Introduction
Osteoarthritis (OA) of the knee joint is one of the leading causes of chronic disability, and conservative approaches to treatment have been shown to have limited long-term efficacy. , While total knee arthroplasty (TKA) represents the final and definitive option in the treatment algorithm for advanced knee OA, up to 44% of patients report some persistent pain after TKA with almost 20% reporting dissatisfaction with their TKA. , In addition, many patients that may otherwise benefit from a TKA are either poor surgical candidates due to medical comorbidities or prefer to avoid surgery.
Radiofrequency ablation (RFA) of the genicular nerves has recently emerged as another treatment option for chronic knee pain. The aim is to block pain signal transmission from the knee with targeted electrodes that generate thermal disruption of the sensory nerves supplying the knee. The procedure has been conceived as a minimally invasive treatment option that can bridge the gap between more conservative treatments (weight loss, physical therapy, oral medications, and intraarticular injections) on one end and TKA on the other. It has also been used for the treatment of persistent postsurgical pain. As such, genicular nerve RFA provides a useful alternative option within the treatment algorithm for chronic knee pain
Knee anatomy
The sensory innervation of the knee joint is complex and comprises various contributions from the femoral, tibial, common peroneal, saphenous, and obturator nerves. , , , These contributions to the knee are referred to as genicular nerves.
A literature review on the innervation of the knee joint found a lack of consensus in anatomic studies regarding the precise pattern of innervation of the knee. The review identified 12 to 13 separate genicular nerves, noting the inconsistency of the reported origins and sensory distribution of these nerves. Nonetheless, radiofrequency ablation of the knee joint has commonly targeted three specific nerves including the superior lateral (SL), superior medial (SM), and inferior medial (IM) genicular nerves. These were initially suggested as suitable targets based on allegedly predictable anatomic location and ease of access using percutaneous fluoroscopic technique. The inferolateral (IL) genicular nerve has been avoided due to its close proximity to the common peroneal nerve and associated risk of motor injury. , To help conceptualize the pattern of innervation, the knee joint capsule can be further subdivided into four quadrants: superomedial, superolateral, inferomedial, and inferolateral. ,
The SM genicular nerve innervates the superomedial quadrant. The SM genicular nerve has been initially described as originating from the tibial nerve after it divides from the sciatic nerve in the popliteal fossa. , Reported variations in origin include the posterior branch of the obturator nerve, the femoral nerve, and the nerve to vastus medialis (NVM), which is a branch of the femoral nerve. , Regardless of origin, the nerve has been reliably found in cadaveric studies to course by the periosteum near the inflexion of the femoral shaft and medial epicondyle in the anteroposterior view ( Fig. 10.1 ), and about the midway point of the femur in the lateral view ( Fig. 10.2 ), though some studies have placed it a bit more posterior in the lateral view. ,
The SL genicular nerve innervates the superolateral quadrant and originates from the common peroneal nerve after it bifurcates from the sciatic nerve, though some studies have found it originating from the sciatic nerve immediately prior to the sciatic nerve bifurcation into the common peroneal and tibial nerves. , Similar to the SM genicular nerve, the SL genicular nerve has been reliably found near the inflexion of the femoral shaft and lateral epicondyle in the anteroposterior view ( Fig. 10.1 ), and about the midway point of the femur in the lateral view ( Fig. 10.2 ), though some studies have placed she SL genicular nerve a bit more posterior in the lateral view. ,
The IM genicular nerve innervates the inferomedial quadrant. , It has been found to variably originate from either the tibial nerve or from the posterior articular nerve branch of the sciatic nerve. , It is reliably located near the inflexion of the tibial shaft and medial tibial epicondyle ( Fig. 10.1 ), at the midway point down the shaft in the lateral view ( Fig. 10.2 ). ,
Though less common, some RFA and nerve block techniques have targeted the infrapatellar branch of the saphenous nerve (IPBSN), which, in addition to the IM genicular nerve, innervates the inferomedial quadrant. , , The saphenous nerve is a purely sensory branch of the femoral nerve and gives rise to the IPBSN in the adductor canal of the medial thigh. , The IPBSN courses anteroinferiorly around the medial knee ( Fig. 10.2 ), giving off one to three small branches between the patella and tibial tuberosity that innervate the inferomedial quadrant of the knee joint. ,
The inferolateral quadrant is innervated by the IL genicular and the recurrent peroneal nerves. As noted previously, due to their close proximity to the common peroneal nerve, these have been avoided as targets for genicular RFA.
