Overview of Upper Limb
The upper limb is characterized by its mobility and ability to grasp, strike, and conduct fine motor skills (manipulation). These characteristics are especially marked in the hand when performing manual activities, such as buttoning a shirt.
Synchronized interplay occurs between the joints of the upper limb to coordinate the intervening segments to perform smooth, efficient motion at the most workable distance or position required for a specific task. Efficiency of hand function results in large part from the ability to place it in the proper position by movements at the scapulothoracic, glenohumeral, elbow, radio-ulnar, and wrist joints.
The upper limb consists of four major segments, which are further subdivided into regions for precise description (Figs. 6.1 and 6.2):
- Shoulder: proximal segment of the limb that overlaps parts of the trunk (thorax and back) and lower lateral neck. It includes the pectoral, scapular, and deltoid regions of the upper limb, and the lateral part (greater supraclavicular fossa) of the lateral cervical region. It overlies half of the pectoral girdle. The pectoral girdle (shoulder girdle) is a bony ring, incomplete posteriorly, formed by the scapulae and clavicles, and completed anteriorly by the manubrium of the sternum (part of the axial skeleton).
- Arm (L. brachium): first segment of the free upper limb (more mobile part of the upper limb independent of the trunk) and the longest segment of the limb. It extends between and connects the shoulder and the elbow, and consists of anterior and posterior regions of the arm, centered around the humerus.
- Forearm (L. antebrachium): second longest segment of the limb. It extends between and connects the elbow and wrist and includes anterior and posterior regions of the forearm overlying the radius and ulna.
- Hand (L. manus): part of the upper limb distal to the forearm that is formed around the carpus, metacarpus, and phalanges. It is composed of the wrist, palm, dorsum of hand, and digits (fingers, including an opposable thumb), and is richly supplied with sensory endings for touch, pain, and temperature.
Figure 6.1. Segments and bones of upper limb.
The joints divide the superior appendicular skeleton, and thus the limb itself, into four main segments: shoulder, arm, forearm, and hand.
Figure 6.2. Regions of upper limb.
For exact description, the upper limb is divided into regions based on the external features (surface anatomy) of the underlying muscular formations, bones, and joints.
Comparison of Upper and Lower Limbs
Developing in a similar fashion, the upper and lower limbs share many common features (see Chapter 5). However, they are sufficiently distinct in structure to enable markedly different functions and abilities. Because the upper limb is not usually involved in weight bearing or motility, its stability has been sacrificed to gain mobility. The upper limb still possesses remarkable strength; and because of the hand’s ability to conform to a paddle or assume a gripping or platform configuration, it may assume a role in motility in certain circumstances.
Both the upper and the lower limbs are connected to the axial skeleton (cranium, vertebral column, and associated thoracic cage) via the bony pectoral and pelvic girdles, respectively. The pelvic girdle consists of the two hip bones connected to the sacrum (see Chapter 5). The pectoral girdle consists of the scapulae and clavicles, connected to the manubrium of the sternum. Both girdles possess a large flat bone located posteriorly, which provides for attachment of proximal muscles, and connects with its contralateral partner anteriorly via small bony braces, the pubic rami and clavicles. However, the flat iliac bones of the pelvic girdle are also connected posteriorly through their primary attachment to the sacrum via the essentially rigid, weight-transferring sacro-iliac joints. This posterior connection to the axial skeleton places the lower limbs inferior to the trunk, enabling them to be supportive as they function primarily in relation to the line of gravity. Furthermore, because the two sides are connected both anteriorly and posteriorly, the pelvic girdle forms a complete rigid ring that limits mobility, making the movements of one limb markedly affect the movements of the other. The pectoral girdle, however, is connected to the trunk only anteriorly, via the sternum, by flexible joints with 3 degrees of freedom. It is an incomplete ring because the scapulae are not connected with each other posteriorly. Thus, the motion of one upper limb is independent of the other, and the limbs are able to operate effectively anterior to the body, at a distance and level that enable precise eye–hand coordination.
In both the upper and the lower limbs, the long bone of the most proximal segment is the largest and is unpaired. The long bones increase progressively in number but decrease in size in the more distal segments of the limb. The second most proximal segment of both limbs (i.e., the leg and forearm) has two parallel bones, although only in the forearm do both articulate with the bone of the proximal segment, and only in the leg do both articulate directly with the distal segment. Although the paired bones of both the leg and forearm flex and extend as a unit, only those of the upper limb are able to move (supinate and pronate) relative to each other; the bones of the leg are fixed in the pronated position.
