12 Blocks at the Knee
12.1 Anatomical Overview
The sciatic nerve (L4–S3) consists of two components, the common fibular nerve (synonym: common peroneal nerve) and the tibial nerve, which are surrounded in the lesser pelvis and thigh by a common connective-tissue sheath and therefore give the impression of a single nerve trunk. The level at which the division into the two branches can take place varies. The common connective-tissue sheath ends at the latest on entry to the popliteal fossa and the nerve divides into the tibial nerve and the common fibular nerve (Fig. 12.1).
Common fibular nerve.
The common fibular nerve (L4–S2) divides below the popliteal fossa into the deep fibular nerve and the superficial fibular nerve. The deep fibular nerve innervates the extensor muscles of the lower leg and foot. The superficial fibular nerve supplies the muscles of the fibular group. The tibial nerve (L4–S3) is responsible for the motor supply of the toe and foot flexors.
The tibial nerve innervates the skin of the lateral lower leg and the sole of the foot and, after joining the communicating branch of the fibular nerve to form the sural nerve, it supplies the lateral border of the heel and foot. The dorsum of the foot is innervated by the superficial fibular nerve, apart from the area between the great toe and second toe (deep fibular nerve).
12.2 Classical Popliteal Block, Posterior Approach
Landmarks and Position
Popliteal fossa, popliteal crease.
The patient lies prone. With the knee extended, puncture is performed at the level of the popliteal crease or slightly cranial to it (Fig. 12.2). The tibial nerve is found about 1 cm lateral to the artery. It is situated at a depth of 1 to 3 cm. To block the common fibular nerve from the same insertion site, the needle is withdrawn under the skin and advanced again further laterally toward the head of the fibula. After around 3 to 4 cm a response will be obtained from the nerve.
12.2.2 Remarks on the Technique
Anesthesia of the sciatic nerve or of its two divisions (fibular nerve and tibial nerve) in the region of the popliteal fossa, known as popliteal block or “knee block,” has often been described. It is a highly effective technique and is easily performed without problems.
The disadvantage of the classical popliteal block in the popliteal fossa or slightly more cranial is the necessity of finding two nerves separately in order to be able to anesthetize the entire foot. The fibular and tibial nerves can be separately blocked in the popliteal fossa or somewhat proximal to it from one puncture site because of the close vicinity of the two nerves. This requires a change in the direction of the needle for selective stimulation of the two nerves.
Double injection technique.
The double injection technique should be a quick procedure, as the risk of intraneural injection increases with the time required (Gligorijevic 2000).
Administration of the local anesthetic after finding the first nerve can result in partial anesthesia of the second nerve, even before it has been localized, because of its proximity. This prevents both an adequate response by the nerve stimulator and the patient′s warning of paresthesia due to inadvertent intraneural injection of the local anesthetic.
The double injection technique leads to a short onset period and an effective block (Bailey et al 1994). Singelyn et al (1991) performed 625 blocks with nerve stimulation in a prospective study; 30 mL of mepivacaine 1% (10 mg/mL) or bupivacaine 0.5% (5 mg/mL) was injected. An adequate block was achieved in 92% of the patients and in another 5% the block was successfully supplemented. The popliteal artery was punctured in two patients (0.3%). Patient satisfaction was 95%. Popliteal block is considered to be a safe technique (Jan et al 2000). An out-of-plane ultrasound-guided puncture a few centimeters distal to the bifurcation has shown that one injection between the two nerves (tibial nerve, common fibular nerve) leads to a fast, reliable block (Perlas et al 2013, Chapter 12.3.5) so that it is not absolutely necessary to block the two nerves separately in this region.
12.3 Distal Block of the Sciatic Nerve
Posterior Approach, Continuous Technique According to Meier (Meier 1996)
The sciatic nerve divides, at the latest where it enters the popliteal fossa, into its two main branches, the tibial nerve and the common fibular nerve. The common fascial sheath can no longer be found in the popliteal fossa. For reasons of efficacy, this suggests that the sciatic nerve should be found and anesthetized as far cranially as possible in the popliteal fossa, that is, before it divides—in other words, a distal sciatic nerve block should be performed (Fig. 12.3).
Above the popliteal crease, the popliteal fossa is bounded laterally by the tendon of biceps femoris, and medially by the semimembranosus and the tendon of semitendinosus. The needle is inserted at the lateral boundary of the popliteal fossa (corresponding to the inside of the biceps femoris tendon) about 8 to 12 cm above the popliteal crease (Fig. 12.4, Fig. 12.5, Fig. 12.6).
The patient lies on his or her side with the leg to be anesthetized on top. The lower leg is flexed at the knee; the upper leg is loosely extended (Fig. 12.5).
Variation of position, preferred position.
The patient lies supine, the leg to be anesthetized is lifted and flexed at the hip and knee, also known as the “lithotomy” position, Fig. 12.7.
The patient is asked to flex the knee. The tendon of the biceps femoris can be readily palpated on the lateral side. The leg is then extended. A line is drawn about 8 to 12 cm proximal and parallel to the popliteal crease. The intersection with the tendon of the biceps femoris marks the insertion site (Fig. 12.6). The insertion site is medial to the tendon of the biceps femoris and lateral to the popliteal vessels (Meier 1996; Fig. 12.8).
Following disinfection, infiltration, and prepuncture of the skin at the insertion site, a 6 to 10 cm long 19.5G needle is connected to a nerve stimulator and advanced proximally and slightly medially at an angle of 30 to 45° to the skin. When the fascia is reached, obvious resistance (“click”) can often be felt.
The sciatic nerve or its divisions are reached after 4 to 6 cm. In obese patients, the distance may be greater than 6 cm. Because of the laterally situated insertion site, the common fibular nerve is usually reached first and then the tibial nerve when the needle is advanced deeper and further medially.
The position of the needle tip is optimal when pronation of the foot with dorsiflexion (fibular division) or a motor response of the tibial nerve (supination of the foot with plantar flexion) can be produced.
Both responses can often be produced by a minimal shift of the needle tip. Then 30 to 40 mL of local anesthetic is injected.
Alternatively, the distal sciatic nerve block can be performed in supine position (with the leg supported with a positioning aid). The patient then does not need to be turned to their side. In the continuous technique, after the local anesthetic is injected the catheter is advanced proximally by the needle 3 cm beyond its tip.
Tips and Tricks
If the tibial nerve is stimulated first, the position of the needle tip should be directed more laterally in order to reach the fibular nerve.
Vascular puncture is not anticipated with this technique of distal sciatic nerve block (Fig. 12.9).
The catheter should not be advanced more than 3 cm past the tip of the needle.
A frequently observed long onset time is possibly caused by fat tissue in the popliteal fossa.