Chapter 7. The Biomechanics of the Human Upper Extremity

After completing this chapter, you will be able to:

• Explain how anatomical structure affects movement capabilities of upper-extremity articulations.
• Identify factors influencing the relative mobility and stability of upper-extremity articulations.
• Identify muscles that are active during specific upper-extremity movements.
• Describe the biomechanical contributions to common injuries of the upper extremity.

The capabilities of the upper extremity are varied and impressive. With the same basic anatomical structure of the arm, forearm, hand, and fingers, major league baseball pitchers hurl fastballs at 40 m/s, swimmers cross the English Channel, gymnasts perform the iron cross, travelers carry briefcases, seamstresses thread needles, and students type on computer keyboards. This chapter reviews the anatomical structures enabling these different types of movement and examines the ways in which the muscles cooperate to achieve the diversity of movement of which the upper extremity is capable.

The shoulder is the most complex joint in the human body, largely because it includes five separate articulations: the glenohumeral joint, the sternoclavicular joint, the acromioclavicular joint, the coracoclavicular joint, and the scapulothoracic joint. The glenohumeral joint is the articulation between the head of the humerus and the glenoid fossa of the scapula, which is the ball-and-socket joint typically considered to be the major shoulder joint. The sternoclavicular and acromioclavicular joints provide mobility for the clavicle and the scapula—the bones of the shoulder girdle.

Sternoclavicular Joint

The proximal end of the clavicle articulates with the clavicular notch of the manubrium of the sternum and with the cartilage of the first rib to form the sternoclavicular joint. This joint provides the major axis of rotation for movements of the clavicle and scapula (Figure 7-1). The sternoclavicular (SC) joint is a modified ball and socket, with frontal and transverse plane motion freely permitted and some forward and backward sagittal plane rotation allowed. A fibrocartilaginous articular disc improves the fit of the articulating bone surfaces and serves as a shock absorber. Rotation occurs at the SC joint during motions such as shrugging the shoulders, elevating the arms above the head, and swimming. The close-packed position for the SC joint occurs with maximal shoulder elevation.

Acromioclavicular Joint

The articulation of the acromion process of the scapula with the distal end of the clavicle is known as the acromioclavicular joint. It is classified as an irregular diarthrodial joint, although the joint’s structure allows limited motion in all three planes. There is a significant amount of anatomical variation in the acromioclavicular (AC) joint from individual to individual, with as many as five different morphological types identified (61). Rotation occurs at the AC joint during arm elevation. The close-packed position of the AC joint occurs when the humerus is abducted to 90˚.

Coracoclavicular Joint

The coracoclavicular joint is a syndesmosis, formed where the coracoid process of the scapula and the inferior surface of the clavicle are bound together by the coracoclavicular ligament. This joint permits little movement. The coracoclavicular and acromioclavicular joints are shown in Figure 7-2.

Glenohumeral Joint

The glenohumeral joint is the most freely moving joint in the human body, enabling flexion, extension, hyperextension, abduction, adduction, horizontal abduction and adduction, and medial and lateral rotation of the humerus (Figure 7-3). The almost hemispherical head of the humerus has three to four times the amount of surface area as the shallow glenoid fossa of the scapula with which it articulates. The glenoid fossa is also less curved than the surface of the humeral head, enabling the humerus to move linearly across the surface of the glenoid fossa in addition to its extensive rotational capability (62). There are anatomical variations in the shape of the glenoid fossa from person to person, with an oval or egg-shaped cavity in about 45% of the population and a pear-shaped cavity in the remaining 55% (65). With passive rotation of the arm, large translations of the humeral head on the glenoid fossa are present at the extremes of the range of motion (39). The muscle forces during active rotation tend to limit ranges of motion at the shoulder, thereby limiting the humeral translation that occurs (39).

At the perimeter of the glenoid fossa is a lip or labrum composed of part of the joint capsule, the tendon of the long head of the biceps brachii, and the glenohumeral ligaments. This rim of dense collagenous tissue is triangular in cross-section and is attached to the periphery of the fossa. The labrum deepens the fossa and adds stability to the joint. The capsule surrounding the glenohumeral joint is shown in Figure 7-4. Several ligaments merge with the glenohumeral joint capsule, including the superior, middle, and inferior glenohumeral ligaments on the anterior side of the joint and the coracohumeral ligament on the superior side.

The tendons of four muscles also join the joint capsule. These are known as the rotator cuff muscles because they contribute to rotation of the humerus and because their tendons form a collagenous cuff around the glenohumeral joint. These include supraspinatus, infraspinatus, teres minor, and subscapularis, and are also sometimes referred to as the SITS muscles after the first letter of the muscles’ names. Supraspinatus, infraspinatus, and teres minor participate in lateral rotation, and subscapularis contributes to medial rotation. The muscles of the lateral rotator group exchange muscle bundles with one another, which increases their ability to quickly develop tension and functional power (29). The rotator cuff surrounds the shoulder on the posterior, superior, and anterior sides. Tension in the rotator cuff muscles pulls the head of the humerus toward the glenoid fossa, contributing significantly to the joint’s minimal stability. It has been shown that the rotator cuff muscles and the biceps are activated to provide shoulder stability prior to motion of the humerus (12). Negative pressure within the capsule of the glenohumeral joint also helps to stabilize the joint (32). The joint is most stable in its close-packed position, when the humerus is abducted and laterally rotated.

Scapulothoracic Joint

Because the scapula can move in both sagittal and frontal planes with respect to the trunk, the region between the anterior scapula and the thoracic wall is sometimes referred to as the scapulothoracic joint. The muscles attaching to the scapula perform two functions. First, they can contract to stabilize the shoulder region. For example, when a suitcase is lifted from the floor, the levator scapula, trapezius, and rhomboids develop tension to support the scapula, and in turn the entire shoulder, through the acromioclavicular joint. Second, the scapular muscles can facilitate movements of the upper extremity through appropriate positioning of the glenohumeral joint. During an overhand throw, for example, the rhomboids contract to move the entire shoulder posteriorly as the humerus is horizontally abducted and externally rotated during the preparatory phase. As the arm and hand then move forward to execute the throw, tension in the rhomboids is released to permit forward movement of the glenohumeral joint.

Bursae

Several small, fibrous sacs that secrete synovial fluid internally in a fashion similar to that of a joint capsule are located in the shoulder region. These sacs, known as bursae, cushion and reduce friction between layers of collagenous tissues. The shoulder is surrounded by several bursae, including the subscapularis, subcoracoid, and subacromial.

