Surface Anatomy

For hundreds of years, artists have been intrigued by the surface anatomy of the human hand.

Artists frequently study the deeper anatomic structures of the hand in order to better understand the details of an eloquent surface anatomy. An artistic display of an extraordinary collection of famous examples of hand sculptures was presented in the exhibit “Essentially the Hand” in 2012 at Caffé Pedrocchi in Padua, Italy. Understanding surface anatomy, crease location, bony landmarks, and their relationship to deep anatomical structures is critical knowledge for those examining and caring for the human hand.

When studying the surface anatomy of the hand, the visible and easily palpable bony landmarks, the palmar skin creases and the topographical reference lines like Kaplan's Cardinal line are important guides to localizing the deeper structures of the hand’s anatomy. While there is no substitute for meticulously accurate surgical dissection when opening an operative incision, these landmarks still help even the experienced surgeon to plan and execute the appropriate incision for each unique surgical patient.^1-5

Bony landmarks of the hand and wrist

1. Distal phalanx
2. Middle phalanx
3. Proximal phalanx
4. Index, long, ring, and little metacarpals
5. Thumb distal phalanx
6. Thumb proximal phalanx
7. Thumb metacarpal
8. Trapezial ridge
9. Pisiform
10. Hook of the hamate
11. Scaphoid tuberosity and scaphoid waist within the anatomic snuffbox
12. Lister’s tubercle
13. Radial styloid
14. Ulnar styloid
15. Carpal boss

