Distal sciatic nerve blocks
Lateral distal approach
Superior edge of the patella, vastus lateralis muscle, long head of the biceps femoris muscle
This technique affords a block of the sciatic nerve just superior to its bifurcation without any complicated positioning. The patient is supine on his back, with the leg in a neutral position; padding is placed distally under the lower leg to allow the knee to hang suspended. The compartment between the vastus lateralis muscle and the sinewy part of the biceps femoris muscle is identified by palpation approx. 5 cm (3 – 8 cm) above the patella. This site is marked. A needle of 50 mm in length is usually sufficient for puncture. The insertion direction points 30° dorsally and 5 – 10° cranially. Again, the positive stimulatory response is elicited from muscles on the lower leg or foot innervated by the peroneal or tibial nerves. Once the threshold electrical current is reached, 30 – 50 ml of local anaesthetic are injected. Compared to the proximal sciatic nerve blocks, onset of action is significantly longer, between 20 – 40 min. A pain catheter can be positioned easily. Under certain circumstances, it may be helpful for localizing the muscle compartment to actively tense the ventral and dorsal thigh muscles against resistance. One common error is made by searching for the nerve too ventrally and too deeply. (The nerve‘s position is always more superficial and dorsal than one thinks).
Ultrasound-guided distal sciatic nerve block
Here too, as already described with regard to proximal subgluteal sciatic nerve blocks, the patient‘s lower leg is kept elevated for this distal nerve block (see photo). The sciatic nerve, that is, its end branches (peroneal and tibial nerves) appear clearly near the popliteal space on the dorsal side of the distal thigh. The primary anatomic landmark here is the medial border of the long head of the femoral biceps muscle. The nerve is scanned in cross-section, starting dorsally. Should it already be distinct, it can usually be traced as far as the bifurcation by shifting the transducer proximally.
Distal dorsal approach
Popliteal fossa between the semitendinosus muscle and biceps femoris muscle, medial epicondyle of femur and the lateral epicondyle of femur, popliteal artery
The patient is either in the prone position or lying on the side that is not to be anaesthetised. The upper leg must then be well extended. With the patient in one of these positions, the popliteal fossa is first identified and then demarcated medial to the semitendinosus muscle and lateral to the biceps femoris muscle. Drawing an isosceles triangle between the medial and lateral condyles of the femur with its tip pointing in the cranial direction can help you find the median line through the popliteal fossa. The puncture site is located 5 – 8 cm cranial from the base of this triangle and about 1 cm lateral of the median line. The puncture site is located approximately 1 cm lateral of the site where the popliteal artery is palpable. After disinfection and infiltration of the puncture site, insert the stimulation needle (Stimuplex® D 50 mm or 80 mm) and advance it in the cranial direction at a flat angle to the skin (approx. 30°). At a depth of about 3 – 5 cm, muscle contractions are triggered in the supply area of the peroneal nerve or the tibial nerve.