A cushion for use with orthopedic casts for encasing wrists, forearms and/or hands, when a finger, such as the thumb, is not itself encased in the cast, is provided. The cushion is manufactured from a moisture-proof, non-absorbent, closed-cell synthetic material, in the shape of a portion of a section of a hollow tube, so that when the cushion is placed against a flat surface, an inner surface of the cushion encloses a cavity or an empty space open at one end of the cushion. When the supplemental cushion is in use, the webbing that connects the human thumb and index finger is positioned in that empty space, and more particularly encased within the empty space of the cushion, so that chaffing and injury to that webbing during thumb movement and extension are minimized, while the wrist, forearm and/or hand are encased in the cast.