Session 2b - Muscles Of The Forarm And The Carpal Tunnel

Question 1

How are the muscles of the anterior compartment arranged?

Answer 1

There are eight muscles in the anterior compartment.
• They are arranged in three layers: superficial, middle, and deep.
• Most of them act as flexors of the wrist, fingers, or thumb.
• Most of them are innervated by the median nerve.

Question 2

What are the muscles of the superficial layer?

Answer 2

Superficial Layer
There are four muscles in the superficial layer. From lateral to medial these are:
• Pronator teres
• Flexor carpi radialis
• Palmaris longus
• Flexor carpi ulnaris

Question 3

What are superficial muscles attached to?

Answer 3

These superficial muscles are attached proximally to the medial epicondyle of the humerus. As most of these muscles are flexors, this region of the humerus is also commonly referred to as the ‘common flexor origin’.

Question 4

What are the muscles of superficial layer?

Answer 4

• Pronator teres – as its name suggests – is a pronator (of the proximal radioulnar joint), rather than a flexor.
• Flexor carpi radialis (FCR) flexes and abducts the wrist. It inserts onto the radial side of the carpus and hand, hence its name.
• Palmaris longus has a small muscle belly but a long, thin, easily recognisable tendon when present (approximately 15% of us do not have one). Its tendon inserts into the fascia of the palm of the hand.
• Flexor carpi ulnaris (FCU) flexes and adducts the wrist. It inserts onto the ulnar side of the carpus and hand. This muscle is another exception to the general rule, as it is innervated by the ulnar nerve, not the median.

Question 5

What muscle is in the middle layer?

Answer 5

There is one muscle in the middle layer: flexor digitorum superficialis (FDS). It gives rise to four tendons. Its name tells us that it is a flexor of the digits – so we can deduce that its tendons must travel beyond the wrist, into the hand and to the fingers (digits 2-5). We will look at the insertion points of the tendons in more detail in the next session. It is innervated by the median nerve, which travels between flexor digitorum superficialis and one of the deep muscles, flexor digitorum profundus.

Question 6

What muscles are in the deep layer?

Answer 6

There are three muscles in the deep layer:
• Flexor digitorum profundus
• Flexor pollicis longus
• Pronator quadratus

Question 7

Where is the flexor digitorum profundus (FDP)?

Answer 7

Flexor digitorum profundus (FDP) is a flexor of the digits and is located deep to flexor digitorum superficialis. The word ‘profundus’ is derived from the Latin word for ‘deep’. It too gives rise to four tendons, which travel into the hand and to the fingers (digits 2-5). The tendons of superficialis and profundus are closely related in the hand and digits. The muscle is interesting as it has a dual innervation.

Question 8

What does the FDP muscle give rise to?

Answer 8

• The lateral half of the muscle, which gives rise to the tendons that travel to the index and middle fingers, is innervated by the median nerve.
• The medial half of the muscle, which gives rise to the tendons that travel to the ring and little fingers, is innervated by the ulnar nerve.
• This is clinically important as injuries to either the ulnar or median nerve only affect one half of the muscle.

Question 9

What is flexor pollicis longus (FPL)?

Answer 9

Flexor pollicis longus (FPL) flexes the thumb (pollex is the Latin word for thumb). ‘Longus’ distinguishes it from another muscle, flexor pollicis brevis, which is much smaller and located within the hand.

Question 10

Where is the pronator quadratus?

Answer 10

Pronator quadratus is the deepest forearm muscle (it is considered a fourth layer by some). It is square-shaped (‘quadratus’) and is located over the distal ends of the radius and ulnar. It pronates the distal radioulnar joint.

Question 11

What are all anterior forearm muscles?

Answer 11

all the anterior forearm muscles are:
• flexors, except for pronator teres and pronator quadratus.
• innervated by the median nerve, except FCU and the medial half of FDP.

Question 12

What are the posterior forearm muscles like?

Answer 12

We can make some general statements about the posterior forearm muscles as a group:
• they are arranged in two layers: superficial and deep.
• most of them are extensors of the wrist, digits, or thumb.
• they are all innervated by the radial nerve.

Question 13

What are the superficial muscles?

Answer 13

Superficial Layer
There are seven superficial muscles. They are:
• Brachioradialis
• Extensor carpi radialis longus
• Extensor carpi radialis brevis
• Extensor digitorum
• Extensor digiti minimi
• Extensor carpi ulnaris
• Anconeus

Question 14

What is known as the common extensor origin?

Answer 14

As a general rule, these muscles are attached proximally to the lateral epicondyle of the humerus and, as most of them are extensors, their origin is known as the ‘common extensor origin’.

