You need to consult a medical/dental malpractice lawyer immediately. The statute of limitations for bringing a medical malpractice claim is one year from the date "the cause of action accrued." The law in Ohio, I believe, is that a the statute of limitations begins to run when a reasonable person knew or should have know of the possible existence of a claim. At best, your attorney can argue that the statute began to run when the specialist identified the problems you were having was the result of the allegedly defective work by the dentist.
Tell the collection agency that because you dispute that you owe the debt because of the enormous expense you incurred fixing his defective work, you refuse to pay. You should also consult with an attorney to find out what you can do to protect your credit history so that potential creditors will know that you aren't a deadbeat for not paying this debt (if it's been reported.)
Yes you have rights and you should get an OH personal injury lawyer.
You might find my Legal Guide helpful, "Successful Start for a Personal Injury Lawsuit"
Because this matter is so important you should really get a lawyer.
You might find my Legal Guide helpful "How to Choose A Lawyer For You"
You might find my Legal Guide helpful " What Do I Tell My Lawyer"
No one can know what the record is in the case because online we cannot see your documents. You need a lawyer. Check with a lawyer in your locale to discuss more of the details.
Good luck to you.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney with whom you have established an attorney client relationship and all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.
A roundup of the best tips and legal advice.