Introduction

Dental implant treatments became one of the main solutions for the treatment of partial and complete edentulous patients for several years. Implant supported fixed prosthesis is an emerging treatment method with high success rates (1). In certain cases, surgical placement of implant is modified by limitations like poor bone quality, minimal bone volume, and anatomical limitations of alveolar bone. Several techniques and procedures have been presented to address these limitations in the past decades. Management of atrophic ridges such as bone augmentation and grafting procedures, surgical displacement of inferior mandibular nerve and using zygoma for implant placement are such examples of these techniques (2-4). However, there are disadvantages such as long healing period for bone augmentation procedures, high costs, and surgical complications related to these grafting procedures (5). Long distal cantilevers on posterior regions could be destructive to bone and implant superstructures (6). Recently, inclined implants have been proposed in rehabilitation of edentulous ridges. Inclining implants toward the distal in the mandible could help avoiding the mental foramen area and prevent nerve damage. In maxilla, this could help avoiding the maxillary sinus and cancel the need for further grafting procedures in sinus cavity. With inclined placement of the implants, conventional size implants could be placed in bone instead of the short length implants to provide maximum contact with cortical bone (7-10). As a result, primary stability of these implants would improve (2, 11-13). The objective of present study was to assess stress distribution in bone under chewing forces in titled longer implants using 3-dimensional finite elements stress analysis method.

Materials and Methods

Three dimensional finite element analysis (FEA) model was used to evaluate stress distribution and stress concentration level in models. Three implants supporting a five-unit restoration were analyzed in straight angle and different inclination degrees. A graphic processing program (Rhinoceros 4.0, McNeel, Seattle, USA and Algor Fempro, Algor Inc., USA) was used to create cortical and cancellous bone models as well as implants and superstructures. Implant and abutment geometries were created for this study. Seven finite element models were designed using 4.0 mm diameter and 8.0 mm long implants. The 8th model was designed using a 4.0 mm diameter and 10.0 mm long implant. Implants were positioned vertically in first model and in other models, implants inclined to distal with a 6 ° increasing angle in each sample compared to previous one. Parallelism between implants was maintained. The implant

Superstructure was made using Chromium-Cobalt (Cr-Co) alloy with a thickness of 0.8 mm and only infrastructure was modeled. Five unit fixed restorations were placed on abutments in virtual environment. Since the entire mandible was not required in the study, the corresponding region was modeled as a box. Each model included approximately 30800 nodes. It was designed to use 8 node elements as much as possible. The calculation of each node displacement verified stress on the structure. The exterior nodes were fixed in all directions as boundary condition. All materials were considered to be isotropic, homogenous, and linearly elastic. Elastic properties of structures used in this study and their Poisson ratios were indicated in the Table I (14-16). An oblique load of 100 N was applied on superstructures with an angle of 45°. The Stress levels were calculated using von Mises stress values. The maximum values were used as a reference.

Table I: Materials and Bone Properties

 Results

When maximum principal stress is applied based on implant inclination, stress concentrations were mostly observed in buccal cortical bone. (Fig. 1, 2) When implants were inclined up to 12 °, stress was observed around implant number 45 and it increased as the angle increased (Fig. 3). At 18 ° angle and higher, stress decreased and stress-concentration regions were observed differently. Stress was concentrated in buccal of implant number 45 at 18 ° and 24 ° inclination (Fig. 4, 5), in buccal of the implant number 47 at 30 ° (Fig. 6), and in buccal of the implant number 43 at 37 °. (Fig. 7) In the 10 mm long implant model, stress values decreased at all sites while only stress in buccal of implant number 45 increased at 37 ° angle. (Fig. 8) When minimum principal stress is applied based on implant inclination, stress concentrations were observed mostly in lingual cortical bone. In vertical implants, stress was first concentrated in lingual of implant number 43 and at 6 ° and 12 ° stress was observed in lingual of implant number 45. At 18 ° and the subsequent angles, stress was again concentrated in lingual of implant number 43. Starting from 24 degrees implants, an increase in stress was observed with an increase in the angle. However, at 30 degrees angles and more, stress levels started to decrease. Comparison of the model with 8 mm implant and the model with 10 mm implant at 37 ° demonstrated that all stress values were reduced in longer implants. When von Mises stress on implants was examined, stress values up to 12 ° angle were concentrated around implant 45. At 18° angles and higher, stress values were concentrated in lingual area of implant 43. In models with angles up to 24 °, an increase in the stress value in proportion to the angle was observed. At 30 degree and higher implants a decrease in stress around implants was observed. Comparison of 8 mm implant model with 10 mm implant model at 37 ° demonstrated that all von Mises stress values decreased when long implant was used. Comparison of stress values for implants demonstrated that inclination of implants and extension of the implant length reduced stress values.