Objectives: Clinical polishing leads to reduction of surface flaws sizes and thus to increased mechanical strength. The aim of the present work was to assess fracture strength of a resin composite and of a glass ceramic as a function of surface roughness and to relate the strength data to flaw sizes, microstructural and fractographic examinations.
Methods: Specimens have manufactured out of a resin composite (Tetric EvoCeram, TEC) and out of glass ceramic material (IPS E.max Press, EMP). Different surface roughness levels have been induced using cutting, grinding and polishing techniques and quantified under CLSM. Fracture strength was measured in four-point bending and analyzed using Weibull statistics. Indentation fracture method was used to calculate fracture toughness. Critical flaw sizes were calculated and related to microstructure. Microstructural and fractographic examinations have been performed under SEM.
Results: Fracture strength upon the glass ceramic material decreased from 441.4 to 303.3 MPa (R(a)=150 nm-1.5 microm) and upon the resin composite from 109.8 to 74.0 MPa (R(a)=300 nm-50 microm). EMP exhibited a fracture toughness of K(Ic)=4.14 MPam(0.5) and TEC of K(Ic)=1.89 MPam(0.5). Calculated crack lengths for EMP ranged from 28.1 microm (441.4 MPa) to 59.6 microm (303.3 MPa) and for TEC from 94.3 microm (109.8 MPa) to 207.0 microm (74.0 MPa).
Significance: Dependency of fracture strength on surface roughness is neither determined by crystallite size of the glass ceramic material nor by filler sizes of the resin composite. No significant increase in fracture strength has been observed below 0.65 microm (1000 grit) in EMP. For TEC a threshold value might be determined below 2.1 microm (320 grit). Optimal polishing of a restoration right after placement is strongly recommended to keep an optimum strength performance through the whole clinical lifetime.