A medial branch block (MBB) is a common, minimally invasive procedure used to address chronic back pain originating in the small joints of the spine. The procedure involves the precise injection of medication, typically a local anesthetic, near specific nerves along the spine. This injection temporarily interrupts pain signals being transmitted from the spinal joints to the brain. The MBB serves as an important diagnostic tool to pinpoint the exact source of discomfort. The goal is to confirm if the facet joints are the cause of chronic pain before pursuing more permanent treatment solutions.
Understanding the Target: Medial Branch Nerves
The target of the medial branch block is the medial branch nerves, a pair of small nerves that branch off the main spinal nerve. These nerves are primarily sensory, transmitting information about pain and sensation to the central nervous system. They do not control movement or sensation in the arms or legs, minimizing the risk of motor complications during the block. The medial branch nerves specifically innervate the facet joints, which are small, hinge-like joints located on the back side of the vertebrae.
Facet joints, also called zygapophyseal joints, help guide and limit the spine’s movement, allowing for bending and twisting. When these joints become irritated, inflamed, or undergo degenerative changes, such as from arthritis, the medial branch nerves transmit the resulting pain signals. Each facet joint typically receives innervation from two medial branch nerves, coming from the spinal segment above and the segment at the same level. Inflammation or wear within the joint capsule activates pain-sensing components, causing chronic back pain.
Diagnostic and Therapeutic Roles
The primary purpose of a medial branch block is diagnostic, confirming if the facet joints are the source of the patient’s pain. A successful diagnostic block occurs when the patient experiences a significant, temporary reduction in pain immediately following the injection of the local anesthetic. Clinicians typically look for a pain reduction of 50% or more, though some guidelines require an 80% or greater decrease to confirm the diagnosis. This temporary relief confirms that the blocked nerves were transmitting the pain signals, identifying the facet joint as the pain generator.
A single positive response may not be conclusive due to the risk of false positives, so a second, comparative diagnostic block is often performed. This second block uses a different local anesthetic with a distinct duration of action to validate the results. If pain is consistently relieved following both blocks, the patient is deemed a candidate for longer-term treatment. While the block is largely diagnostic, the injection of anesthetic, sometimes combined with a corticosteroid, can offer temporary therapeutic relief lasting a few days up to a few months. Sustained treatment is contingent upon a successful diagnostic response, often leading to the recommendation of radiofrequency ablation (RFA).
The Medial Branch Block Procedure
The medial branch block is a minimally invasive outpatient procedure that usually takes 15 to 30 minutes to complete. Before the procedure, patients are instructed to fast and may need to temporarily stop taking certain medications, such as blood thinners. The patient is positioned, typically lying prone (face-down), on an examination table. The injection area is then sterilized with an antiseptic solution.
To ensure precise and safe needle placement, the physician uses fluoroscopy, a live X-ray guidance system. Fluoroscopy allows the doctor to visualize the spinal anatomy and the needle’s trajectory in real-time. This guides the needle tip to the small groove where the medial branch nerve crosses the bone. Once the target location is reached, a local anesthetic, such as lidocaine or bupivacaine, is injected near the nerve.
The injection mixture may also contain a steroid to provide an anti-inflammatory effect if the procedure is intended to be therapeutic. Patients may feel a brief burning or pressure sensation during the injection, but the procedure is generally well-tolerated. After the injection is complete, the needle is removed, and a small bandage is applied to the site.
Post-Procedure Recovery and Outcomes
Immediately following the medial branch block, patients are moved to a recovery area for monitoring, typically around 30 minutes. Because a local anesthetic is used, patients are not permitted to drive themselves home and must arrange for transportation. Temporary numbness or weakness in the affected area is common due to the anesthetic. This is often a sign that the medication has successfully reached the target nerves.
Post-procedure recovery involves the patient tracking their pain levels and activities over the next several hours, often using a pain diary. This documentation helps the physician determine if the facet joint was the source of the pain based on the degree and duration of relief. Most patients can resume their normal activities the following day, though strenuous activity should be avoided for the remainder of the day.
The outcome of the medial branch block dictates the subsequent steps in the treatment plan. If the block resulted in the required level of temporary pain relief, the patient is a suitable candidate for a long-term solution, most commonly radiofrequency ablation (RFA). RFA uses heat to temporarily disable the medial branch nerves for a much longer period, often providing relief for six to 12 months. If the block provides little or no relief, the physician will explore other possible causes of back pain, such as a herniated disc or nerve compression.