The subscapularis and subcoracoid bursae are responsible for managing friction of the superficial fibers of the subscapularis muscle against the neck of the scapula, the head of the humerus, and the coracoid process. In 28% of studied cases, these two bursae physically merge into a single wide bursa (11). Given that the subscapularis undergoes significant changes in orientation during movements of the arm at the glenohumeral joint, especially where the upper portion of the muscle coils around the coracoid process, the role of these bursae is important.

The subacromial bursa lies in the subacromial space, between the acromion process of the scapula and the coracoacromial ligament (above) and the glenohumeral joint (below). This bursa cushions the rotator cuff muscles, particularly the supraspinatus, from the overlying bony acromion (Figure 7-5). The subacromial bursa may become irritated when repeatedly compressed during overhead arm action.

Although some amount of glenohumeral motion may occur while the other shoulder articulations remain stabilized, movement of the humerus more commonly involves some movement at all three shoulder joints (Figure 7-6). Elevation of the humerus in all planes is accompanied by approximately 55˚ of lateral rotation (72). As the arm is elevated in both abduction and flexion, rotation of the scapula accounts for part of the total humeral range of motion. Although the absolute positions of the humerus and scapula vary due to anatomical variations among individuals, a general pattern persists (28). During about the first 30˚ of humeral elevation, the contribution of the scapula is only about one-fifth that of the glenohumeral joint (62). As elevation proceeds beyond 30˚, the scapula rotates approximately 1˚ for every 2˚ of movement of the humerus (17, 34, 69). This important coordination of scapular and humeral movements, known as scapulohumeral rhythm, enables a much greater range of motion at the shoulder than if the scapula were fixed. During the first 90˚ of arm elevation (in sagittal, frontal, or diagonal planes), the clavicle is also elevated through approximately 35–45˚ of motion at the sternoclavicular joint (62). Rotation at the acromioclavicular joint occurs during the first 30˚ of humeral elevation and again as the arm is moved from 135˚ to maximum elevation (41). Positioning of the humerus is further facilitated by motions of the spine. When the hands support an external load, the orientation of the scapula and scapulohumeral rhythm are altered (58).

The movement patterns of the scapula are also different in children and in the elderly. As compared to adults, children receive a greater contribution from the scapulothoracic joint during humeral elevation (13). With aging, there is a lessening of scapular rotation, as well as posterior tilt, with glenohumeral abduction (20).

Muscles of the Scapula

The muscles that attach to the scapula are the levator scapula, rhomboids, serratus anterior, pectoralis minor, and subclavius, and the four parts of the trapezius. Figures 7-7 and 7-8 show the directions in which these muscles exert force on the scapula when contracting. Scapular muscles have two general functions. First, they stabilize the scapula so that it forms a rigid base for muscles of the shoulder during the development of tension. For example, when a person carries a briefcase, the levator scapula, trapezius, and rhomboids stabilize the shoulder against the added weight. Second, scapular muscles facilitate movements of the upper extremity by positioning the glenohumeral joint appropriately. For example, during an overhand throw, the rhomboids contract to move the entire shoulder posteriorly as the arm and hand move posteriorly during the preparatory phase. As the arm and hand move anteriorly to deliver the throw, tension in the rhomboids subsides to permit forward movement of the shoulder, facilitating outward rotation of the humerus.

Muscles of the Glenohumeral Joint

Many muscles cross the glenohumeral joint. Because of their attachment sites and lines of pull, some muscles contribute to more than one action of the humerus. A further complication is that the action produced by the development of tension in a muscle may change with the orientation of the humerus because of the shoulder’s large range of motion. With the basic instability of the structure of the glenohumeral joint, a significant portion of the joint’s stability is derived from tension in the muscles and tendons crossing the joint. However, when one of these muscles develops tension, tension development in an antagonist may be required to prevent dislocation of the joint. A review of the muscles of the shoulder is presented in Table 7-1.

Flexion at the Glenohumeral Joint

The muscles crossing the glenohumeral joint anteriorly participate in flexion at the shoulder (Figure 7-9). The prime flexors are the anterior deltoid and the clavicular portion of the pectoralis major. The small coracobrachialis assists with flexion, as does the short head of the biceps brachii. Although the long head of the biceps also crosses the shoulder, it is not active in isolated shoulder motion when the elbow and forearm do not move (43).

Extension at the Glenohumeral Joint

When shoulder extension is unresisted, gravitational force is the primary mover, with eccentric contraction of the flexor muscles controlling or braking the movement. When resistance is present, contraction of the muscles posterior to the glenohumeral joint, particularly the sternocostal pectoralis, latissimus dorsi, and teres major, extend the humerus. The posterior deltoid assists in extension, especially when the humerus is externally rotated. The long head of the triceps brachii also assists, and because the muscle crosses the elbow, its contribution is slightly more effective when the elbow is in flexion. The shoulder extensors are illustrated in Figure 7-10.

Abduction at the Glenohumeral Joint

The middle deltoid and supraspinatus are the major abductors of the humerus. Both muscles cross the shoulder superior to the glenohumeral joint (Figure 7-11). The supraspinatus, which is active through approximately the first 110˚ of motion, initiates abduction. During the contribution of the middle deltoid (occurring from approximately 90˚ to 180˚ of abduction), the infraspinatus, subscapularis, and teres minor neutralize the superiorly dislocating component of force produced by the middle deltoid.

Adduction at the Glenohumeral Joint

As with extension at the shoulder, adduction in the absence of resistance results from gravitational force, with the abductors controlling the speed of motion. With resistance added, the primary adductors are the latissimus dorsi, teres major, and sternocostal pectoralis, which are located on the inferior side of the joint (Figure 7-12). The short head of the biceps and the long head of the triceps contribute minor assistance, and when the arm is elevated above 90˚, the coracobrachialis and subscapularis also assist.

Medial and Lateral Rotation of the Humerus

Medial, or inward, rotation of the humerus results primarily from the action of the subscapularis and teres major, both attaching to the anterior side of the humerus, with the subscapularis having the greatest mechanical advantage for medial rotation (42). Both portions of the pectoralis major, the anterior deltoid, the latissimus dorsi, and the short head of the biceps brachii assist, with the pectoralis major being the primary assistant (8). Muscles attaching to the posterior aspect of the humerus, particularly infraspinatus and teres minor, produce lateral, or outward, rotation, with some assistance from the posterior deltoid.

