A medial branch block is a diagnostic injection that helps determine whether your back or neck pain is coming from your facet joints, the small paired joints along each side of your spine. A tiny amount of numbing medication, typically just half a milliliter, is placed directly on the nerve that carries pain signals from a specific facet joint. If the injection relieves your pain, it confirms which joint is the source and opens the door to longer-lasting treatments.
The procedure is not designed as a permanent fix. Its primary purpose is pinpointing the exact origin of your pain so your doctor can recommend the right next step.
The Nerves Being Targeted
Each facet joint in your spine is served by small nerves called medial branches. These nerves branch off from the main spinal nerve, wrap around a bony groove at the base of the vertebra, and send pain signals from the joint capsule to your brain. The joint capsules are packed with pain receptors, which is why they generate significant discomfort when irritated by inflammation or mechanical stress.
One important detail: each facet joint receives nerve supply from two adjacent medial branches, not just one. That means a block targeting a single joint typically requires injections at two spinal levels. These same nerves also supply the deep muscles that stabilize your lower back (the multifidus muscles), the ligaments between your vertebrae, and surrounding tissue.
Why It’s Primarily a Diagnostic Tool
Medial branch blocks are classified as diagnostic procedures, not therapeutic ones. The numbing medication wears off within hours, so any pain relief is temporary by design. The real value is the information you and your doctor gain from it.
For the block to be considered positive, most clinical standards require at least 80% relief of your primary pain, and that relief must last a duration consistent with the type of anesthetic used. A 50% pain reduction cutoff was once common, but evidence shows that threshold produces a high rate of false positives. Using the stricter 75% to 80% cutoff with two separate block procedures significantly improves diagnostic accuracy.
This is why most insurance policies, including Medicare, require two positive diagnostic blocks before approving further treatment. The second block, performed a few weeks after the first, often uses a different anesthetic to confirm the result. In one study, the false-positive rate after a single block was 27% for the lumbar spine, which is precisely why that confirmatory second block matters.
What Happens During the Procedure
You’ll lie face down on a table while your doctor uses imaging guidance, either fluoroscopy (live X-ray) or CT scanning, to visualize your spine in real time. A reference marker is placed on your skin, and a limited scan identifies the exact bony landmarks where the medial branch nerve runs.
After numbing the skin with a small needle, the doctor advances a thin spinal needle toward the groove where the nerve sits, at the junction between the superior articular process and the transverse process of the vertebra. Spot images are taken after each small adjustment to confirm the needle’s path. Once the needle reaches the target, a tiny amount of contrast dye (about 0.2 mL) is injected to verify correct positioning. Then the anesthetic, commonly lidocaine, is delivered in a volume of about 0.5 mL per site.
The entire procedure typically takes 15 to 30 minutes. You can eat and drink normally the morning of the injection.
How to Prepare
The most important preparation step happens a week beforehand: if you take blood-thinning medications such as warfarin, apixaban, rivaroxaban, clopidogrel, or similar drugs, you’ll need to stop them before the procedure. Talk with the doctor who prescribed those medications as soon as you schedule your block, because the timing of when to stop varies by drug. If you haven’t arranged this at least a week ahead, the injection will likely need to be rescheduled.
Low-dose aspirin and anti-inflammatory medications may or may not need to be paused. Your pain clinic will give you specific instructions based on your situation.
Tracking Your Pain Afterward
After the injection, your doctor will ask you to pay close attention to your pain levels. The numbing medication kicks in quickly, and the key question is simple: how much did your usual pain decrease? You’ll typically be asked to rate your pain before the procedure and then at intervals afterward, noting when relief begins, how complete it is, and when it fades.
This pain diary is essential. The duration of relief matters as much as the degree. Lidocaine, the most commonly used anesthetic, wears off within a few hours. If a different, longer-acting agent like bupivacaine is used for your confirmatory block, relief should last proportionally longer. A mismatch between the expected and actual duration of relief can signal a false positive.
Possible Side Effects and Risks
Medial branch blocks are generally low-risk, but transient complications do occur. In one study of over 700 spinal pain blocks, temporary neurologic effects appeared in about 1.7% of cases for both cervical and lumbar injections. These included brief episodes of muscle weakness, and less commonly, chest discomfort, nausea, low blood pressure, or a slow heart rate. All resolved on their own.
The probable causes of these temporary effects included the anesthetic reaching unintended areas, accidental injection into a blood vessel, or a small leak of medication into the spinal fluid. Serious permanent complications are rare. Soreness at the injection site for a day or two is the most common aftereffect.
What a Positive Result Means for Treatment
If both diagnostic blocks provide at least 80% sustained relief of your primary pain, the next step is typically radiofrequency ablation (RFA). This procedure uses heat to create a lesion on the same medial branch nerve, interrupting its ability to transmit pain signals for a prolonged period. The relief from RFA is not permanent, because nerves eventually regenerate, but it can last months to over a year.
If your diagnostic block does not provide significant relief, RFA at that level is unlikely to help, and your doctor will look for other sources of your pain. This is exactly why the diagnostic step exists: it prevents you from undergoing a more involved procedure that wouldn’t work.
For those who do proceed to RFA and get good results, Medicare and most insurers will cover repeat treatments as long as each one provides at least 50% pain relief lasting at least three months, or at least 50% improvement in your ability to perform daily activities compared to your baseline.