Effect of Preparation Taper on Fit of E4D CAD/CAM Crowns

Objectives: To determine if variation in preparation taper affected the in vitro marginal and internal fit of all-ceramic crowns produced using the CAD/CAM scanning and milling system E4D Dentist^TM (D4D Technologies).

Methods: Typodont tooth #14 was prepared to receive all-ceramic crown for previously reported fit study. Control preparation had total occlusal convergence (TOC) of 10.72° in buccolingual plane. Preparation was modified to a TOC of 20.85°. Both preparations had well-defined rounded shoulder, ~2 mm occlusal reduction, and rounded internal angles. Small indentations were made on root of preparation as a plane of sectioning. Fifty-six impressions of prepared typodont tooth were taken using VPS and 56 die stone models poured. Dies were placed in full-dentate typodont and scanned with E4D laser. IPS e.max® CAD LT A1/C14 crowns (Ivoclar Vivadent) were fabricated per manufacturer's instructions. Crowns were cemented using Multilink® Automix (Ivoclar Vivadent) under constant pressure of 100 N. Samples were embedded in clear acrylic resin and sectioned buccolingually in a consistent plane using the root indentations as guidance (Isomet 1000, Buehler). Sections were evaluated under digital microscope with 50X objective (Olympus) and measured using OmniMet software (Buehler) on 3 locations per buccal and lingual side of section: marginal-edge, mid-axial wall, and cusp-tip. One measurement was made on occlusal table. Mean thicknesses at each location were calculated and compared to results previously obtained from the un-modified preparation. Data analyzed by independent samples t-test using Holm-Bonferroni correction adjusting for multiple comparisons.

Results: Comparison of mean fits at various locations of initial and modified preparations are shown in Table 1.

 

Table 1: Cement thicknesses measured at marginal and internal locations of preps (n=28).
Measurement Location	Buccal				Lingual
	Margin	Mid-Axial	Cusp	Occlusal Table	Cusp	Mid-Axial	Margin
Modified Prep Mean Thickness (µm ± SD)	72.05 ± 61.66	74.76 ± 35.65	138.79 ± 61.55	290.64 ± 74.54	135.61 + 68.62	112.00 ± 34.35	90.23 ± 88.65
Control Prep Mean Thickness (µm ± SD)	67.44 ± 45.46	116.22 ± 45.56	78.56 ± 52.54	151.34 ± 31.58	85.02 + 34.05	146.78 ± 57.18	45.62 ± 30.89
p Value	0.751*	< 0.001	< 0.001	< 0.001	<0.001	0.008	0.017

Conclusions: Six of seven fit measurements were significantly different between modified and control preparations.