Analgesia for knee surgery (motor sparing), surgery to the medial ankle and medial side of calf and foot.
The femoral triangle is bordered superiorly by the inguinal ligament, laterally by the medial border of sartorius and medially by the medial border of adductor longus. Inferiorly, the apex of the triangle is where the medial borders of sartorius and adductor longus meet. The adductor canal (Hunter’s canal) is approximately 15 cm long, extending from the apex of the femoral triangle to the adductor hiatus in the adductor magnus muscle. The canal is bordered by muscles: sartorius; anteromedial, vastus medialis; lateral, adductor longus and magnus; posterior. A relatively thick aponeurosis extending from vastus medialis to adductor magnus covers the canal and has to be crossed in order to enter the canal and access the nerves. The contents of the femoral triangle and adductor canal are the femoral artery and vein, the saphenous nerve and the nerve to vastus medialis muscle. The saphenous nerve descends in the thigh, crossing the superficial femoral artery from lateral to medial before leaving the adductor canal between sartorius and gracilis muscles at the level of the adductor hiatus, accompanied by the saphenous branch of the descending geniculate artery. The saphenous nerve then accompanies the long saphenous vein to the ankle.
Anterior superior iliac spine, sartorius muscle, femoral artery, long saphenous vein.
Patient supine with the leg abducted and the knee slightly bent to expose the inner thigh.
Figure 1: Patient, transducer and needle positioning for a) in plane approach to the distal femoral triangle and b) in plane approach to the adductor canal
With the transducer transversely on the thigh, identify the common femoral artery and vein. This can be traced distally from the groin. Follow the artery in the femoral triangle passing beneath the body of sartorius muscle before entering the adductor canal. The vessels will then move deep to pass through the adductor hiatus. The saphenous nerve may be seen initially lateral to the artery before crossing obliquely, anterior to the artery and then over adductor magnus tendon to pass between sartorius and gracilis muscles to become super cial. The long saphenous vein can then be traced on the medial surface of the leg from posterior and medial to the knee distally to the ankle.
The saphenous nerve and nerve to vastus medialis are seen as hyperechoic structures lying initially lateral to the femoral artery in the femoral triangle. In the lower leg, the saphenous nerve may be seen adjacent to the vein but may be difficult to visualise due to its size.
Figure 2: a) ultrasound image of the femoral artery and nerves deep to sartorius muscle in the femoral triangle b) annotated ultrasound image.
Figure 3: a) ultrasound image of the femoral artery deep to sartorius muscle in the adductor canal b) annotated ultrasound image
1. Low volumes of local anaesthetic placed within the femoral triangle in the mid-thigh will produce a partial sensory block of the knee (anteromedial) and ankle joints along with the medial side of the calf and foot. Local anaesthetic placed in the adductor canal close to the adductor hiatus will block the saphenous nerve and may also ow through the adductor hiatus and affect perforating branches supplying posterior sensation to the knee joint derived mainly from the tibial and obturator nerves.
2. The saphenous nerve is purely sensory. Nerve stimulation eliciting paraesthesia down the medial side of the lower leg can be used to con rm its identity. Direct blockade in the femoral triangle or adductor canal with 10 ml of LA will achieve a reliable block. Alternatively, a peri-venous injection around the saphenous vein will achieve a block in the lower leg.
Plan B/C/D blocks for the lower limb-knee include: Femoral nerve block +/- iPACK block
More information about the adductor canal block and analgesia for knee surgery can be found at the excellent article here
Text and images have been reproduced from the 2nd edition of the RA-UK Handbook, which can be purchased on Amazon, or is received on joining RA-UK. This excellent resource also contains practical descriptions of all of the advanced blocks referenced in the editorial.