Boolean difference leaves open gap I am trying to make a surgical cutting guide (blue) for a jaw bone (white). The top part has to fit on the teeth, the lower part on the side of the jaw bone.

However, when I use the Boolean Difference modifier, it only partially works: The teeth are nicely cut out from the upper part, as you can see below.

But the part that touches the mandible is left open after the boolean, as you can see below.

I guess it might have something to do with the jaw bone model coming from a CT scan so it has an internal (hollow marrow) structure (see below)

I don't know how to solve it though. Is there a way to just 'fill' the jaw bone model and make it completely solid? Thanks for any advice.
Booleans ussually fails on non-manifold mesh.
To see these problematic parts - from within Edit Mode, using Vertex selection, with no geometry selected, Search for &quot;manifold&quot; (Select non-manifold operation appear). For more info see Blender Manual

Q: Is there a way to 'fill' model and make it completely solid?
A: In a meaning - closed mesh without holes - Yes
_
Fill
Search for Fill operator and increase number of vertices in tool properties until the holes are filled. Like here it was value 14.

But N-gons are not the best for boolean operations so probably better to try ...
Grid Fill
Like here. Important for this function is to have a loop of even number of vertices. If you don't have, you can select one edge and subdivide it.
(White vertex in the screen is one I added.)

Or even better to Triangulate whole mesh Ctrl+T
Remesh
Or you can try Remesh modifier &gt; Sharp if there is too much work, but even on hi Octree Depth in wouldn't be precise.