Radiofrequency ablation of the hip: review

Introduction

Chronic hip pain is a common condition with a wide variation of etiologies: osteoarthritis (OA), rheumatoid arthritis, osteonecrosis, infection, avascular necrosis, labral pathologies, metastasis, peripheral neuropathy, and post-total hip arthroplasty (THA) pain (1-5). Femoroacetabular OA (commonly referred to as hip OA) is by far the most common hip pain etiology (4), and thus is the focus of much of the data in this review. The prevalence of hip OA is between 3–10% of the Western population (2,6). Body mass index (BMI) greater than 30 kg/m², older age, large acetabular/femoral head bone marrow lesions, chronic widespread pain, depression, and female sex increase the risk of developing hip pain (6-9). Management includes conservative treatments (weight loss, physical therapy, exercise programs, use of assist devices, topical medications, and oral pharmacotherapy), interventional procedures [trigger point injections, intra-articular injections, radiofrequency ablation (RFA), visco-supplementation, and regenerative therapies], and/or surgery including THA (3,4,10-17). THA is associated with significant costs and morbidity, including revision surgery and persistent post-surgical pain (12,18). The rate of a THA requiring revision is approximately 20% (11), and 11–36% of patients report persistent post-surgical pain (19-21).

RFA of the articular branches of the femoral and obturator nerves is an emerging therapy in the treatment of chronic hip pain (4,12,13) (Figure 1). Hip RFA is a palliative, alternative, or supplementary intervention by nature, because both THA and hip RFA are effective in treating hip pain. In a review of nine studies, Cheney et al. quoted a 30–80% reduction in pain scores with hip RFA (4). In this review we will discuss the pathophysiology of hip pain, its etiologies, its clinical presentation, the role of imaging in diagnosis/planning of treatment, conservative management, the role of surgery, the anatomy/technique of hip RFA, hip RFA efficacy, and RFA adverse events. It is important to highlight the role of hip RFA in treating patients with chronic hip pain, for those who do find relief from conservative management and for those where surgical management is unwise or undesirable.

Figure 1 This image shows the path of the articular branches of the femoral, accessory obturator, and obturator nerves as described in Short et al.’s cadaver study of the innervation of anterior hip capsule (22). Note the articular branches of the femoral nerve are between the AIIS and iliopubic eminence. The articular branches of the accessory obturator nerve course over the iliopubic eminence more medially to the femoral branches. And the obturator nerve exits the obturator canal (formed by the obturator membrane and the bones of the pelvis) and along the inferomedial acetabulum (described as a “tear drop” radiographic landmark) (22,23). In practice, the accessory obturator nerve is often targeted during ablation of the articular branches of the obturator nerve. Created with BioRender.com.

Pathophysiology and clinical presentation

Hip OA is the most common etiology of chronic hip pain (4) and thus a large focus of conservative, ablative, and surgical treatment. Pain from hip OA is the result of complex pathophysiology; it is defined by the Osteoarthritis Research Society International as first showing molecular derangement (abnormal joint tissue metabolism) (11,24,25), followed by anatomic and/or physiologic derangements (cartilage degrading, bone remodeling, osteophyte formation, and loss of normal joint function) (11,24,25), and finally illness (pain) (11,24,25).

Patients with hip joint OA typically present with ipsilateral groin pain (intermittent, worse during and after activity) as well as stiffness (typically is worse in the morning or after activity, typically resolves in <30 minutes) (3,19). On examination, pain is often elicited with internal rotation of the hip. An anteroposterior and lateral radiograph are typically obtained (11).

OA pain is associated with comorbidities secondary to a lack of physical activity, adverse effects of medications, and the effects of pro-inflammatory cytokines (11). Its role in stance as well as gait may explain the more rapid progression of symptoms in hip OA as compared to knee OA (6). Hip OA progresses more rapidly for those with greater hip pain at baseline, poorer functional status, or more severe joint damage at presentation (9). Central sensitization is associated with increased pain severity, lower functional status, and prolongation of pain course (26).

