If there was an agreement between you and the orthodontist in regards to payment, that agreement is enforceable. If you had violated it, the orthodontist could seek all the money from you; if the insurer declined insurance when the orthodontist had done everything right, he could also seek payment from you, since you are ultimately responsible for the bills. But if the problem is that the orthodontist breached his obligations under the agreement by not submitting the claims in a proper or timely way, then the orthodontist should be precluded from collecting from you (apart from the portion you'd pay anyway), since he was the one who breached the agreement or contract. If you can prove the existence of the agreement and the fact that the orthodontist breached it, you may have a good defense to the bills, but may need to bring a legal action to have them dismissed and prevent the orthodontist from trying to collect. You should consult with an attorney. Good luck.