The effect of preparation taper on the retention of cemented cast crowns under lateral fatigue loading.
BACKGROUND
Clinicians have used resistance form as a basis for determining guidelines for preparation design to ensure clinical success of cemented cast restorations. Disagreement on whether clinical success follows the on-off or linear nature of resistance form continues.


OBJECTIVE
The purpose of this study was to evaluate the number of cycles required to dislodge a cemented complete crown casting under a cyclic lateral load as a function of taper and to compare this relationship for the resistive and nonresistive ranges of taper.


METHODS
Three dies were milled from stainless steel at each of the following tapers: 4, 8, 12, 16, 20, 24, 28, and 32 degrees. A gold-palladium metal-ceramic alloy crown was fabricated for each die, cemented, and subjected to lateral cyclic loading until failure or 1,000,000 cycles. The limiting taper for the dies with their given height and base was 26.6 degrees. Dies with taper less than 26.6 degrees had resistance form, whereas dies with taper larger than 26.6 degrees did not. A linear regression (alpha=.05) was used to evaluate the relation of cycles at dislodgement to taper.


RESULTS
The average number of cycles to crown dislodgement or completion for each taper (SD), in units of 10,000, was as follows: 4 degrees, 100 (0); 8 degrees, 100 (0); 12 degrees, 93.54 (16.56); 16 degrees, 61.33 (38.47); 20 degrees, 25.73 (34.67); 24 degrees, 4.33 (7.36); 28 degrees, 0.06 (0.08); and 32 degrees, 0.05 (0.09). The crowns in the resistive area less than 26.6 degrees that demonstrated failure showed a linear regression with a correlation coefficient of -0.995 between the average number of cycles to dislodge the crown and the taper. The slope was significantly different from zero (P=.0048), with a value of -7.58 and a standard error of 0.53.


CONCLUSIONS
The number of cycles required to cause crown dislodgement was linear after 12 degrees in the resistive area and nearly zero for preparations in the nonresistive area. The limiting taper concept closely predicted the transition point where the slope of the graph of cycles to dislodgement as a function of taper abruptly changed.
Cameron
Stephen M
SM
US Army Dental Activity, Fort Gordon, GA, USA. prosthdir@netzero.net
Morris
W Jack
WJ
Keesee
Stephen M
SM
Barsky
Todd B
TB
Parker
M Harry
MH
D002484:Cementation; D003442:Crowns; D017267:Dental Prosthesis Design; D017268:Dental Prosthesis Retention; D019232:Dental Restoration Failure; D006047:Gold Alloys / Q000737:chemistry; D006801:Humans; D008422:Materials Testing; D016876:Metal Ceramic Alloys / Q000737:chemistry; D010165:Palladium / Q000737:chemistry; D010865:Pilot Projects; D013193:Stainless Steel / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D019231:Tooth Preparation, Prosthodontic / Q000379:methods*
D006047:Gold Alloys; D016876:Metal Ceramic Alloys; D013193:Stainless Steel; D010165:Palladium