Pathology of the genitourinary tract, gastrointestinal tract, vascular pathology, anterolateral thigh, lumbosacral spine, and gluteus can be mis-classified as “hip pain” by patients (1,2). Hip pain is difficult to define topographically as it can be associated with both intra-articular and extra-articular structures such as ligaments, labrum, cartilage, synovium, bone, bursa, tendons, and nerves (11,27). Anterior hip pain is localized to the anteromedial thigh/inguinal region (2). It is most commonly related to OA of the hip joint, however a labrum tear of the acetabulum should be considered in young patients when X-ray imaging is non-diagnostic (2).

Lateral hip pain is commonly related to pathology of the iliotibial (IT) band or gluteus medius (2). Buttock pain is commonly referred from the sacroiliac joint (SIJ) or lumbar spine, but can also be associated with femoroacetabular joint disease (2). Lumbosacral disease is a common confounding pathology; excluding SIJ pathology is achieved through physical exam maneuvers such as the thigh thrust test (anteroposterior shear force through the femur), sacral thrust test (anterior shear force directly on the SIJ), pelvic distraction or compression, and Gaenslen’s test (2). When three or more of these tests are positive, there is a combined 91% sensitivity and 78% specificity for SIJ mediated pain (positive and negative likelihood ratios of 4.12 and 0.12, respectively) (2). Other etiologies to consider for each of the hip pain presentations are listed in Table 1.

Imaging for diagnosis and procedural planning

Classic radiographic features of hip OA include narrowing joint space secondary to loss of articular cartilage and labral tears, as well as osseous reactive changes which include sclerosis of subchondral bone and osteophytes (25). These changes can be seen in Figure 2.

Figure 2 This X-ray image highlights hip OA in one patient, especially on the left. Note the superior acetabular reactive hyperostosis and joint space narrowing. OA, osteoarthritis.

While useful for confirming degenerative changes consistent with OA, the degree of radiographic changes and degree of symptoms do not correlate (1-3,12,13,24,26). Kraus et al. conceptually separates OA the disease—defined as abnormal structure and function of the body, with OA the illness—referring to the human response to disease (24).

The Kellgren-Lawrence (K-L) classification of OA (from grade 0–4) is the most widely used grading scale in clinical practice (28,29). This classification uses osteophyte formation, joint space narrowing, subchondral sclerosis, and presence of bone-end deformity (28). Mild OA is classified as grade 2, moderate as grade 3, severe as grade 4 (28). Both Kohn et al. and Schiphof et al. highlight variations and disagreement in how the literature describes different K-L grades of OA (28,29). The most widely used classification for osteonecrosis of the hip is the Ficat classification, however it has criticism towards its reliability (0.39 interobserver reliability) (30).

Conservative treatments

Conservative measures, such as pharmacotherapy, exercise, and physical therapy, are offered prior to interventions. Non-steroidal anti-inflammatory drugs (NSAIDs) are first line (11,31), and provide a greater probability of efficacy than opiates (31). One meta-analysis reports acetaminophen has a significantly smaller effect size when treating hip OA as compared to NSAIDs or opiates (29). Another report corroborates these findings, and highlights the efficacy of duloxetine and the limited effect of gabapentin (11). Intra-articular corticosteroids are no more effective than placebo, and some studies suggest deleterious effects on cartilage (11).

Outside of pharmacology, structured exercise programs improve pain and functional status (11). One randomized trial found a 1.4 point decrease in pain scores after a walking program (11), and another study quoted an 8 point decrease on a 0 to 100 point pain scale (32). In addition to an improvement in pain, functional status improves as well (3,11,15).

The role of surgery

Some patients with chronic hip pain are refractory to conservative and traditional surgical treatments (19-21,33). These patients could benefit from other interventional options, namely hip RFA. It is important to have an understanding of surgical options so as to counsel this group. Hip pathologies such as OA, osteonecrosis and Paget’s disease have been successfully treated with THA (34-37). While THA is an effective treatment, adverse events may occur such as prosthetic failure or other complications (e.g., aseptic loosening, infection, periprosthetic fracture, and dislocation) requiring revision (17,37). Sixty-five percent of THAs are performed in patients ≥65 years of age (16,37,38). Complications have been demonstrated to increase with age, with reports suggesting a 40% increased risk for each decade after the age of 65 years (37). Longer wait times, greater comorbidities, and poor pre and postoperative function have been associated with poorer THA outcomes (37). Persistent pain after THA is not uncommon. Studies suggest an incidence of approximately 11–36% (19-21).

