Diagnostic Genicular Nerve Block Prior to Radiofrequency Ablation for Knee Osteoarthritis Pain

Knee osteoarthritis (OA) is one of the most common conditions and causes of disability in older adults, with an estimated prevalence of symptoms in 20-30% of individuals over 65 years of age. Pain associated with knee OA may have a central nervous system component, but intra-articular chemical pain mediators such as Substance P and Calcitonin-gene related peptide, possible intra-articular ischemic pain due to vasospasm, mechanical compression or irritation of the richly innervated subchondral bone, periosteum, synovium, and joint capsule, as well as peripheral genicular nerve sensitization have largely been implicated. Knee OA, in general, is treated conservatively with weight loss (when indicated), physical therapy, oral analgesic medications, and intra-articular corticosteroid or hyaluronic acid injections. If this approach fails to provide adequate pain relief and functional restoration, patients may be offered total knee arthroplasty (TKA), if they are surgical candidates. While, TKA is generally a safe procedure, like any major open surgery, it is associated with a risk of serious complications; a large cohort study of 83,756 patients demonstrated the annual incidence of venous thromboembolism (0.6%), myocardial infarction (0.5%), stroke (0.5%), and a 90 day mortality (0.7%) to all be significantly higher than the general population. Furthermore, some patients are not candidates for TKA due to co-morbidities such as morbid obesity or cardiopulmonary disease. Radiofrequency ablation (RFA) for knee joint denervation represents a promising intervention for patients with chronic painful knee osteoarthritis who have failed conservative management and are either not willing or not eligible for TKA. During RFA a thermal lesion is created by applying radiofrequency energy through an electrode placed at a target structure. RFA has been used to disrupt sensory afferent nociceptive nerve fibers supplying the zygapophyseal (facet) joints of the spine as well as the sacroiliac joint with excellent pain reduction and functional improvement when performed appropriately. To date, there are only two published studies that have investigated knee joint denervation by RFA for the treatment of chronic painful OA. In a randomized prospective study (n=35), Choi et al. found that genicular nerve RFA resulted in clinically meaningful pain improvement in approximately 60% of patients at 6 month follow-up. Improvement in function was also observed. Bellini and Barbieri report a series of 3 patients with chronic painful knee OA, two of whom who experienced meaningful pain and functional improvement after genicular nerve RFA. There is no evidence-based algorithm established which provides a means of properly selecting which patients would benefit from genicular nerve RFA. In the study performed by Choi et al., only one set of diagnostic genicular nerve blocks with a threshold of 50% pain relief to be considered a "positive response" was used. In contrast, a well developed literature regarding selection of patients for medial branch RFA for facet-mediated pain indicates that two sets of diagnostic medial branch nerve blocks with a threshold of \>80% pain reduction to be considered a "positive response" increased positive predictive value for both meaningful pain and functional outcomes to 80-90%. While the diagnosis of facet-mediated pain can be challenging given the variety of potential pain generators in the spine, diagnostic uncertainly is far less likely when evaluating the knee as a pain generator. In fact, it is unclear that any diagnostic blocks of the genicular nerves are needed given that this increase the potential for complications and healthcare costs compared to proceeding directly to the ablation procedure. It is vital that evidence-based selection criteria are developed for genicular nerve RFA in order to optimize clinical outcomes and minimize healthcare costs.