CHAPTER 9 Lower limb blocks

Femoral nerve block (3-in-1 block)

Landmark technique

A femoral nerve block is easy to master and has a very high success rate even in relatively inexperienced hands. It has a low risk of complications and has a significant clinical applicability in the ED for post-traumatic pain management of injuries of the femur, anterior thigh and knee. When a femoral nerve block is used in combination with a sciatic nerve block, anaesthesia of almost the entire lower limb distal to the mid-thigh can be achieved. The 3-in-1 block refers to the simultaneous blockade of the anterior branch of the obturator nerve, the lateral femoral cutaneous nerve and the femoral nerve with a single injection. This results from the medial and lateral spread of local anaesthetic injected around the femoral nerve.

The femoral nerve arises from the L2, L3 and L4 nerve roots. The nerve descends between the psoas and the iliacus muscles and passes deep to the inguinal ligament into the thigh (Fig. 9.1). At this point the femoral nerve is positioned immediately lateral to and slightly deeper than the femoral artery. The acronym NAVY is a useful reminder of the arrangement of structures from lateral to medial: Nerve, Artery, and Vein, with Y representing the midline.

Fig. 9.1 Anatomical diagram of the course of the femoral nerve in the proximal thigh.

A femoral block produces anaesthesia of the entire anterior thigh and most of the femur and knee joint, as well as to the skin on the medial aspect of the leg below the knee joint via the saphenous nerve.

Preparation

• Position the patient supine, with the hips and knees extended.

• Mark with a pen the important landmarks that are used to determine the target point for the needle insertion (Fig. 9.2):

• The inguinal ligament – extending from the anterior superior iliac spine to the pubic tubercle.

• The femoral skin crease.

• The femoral artery pulsation at the femoral crease and inguinal ligament.

• The needle insertion point – immediately lateral to the femoral artery at the level of the inguinal ligament or femoral crease.

Fig. 9.2 The surface anatomy of the femoral nerve in the proximal thigh. The inguinal ligament (IL) extends from the anterior superior iliac spine (ASIS) to the pubic tubercle (PT), which lies just lateral to the pubic symphysis. The femoral nerve lies at the midpoint of this line just lateral to the femoral artery (FA) and femoral vein (FV). The target point for needle insertion may be at the level of the inguinal ligament (proximal cross) or at the level of the femoral crease (FC).

Technique

• Prepare the field by cleaning the skin with an antiseptic solution and positioning sterile drapes.

• Identify the target area for the initial needle insertion – a point immediately lateral to the arterial pulsation either at the level of the femoral crease or at the level of the inguinal ligament (Fig. 9.3A&B).

• Raise a superficial weal of local anaesthetic at the needle insertion site.

• Puncture the skin at the target point with a 25 mm to 50 mm nerve-block needle. Advance the needle posteriorly and slightly superiorly.

• If you are using a nerve stimulator, insert the needle using the same technique. Appropriate nerve stimulation must cause quadriceps contraction with movement of the patella. If the patella does not move, the sartorius muscle might have been stimulated.

• Aspirate. If no flashback of blood is obtained, inject 20 to 30 mL of local anaesthetic slowly with intermittent aspiration to rule out intravascular injection. Slow injection increases block success and decreases complications. If resistance to injection, severe paraesthesias or cramping pain sensations occur with initial injection, then the needle should be withdrawn by 1 to 2 mm to avoid intraneural injection. Use the higher end of the volume range if the 3-in-1 block is desired.

• Firm digital pressure applied distal to the injection site during injection and for several minutes thereafter increases the proximal, medial and lateral spread of local anaesthetic and increases the chances of achieving a 3-in-1 block.

• This block works well with smaller volumes of local anaesthetic, but larger volumes increase the likelihood of securing a block of the lateral femoral cutaneous nerve with resultant anaesthesia of the anterolateral aspect of the thigh.

Fig. 9.3 The needle puncture site is just lateral to the femoral artery either at the level of the inguinal ligament or at the level of the femoral crease. The needle may be directed directly posteriorly (A) or posteriorly and superiorly (B).

Ultrasound technique

The use of ultrasound assistance for the femoral nerve block increases the success rate from 80% with the blind and nerve stimulator techniques to 95%. If ultrasound is used, then it is not necessary to use a nerve stimulator as well.

Preparation

• Position the patient supine with the hips and knees extended.

• Use a linear high-frequency probe (10 to 15 MHz is ideal) and select an appropriate pre-set application.

• Identify the area to begin the scan – the area between the inguinal ligament and the femoral crease.

• Perform a preliminary non-sterile survey scan to identify the relevant anatomy and optimise the image by adjusting depth of field (about 20 to 30 mm), focus point, and gain. Mark the best probe position on the skin with a pen, if required.