Indications for hip RFA

There are currently no published guidelines on patient selection for hip joint RFA, however it has been utilized for hip OA, labral tears, osteonecrosis, post THA, post-operative dislocation/fracture, and cancer (4,5,13,39-47). Possible candidates for hip RFA include patients who are not optimal surgical candidates due to medical comorbidities, those who refuse surgery, as a bridge to surgery, and those with persistent pain despite prior surgical procedures (4,12,23). Pivec et al. describes poorer outcomes with long wait times for THA, thus hip RFA could be a temporizing measure prior to surgery as well as an alternative pain control procedure (37).

Innervation of the hip joint

The anatomy of the hip joint is complex; the ball and socket articulation connects the pelvis and the femur, supports the upper body, and supports the stresses of gait (48). It has a vast neuromuscular network (48). The innervation of the hip includes three systems: afferent innervation from the joint capsule, intra-osseous innervation and cutaneous innervation (2,22,23,49,50).

Hip OA pain originates from the femoroacetabular joint capsule, which is innervated by articular nerves. The capsule receives innervation from the articular branches of the femoral, obturator accessory, obturator nerves, as well as from the nerve to the quadratus femoris, superior gluteal, and potentially the sciatic and inferior gluteal nerves (4,49). Short et al. studied the hip anatomy of 13 cadavers, found the anterior hip capsule receives innervation from the femoral and obturator nerves in all examples; the accessory obturator nerve contributed to the anterior capsule in 9 of 13 examples (22). Blood vessels often accompany the innervating nerves, which requires vigilance and/or an alternate approach during block/ablation (22,50-52).

Anterior joint innervation (supplied by the hip articular branch of the femoral nerve as well as the obturator nerve) is conceptualized with four quadrants: superolateral, inferolateral, superomedial, and inferomedial (4,22,23,49). High and low branches of the femoral nerve supply the majority of innervation to both lateral and the superomedial quadrants (22). High and low obturator nerve branches innervate the inferomedial quadrant (22). The accessory obturator nerve innervates both medial quadrants (22). Short et al. reported high branches (that were all found on the periosteal surface of the pubis between the anterior inferior iliac spine (AIIS) and the medial portion of the iliopubic eminence) play a greater role in capsular innervation than was discussed previously (22). Radiographic targets for the femoral nerve include the superolateral acetabular margin, the AIIS, or between the AIIS and iliopubic eminence (12,22,23). Both high and low branches of the obturator nerve were found near the inferomedial acetabulum, which has a characteristic “tear drop” on plain film and fluoroscopy in the anteroposterior view (22,23). The accessory obturator nerve is more medial than the femoral nerve, its landmark is the iliopubic eminence (22,23). Thus performing RFA of the femoral nerve, obturator nerve, and accessory obturator nerve utilizes the AIIS, iliopubic eminence and the inferomedial acetabulum. The radiographic targets of the femoral and obturator branches are highlighted in Figure 3.

Figure 3 The circular targets highlight ideal placement for ablation of the nerves innervating the anterior capsule of the hip joint. The image on the left shows placement of the needle to ablate the articular branches of the femoral nerve, in between the AIIS and the iliopubic eminence. The image on the right has two circles, highlighting the multiple burns required for the articular branches of the obturator nerve, as described by Locher et al. (50). In addition, these ablations often include the accessory obturator nerve, as seen in Figure 1. Reprinted with permission from Avanos Medical, Inc. (5404 Windward Pkwy, Alpharetta, GA 30004). AIIS, anterior inferior iliac spine.

The superior gluteal nerve, inferior gluteal nerve, quadratus femoris, and articular branches of the sciatic nerve have been suggested for treatment of posterior capsule hip pain (4,12,13,22).

