The ornamental design for a dentalinstrument, as shown and described.
FIG. 1 is a right isometric view of a dental instrument showing a new design;
FIG. 2 is a left isometric view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right end view thereof;
FIG. 6 is a left end view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines in FIGS. 5 and 6 are included for the purpose of illustrating portions of the dental instrument and form no part of the claimed design.