If you fell behind on payments, they could stop treating you for that reason alone: a dentist or orthodontist is not a charity and is not required to work for free or to extend loans, credit, or subsidies, either directly or by working without pay, to its patients. You or your husband would not keep working at a job where you were not being paid (or maybe you might personally choose to, because you would hope things will get better, but you would not have to keep working there; working without pay would be 100% voluntary and you could refuse to do so); similarly, an orthodontist does not have to take your financial issues into consideration and provide their services without being paid for them. You admit that you owe them money; that is the reason they do not need to keep seeing or treating your family.