As with pretty much everything in the medical world, if you own your own practice yes and if you don't probably not. Generally dentists and orthodontists work Monday through Friday 9 - 5 or sometimes earlier. However, some offices are offering more flexible hours and more "off" hours to accommodate people's schedules. That means weekends, early mornings, and late evenings.
So, if you want to set your own hours, you'll need to open your own practice, more than likely, and you'll need to determine what is best for the business and the bottom line.