A method for treating a bifurcation or trifurcation aneurysm (201) occurring on a first artery, the first artery and a second artery joining to a third artery, the method comprising: inserting a medical device (202) such that it is at least partially located in the first artery and is at least partially located in the third artery; expanding the medical device (202) from a first position to a second position, said medical device (202) is expanded radially outwardly to the second position such that the exterior surface of said medical device (202) engages with the inner surface of the first and third arteries so as to maintain a fluid pathway through said arteries; and positioning the medical device (202) such that a membrane (203) of the medical device (202) is located against an aneurysm neck of the aneurysm (201) to obstruct blood circulation to the aneurysm (201) when the medical device (202) is expanded to the second position, and at least a portion of the membrane (203) is secured to the medical device (202) to maintain the position of the membrane (203) relative to the medical device (202) when expanded to the second position; wherein the membrane (203) is permeable and porous, the size of the pores of the membrane (203) and the ratio of the material surface area of the membrane (203) being such that blood supply to perforators and/or microscopic branches of main brain arteries is permitted to improve healing of the first artery but blood supply to the aneurysm (201) is prevented.