When is surgery definitly needed for a herniated disk?

Not always: Surgery is best reserved for patients not responding to pain management on occasion, but is best supported when there is progressive spinal cord compression, leg or arm weakness, bladder control problems, or several sensory losses.

Definite indications: 2 indications are a disc herniation leading to myleopathy or cauda equina syndrome. A myleopathy is compression of the spinal cord leading to its dysfunction and cauda equina syndome is bowel/bladder control issues due to those nerves being compressed. Otherwise, disc surgery is for those who fail non op care or may be having intractable pain or a progressive deficit in terms of weakness.