|Year : 2023 | Volume
| Issue : 1 | Page : 60-66
Comparative photoelastic stress analysis between all-on-four implant-supported cobalt-chromium framework and carbon fiber reinforced framework with varying cantilever lengths - An in vitro study
Rutvi Shah1, Anandkumar Patil1, Akhil Deshpande2, Swapnil Shankargouda1
1 Department of Prosthodontics and Crown and Bridge, Kaher's KLE VK Institute of Dental Sciences, Belagavi, Karnataka, India
2 Department of Mechanical Engineering, KLS Gogte Institute of Technology, Belagavi, Karnataka, India
|Date of Submission||05-Jan-2022|
|Date of Acceptance||04-May-2022|
|Date of Web Publication||21-Jan-2023|
Dr. Rutvi Shah
Department of Prosthodontics and Crown and Bridge, Kaher's KLE VK Institute of Dental Sciences, Belagavi, Karnataka
Source of Support: None, Conflict of Interest: None
CONTEXT: In implant prosthodontics, metal frameworks are used to rigidly spilt the implants together to provide rigidity and stiffness to the prosthesis. However, due to the limitations associated with the metal framework fabrication, the recent advances have made it possible to fabricate metal-free implant-supported prostheses using fiber-reinforced composite frameworks.
AIM: This study aimed to evaluate and compare photoelastic stresses between all-on-four implant-supported cobalt-chromium (Co-Cr) framework and the carbon fiber-reinforced composite (CFRC) framework at varying cantilever lengths.
MATERIALS AND METHODS: Two photoelastic models of an edentulous mandible were fabricated according to the all-on-four concept. Following this, frameworks with a bilateral cantilever extension of 20 mm, one with CFRC and the other with Co-Cr, were fabricated and were subjected to a progressive load of 120–180 N with an interval of 20 N. The principal stresses (σ) were calculated using the mean maximum fringe order.
STATISTICAL ANALYSIS: The quantitative analysis was performed using the Kruskal–Wallis test and Mann–Whitney U-test, P ≤ 0.05.
RESULTS: The CFRC framework showed mean principal stress values significantly lower than the Co-Cr framework under all loading conditions; however, deformation of the framework was observed at the cantilever lengths of 15 mm and 20 mm.
CONCLUSION: The study and its findings have indicated that the CFRC framework appears suitable for the fabrication of a framework for an “All-on-four” prosthesis. Within the confines of this study, the use of 10 mm as an arbitrary cantilever length recommendation for the CFRC framework and 15 mm for the Co-Cr framework seems reasonable.
Keywords: All-on-four, cantilever length, carbon fiber-reinforced composite, photoelastic stress analysis
|How to cite this article:|
Shah R, Patil A, Deshpande A, Shankargouda S. Comparative photoelastic stress analysis between all-on-four implant-supported cobalt-chromium framework and carbon fiber reinforced framework with varying cantilever lengths - An in vitro study. Indian J Health Sci Biomed Res 2023;16:60-6
|How to cite this URL:|
Shah R, Patil A, Deshpande A, Shankargouda S. Comparative photoelastic stress analysis between all-on-four implant-supported cobalt-chromium framework and carbon fiber reinforced framework with varying cantilever lengths - An in vitro study. Indian J Health Sci Biomed Res [serial online] 2023 [cited 2023 Jan 28];16:60-6. Available from: https://www.ijournalhs.org/text.asp?2023/16/1/60/368336
| Introduction|| |
The “All-on-four” treatment approach was designed in 2003 by Paulo Malo and Malo Clinic team, to rehabilitate edentulous patients with immediately loaded full-arch restorations supported by only four implants in each edentulous arch.
The All-on-four treatment approach involves the placement of four implants in the interforamina region of the jaw or in the anterior part of the maxilla, with two anterior axial implants and two posterior titled implants to support a full-arch immediately loaded fixed prosthesis (FFP). The posterior tilting of the implant avoids the vital anatomic structures, reduces the posterior cantilever, and produces a well-distributed four-point stability leading to both implant and prosthesis success. This allows rehabilitation of atrophic edentulous jaws with fewer bone grafting procedures, less surgical time, and a reduced number of implants.
In addition, in implant prosthodontics, metal frameworks are used to rigidly spilt the implants together to provide rigidity and stiffness to the prosthesis and reduce possible complications such as prosthetic fracture. Stiff prosthesis material distributes the stresses more evenly to the abutments and implants.