Deep structures related to these bony landmarks¹

1. The distal phalanx is easily palpable at the end of each digit. In the fingers and dorsal to the phalanx is the terminal extensor tendon and the fingernail with its nailbed and surrounding nail folds dorsally. Palmarly in the distal phalanx, there is the pulp with its unique fibrofatty honey-comb structure. The insertion of the flexor digitorum profundus also connects to the distal phalanx. In addition, on the volar surface radially and ulnarly, there are two digital arteries and two digital nerves.
2. The middle phalanx is also easy to palpate in the finger between the distal interphalangeal (DIP) and proximal interphalangeal (PIP) joints. Dorsal to the middle phalanx, the extensor tendon is thin and localized immediately underneath the skin. Volar to the middle phalanx is the flexor digitorum profundus tendon and the insertion of the flexor digitorum superficialis tendon. In addition, on the volar surface there are radial and ulnar neurovascular bundles. 
3. The proximal phalanx is palpable between the metacarpophalangeal (MP) and PIP joints. Dorsal to the easily palpable proximal phalanx is the complex extensor mechanism, which is composed of the intrinsic and extrinsic extensor tendons. These tendons combine to make the dorsal extension hood over the proximal phalanx and the PIP joint. Volar to the proximal phalanx, the flexor digitorum profundus and flexor digitorum sublimis are present within the flexor tendon sheath. The radial and ulnar digital neurovascular bundles lie on either side of the flexor sheath. These bundles are held in place dorsally by Cleland’s ligament and volarly by Grayson’s ligaments.
4. The index, long, ring, and little metacarpals are easily palpable on the dorsum of the hand between the MP joints and the CMC joints.  The visible EDC and EIP tendons are easily palpable dorsal to the index metacarpal with the dorsal interosseous muscles in the intrinsic compartment ulnar to the metacarpal and the first dorsal interosseous muscle radial to the metacarpal.  The long and ring metacarpals also have the respective EDC tendons visible dorsal with additional intrinsic compartments adjacent to these metacarpals.  The visible EDC and EDM tendons are easily palpable dorsal to the little metacarpal with the dorsal aspect of the hypothenar muscles ulnar to the little metacarpal. In the palm the metacarpal shafts are relatively difficult to palpate and therefore the volar aspects of the metacarpals are less useful as bony landmarks.
5. The distal phalanx of the thumb is easily palpable between the tip of the thumb and the IP joint of the thumb. Dorsal to the distal phalanx is the insertion of the extensor pollicis longus (EPL) and the thumbnail unit. Volar to the distal phalanx is the pulp of the thumb and insertion of the flexor pollicis longus (FPL).
6. The thumb proximal phalanx is easily palpable between the thumb IP and MP joints. Dorsal to the proximal phalanx is the EPL tendon, which is easily visualized and palpated. The extensor pollicis brevis (EPB) inserts into the dorsal base of the proximal phalanx via its MP joint capsular insertion. Volar to the proximal phalanx, the FPL is present in the thumb’s flexor tendon sheath. The thumb proximal phalanx’s portion of the radial and ulnar neurovascular bundles are also located volarly.
7. The thumb metacarpal is easily palpated between the thumb MP and carpometacarpal (CMC) joints. Dorsal to this bony landmark, the EPL and EPB are easily visualized and palpated. Palmarly, the thenar muscles are located adjacent to the FPL tendon and the thumb’s flexor sheath. The thenar muscles are partially innervated by the median nerve motor branch and partially by the ulnar nerve motor branch.
8. The trapezial ridge is a bony landmark located at the base of the thenar muscles on the radial volar aspect of the wrist. The trapezial ridge is palpable proximally at the point where the flexor carpi radialis (FCR) passes underneath this ridge to insert into the base of the second metacarpal. The trapezial ridge and the scaphoid tuberosity form the radial osseous side of the carpal tunnel. The ridge also provides one of the origin sites for the thenar muscles.
9. The pisiform is located on the ulnar-volar aspect of the wrist. To localize the pisiform, follow the flexor carpi ulnaris (FCU) distally until the bony surface of the pisiform is palpated. The pisiform is mobile, especially in radial and ulnar directions. This mobility can be demonstrated by pressure against the ulnar side of the pisiform from the examiner’s thumb or index finger. The FCU tendon encases the volar aspect of the pisiform as it progresses distally to insert on the base of the fifth metacarpal. Radial to the pisiform is Guyon’s canal, which contains the ulnar artery and ulnar nerve. The pisiform also provides an insertion site for several ligamentous structures. 
10. The hook of the hamate is useful when localizing important deep anatomical structures.  To locate the hook of the hamate, the examiner should place his or her thumb IP crease on the pisiform and then direct the tip of the thumb towards the base of the ring finger.  This maneuver should place the tip of the examiner’s thumb over the tip of the hook of the hamate.  To the ulnar side of the hook of the hamate is the distal portion of Guyon’s canal, which contains the sensory and motor branches of the ulnar nerve and ulnar artery. The deep branch of the ulnar artery passes distal and dorsal to the hook of the hamate as it progresses to the deep surface of the carpal tunnel to become the deep vascular arch. Similarly, the motor branch of the ulnar nerve passes distally, radially, and progressively dorsal to the hook of the hamate as it reaches the innervation site of the abductor pollicis and other ulnar-innervated intrinsic muscles in the radial aspect of the hand.
11. The scaphoid tuberosity is located just dorsal to the FCR tendon and just proximal to the proximal edge of the trapezium ridge. Tenderness here can be associated with FCR tendinitis or with scaphoid fractures.  The scaphoid waist is a second scaphoid bony landmark that is easily palpable. It is located in the anatomic snuffbox between the base of the thumb and radial styloid. The EPL is the dorsal margin of the snuffbox, and the abductor pollicis longus (APL) is the volar margin of the snuffbox. The dorsal branch of the radial artery passes over the waist of the scaphoid distally as it approaches the dorsal interval between the base of the first and second metacarpals. As the artery passes through this interval, it joins the deep palmar arch.
12. Lister’s tubercle is an important bony prominence located on the dorsal radial aspect of the distal radius. Just ulnar to Lister’s tubercle is the third extensor compartment, which contains the EPL tendon. Lister’s tubercle is a fulcrum for the EPL as it changes direction and moves radially to reach the dorsal base of the thumb. Just radial to Lister’s tubercle, the extensor carpi radialis brevis (ECRB) is located in the second extensor compartment. As the ECRB and extensor carpi radialis longus (ECRL) pass distal to Lister’s tubercle, they are located palmarly to the EPL tendon. Just distal to Lister’s tubercle at the level of the radiocarpal joint, there is a slightly soft area that represents the 3-4 wrist arthroscopy portal. Immediately distal to this portal is the location of the scaphoid lunate ligament. A line drawn from Lister’s tubercle to the base of the long metacarpal will pass over the scapholunate ligament. 
13. The radial styloid is located at the proximal end of the anatomic snuffbox at the distal end of the radius. The radial artery passes over the radial styloid volarly. Palpating the radial styloid and simultaneously the ulnar styloid with the opposite hand is a useful manual maneuver for assessing radial length after a close reduction of a distal radius fracture.
14. The ulnar styloid is palpable on the ulnar dorsal aspect of the wrist. Dorsally the extensor carpi ulnaris passes over the ulnar styloid to insert into the base of the fifth metacarpal. Palmarly, the ulnar styloid attaches to the wrist capsular ligaments and triangular fibrocartilage complex (TFCC). The flexor carpi ulnaris also passes palmar to the styloid. The ECU and FCU form a space distal to the tip of the ulnar styloid through which the TFCC and triquetrum can be palpated.
15. The carpal boss is visible in some individuals and palpable in most, located as a dorsal bony prominence on the base of the index and long metacarpals. A carpal boss originates from the common articulation of the index and long metacarpal bases with the capitate. Often, there is an associated bony prominence in this articulation on the distal capitate. In some patients, the carpal boss will be composed of painful “kissing osteophytes” secondary to osteoarthritis. A line drawn from the carpal boss to Lister’s tubercle will pass along the radial edge of the capitate.
16. Finally, when palpating finger and thumb joints, the joint space will always be distal to the highest proximal bony prominence of the metacarpal head at the MP joints, the proximal phalanx head at the PIP joint, and the middle phalanx head at the DIP joint. An exception to this rule is the thumb CMC joint. The base of the thumb metacarpal is palpable and in a more superficial location than the trapezium. The CMC joint is most readily localized by passively maneuvering the thumb metacarpal through an arc of flexion and extension while carefully visualizing the joint space.

