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ORIGINAL RESEARCH  
Year : 2012  |  Volume : 23  |  Issue : 5  |  Page : 603-607
Macro design effects on stress distribution around implants: A photoelastic stress analysis


Ankara University, Faculty of Dentistry, Department of Prosthodontics, Ankara, Turkey

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Date of Submission30-Nov-2011
Date of Decision05-Aug-2011
Date of Acceptance15-Feb-2011
Date of Web Publication19-Feb-2013
 

   Abstract 

Objectives: Biomechanics is one of the main factors for achieving long-term success of implant supported prostheses. Long-term failures mostly depend on biomechanical complications. It is important to distinguish the effects of macro design of the implants.
Materials and Methods: In this study, the photoelastic response of four different types of implants that were inserted with different angulations were comparatively analyzed. The implant types investigated were screw cylinder (ITI, Straumann AG, Basel, Switzerland), stepped cylinder (Frialit2, Friadent GmbH, Manheim, Germany), root form (Camlog Rootline, Alatatec, Wilshelm, Germany), and cylindrical implant, with micro-threads on the implant neck (Astra, AstraTech, Mölndal, Sweden). In the test models, one of the implants was inserted straight, while the other one was aligned mesially with 15° angles. The superstructures were prepared as single crowns. A 150N loading was applied to the restorations throughout the test.
Results:
A comparison of the implant designs showed that there were no significant differences between the straight implants; however, between the inclined implants, the most favorable stress distribution was seen with the stepped cylinder implants. The least favorable stress concentration was observed around the root formed implants. Microthreads around the implant neck appeared to be effective in a homogenous stress distribution. Observations showed that misaligned implants caused less stress than straight implants, but the stress concentrations were not homogenous.
Conclusion: As there were observable differences between the implant types, straight placed cylindrical implants showed better stress distribution characteristics, while inclined tapering implants had better stress distribution characteristics.

Keywords: Biomechanics, implant macro designs, implant supported Fixed Partial Denture (FPDs), inclined implants, photoelastic stress analysis, stress distribution

How to cite this article:
Ozkir SE, Terzioglu H. Macro design effects on stress distribution around implants: A photoelastic stress analysis. Indian J Dent Res 2012;23:603-7

How to cite this URL:
Ozkir SE, Terzioglu H. Macro design effects on stress distribution around implants: A photoelastic stress analysis. Indian J Dent Res [serial online] 2012 [cited 2020 Jan 19];23:603-7. Available from: http://www.ijdr.in/text.asp?2012/23/5/603/107346
Implant design is a crucial factor in implant biomechanics. In each situation certain implant types, shapes and sizes, and restorative schemes might be more or less advantageous. Therefore, selection of a specific implant system should be made after careful consideration of the specific needs of the patient.

Many designs have been introduced to optimize bone and soft tissue loading under conditions of applied axial and oblique direction of compression, tension, and torque. These macroscopic geometric characteristics have helped to distribute applied forces along the implant-tissue interferences. [1] Major stresses occur around implants during mastication. If these stresses increase to higher levels they may lead to bone resorption. [2] To prevent complications like this, it is necessary to understand where the maximum stresses occur during mastication, around the implants. [3]

Using the Photoelastic Stress Analysis Method (PSA), the main objective of this study was to find out whether implant placement in the posterior region, with inclination, had a biomechanical rationale depending on the implant macro design.


   Materials and Methods Top


The photoelastic approach was selected to determine the load transfer differences between the straight placed and inclined implants. The photoelastic process detects tension distribution throughout the structure, providing a general overview of tension behavior, demonstrating quantity, quality, and distribution of forces in an object. On the other hand, although photoelasticity provides good qualitative information on the overall location and concentration of stresses, it produces limited quantitative information.

The models were prepared with photoelastic resin (PL-2; Vishay Measurents Group, Raleigh, USA). A 7 x 5 x 1 cm glass was used to mould the resin and prepare the models. [4],[5] A block model was used instead of a life-size mandible model as it was briefed that smaller models were suitable for parameter studies. [6],[7],[8],[9],[10],[11],[12] The photoelastic model was prepared according to the manufacturer's directions, using the poured resin technique that was used to represent a complete osseointegration. [13]

Four different implant types were included in the study. Implant lengths and diameters were selected as close as possible to each other. The differences of the lengths and the diameters were dependent on the different manufacturers of the systems [Table 1].
Table 1: Implant types used in the study

