Use of 0.5 mL and 1.0 mL of Local Anesthetic for Genicular Nerve Blocks

Recent literature has identified a high false-positive rate of genicular nerve blocks in predicting successful outcome of genicular nerve radiofrequency ablation (RFA) [1]. A new cadaveric study has confirmed a greater number of nerves contributing to the sensory innervation of the anterior knee joint capsule compared with those targeted by genicular nerve RFA—the superior lateral, superior medial, and inferior medial genicular nerves [2–4]. Because the volume of monopolar radiofrequency lesions has been found to range from 0.5 to 1 cm3 in ex vivo bovine liver [5], depending on electrode type (cooled vs conventional), size, and lesioning parameters, these additional nerves identified would not be captured in the burn radius if a single monopolar lesion were created at each target genicular nerve site [1–4]. Additionally, using specific amounts of contrast medium in place of anesthetic in two separate patients, Figure 1a–d (0.5 mL of contrast medium) demonstrates spread beyond the boundaries of typical 0.5- and 1-cm3 monopolar RFA lesions. Further, many providers utilize 1-mL diagnostic anesthetic block volumes [1,2,4], which cover an even greater territory. Figures 1–3 suggest that depending on individual variations in nerve location [3], both the articular branch of the common fibular nerve and the nerve to the vastus medialis are likely anesthetized by 1.0 mL and may also be anesthetized by 0.5 mL of local anesthetic; however, both of these nerves may not be captured by a monopolar radiofrequency lesion in most cases using common protocols [1,2,4]. Additionally, a 1.0-mL volume diagnostic block likely anesthetizes the superior medial genicular nerve, but this nerve may not be captured by a monopolar radiofrequency lesion in some cases using common protocols [1,2,4]. Orthogonal views of the knee in two different patients, with 0.5 mL or 1.0 mL of contrast medium injected at each of the three sites targeted in currently described genicular nerve radiofrequency ablation protocols. These protocols include needle tip placement at the confluence of the femoral shaft and medial and lateral femoral condyles in an anterior-posterior (AP) view to target the superior medial and superior lateral genicular nerves, respectively; the midpoint diameter of the femoral shaft represents the goal needle tip position in a lateral view for both of these nerves [1, 2, 4]. These protocols also include needle tip position at the confluence of the tibial shaft and medial tibial flare in an AP view, as well as the midpoint diameter of the tibial shaft in a lateral view [1, 2, 4]. AP fluoroscopic views of 0.5 mL of contrast medium injected at the (a) superior lateral and superior medial genicular nerves and (b) inferior medial genicular nerve are shown; lateral fluoroscopic views of 0.5 mL of contrast medium injected at the (c) superior lateral and superior medial genicular nerves and (d) inferior medial genicular nerve are shown. The same fluoroscopic views of 1.0 mL of contrast medium injected at the same sites in a different patient’s knee are shown; each row corresponds to the same fluoroscopic view (i.e., a to e, b to f, c to g, and d to h). a and c) The estimated course of the articular branch of the common fibular nerve (CF) and the nerve to vastus medialis (VM) superimposed on anterior-posterior (AP) and lateral radiographs of the knee [3]; the estimated distribution of spread of 0.5 mL (solid white) and 1.0 mL (dotted line) of injectate at the typical target sites during genicular nerve blocks are shown [1, 2, 4]. b and d) The estimated course of the CF and VM on AP and lateral radiographs of the knee [3], as well as the estimated territory of tissue lesioned using cooled radiofrequency ablation (black circles), per typical protocol based on findings in ex vivo bovine liver [1, 2, 4, 5]. Anatomical studies have found variations in the course of the nerves [3], which may affect nerve capture rates. These two-dimensional representations of contrast spread and radiofrequency ablation (RFA) lesion size are based on estimations extrapolated from comparing the width (1.47 mm) and length (4 mm) of the active tip of a cooled RFA electrode positioned at the standard target sites [1, 2, 4] and viewed in relation to the radiographic bony anatomy of the knee (N = 20); measurement error is expected, and future study is required to confirm these estimations. a and c) The estimated course of the superior medial genicular nerve (SM), superior lateral genicular nerve (SL), and inferior medial genicular nerve (IM) superimposed on AP and lateral radiographs of the knee [3]; the estimated distribution of spread of 0.5 mL (solid white) and 1.0 mL (dotted line) of injectate at the typical target sites during genicular nerve blocks are shown [1, 2, 4]. b and d) The estimated course of the SM, SL, and IM superimposed on AP and lateral radiographs of the knee [3], as well as the estimated territory of tissue lesioned using cooled radiofrequency ablation (black circles), per typical protocol based on findings in ex vivo bovine liver [1, 2, 4, 5]. As with Figure 2, these two-dimensional representations of contrast spread and radiofrequency ablation (RFA) lesion size are based on estimations extrapolated from comparing the width (1.47 mm) and length (4 mm) of the active tip of a cooled RFA electrode positioned at the standard target sites [1, 2, 4] and viewed in relation to the radiographic bony anatomy of the knee (N = 20); measurement error is expected, and future study is required to confirm these estimations. Therefore, both 0.5-mL and 1-mL volume diagnostic blocks may result in a higher false-positive rate than alternative protocols yet to be developed. As such, novel diagnostic block and/or RFA lesioning protocols will be required to improve the concordance and subsequent predictive value of genicular nerve blocks.