Regions of the Upper Limb
The upper limb is essential for our daily functioning. It enables us to grip, write, lift and throw among many other movements. The upper limb has been shaped by evolution, into a highly mobile part of the human body. This contrasts with the lower limb, which has developed for stability. In this article we will discuss the regions of the upper limb, as well as the individual components and essential functions. This will be joint by joint, with other structures mentioned throughout.
The scapula bone is otherwise known as the shoulder blade. Although the scapula is located on the posterior side of the body, it is not a part of the human back. It is classified as a part of the upper limb as it is so important for its functioning.
The scapula has a spine, as well as two fossae that lie above and below it. These form the origin of the supraspinatus and infraspinatus respectively. Supraspinatus is a rotator cuff muscle that increases the stability of the humeral head, as well as an abductor of the shoulder. The infraspinatus is a lateral rotator of the shoulder.
The scapular muscles allow us to elevate our arm beyond 90 degrees of abduction. The wing-nut like movement of the scapula allows us to elevate our upper limb above our heads. The rotation of the scapula occurs through precise and coordinated contraction of several muscles including the rhomboids (which are attached to its medial border) in addition to trapezius and levator scapulae among others (both of which are attached to the superior border). The lateral border of the scapula also gives rise to teres major and minor. These muscles are adductors of the humerus.
This region has a rich blood supply via the anastomosis of several arteries including the transverse cervical, dorsal scapular and subscapular. The nerve supply is from the suprascapular nerve as well as the upper and lower subscapular nerves.
The Scapulothoracic joint
This is a physiological joint, as the scapula has no bony attachments posteriorly, and is held in place purely by muscles. These include the rhomboids and trapezius among others. The rotation and elevation of the scapula is essential for the abduction of the upper limb.
The shoulder region must be seen as a complex of several joints, rather than an individual joint. Classically the glenohumeral joint is what anatomists mean when referring to the shoulder joint.
This is clearly a joint designed for mobility rather than stability. The glenoid fossa is a shallow dish like surface, deepened by the labrum that is attached at its periphery. The labrum is said to provide a suction effect to the head of the humerus, as well as deepening the joint socket and therefore increasing stability. The joint is supplied by the medial and lateral circumflex arteries, which branch from the brachial arteries.
This joint is formed between the thin surface of the clavicle laterally and the acromion process of the scapula. It is a gliding synovial joint, which ensures functional continuity between the scapula region and the thoracic wall.
Learning point: The rounded side of the clavicle is the medial surface, and its thin side is the lateral surface. The underside is marked laterally by the conoid tubercle and trapezoid line which give rise to their respective ligaments.
The manubrium is the superior part of the sternum and is attached to the clavicle laterally. It is a highly mobile synovial joint that allows for both anterior posterior and medial lateral movement. This enables the upper limb to reach a large area.
All these joints are best seen as a continuous functional unit rather than a series of isolated joints. They influence each other and work in coordination to mobilize the upper limb effectively.
In a colloquial sense, the term ‘arm’ refers to the whole upper limb. In an anatomical sense, it refers to the upper arm only i.e. above the elbow. The brachial artery, a direct continuation of the axillary, supplies the region. It becomes the brachial at the lower border of teres major.
The elbow joint refers to articulation of the humerus and the two forearm bones. The ulna articulates with the medial condyle of the humerus known as the trochlear. The ulna has the pronounced olecranon process on its posterior surface, which fits into the olecranon fossa of the humerus posteriorly to limit extension. The triceps is the three-headed muscle that extends the elbow.
The head of the radius articulates with the capitulum of the humerus. The neck of the radius also forms part of the proximal radioulnar joint, which is where supination and pronation of the hand occurs. The radius acts as a wheel, and the annular ligament that encircles the radial neck and attaches it to the ulna holds it in place when both movements occur. There are named muscles that perform these movements i.e. pronator teres, supinator and pronator quadratus. Anconeus is a small muscle at the back of the elbow that abducts the ulna in pronation.
