FOREARM
Forearm is part of the upper limb lying between the elbow and the wrist. It takes the shape of a cone which is flattened anteroposteriorly, the transverse diameter is wider close to the elbow this is as a result of the mass of the muscle of the forearm. The transverse diameter increase towards the elbow and decreases downward due to decrease in the mass of muscles.
It is enclosed by a deep fascia which enclosed all the structures. In the elbow it is strengthened anteriorly by the biccipital aponeurosis, and towards the wrist, it forms the flexor and extensor rectinaculum. It sends septa to the bones of the forearm. It also gives attachment to some of the muscles of the forearm.
The muscles of the forearm are morphologically arranged into flexor compartment muscles and extensor compartment muscles. The flexor compartment is limited medially by the post subcutaneous border of the ulnar bone which is marked superficially by the ulnar artery. The extensor compartment is limited posteromedially by the posterior border of the ulnar and laterally by the extent of the radial artery.
FLEXOR COMPARTMENT
It is made up of muscles which are arranged in superficial and deep layers. The superficial layer muscles are five in number and they include:
· Pronator teres
· Flexor carpi radialis
· Flexor carpi ulnaris
· Flexor digitorum superficialis
· Palmaris longus
In the deep group they are three muscles which include:
· Flexor digitorum profundus
· Flexor pollisis longus
· Pronator quadratus.
PRONATOR TERES
Origin:
By means of 2 heads:
1. Common flexor origin at the medial epicondyle of the humerus.
2. The deep head from medial part of the coronoid process of ulnar bone. In between the two heads runs the median nerve which could be compressed resulting to pronator symdrome.
INSERTION: Middle 1/3rd of the lateral surface of the radius.
ACTION: Pronates the forearm and also acts as a weak flexor of the elbow joint.
NERVE SUPPLY: Median nerve
FLEXOR CARPI RADIALIS
ORIGIN: Common flexor origin which is at the medial epicondyle.
INSERTION: Towards its medial half the muscle gives its tendon which passes beneath the flexor retinaculum of the forearm and get attached to the 2nd and 3rd metarcarpal bones
Action:
* Flexes the elbow joint
* With other muscle its flexes the wrist
* With radial extensors it abducts the hand.
Nerve supply: Median nerve.
PALMARIS LONGUS
Origin: Common flexor origin
Insertion: Distal part of the flexor retinaculum and apex of the palmar aponeurosis.
Action: With other muscles it flexes the wrist, it also helps to stretches the palmar aponeurosis.
Nerve Supply: Median nerve
FLEXOR CARPI ULNARIS
Two heads through which the ulnar nerve passes
Origin: Medial epicondyle, and from a wide aponeurosis arising from the upper three-fourths of the posteromedial surface of the ulna and the olecranon.
Insertion: It gets attached into the base of the fifth metacarpal but towards its insertion the pisiform bone lies within its tendon as a sesamoid bone.
Action: It is a flexor of the wrist it also act with ulna extensors to adduct the hand.
Nerve Supply: It is supplied by the ulnar nerve.
FLEXOR DIGITORUM SUPERFICIALIS
ORIGIN: It takes origin from a wider area:
· From the common flexor origin
· Medial ligament of the elbow joint
· Medial border of coronoid process
· From a fibrous arch connecting radius and ulnar
· From the whole length of the anterior oblique line of the radius.
Insertion: Towards the distal half of the arm it gives 4 tendons which are arranged two superficial for the middle and ring fingers and two deep for the index and little finger. These tendons pass beneath the flexor retinaculum and spread out in the palm, at the base of proximal phalanx each of the tendons enters the fibrous flexor sheath and then splits into two and sends decussating fibers which form a tunnel through which the tendon flexor digitorum profundus passes. The split parts unite again but at the base of the middle phalanx it separates and inserts at the shaft of the middle phalanx.
Action: It flexes the proximal interphallangeal joint and the metacarpophallangeal joint indirectly as it passes the joints. It is a weak flexor of the elbow and wrist joints.
Nerve Supply: Median nerve.
DEEP MUSCLE OF THE FLEXOR COMPARTMENT OF THE FOREARM:
FLEXOR DIGITORUM PROFUNDUS
Origin: From the medial surface of the olecranon. From the upper three – fourth of the medial and anterior surfaces of the ulnar and from the ulnar half of the interosseous membrane. It gives up four tendons which passes beneath the flexor retinaculum at the palm they separate and go to the medial four digits.
Insertion: It passes deep to the tube formed by the decussating fibers and get attached to the base of the distal phalanx.
Action: Directly it flexes the distal interphallangeal joint. Indirectly it flexes the distal interphallengeal joints.
Nerve Supply: It is supplied by both the ulnar nerve and median nerve. The ulnar nerve supply the part of the muscle fibers that go to the little and ring fingers
FLEXOR POLLICIS LONGUS
Origin: From the anterior surface of radial bone between the area of attachment of pronator teres and the area of attachment of pronator quadratus. Sometimes it extends upward to the radial tuberosity and from the radial half of the interosseous membrane.
Insertion: It passes beneath the flexor retinaculum and inserts at the base of the distal phalanx of the thumb.
