|Year : 2011 | Volume
| Issue : 2 | Page : 80-85
Effect of two different abutment types on stress distribution in the bone around an implant under two loading conditions
Siddharth Shelat, BS Kularashmi, H Annapoorani, R Chakravarthy
Department of Prosthodontics and Implantology, Meenakshi Ammal Dental College and Hospital, Chennai, India
|Date of Web Publication||30-Dec-2011|
405/406 Shivalaya Towers, 90 Feet Road, Thakur Complex, Kandivali (East), Mumbai - 400 101, Maharasthra
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Aim: To investigate the effect of two different abutment types on stress distribution in the bone around an implant under two loading conditions, vertical load and combined load (vertical + angle of 45°).
Materials and Methods: Implant of 4.2 × 12 mm 2 was used. Two 2-piece implant systems, Internal Hex and External Hex implant-abutment complex were used. The implant-abutment complex was embedded in bone and subjected to static load of 100 N vertically and a combined load (vertical + 45° angulation). Finite Element modeling of bone implant and abutment was done using Ansys Classical 10.0 software.
Results: In external hex system Von Mises stress was 0.784 MPa and 3.502 MPa for spongious bone and 8.60 MPa and 45.126 MPa for compact bone under vertical and combined loading conditions respectively. Internal hex implant system showed values of 0.922 MPa and 2.22 MPa for spongious bone and 6.798 MPa and 26.29 MPa for compact bone for vertical and combined loading conditions.
Conclusion: Internal Hex Implant system generated the lowest maximum Von Mises stress for all loading conditions. The maximum Von Mises stress occurred in the region of the compact bone under all loading conditions irrespective of the type of abutment use. Significant reduction in Von Mises stress was observed at the boundary between compact and spongious bone because of relatively low elastic modulus of spongious bone.
Keywords: External Hex, finite element analysis, Internal Hex, Von Mises stress
|How to cite this article:|
Shelat S, Kularashmi B S, Annapoorani H, Chakravarthy R. Effect of two different abutment types on stress distribution in the bone around an implant under two loading conditions. J Dent Implant 2011;1:80-5
|How to cite this URL:|
Shelat S, Kularashmi B S, Annapoorani H, Chakravarthy R. Effect of two different abutment types on stress distribution in the bone around an implant under two loading conditions. J Dent Implant [serial online] 2011 [cited 2020 Apr 6];1:80-5. Available from: http://www.jdionline.org/text.asp?2011/1/2/80/91284
| Introduction|| |
The extensive variety of implants available today can be categorized and classified in a number of different ways.  The most logical differentiation and distinction are based on implant/abutment interface, body shape and implant- to-bone interface. 
Poorly designed implants can create regions of increased stress in peri-implant bone and induce severe resorption, leading to gradual loosening and finally complete loss of implant. Analysis needs to be done to evaluate stress distribution with various implant systems to improve implant design. 
Load transfer from implant to surrounding bone depends on, type of loading, bone-implant interface, length and diameter of implants, shape and characteristics of implant surface, prosthesis type, and quantity and quality of surrounding bone. 
Stresses and strains generated have been evaluated by methods like photo-elasticity, strain gauge analysis and finite element analysis. Photo-elasticity provides good qualitative information on the overall location and concentration of stresses, but produces limited quantitative information. The strain gauge measurements provide accurate data regarding strains only at the location of the gauge. The finite element method is capable of providing detailed quantitative data at any location within a mathematical model. Two and three-dimensional finite element analysis have been used to evaluate the stresses around various dental implant systems. 
Previous studies have mainly focused on the effect of implant shape on stress distribution in bone and on the mechanics of implant and abutment connections for abutment loosening.  Since the load transfer mechanism of an implant can be altered significantly by the shape of the abutment the objective of this study was to investigate the effect of two different abutment types on stress distribution in the bone around an implant under two loading conditions (vertical load and combined load (vertical + 45° angulation). 
| Materials and Methods|| |
Implant with a standard outer diameter 4.2 mm and length 12 mm (Indident Dental Co.) was modeled. Two 2-component implant abutment system a) Internal Hex and b) External Hex were used. Ansys Classical 10.0 Software was used for computer simulation of the components of the study.
