The hand is distal to the forearm, and its skeletal framework includes the carpus, or wrist. In lay usage, however, the word "wrist" is used for the distal end of the forearm, a wrist-watch being worn over the lower ends of the radius and ulna. The position of the hand at rest is shown in figure 11-1. The hand is of enormous importance, particularly the thumb. Many hand injuries result in permanent disabilities.
The fingers (or digits of the hand) are numbered from one to five, beginning with the thumb. In clinical notes the fingers should be identified by name rather than by number: thumb (pollex), and index, middle, ring, and little fingers. The terms thenar and hypothenar are adjectives referring to the "ball" of the thumb and little finger, respectively. General features of the hand that should be noted include the lengths of the fingers, the thin and mobile skin of the dorsum (allowing accumulation of fluid subcutaneously), the thick and anchored skin of the palm, the flexure lines associated with cutaneous movement (but which do not necessarily indicate the sites of joints), the fingerprints, and the nails. The subcutaneous fat that constitutes the tips of the fingers is loculated by fibrous septa and occupies a closed pulp space, which is liable to infection (felon, or whitlow) from a penetrating wound followed by the possibility of damage to the distal phalanx owing to involvement of its blood supply.
The flexor retinaculum is a strong, transverse, fibrous band that confines the flexor tendons of the five fingers, together with their synovial sheaths and the median nerve, to the arch of the carpus, which it converts into the carpal tunnel (figs. 11-2, 11-3 and 11-4). The retinaculum is situated distal to the flexion creases in the skin of the wrist. It is attached laterally to the tubercles of the scaphoid and trapezium and medially to the triquetrum, pisiform, and the hook of the hamate. At least the proximal points of attachment can be felt, and, because the retinaculum measures about 3 cm proximo-distally, the quadrilateral outline of the retinaculum can be mapped. Owing to a variety of known and unknown causes, the median nerve may become constricted beneath the flexor retinaculum, resulting in tingling, numbness, and muscular signs (carpal tunnel syndrome). The condition is treated by dividing the flexor retinaculum.
The palmar aponeurosis is a strong, triangular membrane overlying the tendons in the palm (fig. 11-2). Its apex is continuous with the palmaris longus (when present) and is anchored to the anterior aspect of the flexor retinaculum, from which it can be distinguished by the longitudinal direction of its fibers. The aponeurosis continues distally as four slips (pretendinous bands), which overlie the flexor tendons of the medial four fingers. Flexion deformity (Dupuytren's contracture) of one or more fingers, especially the little and ring fingers, is commonly attributed to thickening and shortening of the palmar aponeurosis, although its causation is not really understood. Severe instances are usually treated by excision of the aponeurosis.
The flexor tendons leave the carpal tunnel and enter a central compartment in the palm (fig. 11-2). This compartment contains the flexor tendons and their synovial sheaths, the lumbrical muscles, and the digital nerves and vessels. The central compartment is bounded (1) on the anterior side by the deep aspect of the palmar aponeurosis, (2) on the posterior aspect by a fat pad on the interosseous fascia and by the fascia anterior to the adductor pollicis muscle, and (3) on the sides by the fascia covering the thenar and hypothenar muscles. Uncontrolled infection (e.g., from a synovial sheath) may rupture into the central compartment and spread proximally into the forearm, anterior to the pronator quadratus muscle and its fascia. Although the central compartment is one space proximally, it is subdivided distally by a large number of sagittal septa. In clinical practice, it is customary to select one of these septa (attached to the third metacarpal) for special consideration and to use it as the basis for a description of two fascial "spaces" of the palm. What are termed the "midpalmar space" medially and the "thenar space" laterally are said to be situated posterior to the flexor tendons and are regarded as important sites for the accumulation of pus. Anatomical investigations, however, do not support the generally accepted clinical anatomy of the palm. (See, for example, F. Bojsen-Mol1er and L. Schmidt, J. Anat., 117:55, 1974)
The flexor tendons, being retained in place by fascial retinacula, are invested by synovial sheaths, which facilitate gliding (fig. 11-3). Three synovial sheaths are found on the antrior wrist: (1) a common synovial flexor sheath envelops all the superficial is and profundus tendons, (2) a sheath surrounds the tendon of the flexor pollicis longus, and (3) the flexor carpi radialis tendon has a short sheath. Untreated infection of the synovial sheaths is liable to impair hand function. The common and pollical sheaths are frequently referred to in clinical writing as the ulnar and radial bursae, respectively. These two sheaths project proximally a short distance above the flexor retinaculum, and they usually communicate with each other in the carpal tunnel. The pollical sheath extends distally almost to its insertion, and that part of the common sheath for the little finger usually does likewise. The ring, middle, and index fingers are provided with digital sheaths that usually extend proximally no further than the necks of their metacarpal bones, leaving a gap of 1 to 3 cm below the common sheath. Hence infection of the synovial sheaths of the thumb or little finger may spread readily into the palm and even into the forearm.