Horizontal Adduction and Abduction at the Glenohumeral Joint

The muscles anterior to the joint, including both heads of the pectoralis major, the anterior deltoid, and the coracobrachialis, produce horizontal adduction, with the short head of the biceps brachii assisting. Muscles posterior to the joint axis affect horizontal abduction. The major horizontal abductors are the middle and posterior portions of the deltoid, infraspinatus, and teres minor, with assistance provided by the teres major and the latissimus dorsi. The major horizontal adductors and abductors are shown in Figures 7-13 and 7-14.

Because the articulations of the shoulder girdle are interconnected, they function to some extent as a unit in bearing loads and absorbing shock. However, because the glenohumeral joint provides direct mechanical support for the arm, it sustains much greater loads than the other shoulder joints.

As indicated in Chapter 3, when analyzing the effect of body position, we may assume that body weight acts at the body’s center of gravity. Likewise, when analyzing the effect of the positions of body segments on a joint such as the shoulder, we assume that the weight of each body segment acts at the segmental center of mass. The moment arm for the entire arm segment with respect to the shoulder is therefore the perpendicular distance between the weight vector (acting at the arm’s center of gravity) and the shoulder (Figure 7-15). When the elbow is in flexion, the effects of the upper arm and the forearm/hand segments must be analyzed separately (Figure 7-16). Sample Problem 7.1 illustrates the effect of arm position on shoulder loading.

Although the weight of the arm is only approximately 5% of body weight, the length of the horizontally extended arm creates large segment moment arms and therefore large torques that must be countered by the shoulder muscles. When these muscles contract to support the extended arm, the glenohumeral joint sustains compressive forces estimated to reach 50% of body weight (63). Although this load is reduced by about half when the elbow is maximally flexed due to the shortened moment arms of the forearm and hand, this can place a rotational torque on the humerus that requires the activation of additional shoulder muscles (Figure 7-17).

Because of the effect of arm position on shoulder loading, ergonomists recommend that workers seated at a desk or a table attempt to position the arms with 20˚ or less of abduction and 25˚ or less of flexion (7). Workers who are required to hold the arms in a sustained position overhead are particularly susceptible to degenerative tendinitis in the biceps and supraspinatus (25).

Muscles that attach to the humerus at small angles with respect to the glenoid fossa contribute primarily to shear as opposed to compression at the joint. These muscles serve the important role of stabilizing the humerus in the glenoid fossa against the contractions of powerful muscles that might otherwise dislocate the joint. For example, when the arm is elevated, the deltoid and rotator cuff muscles act in tandem, with the deltoid producing an upward shear force that is counteracted by the downward shear produced by the rotator cuff (59) (Figure 7-18). Maximum shear force has been found to be present at the glenohumeral joint when the arm is elevated approximately 60˚ (80).

Sample Problem 7.1

Using the simplifying assumptions of Poppen and Walker (38), a free body diagram of the arm and shoulder can be constructed as shown below. If the weight of the arm is 33 N, the moment arm for the total arm segment is 30 cm, and the moment arm for the deltoid muscle (Fm) is 3 cm, how much force must be supplied by the deltoid to maintain the arm in this position? What is the magnitude of the horizontal component of the joint reaction force (Rh)?

Known

Solution

The torque at the shoulder created by the muscle force must equal the torque at the shoulder created by arm weight, yielding a net shoulder torque of zero.

Since the horizontal component of joint reaction force (Rh) and Fm are the only two horizontal forces present, and since the arm is stationary, these forces must be equal and opposite. The magnitude of Rh is therefore the same as the magnitude of Fm.

Note: Both components of the joint reaction force are directed through the joint center, and so have a moment arm of zero with respect to the center of rotation.

The shoulder is susceptible to both traumatic and overuse types of injuries, including 8–13% of all sport-related injuries (25).

Dislocations

The glenohumeral joint is the most commonly dislocated joint in the body (5). The loose structure of the glenohumeral joint enables extreme mobility but provides little stability, and dislocations may occur in anterior, posterior, and inferior directions. The strong coracohumeral ligament usually prevents displacement in the superior direction. Glenohumeral dislocations typically occur when the humerus is abducted and externally rotated, with anterior-inferior dislocations more common than those in other directions. Factors that predispose the joint to dislocations include inadequate size of the glenoid fossa, anterior tilt of the glenoid fossa, inadequate retroversion of the humeral head, and deficits in the rotator cuff muscles (69).

Glenohumeral dislocation may result from sustaining a large external force during an accident, such as in cycling, or during participation in a contact sport such as wrestling or football. Unfortunately, once the joint has been dislocated, the stretching of the surrounding collagenous tissues beyond their elastic limits commonly predisposes it to subsequent dislocations. Glenohumeral capsular laxity may also be present due to genetic factors. Individuals with this condition should strengthen their shoulder muscles before athletic participation (24).

Dislocations or separations of the acromioclavicular joint are also common among wrestlers and football players. When a rigidly outstretched arm sustains the force of a full-body fall, either acromioclavicular separation or fracture of the clavicle is likely to result.

Rotator Cuff Damage

A common injury among workers and athletes who engage in forceful overhead movements typically involving abduction or flexion along with medial rotation is rotator cuff impingement syndrome, also known as subacromial impingement syndrome, or shoulder impingement syndrome. This is the most common disorder of the shoulder, with progressive loss of function and disability (52). The cause is progressive pressure on the rotator cuff tendons by the surrounding bone and soft tissue structures. Symptoms include hypermobility of the anterior shoulder capsule, hypomobility of the posterior capsule, excessive external rotation coupled with limited internal rotation of the humerus, and general ligamentous laxity of the glenohumeral joint (81). This can result in inflammation of the underlying tendons and bursae or, in severe cases, rupture of one of the rotator cuff tendons. The muscle most commonly affected is the supraspinatus, possibly because its blood supply is the most susceptible to pressure (70). This condition is accompanied by pain and tenderness in the superior and anterior shoulder regions, and sometimes by associated shoulder weakness. The symptoms are exacerbated by rotary movements of the humerus, especially those involving elevation and internal rotation.

Activities that may promote the development of shoulder impingement syndrome include throwing (particularly an implement like the javelin), serving in tennis, and swimming (especially the freestyle, backstroke, and butterfly) (2, 60, 73). Among competitive swimmers, the syndrome is known as swimmer’s shoulder. Reports indicate shoulder pain complaints in up to 50% of competitive swimmers (41). Older golfers also frequently develop impaired rotator cuff function secondary to degenerative changes such as osteophyte formation that impinges upon the subacromial space (3).

