Functional Anatomy of the Upper Extremity
Peter B. J. Wu, M.D., M.P.H.
This chapter is intended as a concise review of the functional anatomy of the upper extremity. It is neither comprehensive nor exhaustive. Readers are encouraged to refer to references for more details. Neuroanatomy and nerve entrapment syndromes will be addressed in different chapters in this issue.
For convenience, the upper extremity will be divided into the wrist and hand, the elbow, and the shoulder complexes for convenience. The shoulder complex has the most mobile joint in the body and allows the hand to reach any part of the body for basic activities of daily living. It also provides stability and muscle control for propulsive and precision motions in athletic and occupational activities. The elbow complex provides stability to the shoulder to position the hand in space for functional use. The wrist and hand complex is the most active and unique part of the upper extremity. It is the effector organ of the upper extremity with its versatile prehensile abilities. Each complex affects and is affected by others to accomplish the numerous, complex and well coordinated daily tasks of the human upper extremity. Thus, these three complexes should be regarded as one functional unit.
The wrist and hand complex
The skin overlying the palmar aponeurosis, anchored by the numerous strong fasciculi, is relatively immobile. In contrast, the skin on the dorsum of the hand is loose. Most of the lymph from the hand drains into lymph channels located in the loose areolar layer on the dorsum of the hand. This explains the frequent occurrence of lymphedematous swelling on the back of the hand.(19) The hand has transverse creases which represent adherence between skin and fascia with no intervening adipose tissue. The proximal and distal palmar creases overlay the longitudinal fibers of the palmar fascia and are frequently involved in early Dupuytren's disease. Dupuytren's disease is characterized by a diffuse thickening of the deep fascia of the palm and fingers with the ring and small fingers most commonly involved.(42) The only transverse crease corresponding with an underlying joint is the middle digital crease. The distal digital crease lies just proximal to the distal interphalangeal joint (DIP) and the proximal digital crease lies over the middle of the proximal phalanx. The hand forms two transverse (carpal and metacarpal) and one longitudinal arches for grasping objects.(Fig. 1)
The skeleton of the hand and wrist consists of 19 long bones (5 metacarpals and 14 phalanges) and 8 carpal bones. The five polyarticulated rays are made up of the metacarpals and phalanges. Littler (21) identified the skeleton of the hand as the fixed and the mobile elements. The fixed elements consists of the distal carpal row and the second and third metacarpals. The mobile elements consists of the phalanges, thumb, and the ulnar two metacarpals.The first metacarpal makes an angle of about 45 degrees with the second metacarpal in the sagittal plane and is independent. This allows the thumb ray to oppose the other four digital rays. The thumb movements include flexion, extension (in the plane of the palm), abduction (in a plane 90° to that of the plane of the palm), and adduction. Opposition of the thumb is a combination of all motions. It begins as extension proceeding to abduction, then becomes flexion into adduction.(7) The wide range of motion of the thumb provides half of the hand function. The oblique positioning of the ulnar digits explains their convergence toward the scaphoid tubercle with flexion at the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints. The eight carpal bones are arranged in two rows. The pisiform bone is regarded as a sesamoid bone and the proximal carpal row is made up by the scaphoid, the lunate, and the triquetrum. The proximal row forms the radiocarpal joint with its articulation with the radius and is very mobile. The scaphoid is the largest of the proximal row. There are no intraosseous anastomoses between the arteries to the scaphoid tuberosity and the remainder of the intraosseous vessels. Therefore, the proximal part of the scaphoid is susceptible to avascular necrosis if it is not supplied by a direct arterial branch in the event of a fracture.(46) Post-traumatic avascular necrosis has also been described in the lunate, the capitate and the pisiform for similar reasons.(1) The distal row consists of the trapezium, the trapezoid, the capitate, and the hamate. The capitate is the largest of the carpal bones and is the keystone of the distal row. Unlike the proximal row, the distal carpal row is stable and rigid. Most of the wrist movement occurs in the proximal carpal row. Carpal stability is provided by the close-packed bony configuration of the carpal bones and the interosseous ligaments.
