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ORIGINAL ARTICLE
Year : 2020  |  Volume : 23  |  Issue : 4  |  Page : 456-463

Effects of all-on-four implant designs in mandible on implants and the surrounding bone: A 3-D finite element analysis


Department of Prosthodontics, Faculty of Dentistry, Afyonkarahisar Health Science University, Turkey

Date of Submission03-Sep-2019
Date of Acceptance05-Feb-2020
Date of Web Publication4-Apr-2020

Correspondence Address:
Dr. G Deste
Department of Prosthodontics, Faculty of Dentistry, Afyonkarahisar Health Science University, Afyonkarahisar
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_471_19

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   Abstract 


Aims: The purpose of this study was to observe the stresses of all-on-four implant designs in an edentulous mandible in the implant, surrounding bone, and monolithic ceramics. Materials and Methods: In mandibular all-on-four implant models, anterior implants were placed vertically, and posterior implants were differently inclined. On the full-arch fixed prosthetic restoration monolithic zirconia framework, monolithic lithium disilicate was prepared as the superstructure. Model 1M (1M–15.5); posterior implants angled at 15° to the occlusion plane and a cantilever length of 5 mm, Model 2M; (2M–15.9), Model 3M; (3M–30.5), and Model 4M; (4M–30.9) were prepared. A total of 300 N bilateral force was applied at an angle of 30° and oblique to the occlusion plane. Stress values on dental implants, abutments, the surrounding bone, and prosthetic restorations were calculated. Results: The highest stress concentration was observed in the 2nd connector region between the canine and the 1st premolar tooth in the monolithic zirconia frameworks (457.21 MPa). Stress concentration in the cortical bone was 60.93 MPa in posterior implants. Stress was higher in posterior angled implants than straight implants. Stress at posterior angulation increased by 21 MPa in implants angled at 15°. Conclusion: In bilateral loading, the force applied to anterior implants does not have a significant effect on the bone structure. Stress concentration increases in posterior angled implants and surrounding bone. Moreover, stress concentration increases as the length of the cantilever, the weakest part in all-on-four implants, increases. As posterior implant angulation increases, stress concentration level and localization are affected.

Keywords: Ceramics, dental implants, finite element analysis


How to cite this article:
Deste G, Durkan R. Effects of all-on-four implant designs in mandible on implants and the surrounding bone: A 3-D finite element analysis. Niger J Clin Pract 2020;23:456-63

How to cite this URL:
Deste G, Durkan R. Effects of all-on-four implant designs in mandible on implants and the surrounding bone: A 3-D finite element analysis. Niger J Clin Pract [serial online] 2020 [cited 2020 Nov 24];23:456-63. Available from: https://www.njcponline.com/text.asp?2020/23/4/456/281921




   Introduction Top


It is common to use all-on-four implant designs in the treatment of edentulous mandible.[1],[2],[3],[4] However, there are some complications in implants; in the case of an extremely resorbed edentulous mandible, problems can be due to implants, cantilever length, superstructure, or bone.[5],[6] Many complications, including implant design problems (all-on-four implant technique, number of mandibular implants, type, angulation, implant localization), problems related to superstructure (type of ceramic in the superstructure, metal-ceramic, metal-acrylic or ceramic-ceramic superstructure), biological problems (crestal bone loss, location of the mental foramen, osseointegration failures), or esthetic problems (fractures, cervical marginal gaps, discoloration), can be observed.[7],[8],[9],[10],[11]

In retrospective studies conducted on mandibular all-on-four implants and superstructures, many complications related to implant angulation, cantilever extension, bone resorption, and superstructure materials have been reported.[1],[3],[4] Therefore, mandibular all-on-four implant and fixed prosthetic superstructure applications are very important.[12] In particular, extreme resorption of the posterior mandible and the presence of a mandibular canal play a critical role in implant placement in this region. It is essential to perform mandibular fixed full-arch prostheses with fewer implants, less traumatic surgical procedures and maximum esthetics.[13],[14] Prosthetic rehabilitation is important in mandibular implant treatment planning for peri-implant bone preservation and reliable implant loading.[15],[16]

