Accuracy of Digital Model Articulation for Chairside CAD/CAM Restorations

Objectives: This study evaluated the accuracy of two scanning techniques used for virtual model alignment when designing restorations with a chairside CAD/CAM system. 

Methods:  A master model was developed with a crown preparation (tooth #14) and an occlusal stop distal to the preparation (Distal Stop) and adjacent crown preparations (teeth #2 and #3) with no occlusal stop distal to the preparations (Distal Extension).  For each test group, the maxillary and mandibular teeth were scanned with a CEREC AC (Sirona Dental Systems) to create ten independent data files.  One group for each crown preparation used a bite registration scanning technique and another group used the buccal bite scanning technique to virtually align the models for the crown design.  Each file was used to design and mill a ceramic crown (IPS Empress CAD/Ivoclar) that was seated on the master model with a silicon material.  The vertical dimension was recorded with a polyvinylsiloxane bite registration material to measure variation from the control model.  The bite registration was sectioned mesiodistally and measured in cross-section at 20X using a measuring microscope and 3D software program.

Results:

  

Occlusal vertical dimension (microns)	Distal Extension	Distal Stop
Master (control)	3125.12+28.95 a	3032.17+12.08 a
Bite Registration Scan	3279.52+69.41 b	3175.06+53.08 b
Buccal Bite Scan	3104.37+35.91 a	3048.51+44.86 a

The data was analyzed using a two-way ANOVA and significant differences were determined with a Tukey test (p<0.05).  Letters indicate significantly different groups.

Conclusions:

There was no significant difference in the occlusal vertical dimension between the master model and the crowns made with the buccal bite scan technique.  There was a significant difference in the occlusal vertical dimension between the master model and the crowns made with the bite registration scanning technique.  The buccal bite scanning technique resulted in a more accurate occlusal mounting compared to the bite registration technique.