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Keywords:

  • All-on-four;
  • cantilever;
  • fixed prosthesis;
  • immediate loading;
  • tilted implants

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Background:  The aim of this study was to evaluate if tilting of the distal implant at different angulations (30° and 40°) with different cantilever lengths (4 mm and 12 mm) affects the stress and strain distribution in an ‘all-on-four’ situation.

Methods:  A completely edentulous mandible was modelled with four tapered implants placed within the interforaminal region to receive an all acrylic fixed prosthesis. The two posterior implants were tilted at an angle of 30° and 40°. The prosthesis cantilever was given two different variables of 4 mm and 12 mm. For all models, the equivalent von Mises stress and strain was analysed using three-dimensional finite element analysis.

Results:  Statistical significance (p < 0.05) was seen when a comparison was made for the stress developed on the implant and cortical bone between the 30° and 40° distally tilted posterior implants in both situations. No significance was seen in the trabecular bone and on the strain developed in these situations.

Conclusions:  The study shows that increasing the tilt of the distal implants does not increase the stress significantly. It also shows that the architecture of the mandible plays a major role during treatment planning of a completely edentulous patient.


Abbreviation
FEA

finite element analysis

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

A major problem facing dentistry is that approximately 20% of our population is edentulous. An excessive loss of the residual alveolar ridge makes it difficult to provide a prosthesis that meets the needs of these dental patients. Studies have demonstrated a successful outcome with implant-supported, complete arch fixed restorations for the rehabilitation of the edentulous patient.1 The results from a long term clinical study by Eliasson et al.2 indicated that four implants may be sufficient in the edentulous mandible when properly placed anterior to the mental foramina area.

As the placement of implants with significant load bearing capacity often may be restricted to the anterior portions of the arch, cantilevers distal to the most posterior implants are often required. The presence of a load bearing cantilever increases the forces distributed to the implants, possibly up to two or three times the applied load on a single implant, due to the bending moments. To reduce the loading of the terminal implant some have advocated placing short implants distal to the mental foramina and having the cantilever segments rest on the implants without being connected.3

Another concept which has been put forward by various authors is tilting of the posterior implants to decrease the cantilever length. By inclining the posterior implants distally the prosthesis may have adequate support in the premolar and molar areas. In addition to broadening the prosthetic base, the tilting may also allow for improved cortical anchorage and primary stability, short cantilever length, large inter-implant distance as well as the use of longer implants.

The all-on-four concept was developed to create a fast and cost-effective treatment for edentulous patients with a fixed restoration.4,5 This involves the placement of four implants as support for a full arch prosthesis. The four implants, of which the two most posterior implants are tilted distally, are immediately loaded with a screw-retained acrylic prosthesis. Tilting of the posterior implants allows the final prosthesis to hold as many as 12 teeth with only short cantilevers.

The purpose of this study was to evaluate the stress and strain produced on the implant and in the surrounding bone while using the standardized concept of ‘all-on-four’ for an immediately loaded implant supported fixed full arch prosthesis using three-dimensional finite element analysis (3-D FEA).

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

A 3-D finite element model of a completely edentulous mandible, with four tapered implants, placed within the interforamina region to receive an all acrylic fixed full arch prosthesis was used in this study (Fig. 1 and 2).

image

Figure 1.  Model showing all-on-four situation (30° angulation of the distal implants).

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Figure 2.  Model showing all-on-four situation (40° angulation of the distal implants).

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Computerized tomography of the mandible was used to obtain the finite element model of the mandibular bone. The 3-D tetrahedral structural solid finite elements using Pro/Engineer Wildfire 2.0 software was used to model the bone, implant, abutment and the occlusal surface material. All materials used in the models were considered to be isotropic, homogenous and linearly elastic. Material properties of the materials used in the model are presented in Table 1.

Table 1. Material properties of materials used in the model
MaterialElastic modulus (GPa)Poisson’s ratio
Cortical bone13.00.3
Trabecular bone5.50.3
Woven bone3.00.3
Titanium implant and abutment102.00.35
Acrylic resin (prosthesis)2.70.35

As there are no universally accepted properties of the biologic materials available in the literature, it was therefore decided to accept median values of those reported in the literature.