In conventional implant applications, mandibular nerve repositioning operations may require surgical procedures such as crest augmentations and the number of implants is 6 and/or above. Metal-supported ceramic superstructures are used in full-arch fixed mandibular implant-supported prostheses.[17] The recovery time is prolonged in a surgical operation. The high number of implants brings an additional burden to the patient economically.[18] Mandibular nerve repositioning and crest augmentations cause additional surgical procedures and poor patient performance. Unesthetic results and biological problems occur in metal-supported ceramic fixed superstructures.[5] Mandibular all-on-four implant restorations have many advantages.[11],[19],[20] These include the use of a small number of implants, shorter length of the cantilever, and one-piece full-arch fixed prosthesis. With angled implant placement, advanced surgical techniques such as mandibular nerve repositioning operations and crest augmentation are not needed.[11],[19],[20] Because of these advantages, mandibular all-on-four implants are preferred by clinicians over conventional implant applications.[11],[19],[20] However, general rules regarding the cantilever length, degree of posterior implant angulation, and superstructure selection in the mandibular all-on-four implant system could not be established.[21] When making mandibular all-on-four implants, it is important to protect the mandibular bone, implants, abutments, surrounding soft tissues, and prosthetic superstructures.[22] The clinician's experience, patient compliance and tolerance, the type of prosthetic restoration material, the size of the cantilever extension, the location of the mental foramen, and the bone type are the major factors that need to be considered when deciding on the application of mandibular all-on-four implants.[23]

Many researchers have conducted studies on the application of full-arch fixed restorations on mandibular all-on-four implant designs. Posterior implant angulation between 15-45°, different cantilever lengths between 2–15 mm, metal-ceramic or metal-acrylic superstructure applications were used.[24],[25] In particular, superstructure applications cause negative esthetic and biological results.[26] In mandibular all-on-four implants, the cantilever length together with the implant and surgical procedures should be as short as possible, the superstructure should be high strength, esthetic, and biocompatible.[19],[27] If necessary, the occlusal forces should be balanced to protect the implant and surrounding tissues.[19],[27] Many studies have been carried out on the implant-supported mandibular all-on-four prosthesis. Most of the studies have focused on stresses on posterior implants, bone, and superstructure.[19],[27]

Computer-aided design and computer-aided manufacturing (CAD/CAM) has expanded to the field of application for zirconia restorations.[28] Monolithic zirconia ceramics used in the construction of complete arch implant-supported prostheses are connected to the implant with high precision in terms of their production with CAD/CAM. Due to its translucent properties, small particle size and high physical properties in monolithic zirconia ceramics, it is recommended to use full-arch implant-supported prosthetic restorations.[29] Lithium disilicate glass-ceramic with high esthetic properties can be used as superstructure ceramic on monolithic zirconia. Thus, by combining the physical properties of monolithic zirconia and esthetic properties of glass ceramics, high-quality restorations are produced.

The aim of this study is to determine stress distribution in the mandibular cortical and spongious bone, prosthetic restoration, anterior and posterior implants by the three-dimensional finite element analysis models (3D-FEA) method in mandibular all-on-four implant designs with different posterior implant angulations and two different cantilever lengths, within the planning of full-arch fixed prosthesis with monolithic lithium disilicate superstructure on monolithic zirconia framework as the prosthetic restoration. The null hypothesis of this study is that different mandibular all-on-four implant designs have significant effects on implant complex, bone, and prosthetic superstructure.


   Materials and Methods Top


In 3D-FEA, anterior implants had the straight and posterior implants had 15° and 30° mesiodistal inclination. Two different cantilever lengths; 5 mm and 9 mm, were designed in four completely edentulous mandibular models. In the study, four completely edentulous mandibular models were planned according to the all-on-four implant design. These are as follows:

1M model (1M-15.5) All-on-four design mandibular treatment planning: Two anterior implants were placed in the lateral incisor region as double-sided and vertically. The posterior implants were distally tilted at a 15° angle with respect to the occlusion plane. The cantilever length was determined to be 5 mm. The full-arch implant-supported fixed prosthesis was made in monolithic zirconia framework and monolithic lithium disilicate superstructure.

