Analysis of Stress Distribution on the Bone around an Implant Placed in the Anterior Maxilla Using Three Different Abutment Angulations by Means of Finite Element Analysis.

AIM: the aim of the study was to study the effect of stress distribution on the bone around an implant using angled abutments by means of finite element analysis in the anterior maxillary region.
MATERIALS AND METHODS: A three-dimensional (3D) model of the of patient's maxilla of right central incisor: tooth, bone, crown, implant, and abutment system were used in this study. The models were designed for three situations with straight abutment, i.e. 0°, 15°, and 20° angled abutment. The load of 178N was applied on the cingulum area of the prosthesis at an angle of 130° in relationship with the long axis of the implant. After applying the static loads on each model, the stress generated in the bone and the implant was recorded. The results will be analyzed by analysis of variance test.
RESULTS: The cortical and cancellous on Mises stresses in 20° abutment model were found to be maximum as compared to 15° abutment followed by 0° abutment. The stress was concentrated in the implant-abutment joint area. The overall stresses in 20° abutment model were found to more than 15° abutment followed by 0° abutment. The magnitude of stresses increased as the angulations increased.
CONCLUSION: From the conclusions of this study, the stress is more multiplied in angled abutments, hence care needs to be taken when restoring implants using angled abutments, especially in patients with heavy masticatory load or when planning for cantilevering of restorations in these angled implant restoration. Copyright:

INTRODUCTION

Clinicians have long sought to provide their patients with an artificial analog of the natural teeth and a wide variety of materials and techniques have been used for this. However, it has not been possible to replicate the periodontal tissues, and alternative strategies have therefore been adopted. Dental implants have been proven to be an effective way of restoring the masticatory ability of completely or partially edentulous patients.[1] A dental implant restores a lost tooth so that it looks, feels, fits, and functions like a natural tooth. These have been based on the principles of creating and maintaining an interface between the implant and the surrounding bone, which is capable of load transmission, associated with healthy adjacent tissues, predictable in outcome, and with a high success rate.[2]

It was observed that implants placed in the anterior mandible of humans had a high success rate. Lower survival rates were observed for implants placed in the anterior maxilla.[3] When teeth are lost in the anterior maxilla, the pattern of bone loss cannot be accurately predicted. This change in bone morphology often dictates the placement of implants with the long axis in different and exaggerated angulations to satisfy space and esthetic needs.[3] When clinicians deal with undesirable implant orientation caused by poor bone conditions, dental implants are not placed parallel to adjacent teeth or contiguous implants, the clinician can use angled abutments in order to achieve proper restorative contours.[4]

The use of angled abutments facilitates paralleling nonaligned implants, thereby making prosthesis fabrication easier. These abutments also can aid the clinician in avoiding anatomical structures when placing the implants.[4] However, increased stress on bone has been thought to be associated with the use of angled abutments.

Finite element analysis (FEA) is a computer simulation technique used to assess stresses and strains placed on solid objects which involves a numerical approximation of physical properties.[4] Vertical and transverse loads from mastication induce axial forces and bending moments and result in stress gradients in the implant as well as in the bone. FEA allows researchers to predict stress distribution in the contact area of the implants with cortical bone and around the apex of the implants in trabecular bone.[5] It also helps in evaluating a qualitative solution of the interaction between prosthesis, implant, and surrounding bone. The finite element method (FEM) can present possible changes in shape, based on stress and strain values when the specific material properties, the forces applied, and the boundary conditions are predetermined.[6] Hence, effect of stress distribution on the bone around an implant using angled abutments by means of FEA in the anterior maxilla is the ultimate aim of the study.

MATERIALS AND METHODS

A three-dimensional (3D) model of the of patient's maxilla of right central incisor: tooth, bone, crown, implant, and abutment system were used in this study. The analytic model used in this study was developed using reverse engineering technique, i.e. extracting the dimensional details of the physical parts using precision measuring instruments using ANSYS 12.1.0 software (ANSYS, Canonsburg, PA, USA). In the present study, the cortical bone for the maxilla was modeled as a 0.5-mm layer, which better represents a clinical situation. A three-dimensional finite element models will be designed using ANSYS 12.1 software. E-max crowns were used in the study along with the zirconia abutment as they are esthetically superior than the other restorative options in the anterior region. Most commonly used implant diameter 3.5 mm was used. The models were designed for three situations with straight abutment, i.e. 0°, 15°, and 20° angled abutment. [Figures 1, 2 and 3] A conical, square thread implant was created. When the geometry of model was complete, a specialized mesh generation procedure was used to discretize the model. The 3D finite element model corresponding to the geometric model was meshed using hypermesh software (ANSYS 12.1 version software). The load of 178N was applied on the cingulum area of the prosthesis at an angle of 130° in relationship with the long axis of the implant. After applying the static loads on each model, the stress generated in the bone, and the implant was recorded. The results will be analyzed by analysis of variance test.

