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Table of Contents
ORIGINAL ARTICLE
Year : 2012  |  Volume : 2  |  Issue : 1  |  Page : 2-8

Three-dimensional finite element analysis of effect of prosthetic materials and short implant biomechanics on D4 bone under immediate loading


Department of Periodontology and Implantology, H.K.E. Society's, S. Nijalingappa Institute of Dental Sciences and Research, Gulbarga, Karnataka, India

Date of Web Publication24-May-2012

Correspondence Address:
Shrikar R Desai
Department of Periodontiology and Implantology, H.K.E. Society's, S. Nijalingappa Institute of Dental Sciences and Research, Gulbarga - 585 105, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0974-6781.96556

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   Abstract 

Aim: The aim of the study is to evaluate the influence of maxillary cortical bone thickness (CBT) and crown prosthesis on stress distribution at bone bone-implant interface in single immediately loaded short and wide wide-diameter implants.
Materials and Methods: A three dimensional finite element model of a maxillary posterior section of bone (Type 4) with CBT of 0.5 mm was used in this study. Dental implant of length 7 mm length and diameter 5 mm diameter was modeled and inserted into maxillary models. Porcelain fused to metal (PFM) and acrylics were used for crown prosthesis. A total average occlusal force of 100 N was applied at the central pit of the crown in oblique (15° to the vertical) direction buccolingually.
Results: The micro micro-movements and Von Mises stresses were found to be lower for PFM crown than for acrylic crown on the short implant.
Conclusion: The peri-implant stresses on the cortical bone were reduced for PFM crown. Short and wide implant can be placed in D4 bone quality with thin CBT under immediately loading protocol using PFM crowns.

Keywords: Alveolar bone loss, immediate dental implant loading, maxilla


How to cite this article:
Desai SR, Singh R, Karthikeyan I, Reetika G, Jyothilaxmi. Three-dimensional finite element analysis of effect of prosthetic materials and short implant biomechanics on D4 bone under immediate loading. J Dent Implant 2012;2:2-8

How to cite this URL:
Desai SR, Singh R, Karthikeyan I, Reetika G, Jyothilaxmi. Three-dimensional finite element analysis of effect of prosthetic materials and short implant biomechanics on D4 bone under immediate loading. J Dent Implant [serial online] 2012 [cited 2022 Jan 25];2:2-8. Available from: https://www.jdionline.org/text.asp?2012/2/1/2/96556


   Introduction Top


For replacing the missing teeth, the use of dental implants has now become a widely recognized and practiced treatment modality. The complications related to implant placement and survival are constantly decreasing and the success rate is increasing. The classic two-stage protocol of implant placement is associated with longer treatment time, multiple patient visits, and higher treatment expenses, and the elimination of the healing period offers advantages in terms of cost of treatment and convenience to patients. [1] Thus, immediate loading protocol of dental implants has attracted ever-growing attention in the literature as well as in clinical practice. The main advantage of immediate implant loading is the significantly reduced time interval between implant surgery and prosthetic rehabilitation. The patients do not undergo the emotional and functional stress of being edentulous when they are treated under immediate implant loading protocol. [2],[3]

The finite element method (FEM) has been a useful tool in studying the bone-to-implant interface under mechanical loading. [4] By using finite element analysis (FEA) in immediate as well as delayed loading protocols, it has been found that the highest risk of bone resorption occurs in the neck region of an implant [5],[6],[7] Bone loss usually begins at the crestal area of the cortical bone and can progress toward the apical region, jeopardizing the longevity of the implant and prosthesis. [8] Bone quality and cortical bone thickness (CBT) is are an important factors determining the primary stability of implants. The classification scheme for bone quality proposed by Lekholm and Zarb [9] has since been accepted by clinicians and investigators as standard in evaluating patients for implant placement. In this system, the sites are categorized into 1 to 4 groups on the basis ofbased on the jawbone quality. In Type 1 (D1) bone quality, the entire jaw comprises homogenous compact bone. In Type 2 (D2) bone quality, a thick layer (2 mm) of compact bone surrounds a core of dense trabecular bone. In Type 3 (D3) bone quality, a thin layer (1 mm) of cortical bone surrounds a core of dense trabecular bone of favorable strength. In Type 4 (D4) bone quality, a thin layer (1 mm) of cortical bone surrounds a core of low-density trabecular bone. [10],[11],[12],[13] Jaffin and Berman reported that 55% of all implant failures occurred in D4 bone, with an overall 35% failure. [14]