Action: Directly it flexes the distal interphallangeal joint of the thumb. It is also involved in other pollicis movement.
PRONATOR QUADRATUS:
ORIGIN: Arises from the distal ¼ of the antero-lateral aspect of the ulnar at the pronator ridge.
Insertion: Distal ¼ anterior border and surface of the radius. It extends deeper into a triangular area above the ulnar notch and the styliod process.
Action: Principal pronator muscle of the forearm while the deeper fibers help to maintain the apposition of the lower ends of the radius and ulna.
Nerve Supply: Median nerve.
EXTENSOR COMPARTMENT
It is limited medially by the posterior border of the subcutaneous part of ulnar bone and laterally by the course of the radial artery. It contains 12 muscles which are arranged into superficial and deep groups. The superficial group is further subdivided into a lateral group and a posterior group. Some of the superficial group muscles which are about seven in number share a common extensor tendon which is attached anterior to the lateral epicondyle, others originate from the lateral supracondylar ridge and from the lateral intermuscular septum. The lateral group of superficial extensor muscles include:
1. Brachioradialis
2. Extensor carpi radialis longus
3. Extensor carpi radialis brevis
POSTERIOR SUPERFICIAL MUSCLE
1. Extensor Digitorum
2. Extensor Digiti minimi
3. Extensor Carpi Ulnaris
4. Anconeus
Deep group of extensor muscles include:
1. Abductor pollicis longus
2. Extensor Pollicis longus
3. Extensor pollicis brevis
4. Extensor indicis
5. Supinator muscle.
BRACHIORADIALIS
Origin: It is an extensor muscle with extensor nerve developmentally; it is an extensor compartment muscle but acts as a flexor muscle. It takes origin from the upper lateral 2/3rd of the supracondylar ridge. It passes down overlying the radial nerve.
Insertion: It gets attached at the base of the styloid process of the radial bone.
Action: Flexes the elbow joint though it can act in active extension and active flexion of the elbow joint.
Nerve Supply: Radial nerve.
EXTENSOR CARPI RADIALIS LONGUS
Origin: From the lower 1/3rd of the supracondylar ridge. And from the lateral intermuscular septum.
Insertion: It forms a flat tendon which is attached to the dorsal surface of the base of the 2nd metacarpal bone.
Action: It acts as a wrist extensor and its very important in the making of a fist.
Nerve supply: It is supplied by the radial nerve.
EXTENSOR CARPI RADIALIS BREVIS:
Origin: From the common extensors origin and run beneath the extensor carpi radialis longus.
Insertion: It forms a flat tendon which is attached to the 3rd metacarpal bone, It has a bursa on its superior and inferior ends. The inflammation of the superior end is referred to as tennis elbow.
Action: Extension of the wrist joint and it is also important in fist formation.
Nerve supply: It is supplied by the radial nerve.
EXTENSOR DIGITORUM:
Origin: It arises from the common extensor origin which is the anterior surface of the lateral epicondyle of the humerus.
Insertion: It gives rise to four tendons for each medial four digits which passes beneath the extensor retinaculum. On getting to the base of the corresponding proximal phalanx, each of the tendon divides into three slips the central slip get inserted into the middle phalanx, the other slips will diverge along the central slip, each will accept attachment of the interosseous and lumbrical muscles. This is referred to as dorsal extensor expansion. Towards the middle phalanx they converge and get attached to the base of the distal phalanx.
Action: Extends the interphallangeal joints.
EXTENSOR DIGITI MINIMI
Origin: It takes origin from the common extensor origin.
Insertion: It becomes tendinous passing beneath the extensor retinaculum up to the base of the 5th metacarpal bone, where it divides into two as it pass to the digiti minimi to join the dorsal expansion of the digit.
Action: extension of the little finger.
Nerve Supply: It is supplied by the radial nerve.
EXTENSOR CARPI ULNARIS
Origin: It takes origin from the common extensor tendon and from a common aponeurosis which it shares with the flexor carpi ulnaris.
Insertion: It is inserted into the base of the 5th metacarpal bone.
Action: Adduction of the wrist. It is also important in making a fist.
Nerve supply: It is supplied by the radial nerve.
ANCONEUS
Origin: Arises from the lowest part of the lateral epicondyle and is inserted in to the proximal part of the lateral side of the olecranon.
Action: Weak cubital extensor. (weak extensor). It is very minute in action. It is supplied by the radial nerve.
DEEP GROUP OF MUSCLES:
ABDUCTOR POLLICIS LONGUS
Origin: It takes origin from an oblique area on the radius and ulna and from the intervening interosseous membrane.
Insertion: It is inserted into the dorsal aspect of the base of the 1st metacarpal bone by means of a round tendon which splits into three separate bands. One of the bands extends beyond the metacarpal bone and then gives attachment to the abductor pollicis brevis.
Action: It helps in abducting and flexing the wrist joint it also abducts the thumb.
Nerve supply: It is supplied by the radial nerve.
EXTENSOR POLLICIS BREVIS
Origin: From the radial bone below the origin of the abductor pollicis longus and the adjoining interosseous membrane.
