Filler which is surface-modified with a compound of the formula [(PG)—R1—Z]n—SP—[Y—R2—(AG)]m, in which AG is —COOH, —P(═O)(OH)2, —O—P(═O)(OH)2, —SO2OH, —SiR3R4X, or a chelating group; R1 is a C1-C3 alkylene group or cyclopropylene group or is absent; R2 is a C1-C10 alkylene group or is absent; R3 is a C1-C8 alkyl group, chlorine or OR5; R4 is a C1-C8 alkyl group, phenyl, chlorine or OR5; R5 is a C1-C6 alkyl group; X is OR5 or chlorine; Y is O, S, CO—NH, O—CO—NH, NH—CO—NH or is absent, Y being absent if R2 is absent; Z is O, S, CO—NR6, O—CO—NH, NH—CO—NH or is absent, R6 being H or C1-C6 alkyl; PG is a radically polymerizable group of the formula in which R9 is H, C1-C3 alkyl, C1-C3 hydroxyalkyl or COOR10; R10 is H, C1-C10 alkyl, 1,6-dimethylphenyl or mesityl; R11 is H or phenyl; m is 1 or 2; n is 1, 2, 3 or 4; SP is absent or is an (n+m)-valent linear or branched aliphatic C1-C30 radical, in which the carbon chain can be interrupted by O, S, CO—NH, O—CO—NH or NH—CO—NH, an (n+m)-valent aromatic C6-C18 radical, an (n+m)-valent cycloaliphatic C3-C18 radical or an (n+m)-valent heterocyclic C3-C18 radical, where the radicals can be substituted or unsubstituted; in which, if AG is a silyl radical of the formula —SiR3R4X and PG is a vinyl group of the formula H2C═CH—, R1 and Z are not absent simultaneously.