Is it an HIPAA violation or not if a dentist gives my medical records without asking me to his expert witness ?

If litigation is ongoing this would not be a HIPAA violation. Even if this was a HIPAA violation you as the aggrieved party would have no right to legal action as HIPAA violations do not give rise to individual causes of action.

Most likely, no. How would the dentist defend himself without​ this ability? Otherwise​, you deny access and court throws your case out. You certainly can't expect to sue and be allowed to hide this information.

What do you mean "his expert witness"? Do you mean you filed a lawsuit for medical malpractice and the defendant (your dentist) is having your records reviewed by an outside expert? If that is what you mean, then no it is not a violation of HIPAA. You put your medical condition at issue, the defendant is therefore entitled to investigate your claim including having your records reviewed by a qualified medical expert.

Did you sue the dentist? Do you know why your records were provided to an expert witness b y your dentist? This information would be helpful in order to answer your question rather than causing us to assume facts and ultimately guess because we don't know the facts.. Also if you sued the dentist you should ask your own attorney this question r ather than outside attorneys who have have been provided limited/incomplete information.