Regional anesthesia (RA) techniques have become increasingly important in modern anesthetic practice. For anesthesiologists who perform RA procedures infrequently, maintaining a working knowledge of relevant anatomy is crucial for effective and safe practice. This summary aims to provide an overview of essential anatomical concepts and structures important in common RA procedures.

1. The Neck

The Origin of the Brachial Plexus: The Roots and Trunci

The brachial plexus provides motor and sensory innervation of the upper limb. It commonly originates from the ventral rami of the spinal nerves C5 to C8 and the first thoracic spinal nerve, T1. In some cases, there may be contributions from C4 (prefixed plexus) or T2 (postfixed plexus). These anterior rami are more commonly known as ‘roots’, which is the common term used in the literature describing ultrasound guided regional anesthesia. These nerve roots emerge from the intervertebral foramina and pass between the anterior and middle scalene muscles. From the roots the three trunks of the brachial plexus are formed in the posterior triangle of the neck: C5 and C6 unite to form the upper trunk, C7 continues as the middle trunk whereas C8 and T1 join to form the lower trunk. In order to perform awake shoulder surgery, the C5 and C6 roots need to be blocked.

2. Periclavicular area

Brachial plexus: Divisions and Cords

Each of the three trunks (upper, middle, and lower) splits into anterior and posterior divisions. This division occurs behind the clavicle. Anterior division primarily innervates the flexor compartments and posterior division mainly supplies the extensor compartments of the upper limb. The divisions then regroup to form three cords, named according to their relationship to the axillary artery. The cords are formed at the lateral border of the first rib and extend into the axilla. The lateral cord is formed by the anterior divisions of the upper and middle trunks (C5-C7) and gives rise to the musculocutaneous nerve and lateral root of the median nerve, the medial cord, which is a continuation of the anterior division of the lower trunk (C8-T1) and contributes to the ulnar nerve, medial root of the median nerve, and medial cutaneous nerves of the arm and forearm and finally the posterior cord which is formed by the posterior division (C5-T1) and forms the axillary and radial nerves.

3. The Arm

Major branches of the brachial plexus

In the axilla, the major branches of the brachial plexus are arranged around the axillary artery. The musculocutaneous nerve typically pierces the coracobrachialis muscle and runs between the biceps and brachialis. The median nerve lies anterior to the axillary artery. The ulnar nerve is positioned medial to the axillary artery and the radial nerve, the largest branch, is found posterior to the axillary artery, and courses into the posterior compartment of the arm through the triangular interval. The axillary nervewraps around the surgical neck of the humerus with the posterior circumflex humeral vessel. Musculocutaneous nerve innervates biceps brachii, brachialis, and coracobrachialis and provides sensory innervation to the lateral forearm. The median nerve provides motor supply to the most anterior forearm flexors and thenar muscles and sensory innervation to the lateral palm, thumb, index, middle, and lateral half of ring finger. The ulnar nerve innervates hypothenar muscles, interossei, and some intrinsic hand muscle and provides sensory supply to the medial palm, little finger, and medial half of ring finger. The radial nerve provides motor supply to the posterior arm and forearm extensors and sensory innervation to the posterior arm and forearm, as well as areas of hand dorsolaterally. However, recent research has shown that the sensory innervation of the hand seems to be very variable, thereby complicating the testing of a success of a regional anesthetic block. Finally, the axillary nerve innervates the deltoid and teres minor muscles and provides sensory supply to the lateral shoulder area.

4. Thoracic wall

The pectoralis major muscle forms the most superficial layer, originating from the clavicle, sternum, and upper ribs, and inserting onto the humerus. Deep to this lies the pectoralis minor, originating from ribs 3-5 and inserting on the coracoid process. The serratus anterior muscle originates from the lateral aspects of the upper 8-9 ribs and inserts on the medial border of the scapula. It lies on the lateral chest wall, deep to the pectoralis muscles.

The lateral pectoral nerve (C5-C7) innervates pectoralis major, medial pectoral nerve (C8-T1) pectoralis minor and partially pectoralis major muscles, whereas the long thoracic nerve (C5-C7) provides motor supply to the serratus anterior muscle. Note that these nerves originate from the brachial plexus. Intercostal nerves, the ventral rami of thoracic spinal nerves T1-T11, run in the intercostal spaces between ribs and provide sensory innervation to the chest wall.

The axillary vessels and their branches are important landmarks in the ultrasound guided thoracic wall blocks. The thoracoacromial artery emerges from the axillary artery, pierces the clavipectoral fascia, and divides into four branches (acromial, clavicular, deltoid, and pectoral). The pectoral branch is particularly relevant to the PECS I Block, running between pectoralis major and minor. The lateral thoracic artery descends along the lateral border of pectoralis minor, supplying it and the lateral chest wall and is an important landmark for PECS II block. The long thoracic artery, being a key reference for the serratus anterior block, runs along the lateral chest wall, parallel to the long thoracic nerve and supplies the serratus anterior muscle. Internal thoracic artery is a notable mention, while not directly in the block area, its perforating branches contribute to breast and anterior chest wall blood supply.

