The ornamental design for the dental appliance, as shown and described.
FIG. 1 is a top and side perspective view of the bottom piece of the dental appliance, the disclaimed teeth being drawn in dotted lines;
FIG. 2 is a top view of the bottom piece of the dental appliance;
FIG. 3 is a bottom view of the bottom piece of the dental appliance;
FIG. 4 is a side view of the bottom piece of the dental appliance, the reverse side being a mirror image thereof;
FIG. 5 is a bottom and side perspective view of the bottom piece of the dental appliance;
FIG. 6 is a back elevation view of the bottom piece of the dental appliance; and,
FIG. 7 is a front elevation view of the bottom piece of the dental appliance.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.