2M model (2M-15.9) All-on-four design mandibular treatment planning: Two anterior implants were placed in the lateral incisor region as double-sided and vertically. The posterior implants were distally tilted at a 15° angle with respect to the occlusion plane. The cantilever length was determined to be 9 mm. The full-arch implant-supported fixed prosthesis was made in monolithic zirconia framework and monolithic lithium disilicate superstructure.

3M model (3M-30.5) All-on-four design mandibular treatment planning: Two anterior implants were placed in the lateral incisor region as double-sided and vertically. The posterior implants were distally tilted at a 30° angle with respect to the occlusion plane. The cantilever length was determined to be 5 mm. The full-arch implant-supported fixed prosthesis was made in monolithic zirconia framework and monolithic lithium disilicate superstructure.

4M model (4M-30.9) All-on-four design mandibular treatment planning: Two anterior implants were placed in the lateral incisor region as double-sided and vertically. The posterior implants were distally tilted at a 30° angle with respect to the occlusion plane. The cantilever length was determined to be 9 mm. The full-arch implant-supported fixed prosthesis was made in monolithic zirconia framework and monolithic lithium disilicate superstructure.

Ethics committee approval was received for the edentulous mandibular model from Afyon Kocatepe University Clinical Research Ethics Committee with approval number 2017/5-151 and approval date 05/05/2017.

While preparing mathematical models, cortical and spongious bones, osseointegrated implants and other components, a graphic processing program (Rhinoceros 4.0, McNeel, Seattle and Ansys 11.0, Ansys Inc., Canonsburg, PA, USA) was used.

In this study, Nobel-Speedy Groovy branded implants and abutments, which are widely used and available in the markets, were used because of the possibilities of comparison with other studies.[15],[27] Anterior implants were modeled with a diameter of 4.0 mm and a length of 11.5 mm (ø 4.0 mm × 11.5 mm) (Nobel-Speedy Groovy) while posterior implants were modeled with a diameter of 4.0 mm and a length of 13.0 mm (ø 4.0 mm × 13 mm) (Nobel-Speedy Groovy). The thread pitch of the implants was 0.6 mm, and the thread depth was 0.2 mm. The abutments were the regular diameter abutments with a diameter of 4.0 mm and a length of 5.0 mm (ø 4 mm × 5 mm). The implant-abutment connection was modeled with the internal hexagonal design.

The two posterior implants were placed 5 mm anterior to the mental foramen. The two posterior implants were tilted at an angle of 15° and 30°, while the two anterior implants were placed as far away from each other as possible, allowing a safe distance of 5 mm from the posterior implants, at an angle parallel to the long axis of the bone.[30] In the measurements made from the abutment center, the distance between the anterior implant was 13.6 mm while the distance between the posterior implant was 46 mm in models with a 5 mm cantilever and 40.6 mm in models with a 9 mm cantilever. The distance between mesial and distal implants was 24.5 mm in models with a 5 mm cantilever and 19.6 mm in models with a 9 mm cantilever. Moreover, 15° and 30° angled implants and abutments were modeled for posterior region implants. The fixed prosthesis was planned as a full-arch in one piece. The parabol-shaped arch length was 78 mm.

