a method of injection molding an orthopedic device comprising : injection molding a polymeric orthopedic device having a length along a longitudinal axis ; and deliberately inducing , in the device , a material property change along the longitudinal axis by varying a molding parameter during the injection molding . the material property may be , for example , elastic modulus and / or density . the molding parameter varied during the injection molding may be a supply material pressure . the orthopedic device may be an elongated rod - shaped element , such as a spinal fixation rod , which may or may not have a substantially uniform cross - section , and may or may not have a metallic core . a corresponding apparatus is also described .