A spinal injection is an invasive procedure that is used for both diagnosing and treating low back pain. The purpose of a spinal injection is to deliver medicine directly to the specific site of pain.

When you have a diagnostic injection, the goal is to find out if the medication relieves your pain. If it relieves your pain in the area your doctor suspects, based on your physical exam and medical history, is the source, she can confirm that indeed the diagnosis. Pain relief from diagnostic injections is generally temporary — usually just a few hours.

When you have an injection as a treatment, the relief tends to last longer.

Here are 4 commonly given spinal injections.

Selective Nerve Root Block

Spinal column, illustration — BSIP / UIG / Getty Images

Selective nerve root blocks are one of the most commonly given spinal injections. They can be used to treat or diagnose back and/or leg pain due to damaged nerve roots. A nerve root is a collection of nerves that have branched off the main spinal cord on their way out to serve the rest of the body. You have nerve roots on either side of the spinal column at each spinal level. Should a nerve root become compressed or irritated, leg pain may result. Most people call this sciatica, but its technical name is radiculopathy.

Lumbar Facet Joint Nerve Blocks

Human Spine, Pelvis, Chiropractic, Orthopedic, Medical Model, Heathcare, Isolated — 1Photodiva / Getty Images

If based on your physical exam, your doctor suspects your pain is coming from the joints at the back of your spine, she may recommend trying a facet injection. The facet joints are small joints located at each vertebral level — one on each side; they add integrity to the spine by limiting excessive movement. The facet joint is a common site for spinal arthritis to develop.

But buyer beware. Clinical guidelines for the treatment of low back pain that was produced in Feb 2014 by the Agency for Healthcare Research and Quality reports that for the most part, facet joint injections are not proven pain relievers. The AHRQ says that alone, a facet injection alone is not likely to give you long-term results, nor is it particularly accurate for diagnosing.

Injections into Your Sacroiliac Joints

Doctor pointing to Sacroiliac Joint on skeleton — Jan-Otto / Getty Images

Several types of sacroiliac joint injections exist; namely, intraarticular steroid injections (also the type given for painful facet joints) and periarticular injections, as well as pulsed and conventional radiofrequency neurotomy. Botox and steroids are two medications commonly used for periarticular injections.

Intraarticular means inside the joint while periarticular means around the joint. Radiofrequency neurotomy is a procedure that introduces heat into the joint in order to interrupt the functioning of the painful nerve.

The AHRQ says that only limited evidence supports the use of the intraarticular and periarticular types of sacroiliac injections.

That said, as a diagnostic tool, intraarticular sacroiliac joint injections with local anesthetics that relieve 75% to 100% of your usual pain have good science behind them, according to the AHRQ.

The AHRQ also says that evidence both pulsed and conventional radiofrequency neurotomy is limited, but fair evidence for cold radiofrequency neurotomy.

Epidural Steroid Injections

If you have a herniated disc or disc radiculitis, your doctor may suggest an epidural injection.

A spinal epidural injection delivers steroid medication into the epidural space, an area that is between the spinal cord and the vertebral canal, and near the very sensitive spinal cord.

For herniated disc or disc radiculitis, the AHRQ recommends any one of the 3 types of epidurals: The caudal, interlaminar or transforaminal approaches, stating there's good evidence for each. By the way, these fancy words refer to the direction into which the needle is inserted.

Discogenic pain (i.e., pain that arises from inside the disc rather than being due to an injury or herniation) is another reason your doctor may suggest an epidural, but the AHRQ rates the evidence for this use as only fair.

Epidurals into the facet joints are also done, although the AHRQ says the evidence for its effectiveness is only fair or limited, depending on which approach is taken.

Other reasons spinal epidurals are given include spinal stenosis post surgery syndrome.