Skin crease landmarks^2,5

1. Distal digital crease
2. Middle digital crease
3. Proximal digital crease
4. Distal palmar crease
5. Proximal palmar crease
6. Thenar crease
7. Distal thumb crease
8. Proximal thumb crease
9. Distal wrist crease
10. Topographical line over the metacarpal necks

Deep structures related to skin crease landmarks

1. The distal palmar finger creases are slightly proximal to DIP joints (about 7mm).
2. The middle palmar finger creases are just proximal to the PIP joints (about 1.6-2.6mm).²
3. The proximal flexion creases distal to MP joint is positioned approximately at the mid-point of the proximal phalanx and aligned with the web, which is always located approximately at the middle of the proximal phalanx NOTat the MP joint level.
4. The thumb distal and proximal creases are at the IP and MP thumb joint levels.
5. The distal palmar crease and the proximal palmar crease are aligned with the necks of the metacarpals index, long, ring and little. These creases are just proximal to the MP joints of the index, long, ring, and little fingers.
6. The thenar crease (or intra-thenar crease) commonly is located over the base of long metacarpal. As the thenar crease crosses the Cardinal line, it is superficial to the branching point of the median motor nerve from the main median nervein most patients.
7. The distal wrist crease is the most consistent wrist crease and is typically located over the proximal carpal row, waist of the scaphoid and pisiform.

Kaplan’s Cardinal Line and other topographical lines^1,3,4

1. Kaplan’s Cardinal line is an important topographic line of the human palm.
2. A second palmar topographical starts parallel to the radial border of the long finger and runs proximally towards the scaphoid tuberosity and the distal end of the FCR tendon.
3. A third palmar topographical line starts at the ulnar border of the ring finger and proceeds proximally to the hook of the hamate.
4. A fourth palmar topographical line can be drawn from the radial edge of the proximal palmar crease to the ulnar edge of the distal palmar crease.
5. A fifth palmar topographical line starts from the ulnar side of the fifth finger and goes to the hook of the hamate.
6. A sixth palmar topographical line starts at the radial side of the index finger proceeds to the point of origin of the median motor branch, i.e. where the third line crosses the Cardinal line.
7. The mid-axial line of the fingers proceeds from the dorsal edge of the proximal finger crease to the edge of the middle crease and finally to the dorsal edge of the distal palmar flexion crease.

Deep structures defined by Kaplan’s Cardinal line and other secondary topographical lines