Wrist ligaments consists of intrinsic intercarpal ligaments and extrinsic ligaments which run between the carpal bones and the radius, ulna and metacarpals. The volar ligaments are thick and strong and prevent dorsal instability. On the contrary, the dorsal ligaments are thin and less well developed.(44)
The four volar plates of the MCP joints of the fingers are tied together by the transverse intermetacarpal ligament. The MCP and IP joints have the same volar plates and collateral ligaments. These collateral ligaments provide important joint stability.
The wrist contains the radiocarpal and the midcarpal joints. The radiocarpal joint is formed between the radial head and the proximal row of carpal bones. It is an ellipsoidal joint and allows adduction/abduction, flexion/extension, and circumduction. The midcarpal joint lies between the proximal and distal carpal rows. It acts as a hinge joint with one degree of freedom. The distal radioulnar joint is a pivot joint between the head of the ulna and the ulnar notch of the radius. It provides supination and pronation which is the most important functional movement of the wrist. The triangular fibrocartilage complex (TFCC) serves as a cushion for the ulnar carpus and is the most important stabilizer of the distal radioulnar joint.(24,41) (Fig.2)
The PIP and DIP joints are both condylar and move as hinge joints in flexion and extension. The collateral ligaments are loose and redundant with the metacarpophalangeal (MCP) joints in extension and hyperextension, allowing maximum medial and lateral deviation. The integrity of these ligaments can therefore be best assessed in flexion. The side-to-side mobility and rotational movement of the MCP joint facilitates efficient grasp.(3) The carpometacarpal (CMC) joints of the fingers are plane synovial joints with one degree of freedom allowing for small amount of flexion and extension. The CMC joint (metacarpotrapezial joint) of the thumb, on the other hand, is a saddle-shaped joint with two degrees of freedom allowing for the motions of flexion and extension, abduction and adduction, and conjunctional rotation.(25) The interphalangeal joints are true hinge joints and their action is limited to flexion and extension.
Intrinsic muscles of the hand
The intrinsic muscles of the hand include the muscles of the thenar and hypothenar eminences, the interossei, and the lumbricals. The function of the four dorsal interossei muscles is to abduct fingers from the midline. The three palmar interossei muscles adduct the index, ring, and small fingers toward the middle finger. The actions of the interossei on the interphalangeal joints are dependent on the position of the MCP joint. The interosseous hood is put into tension across the extended MCP joint. Therefore, the interossei extend the PIP and DIP joints when the MCP joint is in extension. On the other hand, the hood passes distal to the MCP joint with the tension on the proximal phalanx when the MCP joint is in flexion. The interossei thus assist flexion of the DIP and PIP joints. The four lumbrical muscles arise from the radial aspect of the tendons of the flexor digitorum profundus. Their primary action is to extend the DIP and PIP joints either the MCP joint is in flexion or flexion. The lumbricals could possibly flex the MCP joint. However, the contribution of the lumbricals to the flexion of the MCP joints is not significant when compared with the profundus.(6) The lumbrical tendons course palmar to the axis of the MP joint and prevent its hyperextension.
Extrinsic muscles of the hand
Unlike other fingers, the index and small fingers both have
dual extensor system, i.e., extensor indicis proprius and extensor digiti minimi in addition to the extensor digitorum communis (EDC). These two muscles tendons lie on the ulnar side of the individual EDC tendon.(9) Unlike other fingers, they are capable of performing isolated extension. Every finger has the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) as its two long flexor tendons. There are five annular (A) pulleys and four cruciate (C) pulleys which prevent bowstringing of the flexor tendons across the joints.(FIG.3) Repeated trauma to the flexor tendon causes thickening of the ligament sheath and formation of a nodule. This results in trigger finger which occurs most frequently at the A1 pulley in the middle or ring fingers. The FDP of the index finger has independent function while the other three fingers tend to move together due to intermuscular and intertendinous connections.(6) The FDS function can thus be isolated when adjacent fingers are held in extension to eliminate the action of the FDP. It has been demonstrated that the FDP is the prime mover of the unloaded flexion movement of the fingers.(22) The superficialis comes in to act when more strength is needed with resistance.