The thickness of cortical bone also affects bone stresses and strains in implants. Previous studies indicate that thicker cortical bone reduces stress concentrations around implants, [15],[16] especially when it is increased from 0.5 mm to 2.5 mm. [17] The connection between osseointegrated implants and the surrounding bone is direct and relatively stiff; therefore, it may be assumed that an impact load applied to the implants will be transferred to the bone directly, causing bone microdamage and then marginal bone loss. [18] Thus, resilient occlusal materials such as acrylic resin [19],[20] have been recommended, especially in patients with inadequate marginal cortical bone, to reduce the impact effects arising from masticatory forces. Ismail et al.et al. [21] analyzed the influence of the occlusal materials (porcelain, precious and non-precious alloy, acrylic or composite resin) on the stress in bone and implant, and they reported similar results for all the investigated materials. In models of single implant-supported prostheses [22] and implant implant-supported complete arch prostheses, [23] occlusal material did not influence bone stress,; but in the model of the implant-supported complete arch prosthesis, it did influence retaining screw stress. [23],[24]

Short and wide-diameter implant has been suggested to restore tooth loss in the posterior region, where the dimension of the alveolus is greater than the diameter of a standard implant (3.75 mm). [25],[26] The jaw anatomy limits the choice of implant length. The available bone height is restricted by the presence of the inferior alveolar nerve and mental foramen in the mandible and the maxillary sinus in the maxilla. [25],[26],[27] The short- and mid-term performance of wide-diameter implants was reviewed by Davarpanah et al. and success rates between 82 and 97.7% were reported. [28] Studies [29],[30],[31],[32] have indicated that the highest failure rates occurred with the use of 7-mm implant, when placed in maxilla and in poor bone quality.

The three-dimensional (3D) FEA is considered an appropriate method for investigation of the stress throughout a 3D structure, and therefore this method was selected for bone and implants' stress evaluation in this study. Till date, there are no reports regarding the influence of different prosthetic materials along with CBT on stress distribution and micro micro-movements of short and wide wide-diameter (7 × 5 mm) implant placed in poor bone quality (D4) in maxilla under immediate loading protocol.

The objective of this study is was to evaluate the micro-movements of implant and stress distribution under the influence of use of different prosthetic materials on peri-implant bone, using immediately loaded short and wide wide-diameter implant (7 × 5mm) by 3D FEA.


   Materials and Methods Top


3D finite element models of maxillary section of bone with a missing first molar tooth were created. The bone was modeled as a cancellous core D4 bone surrounded by a 0.5-mm mm-thick cortical layer in the upper part. An implant of length 7 mm and diameter 5 mm was used for the study. To simulate the maxillary molar tooth, porcelain fused to metal (PFM) and acrylic resin crowns were used. PFM crown had an inner metal framework of 0.4 mm thickness and outer porcelain superstructure of 0.3 mm thickness, with a layer of cement between the two. The crown was assigned the standard dimensions of 10 mm mesiodistally and 11 mm buccolingually. The cervico-occlusal height of the crown was kept as 7.5 mm. [33] All the finite element models were created using a software program named ANSYS WORK BENCH, version 11.

Material properties

All materials used in the models were considered to be isotropic, homogeneous, and linearly elastic. The elastic properties used were taken from the literature [34],[35],[36],[37],[38],[39],[40],[41] [Table 1].
Table 1: Material properties

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Interface condition

To simulate the interface of an immediately loaded implant, a frictional coefficient of 0.6 [42] was applied at the bone bone-implant interface.

