My 10-year-old son, who has autism, is also totally terrified of the dentist. We used to sedate him (slightly)to just get him in the door. Then we had to put him in a papoose -- and I had to straddle him and hold his arms and head while his father held down the legs and feet.
Then one of his behavior therapists suggested that we make a 'book' about his brothers going to the dentist. I took the older boys to the dentist (they already had appointments) and took pictures the whole time: pictures walking calmly towards the building, opening the door while smiling, the receptionist greeting the boys, in the xray chair, the xray machine, the xray tech, and so on. I took pictures of every single instrument the dentist might use on my son and every person that my son might need.
I printed the photos and stuck them into a simple photo album -- not a single embellishment, journal block or paper. Just pictures in sequence. I gave it to my son.
He devoured the book, pointing to the tools and the people and looking at me to ask what/who they were (he has limited speech). I labeld each tool and named each person; much of that was from my imagination ("that's a tooth scratcher", "wind blower". "Her name is Lala, like the Teletubby.")
It worked like a charm: no sedation, still a papoose but no need to hold him down. t was amazing, best intervention I've ever used. Took a little more work but the results were wonderful. We still continue to 'read' the book from time to time and it's been 3 months since the check-up.