38 CFR 4.58: Arthritis due to strain.

With service incurred lower extremity amputation or shortening, a disabling arthritis, developing in the same extremity, or in both lower extremities, with indications of earlier, or more severe, arthritis in the injured extremity, including also arthritis of the lumbosacral joints and lumbar spine, if associated with the leg amputation or shortening, will be considered as service incurred, provided, however, that arthritis affecting joints not directly subject to strain as a result of the service incurred amputation will not be granted service connection. This will generally require separate evaluation of the arthritis in the joints directly subject to strain. Amputation, or injury to an upper extremity, is not considered as a causative factor with subsequently developing arthritis, except in joints subject to direct strain or actually injured.