Hi @Fede,
I can’t guarantee this method will work - it depends, crucially, on the external surface of the mesh ( effectively what you can see ) having no entry into the interior surfaces, by way of a joining hole say.
You can do this proceedure in b2.82 but I suggest you actually download the b2.9 Alpha as the method can be done non-destructively in that version - as a modifier 
In b2.82 you will have to use scuplt mode. Be warned, if you get the numbers wrong you will likely crash blender, it’s going to produce MILLIONS of polygons.
I am assuming that the model is real world scale, in which case I suggest you switch your units over to mm for clarity in the Voxel Size parameter.
So, with your model selected go into scuplt mode. You are going to remesh the entire thing with Voxels. At the top of the scuplt window there is this menu:
Set the Voxel size to say 5 mm. Press the Remesh button. It could take some time for your computer to calculate - be patient. This will rebuild the entire geometry using only the external surface. As you have used 5mm it will look totally decimated. But this is just so you can check that the outer surface genuinely has no link to the interior ‘bubbles’. Take a look inside your new mesh ( using undo is the only way to get back to the original ). If you find that you still have any interior geometry, then your jawbone does have a hole that connects the outer surface to the inner ones by a valid hole. You will therefore have to return to the original mesh and close these holes off. You don’t need to actually disconnect the interior faces from the outer mesh, but be sure to create a face that externally does seal the hole. Don’t worry about the fact this is bad modelling practice, voxelization will fix it.
Repeat the process of Voxelization, ( but reduce the Voxel size by 1mm each time) until you are certain there is no longer interior faces. Then, again starting from your now ‘sealed’ original mesh dial the Voxel size down to 1mm or less, but as you will have discovered each reduction in voxel size dramatically increases both the time it takes to calculate but also the number of polygons. Be very careful with numbers below 1mm. I don’t know what resolution you need for the printer, but after you get a good result you might need to use the regular Decimate modifier to help reduce the total poly count.
This is the proceedure in b2.82. In the b2.9 Alpha, the Voxel Remesher is also a Modifier, so you can more easily delete or change the parameters without having to endlessly undo to get back to your original mesh. However, NEVER use the spinner ( mouse movement ) to change the voxel size in the modifier as it applies the new parameters immediately and if you move too far the size value might be so small it’s trying to create a mesh with a resolutiuon of fractions of a mm. This will not go well 
As I said, I can’t guarantee this will work. But it’s worth a shot.
Dj