Anatomical factors believed to predispose a person to impingement syndrome include a flat acromion with only a small inclination, bony spurs at the acromioclavicular joint secondary to osteoarthritis, and a superiorly positioned humeral head (19, 65, 78). A number of theories have been proposed regarding the biomechanical causes of rotator cuff problems. The impingement theory suggests that a genetic factor results in the formation of too narrow a space between the acromion process of the scapula and the head of the humerus. In this situation, the rotator cuff and associated bursae are pinched between the acromion, the acromioclavicular ligament, and the humeral head each time the arm is elevated, with the resulting friction causing irritation and wear. An alternative theory proposes that the major factor is inflammation of the supraspinatus tendon caused by repeated overstretching of the muscle–tendon unit. When the rotator cuff tendons become stretched and weakened, they cannot perform their normal function of holding the humeral head in the glenoid fossa. Consequently, the deltoid muscles pull the humeral head up too high during abduction, resulting in impingement and subsequent wear and tear on the rotator cuff.

The problem is relatively common among swimmers. Research has shown that during the recovery phase of swimming, when the arm is elevated above the shoulder while being medially rotated, the serratus anterior rotates the scapula so that the supraspinatus, infraspinatus, and middle deltoid may freely abduct the humerus (56). Each arm is in this position during an average of approximately 12% of the stroke cycle among collegiate male swimmers (82). It has been hypothesized that if the serratus, which develops nearly maximum tension to accomplish this task, becomes fatigued, the scapula may not be rotated sufficiently to abduct the humerus freely, and impingement may develop (56). Swimming technique appears to be related to the likelihood of developing shoulder impingement in swimmers, with a large amount of medial rotation of the arm during the pull phase, late initiation of lateral rotation of the arm during the overhead phase, and breathing exclusively on one side all implicated as contributing factors (83).

Rotational Injuries

Tears of the labrum, the rotator cuff muscles, and the biceps brachii tendon are among the injuries that may result from repeated, forceful rotation at the shoulder. Throwing, serving in tennis, and spiking in volleyball are examples of forceful rotational movements. If the attaching muscles do not sufficiently stabilize the humerus, it can articulate with the glenoid labrum rather than with the glenoid fossa, contributing to wear on the labrum. Most tears are located in the anterior-superior region of the labrum. Tears of the rotator cuff, primarily of the supraspinatus, have been attributed to the extreme tension requirements placed on the muscle group during the deceleration phase of a vigorous rotational activity. There is a region of low vascularity near the insertion of the supraspinatus tendon, which is the most common site of rotator cuff inflammation and tears (49). The fact that vascularization diminishes with aging may explain why rotator cuff problems occur more frequently in individuals over 40 years of age (49). Tears of the biceps brachii tendon at the site of its attachment to the glenoid fossa may result from the forceful development of tension in the biceps when it negatively accelerates the rate of elbow extension during throwing (51).

Other pathologies of the shoulder attributed to throwing movements are calcifications of the soft tissues of the joint and degenerative changes in the articular surfaces (65). Bursitis, the inflammation of one or more bursae, is another overuse syndrome, generally caused by friction within the bursa (66).

Subscapular Neuropathy

Subscapular neuropathy, or subscapular nerve palsy, most commonly occurs in athletes involved in overhead activities and weight lifting (14). It has been reported in volleyball, baseball, football, and racquetball players, as well as backpackers, gymnasts, and dancers. The condition arises from compression of the subscapular nerve, which occurs most commonly at the suprascapular notch. In volleyball players, it has been attributed to the repeated stretching of the nerve during extreme shoulder abduction and lateral rotation, such as occurs during the serving motion (22).

Although the elbow is generally considered a simple hinge joint, it is actually categorized as a trochoginglymus joint that encompasses three articulations: the humeroulnar, humeroradial, and proximal radioulnar joints (1). All are enclosed in the same joint capsule, which is reinforced by the anterior and posterior radial collateral and ulnar collateral ligaments. Bony structure provides about half of the elbow’s stability, with the remaining stability provided by the joint capsule and the ulnar and radial ligament complexes (53).

Humeroulnar Joint

The hinge joint at the elbow is the humeroulnar joint, where the ovular trochlea of the humerus articulates with the reciprocally shaped trochlear fossa of the ulna (Figure 7-19). Flexion and extension are the primary movements, although in some individuals, a small amount of hyperextension is allowed. The joint is most stable in the close-packed position of extension.

Humeroradial Joint

The humeroradial joint is immediately lateral to the humeroulnar joint and is formed between the spherical capitellum of the humerus and the proximal end of the radius (Figure 7-19). Although the humeroradial articulation is classified as a gliding joint, the immediately adjacent humeroulnar joint restricts motion to the sagittal plane. In the close-packed position, the elbow is flexed at 90˚ and the forearm is supinated about 5˚.

Proximal Radioulnar Joint

The annular ligament binds the head of the radius to the radial notch of the ulna, forming the proximal radioulnar joint. This is a pivot joint, with forearm pronation and supination occurring as the radius rolls medially and laterally over the ulna (Figure 7-20). The close-packed position is at 5˚ of forearm supination.

Carrying Angle

The angle between the longitudinal axes of the humerus and the ulna when the arm is in anatomical position is referred to as the carrying angle. The size of the carrying angle ranges from 10˚ to 15˚ in adults and tends to be larger in females than in males. The carrying angle changes with skeletal growth and is always greater on the side of the dominant hand (57). No particular functional significance has been associated with the carrying angle.

Muscles Crossing the Elbow

Numerous muscles cross the elbow, including those that also cross the shoulder or extend into the hands and fingers. The muscles classified as primary movers of the elbow are summarized in Table 7-2.

Flexion and Extension

The muscles crossing the anterior side of the elbow are the elbow flexors (Figure 7-21). The strongest of the elbow flexors is the brachialis. Since the distal attachment of the brachialis is the coronoid process of the ulna, the muscle is equally effective when the forearm is in supination or pronation. Because it is the major forearm flexor, the brachialis has been called the workhorse of the elbow (27).

Another elbow flexor is the biceps brachii, with both long and short heads attached to the radial tuberosity by a single common tendon. The muscle contributes effectively to flexion when the forearm is supinated, because it is slightly stretched. When the forearm is pronated, the muscle is less taut and consequently less effective.