The muscles intrinsic to the hand are those of the thumb (thenar muscles) and of the little finger (hypothenar muscles), the palmar and dorsal interossei, and the lumbricals. They are supplied by the median and ulnar nerves, which carry fibers of the T1 segment of the spinal cord. These muscles are of great functional importance, particularly those associated with the thumb. Their individual innervations and actions are significant in evaluating peripheral nerve injuries.
Thenar Muscles (table 11-1). The short muscles of the thumb (fig. 11-5) are the abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, adductor pollicis, and the first palmar and dorsal interossei. The short abductor forms the lateral side of the thenar eminence, and the flexor (with which it is often fused) forms the medial side, whereas the opponens is deep to the other muscles. The three muscles are supplied by the important recurrent branch of the median nerve, which lies very superficially at the distal border of the flexor retinaculum (see figs. 10-2 and 11-9). The flexor is generally said to possess a variously defined deep head, e.g., arising in common with the oblique head of the adductor and supplied by the ulnar nerve. The adductor is more deeply placed and has two heads of origin. Two muscles, the abductor and flexor brevis, are supplied by the median nerve and are inserted in common on the lateral side of the proximal phalanx. Two other muscles, the adductor and first palmar interosseus, are supplied by the ulnar nerve and are inserted in common on the medial side of the proximal phalanx.
Hypothenar Muscles (table 11-2). The short muscles of the little finger (see fig. 11-4) are the abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi. They are supplied by the ulnar nerve. The opponens is deep to the others and is not well named in that the fifth finger cannot be opposed. The hypothenar muscles are covered proximally by subcutaneous, transverse fibers termed the palmaris brevis.
Lumbricals and Interossei (table 11-3). The lumbricals and interossei (figs. 11-4 and 11-6) are a dozen small, important muscles that are inserted mainly into the extensor expansion (fig. 11-7). The lumbricals are associated with the tendons of the flexor digitorum profundus in the palm. The palmar interossei lie on the palmar surfaces of the metacarpals, whereas the dorsal interossei are more truly interosseous, i.e., between the bones. Each group of four muscles is numbered from one to four, from lateral to medial. It should be noted that the word interosseous is an adjective, whereas interosseus (plural: interossei) is a noun.
Each of the first two lumbricals arises by a single head from the lateral two tendons of the deep flexor, whereas each of the last two lumbricals arises by two heads from the adjacent sides of the medial three tendons. Each lumbrical has usually the same innervation as its deep tendon, i.e., the first two from the median and the last two from the ulnar. The lumbricals are inserted into the lateral (radial) sides of the extensor expansions of fingers 2 to 5, and they are chiefly extensors at the interphalangeal joints. Variations in attachments are common.
The four palmar interossei adduct the fingers toward an axis through the middle finger. Each arises from the metacarpal shaft of the digit that it adducts (see fig. 11-6). They are inserted into the extensor expansions.
The extensor expansion, or dorsal aponeurosis. is a fibrous sheet on the posterior aspect of each finger, and it contains a hood of transverse fibers (fig. 11-7). The expansion is penetrated by the extensor tendon, which then divides into three slips, a central slip to the base of the middle phalanx and two collateral bands, which, fused with expansions from the interossei and lumbricals, unite and proceed to the base of the distal phalanx. The lumbricals and palmar interossei are inserted into the hood, whereas the dorsal interossei are generally inserted into both the hood and the proximal phalanx deep to the hood. The arrangement of the expansion deep to the hood is very complicated.
The precise details of lumbrical and interosseous movement and their correlation with the actions of the long flexors and extensors of the digits are still not clear.
