CAD/CAM in Restorative Dentistry is introduced in this series by Dr. Alessandro Devigus, Switzerland. In part 2 he explains the preparation rules which are the basis for success of restorations for the posterior region done by Computer Aided Design (CAD) and Computer Added Manufacturing (CAM). The preparation requirements should be maintained due to the following case to ensure optimal fitting of a CEREC-inlay for a second left molar in the lower jaw avoiding restoration failures and fractures.
The preparation design in accordance with the CAD/CAM-technique reduces physical stress influencing the strength and durability of the all-ceramic restoration. The defect oriented preparation along the cavity walls is performed with a cylindrical diamond bur preserving as much tooth as possible. Before the treatment starts the insertion direction is determined with the rotary instrument. The occlusal isthmus is prepared with the correct angle thinning out the composite material to the approximal areas.
A fine diamond bur is used to loosen and break pieces of the composite build-up that is still left in the approximal box ensuring the protection of the neighbouring teeth. For the optical impression the cavity margin should no longer touch the adjacent teeth. Secondary caries is excavated very slowly with a rose head leaving undermining areas to be filled up with adhesive material in the insertion procedure. The bottom of the approximal box is smoothened with a bur only diamond coated at the tip avoiding damage of the adjacent teeth. The filling of the neighbouring premolar is recontured and smoothened with a flexible disc creating ideal contact conditions for the CAD/CAM-inlay.
The preparation depth is checked with a periodontal probe. The thickness in the fissure region should not be less than a minimum of 1.5 mm. The rounded preparation design with a clear external margin in tooth structure and the optimum thickness for a CEREC-inlay in all dimensions guarantees predictable clinical success with CAD/CAM.