Genicular Nerve Blocks: Field Tips on Prognostic Value and Technical Considerations

Radiofrequency ablation (RFA) of the genicular nerves is an increasingly common approach to managing chronic knee pain. In recent years, investigators have sought to identify genicular RFA protocols that may provide more significant and durable improvement in pain and function by targeting additional nerves and accounting for the variability of genicular nerve location between individuals.^1-3

Compared to a pivotal sham-controlled trial that targeted three genicular nerves,⁴it is apparent from both recent and historical dissection studies that several additional nerves innervate the anterior knee joint capsule.^5-8 Feasibility research suggests that many of these additional genicular nerves can be safety targeted by RFA.^1,9,10 

The increasing variation in protocols for genicular RFA raises important questions about the prognostic value, best practices, and healthcare cost burden of genicular nerve blocks (GNB) to aid in the appropriate selection of patients for genicular nerve RFA. Herein, the authors review the latest thinking and evidence behind each.

The increasing variation in protocols for genicular RFA raises important questions about the prognostic value, best practices, and healthcare cost burden of genicular nerve blocks (GNB) to aid in the appropriate selection of patients for genicular nerve RFA. (Image: istock)

Prognostic Value of Genicular Nerve Blocks

Although prognostic GNBs are commonly performed, evidence of their predictive value for genicular nerve RFA success is limited. Within the recent outcome literature, treatment success rates are similar between studies that used prognostic GNBs and those that did not.^2,3,11-14 Furthermore, a recent RCT demonstrated a lack of correlation between positive diagnostic blocks (when defined as ≥ 50% pain relief for the expected duration of the local anesthetic used) versus no GNB at all and treatment success associated with genicular RFA.¹⁵

While it is clear that commonly used protocols for GNBs do not likely provide significant prognostic value, further research may identify techniques that do. First, the prognostic value of GNBs might be improved by increasing the threshold for considering the block “positive.”

In a recent study, 42% of individuals who reported ≥ 50% reduction in knee pain after GNB responded to genicular nerve RFA (success was defined as ≥ 50% reduction in knee pain), whereas 60% of individuals reporting post-GNB pain reductions of ≥ 90% responded to genicular nerve RFA.¹⁵ Although further research is needed to more carefully establish the predictive validity of higher thresholds to consider a GNB “positive,” clinicians may consider increasing such thresholds when selecting patients for genicular RFA candidacy.

Injectate Volume

In addition to potential alterations in block thresholds for defining a “positive” prognostic GNB,  the effect of GNB injectate volume must be considered. Currently, recommendations for local anesthetic injectate volume during GNB are not standardized. Most commonly, investigators have reported use of 1 mL^2,3,15 of injectate volume per nerve target during GNBs, but others have used up to 2 mL⁴ of local anesthetic per target site.

By contrast, the observed volume of a typical genicular nerve RFA lesion is between 0.5 and 1 cubic centimeter, which is contained in an approximate sphere or three-dimensional ellipse.¹⁶A s a liquid, GNB injectate does not typically spread in the distribution of a sphere or three-dimensional ellipse, but rather, follows tissue planes of least resistance.¹⁷ The discrepancy between the territory of tissue anesthetized by a GNB and that captured by an RFA lesion, thus, likely contributes to false-positive responses to prognostic GNBs. We recommend that no more than 0.5 mL be used per genicular nerve target in order to improve the prognostic value of GNBs, but future research may indicate that even smaller injectate volumes may be more appropriate as has been demonstrated for lumbar medial branch nerve blocks.¹⁸

Target Sites

Finally, the predictive value of GNBs is based on the assumption that the local anesthetic agent anesthetizes exactly (and exclusively) the nerve or nerves that the physician seeks to subsequently ablate. However, in light of both recent neuroanatomical research,^5-8,19 and the clear need for updating genicular nerve RFA targets in order to improve patient outcomes, GNB target sites must be concordantly updated.

Recent dissections of the anterior knee joint by Tran et al,⁸ indicate considerable variation in genicular nerve courses between individuals and establish the presence of additional nerves not commonly targeted for GNB or RFA. Further, Tran et al⁸ reviewed previous anatomical studies of the anterior knee and found significant variation in reported genicular nerve locations between studies.

Based on this review and others, it appears potentially useful to target the nerve to the vastus medialis, nerve to the vastus intermedius, nerve to the vastus lateralis, recurrent fibular nerve, and the infrapatellar branch of the saphenous nerve in addition to accounting for variability between individuals in the course of the commonly targeted nerves (superior medial genicular nerve, superior lateral genicular nerve, and inferior medial genicular nerve).^1,9,10,20

Indeed, we may learn from prior extensive work validating medial branch nerve blocks, which has thoroughly delineated the predictive value of these blocks when specific protocols are used. While debate exists regarding optimal block thresholds to consider the test “positive,” and the number of blocks that is optimal by which to select patients for medial branch nerve RFA (taking total healthcare costs into consideration),^21-25 the literature unequivocally demonstrates superior treatment outcomes when higher thresholds of relief from the block and dual comparative blocks are used.^26-30

It is likely that with further research, it will be possible to identify GNB protocols associated with meaningful predictive value. Such research would subsequently inform healthcare economic research on the cost utility of updated protocols for GNBs in the context of successful genicular nerve RFA outcomes.

Patient Selection

Until a more rigorous GNB protocol is established, careful selection by clinical evaluation is necessary. Severe knee osteoarthritis consistent with Grade 4 on the Kellgren-Lawrence scale, prior total knee arthroplasty, and prior knee arthroscopy with or without meniscal repair may be associated with a lower likelihood of genicular nerve RFA treatment success.^31,32

Prior investigation suggests that the duration of knee pain, baseline pain score, and greater baseline analgesic medication use do not have a significant relationship with the likelihood of a successful outcomes associated with genicular nerve RFA. In other words, the physician should not withhold treatment of chronic knee pain by genicular nerve RFA based on these factors when evaluating a patient’s medical history or clinical presentation.

Further research will be necessary to confirm these findings and to identify additional demographic, clinical, and imaging factors that may be associated with predictive value regarding the chances of successful treatment of chronic knee pain by genicular nerve RFA. Until these factors are further explored and GNB protocols adopt more comprehensive and precise measures for genicular nerve capture, categorical assessments of the overall viability of GNBs to predict genicular nerve RFA success will be limited.

Conclusion

In summary, we recommend using a GNB threshold of at least 80% improvement in pain to consider the block “positive” (with consideration of potentially increasing this to ≥90% improvement in pain), use of no more than 0.5 mL of local anesthetic per target site, and incorporation of additional genicular nerves during GNBs. Further, physicians should consider accounting for the genicular nerve course variability of the additionally targeted nerves between individuals. Attention to these protocol adjustments may improve the prognostic value of GNBs.

Physicians should also consider demographic and clinical factors that may predict the likelihood of success of genicular nerve RFA for the treatment of chronic knee pain, including severe knee osteoarthritis, prior total knee arthroplasty, and prior knee arthroscopy with or without meniscal repair.

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