Yes, orthodontist expenses count as medical expenses if the expenses were for you or your dependents. The IRS defines medical expenses as "the costs of diagnosis, cure, mitigation, treatment, or prevention of disease, and for the purpose of affecting any part or function of the body. " Please note that only medical expenses in excess of 7.5% of AGI will be deductible.
For more information, please see Publication 502 What Are Medical Expenses?