The monolithic lithium disilicate glass-ceramic superstructure was designed as a one-piece fixed prosthesis on monolithic zirconia framework which can be obtained with CAD/CAM by making different prosthesis planning [Figure 1]a. A full-arch fixed bridge prosthesis was modeled as mandibular teeth. Tooth sizes were taken from Wheeler's Dental Anatomy  Atlas More Details.[31] Monolithic zirconia framework height was 8 mm, the superstructure was 2 mm so the crown height was 10 mm and the framework thickness was 8 mm. The framework thickness was 1 mm in the cervico-buccal and lingual location and 3 mm in the occlusal location [Figure 1]b. In implant supports, crowns were in contact with the cortical bone and abutment collar region, while ridge-lap contact with the cortical bone was designed for crowns with the body. A 3 mm gap was determined between the prosthesis and cortical bone. The prostheses are designed with screws.
Figure 1: Fixed prosthesis model and implant positions. (a) monolithic lithium disilicate superstructure, (b) monolithic zirconia framework

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Anatomically, the mandibular body was modeled according to physiological conditions. The mandibular bone, 2 mm spongious bone, and the surrounding 2 mm cortical bone thickness were simulated. Modeling of implants, surrounding bone structures and prosthetic superstructure was performed to generate a biomechanical response for the purpose of the study.

Each mathematical model contained approximately 227 360–228 265 nodes.[32] As the assumptions in the computer system, implants and abutments were accepted to be combined and homogeneous. Implants and abutments of titanium alloy with high mechanical properties were accepted to have elastic mechanical properties. Implant, abutment, cortical, and spongious bone were assumed to have isotropic mechanical properties. Mandibular models were limited in the X, Y, and Z directions. The mandibular FEA model was fixed to have zero degree of freedom (0-DOF) along the outer borders of the rear and bottom surfaces [Figure 2]a. Material properties, implant components, and bone properties are presented in [Table 1].
Figure 2: (a) limiting the mandible from the back and bottom surfaces, (b) Image of bilateral loading on mathematical model

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Table 1: Elastic modulus values and Poisson ratios of the data input materials

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Since the ideal organic material properties were impossible, all materials were accepted to be isotropic, homogeneous, and linearly elastic. The implants were assumed to be 100% osseointegrated. Elastic properties of materials (Young's modulus (E) and Poisson's ratio (μ)) were determined with reference to previous studies [Table 1].[33],[34],[35],[36],[37],[38],[39]

As the bilateral loading, including 100 N from the tops of the buccal tubercule of the 1st and 2nd premolar teeth of both posterior regions and 50 N from the mesial and distal buccal tubercule of the 1st molar tooth, a total of 300 N force from the right posterior region and 300 N force from the left posterior region were simultaneously and bilaterally applied and oblique force was applied at an angle of 30° to the occlusion plane [Figure 2]b. The force was applied according to a previous study considering the occlusal force average applied in bilateral loading.[40]

Stress levels were determined by using von Mises stress values, as commonly reported in other finite element analysis studies.[40],[41] In particular, maximum stress values were compared with stress distributions in brittle materials. Cross-sectional views, the amount of stress in the nodes and stress distributions are evaluated and interpreted.


   Results Top


Maximum principal stress values in the cortical bone were 1M: 41.44 MPa; 2M-15.9: 51.72 MPa; 3M-30.5: 48.03 MPa; and 4M-30.9: 60.93 MPa. Accordingly, the highest stress concentration values were observed in the 4M model and the lowest in the 1M model [Figure 3]. While the stress values in the cortical bone of the anterior implants were lower, the force efficiency applied in the region of the posterior implants increased the maximum principal stress values in the cortical bone. Higher stress was observed in angled implants with longer cantilever lengths.
Figure 3: Stress concentrations (MPa) in the cortical bone were high in posterior implant regions. The highest stress values were observed in this region. (a) 1M model (1M–15.5), (b) 2M model (2M–15.9), (c) 3M model (3M–30.5), (d) 4M (4M–30.9)

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Stress values in the spongious bone were 1M: 9.44 MPa; 2M: 12.28 MPa; 3M: 9.6 MPa; and 4M: 3.19 MPa. The highest stress values were observed in the 2M model and the lowest in the 4M model [Figure 4]. Higher stress values were observed in the spongious bone around the posterior implants compared to the anterior implants.
Figure 4: Stress concentrations (MPa) in the surrounding spongious bone. (a) 1M model (1M–15.5), (b) 2M model (2M–15.9), (c) 3M model (3M–30.5), (d) 4M (4M–30.9)

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The maximum principal stress (tensile stress) values in the cortical bone were higher than the stress values in the spongious bone in all models.