Loading conditions

Loads of 100 N were applied in an oblique direction, i.e. at an angle of 15° to the vertical, and in bucco-palatal direction. The forces were applied at the central pit of the first molar tooth. Implant length, bone model, maxillary molar tooth, mesh, and occlusal force are shown in [Figure 1].
Figure 1: Implant and bone models, Meshmesh, molar tooth, and occlusal force

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The parameters analyzed were

  1. total deformation (micro-movement) and
  2. Von Mises stress



   Results Top


The results of total deformation (Micromicro-movement) and Von Mises stress were found to be lower for PFM crown than those for acrylic crown. The values for total deformation were 0.021 and 0.013 mm for acrylic and PFM crowns, respectively [Figure 2]. Von Mises stress values were 108.12 MPa for acrylic crown and 88.22 MPa for PFM crown [Figure 3]. values were tabulated using graphs as shown in [Figure 4].
Figure 2: Total deformation of implant

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Figure 3: 0.5 mm cortical Von Mises stresses

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Figure 4: Graphs of total deformation, Von Mises stress

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   Discussion Top


The present study evaluates the effects of bone quality on 0.5 5-mm CBT and crown prosthesis (acrylic and PFM) on immediately loaded 7 mm × 5 mm implant placed in D4 bone.

FEA, a computer computer-based technique, calculates the behavior of engineering structures and their strength numerically. In the FEM, a structure is broken down into many small simple blocks or elements. A simple set of equations describes the behavior of an individual element relatively. The structure is built fully by joining together these set of elements, so the behavior of the whole structure is described by an extremely large set of equations which are actually the equations describing the behavior of individual elements joined together. The behavior of individual elements is assessed by computer from the solutions. Hence, the stress and deflection of all parts of the structure can be calculated. [43]

The most valid approach for implant placement is using a two two-stage protocol, but recently immediate functional loading has gained importance and comparable results have been obtained in single single-stage surgical procedure. [44] The expected advantageous results for immediate function are good primary stability, controlled loading conditions, and osseoconductive implant surface. [45]

When applying FEA to the dental implants, it is important to consider oblique occlusal force because it representss more realistic occlusal directions and, for a given force, will result in localized stress in cortical bone. [46] A 100 N force which was applied at central fossa, 15° to the vertical in bucco-palatal direction, was used in the present study.

According to Brunski and colleagues, [47],[48],[49],[50] implants can be loaded early or immediately if micro-movements above a threshold of 100 μm can be avoided during the healing phase. Stronger movements would lead to soft tissue ingrowth at the interface rather than to the desired bone apposition. Placing an immediately loaded implant in a thin cortical shell might exacerbate the disproportionate strain distribution in surrounding bone. A thin cortical bone might increase the sliding distance and gap distance between immediately loaded implant and the bone. The resulting enhanced micromotion might promote the ingrowth of a thin fibrous layer at the bone bone-implant interface instead of osseointegration. [46]

In the present study, there was a decrease in the total deformation for PFM crown by 36.13%. All implant displacement values measured in the current study were within the reported acceptable limits of micro-movement, i.e. below 100 μm. This study shows minimal micro-movements for PFM crown than for acrylic crown.