Question 15

What is the brachioradialis like?

Answer 15

Brachioradialis is an exception to some of the rules of posterior compartment muscles. It is located on the boundary between the posterior and anterior compartments. It originates from the humerus, proximal to the lateral epicondyle, and inserts on the distal radius. It acts as a weak flexor of the elbow joint and hence functions as an anterior compartment muscle of the arm. However, it is innervated by the radial nerve.

Question 16

What are the Extensor carpi radialis longus (ECRL)?

Answer 16

Extensor carpi radialis longus (ECRL) and brevis (ECRB) are located on the radial side of the posterior compartment. ECRL inserts onto the 2nd metacarpal and ECRB inserts onto the 3rd metacarpal, hence they extend and abduct the wrist. Brevis is the Latin word for ‘short’.

Question 17

What are the extensor digitorums like (ED)?

Answer 17

Extensor digitorum (ED) extends the digits via four long tendons that insert onto the dorsal aspects of the fingers (digits 2-5). The tendons of ED are connected by fibrous bands – this makes it difficult to fully extend the middle or ring fingers independently.

Question 18

What are extensor digiti minimi (EDM) like?

Answer 18

Extensor digiti minimi (EDM) extends the little finger via its insertion onto the dorsum of the little finger.

Question 19

What are the extensor carpi ulnaris (ECU) like?

Answer 19

Extensor carpi ulnaris (ECU) is the most medial of the superficial muscles. It extends and adducts the wrist via its insertion onto the 5th metacarpal.

Question 20

Where do these muscles travel at the wrist?

Answer 20

At the wrist, the tendons of these muscles travel under a band of tissue, the extensor retinaculum. It prevents the tendons from bowing when the wrist is extended.

Question 21

What is the anconeus?

Answer 21

Anconeus is another small muscle in the superficial posterior compartment of the forearm. It is found proximally, near the olecranon so it is sometimes considered to be part of the posterior compartment of the arm instead. Its small size and position spanning from the lateral epicondyle of the humerus to the olecranon means it acts as a very weak extensor of the elbow. It is also innervated by the radial nerve.

Question 22

What are the five deep muscle layers?

Answer 22

Deep Layer ​
There are five deep muscles. From lateral to medial they are:
• Supinator
• Abductor pollicis longus
• Extensor pollicis brevis
• Extensor pollicis longus
• Extensor indicis.

Question 23

Where do the deep muscles of the posterior compartment attach to?

Answer 23

Except for supinator, they attach proximally to the forearm bones and the interosseous membrane. Two deep muscles are exceptions to the general rules as they are not extensors:
• Supinator supinates the forearm and is attached proximally to the humerus. It wraps around the proximal part of the radius.
• Abductor pollicis longus abducts the thumb. It inserts on the 1st metacarpal.

Question 24

What do the EPB and EPL do?

Answer 24

Extensor pollicis brevis (EPB) and extensor pollicis longus (EPL) extend the thumb. EPB inserts on the proximal phalanx, so extends the metacarpophalangeal joint. EPL inserts on the distal phalanx, so is the only muscle capable of extending the interphalangeal joint of the thumb. Although their proximal muscle attachments are deep in the forearm, the tendons of EPB and APL lie superficially at the wrist as they wrap around the distal radius.

Question 25

What is the insertion of the Extensor indicis?

Answer 25

Extensor indicis (EI) inserts on the dorsum of the index finger, allowing independent extension of this digit.

Question 26

What do the muscles of the posterior forearm share?

Answer 26

To summarise, all the posterior forearm muscles are:
• extensors, except for brachioradialis, supinator and abductor pollicis longus.
• innervated by the radial nerve.
The muscles that cross the wrist also contribute to wrist extension.

Question 27

What are the main arteries of the forearm?

Answer 27

The brachial artery bifurcates in the cubital fossa into two terminal branches – the radial artery and the ulnar artery.

Question 28

What course does the radial artery take?

Answer 28

The radial artery travels along the lateral aspect of the forearm and it can usually be easily palpated at the wrist by locating the tendon of flexor carpi radialis at the level of the distal radius and palpating just laterally to this.

Question 29

What course does the ulnar artery take?

Answer 29

• The ulnar artery travels along the medial aspect of the forearm. It can also be palpated, but not easily as it is located deep to the flexor carpi ulnaris tendon.

Question 30

What do teh radial and ulnar arteries form in the hand?

Answer 30

The radial and ulnar arteries enter the hand. They anastomose in the palm of the hand to form palmar arches. These anastomoses ensure that the hand remains adequately perfused in the event that either artery becomes occluded or injured.

