Your arborvitaes will grow new leaders. The new leader will come out of the top of the tree and be a little offset from the trunk of the tree. So it will have a little zig-zag. By little I mean an inch or two. Over time that leader will become the new trunk. Over a few years, as your tree gets bigger and fuller you will notice the zig-zag less and less. Eventually you won't notice it at all.
For these trees when you cut the top off, it makes the bottom of the tree grow more. Instead of growing up, they grow out. But eventually, as the new leader takes over, the tree will resume its normal shape.
I hope I've relieved some of your concern. Please ask more questions as you think of them.
