Mechanical Properties of Abutments: Resin-Bonded Glass Fiber-Reinforced Versus Titanium Mirko Andreasi Bassi1/Rosells Bedini2/Raffaela Pecci3/Pietro Ioppolo4/ Dorina Laritano5/Francesco Carinci6[Au: Please provide academic credentials for each author.] Purpose: The clinical success and longevity of endosseous implants, after their prosthetic finalization, mainly depends on mechanical factors. Excessive mechanical stress has been shown to cause initial bone loss around implants in the presence of a rigid implant-prosthetic connection. The implant abutments are manufactured with high elastic modulus materials such as titanium, steel, precious alloys, or esthetic ceramics. These materials do not absorb any type of shock from the chewing loads or ensure protection of the bone-fixture interface, especially when the esthetic restorative material is ceramic rather than composite resin. Materials and Methods: The mechanical resistance to cyclical load was evaluated in a tooth-colored fiber-reinforced abutment prototype (TCFRA) and compared to that of a similarly shaped titanium abutment (TA). Eight TCFRAs and eight TAs were adhesively cemented on as many titanium implants. The swinging the two types of abutments showed during the application of sinusoidal load was also analyzed. Results: In the TA group, fracture and deformation occurred in 12.5% of samples, while debonding occurred in 62.5%. In the TCFRA group, only debonding was present, in 37.5% of samples. In comparison to the TAs, the TCFRAs exhibited greater swinging during the application of sinusoidal load. In the TA group extrusion prevailed, whereas in the TCFRA group intrusion was more frequent. Conclusion: TCFRA demonstrated a greater elasticity than did TAs to the flexural load, absorbing part of the transversal load applied on the fixture during the chewing function and thus reducing the stress on the bone-implant interface. Int J Prosthodont 2016;29:xxx–xxx. doi: 10.11607/ijp.4169
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reatment outcomes in implant therapy may be influenced by prosthetic components’ mechanical properties. For example, the elastic modulus of commonly used implant abutments made of titanium, steel, precious alloys, or esthetic ceramics2–3 precludes shock absorbption of the stresses of occlusal loading and associated protection of an osseointegrated interface. This is certainly the case when the selected esthetic restorative material is a ceramic as opposed to a composite resin one.4–5
1Technologies
and Health Department, Istituto Superiore di Sanità, Rome, Italy. 2Dental Sciences, Sapienza University of Rome, Rome, Italy. 3Dental Sciences, Sapienza University of Rome, Rome, Italy. 4Technologies and Health Department, Istituto Superiore di Sanità, Rome, Italy. 5Centro di Neuroscienze di Milano NeuroMi, Università di Milano–Bicocca, Milan, Italy. 6Maxillofacial Surgery, University of Ferrara, Ferrara, Italy. [Au: Please provide a professional title for each author.] Correspondence to: Prof Francesco Carinci. Email:
[email protected] [Au: Please provide a mailing address for correspondence.] ©2016 by Quintessence Publishing Co Inc.
In this preliminary study, a cyclic stress protocol was employed to compare the mechanical performance of a tooth-colored fiber-reinforced abutment prototype (TCFRA) to a similarly shaped titanium abutment.
Materials and Methods Eight TCFRAs were expressly selected for this experimental work. [Au: Edits ok?] The TCFRA was composed of epoxy resin reinforced with pretensioned glass fibers. Eight titanium abutments (Ti grade 2) with the same shape and dimension of the TCFRAs were also chosen (Fig 1). Sixteen cylindrical self-tapping titanium implants (13 mm length, 3.8 mm diameter; Ti grade 2) were fixed on as many trunk-conic form, self-curing acrylic resin supports (Resin Tray) through a specially made decomposable Denril mold. A cylindrical brass cup was adhesively cemented on each abutment, followed by the adhesive cementation of the abutments on the implants (Panavia 21, Kuraray). After 24 hours the specimens were placed on as many radiographic films (4.1 mm × 3.1 mm; Ultra Speed, Kodak) before being irradiated for 0.14 seconds with a radiographic device at a voltage of 70 kV and an electric intensity of 15mA (×70, [Au:
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Mechanical Properties of Abutments
Fig 1 The titanium abutment and the experimental abutment.
Fig 2 X-ray of a sample. 0 = before fatigue tests; 1 = after the first cycle; 2 = after the second cycle; 3 = after the third cycle.
Fig 3 Measure of possible micromovements on every radiographic image by means of specific software. Fig 4 (right) A specimen during the cyclical solicitation. A grid graph was used adjacent to the sample to quantify the excursion of the abutment and to calibrate the digital image analysis software.
What is the manufacturer’s name?]) set to a focal distance of 25 cm from the film. The radiographic films were developed and fixed with standard technique, and subsequently converted to digital files through a flatbed scanner with a resolution of 1200 dpi (Epson Perfection V700 Photo Scanner, Epson). Afterward, all specimens were placed in a gripping system with an inclined plane of 30 degrees and submitted to three series of fatigue cyclic loading by means of a pneumatic instrument (Lloyd SiPlan, Lloyd Instruments) equipped with a load cell of 5 kN. Every series of loading consisted of 200,000 cycles of sinusoidal load, applied with a frequency of 2Hz. The compressive load ranged between 14 N and 140 N for the first series), 19 N and190 N for the second cycle), and 34 N and 340 N for the third cycle. After every series of loading, the specimens were xrayed and converted to digital images again to underline possible micromovements of the abutment (Fig 2). The images were submitted to digital analysis by means of a specific software (Image Pro Plus 4.1, Media Cybernetics) for quantitative evaluation of possible abutment micromovements against the fixture. For this purpose, the distances between the right and left bottom limit of the brass cup and the homologous limit of the profile of the first fixture coil were
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The International Journal of Prosthodontics
[Au: Please provide larger, high resolution images.]
