The effect of implant design on the stress distribution in a three-unit implant-supported distal cantilever fixed partial denture: A three-dimensional finite-element analysis :[PAUTHORS], Indian Journal of Dental Research (IJDR)

ORIGINAL RESEARCH
Year : 2012 | Volume : 23 | Issue : 2 | Page : 129--134

The effect of implant design on the stress distribution in a three-unit implant-supported distal cantilever fixed partial denture: A three-dimensional finite-element analysis

Pradeep Koka¹, Abhijita Mohapatra², Ponsekar Abraham Anandapandian³, K Murugesan³, M Vasanthakumar³,
¹ Department of Prosthodontics, Sri Sai College of Dental Surgery, Vikarabad. Andhra Pradesh, India
² Department of Prosthodontics, Institute of Dental Sciences, SOA University, Bhubaneswar, Orissa, India
³ Department of Prosthodontics, SRM Dental College, SRM University Chennai, India, India

Correspondence Address:
Ponsekar Abraham Anandapandian
Department of Prosthodontics, SRM Dental College, SRM University Chennai, India
India

Abstract

Context: Implant design influences the stress distribution in an implant-supported distal cantilever fixed partial denture and supporting bone tissue. Aim: The purpose of this study was to investigate the effect of implant design on the stress distribution in the framework, implant, and surrounding bone, using a three-dimensional finite-element analysis. Materials and Methods: A three-dimensional finite-element model of a mandibular section of bone with implants placed in the first and second premolar region was created to support a distal cantilever fixed partial denture. A one-piece and two-piece implant and its suprastructure were simulated into wire frame models using Pro engineer (Pro E) program. Four models were created in this study. Results: Comparative analysis of all models showed that the maximum stress overall was in the cervical portion of the secondary abutment. When used in combination, the maximum stress was when the two-piece implant was used as secondary abutment. The one-piece implant showed less stress compared to its counterpart when used as secondary abutment. The maximum stress distribution in the bone was around the neck region of the secondary implant. Conclusion: Within the limitations of this study, it can be concluded that stress distribution is better in a one-piece implant design when compared with the two-piece implant design, with stress concentration being more at the junction of the abutment and the implant fixture in the two-piece implant. When implants are used as abutments (either primary or secondary), irrespective of their position and design, the secondary implant shows the maximum amount of stresses.

How to cite this article:
Koka P, Mohapatra A, Anandapandian PA, Murugesan K, Vasanthakumar M. The effect of implant design on the stress distribution in a three-unit implant-supported distal cantilever fixed partial denture: A three-dimensional finite-element analysis.Indian J Dent Res 2012;23:129-134

How to cite this URL:
Koka P, Mohapatra A, Anandapandian PA, Murugesan K, Vasanthakumar M. The effect of implant design on the stress distribution in a three-unit implant-supported distal cantilever fixed partial denture: A three-dimensional finite-element analysis. Indian J Dent Res [serial online] 2012 [cited 2023 Jun 1 ];23:129-134
Available from: https://www.ijdr.in/text.asp?2012/23/2/129/100413

Full Text

The absence of a distal abutment is a difficult and challenging clinical situation for the dentist trying to achieve occlusal stability, as it can lead to temporomandibular joint dysfunction. Since dental implants were introduced for the rehabilitation of edentulous patients in the late 1960s, awareness and demand for this form of therapy has increased. The use of osseointegrated implants as abutments for fixed partial dentures has become a treatment option for partially edentulous patients, especially for those dissatisfied with a distal extension removable partial denture. [1]

Implant placement is being dictated by the prosthodontic design and so it was logical to develop a prosthetic design that minimized stress concentration, alignment problems, and esthetic restrictions, and avoid anatomic complications. [2],[3]

Endosseous, root-form dental implants distribute occlusal stresses into the supporting bone as a function of their overall design and the amount of bone-to-implant interface achieved. Thanks to the groundbreaking research findings of Branemark describing the direct contact between bone and titanium endosseous implants, and the development of various endosseous implant designs such as one-piece and two-piece implant designs, using these as definitive treatment options is now relatively easier.

Unlike the conventional two-piece implant design, the one-piece implant offers several advantages, such as elimination of second-stage surgery, the use of an interim removable partial denture, and reduction in treatment cost. In the two-piece implant design, because of the microgap between the implant abutment and the fixture, the chances of plaque accumulation is greater. In contrast, the one-piece implant design has a smooth metal surface at the crestal portion (Crest Module) of the implant. The smooth crest module is easier to clean and collects less plaque than a rough surface. A one-piece tapered implant in the form of a threaded screw transmits axial tensile forces or compressive loads and induces a component of compressive load in the bone-implant interface. A tapered threaded implant provides surgical advantages during initial insertion because it inserts down within the osteotomy, engaging bone. [4]

The clinical advantage of the one-piece implant design is well documented but its biomechanical advantages over the two-piece implant require investigation. The literature shows many studies on force transmission to a two-piece implant-supported cantilever partial denture but a comprehensive study evaluating the effect of type of implant design on the framework, implant, and the surrounding bone in a cantilever fixed partial denture has not yet been done. The aim of this biomechanical analysis was to compare and evaluate the level of stresses generated by different types of implant designs and transmitted to the framework, implant, and the surrounding bone in a cantilever fixed partial denture by finite-element analysis.

