What are the causes of sciatica? is it always a herniated disc problem?

No: Sciatica is common term for pain going down the back of the leg. Other things can mimick this such as sacroiliac joint pain, zpiriformis tender point ( this muscle rund diagonally across the buttock), or referred pain from the lower joints in your back.

Nerve irritation: It is due to irritation of one of the nerve roots which combine to form the sciatic nerve.

Not a disk: The vast majority of people with sciatic pain (pain radiating from the lower back or buttock down the back of the leg) do not have a herniated disk. The most common cause is muscle spasm. Many people have slightly herniated disks and surgeons are only too happy to take the disk out, but most of the of back surgeries are unnecessary. The best treatment is physical therapy and staying in good shape.

Lay term: Sciatica, when used by medical professionals, is synonymous with radicular pain.. However, lay people use it to describe all back and/or leg pain. So before we address causes, lets start with definitions - please tell us what pain you mean when you use the term "sciatica"?

No: Sciatica is really a misnomer. Most causes of leg pain are more appropriately called radiculopathy, because the compression occurs in the spinal canal and the sciatic nerve doesn't form until much lower in the body. Other causes "sciatic" pain include, spinal stenosis, tumors, pyriformis syndrome, pelvic pathology, neuropathy, and cervical spine pathology.

Spine Pain Options: This pain in the distribution as you suggested is the result of an irritated nerve or facet joints or other injury typically in the lumbar spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease, etc requiring further evaluation by a spine specialist and may be candidate for facet injections/radiofrequency ablation and epidural steroid injection.