• Place the probe transversely midway between the anterior superior iliac spine and the pubic tubercle (Fig. 9.4). Identify the pulsating femoral artery (lateral) and the compressible femoral vein (medially) with the assistance of colour Doppler if required (Fig. 9.5A–C). The femoral nerve is a triangular, often poorly defined, hyperechoic structure immediately lateral to the femoral artery. The femoral nerve may not be visibly distinct until the circumferential spread of local anaesthetic around the nerve. The position of the femoral nerve is generally easy to locate in this region.

• The lateral femoral cutaneous nerve can often be identified during the survey scan: place the lateral edge of the probe against the anterior superior iliac spine just distal to the inguinal ligament. The nerve may be identified as a small hyperechoic structure between the fascia lata and the fascia iliaca, superficial to the sartorius muscle.

• Prepare the field by cleaning the skin with an antiseptic solution and positioning sterile drapes. Cover the probe with a sterile probe-sheath and apply sterile ultrasound gel to the area between the inguinal ligament and the femoral crease.

Fig. 9.4 The high-frequency linear probe is placed transversely on the anterior proximal thigh, at the level of either the inguinal ligament or the femoral crease. The initial ultrasound landmark to locate is the pulsatile femoral artery.

Fig. 9.5 The femoral artery and vein can be clearly seen and distinguished using colour Doppler (A). The femoral vein (FV) lies closest to the midline, the femoral artery (FA) lateral to the vein, and the femoral nerve most lateral (arrows and arrowhead) (B) and (C). The femoral nerve is not always clearly visible before the injection of local anaesthetic, but its position can be precisely inferred from the position of the femoral artery.

Technique

In-plane approach

• Identify the position of the femoral nerve in its transverse axis in an area convenient to block.

• Raise a weal of local anaesthetic at the needle insertion target with a 27G needle.

• Insert a 25 mm to 50 mm 22G nerve-block needle on the lateral end of the ultrasound probe.

• Advance the needle parallel to the probe towards the edge of the nerve while visualising the entire length of the needle in real time (Fig. 9.6).

• The lateral femoral cutaneous nerve may be blocked separately once the femoral nerve has been blocked.

Fig. 9.6 To perform the in-plane approach, the needle is inserted from the lateral aspect of the probe and advanced under real-time guidance towards the lateral aspect of the nerve.

Out-of-plane approach

• Identify the position of the femoral nerve in its transverse axis in an area convenient to block.

• Line up the nerve target at the midpoint of the screen. The needle insertion point will correspond to the exact centre of the transducer.

• Raise a weal of local anaesthetic at the needle insertion target with a 27G needle.

• Insert a 25 mm to 50 mm 22G nerve-block needle on the inferior (distal) side of the ultrasound probe.

• Observe local tissue and needle movement as the needle is advanced towards the target. Aiming for the side of the nerve bundle rather than the centre makes needle placement more accurate (Fig. 9.7).

• Clear identification of the needle tip may require the probe to be angled back and forth.

• The femoral cutaneous nerve of the thigh may be blocked separately once the femoral nerve has been blocked.

Fig. 9.7 The out-of-plane approach is performed from inferiorly and is very similar to the conventional technique. Aim for the lateral aspect of the femoral nerve. This is a very easy nerve block approach and is a good one for novices to attempt.

The injection process

• Slowly inject local anaesthetic around the femoral nerve by positioning the needle adjacent to the nerve. Aspirate frequently to avoid inadvertent intravascular injection. If resistance to injection, severe paraesthesias or severe cramping pain are provoked in the limb during injection, then immediately withdraw the needle by 1 to 2 mm to avoid intraneural injection.

• Observe the local anaesthetic spread during injection. A hypoechoic collection will appear adjacent to and then spread around the nerve. Observe sheath distension and the formation of a hypoechoic ring of local anaesthetic solution around the hyperechoic nerve structures. Check to see if the local anaesthetic has spread around the lateral femoral cutaneous nerve – if not, it may need to be blocked separately.

• A lack of expansion of the tissue may indicate intravascular injection – aspirate and reposition the needle if necessary.

• Reposition the needle at least once to ensure complete circumferential local anaesthetic spread around the nerve roots.

• Scan proximally and distally to assess the extent of local anaesthetic spread.

Lateral femoral cutaneous nerve block

The lateral femoral cutaneous nerve arises from the L2 and L3 nerve roots and provides sensation to the lateral aspect of the thigh. It enters the thigh a variable distance medial to the anterior superior iliac spine (most commonly 10 to 15 mm, but as much as 50 mm) and deep to the inguinal ligament between the layers of the fascia lata and the fascia of the iliacus muscle. This block may be performed on its own or to complement a femoral nerve block.

