I am 23 and have had braces twice to make space for implants on either side of my two front teeth. I now wear a retainer with two pontics. My dentist is now saying that there still is not enough space. I refuse to go back into braces a 3rd time when I know later I will have to pay for bone graphs and then pay for the implants themselves. What is the minimum amount of space in mm needed? I have about 3mm of space, but dentist says I need 5mm. Have narrower implants been made?