The brachioradialis is a third contributor to flexion at the elbow. This muscle is most effective when the forearm is in a neutral position (midway between full pronation and full supination) because of its distal attachment to the base of the styloid process on the lateral radius. In this position, the muscle is in slight stretch, and the radial attachment is centered in front of the elbow joint.

The major extensor of the elbow is the triceps, which crosses the posterior aspect of the joint (Figure 7-22). Although the three heads have separate proximal attachments, they attach to the olecranon process of the ulna through a common distal tendon. Even though the distal attachment is relatively close to the axis of rotation at the elbow, the size and strength of the muscle make it effective as an elbow extensor. The relatively small anconeus muscle, which courses from the posterior surface of the lateral epicondyle of the humerus to the lateral olecranon and posterior proximal ulna, also assists with extension. Research has shown that the lateral and medial heads of the triceps contribute 70–90% of the extension moment, with approximately 15% contributed by the anconeus (84). The long head of the triceps, crossing both the elbow and the shoulder, was found to contribute significantly less than the other muscles (84).

Pronation and Supination

Pronation and supination of the forearm involve rotation of the radius around the ulna. There are three radioulnar articulations: the proximal, middle, and distal radioulnar joints. Both the proximal and the distal joints are pivot joints, and the middle radioulnar joint is a syndesmosis at which an elastic, interconnecting membrane permits supination and pronation but prevents longitudinal displacement of the bones.

The major pronator is the pronator quadratus, which attaches to the distal ulna and radius (Figure 7-23). When pronation is resisted or rapid, the pronator teres crossing the proximal radioulnar joint assists.

As the name suggests, the supinator is the muscle primarily responsible for supination (Figure 7-24). It is attached to the lateral epicondyle of the humerus and to the lateral proximal third of the radius. When the elbow is in flexion, tension in the supinator lessens, and the biceps assists with supination. When the elbow is flexed to 90˚ or less, the biceps is positioned to serve as a strong supinator (75).

Although the elbow is not considered to be a weight-bearing joint, it regularly sustains large loads during daily activities. For example, research shows that the compressive load at the elbow reaches an estimated 300 N (67 lb) during activities such as dressing and eating, 1700 N (382 lb) when the body is supported by the arms when rising from a chair, and 1900 N (427 lb) when the individual is pulling a table across the floor (35). During moderate levels of activity, a modeling study predicts humeroulnar forces of up to 1600 N and humeroradial forces of up to 800 N (6). During push-up exercises peak forces on each elbow are calculated to reach 45% of body weight (16). Greater posterior and varus forces are generated at the elbow when the hands are medially rotated as compared to being in the neutral or laterally rotated positions (48).

Even greater loads are present during the execution of selected sport skills. During baseball pitching, the elbow undergoes a valgus torque of as much as 64 N-m, with muscle force as large as 1000 N required to prevent dislocation (47). The amount of valgus torque generated is most closely related to the pitcher’s body weight (68). During the execution of gymnastic skills such as the handspring and the vault, the elbow functions as a weight-bearing joint. Research indicates that maximal isometric flexion when the elbow is fully extended can produce joint compressionforces of as much as two times body weight (30).

Since the attachment of the triceps tendon to the ulna is closer to the elbow joint center than the attachments of the brachialis on the ulna and the biceps on the radius, the extensor moment arm is shorter than the flexor moment arm. This means that the elbow extensors must generate more force than the elbow flexors to produce the same amount of joint torque. This translates to larger compression forces at the elbow during extension than during flexion when movements of comparable speed and force requirements are executed. Sample Problem 7.2 illustrates the relationship between moment arm and torque at the elbow.

Because of the shape of the olecranon process, the triceps moment arm also varies with the position of the elbow. As shown in Figure 7-25, the triceps moment arm is larger when the arm is fully extended than when it is flexed past 90˚.

Sample Problem 7.2

How much force must be produced by the brachioradialis and biceps (Fm) to maintain the 15 N forearm and hand in the position shown below, given moment arms of 5 cm for the muscles and 15 cm for the forearm/hand weight? What is the magnitude of the joint reaction force?

Known

Solution

The torque at the elbow created by the muscle force must equal the torque at the elbow created by forearm/hand weight, yielding a net elbow torque of zero.

Since the arm is stationary, the sum of all of the acting vertical forces must be equal to zero. In writing the force equation, it is convenient to regard upward as the positive direction.

Although the elbow is a stable joint reinforced by large, strong ligaments, the large loads placed on the joint during daily activities and sport participation render it susceptible to dislocations and overuse injuries.

Sprains and Dislocations

Forced hyperextension of the elbow can cause posterior displacement of the coronoid process of the ulna with respect to the trochlea of the humerus. Such displacement stretches the ulnar collateral ligament, which may rupture (sprain) anteriorly.

Continued hyperextension of the elbow can cause the distal humerus to slide over the coronoid process of the ulna, resulting in dislocation. Although dislocations of the elbow do not occur as frequently as dislocations of the glenohumeral joint, elbow dislocations occurring in individuals under age 30 are most likely to arise during participation in sports (38). The mechanism involved typically is a fall on an outstretched hand or a forceful, twisting blow. The subsequent stability of a once-dislocated elbow is impaired, particularly if the dislocation was accompanied by humeral fracture or rupture of the ulnar collateral ligament (38). Because of the large number of nerves and blood vessels passing through the elbow, elbow dislocations are of particular concern.

Elbow dislocations in young children age 1–3 are sometimes referred to as “nursemaid’s elbow” or “pulled elbow.” Adults should avoid lifting or swinging young children by the hands, wrists, or forearms, as this type of injury can result.

Overuse Injuries

With the exception of the knee, the elbow is the joint most commonly affected by overuse injuries (36). Stress injuries to the collagenous tissues at the elbow are progressive. The first symptoms are inflammation and swelling, followed by scarring of the soft tissues. If the condition progresses further, calcium deposits accumulate and ossification of the ligaments ensues.

Lateral epicondylitis involves inflammation or microdamage to the tissues on the lateral side of the distal humerus, including the tendinous attachment of the extensor carpi radialis brevis and possibly that of the extensor digitorum. Although a host of factors may contribute to the development of the condition, overuse of the wrist extensors is cited as a major culprit (26).

Because of the relatively high incidence of lateral epicondylitis among tennis players, the injury is commonly referred to as tennis elbow. A reported 30–40% of tennis players develop lateral epicondylitis, with onset typically in players age 35–50 (36). The amount of force to which the lateral aspect of the elbow is subjected during tennis play increases with poor technique and improper equipment. For example, hitting off-center shots and using an overstrung racquet increase the amount of force transmitted to the elbow (26). Activities such as swimming, fencing, and hammering can contribute to lateral epicondylitis as well (27).

