My Skull is Flat on Top

Hi, I have always thought my head looked abnormal. I have recently noticed my skull is flat on top and not rounded like everyone else. This makes me have a flat top when i wear my hair tied back and makes my whole face look short. what causes this and what can be done to fix this? I have seen cranial implants dome shaped to reconstructive surgeries, is this an option. please any advice and information on surgery would be helpful. If an implant was placed how big of an opening would there be?