Surface anatomy markings

When it comes to performing the nerve blocks, it is crucial to be able to palpate and accurately locate bony landmarks, since these are the reference points we use for determining the correct site for needle insertion. The following is a description of the bony landmarks used for femoral and sciatic nerve blocks. They are shown in the diagrams of the nerve block techniques.

Anterior superior iliac spine following the iliac crest (ridge of the pelvic bones) from the flanks forwards, it ends in an obvious bony prominence, at the side of the lower abdomen. This is the anterior superior iliac spine.

Pubic tubercle is the bony prominence that can be felt at the inner (medial) end of the groin crease. It is about 2 - 4 cm from the midline, at the top of the genital area.

Posterior superior iliac spine is the bony prominence at the posterior end of the iliac crest. It is directly caudal to the "sacral dimple"- that depression in the skin visible cranial to (above) the buttocks, on each side, close to the midline.

Greater trochanter this bony landmark is part of the lateral femur, just below the hip joint. It is easy to find at the top of the thigh, protruding directly laterally. With the patient on their side, it represents the highest point on the upper thigh. In obese patients try internally and externally rotating the hip, as this makes the greater trochanter more visible.

The sacral cornu are two bony prominences either side of the midline just at the top end of the natal cleft. One can readily palpate a narrow depression between them - the sacral hiatus. (see figure 3)

The ischial tuberosity is that part of the pelvic bone structure that can be felt posteriorly, on the medial side of the base of the buttock. It is the bony structure that we "sit on." 

Indications for specific nerve blocks

From the outline of the areas covered by each nerve, the reader should know which blocks would be useful in a given situation. Two points are worth emphasising. The knee joint has significant contributions from femoral, obturator and sciatic nerves and significant injury or surgery to this joint will require that all these be blocked. (For the hip, it is nearly always sufficient to perform a 3-in-1 lumbar plexus block even though there is a small contribution from the sciatic nerve.) Secondly, the area covered by the different nerves may vary considerably and if in doubt, it is best to block both main nerve trunks.

The following are some examples of the possible uses.

Femoral nerve blocks:

• operations on the anterior thigh, such as repair of large lacerations.
• pain relief for fractures of the shaft of the femur, particularly more proximal fractures.

Lumbar plexus (3-in-1) block:

• all the uses of a femoral nerve block, plus the following:
• pain relief and anaesthesia for hip injuries such as dislocations and fractures of the neck of the femur. (Major hip surgery will also require a sciatic nerve block.)
• anaesthesia for operations on the lateral thigh such as harvesting of skin grafts, or muscle biopsies.
• pain relief for injuries and operations on the knee; extensive injuries and full knee anaesthesia require a sciatic nerve block also
• this block extends the field of a simple femoral nerve block considerably and is no more difficult to perform.

Sciatic nerve block:

• pain relief or anaesthesia for injuries or operations on the sole of the foot or any of the toes, such as toe amputation (amputation of the big toe may require supplementation at the medial maleolus as well, because the distribution of the saphenous nerve occasionally extends down the medial side of the big toe).
• the distribution of the sciatic nerve means that it has fairly limited application as a block on its own and is most often combined with a femoral or 3-in-1 block.

Combined sciatic and femoral or 3-in-1 block:

• with this combination pain relief and anaesthesia can be provided for almost any injury or operation from the upper thigh downwards.
• one area sometimes not covered is the upper, inner thigh, and possibly the posterior thigh. This may be a problem with tourniquets applied high on the leg and in this situation some supplementary parenteral analgesia or sedation can be useful.
• it may be difficult to provide adequate anaesthesia for major hip surgery, although the blocks described will provide good postoperative analgesia.

Planning the dose of local anaesthetic and dealing with possible side effects

The above discussion will indicate that there are often situations in which one wishes to perform a combined sciatic and 3-in-1 block at the same time. This will necessitate using large volumes of local anaesthetic and the total dose administered may often be at the limit of recommended safe doses. It is important to be able to adjust the concentration of the solution injected when using large volumes, in order to keep the total dose at an acceptable level. (See local anaesthetic, drugs and dosage.)

Local complications of local anaesthetic blocks:

The most important is damage to the nerve. Permanent nerve damage is very rare. It may be caused by accidentally injecting local anaesthetic within the nerve itself (intraneural) or by traumatising the nerve with the needle point. Two signs of intraneural injection are severe pain on attempted injection and marked resistance to injection. (For the patient to respond to the pain of intraneural injection he or she must be awake, or only slightly sedated.) Either of these warning signs should prompt the operator to stop injecting and reposition the needle. Intraneural injection may also be less likely if a short-bevel needle is used ¹. Paraesthesia is the "electric shock-like" feeling felt as the nerve is touched by the needle. It should be a warning sign that nerve damage may occur if the needle is inserted further.

It is also possible to cause a haematoma by puncturing an artery with the needle - most commonly this will be the femoral artery. This is rarely of any significance. If the femoral artery is punctured then firm pressure applied to the site for 5 minutes will minimise the haematoma. 

Performing the nerve blocks - patient preparation and techniques

When performing any of the blocks that are described here, the steps taken to safely prepare the patient should be carefully followed.

Preparing the patient

• Consent - explain the entire procedure to the patient. This will help to relieve any anxiety and increase co-operation.
• Fasting - if an elective procedure is planned, then the patient should be fasted similar to having a general anaesthetic. This increases safety in the event that a general anaesthetic or resuscitation is required.
• Monitoring - the potential complications described in the preceding section mean that monitoring is essential. If available, ECG and blood pressure monitoring should be used. If sedation is planned then a pulse oximeter should also be used. In every case, the most useful monitor is to maintain careful, continuous observation of the patient throughout. An assistant can be invaluable in helping with this.
• Intravenous access - because of the possible complications, should be intravenous access secured before any block is performed. This also allows administration of intravenous fluids, sedative agents and resuscitation drugs if required.
• Positioning - take care with positioning the patient for the block and make sure they are as comfortable as possible as this will make the block easier to perform.
• Identify the bony landmarks - these are described in the anatomy section.
• Clean the site - the skin over the block site should be cleaned with an antiseptic agent and surrounded with sterile drapes. The operator should wash their hands and wear sterile gloves.
• Perform the block!
• Allow time for the local anaesthetic to take effect - at least 15 - 20 minutes will be required for surgical anaesthesia.With the weaker concentrations of bupivacaine, 30 - 45 minutes may be required. 

(Continued ...)

---

© World Federation of Societies of Anaesthesiologists WWW implementation by the NDA Web Team, Oxford

---