Figure 1

The stress distribution after application of load in 0° abutment model in the central incisor region

Figure 2

The stress distribution after application of load in 15° abutment model in the central incisor region

Figure 3

The stress distribution after application of load in 20° abutment model in the central incisor region

RESULTS

Stress distribution pattern generated in the FE models comes in numerical values and in color coding. Maximum values of Von Mises stress is denoted by red color and minimum value by blue color. In between the values are represented by bluish green, green, greenish yellow, and yellowish red in the ascending order of stress distribution [Graph 1]. The color plots obtained were studied and the maximum Von Mises stresses were noted and tabulated for each condition [Table 1].

Graph 1

Stress on abutment, implant cortical bone and cancellous bone with abutment having 0°, 15°, and 20° angulation

Table 1

The values of von Mises stress in implant, abutment, cortical, and cancellous bone in 0° abutment, 15° abutment, 20° abutment after application of loads

Part	Straight case	15° case	20° case
Abutment	42.54	46.84	76.28
Implant	51.4	63.77	91.83
Cortical bone	27.11	37.18	48.2
Cancellous bone	8.39	10.13	14.85

DISCUSSION

The long-term success of prosthesis depends on the osseointegrated fixtures, anatomical conditions of the implant site, the surgical technique, osseointegration, and the distribution of forces during the function. Diameter, length, position, and number of implants have significant effects in the stress distribution on the prosthetic superstructure, on the implant components, and on the supporting bone.[2]

The major factor leading to late failure of implant is inappropriate selection of implants and lack of understanding of biomechanical concepts. It was observed that implants placed in the anterior mandible of humans had a high success rate. Lower survival rates were observed for implants placed in the anterior maxilla. When teeth are lost in the anterior maxilla, the pattern of bone loss cannot be accurately predicted. This change in bone morphology often dictates placement of implants with the long axis in different and exaggerated angulations to satisfy space and esthetic needs.[3] Gelb and Lazzara stated in their study that the use of preangulated abutments should be the treatment of choice when anatomic limitations preclude the axial placement of an implant in using the angled abutment.[7]

In the current study, the most frequently used implant and angled abutments in the clinical practice in anterior maxilla are used to study the stress distribution in bone around these implants. Thus, the implants of different angled abutments are compared in this FEA study.

Today, FEA enjoys a position of predominance among the computational methods to occur in this century. Most of the practical problems are solved by numerical methods, which provide approximate but acceptable solutions. With the advent of computers, one of the most powerful techniques that has emerged from the realm of engineering analysis is the FEM. The method can be used for the analysis of structures that are solid of complex shapes and complicated boundary conditions. The basic concept of FEA is differentiation of a structure into finite number of elements, connected at finite number of points called nodes. The material properties and the governing relationships are considered over these elements and expressed in terms of nodal displacement at nodes. A structure is broken down into many small simple blocks or elements that can be described with a relatively simple set of equations. Just as the set of elements would be joined together to build the whole structure, the equations describing the behaviors of the individual elements are joined into an extremely large set of equations that describe the behavior of the whole structure.[8]

A section of patient's maxilla of right central incisor (a three-dimensional model of bone) region was generated. Several authors have reviewed the literature on FE analysis of dental implants[8910] and stressed the importance of modeling bone as an anisotropic material. Clelland et al.[11] created a 3D model of the anterior maxilla with a 1.5- and 3.0-mm-thick cortical layer with isotropic characteristics, which does not represent type 3 bone with a thin cortical layer. In the present study, the cortical bone for the maxilla was modeled as a 0.5-mm layer, which better represents a clinical situation.

The interincisal mean angle used in this study was 130°. Schaeffer assumed that the force applied to the palatal surface of the maxillary prosthesis would be parallel to the long axis of the mandibular incisor. Therefore, the load applied on the cingulum area of the prosthesis had an angle of 130° in relationship with the long axis of the implant.[12] The magnitude of the force of 178 N used was also within the range of mean values reported in the literature.[31314] After applying the static loads on each model, the stress generated in the bone and the implant was recorded.

Papavasiliou et al.[15] also concluded that the stresses were concentrated more toward cortical bone in his study. When the bite force is applied to an angled abutment, as abutment angle increases, the stress generated in bone increased and appeared to have been concentrated in certain areas, mainly the cortical area of the crestal bone. Compared to the stress transferred through a 0° abutment (straight), the Von Mises stress resultant from implant loading were 12% and 18% greater for abutment angles of 15° and 25°, respectively. Vertical abutment model showed that the load was evenly distributed on the buccal and lingual surfaces of implant, whereas in an angled abutment model, the loads did not travel along the long axis of implant.[3] Brosh et al.[16] attached strain gauges to implant surfaces and embedded these implants in photoelastic acrylic resins to investigate stress transfer from angled abutments. It concluded that only an 11% increase in shear stress detected when the abutment angulations were increased from 0° to 25°, very much similar to the findings of the present study and also the findings of other relevant studies in the literature.[2345614]

Hsu et al.[1] in his study concluded that, when off-axis loading is applied to an implant, the magnitude of the stress will be increased three times or more.