Zarb and Schmitt [51] stated that bone structure is the most important factor in selecting the most favorable treatment outcome in implant dentistry. With the use of 3D FEA, Sevimay et al.et al. [52] investigated the effect of different bone qualities on stress distribution in an implant implant-supported crown. They showed the presence of lower stresses for D1 and D2 bone qualities and increased stresses for D3 and D4 bone qualities because the trabecular bone was weaker and less resistant to deformation. The CBT in dentate and edentulous patients is a critical component of treatment planning in dental implant therapy. Several studies have considered the influence of CBT, [53],[54] , but most of them in delayed loading protocol. In the present study, Von Mises stresses were lower for PFM crown by 18.4%. Thicker cortical bone reduces stress concentrations around the implants; [15],[16] however, increasing the CBT above 2.5 mm had less effect on stress reduction, compared with an increase in CBT from 0.5 to 2.5 mm. These findings indicate that the bone stress and strain do not reduce linearly as CBT increases, with a limited range of CBT (<2.5 mm) being sufficient to provide a superior outcome in terms of decreasing the bone stress and strain around the implants. [17] The above studies did not evaluate the combined role of prostheses and CBT on the peri-implant bone stresses and strains of short implant. The results of the present study partly agree with the above study in having greater reduction of stress with PFM crown compared to acrylic crown in 0.5 5-mm CBT model in D4 bone.

Theoretical assumptions suggest that use of acrylic resin for the occlusal surfaces of a prosthesis would protect the connection between implant and bone. [55] Indeed, damping of the peak force transmitted to implants has been reported by in vitro studies using impact forces on resin resin-veneered superstructures. [56],[57] However, no significant differences were found between in vivo strain-gauge measurements of peak force at the abutment level with ceramic occlusal surfaces versus acrylic resin surfaces. [57] Similarly, no significant differences have been found in the clinical, radiologic, or histometric parameters of the peri-implant tissues (e.g. bone loss) supporting acrylic resin-veneered metal and ceramo-metal fixed prostheses in a dog model. [58] In comparison of the bone stress under static loading with occlusal materials such as acrylic resin, gold alloy, and porcelain, most FEA have shown no difference between the materials for single crowns and only very small differences for fixed prostheses. [23],[59],[60] The study done by Benzing et al. [61] showed that high-rigidity prostheses should be recommended because alloys with low elastic moduli used for the superstructure are predicted to demonstrate larger stresses at the bone-implant interface on the loading side than rigid alloys used for a superstructure with the same geometry. In the present study, PFM crown reduced stresses around the implant as compared to acrylic crown. The study results are in accordance with the above study as low modulus of elasticity (acrylic) transfers larger stress at bone bone-implant interface. A surgeon can encounter D4 bone quality when placing an implant, especially in posterior maxillary region. The present FEA study demonstrates successful placement of short and wide wide-diameter implant in maxilla in poor bone quality (D4) under immediate loading protocol. It was not possible to demonstrate a protective role of resin in comparison to PFM crown for reducing maximum stresses at bone bone-implant interface in poor bone quality. The results of the present study shows that a prosthetic material with higher modulus of elasticity (PFM) transmits less stresses at peri peri-implant bone levels than less rigid materials (acrylic).

One of the limitations of this study is the simplified geometry of the bone model. Even though the strength of a bone block is similar to that of jaw bone, the strain patterns might vary with the bone geometry. In addition, the material properties of the FE maxillary model were assumed to be isotropic and homogenous. The consideration of the anisotropic and inhomogeneous properties is still needed in future studies. Another limitation was the use of a static occlusal force in the FE simulations. Although oblique loading has been suggested to represent a realistic occlusal load, [4] chewing movement, especially with dynamic loading simulations, needs to be considered in future investigations.


   Conclusion Top


This study evaluates the effects of bone quality, CBT of 0.5 mm, and crown prosthesis (acrylic and PFM) on immediately loaded short and wide diameter (7 mm × 5 mm) implant placed in D4 bone. Within the limitations of the 3D FEA study, the following conclusions can be drawn:

  1. The implant displacement values for acrylic and PFM crowns were within the acceptable limits of micro-movement, i.e. below 100 μm, and were found to be lower for PFM crown.
  2. Usage of PFM crown might reduce the stresses compared to using acrylic crown around 0.5 mm CBT.
  3. Short and wide implant can be placed in D4 bone quality with thin CBT under immediately loading protocol using PFM crowns.


Further randomized clinical trials are needed to validate the results of FEM study.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

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