The aim of this study was to evaluate the stress distribution of the short dental implants and bone-to-implant contact ratios in the posterior maxilla using 3-dimensional (3D) finite element models. Two different 3D maxillary posterior bone segments were modeled. Group 1 was composed of a bone segment consisting of cortical bone and type IV cancellous bone with 100% bone-to-implant contact. Group 2 was composed of a bone segment consisting of cortical bone and type IV cancellous bone including spherical bone design and homogenous tubular hollow spaced structures with 30% spherical porosities and 70% bone-to-implant contact ratio. Four-millimeter-diameter and 5-mm-height dental implants were assumed to be osseointegrated and placed at the center of the segments. Lateral occlusal bite force (300 N) was applied at a 25° inclination to the implants long axis. The maximum von Mises stresses in cortical and cancellous bones and implant-abutment complex were calculated. The von Mises stress values on the implants and the cancellous bone around the implants of the 70% bone-to-implant contact group were almost 3 times higher compared with the values of the 100% bone-to-implant contact group. For clinical reality, use of the 70% model for finite element analysis simulation of the posterior maxilla region better represents real alveolar bone and the increased stress and strain distributions evaluated on the cortical and cancellous bone around the dental implants.
Dental implants are widely used for complete and partial edentulism owing to reliable functional and esthetic results.1–4 The success of dental implants is dependent on factors such as bone quality and quantity, implant design, loading type, the load-bearing properties of the implant structure, nature of implant-bone interface, and surgical technique.5,6 Among these, the quality and quantity of alveolar bone are a prognostic factor for the efficacy of implant-supported dental restorations because they affect the implant-bone interface. The thickness of the crestal bone and density of the cancellous bone affect bone strains around implants. As the posterior maxilla is composed of type III and IV bone,5,7 the bone-implant interface area is reduced, leading to crestal bone loss under mechanical loading. Type IV bone is composed of a thin layer of cortical bone with low-density and poor-strength cancellous bone as classified by Lekholm et al.8 The presence of the maxillary sinus also limits the use of long implants, reducing the osseointegration of the implant to the bone, which diminishes stability.9
Three-dimensional (3D) finite element analysis (FEA) is a theoretical technique performed for the analysis of stress distributions that is advantageous over other biomechanical methods as it simulates complex geometric shapes and material properties. FEA has been widely applied to implant dentistry to evaluate the deformities of alveolar bone as a result of mechanical overloading of dental implants, stress, and strain distributions. Although FEA studies performed to date have applied 100% bone-to-implant contact ratio, this is unlikely to reflect the clinical reality. Geometric parameters of the cancellous bone have been estimated to include ~30% porosities including spherical and tubular hollow spaces at the posterior maxilla region, therefore reducing bone-to-implant contact ratio.9,10
This study aimed to evaluate the stress distribution of the short dental implant and different bone-to-implant contact ratios in the posterior maxilla, which has a spherical bone design of porous and cellular structure in 3D finite element models.
Materials and Methods
Two different 3D maxillary posterior bone segments were modeled using the MSC MENTAT 2010 Software (MSC Corporation, Santa Ana, Calif). The mathematic model consisted of an average of 650 000 elements and 120 000 nodes. Young's moduli of cortical bone, cancellous bone, and titanium were 14.8, 1.10, and 105 GPa, respectively. Poisson's ratio of the bone was assumed to be 0.3 for bone and 0.33 for titanium.
In each model, the alveolar bone segments were 6 mm in length mesiodistally, 6 mm in width buccopalatinally, and 7 mm in height vertically. The cortical bone was simulated with thickness of 0.75 mm (Figure 1a). Two groups were created to make the mechanical loading experiment. Group 1 was composed of a bone segment consisting of cortical bone and type IV cancellous bone with 100% bone-to-implant contact. Group 2 was analoged as a bone segment consisting of cortical bone and type IV cancellous bone including homogenous tubular hollow-spaced structures with 30% spherical porosities and 70% bone-to-implant contact ratio.
Using 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 instructions, the implants were inserted 0.5 mm deeper than the crestal level. The models were resorted with a crown cemented on the abutment. An oblique occlusal bite force of 300 N was applied at a 25° inclination to the bucco-palatinal axis of the implants (Figure 1b).
The von Mises stress distributions of the implants and maximum (maximum tension) and minimum (maximum compression) principal stresses values for the cortical and cancellous bone were obtained and evaluated.
For the cortical bone-implant interface on the side of the stress concentration under oblique load, the von Mises stress values were 1.83 and 1.90 MPa for groups 1 and 2, respectively. The maximum principal stress values were 2.09 and 2.11 MPa and the minimum principal stress values were −2.27 and −2.37 MPa for groups 1 and 2, respectively (the Table: Figures 3 through 5).
For cancellous bone, the von Mises stress, maximum principal stress, and minimum principal stress values of groups 1 and 2 were estimated as 0.35 and 0.88 MPa, 0.31 and 0.90 MPa, and −0.34 and −0.85 MPa, respectively (the Table; Figures 3 through 6).