In the present study, the resulting stresses were mainly concentrated within the cortical bone on the crest of the alveolar ridge, similar to the findings of Kao et al.[17] It is important to mention that similar conclusions of several studies regarding the location of the maximum stresses exactly on the cortical layer[15161819202122] may be closely related to the material properties assigned to the bone model.[8]

Models of the mandible or maxilla that have a cancellous core with a Young's modulus an order of magnitude less than that of the cortical layer may behave as if the implants were only supported by cortical bone.[6789101123242526272829] Cortical bone would absorb most of the stresses, while the reaction forces of the cancellous bone upon the loaded implant would be underestimated. By assigning to the cortical and cancellous bones properties that are not so different, it can be seen that the highest strains observed on the bone are at the coronal third of the implant-bone interface. However, they are not limited to the cortical layer; they are also shared with the cancellous core.[3]

In the present study, all three models have shown that the maximum stress was at the bone-implant interface at the level of cortical bone. This is due to stress concentration at interface as there was a significant difference in modulus of adjacent materials. It was also found that there was less stress in the apical region of the implant and that can be explained in a way that the amount of bone directly in contact with the apical surface of a loaded implant was much less than that of remaining part of the implant, and hence the apical region of the implant within the cancellous bone had little stress-induced stimulation.[23]

Clelland et al.[11] loaded differently angled abutments in the anterior maxilla. The authors applied a masticatory force along the long axis of the abutments tested. That force direction would simulate a clinical situation, in which the incisors are in an edge-to-edge position. Since masticatory forces decrease significantly when the mandible is in an eccentric position and there is complete disocclusion of all molars, the magnitude of the results may be overestimated.

Applying the load near the cingulum area with a buccal apical direction simulates a clinical situation, in which the mandibular incisors close on the lingual surfaces of the maxillary incisors, with the mandible near centric occlusion. It also creates a better condition to assess the behavior of implants in the anterior maxilla. Although the load was directed at a greater angle with relation to the direction of the implant, it was observed that most deformation on the bone was still within the physiologic limits proposed in the literature.[30]

The result of FE simulations revealed that the von Mises stress mainly concentrated on the buccal and lingual surfaces of cortical bone around the implant neck with oblique loading at an angle of 130°. Results reported by some literature based on complete osseointegration on the implant–bone interface are very similar to our data. Several authors using FEA have found that the highest risk of bone resorption occurs in the neck region of an implant.[2431] The stresses were concentrated in the cortical bone around the implant neck, which is probably due to the fact that the elastic modulus of cortical bone is higher than cancellous bone and that cortical bone is much stronger and more resistant to deformation.,[2432] The von Mises strain mainly concentrated on the buccal surface of cancellous bone adjacent to the implant neck with oblique loading at an angle of 130°. Nonaxial loading has often been related to marginal bone loss, failure of osseointegration, failure of the implant and/or the prosthetic components, and failure of the cement seal on the natural tooth if connected to natural teeth.[2533] It has long been recognized that both implant and bone should be stressed within a certain range for physiological homeostasis. Overload will cause bone resorption or failure of the implant, whereas bone underloading may lead to disuse atrophy and subsequent bone loss. Usually, the stress levels that actually cause biological response, such as resorption and remodeling of the bone, are not comprehensively known. Therefore, the data of stress provided from the FEA need substantiation by clinical research.[24]

Hence, by increasing the angle of the abutment of the implant selected, stresses are increasing in concentration. The data from Himmlová et al.,[33] who computed values of von Mises stress at the implant–bone interface for all variations in implant diameter using FEA, were the same as our except for where supposed implant and bone bonded completely. Maximum amount of stress concentration was observed in the cortical bone irrespective of the angulation of implant. Cortical bone plays a major role in the dissipation of the stress. There was favorable distribution of stress and strain pattern during oblique loading. Therefore, successful long-term results will be achieved in the selection of 130° angled abutment and 3.5 mm diameter implant in favorable clinical situations.

Within the limitations of the present study and on the basis of results obtained, it can be summarized that the cortical and cancellous von Mises stresses in 20° abutment model were found to be maximum as compared to 15° abutment followed by 0° abutment. The stress was concentrated in the cervical region of bone. The implant von Mises stresses in 20° abutment model were found to be maximum as compared to 15° abutment followed by 0° abutment. The stress was concentrated in the implant-abutment joint area. The overall stresses in 20° abutment model were found to be maximum as compared to 15° abutment followed by 0° abutment. The magnitude of stresses increased as the angulations increased.

CONCLUSION

From the conclusions of this study, the stress is more multiplied in angled abutments; hence, care needs to be taken when restoring implants using angled abutments, especially in patients with heavy masticatory load or when planning for cantilevering of restorations in these angled implant restoration.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.