There are accounts of soldiers who have lost half of their brain (one hemisphere) and the shape of their head shows this. I think these poor soldiers have lifelong problems with thought processes, impulse control, and such, but they are alive.

Is the "dent" important to your plot? Might a growth work as well? A swelling or a bone deformity that results from the accident and looks like a horn? (I think Flannery O'Connor did this in one of her short stories.)