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For standardization of straight and inclined implants; a special apparatus was manufactured by computer-aided design [Figure 1] and [Figure 2]. This apparatus had two pieces. The main part had holes on the body, to provide the movement of the second part that held the implants. The second part had two extending parts, which sat inside the abutments' holes. Parallelization of the abutments was important for standardization of the inclined placed implants. This apparatus helped to hold the inclined implants for 24 hours during the polymerization of the photoelastic resin.
Figure 1: The computer-aided design of the apparatus that was used to hold the implants during polymerization (CAD)

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Figure 2: The computer-aided design of the apparatus that was used to hold the implants during polymerization (CAM)

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Conventional restorative techniques were used to fabricate the fixed restorations. Impressions were taken with polyvinyl siloxan impression material (Affinis, Coltené Whaledent AG, Altstätten, Switzerland) and the restorations were fabricated on stone casts. The Ni-Cr alloy was used for metal substructure and completed with conventional porcelain (Vita Omega Metalkeramik Vita Zahanfabrik Badsackingen Germany). The dimensions of the restorations were kept constant with a silicone putty matrix.

The restorations were cemented with temporary cement (RelyX Temp NE, 3M ESPE, Seefeld, Germany) to prevent any movement of the restoration while loading. [4],[14]

Vertical loads of 150N were applied on the central fossa of the restoration. A pin pointed apparatus was used for application of the load. This load was selected because it was within the realistic load levels and provided a satisfactory photoelastic response. [4],[9],[12],[13],[15],[16],[17]

The models were immersed in a mineral oil tank to minimize surface refraction and facilitate photoelastic observation. The load-induced stresses were monitored in the field of a circular polariscope and the images were captured at the loading area with a digital camera (Panasonic DMC-FZ30EG, Matsushita Electric Industrial Co. Ltd., Japan). Each loading was repeated at least twice, to ensure reproducibility of the results.

The evaluation of the stresses was made according to the concentration, number, and localization of the fringe orders formed around the implants [Figure 3].
Figure 3: Fringe orders

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   Results Top


The stress concentration sites were similar as the straight placed implants were loaded. The highest stress concentrations were at the apical sites of the implants. There were also stress concentrations at the marginal sites of the implants. The lowest stresses were observed at the medium one-third of the implants [Figure 4].
Figure 4: Loading on straight implants. Step-cylinder (Group a), screw-cylinder with microthreads (Group b), root form (Group c), and screw cylinder (Group d) (in clockwise direction)

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The highest stress concentrations at the apical site were observed at the root form (Group c) and stepped cylinder type (Group a) implants. The screw cylinder (Group d) and screw cylinder with microthreads around the implant neck (Group b) followed these. The tapering design of the root form and stepped cylinder type implants showed slightly higher stress concentrations at the apical site than the screw cylinder type implants.

The microthreads around the implant neck helped to distribute the stresses more homogenously. The bigger threads of the screw cylinder implant on the body also helped to distribute the stresses better than the smaller thread forms.

When the inclined implants were loaded, the stress concentrations were observed mostly at the side of the inclination. The highest stress concentrations were observed at the apical side and the mesial side of the marginal site of the implants. Higher stresses were observed at the medium one-third of the implants than the straight placed implants. At the apical site similar stress formations were observed around the root form (Group c) and stepped cylinder type (Group a) implants. The lowest stress concentration at the marginal site was observed around the stepped cylinder implant (Group a), while the highest concentration was observed around the root form implant (Group c) [Figure 5].
Figure 5: Loading on inclined implants Step-cylinder (Group a), screw-cylinder with microthreads (Group b), root form (Group c), and screw cylinder (Group d) (in clockwise direction)

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At the distal side of the implants, the stresses were mostly concentrated at the apical site. The highest stress observed at the apical site was around the root form implant (Group c). The stress concentrations around the stepped cylinder implant were lower than the root form implants.

The lowest stress was observed at the screw cylinder implant with microthreads around the implant neck (Group b). No stress was observed at the medium one-third of the implants, except around the screw cylinder implant (Group d). At the marginal site no stress was observed around the stepped cylinder and screw cylinder with micro thread implants.

The most uniform stress distribution was observed around the screw cylinder implant.


   Discussion Top


As the stress distribution around the dental implants cannot be measured by sensors, their quality and quantity inside the bone is not known. [4] On account of these difficulties, biomechanical studies are mostly done in vitro. Photoelastic stress analysis is used extensively to study the biomechanics of stress transfer in dentistry. However, the method has some limitations. The resin that is used to simulate the bone is homogenous and has isotropic characteristics, but the bone is not homogenous or anisotropic. Photoelastic resin increases the stresses also. On account of these reasons, the results of the studies do not resemble the actual values. However, in light of the answers taken from the studies, information about the behavior of the implants and the bone under stress can be achieved.