Question 31

What are the veins of the forearm?

Answer 31

There are both superficial and deep veins in the upper limb, which communicate with each other. Ultimately, all venous blood drains to the axillary vein. Two important superficial veins of the upper limb are the cephalic vein and the basilic vein.

Question 32

What course does the cephalic and basilic veins take?

Answer 32

The cephalic vein courses laterally in the forearm and the basilic vein courses medially.
They are typically connected to each other in the region of the cubital fossa by the median cubital vein.

They are commonly used for venepuncture and intravenous access.

The basilic vein courses proximally into the arm. It receives the deep veins of the arm to form the axillary vein. The cephalic vein courses proximally in the lateral aspect of the arm and drains into the axillary vein after passing through the deltopectoral groove.

Question 33

Where do deep veins course?

Answer 33

Deep veins accompany arteries, and they are often paired. For example, two brachial veins accompany the brachial artery. Deep veins of the upper limb ultimately drain to the axillary vein.

Question 34

What is the carpal tunnel?

Answer 34

The carpal tunnel is a narrow passageway at the wrist. Its floor and sides are formed by the carpal bones. A fibrous band called the flexor retinaculum completes the tunnel, forming the roof, and is attached to the scaphoid and trapezium laterally and to the hook of the hamate and pisiform medially.

Question 35

What a re the tendons of the anterior forearm muscles?

Answer 35

The tendons of the anterior forearm muscles that insert onto the digits travel through the carpal tunnel. These are the tendons of:
• flexor digitorum superficialis (4 tendons, to digits 2-5).
• flexor digitorum profundus (4 tendons, to digits 2-5).
• flexor pollicis longus (1 tendon to the thumb – the 1st digit).

Question 36

What is contained in the carpal tunnel?

Answer 36

The median nerve also travels through the carpal tunnel. The carpal tunnel is very narrow, so the tendons and median nerve are tightly packed into it. Any condition that further reduces space in the carpal tunnel, such as swelling of the tendons or arthritis between the joints of the carpal bones, will decrease the space and increase the pressure in the carpal tunnel. This can compress the median nerve and cause carpal tunnel syndrome (CTS).

Question 37

What does a patient with carpal tunnel syndrome present with?

Answer 37

• impaired or altered sensation over the skin of the hand supplied by the median nerve. The patient may experience tingling, numbness, or pain in the hand.
• weakness of the hand muscles supplied by the median nerve – particularly the small muscles of the thumb.

Question 38

Why is it important to treat carpal tunnel syndrome?

Answer 38

It is important to recognise and treat CTS. If left untreated, the small muscles of the thumb may atrophy and weakness may be permanent, which has serious consequences for a patient. The flexor retinaculum is divided to alleviate the compression. The radial artery, ulnar artery and ulnar nerve do not travel through the carpal tunnel.

Question 39

What are the flexor tendons at risk of?

Answer 39

The flexor tendons are at risk from lacerations over the anterior forearm and wrist. In patients with such injuries, it is important to test the movements of the wrist and fingers to ascertain whether any tendons have been injured. Patients with confirmed or suspected tendon injuries require a surgical assessment. Failure to recognise tendon injuries may leave patients with permanent impairment.

Question 40

What is wrist drop?

Answer 40

This describes an inability to extend the wrist (and fingers) due to weakness or paralysis of the posterior forearm muscles. It results from injury to the radial nerve proximal to the forearm. It is typically caused by a mid-shaft humeral fracture, as the radial nerve lies close to the bone here. Sensation is also impaired over the lateral aspect of the dorsum of the hand (i.e. in the regions of skin of the hand supplied by the radial nerve).

Question 41

What is lateral and medial epicondylitis?

Answer 41

This is inflammation of the tendinous insertions of the superficial extensor muscles in the forearm at the lateral epicondyle, or the superficial flexor muscles in the forearm at the medial epicondyle. It tends to be caused by repetitive use and strain of the muscles, such as during a tennis serve for lateral epicondylitis (‘tennis elbow’), or a golf swing for medial epicondylitis (‘golfer’s elbow’). Pain is felt around the affected epicondyle and may radiate down the forearm.

Question 42

What is ABG sampling? (Arterial blood gas)

Answer 42

The vast majority of blood tests require venous blood, which can be relatively easily taken from the superficial veins in the forearm and hand as discussed. However, to accurately assess a patient’s blood-oxygen and blood-carbon dioxide level, a sample of arterial blood is required. Most often, this is taken from the radial artery. As it a deep structure, clinicians must know the anatomy well to be able to use one hand to palpate the pulse and use the other to direct a needle into the radial artery to acquire the sample.