|Year : 2019 | Volume
| Issue : 3 | Page : 143-149
Effect of different force magnitudes on the photoelastic stress in overdenture retained by two implants
Júlia Trevizam Campana1, Marcelo Ferraz Mesquista1, Valentim Adelino Ricardo Barão1, Mauro Antonio De Arruda Nóbilo1, Carmem Silvia Pfeifer2, Rafael Leonardo Xediek Consani1
1 Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas, Piracicaba, SP, Brazil
2 Department of Restorative Dentistry, Division of Biomaterials and Biomechanics, Oregon Health and Science University, Portland, OR, USA
|Date of Web Publication||3-Jul-2019|
Rafael Leonardo Xediek Consani
901 Limeira Ave., 13414-903 Piracicaba, SP
Source of Support: None, Conflict of Interest: None
Aim: The aim of this study was to evaluate the photoelastic stress on mandibular overdenture retained by single implant when submitted to occlusal forces exerted by maxillary conventional denture in occlusion, or single axial force on first molars. Materials and Methods: Occlusal forces of 10, 20 and 30 kgf were exerted on the maxillary denture in occlusion with the mandibular overdenture adapted in the photoelastic model. Axial forces with same magnitudes were also individually exerted on the right and left first molars of the overdenture. Qualitative analysis were made in images obtained with polariscope, and quantitative analysis with the FRINGES program. Results: Qualitative analysis showed that the stress was located predominantly around the implant in all force magnitudes. Increase of occlusal force promoted higher stress on the implant. Increased axial single force on the left and right first molars caused higher stress on the medial region of the mandible body, and higher stress was induced in the side of loading. Quantitative analysis showed that the occlusal force promoted the following values: 10 kgf (T = 252.58; N = 0.54); 20 kgf (T = 1033.87; N = 2.21) and 30 kgf (T = 1009.99; N = 2.16); axial force on left molar promoted the following values: 10 kgf (T = 256.95; N = 0.55); 20 kgf (T = 265.07; N = 0.72) and 30 kgf (T = 266.38; N = 0.70); and axial force on right molar promoted the following values: 10 kgf (T = 986.11; N = 2.11); 20 kgf (T = 969.30; N = 2.07) and 30 kgf (T = 1012.68; N = 2.16). Conclusion: In conclusion, the stresses were concentrated at around the implant and in the medial region of the mandible body when the overdentures were submitted to occlusal forces; axial forces on the molars promoted stress at the implant and the mandible body in the side of loading, and the increase of the force promoted higher stress in both loading types.
Keywords: Implant, loading, overdenture, photoelastic analysis, stress
|How to cite this article:|
Campana JT, Mesquista MF, Ricardo Barão VA, Nóbilo MA, Pfeifer CS, Xediek Consani RL. Effect of different force magnitudes on the photoelastic stress in overdenture retained by two implants. Indian J Dent Sci 2019;11:143-9
|How to cite this URL:|
Campana JT, Mesquista MF, Ricardo Barão VA, Nóbilo MA, Pfeifer CS, Xediek Consani RL. Effect of different force magnitudes on the photoelastic stress in overdenture retained by two implants. Indian J Dent Sci [serial online] 2019 [cited 2019 Sep 17];11:143-9. Available from: http://www.ijds.in/text.asp?2019/11/3/143/261945
| Introduction|| |
Caries and periodontal diseases increase demand for prosthetic treatments that restore esthetic, phonetics, mastication, and comfort for the patients. Complete conventional dentures have been the treatment commonly offered for edentulous patients; however, alveolar bone loss decreases the denture retention and stability, resulting in discomfort for the users.
The continuous reduction of the alveolar ridge appears to be a proceeding that occurs in all denture wearers, being considered a normal physiological process; however, the magnitude of the alteration depends upon multiple variables. Furthermore, intermittent pressure on the denture causes greater alveolar bone resorption when compared to continuous pressure although the resorption intensity appears more related to the use of the prosthesis than to disuse atrophy.
