Is Filing Down the Backs of Zirconia Crowns Bad?

I recently had three zirconia crowns placed on my front three teeth. Due to issues with my bite the dentist had to sand down the backs of the crowns after they had already been permanently cemented. He polished them, but I am still afraid that some of the zirconia may have been exposed during this process. Do I have anything to worry about. I am finally confident with the way my teeth look after years of braces, bonding, crowns, and adjustments. I would hate to have more problems.