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Basic Anatomy
The arm and hand are divided into volar or palmar, and also dorsal, aspects. Distal to the elbow,
structures are termed radial or ulnar to the middle finger axis rather than lateral and medial, respectively,
because with forearm pronation and supination, the latter terms become confusing. The nomenclature of
digits has become standardized. The hand has five digits, namely the thumb and four fingers (the thumb
is not called a finger). The four fingers are respectively termed the index, long (middle), ring, and small
(little) fingers. The use of numbers to designate digits is no longer accepted (Fig. 70-1). Within the hand,
those structures close to the fingertips are termed distal, whereas those further up toward the wrist are
termed proximal. Motion in a palmar direction is flexion, whereas dorsal motion is
termed extension. Finger motion away from the long finger axis is termedabduction, whereas motion
toward the axis of the long finger is termed adduction. The description of the motion of the thumb is
sometimes confusing. Extension of the thumb is in the plane of the palm of the hand, whereas palmar
abduction of the thumb is the motion that occurs at 90 degrees away from the plane of the palm. Finally,
side to side motion of the wrist is termed radial and ulnar deviation.
Surface anatomy of the hand. A, Hand surfaces and nomenclature. B, Skin creases
of the hand superimposed on the skeletal structures.
FIGURE 70-1
Intrinsic muscles of the hand are those that have their origins and insertions in the hand, whereas the
extrinsic muscles have their muscle bellies in the forearm and their tendon insertions in the hand. The
intrinsic muscles that make up the thenar eminence are the abductor pollicis brevis (APB), flexor pollicis
brevis (FPB), opponens pollicis (OP), and adductor pollicis (AP). There are four dorsal interossei that
arise from adjacent sides of each metacarpal and provide abduction of the metacarpophalangeal (MP)
joints of the index, middle, and ring fingers. There are three palmar interossei that adduct the index, ring,
and little fingers toward the middle finger. Four lumbricals originate on the flexor digitorum profundus
(FDP) tendons in the palm and insert on the radial sides of the extensor mechanisms of the four fingers.
Together with the interossei, these bring about flexion of the MP joints and extension of the
interphalangeal (IP) joints of the fingers (Fig. 70-2). The FPB flexes the thumb at the MP joint, in contrast
with the extrinsic flexor pollicis longus (FPL), which flexes the thumb IP joint.
Outline of first dorsal interosseous muscle on the index finger shows how it passes
volar to the fulcrum of flexion of the metacarpophalangeal joint (MP) and dorsal to the IP joints.
Interossei flex MP joints and extend proximal and distal IP joints. The long extrinsic extensor
tendon passes dorsal to all joints.
FIGURE 70-2
The hypothenar muscles consist of the flexor digiti minimi (FDM), which flexes the little finger at the MP
joint, as well as the abductor digiti minimi (ADM) and opponens digiti minimi (ODM). A small muscle
called the palmaris brevis is located transversally in the subcutaneous tissue at the base of the
hypothenar imminence. It is innervated by the ulnar nerve, puckers the skin, and helps in cupping the skin
of the palm during grip (Table 70-1).
Table 70-1 -- Intrinsic Muscles of the Hand
MUSCLE
INNERVATION* FUNCTION
Abductor pollicis
brevis (APB)
Median
Abducts the thumb
MUSCLE
INNERVATION* FUNCTION
Flexor pollicis brevis
Median
(FPB)
Flexes the thumb
Opponens pollicis
(OP)
Median
Opposes the thumb
Lumbricals
Median and
ulnar
Flexes metacarpal phalangeal (MCP) joints and
extends interphalangeal (IP) joints
Palmaris brevis
Ulnar
Wrinkles the skin on the medial (ulnar) side of the
palm
Adductor pollicis
(AdP)
Ulnar
Adducts the thumb
Abductor digiti
minimi (ADM)
Ulnar
Abducts the small finger
Flexor digiti minimi
(FDM)
Ulnar
Flexes the small digit
Opponens digiti
minimi (ODM)
Ulnar
Opposes the small finger
Dorsal interossei
Ulnar
Abducts the fingers; flexes MCP joints and
extends the IP joints
Palmar interossei
Ulnar
Adducts the fingers; flexes MCP joints and
extends the IP joints
* All the thenar intrinsic muscles are supplied by the median nerve except the AdP; all the remaining intrinsic muscles are supplied
by the ulnar nerve except the two radial lumbricals.
