Effect of Anterior Implant Position on Biomechanical Performance in the Maxillary All-on-Four Treatment: A 3-D Finite Element Analysis

With implant treatment having achieved successful results in providing function and aesthetics, implant-supported prostheses are now being applied as an alternative to traditional prostheses, and immediate loading procedures for edentulous jaws have become popular among clinicians and patients.^1–3  High survival rates and a low incidence of complications indicate the success of implant treatment regardless of the type of loading.^4,5 

In totally edentulous patients, excessive bone resorption is usually seen in the alveolar ridge. This resorption may occur due to physiological or pathological factors.⁶  The treatment of edentulous patients with implant-supported prostheses is complicated because of their poor bone quality, insufficient residual bone volume, and anatomical limitations in the posterior region.^1,7–9  Graft applications or distraction procedures may be required to increase bone height and width before implant treatment is performed in jaws with atrophy or bone defects.^10,11  These applications require increased treatment time and are costly; furthermore, site complications can be caused in donors and recipients. In addition, due to the low bone density in the maxilla, immediate loading in the maxilla appears to be a greater challenge than in the mandible, and implant anchorage in the edentulous maxilla is limited, especially due to bone resorption in the posterior region of the maxillary arch.⁸  The “all-on-four” technique was developed by Malo et al¹²  to provide a fixed prosthesis for such patients.

The all-on-four concept defines a full arch, with fixed prosthesis supported by 4 implants placed as cornerstones: 2 anteriorly and 2 posteriorly.^7,12  The implants in the anterior region are placed in the incisor area in the lower and upper jaws, and those in the posterior region are placed anterior to the mental foramen in the lower jaw and parallel to the anterior wall of the maxillary sinus in the upper jaw, inclined distally. In addition to achieving a more posterior implant position by distal angling of the implant, stronger implant anchorage can be achieved by making use of the cortical bone of the sinus and nasal fossa wall.^8,13  With this implant arrangement, a greater interimplant distance, shorter cantilever length, and more favorable stress distribution can be achieved.⁸ 

With regard to the all-on-four concept, cantilever length, implant diameter, loading protocols, and positions and angles of the posterior implants have been investigated. However, few studies have evaluated the position of anterior implants in the all-on-four concept.^13–15  The aim of this study was to examine the effect of different anterior implant positions in the all-on-four concept on the stress distribution in cortical and trabecular bone, implants, abutments, screws, and prostheses using finite element analysis (FEA). The null hypothesis of the study is that stress values and stress distribution on the bone, implant, and prosthetic components are not affected by the positions of anterior implants in the all-on-four concept.

In this study, 3 different models were created using the all-on-four concept in a completely edentulous atrophic maxilla (Figure 1).

Model 1: Posterior implants were placed just anterior to the wall of the maxillary sinus and tilted distally about 30° to the occlusal plane; anterior implants were placed axially in the central incisor areas bilaterally.

Model 2: Posterior implants were placed just anterior to the wall of the maxillary sinus and tilted distally about 30° to the occlusal plane; anterior implants were placed axially in the lateral incisor areas bilaterally.

Model 3: Posterior implants were placed just anterior to the wall of the maxillary sinus and tilted distally about 30° to the occlusal plane; anterior implants were placed axially in the canine areas bilaterally.

A 3-dimensional (3D) model of the edentulous maxilla was produced from the clinical computed tomography data of a patient meeting the appropriate criteria. Solid modelling of cortical bone, trabecular bone, and gingival soft tissue were performed using surface modelling techniques in the Solidworks program (Solidworks, Inc, USA) with reference to the maxilla model (Figure 2).

Each implant component (implants, abutments, screws) used in the study was modelled and adapted individually in accordance with its actual dimensions using the Solidworks program. The implants (Straumann bone level tapered [BLT] [Institut Straumann AG, Basel, Switzerland]) used in the study were 10-mm long and 4.1-mm in diameter (Figure 3).

The prosthetic frameworks and crowns were scanned with a 3D optical scanner (Dental Wings 7 Series [Model DW-7-140 / Dental Wings, Inc, Montreal, Quebec, Canada]), and the data obtained were transferred to the Geomagic Design X (3D Systems, Inc) program in “.stl” format. Solid models of the scan images were obtained using various solidification methods. The frameworks were modelled as chrome–cobalt alloy, and the crowns were individually modelled as monolithic zirconia with contact points provided. The prosthetic cantilever length was planned as 10 mm. The prosthetic components, implant components, and bone tissues were merged and adapted using the Solidworks program; cortical bone, trabecular bone, and implant and prosthetic elements were transferred to the model with their actual morphology (Figure 4).

