Dimensional stability of record bases fabricated from light-polymerized composite using two methods

Abstract

Statement of problem. Record bases fabricated from resin-based composite, which undergoes polymerization shrinkage during curing, can distort during curing to produce significant gaps along the posterior border. Purpose. This study evaluated the pattern of gap formation that occurs over the palatal area of a maxillary record base fabricated from light-cured composite (Triad), and determined whether changes in the fabrication method affects the size and pattern of gap formation. Effects of composite width and passage of time on gap formation were also analyzed. Material and methods. Twenty record bases were fabricated with two methods: (1) according to the manufacturer's directions, the entire surface of the record base was exposed to the curing light; and (2) the palatal area of the record base was covered with foil to prevent light penetration, and the base was light cured in three stages with readaptation of the uncured composite between stages. Ten record bases from each group were tested 1 hour after fabrication; the remaining 10 from each group were tested 7 days after fabrication. Measurements of adaptation were made at three points representing the right and left ridge crests and midpalatal areas. The first measurement was taken at the posterior border and subsequent measurements were made at 5 mm increments after reduction of the posterior border of the cast and record base with a model trimmer. Results. Dimensional changes were observed in both procedures. The largest discrepancy occurred at the posterior border of record bases fabricated using method 1. Mean gap formation at the posterior border in this group after 1 hour was 454, 729, and 334 μm and 483, 841, and 330 μm after 7 days at points A, B, and C, respectively. Record bases fabricated with procedure 2 demonstrated mean gap sizes at the posterior border after 1 hour of 219, 421, and 208 μm and 267, 403, and 192 μm after 7 days at points A, B, and C, respectively. No significant influence of composite width on gap size was found. Conclusion. The average gap size due to stage curing (method 2) was never greater than the corresponding gap that arose from the use of the manufacturer's recommendations (method 1). Limiting the amount of surface area exposed to the curing light and readapting the uncured composite to the cast between curing episodes can significantly reduce the mean gap size occurring at the posterior border. (J Prosthet Dent 1998;79:399-403.)