Since you had previous trauma to your skull it is important to see what is causing tthis area of depression. If an implant is placed it need to have inrowth into the implant so it doesn't move and also need to blend at the edges so it is not visible. There are many options of fillers including carbinated calcium phosphet cement, and mutiple comercially available curshed bone fillers, including bone pastes, and osteoconductive graft subsitutes. Fat grafing is another possibility. Neurosurgeons probably have the greatest experince in this area although may not have the greatest concern about the final aesthetic appearanceso also consult with a Board Certified Plastic Surgeion about this many options.