An implantable device capable of regulating flow of material through a body passage. The implantable device may include at least one retention member having a surface curved to resist migration of the implantable device with respect to the body passage. The implantable device may be provided with features providing a cushioning effect with respect to tissue at which the implantable device is implanted. The length of the implantable device and/or the configuration of at least one retention member on the implantable device may be adjustable. A removal element may be provided to facilitate removal of the implantable device.