A medical device for implantation in a knee joint of a human patient is provided. The medical device comprises: an inner surface and an outer surface. The inner surface comprises: a first point, a second point, a third point, a fourth point, a fifth point, and a sixth point, all points located on different places along a length axis of said inner surface. A first straight line, reaching from the first point to the second point is parallel to a second straight line reaching from the third point to the fourth point, which in turn is parallel to a third straight line reaching from the fifth point to the sixth point. The first and third straight lines are of equal length and the second straight line is longer than the first, furthermore the third straight line is positioned between the first and third straight lines.