Photoelastic stress analysis (PSA) and finite element stress analysis (FEA) are two different methods to study the biomechanical behaviors of dental implants. The main difference between these two methods is the localization of the stress concentrations. In an FEA study, stress concentrations are higher, mostly around the implant neck, while in PSA studies, the stresses at the implant apex tend to be higher. Photoelastic analysis has been shown to provide more qualitative results regarding tension distribution, whereas, the finite element method provides detailed information about the type of tension. Hence, it would be advisable to use both methods, because they are complementary to each other. [18]

Well-balanced stress distribution around the implants may be achieved by vertical loading. However, elimination of lateral forces is impossible as they may be explained as lateral components of occlusal forces. Previous studies have indicated that destructive stress concentrations occur where the implant is inserted into the bone, the marginal bone. [19] Within these explanations, stress concentrations have to be directed toward the apical site in order to protect the marginal bone.

Stepped cylinder implants did not show much of a difference in vertical loading than the cylinder type implants. Under oblique loading, they showed better stress distribution than the cylinder types. [15] Siegele et al., [20] reported that conical or tapered implants showed a significantly higher stress concentration than the cylinder or screw type implants. In this study there was some increase in stress, especially at the apical site, but it was not prominently higher than the others. Tapering or root form implants were designed to transfer the stresses to apical sites. In this study, they acted according to what they designed for, but the stress concentrations at the apical sites were higher than in the cylindrical types. Besides that, it was emphasized that the clinical outcomes were acceptable. [21]

Thread forms are effective at stress distribution. The increase in the thread number means increase in the surface and contact area. With a wider surface area, mechanical behavior at the bone-implant connection is better. [22],[23] The size of the threads also affects stress distribution. Larger threads help to decrease stress concentrations and distribute stresses more homogenously. In this study, the macro threads of the screw cylinder implant showed better distribution. Also the micro-threads around the neck of screw cylinder implant in the study had a positive effect on the homogenous distribution of the stresses.

Under loading conditions straight placed implants showed symmetrical fringe patterns, while the inclined implants showed non-symmetrical patterns. [6] In this study the results seemed to be parallel with Brosh's study. [6] Fringe patterns were observed at the inclination side. At the inclined implants, vertical loading would not be parallel to the implant axis, which would act like oblique loading and cause non-symmetrical stress distribution and fringe patterns.

Inclined implants are thought to survive longer than the straight ones. [24] Mandibular molars are inclined 10° mesially and the maximum stress values around the straight placed implants are higher than a 10° inclined implant. An inclined implant is said to have greater surface area to support the occlusal plane. [25],[26] In this study, the stresses around the inclined implants were slightly lower than the straight placed implants, but they were not homogenous nor non-symmetric. Localization of the stress concentrations could induce bone resorption.

A number of implant systems with different designs and trade names are presented in the dental market for dentists and patients. At the end of the study, none of the systems is far better or worse than the others at stress concentrations and distribution. Each system has its own advantages. On account of this, for long-term success, careful planning, care, and maintenance should be provided alongside, besides the implant design.


   Conclusion Top


Under the light of these results it can be assumed that screw cylinder implants with microthreads on the implant neck are useful at stress distribution. Stepped cylinder implants have better stress distribution properties when they have to be placed inclined. Screw cylinder implants have acceptable stress distribution properties. Root form implants have greater stress concentrations than the other types when they transfer the stresses to the apical site.

 
   References Top

1.Lemons JE. Biomaterials, biomechanics, tissue healing, and immediate function dental implants. J Oral Implantol 2004;5:318-24.  Back to cited text no. 1
    
2.Kopp CD. Overdentures and osseointegration: Case studies and treatment planning. Dent Clin North Am 1990;34:729-39.  Back to cited text no. 2
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11.Tada S, Stegaroýu R, Kitamura E, Miyakawa O, Kusakari H. Influence of implant design and bone quality on stress/strain distribution in bone around implants: A 3-dimensional finite element analysis. Int J Oral Maxilofac Implants 2003;18:357-68.  Back to cited text no. 11
    
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16.Huang H, Huang J, Ko C, Hsu J, Chang C, Chen MY. Effects of splinted prosthesis supported a wide implant or two implants: A three dimensional finite element analysis. Clin Oral Implant Res 2005;16:466-72.  Back to cited text no. 16
    
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Correspondence Address:
Serhat Emre Ozkir
Ankara University, Faculty of Dentistry, Department of Prosthodontics, Ankara
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.107346

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