Problem slicing a complex 3D-Shell Model and filling it up to become solid

[jaebeom_shinUQGBM]

Hello everyone,

I imported a Tibia bone model (igs. file) into my AutoCAD 2025. What I need is the only the head of the tibia bone (the upper part of the Z-axis). So I tried slicing it with a planar surface. I first selected the entire bone, then clicked on my planar surface, but then I can't select the upper part which I want to keep. 

Since this is a hollow object, I want to clsoe the cut surface and fill it in/turn it into a solid after slicing

Can someone help me on this ? (I added the file below)

I've seen the youtube videos and practised on simple objects but doesn't seem to work on this. Or does it ?

[parkr4st]

partial solution attached

UNION the surfaces together to make one surface, then SLICE.

There is an area of small surfaces (yellow) that are not unioned due to overlapping edges, non-meeting edges, i.e. gaps, and other defects in the original work that still need fixing to complete the UNION into one surface to move on to the solid.

[jaebeom_shinUQGBM]

thank you for the solution. I see where the problem is. I downloaded the model from a website. 

Do you know if there is a way to repair those areas directly on AutoCAD?

[parkr4st]

back to the portion that would not union, eXploded it to the original surfaces and UNIONed 1 by 1.

There are two that failed to UNION. (magenta in .dwg 2  One fixed by deleting it and SURFPATCH to create a new one which UNIONed successfully.

The other failed to SURFPATCH and is still magenta in .dwg 3.  There is some very small glitch there somewhere whihc I can't find.  Maybe someone else can do so.

If that surface can be UNIONed then moving to the solid is the next quest.