Introduction

Medial branch blocks (MBB) and radiofrequency ablation (RFA) are two techniques utilized in interventional pain management to diagnose and treat low back pain (LBP) arising from the spinal facet joints. Millions of these procedures are performed in the United States every year, with a reported increase in utilization of 9.7% annually. Medial branch blocks involve injection of local anesthetic near the medial branch nerve, which serves to innervate the respective facet joint. The medial branch block procedure is considered the prognostic screening test of choice to be used before denervation therapy. If a patient responds with an adequate reduction in their pain for the duration of the local anesthetic used in the MBB, then the patient will more likely experience a positive response to the radiofrequency ablation procedure.

There has been significant controversy and debate on many aspects of the subject from incidence of facet joint pain to criteria for diagnosis, patient selection for intervention, and efficacy of treatment. A recent publication by Cohen et al. has established consensus practice guidelines for interventions on lumbar facet joint pain.

Diagnosis

Facetogenic low back pain is difficult to diagnose reliably. Symptomatology, physical exam findings, and response to a medial branch block injection are all factors to determine zygapophysial joint pain. There are no characteristic historical signs or physical examination findings that can predict a positive response of low back pain to medial branch blocks. However, there is a weak association to pain located off midline or tenderness overlying the facet joints to a positive response to intervention. There is also some data to suggest that signs indicating radicular pain (radiating pain extending distal to the knee, worsening pain with cough) may have a negative predictive value for diagnostic blocks.

Standard radiological findings such as plain radiography, computed tomography (CT), and magnetic resonance imaging (MRI) have been found to have weak or no evidence supporting their ability to identify painful lumbar facet joints that will respond to MBB. There is, however, moderate evidence to support the use of a nuclear imaging modality known as single photon emission computed tomography, or SPECT, to identify painful lumbar facets prior to MBB. SPECT involves intravenous injection of a gamma radioisotope, which is then detected by external sensors. Two-dimensional images are acquired in various planes to reconstruct a 3D image displaying a positive signal where biological activity is high, such as in joints with inflammation. This can potentially localize pain to certain facet joints. Scintigraphy is a similar process but only generates two-dimensional imaging and has weak evidence supporting its use.

Medial branch block injections are often referred to as diagnostic, meaning they would be able to identify the specific condition of facetogenic pain. However, they should be more accurately referred to as prognostic, meaning they are more useful in predicting response to treatment. The rationale for this procedure is that if a temporary blockade of the nerve produces a reduction in pain, then a longer interruption in nerve conduction in the form of ablation will provide the same benefit for a longer period.

For the MBB injection to truly be diagnostic, the clinician would need to be certain the anesthetic was only affecting the nerves innervating the facet joint, but this is impossible due to close proximity of additional dorsal rami branches and possible aberrant innervation to the facet joint by these branches. Furthermore, the medial branch nerve itself supplies structures such as the multifidus muscle, interspinal muscle, and the periosteum and ligament of the neural arch. Therefore, blockade of the medial branch cannot be isolated to sole reduction of pain through loss of sensation to the facet joint. As a prognostic indicator, the MBB injection has some similar limitations, such as aberrant innervation to the joint, but can provide useful predictive information about the potential efficacy of RFA. The current recommendation is that only one prognostic block with at least 50% pain reduction is necessary to consider RFA. However, most insurance companies still require two MBB procedures with positive responses before they will cover the RFA procedure.

Contraindications to denervation with RFA can include conditions such as inflammatory facetogenic pain in young, athletic patients in whom denervation of the multifidus and other spinal muscles may result in muscle atrophy and an impact on quality of life. In these patients, MBB can still act as a prognostic tool, and intraarticular injections of corticosteroids may have therapeutic value.

Intraprocedure complications and management

Intraprocedure complications from MBB and RFA include bleeding, infection, allergic reaction, and nerve damage, among others. Vascular penetration and injury may be unavoidable, but can be mitigated by aspiration through the needle when it is at its final placement and by understanding the vascular anatomy. Neuritis is a potential complication after ablation. It is thought that postprocedure steroid injection may prevent development of neuritis after radiofrequency ablation. Improper needle trajectory and needle placement may result in spinal nerve root injury, paraspinal muscle atrophy, disc degeneration, and vascular injury. Obtaining good fluoroscopy views will aid in proper needle placement and avoid these complications. It is recommended to utilize fluoroscopic views that obtain true anteroposterior, ipsilateral oblique (‘Scotty-dog’), and true lateral views during placement of MBB needles and RFA cannulas. This fluoroscopic technique is designed to ensure that the needle tips are outside of the neural foramina, minimizing the risk of spinal nerve root injury. In addition, the clinician should ensure the absence of radicular symptoms of sensorimotor responses to test stimulation prior to RFA. Sensorimotor testing also reduces the probability of injury to the spinal cord and spinal nerve roots.

Prior to consideration for prognostic facet blocks, current guidelines recommend a period of 3 months of conservative treatment, including, but not limited to, nutrition improvement, exercise, treatment with medication (NSAIDs, antidepressants), physical therapy (PT), chiropractic care, or spinal manipulation if indicated. There is consistent evidence at this time of only small effect sizes with conservative care, including PT, but due to the relatively low risk and cost of such treatments, the recommendation remains to begin with conservative care. For this reason, insurance providers are increasingly mandating a trial of conservative management before interventional facet procedures will be approved and covered. ^,

Implantable devices such as deep brain/spinal stimulators, pacemakers, cardioverter defibrillators, and adjacent spinal instrumentation can be affected by the RFA probe. Consensus guidelines recommend consulting with the relevant healthcare teams (e.g., cardiology, pain management, neurology) involved in the management of these devices for their input. Some standard procedures to mitigate these risks involve turning off neurostimulators, placing pacemakers in asynchronous mode, and using grounding pads placed away from the devices in question.

Tissue burns are an inherent risk of RFA. Tissue burns are most likely to result from equipment malfunction, improper grounding pad placement, or extension of the lesioning site. Extension of the lesioning site occurs when there is a lack of tissue between the lesion site and either the skin or paraspinal muscles, and usually occurs with larger lesions. Inadvertent lesion extension into the paraspinal muscles may present as increased postprocedure-related pain. Methods to minimize these risks include using a large electrical dispersive pad composed of conductive metal and adhesive polymer gel and positioning the grounding pad’s longest side to face the RFA probe, assuring the equipment is functioning properly, and placing grounding pads appropriately. Grounding pads should also be placed on dry, shaven areas devoid of scars or tattoos on the lower extremities. ^,

Multiplanar fluoroscopic views should be used to ensure that the RFA probe is not in contact with pedicle screws if present to avoid thermal injury to surrounding tissues. Anticoagulation should be continued for MBB and RFA in general. If a case warrants discontinuation for some reason it should be discussed with the patients’ managing provider. ^,

General considerations