In combination with dorsiflexion at the wrist, the interossei and lumbricals produce the "Z-position" (fig. 11-8), i.e., flexion at the metacarpophalangeal joints by the interossei and extension at the interphalangeal joints by the lumbricals. It should be noted that, in section of the ulnar nerve, with consequent paralysis of the interossei and medial two lumbricals, the proximal phalanges are hyperextended by the unopposed long extensors and the middle and distal phalanges are hyperflexed (less so in fingers 2 and 3) by the unopposed long flexors. The result is a "claw hand" (fig. 11-8), which is the opposite of the Z-position.
The hand is supplied by the median, ulnar, and radial nerves (fig. 11-9). The motor fibers to the intrinsic muscles of the hand, which are carried by the median and ulnar nerves, are derived from the T1 segment of the spinal cord.
At the end of its descent along the anterior forearm, the median nerve is found midway between the styloid processes of the radius and ulna, medial to the flexor carpi radialis tendon and lateral to the palmaris longus tendon when this is present (see fig. 10-2). The nerve enters the hand by passing through the carpal tunnel, posterior to the flexor retinaculum and anterior to the flexor tendons to the index finger. It spreads into its terminal branches under cover of the palmar aponeurosis and the superficial palmar arch. An important recurrent branch emerges almost immediately distal to the flexor retinaculum, is dangerously superficial, and supplies the abductor and flexor pollicis brevis and the opponens (see fig. 10-2). The median nerve gives palmar digital nerve branches to three and a half fingers. These supply also the first two or three lumbricals and send branches dorsally to supply the posterior aspects of the distal portions of the fingers. The median nerve (see fig. 11-2) may be compressed in the carpal tunnel (carpal tunnel syndrome), resulting in weakness of the thenar muscles and sensory loss in the distribution of the digital branches.
The median nerve has a complicated arrangement of fibers, and it characteristically supplies bones, joints, ligaments, the interosseous membrane, and blood vessels. Injuries to the median nerve may be followed by severe, chronic sensory and trophic disturbances.
In the middle of the forearm, the ulnar nerve gives off a dorsal branch, which descends between the ulna and the flexor carpi ulnaris and reaches the medial side of the hand. It supplies dorsal digital nerves to usually two and a half or one and a half fingers. The ulnar nerve enters the hand anterior to the flexor retinaculum (hence outside the carpal canal) and lateral to the pisiform. The ulnar artery lies on its lateral side, and both are sometimes covered by a superficial part of the retinaculum. The nerve then divides into superficial and deep branches. The superficial branch supplies the palmaris brevis and gives palmar digital nerves to one and a half fingers. The deep branch of the ulnar nerve passes between the abductor and flexor digiti minimi, both of which it supplies. It innervates the opponens also, then curves around the hook of the hamate and laterally across the palm with the deep palmar arch. There it supplies all the interossei, the medial two lumbricals, the adductor pollicis, and usually the flexor pollicis brevis, in which it ends. Many fibers go to the joints of the hand. The ulnar nerve is distributed to the muscles responsible for the finer movements of the fingers; hence injuries to it cause severe disabilities.
Some distance proximal to the wrist, the superficial branch of the radial nerve leaves the lateral side of the radial artery, passes deep to the brachioradialis, and supplies dorsal digital nerves to two and a half fingers (see fig. 10-4).
In summary, the median nerve tends to supply the thenar muscles, and the ulnar nerve most of the other short muscles of the hand. The dividing line between the two distributions, however, is variable, and either nerve may invade the territory of the other.
The ulnar nerve supplies usually one and a half fingers on the anterior side and two and a half fingers posteriorly. The other fingers are supplied by the median anteriorly and by the radial posteriorly. The median nerve, however, invades the posterior aspect of several fingers distally. Variations are common, and the ulnar distribution varies inversely with that of the median anteriorly and with that of the radial posteriorly.
The radial artery leaves the forearm by curving dorsally to enter the anatomical snuffbox (see fig. 10-4). It then enters the palm between the heads of the first dorsal interosseus. Next it turns medially between the heads of the adductor pollicis and anastomoses with the deep branch of the ulnar artery to form the deep palmar arch.
