Posterior Forearm and Dorsal Hand

Question 1

Posterior Compartment of the Forearm Rules

Answer 1

• Superficial group (cross elbow) and deep group (most do not cross elbow)
• Innervation: radial nerve (C5-8)
• Actions:
  1) Extend wrist joint and fingers (e.g., at MCP and IP joints)
  2) Abduct (radial deviation)/Adduct (ulnar deviation) wrist joint
  3) Supinate forearm (palm up)
  4) Extend, abduct, reposition the thumb (e.g., at CMC, MCP, and IP joints)

Question 2

Describe the Extensor retinaculum

Answer 2

• Band of deep fascia that holds long extensor tendons in place as they cross the wrist
• Tendons are surrounded by synovial sheaths (allow tendons to move freely)
• Synovial (“ganglionic”) cysts (fluid-filled swellings) can develop within the synovial sheaths; the cause is unknown, but movement enlarges the cysts. Common site of occurrence: synovial sheath of the extensor carpi radialis brevis tendon.

Question 3

Superficial Group, Posterior Compartment of Forearm Rules

Answer 3

• Innervation: radial nerve
• Common origin: Lateral Epicondyle of Humerus
• Lateral epicondylitis (tennis elbow): inflammation of common extensor origin and periosteum of the lateral epicondyle

Question 4

What are the muscles of the Superficial Group, Posterior Compartment of Forearm?

Answer 4

1) Brachioradialis
2) Extensor carpi radialis longus & Extensor carpi radialis brevis
3) Extensor carpi ulnaris
4) Extensor digitorum
5) Extensor digiti minimi

Question 5

Brachioradialis Insertions and actions

Answer 5

Insertion: distal radius
Action: flex elbow joint (exception to posterior compartment action rule)
Note: brachioradialis reflex test can be used to assess the radial nerve and the C6 spinal level

Question 6

Extensor carpi radialis longus & Extensor carpi radialis brevis Insertions and actions

Answer 6

Insertion: radial (lateral) side of hand
Action: extend and abduct wrist joint

Question 7

Extensor carpi ulnaris Insertions and actions

Answer 7

Insertion: ulnar (medial) side of hand
Action: extend and adduct wrist joint

Question 8

Extensor digitorum Insertions and actions

Answer 8

Insertion: forms extensor expansions of digits 2-5
Action: extend digits 2-5 (MCP, PIP, and DIP joints), extend wrist joint

Question 9

Extensor digiti minimi Insertions and actions

Answer 9

Insertion: helps form extensor expansion of digit 5
Action: extend digit 5 (MCP, PIP, and DIP joints)

Question 10

Deep Group, Posterior Compartment of Forearm Rules and muscles

Answer 10

• Origins: ulna, radius, interosseous membrane
• Innervation: radial nerve
  1) Abductor pollicis longus
  2) Extensor pollicis brevis
  3) Extensor pollicis longus
  4) Extensor indicis
  5) Supinator

Question 11

Describe Abductor pollicis longus

Answer 11

Insertion: base of MC1
Action: abduct and extend thumb (at CMC joint)

Question 12

Describe pollicis brevis

Answer 12

Insertion: proximal phalanx of pollux
Action: extend thumb (CMC and MCP joints)

Question 13

Describe pollicis longus

Answer 13

Insertion: distal phalanx of pollux

Action: extend thumb (CMC, MCP and IP joints)

Question 14

Describe Extensor indicis

Answer 14

Insertion: extensor expansion of digit 2
Action: extend digit 2 (MCP, PIP and DIP joints)

Question 15

Describe Supinator

Answer 15

Insertion: proximal radius
Action: supinate forearm (when elbow is flexed or extended)
Note: Biceps brachii also supinates the forearm, but it requires a flexed elbow

