A prosthesis for insertion into a body passage is disclosed. The prosthesis includes a plastic or polymer base material which is compatible with living tissue and which possesses a memory of a predetermined configuration. The base material further has a glass transition temperature at which the prosthesis can be molded intravascularly from the predetermined configuration to a larger-radius implant configuration, which is sized and shaped to conform to an internal anatomy of the body passage to expand a narrow segment of or to occlude an opening of an out pouch of the body passage. The glass transition temperature is greater than a temperature of the body passage so that the prosthesis after being molded can be allowed to cool to the temperature of the body passage.