implants in a patient who apply high forces on his dentition having a parafunctional activity such as bruxism.

8 Bruxism has always had harmful effects on patients. Facial muscles and temporomandibular joint pain have always been seen in patients with bruxism (4, 5, 11, 12). Periodontal disease, teeth attrition and loss are among the consequences of bruxism (4,5,12). Sotto Maior et. al. 2014, made a study to evaluate the factors affecting the stress distribution on short dental implant, he tested different factors such as occlusal loading, C/I ratio, retention system and restorative material. They concluded that the most important factor in increased stress concentration is occlusal loading and it is responsible for 70% of the whole stress concentration around the peri-implant bone. (13) Our results indeed showed that cortical bone stress value had the most increase upon overloading and that the highest stresses were extending mainly mesiobuccally. Eposito had done a review in 1998 about factors associated with loss of dental implants and concluded that marginal bone loss have been noted in patients with parafunctional habits(2). In our results we obtained that cortical bone resorption will start at the mesiobuccal side around the neck of the implant if ever happened. Maminskas et. al. have made a systematic review of the finite element studies in 2016, regarding prosthetic influence and biomechanics on peri-implant strain. They reviewed all the parameters affecting stress concentration and found that mixed angulation loading have increased the stress around the neck of the implant. (14) our study observed that the highest maxillary stress was concentrated around the neck mesiobuccally and maybe at risk of failure due to fracture. Same results were seen in the maxillary abutment, where stresses was concentrated mainly in the middle at the level of cortical bone mesiobuccally which might indicate a high risk fracture point. CONCLUSION There is a higher risk of dental implant failure under parafunctional loading due to the possibility of bone resorption around the neck and vertically and because of the stress concentration at the implant surface itself which increase the danger of implant fracture. It might be necessary upon placing dental implants in patients with bruxism to create a space at the occlusal contact areas in maximum intercuspation in order to decrease the forces exerted on the system during parafunctional loading.

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