[The effect of tooth preparation design on the CEREC all-ceramic crown's marginal adaptation]

To study the effect of tooth preparation on marginal adaptation of all-ceramic crown designed by the Cerec system.

Lower first molar in the standard model was prepared into four types, two occlusal convergences (6 degrees and 12 degrees ) and two marginal shapes (chamfer and shoulder), and then divided into four groups correspondingly with 8 copies of abutment preparation test piece in each group. All-ceramic crown was designed by the Cerec 3 system and inserted with corresponding test piece. Marginal silicon rubber thickness of each axis surface was measured by stereo microscope after dissection. The data were analyzed with SPSS11.0 software package.

No significant difference was observed between edge silicon rubber thickness of 4 groups of test pieces (Group A: 172.8 microm+/-42.6 microm,Group B: 181.9 microm+/-46.6 microm,Group C: 187.1 microm+/-46.5 microm,Group D: 191.7 microm+/-46.4 microm,P>0.05). Significant difference was observed between mesial surface and distal surface, mesial surface and buccal surface, mesial surface and lingual surface, distal surface and buccal surface, distal surface and lingual surface (P<0.05). No significant difference was observed between buccal surface and lingual surface. Silicon rubber thickness was in the range of 278.1 microm+/-28.8 microm and 135.8 microm+/-14.6 microm, with that of distal surface the largest (278.1 microm+/-28.8 microm) and that of buccal surface the smallest (135.8 microm+/-14.6 microm).

All the restorations appear to have acceptable marginal adaptability,which suggests that the marginal adaptation of occlusal convergence with 12 degrees is better than 6 degrees , while the marginal design with chamfer is better than shoulder. Supported by Shanghai Leading Academic Discipline Project (Grant No.T0202).