• The Cardinal line (Kaplan’s Cardinal line) and other associated lines that help define the location of deeper structures of the hand include:
  • Kaplan’s Cardinal line starts at the palmar apex of the first web at the point where the thumb MP flexion crease meets the first web. The line ends ulnarly at a point 1 cm distal to the pisiform, or more accurately, at the center of the hook of the hamate.
  • If a second line is drawn parallel to the radial border of the long finger, it crosses the Cardinal line at the point where the motor branch of the median nerve enters the thenar muscles. The thenar crease usually crosses the Cardinal line at the point where the median motor branch leaves the median nerve. Deep (dorsal) to this median nerve branching point, the motor branch of the ulnar nerve enters the adductor pollicis. The accuracy of this location technique has been debated. Some authors find the median nerve motor branching point is located slightly more proximal and ulnar to the location predicted by the Cardinal line and second topographical line.^6-10
  • If a third line is drawn parallel to the ulnar border of the ring finger, that line will cross the Cardinal line at the tip of the hook of the hamate. This third line should also overlie the common digital nerve and artery going to the ring-little web space. Ulnar to the hook of the hamate is Guyon’s canal, which contains the ulnar artery and ulnar nerve. Just distal to the hook in line with the body of the hamate is the volar aspect of the CMC joint of the fourth and fifth metacarpals.
  • If a fourth line is drawn from the radial edge of the proximal palmar crease (proximal to the base of the index finger) to the ulnar edge of the distal palmar crease (proximal to the base of the little finger), this fourth line will complete the outline of a trapezoidal space that was started by the second line, the third line, and the Cardinal line. The superficial palmar vascular arch is located distally in this space. Just proximal to this space and proximal to the Cardinal line is the approximate location of the deep palmar vascular arch and the motor branch of the ulnar nerve.
  • A fifth line from the ulnar side of the fifth finger to the hook of the hamate defines the path of the ulnar sensory distal nerve of the fifth finger.
  • A sixth line from the radial side of the index finger to the point of origin of the median motor nerve defines the path of the radial sensory digital nerve of the index finger.  This radial index digital nerve is the most superficially located digital nerve in the palm. This index radial digital nerve also lies around the radial aspect of the first lumbrical muscle to the index finger.
  • There are multiple definitions of Kaplan’s Cardinal line. In 1994, 2005, and 2006, several clinical investigators described the ambiguities associated with the Cardinal line.^6,7 One group identified Hurst’s⁷ description of Kaplan’s line as the most accurate. That definition states that an accurate Kaplan’s Cardinal line goes from the apex of the first web (usually where the web crease meets the thumb’s MP flexion crease) to the center of the hook of the hamate.
  • Vella’s team stressed that the location of the motor branch of the median nerve is typically slightly proximal and ulnar to the point defined by Kaplan’s line and the line along the radial border of the long finger. Because of this variability, some surgeons feel Kaplan’s Cardinal line is unreliable for locating the motor branch of the median nerve prior to endoscopic carpal tunnel release.⁸ Earlier in 1994, Cobb, Cooney, and An had noted that Kaplan’s line did not provide pinpoint accuracy and recommended using the hook of the hamate as the major landmark for carpal tunnel surgery. Further, they proposed the index-pisiform line with the ulnar ring finger line as a way to identify the location of the hook of the hamate.
  • In one recent publication, Rodriguez and Strauch¹¹ described the middle finger flexion test to locate the origin of the motor branch of the median nerve in a cadaver study. They flexed the MP and PIP joints of the long finger both 90° to allow the long fingertip center to make a mark on the skin at the thenar eminence. By dissecting the hands, they determined that the center of the long fingertip mark was on average 2 mm ulnar and 1 mm proximal to this point. Of course, this method for locating the motor branch origin can be hampered by joint arthritis, contracture and ligamentous laxity and any ulnar or redial deviation occurring during passive flexion of the long finger.
  • A final topographical line that is useful for identifying deeper structures of the finger is the mid-axial line or mid-lateral line. This line is defined by placing a dot at the top (dorsal end) of each finger creases on either the radial or ulnar side of the finger and then connecting these dots longitudinally to produce a line along the side of the finger. When this line was used for a surgical incision, the incision would enter the finger dorsal to the neurovascular bundle. This incision can be used for accessing the flexor tendon sheath or the PIP joint.

Anatomical surface landmarks on the dorsum of the hand and radial and ulnar borders of the hand

1. Loosely attached mobile skin, thumbnail, and fingernails
2. Transverse skin creases on the dorsum of the thumb IP joint and the DIP and PIP joints of the fingers
3. The dorsal veins
4. The extensors of the fingers
5. The juncturae tendinum
6. The extensors of the thumb
7. The first dorsal interosseous muscle
8. The thenar muscles
9. The hypothenar muscles
10. Bony landmarks are noted above but those on the dorsum of the hand and wrist include the carpal boss, the radial styloid, the ulnar styloid, and Lister’s tubercle
11. Dorsal radial artery pulse in the dorsal apex of the first web