The order of flexion of the phalanges is controlled by a complex mechanism. The sequence of flexion of the fingers is the PIP, the MCP, and lastly, the DIP joints to complete the motion. (45) The FDP and EDC contract simultaneously at the beginning of flexion. The extensor acts as a braking mechanism and prevents the flexion of the DIP. The FDP initiates flexion at the PIP by its action on the A4 pulley. As the PIP joint flexes, the extensor tension in the oblique retinacular ligament gradually decreases, thereby allowing the complete flexion of the DIP joint. Flexion of the PIP joint stretches the lumbrical and interosseous tendons and initiates flexion of the MCP joint by passive viscoelastic force. The interossei can actively assist the MCP joint flexion afterwards. It is to be noted that the FDS and the lumbricals do not contract during unopposed flexion.(3)
The EDC starts finger extension at the level of the MCP joint by its linkage to sagittal bands.(FIG. 4,5) Contraction of the lumbricals prevents hyperextension of the MCP joint and provides stability. The PIP joint extends with traction at the central slip of the extensor and the oblique band of the reticular ligament tightens up at the same time. The DIP joint then extends with the coordinated actions of the EDC and the interossei.
The paralyzed lumbricals are unable to stabilize the MCP joints in low ulnar palsy. Subsequently, the extensors hyperextend the MCP joints. The DIP and PIP joints go into flexion from the passive pull of the flexor digitorum profundus. This results in the claw deformity. The clawing is less apparent if the flexor digitorum profundus is also paralyzed in high ulnar palsy or the tendons disrupted.
The flexor carpi radialis, ulnaris, and FDS act together in wrist flexion without participation from the FDP. The three extensors of the wrist as well as the EDC work synchronously during wrist extension. The extensor carpi ulnaris is a strong ulnar deviator in pronation and supination. It is an effective wrist extrensor only in supination. (46) Long (22) have provided detailed analysis on the activities of the extrinsic and intrinsic hand muscles with different positions of fingers, power grip and precision handling.
The extensor and flexor compartments
The extensor retinaculum at the wrist forms a roof over the six extensor compartments.(Fig.6) Each compartment has a synovial compartment that extends both proximally and distally. Tenosynovitis occurs commonly at the first dorsal compartment, which is well known as de Quervain's syndrome. Tenosynovitis from overuse or trauma involving other compartments has also been described.(17)
The flexor retinaculum distal to the radial joint forms the roof of carpal tunnel. Both the flexor carpi ulnaris and the palmaris longus tendons do not have synovial sheaths and are the only flexor tendons which do not pass through the carpal tunnel. Flexor tenosynovitis in the carpal tunnel can cause median nerve compression and is a common cause of carpal tunnel syndrome.
The elbow complex
The elbow joint complex consists of the humeroradial, humeroulnar, proximal and distal radioulnar joints. The first three joints are enclosed in a single joint capsule. The humeroradial joint is referred to as a hinge/pivot joint as it performs flexion/extension movements with the humeroulnar joint and also pivots to perform rotational movements along with the proximal radioulnar joint. The humeroulnar joint is a modified hinge joint with approximately 5° of internal and external rotation at the extremes of flexion and extension.(43) The congruent articular surfaces of the trochlea of the ulna and the capitulum of the humerus provide stability to the elbow joint and limit its motion in flexion and extension. The proximal radioulnar joint functions with the distal radioulnar joint, which is not in direct association with other elbow joints, as one unit to perform pronation and supination of the forearm. The range of motion of the elbow is 140° flexion from full extension, and from 75° pronation to 85° supination.(2) Most of the activities of daily living can be achieved from 30° to 130° of flexion and 50° of pronation and supination.(26)
Stability of the elbow joint
The strong medial and lateral collateral ligament complexes strengthen the relatively weak articular capsule of the elbow.
The lateral ligament complex is composed of four ligaments, i.e., the radial collateral ligament, the lateral ulnar collateral ligament, the accessory lateral collateral ligament, and the annular ligament.(FIG.7)The medial collateral ligament complex consists of three ligaments, i.e., the anterior oblique ligament, posterior oblique ligament, and transverse ligament.(FIG.8) The medial and lateral collateral ligament complexes provide valgus and varus stabilities respectively. The anconeus is a dynamic stabilizer in response to varus stress. On the other hand, the flexor-pronator group is the primary dynamic stabilizer to valgus stress.(39)
The musculature that moves the elbow can be grouped into four functional groups, i.e., the elbow flexors and extensors, the flexor-pronators, and the extensor-supinators.