The FEA model assumed a state of optimal osseointegration, which means that the cortical and trabecular bone are assumed to be perfectly bonded to the implant
The implant is a self-threaded, cylindrical fixture made of titanium alloy. Components were assigned the isotropic elastic properties of titanium. A three-dimensional finite element model of bone was constructed. A layer of cortical bone 2-mm thick was contoured around the implant neck and the body of implant embedded in spongious bone. A fixed bond, i.e., total osseointegration, between bone and implant along the whole interface was assumed which meant that under the applied load on the implant, relative motion between bone and implant did not occur.
ANSYS CLASSICAL 10.0 software was used for solid modeling, geometric construction, finite element mesh creation and post-processing. Dimensional drawing of the insert from the manufacturer was available. The model was created using solid 45 elements (The element is defined by eight nodes having three degrees of freedom at each node with translations in the nodal x, y and z directions. The element has plasticity, creep, swelling, stress stiffening, large deflection and large strain capabilities). Different regions of the model (spongious bone, compact bone, implant, abutment) were modeled and material properties appropriately assigned.
A load of 100 N (Static Load) was applied along the long axis of the implant vertically and at an angle of 45° to the long axis. Load was applied at the central point of the abutment fixture. The measurements were made along the x, y and z axis.
The values obtained were in Kgf/mm 2 .
The following conversion was used to convert the values into Mpa
1 Kgf/mm 2 = 1 MPa/9.81
Therefore: 1 MPa = 9.81 × 1 Kgf/mm 2
The material properties of the cortical bone, cancellous bone and the implant fixture were
1 GPa = 10 3 MPa
Refer [Figure 1] and [Figure 2] below.
| Results|| |
The results of numerical analysis are shown for Von Mises stresses occurring for External Hex and Internal Hex in a) compact bone, b) spongious bone subjected to vertical load and combined load (vertical + 45° angulation) of magnitude 100 N.
The values for Von Mises stress concentration for each of the three implant abutment complex were obtained.
External Hex implant abutment complex: Von Mises stress was 0.784 MPa for spongious bone under vertical loading conditions and the Von Mises stress were found to be 3.502 MPa under combined load (vertical + 45° angulation). Von Mises stress for compact bone was 8.60 MPa under vertical load and 45.126 MPa under combined load.
Internal Hex implant abutment complex: Von Mises stress was 0.922 MPa for spongious bone under vertical loading conditions and 2.22 MPa under combined load. Von Mises stress for compact bone was found to be 6.798 MPa under vertical loading condition and 26.29 MPa under combined loading condition [Table 1] and [Table 2]. Refer [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9] and [Figure 10] also.
|Figure 3: External Hex Von Mises stress (Spongious bone under vertical load)|
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|Figure 4: External Hex Von Mises stress (Compact bone under vertical load)|
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|Figure 5: External Hex Von Mises stress (Spongious bone under combined load)|
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|Figure 6: External Hex Von Mises stress (Compact bone under combined load)|
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|Figure 7: Internal Hex Von Mises stress (Spongious bone under vertical load)|
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|Figure 8: Internal Hex Von Mises stress (Compact bone under vertical load)|
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|Figure 9: Internal Hex Von Mises stress (Spongious bone under combined load)|
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|Figure 10: Internal Hex Von Mises stress (Compact bone under combined load)|
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|Table 1: Von Mises stress values: External Hex implant abutment complex values in MPa|
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|Table 2: Von Mises stress values: Internal Hex implant abutment complex values in MPa|
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| Discussion|| |
To achieve stable osseointegration for implant restoration, the generation of high stress concentration or distribution in bone should be avoided, since the high level of stress concentration or distribution can induce severe resorption in the surrounding bone, leading to gradual loosening and finally complete loss of implant. Therefore study of the effect of abutment type on stress distribution in bone is important. 
The strength of materials principle states that if the implant supporting tissue has homogenous elastic properties, the axial load transmitted from implant to bone concentrates highly in the upper region of bone and decreases rapidly toward the implant base.
When a vertical load (point load) was applied along the long axis of the implant abutment complex on the top of the abutment fixture Von Mises stress was seen maximum in case of external hex implant abutment system at the compact bone level. This is in accordance with a study conducted by Heoung-Jae Chun et al. 