The wrist and ankle have a similar number of short bones (eight and seven, respectively). Both groups of short bones interrupt a series of long bones that resumes distally with several sets of long bones of similar lengths, with a similar number of joints of essentially the same type. The digits of the upper limb (fingers including the thumb) are the most mobile parts of either limb. However, all other parts of the upper limb are more mobile than the comparable parts of the lower limb.
Bones of Upper Limb
The pectoral girdle and bones of the free part of the upper limb form the superior appendicular skeleton (Fig. 6.3); the pelvic girdle and bones of the free part of the lower limb form the inferior appendicular skeleton. The superior appendicular skeleton articulates with the axial skeleton only at the sternoclavicular joint, allowing great mobility. The clavicles and scapulae of the pectoral girdle are supported, stabilized, and moved by axio-appendicular muscles that attach to the relatively fixed ribs, sternum, and vertebrae of the axial skeleton.
Figure 6.3. Bones of upper limb
The clavicle (collar bone) connects the upper limb to the trunk (Figs. 6.3 and 6.4). The shaft of the clavicle has a double curve in a horizontal plane. Its medial half is convex anteriorly, and its sternal end is enlarged and triangular where it articulates with the manubrium of the sternum at the sternoclavicular (SC) joint. Its lateral half is concave anteriorly, and its acromial end is flat where it articulates with the acromion of the scapula at the acromioclavicular (AC) joint (Figs. 6.3B and 6.4). The medial two thirds of the shaft of the clavicle are convex anteriorly, whereas the lateral third is flattened and concave anteriorly. These curvatures increase the resilience of the clavicle, and give it the appearance of an elongated capital S.
Figure 6.4. Right clavicle.
Prominent features of the superior and inferior surfaces of the clavicle. The bone acts as a mobile strut (supporting brace) connecting the upper limb to the trunk; its length allows the limb to pivot around the trunk.
- Serves as a moveable, crane-like strut (rigid support) from which the scapula and free limb are suspended, keeping them away from the trunk so that the limb has maximum freedom of motion. The strut is movable and allows the scapula to move on the thoracic wall at the “scapulothoracic joint,”1 increasing the range of motion of the limb. Fixing the strut in position, especially after its elevation, enables elevation of the ribs for deep inspiration.
- Forms one of the bony boundaries of the cervico-axillary canal (passageway between the neck and arm), affording protection to the neurovascular bundle supplying the upper limb.
- Transmits shocks (traumatic impacts) from the upper limb to the axial skeleton.
Although designated as a long bone, the clavicle has no medullary (marrow) cavity. It consists of spongy (trabecular) bone with a shell of compact bone.
The superior surface of the clavicle, lying just deep to the skin and platysma (G. flat plate) muscle in the subcutaneous tissue, is smooth.
The inferior surface of the clavicle is rough because strong ligaments bind it to the 1st rib near its sternal end and suspend the scapula from its acromial end. The conoid tubercle, near the acromial end of the clavicle (Fig. 6.4), gives attachment to the conoid ligament, the medial part of the coracoclavicular ligament by which the remainder of the upper limb is passively suspended from the clavicle. Also, near the acromial end of the clavicle is the trapezoid line, to which the trapezoid ligament attaches; it is the lateral part of the coracoclavicular ligament.
The subclavian groove (groove for the subclavius) in the medial third of the shaft of the clavicle is the site of attachment of the subclavius muscle. More medially is the impression for the costoclavicular ligament, a rough, often depressed, oval area that gives attachment to the ligament binding the 1st rib (L. costa) to the clavicle, limiting elevation of the shoulder.
The scapula (shoulder blade) is a triangular flat bone that lies on the posterolateral aspect of the thorax, overlying the 2nd–7th ribs (see Fig. I.11). The convex posterior surface of the scapula is unevenly divided by a thick projecting ridge of bone, the spine of the scapula, into a small supraspinous fossa and a much larger infraspinous fossa (Fig. 6.5A). The concave costal surface of most of the scapula forms a large subscapular fossa. The broad bony surfaces of the three fossae provide attachments for fleshy muscles. The triangular body of the scapula is thin and translucent superior and inferior to the spine of the scapula; although its borders, especially the lateral one, are somewhat thicker. The spine continues laterally as the flat, expanded acromion (G. akros, point), which forms the subcutaneous point of the shoulder and articulates with the acromial end of the clavicle. The deltoid tubercle of the scapular spine is the prominence indicating the medial point of attachment of the deltoid. The spine and acromion serve as levers for the attached muscles, particularly the trapezius.
Figure 6.5. Right scapula.
A.The bony features of the costal and posterior surfaces of the scapula. B.The borders and angles of the scapula. C.The scapula is suspended from the clavicle by the coracoclavicular ligament, at which a balance is achieved among the weight of the scapula and its attached muscles plus the muscular activity medially and the weight of the free limb laterally.