i) The elbow flexors include the biceps, the brachialis, and the brachioradialis. The long head of the biceps arises from the supraglenoid tubercle of the scapula and crosses both the glenohumeral and elbow joints. The biceps is a significant elbow flexor when the forearm is in supination and neutral, but not in pronation. The biceps generates its highest torque and is most effective when the elbow is flexed between 80 and 100°. It is neither effective as an elbow flexor due to short moment arm when the elbow is in extension, nor with simultaneous shoulder and elbow flexion when the long head is shortened over both the shoulder and elbow.(29) It is also not active during supination with an extended forearm unless resistance is given.(2) The brachialis is found active in all positions of the forearm, with and without resistance. The brachioradialis is an elbow flexor and possibly acts as an accessory pronator in full supination and as a supinator in full pronation.(2,16,46)
ii) The triceps brachii and the anconeus serve as primary extensors of the elbow. The long head of the triceps arises from the infraglenoid tubercle of the humerus and crosses both the shoulder and the elbow joints. It is thus ineffective when the shoulder is hyperextended with simultaneous elbow extension. The triceps is most effective and produces the maximum torque at 90° of elbow flexion. The anconeus acts to initiate extension of the elbow and becomes ineffective at full extension. It also provides elbow stability during pronation and supination movements.(34)
iii) The flexor-pronators include the pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, and FDS. These muscles arise from the common flexor tendon of origin at the medial epicondyle, either partially or completely. They provide dynamic stability to the medial elbow and are also weak elbow flexors. Forearm pronation is effected by the pronator teres and the pronator quadratus. The flexor carpi radialis also has a minor role in pronation as well.
iv) The extensor-supinators comprise the extensor carpi radialis longus and brevis, supinator, EDC, extensor digiti minimi, and extensor carpi ulnaris. The supinator, unlike the biceps which is most effective as a supinator when the elbow is in 90° flexion, is unaffected by elbow position. The supinator initiates and carries out the supination movement. The biceps acts only in the face of added resistance. The other muscles of this group have been discussed in the previous section.
Lateral epicondylitis is the most common source of elbow pain in the general population. In most cases of lateral epicondylitis, the lesion involves the junctional tissue at the common extensor tendon, primarily the extensor carpi radialis brevis. Pain with resisted extension of the middle finger with the elbow in full extension along with tenderness at the lateral epicondyle suggests involvement of the extensor carpi radialis brevis. (48) Extensor carpi radialis longus originates from the lower one-third of the lateral supracondylar ridge of the humerus, the front of the lateral intermuscular septum and the common extensor origin attached to the lateral epicondyle. Tenderness at the supracondylar ridge, which is also a common finding, will indicate that the extensor carpi radialis longus is involved. EDC takes its origin from the anterior distal aspect of the lateral epicondyle by the common extensor tendon. The humeral head of the extensor carpi ulnaris arises from the most medial aspect of the common extensor tendon. Involvement of the EDC or the extensor carpi ulnaris is considered to be rare.(4) On the other hand, repetitive overuse of the pronator teres, flexor carpi radialis, and occasionally the flexor carpi ulnaris will result in medial epicondylitis.(20) Point tenderness is thus noted over the above muscles at or distal to medial epicondyle.
The Shoulder complex
The shoulder girdle consists of seven joints, i.e., glenohumeral, suprahumeral, acromioclavicular (AC), scapulocostal, sternoclavicular (SC), costosternal, and costovetertebral joints.(8) (FIG.9) The suprahumeral and scapulocostal joints are functional joints rather than true joints. However, they function as an integral part of the shoulder complex and must be considered in the assessment. The full mobility of the shoulder girdle is hinged on the coordinated and synchronous motion of these individual joints.
The glenohumeral joint is a ball-and-socket joint which has the freedom of moving in an infinite number of axes around the humeral head. The increased mobility of it is accompanied by a loss of stability. The small surface of the glenoid fossa, which is only 1/4 to 1/3 of the size of the large humeral head, implies significant instability.(10) Fortunately, the fibrocartilagenous labrum enlarges the humeral contact area to 75% vertically and 56% transversely. Thus, the stability of the glenohumeral joint is improved.(40)
The long head of the biceps passes over the humeral head within the capsule of the glenohumeral joint and goes under the transverse humeral ligament in the bicipital groove. It then units with the short head, which originates from the coracoid process, at the middle of the arm. The synovial lining of the long head is an extension of the synovium of the glenohumeral joint. Any inflammatory process within the synovium of the joint, such as irritation of the neighboring synovium of the rotator cuff, can involve the tendon. This could partly explain the frequent coexistence of tendinitis of the rotator cuff and the long head of the biceps.(27,28)
The glenoid fossa is tilted downward 5° at rest. The plane of the scapula is angled approximately 30° anterior to the bodyís coronal plane.(35) There is 30° retroversion of the head of the humerus on its shaft. Neutral elevation of the arm follows the plane of the scapula and is termed scaption.
