gridgen ruled surface

jjc

I am using Gridgen 15.00 to generate a grid from a ct images geometry (pulmonnary artery). I would like to extend the main branch with a cylinder. to do so, i have to connect a circle with the imported geometry outlet border. that is to say, to create a ruled surface between the circle and the outlet border. I split this problem in 2 halves, so i deal with a semi circle and a polylines in 3d (one half of the outlet border). The thing is, the control points of the polylines are not coplanar. when I try to create the ruled surface, there is no message of error, but the usual 2 segments connecting the starting and ending points of he 2 rail curves does not appear, and, more over, after clicking on "save ruled surface", the surface does not appears anywhere (in examine, delete, even in the connectors menu). I tried with a 7 control points polylines and a semi circle: if the 7 points are coplanar, everything is fine, but if they are not, i have the same problem. Is there a way to overcome it?