Materials and Methods

Four models were created in this study using PRO E software and three-dimensional FEA code (ANSYS Workbench 11.0, PRO E Software) [Figure 1], [Figure 2], [Figure 3] and [Figure 4].{Figure 1}{Figure 2}{Figure 3}{Figure 4}

The shape of the bone was simplified to a 30-mm-high and 35-mm-wide prism having a quadrangular base and walls of an irregular octagon so that the unification of elements during meshing was simple and effective for saving in the computer memory. The implant was to be placed into the first and second premolar region of the simulated mandible in a vertical position. The implant nearer to the pontic was considered as the primary implant and the implant situated away from the pontic as the secondary implant.

To evaluate the effect of implant design two models, namely, model A, comprising two-piece implants as primary and secondary abutments, and model B, with single-piece implants as primary and secondary abutments were created. model C, comprising a one-piece implant as primary abutment and a two-piece implant as secondary abutment, and model D with two-piece implant as primary abutment and one-piece implant as secondary abutment were created to simulate a situation where a combination of a two-piece and one-piece implant is used. This situation may arise in patients with existing implants who require an additional implant to be placed to support a distal cantilever fixed partial denture.

The 3D implant model was represented with Nobel Biocare implant system available as one-piece and two-piece implants (elastic modulus [E]=1.1 × 10 5 Mpa, Poisson's ratio [ν]=0.32). Solid screw-shaped tapered for one-piece implant and cylindrical threaded for two-piece implant of similar dimensions was used in the designing. The interface between the implant and bone was assumed to be an immovable junction, indicating 100% osseointegration between the implant and the bone. Metal coping and porcelain thickness used in this study were 0.8 mm and 2.00 mm, respectively. [5] The connectors were designed in an elliptical shape of 2.25 mm width and placed just cervical to the junction of the occlusal and middle third. The cement thickness layer was ignored.

All materials were presumed linear elastic, homogeneous, and isotropic. The corresponding elastic properties such as Young's modulus (E) and Poisson ratio (ν) were determined from the literature. The boundary conditions of the model were assigned to simulate the average bone conditions. An average occlusal force of 300 N was determined from the literature. Three-point vertical loads were applied at the following locations on each crown: The tip of the buccal cusp of the premolar and the mesiobuccal cusp of the molar (100 N), the distal fossa (100 N), and the mesial fossa (100 N) [6] [Figure 5].{Figure 5}

Results

When a vertical load of 300 N was applied onto the three-unit cantilever fixed partial denture supported by implants as primary abutment and secondary abutment, a comparison of model A and model B gave the following results [Table 1]. Overall, the maximum stress was in the cervical portion of the secondary abutment in model A. Model A (129.4 MPa) showed more stress when compared to model B (101.2 MPa). Model A (56 MPa) and model B (48.9 MPa) showed almost similar stress distribution in the implant abutment, with a slightly higher stress level in model A. In the connector region the maximum stress was between the primary and secondary abutment in model A (95.5 MPa) but in model B the maximum stress (96.9 MPa) was in the connector region between the primary abutment and the pontic. Model A (13 MPa) and model B (15.2 MPa) showed almost similar stress distribution in the bone around the neck region of the secondary implant, with a slightly higher stress level in model B.{Table 1}

The results of comparison between model C and model D are shown in [Table 1]. Overall, the maximum stress was in the cervical portion of the secondary abutment. Model D (124.3 MPa) showed more stress when compared to model C (102.8 MPa). Model C (47.4 MPa) and model D (50.9 MPa) showed almost similar stress distribution in the implant abutment, with a slightly higher stress level in model D. In the connector region the maximum stress was between the primary and secondary abutment in both model C (102.8 MPa) and model D (81.9 MPa). Model C (16.2 MPa) and model D (12.0 MPa) showed almost similar stress distribution in the bone around the neck region of the secondary implant, with a slightly higher stress level in model C.

Discussion

Cantilever fixed partial dentures supported by dental implants induce excessive stress concentration in the supporting alveolar bone, causing bone resorption under functional occlusal loads. This causes stress concentration in the cervical region of implants, compromising the longevity of the implant-supported prosthesis. [7]

Different features of implants will influence the stresses generated and thereby influence the success and outcome of a fixed partial denture. To gain insights into the biomechanics of oral implants, it is crucial to obtain an integrated understanding of the bone behavior around oral implants. Load transfer from implants to surrounding bone depends on factors such as thread pitch, thread shape, thread depth, length and diameter of the implants, shape of the implant, the bone-implant interface, prosthesis type and material, type of loading, and quantity and quality of the surrounding bone. [8]