Denture better adapted may recover part of the masticatory function and minimize the atrophy; however, the atrophy process is not interrupted. Thus, it is not known with certainty what are the factors that most influence the loss of the alveolar bone crest. The advent of osseointegrated dental implants enhanced the quality of life for patients with conventional implant-supported prostheses, improving the oral function due to better retention, stability, and chewing efficiency.
There are basic principles for the treatment with implants to retain overdentures in the edentulous mandible. Although treatments are feasible and the results are clinically satisfactory, there are advantages and disadvantages in each specific case. It has been suggested the use of overdentures retained by a single implant placed in the interforame region of the mandible with severe alveolar bone resorption and impossibility to install two or more implants in the distal region. In addition, there is a report suggesting that single implant may result in high success rate, when compared to overdentures retained by multiple implants. Conversely, a previous study reported an unacceptably high failure rate (37.5%) for overdentures retained by a single implant.
Deformations may occur at peri-implant region when the stresses exceed the physiological limit of the alveolar bone causing microfractures and unwanted bone loss. Moreover, the bone density and the mineralized bone-to-implant contact were higher adjacent to laterally loaded implants than at unactivated sites. Thus, it was suggested that a static load applied to implants in the lateral direction causes in a structural adaptation of the peri-implant bone.
The photoelastic analysis evaluates the stresses in complex mechanical structures. This method is interesting because it observes the distribution of stress throughout the structure, enabling a general insight into the model mechanical behavior. In addition, it provides visual display of tensions of a particular model with the aid of the polariscope. Two types of fringes (stress) are revealed: (i) colored patterns (clear) which are the isochromatic fringes, representing the intensity of the stresses, and (ii) the dark lines, isoclínicas calls, overlapping the colored fringes related to direction of the tension. For application in dentistry, the main information required is location and intensity of stress which may be measured and/or photographed.
The aim of this study was to evaluate the photoelastic stresses in the mandibular overdenture retained by a single implant when submitted to occlusal forces exerted by maxillary conventional denture or to single axial forces on the first molars. The hypothesis tested was that different force types and force magnitudes would promote different stresses on overdenture supported by single implant.
| Materials and Methods|| |
A conventional maxillary denture and a mandibular overdenture were traditionally made with thermo-activated acrylic resin (QC-20; Dentsply, Petropolis, RJ, Brazil). Acrylic resin record bases (VipiCril Plus; Vipi Dental Products, Pirassununga, SP, Brazil) and wax occlusal rims (Kota, Sao Paulo, SP, Brazil) were used for maxillomandibular relation in a semi-adjustable articulator (A7 Plus; Bioart, Sao Carlos, SP, Brazil).
The arrangement of the artificial teeth (Vivadent PE and Orthosit PE; Ivoclar Vivadent, Barueri, SP, Brazil) was made in the maxillary and mandibular wax occlusal rims. O'ring attachment system supported by hexagon external implant with 4.1 mm in diameter and 10 mm in length and corresponding transfer system (Conexao Prosthesis System; Aruja, SP, Brazil) were used in the study. They were placed on the mandibular Type IV dental stone cast (Durone; Dentsply), and the transfer system screwed in the impression copy using an acrylic resin customized tray (VipiMold; Vipi) fabricated with an access opening for impression transfer system.
The dental stone cast with the components was replicated with silicone impression material (Silibor; Classico Dental Products, Sao Paulo, SP, Brazil). After 24 h, the impression copy was released from the fixation screw, and the implant placed in the silicone mold used to make the photoelastic model. The photoelastic resin (Araldite; Huntsman, Sao Paulo, SP, Brazil) is composed of a reactive liquid Gy-279 BR (derived from bisphenol A) and a hardener HY 2964 (derived from cyclo-aliphatic amine). The amount of the resin was calculated following the manufacturer's recommendation (100 parts of GY279-48 parts of HY2964). The curing occurs at room temperature allowing the production of transparent photoelastic models.