Technique

The main targets for hip RFA include the femoral nerve, obturator nerve, and accessory obturator nerve (4,12,22,49), whose radiographic targets are described above. These nerves provide sensation to the anterior joint capsule (4,22,49). Posterior hip capsule ablation has been studied less rigorously, but can be ablated by targeting the quadratus femoris, superior gluteal, sciatic, and inferior gluteal nerves (13). Only one series discussed targeting these nerves (53). Locher et al. initially described a lateral approach to the articular branches of the obturator nerve, first finding the “teardrop” landmark for the obturator nerve and entering the skin at the posterior-lateral edge of that landmark (50). The femoral artery is palpated and a more medial skin puncture is chosen if the vessel is in the needle’s path (50). The fluoroscopy is adjusted to a 70° angle from the anteroposterior (AP) view, to be in parallel with the articular branches (50). The branches of the obturator nerve require multiple points of ablation, and thus the needle is adjusted 2–3 mm for a subsequent burn (50). A more recent description from Tinnirello et al. approaches the skin puncture by inserting the needle 2 cm lateral to the pulsation of the femoral artery (52), before advancing the needle to the femoral and obturator nerve’s respective landmark. The RFA of the patient in Figure 2 can be seen in Figure 4.

Figure 4 This fluoroscopy image shows a view of the RFA of the patient in Figures 2,3. The targets of this ablation are the articular branches of the femoral nerve (superior catheter) and the articular branches of the obturator nerve (inferior catheter). Note the articular branches of the obturator nerve often require multiple burns, as described by Locher et al. (50). RFA, radiofrequency ablation.

The use of ultrasound can allow for an anterior needle approach with visualization of the femoral artery and vein (23,51). Feigin and Peng describe accessing the femoral nerve by first locating the ASIS on ultrasound, scanning caudally to find the AIIS, and rotating the probe to see both the AIIS and the iliopubic eminence (23). For the obturator nerve, the ultrasound probe is first placed on the hip and adjusted to align the femoral neck, femoral head, and acetabulum (23). The probe is moved inferior and medial until the femoral head and neck disappear, thus the probe has located the inferomedial acetabulum (23). In contrast, Stone et al. described an anterior approach, guiding the needle between the femoral artery and vein with ultrasound guidance (51), with confirmation via sensory and motor stimulation. Traditional RFA (75–90° for 1–3 min) (4,12), pulsed RFA (42° for 2–5 min) (12,13), and “cooled” RFA (60° for 2.5–3 min) (12) have been described. In palliative cases (metastasis or inoperative fracture), alcohol and phenol can be used for chemical ablation, 1 mL of neurolytic agent is injected after local anesthetic (23).

Diagnostic block usage was mixed (4,13,23). Bhatia et al. challenged the idea of a “diagnostic block” and instead described the blocks as “prognostic”, because the femoral and obturator articular branches are likely only part of the nociceptive innervation of the joint (12). Prognostic blocks of the patient in Figure 2 can be seen in Figure 5. Volumes of long acting local anesthetic ranged from 0.5–3 mL of bupivacaine or 5–7 mL of ropivacaine, and only one report set a qualitative goal post of “>50% relief” for a successful test block (4). Some studies did not perform a test block and instead immediately performed RF ablation (4,12).

Figure 5 These pictures show ideal placement of a prognostic block of the high and low branches of the femoral nerve (left image), and the high and low branches of the obturator nerve (right image) in the same patient as Figure 2. Note the left image shows the catheter in between the AIIS and iliopubic eminence, at the superomedial acetabulum. The image on the right shows the catheter at the inferomedial acetabulum (colloquially known as the “tear drop” radiographic landmark). AIIS, anterior inferior iliac spine.

Efficacy

RFA was traditionally utilized to treat facet mediated pain (13,54). It has emerged as a treatment option for patients with joint-mediated pain, including the knee, SIJ, hip, and shoulder (54,55). Okada first described RFA of the innervating articular branches in 1981, and subsequently published a case series suggesting successful treatment of 15 patients targeting the femoral and obturator branches anteriorly and the sciatic posteriorly (53).

Since that time, several studies have reported improvement in pain following RFA for the hip joint. Both Cheney et al. and Bhatia et al. discussed these outcomes in narrative reviews of hip RFA (4,12), citing similar studies. Multiple studies (39-41,43-46) have reported 40% to >80% pain reduction in their cohorts (3 to 25 patients) at points of follow-up ≥6 months. In the same studies, 44–100% of patients had >50% relief and were classified as “responders” (39-41,43-46). The variation depends on the time the patient was surveyed, between 1- and 8-month post-procedure (39-41,43-45). Kapural et al. followed up their initial report with a large retrospective study of 136 patients; 178 ablations in 84 patients had 12 months of follow up (38% loss to follow up) (42). They report 69% of patients had >50% relief at 6 months, 52% had >50% relief at 12 months (42). Their average VAS pain score reduction (on a scale of 10) was 3.44±2.5 at 6 months and 4.23±2.5 at 12 months. They reported 82 repeat procedures on 36 patients, with similar (P=0.197 in regards to average VAS reduction) pain relief (42), which is contrary to previous case reports.