Preparation

• Position the patient supine with the hips and knees extended.

• Mark with a pen the important landmarks that are used to determine the target point for the needle insertion (Figs 9.8, 9.9):

• The anterior superior iliac spine.

• The needle insertion point – 20 mm medial and 20 mm inferior (distal) to the anterior superior iliac spine.

Fig. 9.8 The lateral femoral cutaneous nerve of the thigh may be blocked at a point 20 mm medial and 20 mm inferior to the anterior superior iliac spine. A fan-type of injection to disperse the local anaesthetic increases the chances of block success.

Fig. 9.9 The lateral femoral cutaneous nerve of the thigh can be visualised on ultrasound by positioning the probe immediately medial and inferior to the anterior superior iliac spine. The nerve can be seen between the fascia lata (FL) and the fascia iliaca (FI). It may be blocked using an in-plane or out-of-plane technique. If the nerve cannot be visualised, local anaesthetic can be deposited between the fascial layers about 15 to 20 mm medial to the anterior superior iliac spine.

Sciatic nerve block

Sciatic blockade (via the posterior, anterior or popliteal approach) has the potential to be one of the most commonly used regional anaesthetic techniques in the ED and can be invaluable for pain management following trauma to the lower limb (Fig. 9.10). This block is relatively easy and is associated with a high success rate when properly performed. It is particularly well suited for injuries to the leg, ankle, and foot. It provides complete anaesthesia of the leg below the knee with the exception of a medial strip of skin which is innervated by the saphenous nerve. When combined with a femoral nerve block or 3-in-1 block, anaesthesia of almost the entire lower limb distal to the mid-thigh is achieved. If spinal immobilisation procedures are required, rather use the anterior or the popliteal approach, which require less movement of the patient and the injured limb.

Fig. 9.10 (A) Posterior view of the course of the sciatic nerve in the pelvis and proximal thigh. The red line indicates the landmarks for the posterior approach to this block. (B) Anterior view of the course of the sciatic nerve in the proximal thigh. The landmarks used for the anterior approach to this block are indicated by the red lines. (C) Cross-sectional anatomy of the proximal thigh.

Posterior approach landmark technique

The traditional posterior approach to sciatic nerve blockade is relatively simple and successful but it has the disadvantage of requiring a significant repositioning of the patient, which might be difficult with an injured limb.

Preparation

• Position the patient in the lateral decubitus position with the side to be blocked uppermost. Flex the hip and knee to 90°.

• It is important that the patient remain in the same position once the landmarks have been marked. Small positional changes can result in a significant shift of the landmarks due to the movement of the skin, which can lead to difficulty in localising the sciatic nerve.

• Mark the important landmarks:

• The greater trochanter.

• The posterior superior iliac spine.

• The landmarks are easy to identify in most patients, but take care to palpate carefully when the landmarks are obscured in overweight or obese individuals.

• Mark the innermost aspects of both landmarks. Marking the outer aspects will lead to inaccurate estimation of the position of the sciatic nerve.

• Approach the greater trochanter from the posterior side when identifying and marking.

• Approach the posterior superior iliac spine from the anterior side when identifying and marking.

• Draw a line from the greater trochanter to the posterior superior iliac spine; mark the midpoint of this line; the target point for needle insertion is 30 to 50 mm distal to this midpoint mark, perpendicular to the line (Fig. 9.11).

Fig. 9.11 With the patient in the lateral decubitus position, mark the midpoint of a line between the posterior aspect of the greater trochanter (GT) and the anterior aspect of the posterior superior iliac spine (PSIS). The target for needle puncture is a point 40 mm (30 to 50 mm) distal and perpendicular to this midpoint.

Technique

• Prepare the field by cleaning the skin with an antiseptic solution and positioning sterile drapes.

• Identify the target area for the initial needle insertion – a point 30 to 50 mm distal and perpendicular to the midpoint of a line joining the inner aspects of the greater trochanter and the posterior superior iliac spine.

• Raise a superficial weal of local anaesthetic at the needle insertion site with a 27G needle.

• Puncture the skin at the target point with a 75 mm to 100 mm nerve-block needle at a perpendicular angle to the spherical skin plane. Keep the needle at this angle while advancing it. The sciatic nerve should be encountered at 50 to 80 mm depth (Fig. 9.12).

• If you are using a nerve stimulator, use the same needle insertion technique and, once the target depth has been reached, look for twitches of the hamstrings, calf, foot, or toes. Gluteal twitches indicate that the needle is still too superficial. The use of nerve stimulation for the sciatic nerve can at times be misleading as the sciatic nerve is predominantly composed of sensory fibres. The needle may completely penetrate the nerve without producing muscle twitches!