measured (Fig 3) on each radiographic image. The condition of each abutment was evaluated at the end of the third and the last cycle of loading and a score was assigned, according to the presence (score = 1) or absence (score = 0) of the following phenomena: abutment debonding, visible deformation of the abutment, or fracture of the abutment. To quantify the excursion of the abutment and to calibrate the digital image analysis software, a grid graph was used (Fig 4). The grid was placed adjacent to the sample that resulted and interposed between this and the camcorder. [Au: Edits ok?] The frames, coinciding with the upper and lower limits of the load, were then compared, measuring the excursions shown by the anterior-superior edge of the coping cylindrical brass cup against a fixed reference point chosen on the grid graph (Fig 4). The swinging values for each range of stress chosen were compared within each group and among the groups, using the Newman-Keuls test.
Results In the TA group, both fracture and deformation occurred in 12.5% of samples, while debonding was found in 62.5% of samples. In the TCFRA group, only
Bassi et al
debonding was present, occurring in 37.5% of samples. In comparison to the TAs, the TCFRAs exhibited significantly (P < .05) greater swinging during the application of sinusoidal load. In TAs the swinging was 2.4 ± 0.2 μm, 6.1 ± 0.3 μm, 22.7 ± 0.7 μm, respectively, in the three intervals of increasing compressive load applied, while in the case of the TCFRAs the swinging was 57.8 ± 2.7 μm, 89.9 ± 1.5μm, and 115.6 ± 3.1 μm, respectively, in the above-mentioned three intervals. In the TA group extrusion prevailed (78.3%), whereas in the TCFRA group intrusion was more frequent (70.8%).
Discussion The excessive stress transmitted to the bone-implant interface is one of the etiological causes of crestal bone resorption in load conditions.1 The titanium has an elastic modulus about 5 times higher than that of cortical bone, facilitating the concentration of stress at the level of the crestal bone. The periodontal ligament that surrounds the natural tooth works as a viscoelastic shock absorber, able to reduce the flow stress to the crestal bone, thus lengthening the time in which the load is dissipated.2,3 A flexible abutment could work in a similar way. In fact, due to its anisotropic nature, the TCFRA deforms itself, within its elastic limit, absorbing part of the applied load. The higher flexibility exhibited by this experimental abutment could theoretically prevent excessive concentration of stress in the implant-bone interface. The TCFRA exhibits marked viscoelastic properties that are certainly useful in the dissipation of chewing loads, especially if the stresses are not parallel to the axis of the fixture. The use of this type of abutment could be beneficial not only in normal situations but also in cases of parafunctional habits such as clenching and bruxism. The lower percentage rate of debonding exhibited by TCFRAs (37.5% compared to 62.5%) could be attributed to the experimental nature of their resinous matrix that makes them more likely to bond to the composite resin cement. Their greater intrinsic roughness could also have a role in this phenomenon. The high rate of extrusion exhibited by titanium abutments could be attributed to the debonding that is consequent to cyclical solicitation. The prevalence of intrusion in the TCFRA group is probably due to the anisotropic structure of the abutment. In fact, the orientation of the fibers gives a higher elasticity in bending and a lower resistance under compression compared to titanium abutments. The intrusive behavior of the TCFRA could theoretically avoid crestal bone resorption in the case of occlusal overload. In fact, the natural teeth exhibit a greater ability to perceive any interference in respect to implant-supported prosthetic teeth.4,5 Furthermore, the latter perceive
noxious stimuli (pressure) in a delayed and attenuated manner compared to natural teeth, which are known to have a quicker, acute response.4,5 In both groups, the failure of the adhesive bond occurred at the cement-abutment interface, probably due to the cemented portion of the abutment not having a satisfactory macroretentive structure at the inner surface of the fixture that was instead threaded. The trend toward debonding in titanium abutments can probably be reduced by means of thin transverse grooves at the level of their cemented portion to obtain a valid macroretentive structure. The tendency toward debonding in TCFRAs can be reduced in a similar manner. Although both the composite cement and the adhesive system used are particularly suitable for the cementation of passivated metals and fiber posts, adhesive systems and composite cements developed specifically for coupling the titanium with this type of experimental abutment could improve the quality and duration of the bond. The experimental abutments theoretically exhibit the best esthetic properties, avoiding the shining effect associated with metal abutments. For this reason, they could be advantageous with full ceramic crowns.
Conclusion The greater elasticity of TCFRA to the flexural load allows them to absorb part of the transversal load applied on the fixture during the chewing function, thus reducing stress on the bone-implant interface.
Acknowledgments All authors contributed equally to this manuscript The authors reported no conflicts of interest related to this study.
References 1. Bidez MW, Misch CE. Force transfer in implant dentistry: Basic concepts and principles. Oral Implant 1992;18:264–274. 2. Papavasiliou G, Kamposiora P, Bayne SC, Felton DA. Three dimensional finite elements analysis of stress distribution around single tooth implants as a function of bony support prosthesis type and loading during function. J Prosthet Dent 1996;76: 633–640. 3. Ko CC, Kohn DH, Hollister SJ. Micromechanics of implants/ tissue interfaces. J Oral Implantol 1992;18:220–230. 4. Skalak R. Biomechanical considerations in osseointegrated prostheses. J Prosthet Dent 1983;49:843–848. 5. Rosentritt M, Behr M, Lang R, Handel G. Experimental design of FPD made of all-ceramics and fibre-reinforced composite. Dent Mater 2000;16:159–165.
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