medical bills and collection agency

What is the name of your state? Missouri
We accrued bills for medical treatment approved by a judge in a workmans comp. case. After four years the case was settled for little or nothing and the bills were ruled not to be covered. Creditors immediately demanded payment. We are both on SS disability, we sent what we could afford, every month, even W/O getting billed. After more than 2 yrs. doctors sent our ck back saying they would no longer accept it, that we should send it to the collection agency they had sold it to 2 1/2 years earlier.
We knew they had sold acct, but sent payment to docs. on advice from someone who had been through it. Collection agency had stopped calling when we told them we were on SS and did not own anything.
Are we still liable for this bill?