Because the acromion is a lateral extension of the scapula, the AC joint is placed lateral to the mass of the scapula and its attached muscles (Fig. 6.5C). The glenohumeral (shoulder) joint on which these muscles operate is almost directly inferior to the AC joint; thus the scapular mass is balanced with that of the free limb, and the suspending structure (coracoclavicular ligament) lies between the two masses.
Superolaterally, the lateral surface of the scapula has a glenoid cavity (G. socket), which receives and articulates with the head of the humerus at the glenohumeral joint (Fig. 6.5A & C). The glenoid cavity is a shallow, concave, oval fossa (L. fossa ovalis), directed anterolaterally and slightly superiorly—that is considerably smaller than the ball (head of the humerus) for which it serves as a socket. The beak-like coracoid process (G. korak-ode?s, like a crow’s beak) is superior to the glenoid cavity, and projects anterolaterally. This process also resembles in size, shape, and direction a bent finger pointing to the shoulder, the knuckle of which provides the inferior attachment for the passively supporting coracoclavicular ligament.
The scapula has medial, lateral, and superior borders and superior, lateral, and inferior angles (Fig. 6.5B). When the scapular body is in the anatomical position, the thin medial border of the scapula runs parallel to and approximately 5 cm lateral to the spinous processes of the thoracic vertebrae; hence it is often called the vertebral border (Fig. 6.5B). From the inferior angle, the lateral border of the scapula runs superolaterally toward the apex of the axilla; hence it is often called the axillary border. The lateral border is made up of a thick bar of bone that prevents buckling of this stress-bearing region of the scapula.
The lateral border terminates in the truncated lateral angle of the scapula, the thickest part of the bone that bears the broadened head of the scapula (Fig. 6.5A & B). The glenoid cavity is the primary feature of the head. The shallow constriction between the head and body defines the neck of the scapula. The superior border of the scapula is marked near the junction of its medial two thirds and lateral third by the suprascapular notch, which is located where the superior border joins the base of the coracoid process. The superior border is the thinnest and shortest of the three borders.
The scapula is capable of considerable movement on the thoracic wall at the physiological scapulothoracic joint, providing the base from which the upper limb operates. These movements, enabling the arm to move freely, are discussed later in this chapter with the muscles that move the scapula.
The humerus (arm bone), the largest bone in the upper limb, articulates with the scapula at the glenohumeral joint, and the radius and ulna at the elbow joint (Figs. 6.1, 6.3, and 6.5C). The proximal end of the humerus has a head, surgical and anatomical necks, and greater and lesser tubercles. The spherical head of the humerus articulates with the glenoid cavity of the scapula. The anatomical neck of the humerus is formed by the groove circumscribing the head and separating it from the greater and lesser tubercles. It indicates the line of attachment of the glenohumeral joint capsule. The surgical neck of the humerus, a common site of fracture, is the narrow part distal to the head and tubercles (Fig. 6.3B).
The junction of the head and neck with the shaft of the humerus is indicated by the greater and lesser tubercles, which provide attachment and leverage to some scapulohumeral muscles (Fig. 6.3A & B). The greater tubercle is at the lateral margin of the humerus, whereas the lesser tubercle projects anteriorly from the bone. The intertubercular sulcus (bicipital groove) separates the tubercles, and provides protected passage for the slender tendon of the long head of the biceps muscle.
The shaft of the humerus has two prominent features: the deltoid tuberosity laterally, for attachment of the deltoid muscle, and the oblique radial groove (groove for radial nerve, spiral groove) posteriorly, in which the radial nerve and profunda brachii artery lie as they pass anterior to the long head and between the medial and the lateral heads of the triceps brachii muscle. The inferior end of the humeral shaft widens as the sharp medial and lateral supra-epicondylar (supracondylar) ridges form, and then end distally in the especially prominent medial epicondyle and the lateral epicondyle, providing for muscle attachment.
The distal end of the humerus—including the trochlea, capitulum, olecranon, coronoid, and radial fossae—makes up the condyle of the humerus (Fig. 6.6). The condyle has two articular surfaces: a lateral capitulum (L. little head) for articulation with the head of the radius, and a medial, spool-shaped or pulley-like trochlea (L. pulley) for articulation with the proximal end (trochlear notch) of the ulna. Two hollows, or fossae, occur back to back superior to the trochlea, making the condyle quite thin between the epicondyles. Anteriorly, the coronoid fossa receives the coronoid process of the ulna during full flexion of the elbow. Posteriorly, the olecranon fossa accommodates the olecranon of the ulna during full extension of the elbow. Superior to the capitulum anteriorly, a shallower radial fossa accommodates the edge of the head of the radius when the forearm is fully flexed.