The capture of the O'ring attachment system that was previously positioned on the photoelastic model was made by using self-curing acrylic resin (VipiFlash, Vipi), and the capture material region was polished to stay as translucent as other regions of the overdenture.
The photoelastic model [Figure 1] was submitted to occlusal forces of 10, 20, and 30 kgf exerted on the maxillary conventional denture positioned in occlusion with the mandibular overdenture  and the stress evaluated in different mandible locations (frontal, and left and right sides). Axial single forces with similar magnitudes were individually exerted on the first right or left molar and evaluated in these same positions. During chewing, there are loads of centric occlusion and lateral movements. Even if the rehabilitation is considered symmetrical because the implant is placed in the midline, the entire overdenture making process (resin bases, teeth arrangement, and processing technique) and relation with the conventional maxillary denture may conceal small differences between the two sides of the prostheses that could influence differently the magnitude of the concentrated stresses. In addition, laterality movements during clinical occlusion are different for each side and depend or are influenced by the patient's normal or parafunctional habits.
|Figure 1: Mandibular photoelastic model with single implant placed in the midline|
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The most relevant positions were observed for each force type, and the points from R1 to R11 were selected along the mandible. For the occlusal force, three positions (P1 – right, P3 – frontal, and P5 – left) were selected, and for the axial single force, one position for the right molar load (P1) and another for the left molar load (P5) were selected. The region evaluated for each location was standardized so that it did not alter the location between one image and another. In this way, it was possible to choose standard points in the different structures for analysis of the maximum shear stress.
A circular polariscope (PTH-A-01 model; Federal University of Uberlandia, MG, Brazil) analyzed the stresses, and a digital camera (Canon EOS XSI; New York, NY, USA) takes the images. The color pattern versus fringe order analysis was according to the schematic demonstration of isochromatic fringe order for maximum shear stress in FRINGES program (MatLab environment; Federal University of Uberlandia), based in the comparison among fringe orders.
The images of the photoelastic model and the optical constant (Kσ =0.468 kgf/mm) of the photoelastic resin were inserted in the FRINGES program. Based on the equations inserted and the fringe orders informed by the examiners, the FRINGES program provided the maximum shear stress (T) for predetermined points. The force was applied so that the fringe orders in each position did not exceed the fringe order 4. After loading, each image was analyzed and the values of the fringe orders and shear stress were obtained for each point, and the mean values for each position and force were also calculated.
Stresses were evaluated to identify the fringe orders and compare the concentrations occurred on implant and simulated alveolar bone (photoelastic model). In this condition, the study considered the following methodological aspects: one photoelastic model was used, and Adobe Photoshop 7.0 software analyzed the photoelastic images. Images of the stresses permitted to verify the passivity of the structures after overdenture fixation or when the force was applied., The method recorded the frange orders and the direction of stress propagation.
Two evaluators analyzed the tensions. When there was no agreement between them or there were doubts about the reliability of the results, the third evaluator was consulted to settle the doubts and to establish agreement among the three examiners.
| Results|| |
[Figure 2] shows the frontal view of the photoelastic model with a single implant placed in the midline of the mandible and submitted to occlusal forces of 10, 20, and 30 kgf, respectively. Similar concentration of stresses occurred around the implant for the three force magnitudes, mainly at middle third of the implant. Increased force promoted higher stress on the implant apex.
|Figure 2: Frontal view of the photoelastic model with single implant placed in the midline of the mandible, and submitted to occlusal forces (10, 20, or 30 kgf, respectively)|
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[Figure 3] shows the left-sided view of the photoelastic model with single implant placed in the midline of the mandible and submitted to occlusal forces of 10, 20, and 30 kgf, respectively. Stress distribution occurred at the left side of the implant and along the mandible body. The stress was greater with the increase of the force.
|Figure 3: Left side view of the photoelastic model with single implant placed in the midline of the mandible, and submitted to occlusal forces (10, 20, and 30 kgf, respectively)|
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[Figure 4] shows the right-sided view of the photoelastic model with single implant placed in the midline of the mandible and submitted to occlusal forces of 10, 20 and 30 kgf, respectively. Stress distribution occurred at the right side of the implant and along the mandible body. The stress was greater with the increase of the force.