Bone loss greater around cylindrical shaped implants compared to screw-shaped implants. An implant's elastic behavior is not the only governing factor. An implant's geometry is important in properly distributing stress from implant to the bone according to Rieger MR et al. 
External Hex showed a Von Mises stress of 8.60 MPa compared to 6.78 MPa of Internal Hex. In case of External Hex and Internal Hex implant abutment complex system the load is concentrated maximum at the compact bone level. Very minimal stress is distributed along the length of the implant into the surrounding spongy bone. Marginal bone loss around implant reported. In External Hex system the maximum stress is concentrated at the implant abutment interface because of the reduced load transfer area between abutment and implant thus the stress concentration at spongy bone level is comparatively less.  This is in accordance with a study conducted by Heoung-Jae Chun et al. 
Stress induced by occlusal load are initially transferred from implant to cervical bone. Small amount of remaining spreads to cancellous bone, higher stress values observed in compact bone because of higher modulus of elasticity compared to spongious bone. Bone loss initiated in implant neck region. Spongious bone shows lower stress concentrations than compact bone because of the low density of bone compared to compact bone. This shows most of the stress concentration takes place at the compact bone level. Significant reduction in Von Mises stresses for all the systems was seen between compact bone and spongy bone because of low elastic modulus of spongious bone. 
According to animal and clinical studies bone loss around implants that may lead to implant failures was associated in many cases with unfavorable loading conditions. Suggesting that implant shape, diameter, length, prosthesis and abutment type is not the only reason for marginal bone loss followed by implant failure. External Hex implant abutment complex generated the maximum Von Mises stress under vertical loading condition and Internal Hex generated the least stress. Spongy bone under vertical load shows more stress concentration at the apex of the implant in case of Internal Hex implant abutment system. This is in accordance with a study conducted by Heoung-Jae Chun et al. 
There is higher Von Mises stress concentration in spongy bone in Internal Hex implant abutment complex system compared to External Hex which showed Von Mises stress value of 0.784MPa. Von Mises stress was 0.92MPa for Internal Hex implant abutment complex.
The present study expressed results in both equivalent stress (Σ E Max) and principal stress (Σ1 ). But the former was preferred for the comparison between models. The equivalent stress is a scalar quantity that includes all components of the stress strain tensor and allows comprehensive comparison between models. The implant and abutment function as one unit distributing the force evenly along the length of the implant showing comparable values.
No component of load acts singularly in a functional situation. There is a combination of load acting in vertical as well as horizontal direction. In this study there has been a combination of vertical and horizontal load acting at the same time and its effect on the stress concentration in bone is documented.
The addition of horizontal component of inclined load generates increase of moment and eventually increases the compressive load to a level higher than the compressive load generated by only the vertical component of load at the compact bone.  Maximum stress concentration is seen in relation to External Hex and least by Internal Hex implant abutment complex.
External Hex shows a Von Mises stress of 3.502 MPa in spongious bone and 45.126 MPa in compact bone. Internal Hex with values of 2.22 MPa and 26.29 MPa for spongious and compact bone, respectively. In case of Internal Hex, the Hex being within the implant body makes the implant abutment complex to behave as one unit compared to External Hex thus producing the least amount of stress concentration. When comparing the Internal and External Hex implant abutment complex the External Hex system shows higher Von Mises stress values as the Hex is not within the implant body but is at the implant abutment interface and also the surface area covered between the implant and abutment is greater in case of Internal Hex system.
| Conclusions|| |
Within the limitations of the study, the following conclusions can be drawn:
- The maximum Von Mises stress occurred in the region of the compact bone under all loading conditions irrespective of the type of abutment used.
- Von Mises stress when monitored for vertical and combined load.
Conditions, showed that there was a gradual decrease from marginal bone level to apex of implant
- Significant reduction in Von Mises stress was observed at the boundary between compact and spongious bone because of relatively low elastic modulus of spongious bone.
- Size of the contact area between the abutment and the implant significantly influenced the stress distribution and magnitude of Von Mises stress generated in bone for all implant systems.
- Under vertical loading condition (point load) both implant abutment complexes the differences in stress concentration in compact bone were not significant
- Under combined (vertical + 45° angle) loading condition there was a significant difference in Von Mises stress concentration
- Internal Hex implant system generated the lowest maximum Von Mises stress for all loading conditions.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1], [Table 2]