Solid models made in the Solidworks program were transferred to Ansys 18.1 software (ANSYS, Inc, Canonsburg, PA), preserving the 3D coordinates. Elasticity modulus and Poisson ratio values were defined for each of the structures whose mathematical models were created with Ansys 18.1 software (Table 1). In the mathematical models, an average of 5,065,655 nodes and 3,121,298 elements were used. In all models, it was assumed that the implants were fully osseointegrated to the bone and there was a tight connection between the bone and the implants along the entire interface. In addition, all models were accepted as comprising homogeneous, isotropic, and linear elastic materials.

The models were fixed without movement in all directions, and all structures were modelled as tightly bonded. Occlusal forces were applied in the palatal–buccal direction with 45° inclination bilaterally to canines, premolars and first molars at 100 N, 150 N, 150 N, and 200 N, respectively (Figure 5).

Von Mises (vM) stress values were used to evaluate the stresses on the implants, abutments, screws, frameworks, and crowns; in addition, the vM, minimum principal (Pmin), and maximum principal (Pmax) stress values were used in evaluating the stresses in trabecular and cortical bone. For this evaluation, the highest stress values occurring in the structures were considered, and the values were recorded as megapascals (MPa).

Cortical bone exhibited a higher stress concentration than trabecular bone. The stresses were concentrated around the posterior implant socket (Figures 6 and 7). The models showing maximum vM stresses in trabecular bone were, in order, model 2 > model 1 > model 3, and the Pmin and Pmax values were model 1 > model 2 > model 3. In cortical bone, the Pmin values were model 3 > model 1 > model 2, and vM and Pmax were model 1 > model 2 > model 3 (Table 2).

In all models, the maximum vM stresses were concentrated on the neck of the posterior implants; accordingly, more stress was located at the posterior abutment and abutment screws. Maximum vM stresses on abutments was concentrated on the inside grooves, whereas for abutment screws, these stresses were concentrated in the neck area (Figure 8). In all models, the maximum vM stresses on the implant, abutment, and abutment screws were ranked as model 1 > model 3 > model 2 (Table 3).

The vM stresses were concentrated in the anterior abutment region and midline in the frameworks, whereas they were concentrated in the cervical region of the anterior teeth in the crowns (Figure 9). The maximum vM stresses in the framework were ranked as model 1 > model 2 > model 3, and those in the crowns as model 2 > model 3 > model 1 (Table 3).

The maximum vM values in all structures were ranked as trabecular bone < screws < cortical bone < implants < abutments < frameworks < crowns (Table 3).

Functional and parafunctional forces in the stomatognathic system cause stress in bone, teeth, soft tissues, and dental materials. The determination and analysis of the distribution of stresses is an important key in increasing the success of prosthetic restorations and contributing to their development. In this study, the effect of different anterior implant positions of the all-on-four concept on stress distribution in cortical and trabecular bone, implants, abutments, screws, and prostheses was investigated by FEA. The null hypothesis was rejected, as the stress values and distributions were affected by anterior implant positions.

Bone tissue does not show a homogeneous density and structure and is not isotropic.¹⁶  Due to this complex structure of the jaw bones, it is difficult to prepare geometric models with high accuracy. For this reason, in our study, as in most FEA studies,^10,17–21  all materials were accepted as homogeneous, isotropic, and linear elastic for comparison.

Although previous histological studies²²  have reported that bone–implant contact varies between 30% and 70%, implant–bone contact was considered 100% in our study, in line with previous studies.^10,17,23 

FEA studies have reported similar stresses under vertical loads in different models, with oblique loads creating distinct differences.²⁴  In addition, the use of combined loads has been recommended because they reflect bite directions more realistically and impose greater stress on cortical bone.²⁵  In our study, forces were applied bilaterally to the occlusal surfaces of canines, premolars and first molars at an angle of 45° in the palatal–buccal direction.

Previous FEAs have shown that principal stress values can be used for fragile materials (graft materials, bone, porcelains) and vM stress values for retractable materials such as metals.²⁶  Based on this proposition, some researchers have used vM values for implant and prosthetic components analysis and Pmax and Pmin values for bone tissue analysis.^10,23  On the other hand, it has been reported that vM stress is a sufficient and common criterion for general stress analyses of flexible materials and can provide information about stress values that occur in the whole structure.^17,22,27,28  Based on this proposition, in some studies,^13,17,24,29  vM values have been used in the analysis of all implants, prosthetic components, and bone tissues. In our study, vM values were used in all structures to compare bone, implant, and prosthetic components using the same parameter. Furthermore, principal stress values in bone tissue were analyzed and interpreted comparatively. Although for vM, Pmax, and Pmin, the values in bone tissue differed in the results obtained, no significant differences were found in the regions where these 3 stress types were concentrated.

