Focusing on numbers of access-hole showing a disappearance of the overfilling in an early period (up to T = 3 M), groups CR and M4M exhibited 82% (23/28) and 39.3% (11/28), respectively (Table 4). This can be explained by the differences of mechanical properties of both materials. The elastic moduli of M4M and CR are 1.9 and 7.9 MPa, respectively. Flexural strength of M4M is 66 MPa which is comparable to that of an acrylic resin (60 MPa) and almost half compared to CR (115 MPa) [31, 32]. The low flexural strength of M4M possibly absorbed the occlusal stresses, and the overfilling stayed longer compared to CR.

According to the results of contact mode and the surface areas changes, it was suggested that there were no correlations among the occlusal contact modes and the surface areas reduction up to 12 months in vivo.

As mentioned in the previous study [12], the margin of M4M group had a better adhesion to the surrounding access hole ceramic and no gap formation was observed between the cavity and the filling material, due to the specific polymerization mode (TBB initiator). The bond strengths on the glazed feldspathic ceramic were 7.6 ± 2.2 MPa for group CR, and 8.6 ± 1.0 MPa for group M4M. In some cases, adhesive failure mode was detected at the margin with group CR. The polymerization shrinkage of the CR might have deteriorated the adhesion quality at the margin [33].

The elastic modulus of a filling material and its flexural strength influence the wear resistance at the marginal zone [31]. The difference of the polymerization mode modifies the stress distribution at the marginal areas. M4M is polymerized with the TBB auto-polymerizing catalyst; therefore it shows low shrinkage at the marginal surface and possesses an advantage of minimizing gap formation [34]. The marginal areas of CR might receive stress concentration, and subsequent micro-gap formation possibly occurred due to polymerization shrinkage during the photo-polymerization. The wear values of filling material itself influence the wear pattern. The 3-body wear values are 1.5 mm [3] for the acrylic resin, 0.9 mm [3] for the M4M and 0.3 mm [3] for the CR. The toothbrush wear test shows the same tendency [34]. Although, CR contains greater amounts of filler (TMPT fillers) compared to M4M (CR:71.5 wt%, M4M:<10 wt%), M4M has an aptitude for lower brittleness compared to CR as its matrix consists of acrylic resin which possesses flexibility [35].