The effect of two different milling instrument sets on CAD proposed cement thickness and fit surface of chairside CAD crowns

Aim To investigate the influence of milling instrument software presets on the proposed internal fit of anterior and posterior chairside fabricated CAD/CAM crowns.

Methods A total of 24 plastic upper right central incisors (#11) and 24 plastic lower left first molars (#36) were prepared by two practitioners and scanned with an Omnicam  intraoral scanner (Dentsply Sirona Inc.). Crowns were designed with inLab CAD software (SW 19.0, Dentsply Sirona Inc.) for the MC XL milling machine. Two design strategies were used: Step Bur 12 with Cylinder Pointed Bur 12S (Group 12) and Step Bur 12S with Cylinder Pointed Bur 12S (Group 12S). The maximum, mean and standard deviation of the planned fitting surface (taking into account bur shape) for Group 12 and Group 12S were compared against the ideal fitting surface. Paired t-tests were used to assess statistical significance (p<0.05).

Results Group 12 performed significantly better (p<0.001), with proposals deviating less from the fitting surface, than group 12S for both anterior and posterior crowns. Group 12S showed the highest deviations in the designs proposed for the incisors, reporting maximum distances of 531 (±107) μm, where Group 12 reported 369 (±112) μm. The maximum distances for the molars were 326.91 (±65.53) µm for Group 12S as opposed to 245.41 (±49.18) µm for Group 12.

Conclusion Within the limitations of this study, bur set 12 produced crown proposals with less deviation in cement thickness than those for the 12S burs for upper central incisor and lower first molar crowns. Both methods showed deviations of >0.3 mm incisally, which might lead to thinner crowns and increased risk of fracture. When preparing teeth for milled chairside CADCAM crowns using an MC XL miller, a further occlusal/incisal reduction of up to 0.5 mm beyond manufacturers guidelines must be performed to ensure the correct crown thickness is maintained. Step Bur 12 should be used in preference to Step Bur 12S, to minimise, but not eliminate, this over-milling.