The ornamental design for the orthodontic appliance, as shown and described.
FIG. 1 is a top, right side perspective view of the orthodontic appliance.
FIG. 2 is a front elevational view of the design of FIG. 1. The rear elevational view is symmetrical with the front elevational view.
FIG. 3 is a right side elevational view of the design of FIG. 1. The left side elevational view is symmetrical with the right side elevational view.
FIG. 4 is a top plan view of the design of FIG. 1; and,
FIG. 5 is a bottom plan view of the design of FIG. 1.
The broken lines shown in the figures are included for the purpose of illustrating portions of the orthodontic appliance and form no part of the claimed design.