Von Mises stresses in anterior implants were 1M: 95.66 MPa; 2M: 104.25 MPa; 3M: 95.98 MPa; and 4M: 105.06 MPa. The highest stress values were observed in the 4M model and the lowest in the 1M model [Figure 5]. Stress values were close to each other. Von Mises stresses in posterior implants were 1M: 168.21 MPa; 2M: 185.97 MPa; 3M: 111.85 MPa; and 4M: 164.63 MPa. The highest stress was observed in the 2M model and the lowest in the 3M model [Figure 5].
Figure 5: Stress concentrations (MPa) were high in the implant neck in angled implants. (a) 1M model (1M–15.5), (b) 2M model (2M–15.9), (c) 3M model (3M-–30.5), (d) 4M (4M–30.9)

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Stress concentrations in anterior straight implants were observed in the implant neck and at similar values. In posterior angled implants, stress concentration was observed in the implant neck. Higher stress was observed in angled implants with longer cantilever lengths. Stress values decreased as posterior implant angulation increased. The stress in the angled implant was higher than the stress in the straight implant. The highest stress concentration was observed in the collar of the implant with the angled implant. The stress concentrations in the cortical bone were lower than the implants and their components in all models.

Stress values at the selected connector points in the framework were 1M: 457.21 MPa; 2M: 381.14 MPa; 3M: 447.56 MPa; and 4M: 397.14 MPa. The highest stresses were concentrated in the 2nd connector area between the canine and the 1st premolar tooth in all models. The highest stresses were obtained in the 1M model and the lowest stresses were obtained in the 2M model [Figure 6].
Figure 6: Stress concentrations (MPa) at the selected node points in monolithic zirconia frameworks. (a) 1M model (1M–15.5), (b) 2M model (2M–15.9), (c) 3M model (3M–30.5), (d) 4M (4M–30.9)

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   Discussion Top


Nowadays, advances in implant treatment methods and ceramic materials have increased the interest and confidence of patients in implant-supported prosthetic treatments. Especially in the case of complete edentulousness, elderly patients prefer implant-supported prostheses more. In the case of a completely edentulous mandible, the difficulties of patients using a removable prosthesis and getting used to the prosthesis increased the implant-supported prosthetic treatments. Mandibular implant-supported prostheses are designed as fixed or removable.[27] It is accepted that full-arch implant-supported fixed prosthetic restorations are better in terms of aesthetics, function, and phonation than removable prostheses.[25] Patients prefer less surgical procedures and more economical treatment methods in a shorter period of time. Thus, full-arch all-on-four implant-supported fixed prosthetic restorations used in both maxillary and mandibular complete edentulous cases have come to the forefront.[22] Two of the four implants placed in the interforaminal region are applied vertically to the anterior canine-lateral teeth and the others are applied bilaterally to the posterior region with a 15°–45° distal angle to the anterior of the mental foramen to prevent the limitations of the mandibular canal.[7],[11]

Different types of loading models were used in the analysis of all-on-four implant-supported full-arch mandibular fixed restorations.[40],[41] Bilateral balanced occlusion simulation was used in this study. Bilateral balanced occlusion is a type of occlusion applied in maxillary and mandibular complete edentulousness, especially in elderly patients. In this study, in the bilateral loading performed for bilateral balanced occlusion simulation, 300 N force was applied from the right and left posterior teeth at an angle of 30° in the buccolingual direction from the buccal tubercules of the 1st premolar, 2nd premolar, and 1st molar teeth.[40]

In the all-on-four implant design, in the photoelastic stress analysis performed by placing the implants at 0°, 15°, 30°, and 45° distal angles, it was emphasized that stresses occur in 0°, 15° and 30° angled implants similarly but stresses increase in 45° angled implants and therefore the inclination should not be increased in posterior angled implants.[24] Photoelastic and SESA analyses indicate that stresses occur around the distal implant.[11],[24],[40],[41] In this study, high stresses were observed around the distal implant in all loading protocols and similar results were obtained as the present studies.

