Adductor canal block ("subsartorial saphenous nerve block")
Introduction: The saphenous nerve is a pure sensory nerve. It innervates the medial side of the lower leg and foot; albeit with significant interpatient variability. Saphenous nerve block is usually performed in combination with a sciatic block to provide complete anesthesia/analgesia to the foot and ankle. Recent evidence suggests that for knee surgery, when performed at the adductor canal ("adductor canal block" or "subsartorial saphenous nerve block"), the block has a morphine sparing and analgesic effect as well as faciitating mobilisation. When used for this indication, saphenous block has the theoretical advantage of avoiding quadriceps weakness. However, to date, evidence supporting its use in knee surgery is limited. The adductor canal block will also not provide analgesia after knee surgery as good as that achieved by a more proximal femoral/obturator block (see below for reasons why). 1
Skill level: moderate due to the difficulty often encountered visualising the femoral artery/saphenous nerve.
Procedure time: 1-5 mins depending on the level saphenous block is performed and operator experience
1. Foot/ankle surgery – at the medial malleolar level (usually combined with sciatic nerve block or as part of the ankle block).
2. Knee/distal leg surgery (e.g. meniscectomy, ACLR, TKJR) as an alternative to femoral block.
Sedation: e.g. midazolam 2 mg + alfentanil 250-500 mcg or under GA
Surface landmarks: mark the side to be blocked
Anatomy: 2, 3
The adductor canal (subsartorial canal) is an aponeurosis covered tunnel in the middle third of the thigh (Fig. 1). The canal contains the femoral artery, femoral vein, the posterior branch of the obturator nerve, and branches of the femoral nerve (specifically the saphenous nerve and nerve to the vastus medialis). It is bordered by 3 muscles: anterolaterally the quadriceps muscle (specifically vastus medialis); posteriorly adductor magnus, and medially the sartorius muscle. It extends from the upper/anterior thigh femoral structures to the lower/medial thigh adductor hiatus – the distal opening in the adductor magnus muscle approx. 13 cm proximal to the knee. The femoral vessels leave the canal at the adductor hiatus, where they dive deeply, and this abrupt change in femoral artery depth is a useful indicator of the distal limit of the canal, and therefore the optimal level to form the adductor canal (subsartorial) block.
Fig 1. Transverse section of the the thigh/hamstrings showing the boundaries of the adductor canal: anterolaterally the quadriceps (specifically vastus medialis), posteriorly adductor magnus and medially the sartorius muscle. Patient is on the side with the medial thigh up.
The adductor (subsartorial) canal block will not provide analgesia after knee surgery as complete as that achieved when using a conventional inguinal level femoral block. This is because the posterior division of the femoral nerve sends branches to the quadriceps muscle (particulary vastus lateralis, intermedius) at varying levels proximal to the adductor canal. Each branch gives rise to articular filaments, which together with the saphenous termination of the femoral nerve, provide sensory innervation to the knee joint. The clinical relevance of these proximally arising femoral sensory nerves, in terms of mediating pain after knee surgery (compared to the saphenous nerve), is at present unknown.
Note: to perform saphenous block at the medial malleolar level, see ankle block (nerve is posterior to the saphenous vein)
Needle: 8 cm 18G Tuohy epidural needle.
1. Linear or curvilinear probe e.g. Sonosite L38.
2. 8-10 cm Tuohy needle.
3. 20 mL LA filled syringe
Procedure (Fig. 2, 3, Video 1, 2)
1. If performing awake, under US guidance, infiltrate the skin, subcutaneous fat, and muscle along the intended needle tract down to the femoral artery/saphenous nerve using an in-plane needle-probe alignment, a 22-25G hypodermic needle and 10 mL lidocaine 1%.
2. Standing on the patient's right side for the right leg, and the patients left side for the left leg, the needle entry point is over the medial thigh.
3. Nerve stimulation is rarely helpful for saphenous block (a pure sensory nerve).
4. Place probe on the medial/inner aspect of the mid thigh and locate the femoral artery (Fig. 2). Trace the artery distally to locate the point just before it starts to dive down to form the popliteal artery, which is approximately 13 cm proximal to the knee. At this location, the vastus medialis muscle lies anterolateral, the adductor magnus muscle posteromedial and the sartorius muscle medial (Fig. 3). The appropriate probe position is just proximal to where the femoral artery "dives" posteriorly. The saphenous nerve may or may not be visible posteromedial or anterolateral to the femoral artery.
5. With a 10-20 ml LA filled syringe attached directly to the Tuohy needle, advance towards the femoral artery and pierce the fascia on the inner aspect of the sartorius muscle, ideally anterolateral and posteromedial to the femoral artery. Penetration of the fascia with a Tuophy needle requires short, sharp, needle thrusts and is facilitated by orientation of the bevel towards the fascia (i.e. posteriorly).
6. Deposit 7.5-10 mL LA at these two points.
Fig. 2. Subsartorial saphenous nerve block (adductor canal block). Access to the medial side of the thigh is facilitated by slight external leg rotation. A needle entry point as close as possible to the probe (i.e. as posteriorly as feasible) facilitates access to the posteromedial side of the femoral artery.
Fig. 3. Saphenous block (adductor canal block) ultrasound image. Blue arrows=approx. direction of needle trajectory. VM=vastus medialis (anterolateral to artery). FA=femoral artery. FV=position of femoral vein. Aim to pierce the fasicia on the inner aspect of the sartorius muscle, before depositing LA ideally in a two point location anterolaterally + posteromedially to the femoral artery (indicated by the two arrows). In the distal thigh, the saphenous nerve is more commonly located between the sartorius muscle and the adductors (i.e.posteromedial to the artery)
Local anesthetic regimen: 15-20 mL LA e.g ropivacaine 0.50%.
Specific Complications: None of note.
1. Consider the out-of-plane approach, which faciltates needle access to both the anterolateral and posteromedial aspects of the femoral artery.
2. An alternative to the sub-sartorial approach for saphenous nerve block is to inject just distal to the knee. LA is injected around the saphenous vein, typically one hands breadth posterior to the patella.
Video 1. Adductor canal block
1. Tsai PB, Karnwal A, Kakazu C, et al. Efficacy of an ultrasound-guided subsartorial approach to saphenous nerve block: a case series. Can J Anaesth 2010;57:683-8.
2. Manickam B, Perlas A, Duggan E, et al. Feasibility and efficacy of ultrasound-guided block of the saphenous nerve in the adductor canal. Reg Anesth Pain Med 2009;34:578-80.
3. Saranteas T, Anagnostis G, Paraskeuopoulos T, et al. Anatomy and clinical implications of the ultrasound-guided subsartorial saphenous nerve block. Reg Anesth Pain Med 2011;36:399-402.