The term of scapulohumeral rhythm describes a synchronized movement from the glenohumeral joint and the scapula on the thorax for arm elevation. This movement requires coordinated motion to have the humerus to pass under the coracoacromial arch. Scapular gliding is highly variable at the onset of arm elevation. It varies from person to person and is characteristic for each individual. It may be absent, minimal, reversed, or rarely oscillatory.(11,15) The lack of active participation from scapula continues until the first 60° of flexion and 30° abduction. Both humerus and scapula move continuously and synchronously thereafter. The ratio of humeral to scapular motion is from 1.25 to 3:1 in different studies. (11,12,15,38,40) There is also a decrease of scapular movement in the terminal arc past 120°. Of note, there is 5° AC rotation during the first 30° of arm elevation. Clavicular elevation at the SC joint is the major arc of motion relating to the scapular motion until the arm reaches 120°. The full scapular rotation can not be completed without the interplay of the AC joint and clavicular rotation about its long axis beyond that.
Rotator cuff impingement
The arm can be abducted passively to 120° with movement exclusively at the glenohumeral joint. After 120°, abduction is blocked by the greater tuberosity of the humerus impinging upon the acromial process and the coracoacromial ligament. Only 60° of abduction is possible with the humerus in internal rotation. The rotator cuff tendons and the subacromial bursa occupy the subacromial space which is between the coracoacromial arch and the humeral head. Different pathologies in the acromion, the coracoacromial ligament, and the AC joint will make the shoulder susceptible to primary impingement of rotator cuff and associated subacromial bursitis.(13) Biomechanically, rotator cuff muscles provide a compressive force centralizing the humeral head within the glenoid socket throughout the range of arm elevation.(36) The deltoid, on the contrary, creates more upward shear forces which threatens the joint stability by the sliding strains until the arm is at about 90° in elevation.(35) With chronic overuse and fatigue the stabilizing force of the rotator cuff will become insufficient to counteract the shearing force from the deltoid. As a result, increased humeral head displacement will result in secondary mechanical impingement of the rotator cuff. Selective strengthening of the rotator cuff is thus considered critical in the unstable shoulder and rotator cuff impingement syndrome.(49)
Stability of the shoulder joint
Ligaments provide the critical static restraints in shoulder joint stability.(23) It includes the superior glenohumeral ligament (SGHL), middle glenohumeral ligament (MGHL), inferior glenohumeral ligament complex (IGHLC), and the coracohumeral ligament. The IGHLC consists of an anterior band, a posterior band, and an axillary pouch.(30) (FIG.10) The superior, middle, and inferior glenohumeral ligaments reinforced the anterior wall of the articular capsule. It has been demonstrated that at 0° and 90° of abduction, subscapularis and the IGHL provide anterior restraint respectively.(31) At 45° of abduction, the subscapularis, MGHL, and anterior band of the IGHLC provide stability.(32,47) The SGHL restricts inferior translation in the adducted shoulder.(5) The primary anatomical structure responsible for posterior shoulder stability in the 90° abducted shoulder is the IGHLC. It has also been suggested that the posterior rotator cuff muscles are static stabilizers at all positions of abduction.(33)
The rotator cuff muscle tendons provide mainly dynamic stability by compressing the humeral head into the glenoid, and secondarily as static barrier to limit translation of the humeral head in either the anterior, posterior, or superior-inferior direction. The long head of the biceps functions to stabilize the humeral head in the joint during flexion and supination of the forearm.(18)
All the shoulder muscles are silent in EMG in the relaxed standing position. The shoulder girdle has wide range of mobility and is capable of carrying out numerous daily tasks with various movement patterns. Table 1 is a summary of the main muscles involved with the different scapular and shoulder movements. (14,37)