Question 16

Describe the Dorsal Hand

Answer 16

-Long extensor tendons only (no intrinsic muscles)
-Dorsal interossei are not part of this compartment, but can be seen deep to the extensor tendons; the dorsal interossei reside in the deepest compartment of the palmar hand
-Anatomical Snuff Box
o Depression proximal to the thumb, on the dorsal hand
o Most prominent when thumb is fully extended
o Lateral border: tendons of abductor pollicis longus and extensor pollicis brevis
o Medial border: tendon of extensor pollicis longus
o Floor: scaphoid and trapezium
o Contents: radial artery
The scaphoid bone can fracture as a result of landing on an outstretched hand. This type of fracture presents as tenderness/pain upon palpation of the anatomical snuff box.

Question 17

Describe the Anatomical Snuff Box

Answer 17

• Depression proximal to the thumb, on the dorsal hand
• Most prominent when thumb is fully extended
• Lateral border: tendons of abductor pollicis longus and extensor pollicis brevis
• Medial border: tendon of extensor pollicis longus
• Floor: scaphoid and trapezium
• Contents: radial artery
• The scaphoid bone can fracture as a result of landing on an outstretched hand. This type of fracture presents as tenderness/pain upon palpation of the anatomical snuff box.

Question 18

Describe the path of the Radial nerve

Answer 18

• Travels in radial groove of the humerus with the deep artery & vein of the arm
• Innervates all the muscles in the posterior compartment of the arm
• Innervates the skin of the posterior arm
• Posterior cutaneous nerve of the forearm branches off in the arm (innervates the skin on the posterior forearm)
• Divides into deep and superficial branches near the elbow
• Deep branch (called the posterior interosseous nerve once it emerges from supinator) innervates the muscles of the posterior compartment of forearm
• Superficial branch provides sensory innervation for skin of lateral wrist (including skin over the anatomical snuff box) and lateral aspect of dorsal hand

Question 19

Describe what would happen if you got a Proximal Lesion of the Radial Nerve (e.g., in the axilla):

Answer 19

1) Loss of elbow extension
2) Weakened elbow flexion (biceps brachii and brachialis still intact as they are innervated by the musculocutaneous nerve)
3) Weakened supination (biceps brachii still intact as it is innervated by the musculocutaneous nerve)
4) Loss of extension at the wrist and MCP joints (“wrist drop”)
5) Weakened abduction and adduction of the wrist (flexor carpi radialis (median nerve) and flexor carpi ulnaris (ulnar nerve) still intact)
6) Weakened extension at the PIP and DIP joints of digits 2-5 (lumbricals still intact as they are innervated by the median nerve or the ulnar nerve)
7) Weakened abduction of the thumb (abductor pollicis brevis (median nerve) still intact) and loss of thumb extension
8) Skin anesthetized along posterior arm, forearm, and dorsolateral wrist and hand

Question 20

Blood Vessels of the Posterior Forearm (Radial and its branches)

Answer 20

• Radial artery supplies the lateral aspect of the posterior forearm (it also supplies the lateral aspect of the anterior forearm)
• Posterior interosseous artery (branch of common interosseous artery, which is a branch of the ulnar) supplies superficial and deep aspects of the posterior forearm
• Anterior interosseous artery (also a branch of common interosseous artery) supplies the deep aspect of the distal posterior forearm and the dorsal hand (it also supplies the deep aspect of the anterior forearm)

Question 21

Blood Vessels of the Posterior Dorsal Hand

Answer 21

• Ulnar, radial, and anterior interosseous arteries form the dorsal carpal arch
• Dorsal carpal arch provides blood to the digits, but the palmar arches (superficial and deep) provide the majority of the blood to the digits

Question 22

Describe the veins of the Posterior Forearm and Dorsal Hand

Answer 22

• Deep veins accompany arteries and have the same names (e.g., radial, posterior interosseous); typically paired and communicate with superficial veins
• Note: most of the hand is drained by the superficial veins on the dorsal hand which eventually drain into the cephalic and basilic veins