Metal frameworks have long been considered the gold standard in full-arch FFPs to rigidly splint the implants together, and to protect implants from micromotion and overloads. Full-arch implant-supported prostheses (ISPs) frameworks can be made of different alloys such as gold alloys, titanium, cobalt-chromium (Co-Cr), and nickel-chromium. However, metal framework fabrication is expensive, time-consuming, technique-sensitive with high melting point and high casting shrinkage. Hence, to overcome these drawbacks, recent improvements in composite materials have made it possible to fabricate metal-free prostheses using fiber-reinforced frameworks.,
The prosthodontic frameworks made of carbon fiber-reinforced composites (CFRC) appear to be a suitable substitute for traditional metal frameworks in full-arch ISPs. They offer high stiffness and biocompatibility, are easy to fabricate, have high fracture strength and creep resistance, provide good adhesion with the veneering acrylic resin, and no expensive machineries are required for their fabrication by the manual process, and are lightweight in nature., The above properties make CFRC to be an excellent material for the fabrication of frameworks for full-arch ISPs.
The choice of implant framework material is important for success. However, the studies evaluating the effect of reinforcement of carbon fiber in full-arch ISPs are scarce, inspite of its excellent mechanical and biological properties. Hence, in the present study, the stresses in the all-on-four implant-supported CFRC framework were analyzed and compared with the Co-Cr framework at varying cantilever lengths using photoelastic stress analysis. The null hypothesis was that there is no difference in stress distribution between All-on-four implant-supported Co-Cr framework and the CFRC framework at different cantilever lengths.
| Materials and Methods|| |
Ethical Clearance was obtained from the institutional review board of KAHER'S KLE V.K. Institute of Dental Sciences, Belagavi with Ref no 1330 dated 12-01-2020. Two photoelastic models of an edentulous mandible [Figure 1] were fabricated using epoxy resin CY 230 and hardener HY 951 (Araldite casting system [Huntsman Corporation]) with two implants placed vertically in the anterior region and two placed at an angulation of 30° in the posterior region using an custom made All-on-four surgical guide. The models were devoid of the land area, limiting structures, and tongue space, which could interfere with the photoelastic stress analysis.
|Figure 1: Two photoelastic models fabricated following the “All-on-four” concept|
Click here to view
The finished photoelastic models were scanned using an extraoral scanner, and obtained stereolithographic files were imported into the exocad designing software. Two identical ISPs bar frameworks with a bilateral cantilever extension of 20 mm, one with CFRC and the other one with Co-Cr, were designed in the exocad designing software [Figure 2] and [Figure 3] and were milled [Figure 4] and [Figure 5]. The frameworks were verified for passive fit using the single screw test, and the retaining screws were torqued to 20 Ncm. Graduated markings were made on both the frameworks at predetermined cantilever lengths of 0 mm, 5 mm, 10 mm, 15 mm, and 20 mm.
|Figure 2: HTML file of the Co-Cr framework designed in the exocad software. Co-Cr: Cobalt-chromium|
Click here to view
|Figure 3: HTML file of the CFRC framework designed in the exocad software. CFRC: Carbon fiber-reinforced composite|
Click here to view
|Figure 4: Fabricated Co-Cr All-on-four framework. Co-Cr: Cobalt-chromium|
Click here to view
|Figure 5: Fabricated CFRC All-on-four framework. CFRC: Carbon fiber-reinforced composite|
Click here to view
For photoelastic stress analysis [Figure 6], [Figure 7], [Figure 8]; the settings of the circular polariscope were adjusted to obtain a bright field arrangement, and the model was placed in the loading frame to analyze the presence of residual stresses. Following that the models were subjected to progressive loading by applying loads of 120 N, 140N, 160 N, and 180 N; to simulate the natural masticatory force. Loads of 120 N–180 N were applied five times at six designated loading points. (A) abutment of the last tilted implant (i.e., at 0 mm cantilever extension), (B) on all four abutments at the same time using a resin platform, and (C) at different cantilever lengths of 5 mm, 10 mm, 15, mm and 20 mm.
|Figure 6: (a-d): Method of interpretation of the fringe orders. (a) 1st order fringe, (b) 2nd order fringe, (c) 3rd order fringe, and (d) 4th order fringe)|
Click here to view
|Figure 7: 4th order fringe along with high crestal and apical stresses seen in Co-Cr framework, at application of 180 N load on 20 mm distal cantilever. Co-Cr: Cobalt-chromium|
Click here to view
|Figure 8: 4th order fringe at application of 160 N load on 15 mm cantilever length in CFRC framework (Note the homogeneous load distribution and absence of crestal stresses). CFRC: Carbon fiber-reinforced composite|
Click here to view
After the impact of the load, the presence of isochromatic fringes was observed and recorded using a digital camera. The stresses were quantified by counting the number of fringes [Figure 6]a, [Figure 6]b, [Figure 6]c, [Figure 6]d. The higher the fringe order, more is the stress magnitude; and closer the fringes, more is the stress concentration. The zero-order fringes are black and indicate no stresses, whereas pink- and green-order fringes indicate the highest stresses.