• Aspirate. If no flashback of blood is obtained, inject 20 to 30 mL of local anaesthetic slowly with intermittent aspiration to rule out intravascular injection. Slow injection increases block success and decreases complications. If resistance to injection, severe paraesthesias or cramping pain sensations occur with initial injection, then the needle should be withdrawn by 1 to 2 mm to avoid intraneural injection.

• Onset of anaesthesia should occur in 15 to 30 minutes.

Fig. 9.12 The needle is inserted perpendicular to the skin and advanced directly in that plane until the sciatic nerve is encountered at a depth of 50 to 80 mm.

Posterior approach ultrasound technique

This block is one of the more difficult of the ultrasound-guided nerve blocks. Although the sciatic nerve is one of the largest peripheral nerves, it is often difficult to visualise because of the depth from the skin and because of the overlying adipose tissue.

Preparation

• Position the patient in the lateral decubitus position with the hip and knee flexed.

• Use a curvilinear probe (2 to 6 MHz) and select an appropriate pre-set application.

• Identify the area to begin the scan – the area between the ischial tuberosity and the greater trochanter (Fig. 9.13). The landmarks are easy to identify in most patients, but take care to visualise them carefully when excess adipose causes a poor image.

• Perform a preliminary non-sterile survey scan to identify the relevant anatomy and optimise the image by adjusting depth of field (about 40 to 80 mm), focus point, and gain. Mark the best probe position on the skin with a pen, if required.

• Place the probe obliquely with the long axis parallel to a line between the ischial tuberosity and the greater trochanter in order to visualise the subgluteal space, which is an echolucent space deep to the gluteus maximus and superficial to the quadratus femoris muscles (Fig. 9.14A&B). The sciatic nerve, which is large, echogenic, wide and flat, always lies just medial to the midpoint of the echogenic fascia connecting the ischial tuberosity and the greater trochanter and appears as if it is protruding into the subgluteal space. There is an echogenic tendinous structure close to the greater trochanter that might be mistaken for the sciatic nerve but the sciatic nerve is always medial to the midpoint of the subgluteal space.

• Prepare the field by cleaning the skin with an antiseptic solution and positioning sterile drapes. Cover the probe with a sterile probe-sheath and apply sterile ultrasound gel to the area to be scanned.

Fig. 9.13 The sciatic nerve is very deep when visualised with ultrasound from the posterior aspect and therefore a curvilinear low-frequency transducer is required. The probe is positioned transversely between the greater trochanter (GT) and the ischial tuberosity (IT) in order to visualise the sciatic nerve. The nerve is often very hard to locate and this block is difficult for novices.

Fig. 9.14 (A)&(B) The sciatic nerve (usually) can be seen as a large hyperechoic oval structure in the medial half of the subgluteal space. The subgluteal space is an echolucent area between two ‘tram-track’ hyperechoic fascial planes running between the ischial tuberosity (IT) and the greater trochanter (GT). The arrows and arrowhead indicate the nerve.

Technique

In-plane approach

• Identify the position of the sciatic nerve in its transverse axis in an area convenient to block between the ischial tuberosity and the greater trochanter.

• Raise a weal of local anaesthetic at the needle insertion target point (the lateral edge of the probe) with a 27G needle.

• Insert a 75 mm to 100 mm 22G nerve-block needle on the lateral end of the ultrasound probe.

• Advance the needle parallel to the probe towards the edge of the nerve (rather than attempting to hit the nerve head-on) while visualising the entire length of the needle in real time (Fig. 9.15). Nerve movement may be noticed as the needle approaches the target.

Fig. 9.15 The in-plane approach is most appropriate for this block as the needle has a long path to travel to the nerve. The needle is inserted at the lateral edge of the probe and guided in real time towards the edge of the nerve. The probe is held steadily between the greater trochanter (GT) and the ischial tuberosity (IT).

Out-of-plane approach

• Identify the position of the sciatic nerve in its transverse axis in an area convenient to block between the ischial tuberosity and the greater trochanter.

• Line up the nerve target at the midpoint of the screen. The needle insertion point will correspond to the exact centre of the transducer.

• Raise a weal of local anaesthetic at the needle insertion target with a 27G needle.

• Insert a 75 mm to 100 mm 22G nerve-block needle on the inferior (distal) side of the ultrasound probe.

• Observe local tissue and needle movement as the needle is advanced towards the target. Aiming for the side of the nerve bundle rather than the centre makes needle placement more accurate.

• Clear identification of the needle tip may require the probe to be angled back and forth (Fig. 9.16).

Fig. 9.16

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