Diagnosis (patient selection)
Genicular nerve RFA has primarily been studied in the context of knee OA or persistent postsurgical pain. , , , The source of chronic knee pain should be identified and other causes of pain such as ligamentous or meniscal injuries addressed appropriately. Clinically, patients with knee OA may present with varying degrees of pain, joint effusion, crepitus, and decreased range of motion. Radiographically, knee OA is commonly categorized according to the Kellgren-Lawrence scale (from 0 to 4) with evidence of joint space narrowing, subchondral cysts and sclerosis, marginal osteophytes, and bone deformities of the femoral condyles and tibial plateau.
Generally, indications for genicular nerve RFA include patients with symptomatic knee OA, patients who are considered poor surgical candidates due to age or medical comorbidities, patients who prefer to avoid surgery, patients who have had a TKA but continue to experience pain, or patients who have had a successful genicular nerve RFA (GNRFA) in the past with recurrence of pain. , The specific severity of knee OA that is most appropriate for a GNRFA has not been established. The procedure is typically done for moderate to severe OA (Kellgren-Lawrence grade 3 to 4), though some studies have also included mild OA (Kellgren-Lawrence grade 2). , While genicular nerve RFA is a minimally invasive procedure, it carries inherent risks and is typically considered after more conservative treatment options have been employed without sufficient pain relief. A prospective candidate for the procedure generally undergoes one or two diagnostic genicular nerve blocks to help determine potential success of an RFA, with a minimum 50% reduction in pain for 24 h used as a standard cutoff. Though the prognostic value of these genicular nerve blocks has been questioned, they remain in use for candidate selection.
There are several important contraindications to a GNRFA that should be considered in the selection process. An active knee infection or soft tissue infection around the knee joint is generally considered an absolute contraindication. Acute knee injuries or an unstable knee joint should be addressed and treated first regardless of any superimposed chronic knee pain. For patients with cardiac devices, nerve stimulators, or other implantable devices, the manufacturers should be consulted regarding operational safety in the context of an RFA. Uncontrolled diabetes mellitus has been associated with poor surgical outcomes, and elective procedures are generally avoided for patients with a hemoglobin A1c >8. Active pregnancy is commonly avoided. Other relative contraindications that should be considered and discussed but are not absolutely limiting include bleeding disorders or use of antithrombotic agents, complicating psychological factors, and chronic pain syndromes.
Diagnostic blocks
Diagnostic genicular nerve blocks are routinely administered as an initial step in candidate selection to determine the potential efficacy of a future nerve RFA. These consist of administering a local anesthetic, either alone or in combination with a steroid, around the site of the genicular nerves, and monitoring for a reduction in pain. A positive diagnostic block is generally regarded as a 50% reduction or more in pain scores for 24 h. The diagnostic blocks are performed under either fluoroscopic or ultrasound guidance. The comparative effectiveness of each technique has not been well studied, with one randomized comparison study finding no differences in outcomes. In addition, the predictive value of a diagnostic block for a successful genicular nerve RFA remains unclear, with one study finding no improvement in the RFA success rate following a diagnostic genicular nerve block.
As discussed previously, the standard targets for genicular nerve blocks are the SM, IM, and SL genicular nerves, while the IL genicular nerve is avoided due to a potential for motor block. , , The fluoroscopic technique takes advantage of the reliable anatomic location of these genicular nerves as they traverse the periosteum near the bilateral epicondyles of the femur and the medial epicondyle of the tibia. From an anterior view of the knee joint, the nerves are generally found near the inflexion point of the shaft as it meets the epicondyles ( Fig. 10.3 ). From a lateral view, they are generally targeted at the midway point between the anterior and posterior edges of the long bones. Based on more recent cadaveric studies, some investigators have recommended a more posterior placement on the lateral view for targeting the SM and SL nerves ( Fig. 10.4 ). The most precise placement remains an area of active study.
The patient is positioned supine, and the target knee is positioned in a slight flexion of about 30 degrees by placing a soft pillow or other device beneath. The knee is prepped in a sterile fashion using chloride- or iodine-based solution and then draped. Anteroposterior images of the knee are obtained, and the sites of initial needle placement are identified ( Fig. 10.5 ). Local anesthetic is administered subcutaneously to form a skin wheal at these sites for patient comfort during the procedure. Depending on the body habitus, 1.5- to 3-inch needles are then advanced at the sites using anteroposterior imaging for accurate placement until the needle tips are in the proper position at this view. 3.5-inch spinal needles may be used if desired. Lateral images are then obtained to ensure proper depth of the needles ( Fig. 10.6 ). Contrast may be used to confirm placement and ensure there is no vascular spread, though observing for no blood on aspiration is also acceptable. The anesthetic is injected once placement is confirmed. The choice of local anesthetic and the optimal volume of injectate have not been studied. Shorter- and longer-acting agents, such as lidocaine and bupivacaine respectively, are often used alone or in combination with a steroid, while volumes between 0.5 cc and 2 cc of injectate at each site have been used in various studies. , , , , All are presently considered acceptable methods.