|Figure 4: Right side view of the photoelastic model with single implant placed in the midline of the mandible and submitted to occlusal forces (10, 20, and 30 kgf, respectively)|
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Force on first left molar
[Figure 5] shows the photoelastic model with single implant placed in the midline of the mandible and submitted to axial forces on the first left molar of 10, 20, and 30 kgf, respectively. Stress distribution occurred along the mandible body. Increase of stress was proportional to force magnitude, and it was more evident in the premolar region and around the implant.
|Figure 5: Photoelastic model with single implant placed in the midline of the mandible, and submitted to axial forces on the first left molar (10, 20, and 30 kgf, respectively)|
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Force on first right molar
[Figure 6] shows the photoelastic model with single implant placed in the midline of the mandible and submitted to axial forces on the first right molar of 10, 20, and 30 kgf, respectively. Even with the increased force magnitude, lower stress occurred at the implant and right side of the mandible body when compared to the left side.
|Figure 6: Photoelastic model with single implant placed in the midline of the mandible, and submitted to axial forces on the right molar (10, 20, and 30 kgf, respectively)|
Click here to view
[Table 1] shows the means of shear stress (T) and fringe order (N) according to force magnitude for overdenture retained by single implant and submitted to occlusal forces. Increased values of shear stress (T) and fringe order (N) were shown with the force magnitude increase, mainly for 20 kgf.
|Table 1: Means of shear stress and fringe order for overdenture retained by single implant submitted to occlusal force|
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| Discussion|| |
Previous case report has shown that overdentures retained by single implant placed on the mandibular midline region is an appropriate treatment for compromised atrophic mandibles with impossibility to place implants on distalized regions. The literature has also shown that photoelastic analysis has been utilized to verify the biomechanical behavior of prostheses during occlusal loading.,,,
Different occlusal forces [Figure 2], [Figure 3], [Figure 4] and axial single forces [Figure 5] and [Figure 6] promoted different stresses on the simulated mandible. Similar stress concentration occurred around the implant for the three forces, and increased magnitude promoted higher stress on the implant [Figure 2]. Stress occurred along the left side of the mandible between implant and distalised region, and the stress was greater with the increase of the force [Figure 3]. Stress distribution occurred along the right side of the mandible between implant and distalised region, and the stress was greater with the increase of the force [Figure 4]. Stress occurred along the mandible body between implant and distalized region is proportional to force and more evident in the premolar region [Figure 5]. Low-stress concentration occurred at the implant and right side of the mandible body when compared to the left side [Figure 6].
Since the current study showed differences in the stresses exerted on overdenture retained by single implant submitted to occlusal forces and axial single forces on the first molars, the work hypothesis was accepted.
A strain-gauge investigation showed that compressive force applied at different sites has significant influence on different mandibular prostheses. In the current study, conversely, the overdentures were occlusally charged with standardized forces exerted by the maxillary conventional denture. By this reason, it is reasonable to infer that the results obtained were not negatively influenced by the location or inclination of the forces. Previous photoelastic analysis also showed that the number of implants was not significant for the stress values on peri-implant region when submitted to axial forces exerted on the right molar; however, different types of implant-retained prostheses promoted different stress values. Finite element analysis showed that vertical forces on the lower incisors of single-implant overdenture promoted implant microrotation; moreover, the overdenture did not present harmful strain on the peri-implant bone. In addition, balanced occlusion is necessary for retention and stability of implant-supported overdenture when occlusal acceptable forces are clinically applied.
Study in vitro using strain-gauge revealed that single-implant overdenture with ball attachments showed biomechanical effect similar to two-implant overdenture in terms of lateral forces to the abutment and denture base movements when submitted to functional force at the molar.