Begg et al.[24] reported that 15° and 30° angled distal implants showed a similar stress pattern as axial implants. The use of angled implants alone increases stress values in the bone, and when they are splinted together with more than one implant, stress values decrease.[42],[43],[44] In this study, it was observed that von Mises stresses decreased when the posterior implant angle increased from 15 to 30°.

Bellini et al.[25] analyzed stresses in the bone in angled implants in the All-on-Four models to which a 100 N load was applied from the cantilever region, and they detected the minimum principal stress to be -24 MPa in the extension of a 15 mm cantilever. The minimum principal stress increased by 33% when the cantilever length increased from 5 mm to 15 mm. The findings of the study carried out by Horita et al.[11] and Rubo et al.[45] showed that there was a direct proportion between the increased cantilever length and increased stress concentrations around the implants. In this study, when the cantilever length increased, the maximum and minimum principal stress values in the cortical and spongious bone and von Mises stress values in implants increased.

Nowadays, monolithic zirconia ceramics are used in fixed prosthetic restorations on implants.[46] In the literature review, since no analysis was carried out with full-arch prosthetic restoration consisting of lithium disilicate superstructure on a monolithic zirconia ceramic framework, comparisons were made with our own study plans. While the monolithic zirconia ceramic framework produced intense stresses, the stresses transmitted to the implant and bone decreased.[45],[46] In this study, the maximum and minimum principal stress values in the cortical and spongious bone were lower than von Mises stresses in the monolithic zirconia ceramic framework. It was stated that more rigid and durable materials are more preferable biomechanically. Furthermore, it was found out that stress values did not exceed the limits of bone resistance in all-on-four treatment planning.

The hypothesis in this study is that different mandibular all-on-four implant designs have significant effects on implant complex, bone, and prosthetic superstructure. The hypothesis was accepted.

Limitations of this study include the limitations that we stated for finite element stress analysis. Mandibular models are accepted as linear, elastic and homogeneous. However, the mandibular bone structure is not homogeneous and due to its anisotropic structure, there are different stress distributions.[47] Complete osseointegration between the implant and bone is accepted but clinically, partial contact between the implant and bone and osseointegration are reported.[6],[48] Furthermore, the stress distributions around the implant vary according to the loading direction in vivo, while the loading direction is kept constant in vitro. In stress analysis, distal loading simulations indicate that there is only bending motion in the prosthetic superstructures but bending moments and rotational moments that occur in the implants affect the superstructure.[49]


   Conclusions Top


As the cantilever length increased, the maximum and minimum principal stresses of the cortical and spongious bone and von Mises stresses of the implants increased in all models.

There was no significant difference in the models between the 15° and 30° posterior implant angulation in terms of maximum and minimum principal stress values in the cortical bone while von Mises stress values in the implants decreased.

The highest principle stress values in the cortical bone were concentrated in the crestal bone.

Von Mises stresses in the implants were concentrated in the 2nd and 3rd threads in the neck region of the posterior implants.

When the selected node points in the monolithic zirconia frameworks were evaluated, the highest stresses occurred in the 1st and 2nd connector regions in the anterior region.

Von Mises stresses that occurred in the selected node points in monolithic zirconia frameworks did not exceed the fracture strength of monolithic zirconia.

In all models, stresses that occurred in the cortical bone were found to be higher than stresses in the spongious bone.

In all models, the highest stress values were obtained in monolithic zirconia frameworks and posterior implants, while the lowest stress values were obtained in the spongious bone.

Financial support and sponsorship

Afyon Kocatepe University Commission of Scientific Research Projects.

Conflicts of interest

There are no conflicts of interest.



 
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