Cuspal Displacement Induced by Bulk Fill Resin Composite Polymerization: Biomechanical Evaluation Using Fiber Bragg Grating Sensors

Hindawi Publishing Corporation International Journal of Biomaterials Volume 2016, Article ID 7134283, 9 pages http://dx.doi.org/10.1155/2016/7134283 Research Article Cuspal Displacement Induced by Bulk Fill Resin Composite Polymerization: Biomechanical Evaluation Using Fiber Bragg Grating Sensors Alexandra Vinagre,1 João Ramos,1 Sofia Alves,1 Ana Messias,1 Nélia Alberto,2 and Rogério Nogueira2 1 Faculty of Medicine, University of Coimbra, Avenida Bissaya Barreto, Blocos de Celas, 3000-075 Coimbra, Portugal Instituto de Telecomunicações (IT), Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 2 Correspondence should be addressed to Alexandra Vinagre; Received 26 November 2015; Revised 18 March 2016; Accepted 21 March 2016 Academic Editor: Feng-Huei Lin Copyright © 2016 Alexandra Vinagre et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Polymerization shrinkage is a major concern to the clinical success of direct composite resin restorations. The aim of this study was to compare the effect of polymerization shrinkage strain of two resin composites on cuspal movement based on the use of fiber Bragg grating (FBG) sensors. Twenty standardized Class II cavities prepared in upper third molars were allocated into two groups (𝑛 = 10). Restorations involved the bulk fill placement of conventional microhybrid resin composite (Esthet∙X HD, Dentsply DeTrey) (Group 1) or flowable “low-shrinkage” resin composite (SDR, Dentsply DeTrey) (Group 2). Two FBG sensors were used per restoration for real-time measurement of cuspal linear deformation and temperature variation. Group comparisons were determined using ANCOVA (𝛼 = 0.05) considering temperature as the covariate. A statistically significant correlation between cuspal deflection, time, and material was observed (𝑝 < 0.01). Cuspal deflection reached 8.8 𝜇m (0.23%) and 7.8 𝜇m (0.20%) in Groups 1 and 2, respectively. When used with bulk fill technique, flowable resin composite SDR induced significantly less cuspal deflection than the conventional resin composite Esthet∙X HD (𝑝 = 0.015) and presented a smoother curve slope during the polymerization. FBG sensors appear to be a valid tool for accurate real-time monitoring of cuspal deformation. 1. Introduction Volumetric shrinkage remains a major drawback to the clinical performance of the resin composite restorations. Shrinkage leads to deformation of the resin composites and generates stress due to the confinement of the resin to the cavity walls generated by the bonding procedure. This shrinkage stress is transferred to the tooth and may lead to cuspal deflection or enamel microcracks, whereas stress at the tooth-composite interface increases the likelihood of interfacial adhesive failures [1]. Cuspal deflection occurs due to the interaction between the polymerization shrinkage stress of the resin composite, the adhesive interface, and the compliance of the cavity wall [2]. Compliance is defined as the change in dimension per unit of force applied or generated, being essentially the inverse of stiffness [1]. Several studies have described it as a valuable method to assess the effects of polymerization shrinkage stress [3–7] and dimensional changes have been reported to range from 4 to 25 𝜇m [4, 6, 8, 9]. The amplitude of this inward cuspal movement can depend on several factors, namely, the size and configuration of the cavity [2, 3, 10]; the properties of the resin composite [2, 4, 5, 9]; the bonding system [3, 5]; the hydration condition of the teeth [2]; and the experimental conditions [4]. Even though different model designs have been used for cusp deflection assessment, such as glass rods, aluminum blocks, or tooth structure, all inherently present with distinct compliance behaviors [4, 11, 12]. In order to overcome this limitation, system compliance similar to that of teeth is necessary to accurately detect stress [4, 11, 12]. Considering substrate structural deformability, both C-factor and resin composite 2 volume seem to have an impact on the substrate compliance. When the substrate is only slightly deformable, the increase of the stress correlates better with the C-factor but if the compliance is higher, the resin composite volume would correlate better with stress development [10]. These findings demand careful data interpretation across studies concerning different methodologies for cuspal deflection assessment. Additionally, the development of inward cuspal deflection can also be related to the strategies employed for managing shrinkage stress of resin composites [1]. These clinical approaches to reduce polymerization shrinkage include incremental placement techniques [2, 4, 11], the use of lowmodulus intermediate liner materials as stress absorbers [4, 7], and modification of the light application methods to reduce curing speed [13]. Also, factors related to resin composite formulations like changes in filler amount, shape or surface treatment, variations in monomer structure or chemistry, and modification of polymerization resin kinetics have been more recently introduced aiming to reduce the polymerization shrinkage [1, 4, 5, 13]. All these strategies encompass a new class of resin composites known as “lowshrinkage resin based composites” that are generally allowed to be placed in a bulk fill mode due to the increased depth of cure, probably related to higher translucency [14]. Bulk filling techniques are undoubtedly more user friendly than the necessary meticulous incremental layering techniques advocated for conventional resin based composites (RBCs) [8], which justifies the growing interest in these so-called “low-shrinkage” RBCs and raises the need for exhaustive studies to clarify their potentialities [5, 8, 14–16]. Many methods have been used to evaluate cuspal deflection, involving technologies that go from linear variable differential transformers (LVDT) [2, 4, 11], strain gauges [9], profilometry [3], or twin channel deflection measuring gauge [6–8], among others. Fiber Bragg grating (FBG) sensors can be used to perform real-time local temperature and strain measurements [17–20]. Fiber optical sensors have the advantage of presenting immunity to electromagnetic interference [21], small dimensions [17–20], high resolution and sensibility, chemical inertness [17–19], biocompatibility [17], long-term stability [20], multiplexing capability, possibility to be embedded in different structures [17, 22], and ability to perform remote measurements [21]. The aim of this study was to compare the cuspal displacement induced by the polymerization shrinkage of a bulk fill resin composite (SDR) and a conventional microhybrid resin composite (Esthet∙X HD) using fiber Bragg grating (FBG) sensors. The null hypothesis stated that there are no significant differences in cuspal displa (...truncated)