In the current study, occlusal or axial forces on the molar promoted different shear stress (T) and fringe order (N) on the simulated mandible [Table 1], [Table 2], [Table 3]. Increased values of (T) and (N) were shown mainly for 20 kgf [Table 1]. Similar increase of (T) and (N) were shown with the load increased [Table 2]. Higher values of (T) and (N) were shown mainly for 30 kgf [Table 3].
|Table 2: Means of shear stress (T) and fringe order (N) for overdenture retained by single implant submitted to axial force on the first left molar|
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|Table 3: Means of shear stress (T) and fringe order (N) for overdenture retained by single implant submitted to axial force on the first right molar|
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Previous study showed that the force transferred to a single mandibular implant is evenly distributed around the implant with low-stress concentration whatever the type of retention system. However, no significant difference was observed in the values obtained with implants of different diameters placed on the midline of the mandible in relation to primary stability. In addition, no clear correlation was established between stability and implant diameter.
The facts aforementioned seem to confirm the results of the current study, mainly when the increase of load was related to stress increased and to the levels obtained in the loading of the first molars. Moreover, chewing promoted lower forces compared to maximum biting in centric occlusion for implants supporting overdentures.
In view of these results, it may be assumed that the stress around the implant occurs because it acts as a stress concentrator. It may also be alleged that the portion mucus-supported assumes significant support to detriment of stress transmitted to the implant and alveolar bone. This fact prevents or hampers that the stress around the implant is transferred and concentrated at alveolar ridge under the area of the overdenture with mucosal support. However, without further studies involving other variables would not be prudent to believe that the increase of the stress occurring on this overdenture may result in higher bone loss impairing the maintaining of the osseointegration in the long term.
Lateral or tangential forces applied to the right or left first molars may promote motion of bascule over the implant, causing additional stress on the implant in the application side. Even with the increased force, lower stress occurred on the right side of the mandible body when compared to the left side, suggesting different biomechanical behavior between the sides of the overdenture. Since the force applied was similar for both overdenture sides, other variables as possible little differences in volume, height, and width existing in the simulated alveolar ridge of the photoelastic model could be responsible for these results. Moreover, this supposition could be verified in studies using finite element analysis and clinical trials. In addition, retention and stability of an overdenture implant-retained are significantly affected by the number and implant distribution and abutment type.
Clinically, the most common mechanical implant complications (incidence >15%) for overdentures were loosening of the retention, implant loss in maxillary, and clip/attachment fracture. Stress was highest on the trabecular and cortical bones in single implant-retained mandibular overdenture while stress across the denture and implant was highest in two implant-retained mandibular overdentures. However, no significant difference was shown in relation to implant survival rate and peri-implant bone loss.
Although previous study has shown that mandibular overdenture retained by two implants promoted better patient satisfaction in the follow-up at 12 months and better masticatory performance than mandibular overdenture retained by single implant, after 5 years in function, meta-analyses revealed that there were nonsignificant differences regarding overall prosthetic complications when mandibular overdentures supported by a single implant were compared with overdentures retained by two implants.
In addition, recent prospective study evaluated the concept of single implant to retain mandibular complete denture related to implant survival and prosthodontic maintenance. Over 10 years, no implant was lost. The most frequent maintenance was activation of the matrix (retention loss and component exchange). Denture base fracture in the attachment area occurred in eight cases and two denture bases fractured twice. The results were promising; however, further studies with a larger abrangency are necessary.
Based on these affirmations and since the mechanical performance and patient's satisfaction in relation to implant-supported prostheses seem to be also related to the time of use, further studies should be developed to establish a correlation between the variables investigated in these study and the mechanical failures in overdentures retained by single implant. Therefore, the existence of possible unequal alignments between occlusal sides of the same overdenture should be considered as a limitation of the study or a possible hidden variable.
| Conclusion|| |
Overdenture supported by single implant showed that:
- The stresses were concentrated at around the implant and in the medial region of the mandible body when the overdentures were submitted to occlusal forces
- Axial forces on the molars promoted stress at the implant and the mandible body in the side of loading
- The increase of the force promoted higher stress in both loading types.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3]