Figure 6.6. Distal end of right humerus.
A and B.The condyle (the boundaries of which are indicated by the dashed line) consists of the capitulum; the trochlea; and the radial, coronoid, and olecranon fossae.
Bones of Forearm
The two forearm bones serve together to form the second unit of an articulated mobile strut (the first unit being the humerus), with a mobile base formed by the shoulder, that positions the hand. However, because this unit is formed by two parallel bones, one of which (the radius) can pivot about the other (the ulna), supination and pronation are possible. This makes it possible to rotate the hand when the elbow is flexed.
The ulna is the stabilizing bone of the forearm and is the medial and longer of the two forearm bones (Figs. 6.7 and 6.8). Its more massive proximal end is specialized for articulation with the humerus proximally, and the head of the radius laterally. For articulation with the humerus, the ulna has two prominent projections: (1) the olecranon, which projects proximally from its posterior aspect (forming the point of the elbow), and serves as a short lever for extension of the elbow, and (2) the coronoid process, which projects anteriorly.
Figure 6.7. Bones of right elbow region.
A.The proximal part of the ulna. B.The bones of the elbow region, demonstrating the relationship of the distal humerus and proximal ulna and radius during extension of the elbow joint. C.The relationship of the humerus and forearm bones during flexion of the elbow joint.
Figure 6.8. Right radius and ulna.
A and B.The radius and ulna are shown in the articulated position, connected by the interosseous membrane. C and D.The features of the distal ends of the forearm bones. E.In cross section, the shafts of the radius and ulna appear almost as mirror images of one another for much of the middle and distal thirds of their lengths.
The olecranon and coronoid processes form the walls of the trochlear notch, which in profile resembles the jaws of a crescent wrench as it “grips” (articulates with) the trochlea of the humerus (Fig. 6.7B & C). The articulation between the ulna and humerus primarily allows only flexion and extension of the elbow joint, although a small amount of abduction–adduction occurs during pronation and supination of the forearm. Inferior to the coronoid process is the tuberosity of the ulna for attachment of the tendon of the brachialis muscle (Fig. 6.7A and 6.8A & B).
On the lateral side of the coronoid process is a smooth, rounded concavity, the radial notch, which receives the broad periphery of the head of the radius. Inferior to the radial notch on the lateral surface of the ulnar shaft is a prominent ridge, the supinator crest. Between it and the distal part of the coronoid process is a concavity, the supinator fossa. The deep part of the supinator muscle attaches to the supinator crest and fossa (6.7A).
The shaft of the ulna is thick and cylindrical proximally, but it tapers, diminishing in diameter, as it continues distally (Fig. 6.8A). At the narrow distal end of the ulna is a small but abrupt enlargement, the disc-like head of the ulna with a small, conical ulnar styloid process. The ulna does not reach—and therefore does not participate in—the wrist (radiocarpal) joint (Fig. 6.8).
The radius is the lateral and shorter of the two forearm bones. Its proximal end includes a short head, neck, and medially directed tuberosity (Fig. 6.8A). Proximally, the smooth superior aspect of the discoid head of the radius is concave for articulation with the capitulum of the humerus during flexion and extension of the elbow joint. The head also articulates peripherally with the radial notch of the ulna; thus the head is covered with articular cartilage.
The neck of the radius is a constriction distal to the head. The oval radial tuberosity is distal to the medial part of the neck, and demarcates the proximal end (head and neck) of the radius from the shaft.
The shaft of the radius, in contrast to that of the ulna, gradually enlarges as it passes distally. The distal end of the radius is essentially four sided when sectioned transversely. Its medial aspect forms a concavity, the ulnar notch (Fig. 6.8C & D), which accommodates the head of the ulna. Its lateral aspect becomes increasingly ridge-like, terminating distally in the radial styloid process.
Projecting dorsally, the dorsal tubercle of the radius lies between otherwise shallow grooves for the passage of the tendons of forearm muscles. The radial styloid process is larger than the ulnar styloid process, and extends farther distally (Fig. 6.8A & B). This relationship is of clinical importance when the ulna and/or the radius is fractured.
Most of the length of the shafts of the radius and ulna are essentially triangular in cross section, with a rounded, superficially directed base and an acute, deeply directed apex (Fig. 6.8A & E). The apex is formed by a section of the sharp interosseous border of the radius or ulna that connects to the thin, fibrous interosseous membrane of the forearm (Fig. 6.8A, B, & E). The majority of the fibers of the interosseous membrane run an oblique course, passing inferiorly from the radius as they extend medially to the ulna (Fig. 6.8A & B). Thus, they are positioned to transmit forces received by the radius (via the hands) to the ulna for transmission to the humerus.