The extrinsic muscles originate proximal to the wrist and comprise the long flexors and extensors of the
wrist and digits. The extensors are located dorsally and are divided into three subgroups. The radialmost
subgroup is termed the mobile wad and comprises the brachioradialis (BR), extensor carpi radialis longus
(ECRL), and extensor carpi radialis brevis (ECRB). The ECRL and ECRB extend the wrist and deviate it
radially. The second group is located in a more superficial layer and comprises three muscles—namely,
the extensor carpi ulnaris (ECU), extensor digiti minimi-quinti (EDM-Q), and extensor digitorum communis
(EDC). The ECU deviates the wrist in an ulnar direction and extends the wrist, whereas the EDM and
EDC extend the MP joints of the fingers. The third and deeper subgroup comprises four muscles, three of
which act on the thumb; the remaining muscle influences the index finger. The abductor pollicis longus
(APL), extensor pollicis longus (EPL), and extensor pollicis brevis (EPB) provide function to the thumb,
and the extensor indicis proprius (EIP) extends the MP joint to the index finger. Last of the deep muscles
is the supinator, which is located proximally in the forearm (Table 70-2).
Table 70-2 -- Extrinsic Muscles of the Dorsal Forearm
MUSCLE
INNERVATION* FUNCTION
Extensor pollicis brevis
(EPB)
Radial
Abducts the hand and extends the thumb
at the proximal phalanx
Abductor pollicis longus
(APL)
Radial
Abducts the hand and thumb
Extensor carpi radialis
longus (ECRL)
Radial
Extends and radially deviates the hand
Extensor carpi radialis
brevis (ECRB)
Radial
Extends and radially deviates the hand
Extensor pollicis longus
(EPL)
Radial
Extends the distal phalanx of the thumb
Extensor digitorum
communis (EDC)
Radial
Extends the fingers and the hand
Extensor indicis proprius
(EIP)
Radial
Extends the index finger
Extensor digiti minimi/quinti
Radial
(EDM/Q)
Extends the small finger
Extensor carpi ulnaris
Radial
Extends and ulnarly deviates the wrist
MUSCLE
(ECU)
INNERVATION* FUNCTION
Supinator
Radial
Supination
Brachioradialis
Radial
Flexes the forearm
* All muscles of the dorsal forearm are innervated by the radial nerve and its respective branches.
The extensor tendons pass through six compartments deep to the extensor retinaculum at the dorsum of
the wrist. From radial to ulnar side, these tendons and compartments are arranged as follows. The first
compartment contains the APL and EPB, which also forms the radial boundary of the so-called anatomic
snuffbox. The second compartment consists of the ECRL and ECRB, and the third compartment (which
also forms the ulnar boundary of the anatomic snuffbox) contains the EPL. The EIP and EDC pass
through the fourth compartment and the EDM through the fifth compartment, where they overlie the distal
radioulnar joint. The sixth compartment contains the ECU (Fig. 70-3).
A, B, Surface anatomy of the six dorsal extensor compartments at the wrist. Note
that the first (APL and EPB) and third (EPL) compartments form the radial and ulnar boundaries,
respectively, of the anatomic snuffbox.
FIGURE 70-3
At the level of the MP joints, the long extrinsic extensor tendons broaden out to form the extensor hood.
The proximal part of the hood at this level is called the sagittal band. It loops around the MP joint and
blends into the volar plate, thus forming a lasso around the base of the proximal phalanx, through which it
extends the MP joint. The insertions of the interossei and lumbricals enter into the extensor hood as the
lateral bands. These lateral bands insert distally and dorsally to the axis of the PIP joint, and it is through
this distal insertion that the intrinsic muscles (the interossei and lumbricals) are flexors of the MP joints
and yet extensors of the IP joints. The extensor hood inserts to the base of the middle phalanx, which is
termed the central slip, and finally proceeds on to the base of the distal phalanx, where it inserts through
the terminal slip, thus extending the distal interphalangeal (DIP) joint (Fig. 70-4).
FIGURE 70-4
Extensor mechanism of the fingers. A, Lateral view. B, Dorsal view.
