Effect of ﬁller size and morphology on viscoelastic stability of resin-composites under dynamic loading Muhammad Kaleem • Julian D. Satterthwaite • David C. Watts Received: 15 September 2011 / Accepted: 16 December 2011 / Published online: 1 February 2012 Ó Springer Science+Business Media, LLC 2012 Abstract Effect of variation in ﬁller particle size, mor- phology and wet conditioning on the viscoelastic stability of resin-composites under dynamic loading was investi- gated. Eight experimental light cured resin-composites were selected. For each of the eight resin-composites, ten cylindrical specimens (4 9 6 mm), divided into two sub- groups (n = 5) were prepared. Group 1 and 2 were loaded dynamically after 1 day of dry storage and 1 week of wet storage, respectively. A cyclic load between 1 and 50 MPa was applied for both groups at a frequency of 0.25 Hz for 30 min to obtain the ‘dynamic’ creep strain (%). Data was analysed by univariate ANOVA. Unimodal spherical and irregular resin-composites showed a signiﬁcant inﬂuence of particle size and shape on dynamic creep under dry con- dition, but not for wet conditions. Irregular ﬁller particles in both unimodal and multimodal resin-composites were more resistant to dynamic creep under wet conditions and showed higher stiffness. 1 Introduction Resin-composite restorative materials are being used more commonly as an alternative to dental amalgam due to improvements in aesthetics, biocompatibility and mechani- cal properties [1, 2]. Resin-composites are also being increasingly used in load-bearing areas in the posterior dentition and therefore, inevitably subjected to masticatory forces of varying magnitude and rate, sometimes reaching to 66 N and in some extreme conditions to 90 N [3]. This may result in the deformation of the restorative material and ultimately in the failure of the restoration. The deformation response of polymer based resin-com- posites can be studied by measuring the strain generated during a speciﬁed period of time by a constant stress (i.e. creep) [4]. Creep, by deﬁnition, requires a constant load applied for a certain period of time on a specimen. However, these conditions occur very rarely in the oral environment. Many studies have been carried out to try and evaluate the viscoelastic stability of resin-composite, which depends on matrix composition. Creep is mainly related to the increased susceptibility of the polymer matrix to viscous deformation: in most studies, a static compressive load was applied to deter- mine the creep behaviour [5–10], but in a few studies torsion and tensile creep modes have also been used [4, 11, 12]. It would be more clinically relevant to examine visco- elastic stability of resin-composites under cyclic loading, as the loading force during chewing is low and repeated. The estimated number of chewing stresses imposed on a dental restoration in the mouth is more than 3 9 10 5 times per year [13]. Under such conditions, there is a possibility of strain recovery during the unloading phases. Creep behaviour of resin-composite has been reported in the literature by Oden et al. [10]. They compared the vis- coelastic stability of resin-composites under uniaxial static and dynamic compression and found the results compara- ble. They also found that the dynamic creep recovery was decreased as the applied load and number of cycles increased, resulting in plastic deformation of the materials. M. Kaleem (&) Dental Materials Department, Army Medical College, National University of Science and Technology, Rawalpindi, Pakistan e-mail: mkaleem-amc@nust.edu.pk; dr_kaleem78@hotmail.com M. Kaleem Á J. D. Satterthwaite Á D. C. Watts Biomaterials Research Group, School of Dentistry, University of Manchester, Manchester, UK D. C. Watts Institute of Materials Science and Technology (IMT), Friedrich-Schiller-University Jena, Jena, Germany 123 J Mater Sci: Mater Med (2012) 23:623–627 DOI 10.1007/s10856-011-4540-z