Outcome and predictors of knee radiofrequency ablation for chronic pain relief: A retrospective cohort study

INTRODUCTION

Knee osteoarthritis is the leading cause of chronic knee pain, with a global prevalence of 22.9% in individuals aged >40 years.[1] The majority of patients may not find satisfactory pain relief with conservative options.[2] Radiofrequency ablation (RFA) of genicular nerves is considered an alternative option, providing a longer duration of relief with low risk.[3] Most studies have targeted three nerves – the superior medial genicular nerve (SMGN), inferomedial genicular nerve (IMGN), and superior lateral genicular nerve (SLGN) – with few studies suggesting higher effectiveness with the inclusion of additional nerves.[3] The primary objectives were to assess the rate and factors associated with successful pain relief after genicular nerve RFA. We also evaluated the rate of false-positive genicular nerve blockade (GNB) and the rate of patients with discordant prolonged pain relief with GNB.

METHODS

This retrospective study was approved by the Hamilton Integrated Research Ethics Board (#2024-17799-C, 21 October 2024). The study was conducted in accordance with the principles of the Declaration of Helsinki (2013) and the Good Clinical Practice guidelines. Being retrospective, the need for written informed consent was waived. Patients who underwent RFA by a single physician from 1 July 2022 to 28 June 2024 were included. Patients were excluded if they had surgically correctable reasons for knee pain (such as meniscal injuries), ongoing infection, bleeding disorders, or severe anxiety about the procedures. After procedural consent, a prognostic GNB was performed under ultrasound guidance without any sedation using a 25-G hypodermic needle, with 0.5–1 mL of 0.5% bupivacaine administered at each site. Outcome of the GNB was discussed and recorded during a telephone follow-up in the next 1–2 weeks. Patients were categorised as positive (>50% relief lasting 10 h or less), negative (<50% relief), and positive discordant prolonged (>50% relief lasting beyond the day of blockade). Patients with positive relief underwent genicular nerve RFA in the operating room under fluoroscopy using 18-G RF needles. We identified three targets for RFA treatment: SMGN, IMGN, and SLGN. A fourth target for the inferior lateral genicular nerve was added in patients with pain involving the inferolateral area [Figure 1].[4] Outcomes of RFA (including RFA success defined as >50% pain relief between 2–4 months) were noted during a follow-up, arranged within the next 2–4 months.

Figure 1.

Radiofrequency needles placed for ablation of knee genicular nerves. (a) Targeting the superior medial and superior lateral genicular nerves; (b) Targeting the inferomedial and inferolateral genicular nerves. ILGN = Inferolateral genicular nerves; IMGN = Inferomedial genicular nerves; SLGN = Superior lateral genicular nerves; SMGN = Superior medial genicular nerves

Study data were extracted from the hospital electronic medical record system. This included demographic details, including age, gender, obesity, details of chronic knee pain (including side, location, severity of knee osteoarthritis using knee radiographs in the past 12 months), history of previous surgeries, response to oral analgesics (> or <50% pain relief), response to intraarticular steroid (> or <50% pain relief), presence of other chronic pain conditions, and history of opioid use. Extracted data were compiled in a REDCap study database for safe storage and analysis.

We hypothesised that obesity, history of previous surgery, and negative or discordant prolonged response to GNB would predict failures. The sample size was estimated based on the number of predictor variables. With an expected success rate of 60%, 75 genicular RFA procedures would result in approximately 30 failed procedures, allowing for the testing of four variables in a multivariable regression model.[5] All statistical analyses were performed in SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). Patient characteristics, baseline information, and the outcomes of interest were summarised using descriptive measures. Study outcomes were reported as rates (%) with 95% confidence intervals (CIs), and statistical significance was considered using a two-sided test with P < 0.05. Factors affecting success were explored using univariate regression analysis, and if P < 0.1, considered for multivariable regression.

RESULTS

A total of 73 knee joints, involving 66 patients (seven patients had both knees treated separately during the study period), were included. Baseline variables, characteristics of chronic knee pain, analgesic use, and previous injections are noted in Table 1. Among 73 knee joints, 13 had RFA of IFGN in addition to SMGN, IMGN, and SLGN. The majority (50%) had medial compartment pain, with 90% having Grade 3 or 4 osteoarthritis. Only a minority reported meaningful response with oral analgesics or previous injections of intra-articular steroid or viscous supplementation. Study outcomes are summarised in Table 2. Among patients with positive GNB, a majority had a prolonged response lasting from more than a day to nearly a month. Among the predictor variables, except for severe pre-procedure knee pain (≥8/10 on a numeric rating scale) and a history of previous total knee replacement, none crossed the threshold of P < 0.1; hence, an adjusted regression analysis was not carried out. We did not observe any serious complications among our patients.

Table 1.

