Measurement of cement thickness under lithium disilicate crowns using an impression material technique

According to the literature, marginal gaps below 120 μm are recommended for conventionally cemented crown restorations. Therefore, the null hypothesis tested was as follows: the upper bound of the 95% confidence interval of the marginal accuracy of chair-side generated lithium disilicate crowns lies below 120 μm. Prior to definite insertion, the accuracy of 20 lithium disilicate crowns (e.max CAD LT) was evaluated by a replica technique. A light-body silicone was used to document the gap between crown and abutment. The thickness of the light-body silicone layer was examined under microscope (MM40, Nikon Corp., Tokyo, Japan; magnification, ×50) at four different landmarks (LM): LM1 = marginal discrepancy, LM2 = mid-axial discrepancy of the inner crown surface, LM3 = axio-occlusal discrepancy, and LM4 = mid-occlusal discrepancy. At LM1, the mean marginal discrepancy revealed 100 μm (SD, ±61 μm); the median was 81 μm. The lower bound of the 95% confidence interval was 90 μm, and the upper bound was 110 μm. The means of internal gaps at LM2, LM3, and LM4 showed 148 μm (SD, ±61 μm), 227 μm (SD, ±83 μm), and 284 μm (SD, ±95 μm), respectively. The lower bounds of the 95% confidence interval revealed values between 137 μm (LM2) and 269 μm (LM4). The upper bounds were between 158 μm (LM2) and 300 μm (LM4). The null hypothesis was not rejected. Within the limits of the study, the chair-side generated lithium disilicate crowns exhibited a sufficient clinical accuracy.