The in vivo evaluation is quite different from that of the in vitro analysis. In the previous study, all the specimens were calibrated to a flat surface with 2.5 mm diameter. In present study, each access hole had a different dimension and configuration. The filling surfaces were often ø3 to 4 mm in diameter. Access-holes located in the occlusal groove area or in the inclined cusp surface induced an overfilling (Fig. 3a–e).

In clinical situations, masticatory stresses are quite different from the standard three-body generalized wear test [30]. Factors associating with wear processes including occlusal force, direction and speed of food excursion are individually different according to the location of the access-hole. Therefore, in this study, both filling groups were arranged in a symmetrical random way. For these reasons, the evaluation of changes in surface areas, marginal wear pattern, and marginal depth of fillings were selected.

The surface areas of filling were reduced with time due to the wear of overflowed material that occurs systematically in a clinical situation. At the baseline (T = 0), M4M group presented the overfilling more frequently compared to the CR group possibly due to the low viscosity of the material during the setting time. M4M is much more fluid than CR, and the setting time duration is longer. The surface reduction by the occlusal wear seems to be smaller when the filling surface comes closer to the vertical inner wall of the access hole (Figs. 3a–e and 6). This particularity is the main difference with the previous in vitro study [12].

This surface reduction due to the wear might have a threshold value in relation with the material thickness at the margin and the toughness of the material regarding the compressive strength against the occlusal loading. A long-term analysis should be conducted to determine if this surface reduction will decrease with time.