The results were obtained using the mean of maximum fringe order obtained from five repetition stresses and using a formula for calculating maximum principal stress (σ) = σ1− σ2 = Nmax Fσ/h. Where σ = principal stress, Nmax = Maximum fringe order, Fσ = Material fringe value (constant), and h = Thickness of the material.
Descriptive statistical measures such as mean, median, and standard deviation were computed and measured for all loading points for both frameworks. To compare the mean of the loading points in the same framework, Kruskal–Wallis test was used; whereas Mann–Whitney U-test was used for comparing loading points between two different frameworks.
| Results|| |
The results obtained in the present study rejected the null hypothesis that there is no difference in stress distribution between all-on-four implant-supported Co-Cr framework and the CFRC framework at different cantilever lengths. The results demonstrated that there was a statistically significant difference seen between the “All-on-four” implant-supported Co-Cr framework and CFRC framework, when evaluated at different cantilever lengths of 0 mm, 5 mm, 10 mm, 15 mm, and 20 mm under 120–180 N progressive loading.
The maximum principal stress (σ) was calculated using a formula, .
For example, The principal stress (σ) at the abutment of the last tilted implant in Co-Cr framework, when subjected to a load to 180 N; leading to an observation of third-order fringe. MegaPascal (MPa) or N/mm2.
In the present study, the principal stresses (σ) observed at different cantilever lengths in Co-Cr framework demonstrated that the principal stresses increased with the increase in the posterior cantilever and with the increase in the progressive loading from 120N to 180 N. The obtained mean values showed that highest mean value (1.5808 MPa) was observed at the cantilever length of 20 mm when subjected to a load of 180 N; whereas the lowest mean value (0.6330 MPa) was observed at the abutment of last tilted implant (with 0 mm cantilever extension), when subjected to a load of 120 N [Graph 1].
Whereas, in the CFRC framework, the principal stresses increased with the increase in the progressive loading from 120N to 180 N and increase in the posterior cantilever from 0 to 15 mm. The cantilever length of 20 mm showed bending movement and deformation of the framework at the application of a load of 40 N, so principal stress values could not be obtained for 20 mm cantilever; also 15 mm cantilever showed bending movement at 180 N, therefore could not be included in the analysis. The obtained mean values show that the highest mean value (1.2607 MPa) was observed at 15 mm cantilever on 160 N load application; whereas the lowest mean value (0.2604 MPa) was observed when all the abutments were loaded simultaneously at 120 N [Graph 2].
[Graph 3] and [Table 1] compare the mean principal stress (σ) values between the Co-Cr framework and CFRC framework by using Mann–Whitney U-test. The comparison of different loading points at 120, 140, 160, and 180N load showed significant differences between both the frameworks (P < 0.05*). The mean principal stress values and mean rank for the CFRC framework were significantly lower as compared to the Co-Cr framework at all the loading points with a level of significance of P < 0.05*.
|Table 1: Comparison of cobalt-chromium and carbon fiber-reinforced composite frameworks at different loading points with 120, 140, 160, and 180N load by Mann–Whitney U-test|
Click here to view
| Discussion|| |
The selection and accuracy of frameworks for implant-supported FFP are essential prerequisites for the osseointegration of dental implants in an immediately loaded prosthesis., In immediate loading protocols, it is crucial to control the implant micromovements in order to ensure osseointegration, which can be accomplished by rigidly splinting the implants together with a stiff substructure framework.,
Traditionally, for the above-mentioned reasons, metal frameworks have been used in full-arch FFPs to rigidly splint the implants together. However, due to the limitations associated with the metal framework fabrication and with the recent advances and improvements in fiber-reinforced materials, metal-free ISPs can be fabricated using fiber-reinforced composite frameworks.
Among the fiber-reinforced composite materials, studies done by Menini et al., Li et al., Pera et al., and Menini et al., have demonstrated that CFRC exhibits excellent mechanical and biological properties, as well as good adhesion with the veneering materials, and appears to be a promising alternative for fabrication of full-arch ISPs.
Additionally, bilaterally cantilevered frameworks are required to replace the posterior occlusion; when implants are placed in between the mandibular mental foramina or maxillary sinuses, to avoid invasive surgical procedures. However, the posterior cantilever length has demonstrated to have a direct clinical impact on the marginal bone loss, as it directly influences the forces transmitted to the implants and thereby to the bone.