The extrinsic flexor muscles are located on the volar aspect of the forearm and are arranged in three
layers. The superficial layer comprises four muscles—pronator teres (PT), flexor carpi radialis (FCR),
flexor carpi ulnaris (FCU), and palmaris longus (PL). The PL muscle may be absent in as many as 10% to
12% of individuals. These muscles originate from the medial humeral epicondyle in the proximal forearm
and function to flex the wrist and pronate the forearm. The intermediate layer consists of the flexor
digitorum superficialis (FDS), which allows independent flexion of the proximal interphalangeal (PIP)
joints of the fingers. In the deep layer, there are three muscles: the FPL, which flexes the IP joint to the
thumb; the FDP, which flexes the DIP joints of the fingers; and a distal quadrangular muscle that spans
between the radius and ulna termed the pronator quadratus, which helps in pronation of the forearm
(Table 70-3).
Table 70-3 -- Extrinsic Muscles of the Volar Forearm
MUSCLE
INNERVATION* FUNCTION
Pronator teres (PT)
Median
Pronation
Flexor carpi radialis (FCR)
Median
Flexion and radial deviation of the wrist
Palmaris longus (PL)
Median
Flexion of the wrist
MUSCLE
INNERVATION* FUNCTION
Flexor carpi ulnaris (FCU)
Ulnar
Flexion and ulnar deviation of the wrist
Flexor digitorum superficialis
Median
(FDS)
Flexion of the proximal interphalangeal
(PIP) joint
Flexor digitorum profundus
(FDP)
Median and
ulnar
Flexion of the distal interphalangeal
(DIP) joint
Pronator quadratus
Median
Pronation
Flexor pollicis longus (FPL)
Median
Flexion of the thumb
* All muscles of the volar forearm are innervated by the median nerve and its branches except the two ulnar digits of the FDP and
FCU, which are innervated by the ulnar nerve.
Nerve supply to the hand is by three nerves, the median, ulnar, and radial nerves. A knowledge of the
surface anatomy of nerves helps when evaluating specific lacerating injuries (Fig. 70-5). The ulnar
attachment to the flexor retinaculum is to the pisiform and hook of the hamate, and the radial attachment
is to the scaphoid and ridge of the trapezium. The median nerve passes through the carpal tunnel
between these landmarks. It gives sensation to the thumb, index finger, middle finger, and radial half of
the ring finger. The palmar cutaneous branch of the median nerve originates from its radial side 5 to 6 cm
proximal to the wrist, providing sensation to the palmar triangle. The ulnar nerve travels to the radial side
of the pisiform and passes to the ulnar side of the hook of the hamate in its passage through Guyon's
canal. It gives sensation to the little finger and ulnar half of the ring finger; the dorsal branch of the ulnar
nerve (arising proximal to the wrist and curving dorsally around the head of the ulna) supplies the same
digits on their dorsal aspects. The superficial radial sensory nerve emerges from under the brachioradialis
in the distal forearm, dividing into two or three branches proximal to the radial styloid, which then proceed
in a subcutaneous course across the anatomic snuffbox, innervating the skin of the dorsum of the first
web space. The number of fingers served by each nerve is variable. However, as an absolute rule, the
palmar surfaces of the index and little fingers are always served by the median and ulnar nerves,
respectively.
FIGURE 70-5 Surface anatomy of median (red) and ulnar (black) nerves. H, Hook of
hamate; P, pisiform; S, scaphoid; T, trapezium.
With regard to the motor supply of these nerves, the ulnar nerve supplies the hypothenar muscles,
interossei, ulnar two lumbricals, adductor pollicis, and deep head of the flexor pollicis brevis. The median
nerve supplies the abductor pollicis brevis, opponens pollicis, radial two lumbricals, and superficial head
of the flexor pollicis brevis. In summary, the median nerve thus supplies all the extrinsic digit flexors and
wrist flexors (except the FDP to the ring and little fingers and the FCU, which are supplied by the ulnar
nerve) and all the thumb intrinsic muscles (except the AP, innervated by the ulnar nerve). The ulnar nerve
supplies all the interossei, all the lumbricals (except the radial two, supplied by the median nerve), and
the adductor of the thumb. The radial nerve innervates all of the wrist, finger, and thumb extrinsic long
extensors.
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