Decreased consumption of analgesics has been reported in several studies, however, this was subjective in nature (12). Two articles examined the use of pulsed RFA, finding improved analgesia and subjective decreases in the need for oral analgesic lasting 3–4 months (5,47). Functional improvement generally improved, but measurement had significant variability (4,12). Some studies used Western Ontario and McMaster Universities Arthritis Index (WOMAC), Harris Hip, and Oxford HipScore (OHS) (43,47); most studies used subjective assessment of functional improvement such as improved ambulation, sleep, and ability to perform activities of daily living (4,12).

While Farì et al. (55) are correct to call for randomized controlled trials to compare RFA to other methods to improve pain, it and the evidence in this review (39-41,43-46) undoubtedly support the use of RFA to improve musculoskeletal pain and function at multiple sites (hip, knee, shoulder, low back, SIJ).

Safety

The percutaneous nature of RFA narrows the list of contraindications, as compared to more invasive procedures. Patient refusal and local/systemic infection are the absolute contraindications (54). Due to significant neurovascular structures surrounding the RF targets, the risk of bleeding and coagulopathy/anticoagulant medication use must be considered. The American Society for Regional Anesthesia and Pain Management (ASRA) guidelines detail when common anticoagulant medications should be held before an elective procedure (56). Local malignancy, abnormal structural anatomy, and allergy to local anesthetic are other circumstances where the provider and patient must weigh the risks and benefits of pursuing RFA (54).

The most frequently reported adverse event for hip RFA is procedural pain from passage of the RF needle through muscle, ligaments, and the periosteal area despite IV analgesics (54). Additional minor complications to note include numbness, bruising, pruritus, erythema, and self-limiting paresthesias (most frequently noted in the distribution of the pudendal, LFCN, or sciatic nerves) (4,54,57-59). While bleeding events have been reported, such as subcutaneous hematoma formation (4,54), there are no documented cases of serious bleeding events or permanent damage from hematoma formation. In their comprehensive review on hip RFA, Cheney et al. detailed only one case where permanent paralysis of the quadriceps muscle occurred after lesioning of the femoral nerve (4). Despite concern over patients developing a Charcot-type joint pathology due to deafferentation of the hip joint, there have been no reports in the literature of neuropathic arthropathy occurring after joint RFA (60,61). The goal of hip RFA is partial deafferentiation, with sparing of motor nerve bundles, which prevents the repeated occurrences of microtrauma that lead to bone resorption and hypertrophy.

Outside of RFA of the hip, there have been cases detailing adverse outcomes from RFA of other targets: facet arthropathy, knee OA, atrial fibrillation, and thyroid nodules. Amongst these, complications include third degree burns (62-64) and grounding pad burns (65,66), from improper RF needle usage and incorrect grounding pad placement, respectively. While not directly targeting the joint space during the procedure, there have also been cases of infection seeding the knee joint after genicular nerve RF (67).

Conclusions

RFA of the hip has a role in caring for patients whose medical comorbidities, choice of intervention, long wait time for hip arthroplasty, or who have persistent pain after arthroplasty (4,12,23). It is an effective procedure to reduce a patient’s pain by 40 to >80% (5,39-47) for 6 months after the procedure. The authors believe hip RFA plays a valuable role in adjunct and neo-adjunct care for those with significant symptoms.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editor (Alaa Abd-Elsayed) for the series “Advances in Radiofrequency Ablation” published in Annals of Palliative Medicine. The article has undergone external peer review.

Peer Review File: Available at https://apm.amegroups.com/article/view/10.21037/apm-23-470/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://apm.amegroups.com/article/view/10.21037/apm-23-470/coif). The series “Advances in Radiofrequency Ablation” was commissioned by the editorial office without any funding or sponsorship. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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