Different authors have given different recommendations for cantilever lengths. Branemark et al. advised cantilever length equivalent to the length of 2–3 premolars. Zarb and Schmitt recommended to work within the limit of a 20-mm distal cantilever, whereas Taylor and Bergman indicated that the maximum cantilever extension should be 20 mm when five to six abutments are involved, and 15 mm when four abutments are involved. Rangert et al. recommended cantilever extension of 15–20 mm for the mandible, and upto 10 mm for the maxilla. The 9th Ticare consensus agreement on the “All-on-four” standard treatment recommends placement of 10–12 teeth in the “All-on-four” implant-supported FFP, depending on the emergence of the implant in the second premolar or the first molar area.
Therefore, different authors have given different recommendations for cantilever length, but the optimal cantilever length as a function of framework material has not been established. Hence, the present study compared the stresses in the recently introduced CFRC framework and the conventional Co-Cr framework at varying cantilever lengths of 0 mm, 5 mm, 10 mm, 15 mm, and 20 mm for the all-on-4 framework. These tested cantilever lengths are in accordance with those recommended by Zarb and Schmitt, Rangert et al. and English.
The results obtained from the present study demonstrated that for both the frameworks (i.e., the Co-Cr framework and CFRC framework); the principal stresses increased significantly with the increase in the posterior cantilever length, and also with the increase in the progressive loading from 120N to 180 N.
In the present study, the CFRC framework showed mean principal stress values and mean rank significantly lower as compared to the Co-Cr framework at all the loading points tested under 120N to 180N progressive load. Furthermore, the CFRC framework showed a more homogeneous stress distribution, with the stresses transmitted apically and distributed more evenly throughout the model; with the absence of stress concentration at the crest of terminal implant [Figure 8].
These results are in accordance with the findings of the studies done by Bahajan and Manocha, and Menini et al. In a study Bahajan and Manocha, revealed that CFRC distributed the applied forces uniformly across the prosthetic framework, providing high strength and rigidity to the framework. This is because in CFRC, the polymeric matrix binding the fibers together, transfers the applied load evenly among them.
However, in the present study, the CFRC framework showed bending movement and deformation of the framework at 20 mm cantilever length at the applied load of 40 N, also at the 15 mm cantilever length at 180 N load application. These results could not be compared with the literature available because of the lack of studies evaluating the effect of cantilever length on the CFRC framework.
In the current study, the Co-Cr framework showed mean principal stress values higher than the CFRC framework at all tested loading points and applied load. Furthermore, the Co-Cr framework did not show any signs of deformation at higher cantilever lengths, i.e., 15 mm and 20 mm, as seen in the CFRC framework. However, high crestal stresses (with the observation of pink and green order fringes indicating the highest stresses) were observed at the distal/terminal implant in the Co-Cr framework at the cantilever lengths of 10 mm and more (i.e., 15 mm and 20 mm) [Figure 7].
Hence, within the limitations of this study, the CFRC framework appears suitable for fabrication of framework for “All-on-four” prosthesis with a recommended short cantilever length of 10 mm. The study has its own limitations of not simulating the intraoral environmental conditions, the effect of multidirectional load applied intraorally, and the effect of the superstructure material on the stress distribution, so future well designed clinical studies can be conducted to evaluate the effect of the newer framework materials and its clinical relevance.
| Conclusion|| |
Within the confines of this study, we can recommend that CFRC framework appears suitable for fabrication of framework for “All-on-four” prosthesis with a recommended short cantilever length of 10 mm, as they demonstrated mean principal stress values and crestal stress significantly lower in comparison to the Co-Cr framework under all loading conditions. However, Co-Cr framework can be recommended in clinical scenarios, necessitating full-arch ISPs requiring distal cantilever >10 mm due to high stiffness and yield strength of the material; also keeping in mind the high risk of failure associated with longer cantilevers.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
Taruna M, Chittaranjan B, Sudheer N, Tella S, Abusaad M. Prosthodontic perspective to all-on-4®
concept for dental implants. J Clin Diagn Res 2014;8:ZE16-9.
Soto-Penaloza D, Zaragozí-Alonso R, Penarrocha-Diago M, Penarrocha-Diago M. The all-on-four treatment concept: Systematic review. J Clin Exp Dent 2017;9:e474-88.
Krekmanov L. Placement of posterior mandibular and maxillary implants in patients with severe bone deficiency: A clinical report of procedure. Int J Oral Maxillofac Implants 2000;15:722-30.