Medial epicondylitis, which has been called Little Leaguer’s elbow, is the same type of injury to the tissues on the medial aspect of the distal humerus. During pitching, the valgusstrain imparted to the medial aspect of the elbow during the initial stage, when the trunk and shoulder are brought forward ahead of the forearm and hand, contributes to development of the condition. Medial epicondyle avulsion fractures have also been attributed to forceful terminal wrist flexion during the follow-through phase of the pitch (36). More commonly, however, throwing injuries to the elbow are chronic rather than acute. Injury or stretching of the ulnar collateral ligament can result in valgus instability, which, with repeated valgus overload during repetitive throwing, can provoke the development of bony changes that further exacerbate the problem (9, 18). Although uncommon among athletes in general, valgus instability is seen with a higher incidence in individuals who throw repetitively (33). Throwing curve balls and sliders requires added forceful rotation of the arm and hand during the delivery, and increases the stresses on the elbow and shoulder. For this reason, these pitches are contraindicated for Little League pitchers. It is recommended that young athletes not progress more than 10% per week in the number of balls thrown (23).

Medial and lateral epicondylitis occur with about equal frequency in golfers, particularly amateurs (3, 71). Among right-handed golfers, lateral epicondylitis occurs more often on the left side, and medial epicondylitis is found more often on the right side (3). Lateral epicondylitis may be related to gripping the club with excessive pronation of the right hand, while medial epicondylitis appears to be associated with repeatedly striking the ground with the club (3).

The wrist is composed of radiocarpal and intercarpal articulations (Figure 7-26). Most wrist motion occurs at the radiocarpal joint, a condyloid joint where the radius articulates with the scaphoid, the lunate, and the triquetrum. The joint allows sagittal plane motions (flexion, extension, and hyperextension) and frontal plane motions (radial deviation and ulnar deviation), as well as circumduction. Its close-packed position is in extension with radial deviation. A cartilaginous disc separates the distal head of the ulna from the lunate and triquetral bones and the radius. Although this articular disc is common to both the radiocarpal joint and the distal radioulnar joint, the two articulations have separate joint capsules. The radiocarpal joint capsule is reinforced by the volar radiocarpal, dorsal radiocarpal, radial collateral, and ulnar collateral ligaments. The intercarpal joints are gliding joints that contribute little to wrist motion.

The fascia around the wrist is thickened into strong fibrous bands called retinacula, which form protective passageways through which tendons, nerves, and blood vessels pass. The flexor retinaculum protects the extrinsic flexor tendons and the median nerves where they cross the palmar side of the wrist. On the dorsal side of the wrist, the extensor retinaculum provides a passageway for the extrinsic extensor tendons.

The wrist is capable of sagittal and frontal plane movements, as well as rotary motion (Figure 7-27). Flexion is motion of the palmar surface of the hand toward the anterior forearm. Extension is the return of the hand to anatomical position, and in hyperextension, the dorsal surface of the hand approaches the posterior forearm. Movement of the hand toward the thumb side of the arm is radial deviation, with movement in the opposite direction designated as ulnar deviation. Rotational movement of the hand through all four directions produces circumduction. Because of the complex structure of the wrist, rotational movements at the wrist are also complex, with different axes of rotation and different mechanisms through which wrist motions occur (55).

Flexion

The muscles responsible for flexion at the wrist are the flexor carpi radialis and the powerful flexor carpi ulnaris (Figure 7-28). The palmaris longus, which is often absent in one or both forearms, contributes to flexion when present. All three muscles have proximal attachments on the medial epicondyle of the humerus. The flexor digitorum superficialis and flexor digitorum profundus can assist with flexion at the wrist when the fingers are completely extended, but when the fingers are in flexion, these muscles cannot develop sufficient tension due to active insufficiency.

Extension and Hyperextension

Extension and hyperextension at the wrist result from contraction of the extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris (Figure 7-29). These muscles originate on the lateral epicondyle of the humerus. The other posterior wrist muscles may also assist with extension, particularly when the fingers are in flexion. Included in this group are the extensor pollicis longus, extensor indicis, extensor digiti minimi, and extensor digitorum (Figure 7-30).

Radial and Ulnar Deviation

Cooperative action of both flexor and extensor muscles produces lateral deviation of the hand at the wrist. The flexor carpi radialis and extensor carpi radialis longus and brevis contract to produce radial deviation, and the flexor carpi ulnaris and extensor carpi ulnaris cause ulnar deviation.

A large number of joints are required to provide the extensive motion capabilities of the hand. Included are the carpometacarpal (CM), intermetacarpal, metacarpophalangeal (MP), and interphalangeal (IP) joints (Figure 7-31). The fingers are referred to as digits one through five, with the first digit being the thumb.

Carpometacarpal and Intermetacarpal Joints

The carpometacarpal (CM) joint of the thumb, the articulation between the trapezium and the first metacarpal, is a classic saddle joint. The other CM joints are generally regarded as gliding joints. All carpometacarpal joints are surrounded by joint capsules, which are reinforced by the dorsal, volar, and interosseous CM ligaments. The irregular intermetacarpal joints share these joint capsules.

Metacarpophalangeal Joints

The metacarpophalangeal (MP) joints are the condyloid joints between the rounded distal heads of the metacarpals and the concave proximal ends of the phalanges. These joints form the knuckles of the hand. Each joint is enclosed in a capsule that is reinforced by strong collateral ligaments. A dorsal ligament also merges with the MP joint of the thumb. Close-packed positions of the MP joints in the fingers and thumb are full flexion and opposition, respectively.

Interphalangeal Joints

The proximal and distal interphalangeal (IP) joints of the fingers and the single interphalangeal joint of the thumb are all hinge joints. An articular capsule joined by volar and collateral ligaments surrounds each IP joint. These joints are most stable in the close-packed position of full extension.

The carpometacarpal (CM) joint of the thumb allows a large range of movement similar to that of a ball-and-socket joint (Figure 7-32). Motion at CM joints two through four is slight due to constraining ligaments, with somewhat more motion permitted at the fifth CM joint.

The metacarpophalangeal (MP) joints of the fingers allow flexion, extension, abduction, adduction, and circumduction for digits two through five, with abduction defined as movement away from the middle finger and adduction being movement toward the middle finger (Figure 7-33). Because the articulating bone surfaces at the metacarpophalangeal joint of the thumb are relatively flat, the joint functions more as a hinge joint, allowing only flexion and extension.

