Dent Mater8:310-319,September,1992 A classification of dental composites according to their morphological and mechanical characteristics G. Willems ~, P. Lambrechts ~, M. Braem 2, J. P. Celis 3, G. Vanherle 1 1Department of Operative Dentistry and Dental Materials, Katholieke Unwers~teitte Leuven, Leuven, Belgium 2Dental Propedeut~cs, Universitair Centrum Antwerpen (RUCA), Antwerp, Belgium 3Department of Metallurgy and Materials Engineering (MTM), Katholieke Universite~tte Leuven, Leuven, Belgium Abstract. The on-going search for a biologically acceptable restorative material has brought a confusing variety of composites on the dental market. In the present study, commercially available composites are categorized as a function of their mean particle size, filler distribution, filler content, Young's modulus, surface roughness, compressive strength, surface hardness, and filler morphology. Out of th~sinformation, it can be concluded that the materials of choice for restoring postenor cavit,es at present are the Ultrafine Compact-Filled Composites because their ~ntrinsic surface roughness, Young's modulus and, indirectly, their filler content, compressive strength, and surface hardness are compa- rable to the same properties of enamel and dentin. The Ultrafine Midway-Filled Compos,tes seem to be very sat,sfactory matenals for antenor use. INTRODUCTION The increased need for esthetic tooth-colored restorative materials has brought a confusing variety of composites onto the dental market. The on-going search for a biologically acceptable material that not only has physical and mechani- cal properties similar to natural tooth tissues but also is economically and manipulatively equivalent to amalgam has dramatically increased the number of dental composites available. To keep track of recently developed materials and to categorize others, several classifications have been proposed. The American Dental Association described two categories of direct filling resins in Specification No. 27 (Council on Dental Materials, American Dental Association, 1977). More elabo- rate rankings are based on the specific filler-size distribution and amount of incorporated filler (Lutz and Phillips, 1983; Albers, 1985; Leinfelder, 1985) as well as on filler appearance and composition (Hosoda et al., 1990). Filler content and size were shown to directly determine the physical and mechanical properties of composite materi- als (Lietal., 1985; Braemetal., 1989; Chung, 1990; Chung and Greener, 1990), ofwhichthe dynamicYoung's modulus (Braem, 1985; Braem et al., 1986), surface hardness (Craig, 1989) and intrinsic surface roughness (Willems et al., 1991a) seem to be the most clinically relevant for their mechanical performance. The purpose of the present study was to rank most of the commercially available composites as a function of their mean particle size, filler size distribution, filler content, Young's modulus, surface roughness, surface hardness, compressive strength, and scanning electron microscope (SEM) appear- ance. MATERIALS AND METHODS Table I lists the 89 commercially available composites inves- tigated in this study. Approximately 3 g of each composite was dissolved in acetone pro analysi (ACS, ISO, 14.100; Merck, Darmstadt, Germany), mixed and centrifuged for 30 rain at 3000 rpm. The remaining solution of acetone and dissolved resin was carefully removed by pipette aspiration without agitating the centrifuged filler particles. This procedure was then repeated a second time to completely remove remnants of resin matrix. The supernatant was again removed and the remaining filler particles dried at 37°C for 12 h. Finally, the powder was ultrasonically agitated to reduce agglomeration of filler particles. All powders were screened under SEM (PSEM 500, Phillips, Eindhoven, The Netherlands) to confirm com- plete filler isolation and to investigate filler morphology. A computer-controlled apparatus (Coulter LS Series 100, Coulter Electronics Inc., Hialeah, FL, USA), using laser- diffraction technology (laser light of 750 nm), was used to determine particle size distribution. This equipment can analyze a sample with a particle range of 0.4 to 800 ~m. Light is diffracted around a particle at angles inversely proportional to the size of the particle. The smaller the particle, the greater will be the angle of diffraction (Fig. 1A). Most laser-diffraction particle-size analyzers have only a single optical system (Fig. 1B ) for collecting and sensing diffracted light. Since one optical system is not capable of capturing the full range of diffracted light (from very large to very small particles), the collecting lens must be changed or adjusted when diffraction angles exceed the lens capabilities. Multiple optical detection sys- tems (Fig. 1C) allow the particle-size analyzer to simulta- neously capture the diffracted light of a wide range of particles varying in size from 0.4 to 800 pro. The diffracted light is collected by a Fourier lens, which focuses the light up to about 15 ° on two sets of detectors, one for low-angle scattering and the other for mid-angle scattering. Another Fourier lens system (multiple optical detection systems) collects the dif- fracted light from about 10° to 35 ° and focuses it onto a third set of high-angle detectors (Fig. 1C). A high-volume pump pushes the particles through the 310 Wlllems et al /Classlhcatlon of dental composztes