In the forearm, the radial artery has an inconstant superficial palmar branch, which joins the superficial palmar arch. Palmar and dorsal carpal branches from the radial artery form networks with corresponding vessels from the ulnar artery and give rise to dorsal metacarpal and digital arteries. Other digital vessels (see fig. 10-2) from the radial artery include those to the thumb (princeps pollicis) and radial side of the index finger (radialis indicis). The deep palmar arch, which lies on the interossei, is formed by the termination of the radial artery and the deep branch of the ulnar artery. It gives several palmar metacarpal arteries.
The ulnar artery enters the hand anterior to the flexor retinaculum, on the lateral side of the pisiform. It then divides into the superficial palmar arch and a deep palmar branch. The latter joins the radial artery to form the deep palmar arch.
The superficial palmar arch (figs. 10-2 and 11-10), which is the main termination of the ulnar artery, is completed on the radial side in a variable manner, e.g., by the radialis indicis. The arch lies on the flexor tendons and the branches of the median nerve, under cover of the palmar aponeurosis. It gives off several palmar digital arteries. The convexity of the superficial palmar arch is approximately at the level of a line drawn across the hand from the distal border of the extended thumb. The anastomoses in the hand are extensive, and severe hemorrhage may occur from wounds of the palmar arches.
The distal radio-ulnar joint is a pivot articulation between the head of the ulna and the ulnar notch of the radius. The articular disc (fig. 11-11) is a strong, triangular, fibrous or fibrocartilaginous plate. Its base is anchored to the radius, and its apex to the root of the styloid process of the ulna. The joint cavity lies between the disc and the head of the ulna and extends proximally (as the recessus sacciformis) between the two bones of the forearm. The disc takes part in the wrist joint but excludes the ulna from that articulation. The movements at the distal radio-ulnar joint are supination and pronation.
The radiocarpal, or wrist, joint is an ellipsoidal articulation formed by the radius and articular disc and the scaphoid, lunate, and triquetrum (fig. 11-11). The position of the wrist joint is indicated approximately by a line connecting the styloid processes of the radius and ulna. The capsule is reinforced by palmar, dorsal, radial collateral, and ulnar collateral ligaments. The synovial membrane does not cover the articular disc. The cavity commonly communicates with those of some of the intercarpal joints but usually not with that of the distal radio-ulnar articulation.
Among the intercarpal joints, the most important is the midcarpal joint, formed between the proximal and distal rows of the carpus, exclusive of the pisiform (fig. 11-11). The three proximal bones form a socket for the hamate and head of the capitate (ellipsoidal joint), and the scaphoid articulates also with the trapezium and trapezoid (plane joint). The pisotriquetral joint is a small synovial articulation.
The radiocarpal and (medial part of the) midcarpal joints are ellipsoidal; hence their movements are basically flexion and extension, abduction and adduction, and combinations of these. The movements are produced by the extrinsic muscles of the hand: the chief flexors are the flexor carpi ulnaris and flexor carpi radialis; the extensors are the extensor carpi radialis longus and brevis and the extensor carpi ulnaris; the chief abductors are the flexor carpi radialis and extensor carpi radialis longus and brevis; the adductors are the extensor and flexor carpi ulnaris.
The metacarpophalangeal joints are ellipsoidal, and the interphalangeal joints are hinge articulations, but their ligamentous arrangements are similar. Each capsule is strengthened by a collateral ligament on each side, and the two ligaments fuse on the anterior aspect of the joint to form a dense, fibrous or fibrocartilaginous pad termed the palmar ligament. The four medial metacarpals are held together by a deep transverse metacarpal ligament related in front to the lumbrical tendons and behind to those of the interossei.
In addition to free movement, the fingers (including the thumb) may be held forcibly against the palm (power grip) to transmit force to an object and to conform to its size and shape, as in holding a ball. Objects can also be rotated by fine movements (precision handling) of the fingers (including the thumb), as in winding a watch, or translated toward or away from the palm, as in threading a needle and pulling the thread through it. Furthermore, objects can be compressed (pinched) between the thumb and index (or index and middle) finger, as in holding a pen while writing. These mechanisms are of great importance.
In power grip, the extrinsic muscles provide the major force, the interossei are used as flexors and rotators at the metacarpophalangeal joints, and the thenar muscles are generally active. In precision handling, specific extrinsic muscles, the interossei and lumbricals, and the thenar triad (abductor and flexor brevis and opponens) are important. In pinching, compression is provided chiefly by the extrinsic muscles, assisted by the thenar musculature and the interossei (fig. 11-12).
