The brain is surprisingly much smaller than the full shape of the head. There is a thick layer between the scalp and the skull, and the thickness of the bone itself. I definitely was shocked the first time I saw a patient with a craniectomy.

There is an incision made in the abdomen and a pocket is made where the skull flap is inserted. Basically the skull sits under the skin. This keeps the bone in an environment that is isotonic and warm, plus the hospital cannot lose the bone.

I'm fairly sure this is not a 'a deformity'. This is the result of a craniectomy. A portion of the skull is removed to perform surgery and to reduce intracranial pressure due to swelling. Often the skull flap is actually placed into the abdomen of the patient until it can be put back. I am an Anesthetist who does a lot of neuro cases.