Bone

The bone was modelled with a height of 17 mm, width minimum of 8 mm and interforamina distance of 46 mm. The cortical bone was modelled at the top and bottom as 2 and 3 mm thick layers, respectively. The trabecular bone was modelled as a 12 mm thick layer between the two cortical layers. Woven bone was also modelled around the implant with a width of 1 mm in the area adjacent to the implant to depict an immediate loading situation.

Implants

A solid 4.3 mm tapered 15 mm long implant was selected for this study. The two posterior implants were placed 5 mm anterior to the mental foramen. The two posterior implants were tilted at an angle of 30° and 40°, 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. A 4.3 mm straight and angulated abutment was used for anterior and posterior implants, respectively. The bone-to-implant contact given in this study was 65% to simulate an immediate loading situation.

Prosthesis

As cited by Ellakwa et al.,6 a minimum overall thickness of 1.5 mm is recommended to resist fracture. A fixed bridge with a cantilever on both ends was depicted in this study as a homogenous block of acrylic resin of 2 mm thickness. The cantilever was given two different variables (4 mm and 12 mm).

Load

A 100 N load was applied to the anterior implant which was placed in the canine region. A 250 N load was applied to the posterior distally tilted implant and the end of the cantilever to depict forces on the premolars and molars, respectively.

Both axial and oblique forces were applied. The bilateral axial forces depicted a clenching situation while the unilateral forces depicted masticatory forces (Fig. 3 and 4). The oblique force was applied unilaterally at an angle of 30° to the long axis (Fig. 5).

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Figure 3.  Bilateral loading.

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image

Figure 4.  Unilateral loading.

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Figure 5.  Oblique force applied at 30°.

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Comparative groups

The comparative groups were: Group 1 – 30° angulated distal implant with 4 mm cantilever; Group 2 – 30° angulated distal implant with 12 mm cantilever; Group 3 – 40° angulated distal implant with 4 mm cantilever; and Group 4 – 40° angulated distal implant with 12 mm cantilever. The loads were simultaneously applied on the canine (anterior implant), the distal implant and the end of the cantilever (molar) region in all four groups.

Meshed models

A FEA of the models was carried out using ANSYS Workbench 11 software (ANSYS Inc, Canonsburg, USA). The ProE models were imported to the ANSYS Workbench 11 software and the models were meshed (Fig. 6).

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Figure 6.  Meshed model.

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Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

An independent student t-test analysis was performed. The results did not show any statistical significance (p > 0.05) between 4 mm and 12 mm cantilever for both 30° and 40° distally tilted posterior implants and also between the 30° and 40° distally tilted posterior implants under all three loading conditions for both stress and strain. But statistical significance (p < 0.005) was seen when a comparison was made for the stress developed on the implant and cortical bone between the 30° and 40° distally tilted posterior implants in both situations (4 mm and 12 mm cantilever). No significance was seen in the trabecular bone and on the strain developed in these situations.

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Figure 7.  Comparison of stresses between 30° and 40° distally tilted distal implant situations.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

The research hypothesis that tilting of splinted implants does not increase the bone stress, and that shortening the prosthesis cantilever length by implant tilting reduces the stress, is supported by this 3-D FEA. Another study conducted by Zampelis et al.7 came to a similar conclusion. The results in our study showed that during oblique loading, stresses were higher than during axial loading in all situations, except where the distal implants are tilted distally by 40° and have a 12 mm cantilever posterior to it. The oblique load produces a bending moment which has the greatest potential to induce a high gradient of stress in the bone surrounding the implant. Also, the rotation induced by buccolingual and mesiodistal loads was responsible for the higher values of the maximum stress under these loads. This was also seen by Skalak as cited by Barbier and Schepers,8 Borchers and Reichart,9 and Stegaroiu et al.10

Thus, in light of the high stress calculated for bucco-lingual forces, when planning and fabricating a superstructure it is important to create an occlusal shape that minimizes lateral force components. The same principle should be considered during occlusal adjustments.

The highest von Mises stress and strain was seen near the distal implants supporting the distal cantilevers. The maximum stress and strain in the implant was found at the implant abutment interface.

