Supplementing retention through crown/preparation modification: an in vitro study

The best treatment for a crown that has come loose but has appropriate marginal fit and form is not clear.

This study was designed to determine whether the retention of such crowns can be increased without remaking the crown or by extensively modifying the tooth preparation.

Ninety cast metal complete crowns, divided into 9 groups of 10, were fabricated to be slightly loose in their internal adaptation to metal dies with an optimal tooth preparation. Horizontal grooves were formed around the circumference of the internal crown surface and the external surface of the metal die, the control being the unaltered crown and die. The crowns were cemented with resin-modified glass ionomer cement and then subjected to a tensile force until they were dislodged. The data were subjected to a 2-way ANOVA to determine the significance of the differences between crowns and dies, and 2 t tests were then used to compare each crown/die combination to the control (α=.05).

The mean retention was significantly higher when 1 horizontal groove was placed inside the crowns (P<.001) and was even higher when 2 grooves were placed inside the crowns (P<.001). Placing 1 or 2 grooves in the metal die or in both the crown and die was not significantly more retentive than placing grooves only in the crown.

Placing 1 or 2 horizontal circumferential grooves into the internal surface increased the retention of metal complete crowns made for optimal tooth preparations. Grooves placed into the crown were as effective as or more effective than grooves placed into the tooth / die.