Lumbar Medial Branch Blocks

The lumbar facet joints (zygapophyseal joints) form the posterolateral articulations connecting the vertebral arch of one vertebra to the arch of the adjacent vertebra. As true synovial joints, each facet joint contains a distinct joint space capable of accommodating between 1 and 1.5 mL of fluid, a synovial membrane, hyaline cartilage surfaces and a fibrous capsule. The facet joint capsule and surrounding structures are innervated. Chemical or mechanical stimulation of the facet joints and their nerve supply elicit back and/or leg pain.

The prevalence of facet-mediated pain varies in different studies from less than 5% to as high as 90%. Studies conducted in well-selected patient populations, using well-defined diagnostic criteria, indicate a prevalence around 15%. Facetogenic pain is predominantly caused by repetitive stress and/or cumulative low-level trauma. The resulting osteoarthritis leads to inflammation, which can cause the facet joint to be filled with fluid and swell, which in turn results in stretching of the joint capsule and subsequent pain generation. Inflammatory changes around the facet joint also can irritate the spinal nerve via foraminal narrowing, resulting in leg pain. The most frequent complaint is axial low back pain. Sometimes, pain may be referred into the groin or thigh. Lumbar paravertebral tenderness is indicative of facetogenic pain.

Each lumbar facet joint has dual innervation, being supplied by 2 medial branch nerves. Each nerve emerges from its intervertebral foramen and enters the posterior compartment of the back by coursing around the neck of the superior articular process. Hugging the neck of the superior articular process, the medial branch passes caudally and slightly dorsally, covered by the mamillo-accessory ligament, hooking medially around the caudal aspect of the root of the superior articular process to enter the multifidus muscle.

Lumbar medial branch blocks are diagnostic injections designed to determine if patients back pain is emanating from the lumbar facet joints.

A positive response is considered as one with at least 80% pain relief of at least 2 hours duration when lidocaine is used, and at least 3 hours or longer when bupivacaine was used. Lumbar medial branch blocks are not intended to provide therapeutic benefit. Patients with positive response to lumbar medial branch blocks are offered radiofrequency denervation treatment for a more sustained response.

Injections are generally avoided in patients with systemic infection or skin infection over puncture site, bleeding disorders or coagulopathy, allergy to local anaesthetics or any of the medications to be administered.

The procedure is usually done on an outpatient basis. The procedure is performed under fluoroscopic guidance to ensure accuracy of needle placement. This involves injection of local anaesthetic around the medial branches innervating the relevant facet joints. Patients who experience good pain relief following diagnostic injections are offered radiofrequency denervation treatment.

Complications are rare, particularly if injections are performed using a precise needle-positioning technique. Severe allergic reactions to local anaesthetics are uncommon. Post-procedural pain flare-up is also uncommon as steroids are not used for these diagnostic injections. Neurological complications including paraesthesias, numbness and paralysis have been described but are rare. Infections including epidural abscess and chemical meningitis can occur but the incidence is very low as the procedure is performed under strict aseptic conditions.