The stress was seen higher on the loading side. In bilateral loading the stress and strain was higher on the right side of the mandible than the left side (though the difference was minimum) in all situations except for the 40° angulated distal implant with 12 mm cantilever situation where it was seen higher on the left side. This may be due to the asymmetry of the model.

In the cortical bone maximum stress and strain was found at the cervical aspect, near the implant neck and in the trabecular bone the maximum stresses were seen apical for axial loadings and cervically for unilateral oblique loadings. Similar findings have been reported by Borchers and Reichart,9 Sevimay et al.,11 Jeong et al.12 and Tashkandi et al.13

The results from our study also showed that stresses are seen distal or mesial on axial loading (both bilateral and unilateral), and buccal or lingual on oblique unilateral loading. High stresses around the distal half of the distal implant, which was found under axial loads, resulted from both the rotation in the vertical plane and the load applied to that implant. Under buccolingual loads, the deformations that occurred as a combined effect of the rotations in the transversal and horizontal planes yielded increased stress around the distal implant and lower stress around the mesial implant. Another reason for stresses being distributed to the buccal or lingual side during oblique loading may be due to thinner buccal and lingual cortical plates. This has also been reported by Sevimay et al.,11 Geng et al.14 and Stegaroiu et al.10

Our results showed that increasing the tilt of the implants does not significantly increase stress and strain when the comparison was made between 30° and 40° situations in various loading situations. This is supported by Zampelis et al.,7 Krekmanov et al.,15 Aparicio et al.,16 Kronstrom17 and Malo et al.4,5,18

Studies by Zampelis et al.7 showed the stress at the most coronal bone-to-implant contact was identical irrespective of the angle of the tilt, demonstrating that tilting of splinted implants does not result in increased stress. The graphical interpretation of their results also shows that the sequential increase in the angle of tilt does not proportionately result in increase in stress. The stress for 30° tilt was higher than stress for 0°, 10°, 20° and 45°. A similar result was also seen in our study.

No significant difference was seen on increasing the cantilever length from 4 mm to 12 mm. Though many studies have shown that stress increases with increase in cantilever length; a 3-D FEA model analysis was done by van Zyl et al.19 for stress analysis of mandibular cantilever prosthesis. They reported that an optimal area of cantilever loading exists up to a cantilever length of 15 mm where low patterns of von Mises stress develop. Extension beyond 15 mm could lead to greater stress in the lingual and buccal cortical plates, which could jeopardize the integrity of the implant. This may explain why there was no significant difference in our study between the two cantilever extensions of 4 mm and 12 mm as they lie within this optimal range.

When the comparison was made between the two groups for implant, cortical bone and trabecular bone individually, a significant difference was seen between the two groups for stress. In implants the stress in 30° tilted distal implants was significantly higher (p < 0.05) than 40° while in cortical bone the stress was significantly higher in 30° than 40° angulated distal implants. This could be because increasing the tilt of the implant results in more acute angle at the distal surface of the implant, which in turn results in increased stress.

The results from our study also revealed that the strain produced in different situations is within the normal physiologic zone except for strain in cortical bone on bilateral loading. This strain lies in the mild overload zone. When strain conditions to the interfacial bone are in mild overload zone, an increased bone modelling response occurs, which results in a reactive woven bone formation that is less mineralized and weaker. Greater stresses may cause the interfacial strain to reach the pathologic overload zone and may cause microfracture of the bone, fibrous tissue formation, and/or bone resorption.

Although our study was conducted in such a way as to simulate the clinical scenario as close as possible, some limitations do exist when not directly dealing with a clinical case. As stated by Lewis and Klineberg, while laboratory and FEA studies provide insight into the way a system works, the results do not necessarily correlate with clinical performance and need to be interpreted with caution.20

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Within the limitations of this study, the findings provide evidence that increasing the tilt of the distal implant does not significantly increase the stress. The study also shows that the architecture of the mandible plays a major role during treatment planning of a completely edentulous patient, especially when considering a cross-arch splinted restoration. This is because the stress and strain on the thinner segment of the mandible will take up more load, resulting in increased bone resorption on that side.

Thus, our study confirms the results seen in clinical situations which report a fairly good success rate. Within the limitations of this study, we conclude that ‘all-on-four’ is a mechanically viable concept and leaves room for further research on other variables that could influence the biomechanical outcome.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References
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