The interphalangeal (IP) joints permit flexion and extension, and in some individuals, slight hyperextension. These are classic hinge joints. Due to passive tension in the [gx id=""]extrinsic muscles[/gx], when the hand is relaxed and the wrist moves from full flexion to full extension, the distal interphalangeal joints go from approximately 12˚ to 31˚ of flexion, and the proximal IP joints go from about 19˚ to 70˚ of flexion (74).

A relatively large number of muscles are responsible for the many precise movements performed by the hand and fingers (Table 7-3). There are 9 [gx id=""]extrinsic muscles[/gx] that cross the wrist and 10 [gx id=""]intrinsic muscles[/gx] with both of their attachments distal to the wrist.

The extrinsic flexor muscles of the hand are more than twice as strong as the strongest of the extrinsic extensor muscles (79). This should come as little surprise, given that the flexor muscles of the hand are used extensively in everyday activities involving gripping, grasping, or pinching movements, while the extensor muscles rarely exert much force. Multidirectional force measurement for the index finger shows the highest force production in flexion with forces generated in extension, abduction, and adduction being about 38%, 98%, and 79%, respectively, of the flexion force (45). The strongest of the extrinsic flexor muscles are the flexor digitorum profundus and the flexordigitorum superficialis, collectively contributing over 80% of all flexion force (46).

The hand is used almost continuously in daily activities and in many sports. Wrist sprains or strains are fairly common and are occasionally accompanied by dislocation of a carpal bone or the distal radius. These types of injuries often result from the natural tendency to sustain the force of a fall on the hyperextended wrist. It has been shown that falls from heights greater than 0.6 m can readily result in wrist fracture, since the peak force sustained exceeds the average fracture force for the distal radius (10). Fracture of the distal radius is the most common type of fracture in the population under 75 years of age and is second only to vertebral fractures among the elderly (10). Fractures of the scaphoid and lunate bones are relatively common for the same reason.

Certain hand/wrist injuries are characteristic of participation in a given sport. Examples are metacarpal (boxer’s) fractures and mallet or drop finger deformity resulting from injury at the distal interphalangeal joints among football receivers and baseball catchers. Forced abduction of the thumb leading to ulnar collateral ligament injury often results from wrestling, football, hockey, and skiing (37). The most common injuries encountered in skateboarding and snowboarding are fractures of or close to the wrist (15). In sport rock climbing, 62% of all injuries are to the elbow, forearm, wrist, and hand, with many injuries specific to the handholds employed (31).

The wrist is the most frequently injured joint among golfers, with right-handed golfers tending to injure the left wrist (3). Both overuse injuries such as De Quervains disease (tendinitis of the extensor pollicis brevis and the abductor pollicis longus) and impact-related injuries are common (54). According to one study, golfers with overuse injuries of the wrist use a larger-than-average range of motion of the wrists during the swing (4).

Carpal tunnel syndrome is a fairly common disorder that occurs most often in women of middle age or older (70). The carpal tunnel is a passageway between the carpal bones and the flexor retinaculum on the palmar side of the wrist. Although the cause of this disorder in a given individual is often unknown, any swelling caused by acute or chronic trauma in the region can compress the median nerve, which passes through the carpal tunnel, thus bringing on the syndrome. Tendon and nerve movement during prolonged repetitive hand movement and incursion of the flexor muscles into the carpal tunnel during wrist extension have been hypothesized as causes for carpal tunnel syndrome (40, 77). Symptoms include pain and numbness along the median nerve, clumsiness of finger function, and eventually weakness and atrophy of the muscles supplied by the median nerve. Workers at jobs requiring repeated forceful wrist flexion and office workers who habitually rest the arms on the palmar sides of the wrist are especially vulnerable to carpal tunnel syndrome. Research indicates that modifications in keyboard design can dramatically affect tendon motion at the wrist, with promising implications for reducing the incidence of overuse injuries (76). The goal of workstation modifications in preventing carpal tunnel syndrome is enabling work with the wrist in neutral position (21, 44). Carpal tunnel syndrome has also been reported among athletes in badminton, baseball, cycling, gymnastics, field hockey, racquetball, rowing, skiing, squash, tennis, and rock climbing (14).

The shoulder is the most complex joint in the human body, with four different articulations contributing to movement. The glenohumeral joint is a loosely structured ball-and-socket joint in which range of movement is substantial and stability is minimal. The sternoclavicular joint enables some movement of the bones of the shoulder girdle, clavicle, and scapula. Movements of the shoulder girdle contribute to optimal positioning of the glenohumeral joint for different humeral movements. Small movements are also provided by the acromioclavicular and coracoclavicular joints.

The humeroulnar articulation controls flexion and extension at the elbow. Pronation and supination of the forearm occur at the proximal and distal radioulnar joints.

The structure of the condyloid joint between the radius and the three carpal bones controls motion at the wrist. Flexion, extension, radial flexion, and ulnar flexion are permitted. The joints of the hand at which most movements occur are the carpometacarpal joint of the thumb, the metacarpophalangeal joints, and the hinges at the interphalangeal articulations.

1. 1. Construct a chart listing all muscles crossing the glenohumeral joint according to whether they are superior, inferior, anterior, or posterior to the joint center. Note that some muscles may fall into more than one category. Identify the action or actions performed by muscles in each of the four categories.

2. 2. Construct a chart listing all muscles crossing the elbow joint according to whether they are medial, lateral, anterior, or posterior to the joint center with the arm in anatomical position. Note that some muscles may fall into more than one category. Identify the action or actions performed by muscles in each of the four categories.

3. 3. Construct a chart listing all muscles crossing the wrist joint according to whether they are medial, lateral, anterior, or posterior to the joint center with the arm in anatomical position. Note that some muscles may fall into more than one category. Identify the action or actions performed by muscles in each of the four categories.

4. 4. List the muscles that develop tension to stabilize the scapula during each of the following activities:

   • a. Carrying a suitcase

   • b. Water-skiing

   • c. Performing a push-up

   • d. Performing a pull-up

5. 5. List the muscles used as agonists, antagonists, stabilizers, and neutralizers during the performance of a push-up.

6. 6. Explain how the use of an overhand as compared to an underhand grip affects an individual’s ability to perform a pull-up.

7. 7. Select a familiar activity and identify the muscles of the upper extremity that are used as agonists during the activity.

