Saphenous Nerve Block for the Assessment of Knee Pain Refractory to Conservative Treatment

Saphenous neuralgia is an underappreciated cause of anterior and medial knee pain. Irritation or entrapment of the saphenous nerve may mimic common conditions such as patellofemoral pain syndrome or present as a complication of surgical interventions at the knee (1,2). The use of an ultrasound-guided block of the nerve can be a very useful diagnostic tool for determining the presence of saphenous neuralgia and its contribution as a pain generator in patients who have been refractory to treatment for more common etiologies. Because the saphenous nerve is most commonly entrapped at either the distal aspect of the adductor (Hunter’s) canal or at the infrapatellar branch of the saphenous nerve (1,2), the clinician may take two different strategies in their approach to the block: 1) block the infrapatellar branch first, and if this does not relieve pain then block the saphenous nerve at the adductor canal, or 2) block the nerve at the adductor canal to determine involvement at either site. The clinician should consider their likely treatment course in the case of a positive response to guide their choice of blocks as knowledge of the specific entrapment site may not be necessary for all treatment options. To perform a block of the saphenous nerve at the distal aspect of the adductor canal, the probe is placed at the medial aspect of the thigh approximately 7 cm to 8 cm proximal to the superior pole of the patella (Fig. 1). The saphenous nerve should be visualized deep to the sartorius muscle. Placing the patient in a lateral decubitus may aid the practitioner’s lateral-to-medial approach. Care should be taken to avoid the geniculate artery; however, it also may help with identification of the nerve at this level via Doppler mode scanning. The block may be confirmed by assessing for anesthesia to the distal medial leg. Once anesthesia is attained, the patient should be asked to attempt positions/tasks that typically exacerbate their pain and compare their pain level to their pre-injection state. A minimum clinically important difference of 20 mm to 30 mm on the visual analog scale should be used as a positive indication of involvement of the saphenous nerve as a pain generator.Figure 1: (A) Positioning of the ultrasound probe at the distal end of the adductor canal; (B) short axis view of the saphenous nerve. Closed arrow, saphenous nerve; open arrows, geniculate artery; Sar, sartorius; VM, vastus medialus; AdM, adductor magnus.Identification of the infrapatellar branch may be accomplished by tracing the course of the saphenous nerve to locate its bifurcation point into the infrapatellar and larger sartorial branches. This bifurcation can be variable in its position and orientation, but typically occurs just proximal to the medial condyle of the femur (Fig. 2) (3). The use of fast scanning techniques can be helpful in both identification of the bifurcation and in tracing the infrapatellar branch distally from the bifurcation. Similar positioning and approach may be used to block this nerve, with confirmation of local anesthesia that spares the distal medial leg. A low volume (1 mL to 2 mL) of anesthetic is recommended at this location to reduce the possibility of confounding the diagnosis.Figure 2: (A) Positioning of the ultrasound probe at the bifurcation of the saphenous nerve; (B) short axis view of the saphenous nerve. Closed arrow, sartorial branch of the saphenous nerve, open arrow, infrapatellar branch of the saphenous nerve.