A new group of restorative materials called "packable" composites has recen
tly been introduced. These products are essentially highly-filled or densel
y-filled hybrid resin composites. One of the many claims made about these m
aterials is that they undergo less polymerization shrinkage than their conv
entional counterparts. This in vitro investigation compared the amount of l
inear shrinkage that occurs within a variety of densely filled resin compos
ites (DFC) and conventional hybrid resin composites when cured with a visib
le halogen light. Six DFC resins (Alert, Ariston, P60, Prodigy, Solitaire a
nd Surefil) and two hybrids (TPH-Spectrum, Z100) were used in this study. D
imensional change was measured in a linear direction using a calibrated lig
ht microscope. Eighty samples of resin composite were tested, resulting in
eight groups of 10 samples (N=10) each. The one-way ANOVA with Student-Newm
an-Keuls post-hoc test was used to compare the shrinkage between groups, an
d Pearson's Correlation was used to test the relationship between filler ch
aracteristics and shrinkage. Alert and P-60 had significantly less shrinkag
e than Solitaire, Ariston, Prodigy, Z-100 and TPH-S. Thus, the shrinkage va
lues of some DFC resins were significantly less and others were no differen
t from conventional hybrid resins. There is a moderate association between
filler volume and shrinkage. Filler size and resin chemistry are other fact
ors that may also effect shrinkage.