Edit to add, if you think of neurosurgeons etc, they use the title "Mr" to distinguish their specialisation from "regular" doctors. As a Dental Surgeon, should a dentist be referred to as "Mr" to.
Surgeons use 'Mr' rather than 'Dr' because traditionally you did an apprenticeship to become a surgeon rather than studied medicine at university. Obviously that has now changed, but it's just a hang-on from yesteryear.