Effectiveness of self-adhesive cements in retaining cast crowns

OBJECTIVE:  Self-adhesive, modified-resin (SAMR) cements are available in paste consistencies, but little is known about their effectiveness in retaining crowns.  The purpose was to evaluate crown retention using SAMR paste-paste cements. 

METHODS:  Cements evaluated were Heraeus iCEM (IC), 3M-ESPE RelyX-Unicem Clicker (UCC),  Bisco-Biscem (BC) and Kerr-Maxcem (MC).  UCC was extruded/hand mixed; others were automixed.  Extracted molars were prepared with flat occlusals, 20° taper and 4-mm axial lengths.  A high-noble ceramometal crown (SpecialWhite, Ceramco) was produced for each preparation.  Total cement contact surface areas were determined (JProsthDent 2006:96:104) and distributions were made to achieve similar areas for the four groups (n=8). Cements were mixed as directed; castings were cemented with a force of 20 kg; stored 24-h; then thermocycled for 5000 cycles (5-55°C). Castings were removed along the path of insertion (Instron,0.5 mm/min).  Cement failure modes were recorded and compared. Data analyzed with ANOVA; C²; a=0.05. 

RESULTS:   Mean crown removal stresses ranged from 2.9-4.9 MPa.  Means not shown to differ are with small letters.  MC exhibited the highest removal stress, differing from the others (P=0.001).  Failure modes were similar, with the cement residing principally on crown internal surfaces (C²=4.5;P=0.2). MC had 38% mixed or cohesive failures.  There were no tooth fractures or specimens with cement residing principally on dentin.  

Cement	Removal Stress-MPa (SD)		Removal Stress Kg/mm2 (SD)	Failure mode: tooth-casting mixed	Failure mode: on casting
IC	2.9   (0.8)	a	29.7   (8.5)		100%
UCC	3.7   (1.0)	a,b	37.8   (10.4)	12.5%	87.5%
BC	4.0   (0.7)	b	41.0   (6.7)	12.5%	87.5%
MC	4.9   (0.8)		49.9   (8.0)	37.5%	62.5%

CONCLUSION:  Removal stresses for these four SAMR paste-paste luting agents are known to be sufficient to retain noble metal castings on standard tooth preparations (JProsthDent 2004: 91:428).  Although marketed as self-adhesive to dentin, results suggest that SAMR cement adhesion to the casting predominates.  Supported by Heraeus.