How much tooth structure is needed for a crown on a molar?

Lower left wisdom tooth w/large amalgam filling w/3 sides of tooth structure left - the outer tooth structure 'formed' w/ amalgam 20+ years-broke off.Dentist said he can crown it& remaining tooth structure very hard/healthy Said today's crowns are different-all ceramic & you don't need much structure.Said the liner used years ago is intact gumline is exposed a little he said yes but not the dentin - he said he will grind it down below the gumline a little. He's75% confident what do u think?