Stability, adaptation and wear of composite and stainless steel crowns

Objectives: In-vitro study to compare fracture resistance (FR) and marginal adaptation (MA) of experimental preformed composite crowns (EXP), MZ100 and stainless steel (SS) crowns.

Methods: Single molar crowns were fabricated with EXP, MZ100 and SS (all 3M ESPE, USA). The roots of human molars were fixed with a 1mm polyether layer to imitate the periodontium. Each test-group (eight crowns) was prepared as listed below.All crowns were thermocycled and mechanically loaded (TMCL:12000x5°C/55°C,2.4 x10⁶x50N,1.66 Hz) with human antagonists,then axially loaded to failure (Zwick 1446; v=1mm/min). Failure detection was set to 10% of Fmax. Occlusal wear was measured in comparison to the unworn surface by a 3-D scanning device (Willytec,G). MA (%perfect margin) was determined in a scanning electron microscope (Phillips Quanta FEG 400, NL) via replica-technique before and after TCML. Statistical analysis was performed with the Mann-Whitney-U-test (P=0.05).

Results:

Crown:	1 MZ100	2 EXP	3 EXP	4 EXP	5 EXP	6 SS
Prep-Situation:	one cusp missing	one cusp missing	one cusp missing	one cusp missing	two cusps missing	one cusp missing
Core-buildup:	Z100 & Single Bond	none	none	Z100 & Single Bond	none	Z100 & Single Bond
Margin liner :	none	Filtek Flow	none	Filtek Flow	none	none
Cement :	RXU100	RXU100	RXU100	RXU100	RXU100	Ketac Cem Easymix
FR (N) Median(Q1/Q3)	3199(3002/3740)	2090(1798/2601)	2621(2427/2882)	3068(2434/3480)	2444(1828/2963)	1500(1014/1540)
MA before TCML[%] Median(Q1/Q3)	89(85/100)	100(91/100)	98(90/100)	84(79/95)	100(96/100)	0(0/0)
MA after TCML[%] Median(Q1/Q3)	88(60/94)	84(74/96)	96(81/100)	75(58/90)	75(45/93)	0(0/0)

MZ100 showed highest FR, followed by EXP and SS crowns. EXP and MZ100 showed best MA, SS crowns displayed no acceptable marginal adaptation. The crown surfaces showed comparable wear rates for MZ 100, EXP and SS crowns.

Conclusion: These results indicate that MZ 100 and EXP crowns may be fit for clinical application in permanent restorations (5 years+). Clinical research is needed to confirm.