Legal question orthodontist treatment?

Orthodontist treatment paid in full by patient ( no insurance). Patient in treatment acquires dental insurance and submits a claim. Insurance gives back 1,000 dollars to patient, they send check to orthodontist office. Office keeps those 1,000 dollars stating that treatment took longer than expected due to few missed appt's which patient paid for individually for! Orthodontist won't give back money and now decides to charge patient 150 dollars for each appt from now on till the next year that treatment will be over. Is this allowed? by the way no one ever told patient that treatment was going to be longer until patient submitted insurance claim! sorry for the lenghty explanation! thank you! legal advice needed!

Legal question orthodontist treatment?
Most dentists provide a written treatment plan and payment plan. This would be considered a contract. If you have one this would be your best evidence and I'd sue them in small claims court.