Maló et al⁸  stated that the greater the distance is between the anterior and posterior implants, the greater is the support that can be provided to the prosthesis. In the literature, the number of studies evaluating the positions of anterior implants in the all-on-four concept is limited. Hussein and Rabie¹⁵  reported that changing the position of the anterior implants from the central tooth to the canine area in the mandible all-on-four concept reduced the maximum stress in the peri-implant bone from 68.6 to 63.9 MPa. In addition, placing anterior implants in the strategic position at the arc corner can reduce stress levels and create a favorable stress distribution by reducing the distance between the anterior implants and the applied load compared with the typical all-on-four. Wu et al¹³  found that in the mandibular all-on-four concept, changing the position of the anterior implants from the anterior tooth to the canine region decreased the vM stresses in the surrounding bone from 24.89 to 23.74 MPa; the stresses were increased to 25.17 MPa when the implants were placed more posteriorly. However, they reported that this change had a small effect and did not provide any benefit in terms of biomechanics. In our study, changing the location of the anterior implants affected the magnitude and distribution of stress on bone tissue, implants, and prosthetic elements. In accordance with the works of Hussein and Rabie¹⁵  and Wu et al,¹³  when the implant position was changed from the central incisor to the canine tooth, the maximum vM stress and Pmax decreased; the rank was model 1 > model 2 > model 3, while the Pmin ranks were model 3 > model 1 > model 2 in cortical bone.

Pmax and Pmin were ranked as model 1 > model 2 > model 3, while vM stress was ranked as model 2 > model 1 > model 3 in trabecular bone, showing very small differences. The highest stress on the implant, abutment, and abutment screws occurred in model 1, and the least stress in model 2. The framework stresses were ranked as model 1 > model 2 > model 3, similar to cortical bone. Across all structures, the highest stresses occurred on the crowns, and the lowest stresses on trabecular bone.

In implant treatment, a better load distribution is expected as the prosthesis support area increases. Maló et al⁸  pointed out the value of increasing the area of prosthesis support by stating that the greater the distance is between the anterior and posterior implants, the more support the prosthesis can provide. In our study, prosthetic support areas were calculated using the surface area of the rectangle formed by the implant positions in the models. Support areas were calculated as 313 mm² for model 1, 339 mm² for model 2, and 300.09 mm² for model 3. However, while stresses are expected to decrease as the support surface area increases, cortical bone and prosthetic frameworks generally had the lowest stresses in model 3 and the highest stresses in model 1. Many factors may have influenced these results. As Hussein and Rabie¹⁵  stated, the placement of anterior implants in the strategic position at the arc corner may have reduced the stress level. Moreover, not only the implant positions but also the position of the occlusal force can influence the stress distribution in the full-arch prosthesis. Rubo and Capello Souza³⁰  reported that stresses are concentrated in the cortical bone around the implant closest to the area where the force is applied. In our study, as no force was applied to the central and lateral incisors, the resultant force was closer to a posterior implant. The concentration of stresses around the posterior implants is consistent with many studies in the literature^10,31,32  and supports the result reported by Rubo and Capello Souza.³⁰  As Hussein and Rabie¹⁵  stated, by placing the anterior implants in the canine area, the distance between the anterior implants and the applied force is reduced. Hence, the occlusal force may be distributed between the posterior and anterior implants, and a more favorable stress distribution may have been formed. Eventually, it is difficult to say which of the anterior implant positions is advantageous in terms of stress distribution.

The main limitations of this study are the limitations of FEA^33,34  and the analysis of the stress distribution, which relied on a single arc form. Moreover, when it comes to implant locations, it should be noted that the arch form may also affect the stress distribution. Additional clinical studies are needed to understand the effects of anterior implant positions on stress distribution.

Within the limits of the study, the following conclusions were reached:

1. Cortical bone exhibited a higher stress concentration than trabecular bone in all groups.

2. The stresses that occurred in the cortical and trabecular bone were concentrated around the socket of the posterior implants.

3. Changing the location of anterior implants affected the magnitude and distribution of stress in bone tissue, implants, and prosthetic elements.

4. Generally, the highest stress on bone tissue, implants, and prosthetic components occurred in model 1, and the lowest in model 3.

Abbreviations

MPa:

Megapascal

3D:

Three dimension

FEA:

Finite element analysis

vM:

von Misses

Pmax:

Maximum principal

Pmin:

Minimum principal

The authors declare no conflicts of interest related to this study.