I often work with complicated osteotomy geometry that doesn’t follow straight lines or curves.

I need to experiment with many shapes to see the outcome. it would be so good if I could draw a closed curve and a plane is fitted through it

thanks

This feature is already available. Your can get the curve coordinate system (plane position, tangent, normal direction) at any curve point. There is also a nice module with a convenient GUI in SlicerVMTK extension: Cross-section analysis module.

Placing curve is not the issue . The request is about having a fitted plane or planes in the closed curve area to perform the cut .

I thought you need to fit the plane orthogonally to the curve. Fitting a plane in the curve points is even simpler. Have your tried using plane fit mode? It draws the plane by fitting it on all the control points that you specify.

never noticed before, and could not figure out how to use it .

not clear how to select points and initiate fitting