The thumb is set at an angle to the plane of the palm; hence its movements are described differently from those of the other fingers. Anteriorward movement of the thumb as a whole, away from the palm, is called abduction (fig. 11-13); the converse movement is called adduction. With the hand on a table, palm up, abduction points the thumb toward the ceiling. A medial movement of the thumb in the plane of the palm is termed flexion; the converse is termed extension. Opposition is the movement whereby the palmar aspect of the thumb touches the palmar aspect of another finger of the same hand. (When the contact is not by their palmar aspects, the movement is merely apposition.) The converse of opposition is reposition. These movements occur at all the joints of the thumb, although opposition takes place mainly at the carpometacarpal joint.
In the other fingers, flexion and extension occur at the metacarpophalangeal and interphalangeal joints (figs. 11-14 and 11-15), and an important rotatory component (produced by the interossei through abduction and adduction) takes place at the metacarpophalangeal joints (fig. 11-14).
The "position of rest," resulting from merely intrinsic muscular tone, is assumed by the inactive hand (see fig. 11-1). When immobilization is necessary, the hand should be placed in the "safe position" in order to limit the development of restrictions of joint motion. This position includes slight wrist extension (30-45 degrees), flexion (70-90 degrees) at the metacarpophalangeal joints and near full extension at the interphalangeal joints.
J. I. James. Common, simple errors in the management of hand injuries. Proc R Soc Med. 1970; 63(1): 69–71. Classic description of procedures and potential complications of hand immobilization.
Jones, F. W., The Principles of Anatomy as Seen in the Hand, 2nd ed., Bailliere, Tindall, and Cox, London, 1942. A very readable classic.
Kaplan, E. B., Functional and Surgical Anatomy of the Hand, 2nd ed., Lippincott, Philadelphia, 1965. A well-known text.
Landsmeer, J. M. F., Atlas of Anatomy of the Hand, Churchill Livingstone, Edinburgh, 1976. A very advanced study with numerous photomicrographs.
Tubiana, R. (ed.), The Hand, vol. I, W. B. Saunders Company, Philadelphia, 1981. A detailed account including much anatomy.
11-1 When the hand has to be immobilized, in which position should it be placed?
11-1 When the hand has to be immobilized, it should be placed in the "position of rest" (see fig. 11-1). When the hand is at rest, the wrist is extended slightly and the fingers are flexed, as when the inactive hand is resting on a horizontal surface. In the "functional position" that precedes activity, the wrist is extended much further, and the metacarpophalangeal joints are extended also.
11-2 What is the clinical importance of the carpal tunnel?
11-3 What is the clinical significance of the palmar aponeurosis?
11-3 Dupuytren's contracture, described by the famous French surgeon in 1832, is commonly attributed to thickening and shortening of the palmar aponeurosis, although the causation is obscure. The little and ring fingers are most affected. The aponeurosis can be excised.
11-4 What are the "midpalmar space" and "thenar space"?
11-5 What are the "ulnar bursa" and the "radial bursa"?
11-6 Which spinal segment supplies the small muscles of the hand?
11-7 What is the innervation of the muscles of the thumb?
11-7 Usually, the thenar triad (abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis) is supplied by the median nerve (recurrent branch; see fig. 10-2). The adductor pollicis, together with the deep head of the flexor brevis and the first palmar interosseus, is supplied by the ulnar nerve. See figure 11-9.
11-8 What are the chief effects of section of the median nerve at the wrist?
11-9 Which muscles produce the "Z-position" of the hand?
11-10 Apart from claw hand, what motor deficits would be expected in ulnar nerve palsy?
11-10 In ulnar nerve lesions, in addition to the interossei and lumbricals 3 and 4, the adductor pollicis is also paralyzed. In attempting to grasp a sheet of paper between thumb and index finger, the thumb is incapable of adequate adduction. This is masked by flexion at the interphalangeal joint of the thumb, which is accentuated when an attempt is made to pull the paper away ("Froment's sign").
11-11 Which nerves would be tested by examining for sensation along the lateral side of the index finger and the medial side of the little finger?