Patient demographics and knee pain characteristics

Patient demographics (n=66)
Age, years (mean, SD)	66 (11.2)
Male (n, %)	21 (32)
BMI, kg/m2 (mean, SD)	33 (6.7)
Obesity (n, %)*	n=65
Normal weight (BMI: 18-24.9)	5 (7.7)
Overweight (BMI: 25-29.9)	15 (23.1)
Mild obesity (BMI: 30-34.9)	21 (32.3)
Moderate obesity (BMI: 35-39.9)	9 (13.8)
Severe obesity (BMI: ≥40)	15 (23.1)
Side of knee pain (n, %)	15 (23.1)
Right side only	24 (36.4)
Left side only	22 (33.3)
Bilateral	20 (30.3)
Other chronic pain needing treatment (n, %)†	24 (37.5)
Pain location (as % of patients with chronic pain)
Head and neck (n, %)	4 (16.7)
Back (n, %)	12 (50)
Upper limb (n, %)	6 (25)
Lower limb (n, %)	11 (45.8)
Pelvis (n, %)	3 (12.5)
Characteristics of chronic knee pain (n=73 knees)
Pain intensity (mean, SD)
At rest	3.44 (2.87)
With movement	8.03 (1.62)
Predominant location of knee pain (n, %)
Medial	57 (78.1)
Lateral	30 (41.1)
Superior	14 (19.2)
Inferior	13 (17.8)
Osteoarthritis grade (n, %)‡
Grade 2	6 (9.7)
Grade 3	32 (51.6)
Grade 4	24 (38.7)
Previous knee surgery (n, %)	24 (36%)
Total knee arthroplasty	10 (13.7)
Other	14 (19.2)
Analgesia with a history of oral analgesics and previous injections
Patients regularly using opioids (n, %)†	9 (12.7)
Response to oral analgesics (>50% relief) (n, %)	5 (7.5)
Patients with a history of intra-articular steroid injection in 73 joints Response to previous intra-articular steroid injection	62 (84.9)
Relief >50% relief	19 (30.6)
Duration >2 months	13 (23.6)
Patients with a history of intra-articular viscous injection in 73 joints Response to previous intra-articular viscous injection	22 (30.1)
Relief >50% relief	6 (27.3)
Duration >2 months	2 (10)

Data expressed as mean (standard deviation) or numbers (percentages). Missing data: *n=8, ^†n=2, ^‡n=11. BMI=body mass index; SD=standard deviation

Table 2.

Study outcomes including potential predictors of knee radiofrequency ablation

Primary and secondary outcomes of success
Outcomes	(n, %) n=73	95% CI
Successful pain relief with RFA (>50% relief between 2–4 months)	43 (64.2)	53%-76%
False-positive genicular block	22 (33.8)	22%-45%
Discordant pain relief with genicular block	38 (56.7)	45%-69%
Duration of block in patients with discordant pain relief with genicular block
>1 day-1 month*	36 (94.7)
>1 month	2 (5.3)
Univariate regression analysis of predictors
Predictor variables	OR (95% CI)	P
Age (for every 5 years increase)	0.90 (0.73–1.40)	0.43
Sex	0.93 (0.33–2.68)	0.90
Obesity (normal-mild-overweight vs moderate-severe)	0.69 (0.23–2.09)	0.51
History of chronic pain in other areas	0.75 (0.26–2.14)	0.59
Pre-procedure pain severity (≥8/10 vs <8/10 on NRS)	0.32 (0.11–0.96)	0.04
Discordant pain relief with genicular block	1.40 (0.47–4.21)	0.38
Marginal pain relief with genicular block (<80% vs ≥80%)	1.39 (0.46–4.21)	0.57
OA grade (Grade 2 or 3 vs Grade 4)	1.14 (0.39–3.35)	0.82
Response to steroid injection (50% relief or less)	1.44 (0.46–4.5)	0.53
Previous total knee replacement	6.09 (0.72–51.37)	0.09
Previous knee surgery (any surgery)	2.25 (0.70–7.20)	0.17

Data expressed as mean (standard deviation) or numbers (percentages). *95% CI was not estimated, considering only two patients had >1 month. CI=confidence interval; NRS=numerical rating scale; OA=osteoarthritis; OR=odds ratio; RFA=radiofrequency ablation

DISCUSSION

Our cohort study of genicular nerve RFA showed a success rate of 64% among older patients with chronic knee pain due to moderate-to-severe osteoarthritis. GNB was false positive in 34% and discordantly prolonged in 57%. In univariate analysis, only severe pre-procedure pain was associated with lower odds of treatment success.

Our results are comparable to those of Choi et al.,[6] as well as recent reports from real-world settings.[7,8] Although obesity, disease severity, and discordant or marginal pain relief with GNB may be associated with failure, we did not observe any such association. Very few studies have evaluated important prognostic factors. Chen et al.[8] observed increased chances of success (>30% relief) with obesity, not being depressed or on opioids, multiple lesions per nerve, and use of cooled RFA. Patients with >80% relief with prognostic blocks also showed increased success with their unadjusted analysis. Caragea et al. noted that only the severity of osteoarthritis predicted success as defined by >50% pain relief.[7] A recent review noted varying success rates based on limited studies.[9]

Emerging evidence seems to indicate that the prognostic value of GNB is limited, and it is best to select patients with severe osteoarthritis and no pre-existing anxiety and depression. As targets, one should consider additional nerves based on pain distribution, in addition to the conventional three nerves. Our study is limited by the available data routinely collected in patients’ charts, apart from potential biases inherent to the study design. We suggest longer follow-up, more objective measures of success, and evaluation of psychosocial factors as predictors of treatment success for future studies.

CONCLUSION

Radiofrequency ablation of the genicular nerves can lead to successful relief of chronic knee pain in two-thirds of patients.

Study data availability

De-identified data may be requested, with reasonable justification, from the authors (email to the corresponding author) and shall be shared upon request.

Authors contributions

HS: Study concept, study methods, writing of manuscript, overall supervision and study integrity

MK: Study concept, protocol drafting, study analysis, manuscript writing

DR: Study methods, data extraction, study manuscript

AD: Study methods, data extraction, study manuscript.

Disclosure of use of artificial intelligence (AI)-assistive or generative tools

The authors confirm that no AI tools or language models (LLMs) were used in the writing or editing of the manuscript.

Declaration of use of permitted tools

The scales, scores, figures, and tables are not copyrighted.

Presentation at conferences/CMEs and abstract publication

Nil.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.