Insertion: It is inserted into the base of the thumb’s proximal phalanx by means of a slender tendon.
Action: It helps to extend the thumb’s proximal phalanx.
Nerve supply: It is supplied by the radial nerve.
EXTENSOR POLLICIS LONGUS
Origin: It originated from the ulnar bone also distal to the origin of the abductor pollicis longus and the adjoining interosseous membrane.
Insertion: It is inserted at the base of the distal phalanx.
Action: It extends the terminal phalanx of the thumb
EXTENSOR INDICIS
Origin: From the ulnar bone just distal to the origin of the extensor pollicis longus and the adjoining interosseous membrane and it passes through the extensor retinaculum beneath the extensor digitorum tendon.
Insertion: Its tendon joins the dorsal expansion of the index finger.
SUPINATOR
Origin: Arises by means of two heads in which the posterior interosseous nerve passes. The superficial head arises from the distal part of the lateral epicondyle and from the anterior part of the radial collateral ligament while the deep head arises from the supinator crest and fossa of the ulna.
Insertion: From the ulnar the fibers meet to be inserted in the radius between the anterior and lateral oblique line.
Action: It supinates the forearm especially when the elbow is extended.
CUBITAL FOSSA
It is an intermuscular space which is a depressed area found anterior to the elbow. It has a lateral and medial border, apex, base, a roof and floor and content.
· The medial boundary is formed by the lateral border of pronator teres.
· The lateral boundary is formed by the medial border of brachioradialis muscle
· The apex lies inferior and corresponds to the point where the brachioradialis over lap the pronator teres.
· The base lies superiorly and corresponds to an imaginary line passing between the lateral epicondyle and medial epicondyle. It is referred to as intercondylar line.
· The floor is formed superiorly by the brachialis and supinator muscle.
· Roof is formed by the deep fascia which is straightened medially by lacerta fibrosus (biccipital aponeurosis).
CONTENTS
Superficial to the roof runs the median cubital vein, the deep contents include from medial to lateral; the median nerve, brachial artery and its terminal branch the ulnar and radial artery, the tendon of biceps and radial nerve.
APPLIED ANATOMY
· It is the most convenient site for measuring blood pressure.
· The median cubital vein is the vein of choice for intravenous infussion
FLEXOR RETINACULUM
The flexor retinaculum is a tough fibrous band which is the condensed part of the deep fascia of the forearm which is found uniting the lateral and medial aspect of the carpal bones. It acts as a restraining band preventing the bowing out of the long flexor tendons around the wrist joint. Flexor retinaculum is attached medially to the pisiform bone and hook of hammate while laterally it is separated into two laminae; a superficial lamina which attaches to the tubercle of the scaphoid and trapezium and a deep lamina which attaches into the groove formed by the trapezius. This deep lamina and the groove formed by the trapezium form a separate tunnel for the tendon of flexor carpi radialis and its synovial sheath. Superiorly the retinaculum is continuous with the deep fascia of the forearm while inferiorly it is continuous with the deep fascia of the palm and the centrally placed palmar aponeurosis.
The flexor retinaculum is about 2 – 3cm in length and width. The bridging of the flexor retinaculum over the carpal bones gives rise to an osteofibrous tunnel which is known as the carpal tunnel. Through this tunnel passes the tendon of flexor digitorum profundus and superficialis, flexor pollicis longus and the median nerve while superficial to the flexor retinaculum passes the tendon of palmaris longus which is inserted into its distal half and the apex of the palmar aponeurosis. The tendon of flexor carpi ulnaris which is inserted into the base of the fifth metacarpal bone via the pisiform bone, the ulnar nerve and ulnar artery which lie lateral to the tendon of flexor carpi ulnaris and the palmar cutaneous branch of ulnar nerve and median nerve all run superficially over the flexor retinaculum.
APPLIED ANATOMY
CARPAL TUNNEL SYNDROME:
The inflammation of the synovial membrane which accompanies the long flexor tendons through the flexor retinaculum leads to compression of the median nerve within the carpal tunnel; this is known as carpal tunnel syndrome. The carpal tunnel is well crowded and the median nerve could easily be compressed in infections involving the synovial sheaths as a result of this compression on the median nerve there will be weakness in the action and wasting of the thenar muscles, there will also be loss of sensation around the radial two-third digits. The pressure caused by inflammation of the synovial sheaths can be relieved by incision of the flexor retinaculum. It should be noted that during carpal tunnel syndrome there is no loss of sensation over the skin of the thenar eminence, this is due to the fact that the median nerve has already given off a palmar cutaneous branch before entering the carpal tunnel.
EXTENSOR RETINACULUM
This is a broad band of condense part of the deep fascia of the forearm. It is about 2.5cm wide and lies obliquely across the extensor surface of the wrist joint.
ANATOMICAL SNUFF BOX
It is a depression found at the root of the thumb especially when the thumb is fully extended. It is bounded lateral by the extensor pollicis brevis and abductor pollicis longus and medial by the extensor pollicis longus, the floor is formed by the scaphoid and trapezium above is the styloid process of the radius and below is the base of the 1st metacarpal bone.
Contents:
Superficial radial artery
Cutaneous branch of the radial nerve