11-11 Sensation along the lateral side of the index finger and the medial side of the little finger is mediated by the median (see fig. 8-11) and ulnar (see fig. 8-12) nerves, respectively. These areas are reliable because they are unlikely to be altered by variation or by overlapping of adjacent nerves.
11-12 Why is a fracture of the base of the first metacarpal serious?
11-12 A fracture of the base of the first metacarpal is serious because it generally involves the carpometacarpal joint of the thumb. This fracture-dislocation, produced by a blow to the tip of the thumb, was described in 1881 by Edward Hallaran Bennett on the basis of his anatomical studies some 14 years before the discovery of x-rays.
Figure 11-1 The position of the hand at rest.
Figure 11-2 Horizontal sections through the hand. A is through the metacarpals and shows the palmar aponeurosis and the "mid palmar" and "thenar" spaces separated from each other by an oblique septum attached to the 3rd metacarpal. Actually there are many such septa tending to subdivide the central compartment. The superficial palmar arch and the digital branches of the median nerve (not shown) lie between the palmar aponeurosis and the flexor tendons. Ab.P.B., abductor pollicis brevis; F.D.P., flexor digitorum profundus; F.D.5., flexor digitorum superficialis; F.P.B., flexor pollicis brevis; F.P.L., flexor pollicis longus; Op.P., opponens pollicis. B is through the distal row of carpal bones and shows the flexor retinaculum and the carpal tunnel (or canal), which contains the median nerve and the flexor tendons. The ulnar nerve and artery pass anterior to the flexor retinaculum, although they are covered by a fascial layer and the palmaris brevis. The extensor tendons have been omitted from both drawings. The inset shows the palmar aponeurosis, palmaris longus, and flexor retinaculum, as well as the levels of the sections. (Modified from Castaing and Soutoul.) For a section at C, see fig. 10-5.
Figure 11-3 The extensor and flexor tendons, and a common arrangement of their synovial sheaths. Note the extensor and flexor retinacula.
Figure 11-4 The intrinsic muscles of the little finger.
Figure 11-5 The intrinsic muscles of the thumb. The opponens is not shown.
Figure 11-7 The extensor aponeurosis of the fourth finger. A, Dorsal view showing the extensor hood. B, Dorsal view with the hood removed. C, Medial view. (Based on Landsmeer.)
Figure 11-8 A, Diagram to show the chief muscles producing extension and flexion of the fingers. ED., extensor digitorum; FD.P., flexor digitorum profundus; FD.5., flexor digitorum superficialis; I., interosseus; L., lumbrical. B, The Z-position of the hand, produced by the interossei and lumbricals. C, Claw hand, due to paralysis of the interossei and lumbricals (following section of the ulnar nerve). The unopposed extensor and flexors produce the opposite of the Z-position.
Figure 11-9 Diagram to show the motor innervation of the hand. Cutaneous innervation is indicated on the fingers. Sensation in the ulnar and median territories may be tested on the fifth and second fingers (at broad arrows), respectively.
Figure 11-10 The arteries of the anterior hand. The superficial palmar arch is sometimes completed on the lateral side by the superficial palmar branch of the radial artery. The anastomosis is shown by dotted lines.
Figure 11-11 Schematic coronal section of the carpus as if the carpus were flat. Note the radiocarpal, intercarpal and carpometacarpal joints.
Figure 11-12 Testing finger movements. Upper, The use of a pinch to test the first dorsal interosseus and adductor pollicis. Adduction by the other interossei can be tested by having the subject hold a piece of paper between his fingers as the examiner tries to pull it away. Middle, The muscles of the thumb can be checked by palpating them as movements are made against resistance. Note the tenseness of the first dorsal interosseus of the examiner. Lower, the hypothenar muscles can be tested by abducting (or flexing) against resistance.
Figure 11-13 Movements of the thumb.
Figure 11-14 Diagram showing a power grip with rotation at metacarpophalangeal joints while squeezing a ball. The phalangeal insertions of the dorsal interossei and abductor digiti minimi are shown. (Based on Landsmeer.)
Figure 11-15 Testing finger movements. Upper, If the metacarpophalangeal and proximal interphalangeal joints are stabilized as shown, flexion at the distal interphalangeal joint (flexor profundus) can be tested. Lower, If the metacarpophalangeal joint is stabilized as shown, flexion at the proximal interphalangeal joint (flexor superficialis) can be tested.
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