What is the resolution of your image (image spacing along each axis)?
If you segment the bone then you can use the Segment Editor module to automatically get the internal cavity and external surface of the bone using Wrap Solidfy effect. This will allow you to get the centerline of either the internal cavity or the external surface of the bone.
We don’t hear much from the developers of this extension, so I’m not sure if it is still maintained, but please report the issue at the extension’s repository and hopefully you’ll get a response.