The ornamental design for a thumb orthosis, as shown and described.
FIG. 1 is a top perspective view of a thumb orthosis, showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right elevation view thereof;
FIG. 5 is a left elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view of the thumb orthosis shown in use.
All broken lines shown in the FIGS. 1 through 7 depict portions of the thumb orthosis that form no part of the claimed design, while the additional broken lines showing a user hand and attachment straps in FIG. 8 depict environmental subject matter that forms no part of the claimed design.