8. 8. Using the diagram in Sample Problem 7.1 as a model, calculate the tension required in the deltoid with a moment arm of 3 cm from the shoulder, given the following weights and moment arms for the upper arm (u), forearm (f), and hand (h) segments: wtu = 19 N, wtf = 11 N, wth = 4 N, du = 12 cm, df = 40 cm, dh = 64 cm. (Answer: 308 N)

9. 9. Which of the three segments in Problem 8 creates the largest torque about the shoulder when the arm is horizontally extended? Explain your answer and discuss the implications for positioning the arm for shoulder-level tasks.

10. 10. Solve Sample Problem 7.2 with the addition of a 10 kg bowling ball held in the hand at a distance of 35 cm from the elbow. (Remember that kg is a unit of mass, not weight!) (Answer: Fm = 732 N, R5619 N)

1. 1. Identify the sequence of movements that occur at the scapulothoracic, shoulder, elbow, and wrist joints during the performance of an overhand throw.

2. 2. Which muscles are most likely to serve as agonists to produce the movements identified in your answer to Problem 1?

3. 3. Select a familiar racket sport and identify the sequence of movements that occur at the shoulder, elbow, and wrist joints during the execution of forehand and backhand strokes.

4. 4. Which muscles are most likely to serve as agonists to produce the movements identified in your answer to Problem 3?

5. 5. Select five resistance-based exercises for the upper extremity, and identify which muscles are the primary movers and which muscles assist during the performance of each exercise.

6. 6. Discuss the importance of the rotator cuff muscles as stabilizers of the glenohumeral joint and movers of the humerus.

7. 7. Discuss possible mechanisms of rotator cuff injury. Include in your discussion the implications of the force–length relationship for muscle (described in Chapter 6).

8. 8. How much tension (Fm) must be supplied by the triceps to stabilize the arm against an external force (Fe) of 200 N, given dm = 2 cm and de = 25 cm? What is the magnitude of the joint reaction force (R)? (Since the forearm is vertical, its weight does not produce torque at the elbow.) (Answer: Fm = 2500 N, R = 2700 N)

   

9. 9. What is the length of the moment arm between the dumbbell and the shoulder when the extended 50 cm arm is positioned at a 60˚ angle? (Answer: 43.3 cm)

   

10. 10. The medial deltoid attaches to the humerus at an angle of 15˚. What are the sizes of the rotary and stabilizing components of muscle force when the total muscle force is 500 N? (Answer: rotary component = 129 N, stabilizing component = 483 N)

    

• The glenohumeral joint is considered to be the shoulder joint.

• The clavicles and the scapulae make up the shoulder girdle.

• Most of the motion of the shoulder girdle takes place at the sternoclavicular joints.

• The extreme mobility of the glenohumeral joint is achieved at the expense of joint stability.

• The scapular muscles perform two functions: (a) stabilizing the scapula when the shoulder complex is loaded, and (b) moving and positioning the scapula to facilitate movement at the glenohumeral joint.

• The development of tension in one shoulder muscle must frequently be accompanied by the development of tension in an antagonist to prevent dislocation of the humeral head.

• When a glenohumeral joint dislocation occurs, the supporting soft tissues are often stretched beyond their elastic limits, thus predisposing the joint to subsequent dislocations.

• The humeroulnar hinge joint is considered to be the elbow joint.

• When pronation and supination of the forearm occurs, the radius pivots around the ulna.

• The radiocarpal joints make up the wrist.

• The large range of movement of the thumb compared to that of the fingers is derived from the structure of the thumb’s carpometacarpal joint.

Barr AE and Bear-Lehman J: Biomechanics of the wrist and hand. In Nordin M and Frankel VH, eds: Basic biomechanics of the musculoskeletal system (3rd ed), Philadelphia, 2001, Lippincott Williams & Wilkins.

Includes comprehensive discussion of wrist and hand anatomy, control, and kinematics, plus sections on interaction of wrist and hand motion and patterns of prehensile hand function.

Cavallo RJ and Speer KP: Shoulder instability and impingement in throwing athletes, Med Sci Sports Exerc 30:S18, 1998.

Describes glenohumeral anatomy, biomechanics of throwing and the pathophysiology of instability, and impingement syndrome, including diagnosis and treatment.

Jazrawi LM, Rokito AS, Birdzell G, and Zuckerman JD: Biomechanics of the elbow. In Nordin M and Frankel VH, eds: Basic biomechanics of the musculoskeletal system (3rd ed), Philadelphia, 2001, Lippincott Williams & Wilkins.

Describes kinematics and kinetics of the elbow, with sample calculations of joint reaction forces.

Prescher A: Anatomical basics, variations, and degenerative changes of the shoulder joint and shoulder girdle, Eur J Rad 35:88, 2000.

Thoroughly reviews shoulder anatomy, degenerative changes associated with aging, and impingement syndrome, with numerous photographic preparations of anatomical structures.

Arthroscopy.com

• www.arthroscopy.com/sports.htm
• Includes information and color graphics on the anatomy and function of the upper extremity.

E-hand.com: The Electronic Handbook of Hand Surgery

• http://www.eatonhand.com/
• Presents a comprehensive list of links related to anatomy, injury, treatment, and research on the hand, including a clip art image gallery and hand surgery slide lecture archives.

Northern Rockies Orthopaedics Specialists

• www.orthopaedic.com
• Provides links to pages describing descriptions of injuries, diagnostic tests, and surgical procedures for the shoulder, elbow, wrist, and hand.

Rothman Institute

• http://www.rothmaninstitute.com/index.html
• Includes information on common sport injuries to the shoulder and elbow.

Southern California Orthopaedic Institute

• www.scoi.com
• Includes links to anatomical descriptions and labeled photographs of the shoulder, elbow, wrist, and hand.

University of Washington Orthopaedic Physicians

• www.orthop.washington.edu
• Provides radiographs and information on common injuries and pathological conditions for the hand and wrist.

Virtual Hospital: Joint Fluoroscopy

• www.vh.org/Providers/Textbooks/JointFluoro/JointFluoroHP.html
• Provides detailed descriptions of anatomical features and includes multiple-view electric joint fluoroscopies of the elbow, lateral elbow, shoulder, and wrist.

“Virtual’’ Medical Center: Anatomy & Histology Center

• http://www.martindalecenter.com/MedicalAnatomy.html
• Contains numerous images, movies, and course links for human anatomy.

Wheeless’ Textbook of Orthopaedics

• www.wheelessonline.com/
• Provides comprehensive, detailed information, graphics, and related literature for all the joints.