5. Abdominal wall

The abdominal wall consists of several muscles, the most superficial is the external oblique muscle, followed by the internal oblique and finally the deepest muscle, the transversus abdominis. Rectus abdominis muscles are paired vertical muscles, separated by the linea alba. Key nerves in this area include the thoracoabdominal nerves (T7-T12), that run between internal oblique and transversus abdominis muscles as well as the Ilioinguinal and iliohypogastric nerves (L1) that course through the transversus abdominis plane in the lower abdomen. The key vessels in this area are the superior and inferior epigastric vessels, which run deep to the rectus abdominis muscle and the deep circumflex iliac vessel, that courses along the inner aspect of the iliac crest. Two fascial planes hold a significance for the occasional regional anesthetist: transversus abdominis plane (TAP) between internal oblique and transversus abdominis muscles and the rectus sheath which surrounds the rectus abdominis muscle.

6. Inguinal crease and the thigh

In lieu of the modern surgery and its push to an early ambulation, the femoral block itself has lost some of its appeal, however anatomy around the inguinal continues to play an important role in the armamentarium of an occasional regional anesthetist.

The Inguinal ligament which runs from the anterior superior iliac spine to the pubic tubercle, represents an important landmark in the performance of the suprainguinal fascia iliaca block as well as the PENG block. The lateral femoral cutaneous nerve passes under or through the inguinal ligament lateral to the anterior superior iliac spine. Two fasciae of the utmost importance in terms of the blocks performed in this area: the fascia lata and the deeper fascia iliaca, which envelops the iliacus and psoas muscles. Femoral nerve lies deep to the fascia iliaca, lateral to the femoral artery, medial to the artery lies the femoral vein.

An important structure in the transition from the inguinal crease to the thigh is the femoral triangle, bounded by the inguinal ligament superiorly, sartorius laterally, and adductor longus medially. The floor of the femoral triangle is formed by the iliopsoas muscle and the roof by the fascia iliaca. Key structures at the tip of the femoral triangle are the femoral artery, a central, hyperechoic structures on ultrasound, medial to it, usually compressible with the ultrasound probe, lies the femoral vein.

7. Popliteal fossa and the foot

The popliteal fossa is a diamond-shaped space behind the knee, bounded superolaterally by the biceps femoris muscle, superomedially by the semimembranosus and semitendinosus muscles, inferolaterally by the lateral head of gastrocnemius muscle and inferomedially by the medial head of gastrocnemius muscle. Within the popliteal fossa, the sciatic nerve typically bifurcates into tibial and common peroneal nerves, though the level of division can vary considerably, anywhere from the lower thigh to the popliteal fossa. The tibial nerve is larger and lies more superficially and is a continuation of the sciatic nerve’s medial component. Common peroneal nerve is smaller and is located lateral to the tibial nerve. The popliteal vein typically lies between the sciatic nerve and the popliteal artery, which is the deepest ultrasound landmark structure, lying closest to the femur bone.

The tibial nerve provides motor innervation to the muscles in the posterior compartment of the leg as well as most of the intrinsic muscles of the foot. Sensory supply of the tibial nerve stretches over the posterior aspect of the leg, sole and lateral aspect of the foot, as well as the toes, with the exception of the webspace between the 1st and 2nd toes, which is innervated by the deep peroneal nerve. This is also the only area on the lower limb where the success of the sensory block of the deep peroneal nerve can be tested, as this nerve is a predominantly motor nerve, innervating the anterior compartment and dorsiflexors of the foot and has a limited sensory distribution.

The superficial peroneal nerve provides motor innervation to the peroneal muscles (evertors) and extensive sensory innervation to the dorsum of the foot, with the exception of the lateral side of the fifth toe, typically innervated by the sural nerve, which is formed from the tibial and the common peroneal nerve and is a purely sensory nerve.

An important exception to the predominantly sciatic sensory supply to the lower leg are the medial aspect of the leg from knee to ankle and in up to 10% of the population the medial aspect of the foot to the base of the big toe, which are innervated by the saphenous nerve, the terminal branch of the femoral nerve.

A solid foundation in relevant anatomy is essential for the safe and effective practice of regional anesthesia, even for occasional practitioners. By focusing on key anatomical concepts and structures, anesthesiologists can enhance their ability to perform successful nerve blocks, interpret imaging findings, and manage potential complications. Ongoing anatomical education and review should be an integral part of maintaining competence in regional anesthesia techniques.