Hand and Wrist Anatomy

Mark K. Solarz

Robert C. Matthias Jr.

INTRODUCTION

Comprehensive knowledge of upper extremity anatomy is the foundation for the diagnosis and treatment of hand and wrist pathology.

Kaplan’s cardinal line (KCL)—originally described in 1953 as the line from the “apex of the interdigital fold between the thumb and index finger … parallel with the middle crease of the hand”
- However, several differing lines have been described using the same name with no consensus among hand surgeons as to which is correct.¹
- Using the original description as a surgical landmark
  - The motor branch of the median nerve lies 15 mm ulnar and proximal to the intersection of KCL and a longitudinal line drawn from the 2nd webspace.¹
  - The superficial palmar arch (SPA) lies 14 mm distal to the intersection between KCL and a longitudinal line drawn from the 3rd webspace.¹
    - The SPA was no closer than 11 mm from any of the four lines considered as KCL in a cadaveric study by Vella et al.¹
  - The distal extent of the transverse carpal ligament lies 5 mm proximal to the intersection of KCL and a longitudinal line drawn from the 3rd webspace.¹
Distal and proximal palmar creases—palmar creases are used by some to estimate the level of the A1 pulley.
- Ring and small at the crease²
- Long between the distal and proximal creases
- Index at the proximal crease
- The distance from the proximal edge of the A1 pulley to the palmodigital crease can be estimated by the distance from the palmodigital crease to the proximal interphalangeal (PIP) crease.³
Pisiform—sesamoid bone within the flexor carpi ulnaris (FCU) tendon, forms the ulnar border of Guyon canal, serves as an attachment point for the transverse carpal ligament
FCU tendon—ulnar artery is palpable deep and radial to the tendon as the artery enters Guyon canal.
Hook of hamate—forms the radial border of Guyon canal and the ulnar border of the carpal tunnel, provides a fulcrum for small and ring finger flexors
Flexor carpi radialis (FCR)—runs through the groove of trapezium, which is palpable along with distal pole of scaphoid, radial artery located immediately radial to the tendon
Lister’s tubercle—Bony prominence on the dorsum of the distal radius, which serves as a fulcrum for the extensor pollicis longus (EPL) tendon, which is located on its ulnar side and curves radially as it passes distal to the tubercle
- The scapholunate (SL) ligament is located about 1 cm distal to Lister’s tubercle.
Anatomic snuffbox—bordered by the EPL and extensor pollicis brevis (EPB), location of the scaphoid

FIGURE 61.1 A and B, Surface anatomy of the hand and wrist. The dotted line represents the original description of Kaplan’s cardinal line. the dashed line denotes the distal palmar crease. On the palmar view, the superficial flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU) tendons are easily palpable on their respective sides of the wrist. the FCU continues to the pisiform (p), and the hook of the hamate (H) is palpable in the palm distal and radial to the pisiform. the FCR tendon continues to its groove in the trapezium (t). On the dorsal wrist view, Lister tubercle (asterisk) is palpable on the distal radius. the anatomic snuffbox (hash) is noted between the extensor pollicis longus (EPL) and extensor pollicis brevis (EPB) tendons.

BONES (Figure 61.2)

Distal Radius (Figure 61.3)

Average radiographic parameters⁴
- Inclination: 23.6°
  - Angle formed by the line between the tip of the radial styloid and the central reference point (CRP: midpoint between the volar ulnar corner and dorsal ulnar corner) and a line perpendicular to the radial shaft
- Height: 11.6 mm
  - Axial length from the tip of the radial styloid and CRP
- Volar tilt: 11°
  - Angle formed on the lateral view between the line connecting the volar and ulnar dorsal corners and the line perpendicular to the radial shaft
- Ulnar variance: 0.6 mm
  - Axial length from the ulnar head and CRP
- Teardrop angle: 70°
  - Angle formed from a line parallel to the subchondral bone of the volar rim of the lunate facet and the radial shaft
Scaphoid facet
Lunate facet—radius of curvature⁵ 10.9 mm
Sigmoid notch—articulates with ulnar head at distal radial ulnar joint (DRUJ), radius of curvature 50% to 100% larger than ulnar head
- Articular contact provides about 20% DRUJ stability⁶

FIGURE 61.2 A,B, Bones of the wrist and hand. Reprinted with permission from Doyle JR, Tornetta P, Einhorn TA. Hand and Wrist. Philadelphia, PA: Lippincott Williams & Wilkins; 2006.

Distal Ulna

Ulnar styloid—base serves as the attachment point for the triangular fibrocartilage complex (TFCC)
Extensor carpi ulnaris (ECU) groove—located at the dorsal-ulnar aspect of the ulnar head
- Average depth of 1.4 mm and width⁷ of 9.0 mm
Ulnocarpal ligament complex—composed of the ulnotriquetral, ulnocapitate, and ulnolunate ligaments
- Originates adjacent to the ulnar styloid at the volar foveal region and the individual ligaments fan out to their respective insertions

FIGURE 61.3 a and B, Osseous anatomy of the distal radius. EPL, extensor pollicis longus; LF, lunate facet; SF, scaphoid facet; SN, sigmoid notch; TFCC, triangular fibrocartilage complex. From Cooney WP. The Wrist: Diagnosis and Operative Treatment. 2nd ed. philadelphia, pa: Lippincott Williams & Wilkins, a Wolters Kluwer Business; 2010. Used with permission of Mayo Foundation for Medical Education and research. All rights reserved.