The forearm is the region between the elbow and wrist joint. It comprises two bones, the radius and the ulna. The brachial artery divides into the radial and ulnar, which run down the forearm on their respective sides. The ulnar artery also gives rise to the anterior and posterior interosseus arteries that supply the deep muscles of the flexor and extensor compartment. The nerve supply is from the ulnar and median nerves (mainly median).
The proximal part of the radius is the head, and the distal part is the base, and vice versa for the ulna. A tough interosseus membrane connects the two bones and distributes the load. The fibers of this membrane are very tough as well as flexible, and are orientated in an obliquely downward direction. There is also a fibrous structure called the oblique cord, which is thought by some to be vestigial, but does offer some structural advantage in connecting the bones.
The wrist joint can be divided into three main parts. There is the radiocarpal joint, the ulnocarpal joint, and the midcarpal joints. The joints derive their arterial supply from the dorsal and palmar carpal arches.
The radius articulates with the scaphoid laterally and the lunate medially. The ulna articulates with a triangular pad of fibrocartilage, which is interposed between it and the triquetrum bone. When extended, the wrist joint creates torque for the long flexors to perform their movements with higher strength. The mid carpal joint is between the proximal and distal row of carpals. It is active in early flexion and extension as well as radial and ulnar deviation. In late flexion and extension, the scaphoid bridges the proximal and distal carpal rows, which consequently move in unison.
The hand is an exquisite example of engineering. The complexity of its movements, as well as its functional use make it perfect for working with tools. The blood supply is derived from the superficial and deep palmar arches from the ulnar and radial arteries respectively. The nerve supply is from the ulnar and median nerves (mainly ulnar).
The carpometacarpal joints are simple synovial joints. But the first carpometacarpal joint is more unique. It is a saddle joint that allows the thumb to oppose with the other fingers. It also enables the thumb to move with a great deal of freedom. Our thumb is much longer when compared to other primates, which signifies the move our ancestors made from the trees to the planes of the African savannah. Climbing was no longer as useful, and throwing, hammering and gripping generally were of more use to our species. The elongated, powerful and highly mobile thumb reflects this.
The other joints to consider are the metacarpophalangeal (MCP) and interphalangeal (IP) joint. The MCP joints allow for flexion, extension, abduction and adduction. The IP joints, however, only allow for flexion and extension. The proximal and distal interphalangeal joints are connected by a collateral ligament, which means that extending or flexing your DIP causes your PIP to move in the same way.
The long thoracic nerve (nerve roots C5, C6, C7) innervates serratus anterior. This muscle protracts the scapula, i.e. it moves the scapula forwards. This muscle is well developed in boxers who need to maximize the reach of their punches and therefore extend their arms and protract their scapulas. When the long thoracic nerve is damaged, the scapula comes away from the thorax, causing a winged appearance.
The clavicle has a narrowing in its middle third. This is where the bone is most commonly fractured. The bone is close to the surface and therefore quite vulnerable. If the fractured segment is left loose, it could shear the subclavian vein (which lies anterior to the scalenus anterior) and result in fatal exsanguination. The subclavian artery lies behind the muscle, and therefore is not as vulnerable.
Radial nerve palsy/Saturday night palsy
The radial nerve supplies the extensor compartment of the arm and forearm. If an individual undergoes axillary compression e.g. while resting their arm on a chair in a drunken state, the nerve may also be compressed. This results in wrist drop, as the wrist extensors are not functioning. Gravity is able to extend the elbow.
Carpal tunnel syndrome
The median nerve supplies the thenar eminence and the first two (radial side) lumbricals in the hand. It runs in the carpal tunnel beneath the flexor retinaculum. If it becomes compressed here, the patient suffers thenar muscle wasting, as well as a burning and tingling sensation in the radial 3 and a half fingers.
The fracture usually occurs from a fall on an outstretched hand. This bone gets its blood supply from a nutrient branch of the radial artery given off in the anatomical snuffbox. The artery enters its distal pole and feeds proximally. This means a fracture in its middle third (the narrowest and therefore most vulnerable section) results in avascular necrosis of the proximal segment. Symptoms include tenderness in the anatomical snuffbox.
When the palmar fascia is thickened, it causes a claw appearance of the medial two fingers and sometimes more. It has an unknown aetiology and is associated with Peyronie’s disease.