The null hypothesis, that there is no difference in the accuracy of restorations with different sintering times, was partially rejected. The mean marginal gap values of 4Y-TZP were increased if the sintering time is shortened while 3Y-TZP groups were not affected by this situation.
A few studies focused on the effect of changes in the sintering time on the marginal fit of monolithic zirconia crowns [23, 26]. Khaledi et al.  reported that different sintering times does not alter the marginal fit of the zirconia copings fabricated from 3Y-TZP presintered blocks. The findings are in accordance with the present study for the 3Y-TZP group. On the other hand, in a previous study focused on 3Y-TZP monolithic crowns it was showed that the marginal gap values of speed sintering (2 hours, 55 minutes in total) groups were higher than the corresponding combinations of standart sintering (9 hours, 50 minutes in total) . The differences between the findings of these studies may be due to the thickness of restorations.
4Y-TZP zirconia was launched in 2015 with the indication of full-contour restoration and framework material in the posterior region due to adequate mechanical strength with improved optical properties. The present study revelaed a statistically significant increase in the marginal gap values in 4Y-TZP group for the speed sintering protocol. The creep rate of polycrystalline ZrO2 with different composition may be more sensitive to creep and distortion during the sintering process . Increasing yttria content influence the number of grain boundaries that controls the major physical properties such as light transmission and flexural strength . Moreover, material composition of monolithic zirconia crowns affect internal fit, crown margin quality .
Zirconia restorations produced by soft machining of the presintered blocks are sintered at temperatures varying between 1350 and 1550◦C, depending on the manufacturer. In general, it can be stated that sintering temperatures of 1,600°C onwards lead to a decrease in flexural strength.14 The sintering conditions have a strong impact on the mechanical properties of the definitive product . Novel speed sintering protocols have been developed to meet the demand for chairside one-visit restorations . Altering the sintering parameters was attempted to shorten the zirconia sintering process by inducing rapid heating rate and lowering the sintered-holding time .
The effect of altering the sintering conditions on the mechanical and optical properties of zirconia frameworks has been the subject of many studies [15, 16, 18, 31–33].The reported results are inconclusive, and whether increasing the sintering temperature and/or prolonged sintering-holding time enhances translucency [32–34] remains unclear [13, 31 31]. For improved productivity, shortened dwell time is beneficial, but it may result in poor mechanical behavior compared with long-term sintering . On the other hand, encouraging results of a previous study showed that the combination of high sintering temperature and short sintering time increases the flexural strength of zirconia . Additionally, it was reported that biaxial flexural strength is not affected by altering the sintering conditions [15, 33]. More research is needed to assess the effect of sintering times and temperatures on the mechanical and optical properties of zirconia prostheses.
Shrinkage occurring in presintered blocks during the sintering process has been studied previously [12, 35]. The milling machine software is programmed to compensate for the shrinkage percentage of the partially sintered zirconia blocks after sintering . Rezende et al.  evaluated dimensional changes brought about by sintering of the Y-TZP blocks and reported a significant difference among the groups for the internal fit of copings. Additionally, the sintering shrinkage rate reported by the manufacturer was different from that obtained experimentally. Limited data are available to compare those with results of the present study. However, it is noteworthy that one of the two groups with a significant difference showed an increase in the gap values, whereas shrinkage occurred in the other group. This may be because the density difference of presintered zirconia blocks affects linear sintering shrinkage .
Non-uniform sintering shrinkage might lead to a misfit of the restorations. The cement space has a significant effect on the marginal fit of monolithic zirconia crowns. Marginal discrepancy values increase when the cement space decreases. As the cement space was set at 50 µm using the software, the marginal discrepancy was 53 µm in this study. Conversely, when the cement space was set at 30 µm, the marginal discrepancy was 85 µm . Larger internal misfit may also affect the success of ceramic crowns . However, there is no consensus on the limit for clinically acceptable values of internal fit. The mean variation in the crowns tested in the present study is in accordance with that reported in previous studies [29, 37]. Additionally, marginal gap values of all groups were less than 120 µm. The widest marginal gap measured was 78±9 µm, and the lowest was 33±6 µm. Larger frameworks might result in a higher misfit of the prosthesis.38 Further studies are required to assess the effect of sintering conditions on the accuracy of new generations of monolithic zirconia restorations.