have a legal case. Is this true?

You would only if this represented a deficit or detriment to your treatment. Had braces myself long ago and what was originally thought to take only a year and a half, took 4 years. Medicine is not precise and simply entending the treatment time would probably not be malpractice. If because of the extended treatment it somehow negatively affected your health, it might.

Simply not having enough flexibility in the schedule to accomodate a patient's need to reschedule is not malpractice.