Carpus

Proximal row
- Scaphoid
  - Serves as crutch to stabilize the proximal and distal rows
  - Blood supply—dorsal carpal branch of the radial artery makes up the primary vascular supply
    - Retrograde pattern predisposes scaphoid to avacsular necrosis (AVN) particularly with proximal pole fractures
    - Superficial palmar branch of the radial artery supplies distal pole
  - SL angle—formed by the longitudinal axis of the scaphoid and the line intersecting the body of the lunate on a lateral radiograph
    - Normal—30° to 60°, DISI deformity: >60°, VISI deformity: <30°
- Lunate
  - Blood supply variants⁸: Y-pattern (59%), X-pattern (10%), I-pattern (31%)
  - Radius of curvature⁵ 10.4 mm
  - Variable distal medial facet for hamate articulation (type 1: absent, type 2: present)
- Triquetrum
  - Articulates with pisiform on volar side
Distal row
- Trapezium
  - Saddle articulation with the thumb metacarpal allows for wide range of motion.
  - Groove on volar surface provides fulcrum for FCR as it passes underneath the trapezium to attach at the second metacarpal base.
- Trapezoid
  - Forms scaphotrapezotrapezoidal (STT) joint with scaphoid and trapezium
  - Articulates with index metacarpal at carpometacarpal joint
- Capitate
  - Blood supply—largely from retrograde flow, though 70% have a specific volar supply to the proximal pole⁹
  - Radius of curvature⁵ 6.1 mm
  - Articulates with long metacarpal at carpometacarpal joint
- Hamate
  - Hook projects volarly—serves as attachment point for transverse carpal ligament, pisohamate ligament, hypothenar muscles
  - Articulates with type 2 lunates
  - Articulates with ring and small metacarpals at carpometacarpal joint

Metacarpals

Medial and lateral sides are concave to serve as attachment points for adjacent interosseous muscles (four dorsal and three palmar).
Metacarpal bases serve as the site of attachment for the abductor pollicis longus (APL; thumb), extensor carpi radialis longus (ECRL; index), extensor carpi radialis brevis (ECRB; middle), extensor carpi ulnaris (small), flexor carpi radialis (index and middle), flexor carpi ulnaris (small).

Phalanges

Each digit has three phalanges (proximal, middle, and distal), other than thumb (proximal and distal).
- Proximal phalanx serves as attachment site for interosseous muscles.
- Middle phalanx serves as attachment site for flexor digitorum superficialis (FDS) and central slip.
- Distal phalanx serves as attachment site for flexor digitorum profundus (FDP) and terminal tendon of the extensor mechanism.

LIGAMENTS

Metacarpophalangeal (MCP) Joint of the Thumb

Ulnar collateral ligament
- Stabilizes the thumb during pinch and grasp
- Origin is 4.2 mm from the dorsal cortex and 5.3 mm from the articular surface on the thumb metacarpal head.
- Insertion is 2.8 mm from the volar surface and 3.4 mm from the articular surface of the proximal phalanx.¹⁵
- Proper collateral ligament is the main stabilizer in midflexion while the accessory collateral ligament and volar plate stabilize the joint in extension.
Radial collateral ligament
- Origin is 3.5 mm from the dorsal cortex and 3.3 mm from the articular surface on the thumb metacarpal head.
- Insertion is 2.8 mm from the volar cortex and 2.6 mm from the articular surface.

MCP Joints of the Fingers (Figure 61.4)

Proper collateral ligaments
- Origin is dorsal to the axis of rotation on the metacarpal head.
  - Along with an increasing diameter of the metacarpal head in the sagittal plane from dorsal to volar, the resulting cam effect tightens the proper collateral ligament in 70° to 90° flexion.
  - Proper collateral ligament is lengthened by 15% when flexing¹⁰ from 0° to 90°.
Accessory collateral ligament
- Inserts onto and suspends the volar plate
- Along with the volar plate, provides stability in extension
Deep transverse metacarpal ligaments
- Attach adjacent volar plates of the index through small MCP joints to prevent ray separation

FIGURE 61.4 Collateral ligaments of the digits. proper collateral ligament, which inserts onto bone, is dorsal to the accessory collateral ligament, which inserts onto the volar plate. reprinted with permission from Doyle JR, Tornetta p, einhorn ta. Hand and Wrist. philadelphia, PA: Lippincott Williams & Wilkins; 2006.

PIP Joints

Proper collateral ligaments
- Crescent-shaped origin is dorsal and proximal within the concavity on the head of the proximal phalanx.
- Insertion includes the majority of the base of the middle phalanx.
- Dorsal fibers of the ligament extend parallel to the axis of the middle phalanx and the volar fibers run in an oblique manner, giving the ligament its fan shape.¹¹
- Proper collateral ligament is tensioned throughout the interphalangeal (IP) range of motion.
Accessory collateral ligament
- Volar to the proper collateral ligament, inserts onto the volar plate
- Tensioned with IP joint extension and relaxes in flexion
- Results in contracture with prolonged immobilization in flexion
Intrinsic-plus splinting
- “Position of Safety”
- MCP joints placed in 70° to 90° of flexion to tension proper collateral ligament, and IP joints placed in full extension to tension accessory collateral ligament and volar plate
- Failure to maintain this position during prolonged splinting results in joint contracture and limitations in motion.