The von Mises stress values on the implants and the cancellous bone around the implants of the 70% bone-to-implant contact group were almost 3 times higher compared with the values of the 100% bone-to-implant contact group (the Table).
Three-dimensional FEA is a well-known and frequently performed method for the analysis of stress distributions that is advantageous over other biomechanical methods as it allows the simulation and evaluation of complex geometric structures, such as dental implant systems and alveolar bone.10,11 There are many variables that need to be considered to achieve reliable FEA results, including type of loading, material properties of the implant and prosthesis, implant geometry, length and diameter, implant surface structure, implant-bone interface, and quality and quantity of the bone.12,13
In our study, a 3D FEA approach was used as it provides the availability of the various different border and loading conditions required for biomechanical evaluation of implant success rate.14 As a load-bearing device that ultimately transfers loads to the peri-implant bone, the long-term success of an implant depends on the stability of the bone-to-implant contact and the stress distributions transferred from the implant to the surrounding cortical and cancellous bones.2,15
The evaluation of FEA is another criteria for correct results. The von Mises stress measure is accepted as the fracture criterion for metal materials based on elastic mechanics.16 Therefore, in our study von Mises stress values are used for evaluation and comparison of the 2 groups. For the alveolar bone, the maxillary posterior region can be considered as the most problematic type III and IV cancellous bone due to a lack of alveolar height caused by the anatomic limitations of the maxillary sinus. Here we test the potential use of short dental implants at the maxillary posterior region, which has type III and IV bone and occasionally type II alveolar bone as an alternative strategy to the long (10 mm) dental implants typically used for this type of bone and region.17
Most the finite element studies performed to date have assumed 100% implant-bone contact, with the model structures assumed to be homogenous, isotropic, and linear elastic.18,19 Although the trabecular bone has been assumed to have a solid pattern inside the inner cortical shell,13 these living tissues are in fact porous, meaning that implants could never be perfectly bonded to the bone.20,21 Sagirkaya et al22 evaluated the histomorphometric bone-to-implant contact of dental implants in humans and concluded that the posterior maxilla had the worst bone-implant contact compared with other quadrants of the jaws. Roberts et al23 found that clinically successful implants had less than half of the intraosseous interface in direct contact with bone, whereas Geng et al13 stated that the lowest levels of cortical osseointegration (<25%) are observed in the posterior maxilla. As we have simulated the posterior maxilla for evaluation of the stress distribution, not only Young's modulus (1.10 GPa, the typical value applied to type IV cancellous bone) but also the 70% bone-to-implant contact ratio could be assessed, allowing comparison between the 2 models because the bone implant contact was found around 70% in different previously reported articles.24–26
The length and diameter of implants on type IV bone under mechanical loading are important determinants of implant success. Li et al5 performed an FEA of implants and concluded that implant diameter >4.0 mm and length >9.0 mm were optimal for a screwed implant in type IV bone. Using the same model, Kong et al27 found that increasing implant diameter could reduce the values of bone strains on type IV bone. The thickness of the crestal bone and density of the cancellous bone affect the bone strains around implants. Although most studies have suggested the use of longer and wider implants,28 clinicians are not always able to insert the optimal size implant because of limitations caused by the maxillary sinus. In the present study, we assessed the potential of short implants manufactured as an alternative to the sinus lifting operation at this part of the jaw.1,29,30
To achieve reliable results from FEA models, axial loads and horizontal forces must be combined at an oblique occlusal force.31 The load delivery angle for previous studies vary from an 11° to 45° inclination to the long axis of implants,1,2,5,19,28,32 so in our study, the average of the other studies (25°) was taken as the inclination degree to the implant long axis. Papavasiliou et al33 performed a detailed FEA analysis of various load types applied to implants and concluded that oblique applied stresses elevated the interfacial stresses by 5–20 times and that these oblique forces highly resolved at facial and lingual parts of the bone. The applied loading results were similar to those obtained in our study, in which 300-N oblique occlusal bite force was applied at a 25° inclination to the bucco-palatinal axis of the implants, and for ideal 70% implant-to-bone contact models, stress values were higher at the cortical bone, compared with 100% implant-to-bone contact models. Moreover, stress values were almost 3 times higher at the cancellous part of the 70% bone model, and the high stresses were concentrated within the buccal and palatinal parts of the peri-implant bone.
Performing a model of 70% bone-to-implant contact surface is more relevant than the currently applied 100% value. Use of the 70% model for FEA simulation of the posterior maxilla region better represents real alveolar bone and the increased stress and strain distributions evaluated on the cortical and cancellous bone around the dental implants. Our findings suggest clinicians must be careful when applying a short dental implant to the posterior maxilla. Furthermore, this model can be used for the analysis of stress distributions, but FEA is truly a theoretical analysis and may not apply to a particular clinical situation.