Menini M, Pesce P, Bevilacqua M, Pera F, Tealdo T, Barberis F, et al.
Effect of framework in an implant-supported full-arch fixed prosthesis: 3D finite element analysis. Int J Prosthodont 2015;28:627-30.
Skalak R. Biomechanical considerations in osseointegrated prostheses. J Prosthet Dent 1983;49:843-8.
Drago C, Howell K. Concepts for designing and fabricating metal implant frameworks for hybrid implant prostheses. J Prosthodont 2012;21:413-24.
Nakamura T, Waki T, Kinuta S, Tanaka H. Strength and elastic modulus of fiber-reinforced composites used for fabricating FPDs. Int J Prosthodont 2003;16:549-53.
van Heumen CC, Tanner J, van Dijken JW, Pikaar R, Lassila LV, Creugers NH, et al.
Five-year survival of 3-unit fiber-reinforced composite fixed partial dentures in the posterior area. Dent Mater 2010;26:954-60.
Menini M, Pesce P, Pera F, Barberis F, Lagazzo A, Bertola L, et al.
Biological and mechanical characterization of carbon fiber frameworks for dental implant applications. Mater Sci Eng C Mater Biol Appl 2017;70:646-55.
Chand S. Review carbon fibers for composites. J Mater Sci 2000;35:1303-13.
Ramesh K, Hariprasad MP, Bhuvanewari S. Digital photoelastic analysis applied to implant dentistry. Opt Lasers Eng 2016;87:204-13.
Shetty P, Konark P, Meshramkar R, Nadiger R. A fast and economical photoelastic model making of the teeth and surrounding structure. IOSR J Dent Med Sci 2013;3:28-33.
Brånemark PI, Engstrand P, Ohrnell LO, Gröndahl K, Nilsson P, Hagberg K, et al.
Brånemark Novum: A new treatment concept for rehabilitation of the edentulous mandible. Preliminary results from a prospective clinical follow-up study. Clin Implant Dent Relat Res 1999;1:2-16.
Brunski JB. Biomechanical factors affecting the bone-dental implant interface. Clin Mater 1992;10:153-201.
Li BB, Lin Y, Cui HY, Hao Q, Xu JB, Di P. Clinical evaluation of “All-on-Four” provisional prostheses reinforced with carbon fibers. Beijing Da Xue Xue Bao Yi Xue Ban 2016;48:133-7.
Pera F, Pesce P, Solimano F, Tealdo T, Pera P, Menini M. Carbon fibre versus metal framework in full-arch immediate loading rehabilitations of the maxilla – A cohort clinical study. J Oral Rehabil 2017;44:392-7.
Menini M, Pera F, Barberis F, Rosenberg G, Bagnasco F, Pesce P. Evaluation of adhesion between carbon fiber frameworks and esthetic veneering materials. Int J Prosthodont 2018;31:453-5.
Branemark PI, Zarb GA, Albrektsson T, Rosen HM. Tissue-Integrated Prostheses: Osseointegration in Clinical Dentistry. Chicago, II: Quintessence; 1985. p. 51-70, 117-28.
White SN, Caputo AA, Anderkvist T. Effect of cantilever length on stress transfer by implant-supported prostheses. J Prosthet Dent 1994;71:493-9.
Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated dental implants: The Toronto study. Part III: Problems and complications encountered. J Prosthet Dent 1990;64:185-94.
Taylor R, Bergman G. Laboratory Techniques for the Branemark System. 1st
ed. Chicago, II: Quintessence; 1990.
Rangert B, Jemt T, Jörneus L. Forces and moments on Branemark implants. Int J Oral Maxillofac Implants 1989;4:241-7.
Penarrocha-Diago M, Penarrocha-Diago M, Zaragozí-Alonso R, Soto-Penaloza D, On Behalf Of The Ticare Consensus M. Consensus statements and clinical recommendations on treatment indications, surgical procedures, prosthetic protocols and complications following All-On-4 standard treatment. 9th
Mozo-Grau Ticare Conference in Quintanilla, Spain. J Clin Exp Dent 2017;9:e712-5.
Maló P, Rangert B, Nobre M. All-on-4 immediate-function concept with Brånemark System implants for completely edentulous maxillae: A 1-year retrospective clinical study. Clin Implant Dent Relat Res 2005;7 Suppl 1:S88-94.
Bahajan S, Manocha L. Carbon fibers. In: Buschow KJ, Cahn R, Mahajan S, editors. Encyclopaedia of Materials: Science and Technology. 2nd
ed. London: Elsevier; 2001. p. 906-16.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]