According to the results of this study, the two-piece implant as secondary abutment showed the maximum amount of stress at the junction of the implant and abutment when used in conjunction with a one- or two-piece implant as primary abutment. The possible reason is that the two-piece implant system comprising a separate abutment and fixture joined by either internal or external hex screw creates a preload or over clamping forces between implant abutment and implant fixture. Pretorqueing is the tension created in a screw, especially in the fluked threading, when it is tightened. On further application of load it creates torsional or bending forces on the neck of the implant. This has a cumulative effect on the preload, leading to microrotation and stress concentration at the hex, which dissipates into the crest of ridge at the implant-tissue interface, leading to inevitable crestal bone loss. [8],[9]

When one-piece implant design is used as secondary and primary abutments and also when it is used in conjunction with a two-piece implant as primary abutment there was less stress concentration (48.9 MPa) than seen in the two-piece implant (56 MPa). In the one-piece implant design with no distinct interface or disjuncture between the abutment and the implant fixture, this dissipates the occlusal load along the implant fixture This is seen as increased force dissipation in the trabecular bone in all the models. [4] Thus, compared with the two-piece design, the one-piece implant distributes stress better within the implant

The common connector design must be thick enough to provide adequate resistance to occlusal loads; however, occlusal and gingival embrasures must be formed so as to ensure an esthetic restoration. Connectors should be more elliptical than circular to better withstand vertical loads. Correia et al. said that introduction of a more elliptical shape, as well as a fillet in the junction of the connector and the abutment, lowers the von Mises stresses. Thus, in the current study, the model developed had a connector height of 2.25 mm, which is approximately one-third of crown length, and was elliptical in shape. [10]

The stress concentration at the connector level in the current study was greatest in between the retainers of the primary and secondary abutments when two-piece implants were used as primary and secondary abutments and as well as when a combination of two-piece implant and one-piece implant was used as primary and secondary abutments. This is in agreement to the studies conducted by Himmel and Lazzara. [11],[12] However, when one-piece implants are used as primary and secondary abutments, the maximum stress in the connector region was between the primary abutment and the pontic. This could be due to the axial, rotational, and torsional forces [5],[10] and also because of the cantilever principle, according to which when the pontic is loaded occlusally, the abutment nearest to the cantilever pontic acts as a fulcrum (F). Whatever force is applied to the cantilever pontic, a force twice as great will be generated on the farthest abutment (resistance) from the cantilever. The force on the cantilever is a compressive one, whereas the force to the distal abutment is a tensile force. The load on the abutment adjacent to the cantilever is the sum of the two components (i.e., force acting on the pontic and force acting on the farthest abutment) and is also compressive. Thus, because of the variation in the thickness in the connector, the stress concentration was more in the connector region between the pontic and the primary abutment. [4]

In all the models the maximum stress distribution in the bone was around the neck region of the secondary implant due to the rigid connection between the bone and the implant. The modulus of elasticity of cortical bone is higher than that of spongy bone and, for this reason, cortical bone is stronger and more resistant to deformation. A consistent observation from the current study was the concentration of maximum stress at the bone-implant interface at the level of cortical bone. The results of this current study are in agreement with the findings of other researchers. [3],[13],[14],[15]

In all the models the maximum stress concentration was observed in the cervical portion of the secondary abutment as shown in [Figure 6], [Figure 7], [Figure 8] and [Figure 9]. With the occlusal forces acting on the cantilever fixed partial denture, the primary implant becomes a fulcrum and the secondary implant is subjected to axial, rotational, and torsional forces, resulting in maximum stress concentration. [11]{Figure 6}{Figure 7}{Figure 8}{Figure 9}

Limitations of Study

The model in this present study made several assumptions regarding the simulated structures. The structures in the model were all assumed to be homogenous, isotropic, and to possess linear elasticity. These properties of the materials modeled in this study, particularly the living tissues, are different from actual biological system. In this model a 100% implant-bone interface was simulated which does not necessarily simulate the clinical situation. Furthermore, the cement layer was not modeled.

Finite-element analysis is based on mathematical calculations after simulation of structures in its environment. Though finite-element analysis provides a very sound theoretical basis for understanding the behaviors of structure in a given environment, it should not be considered in isolation. Actual experimental techniques and clinical trials should follow the finite-element analysis to establish the true nature of any biological system. [16]

Conclusion

When one-piece and two-piece implants are used as primary and secondary abutments or in combination to support a three-unit distal cantilever fixed partial denture, the stress distribution is better with the one-piece implant design than with the two-piece implant design.

When the two-piece implant design and the one-piece implant design are compared, the stress concentration is more at the junction of the abutment and the implant fixture in the two-piece implant.

When implants are used as primary and secondary abutments to support the cantilevered distal fixed partial denture, irrespective of the implant position and implant design, the secondary implant shows the maximum amount of stresses.

One-piece implants and two-piece implants are similar with regard to stress created in the surrounding bone.

Clinical Implication

The results suggest that stress distribution is better when the one-piece implant is used to support a distal cantilever fixed partial denture.

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