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Carpal bones

Medically Reviewed by Anatomy Team

Table of Contents

Carpal bones are a group of eight small, irregularly shaped bones that form the wrist (carpus). These bones are arranged in two rows: a proximal row (closer to the forearm) and a distal row (closer to the hand). The carpal bones collectively provide flexibility and stability to the wrist, enabling a wide range of movements.[2] The eight carpal bones are the scaphoid, lunate, triquetrum, pisiform (proximal row), trapezium, trapezoid, capitate, and hamate (distal row).

Location

The carpal bones are located in the wrist, between the distal ends of the radius and ulna (forearm bones) and the bases of the metacarpal bones (palm).[3] They form the bony framework of the wrist joint.

Structure and Anatomy

The carpal bones are eight small, irregularly shaped bones that are organized into two rows: the proximal row and the distal row.[5] Together, they form the structure of the wrist and provide the framework for its articulation and movement.

General Structure of Carpal Bones

  • Shape and Size
    • Each carpal bone is irregular in shape, with a rough surface for ligament and tendon attachment and smooth areas for articulation.
    • They are classified as short bones, primarily composed of cancellous (spongy) bone surrounded by a thin layer of compact bone.[7]
  • Arrangement
    • Proximal Row: Scaphoid, lunate, triquetrum, pisiform (from lateral to medial).
    • Distal Row: Trapezium, trapezoid, capitate, hamate (from lateral to medial).
    • These bones form a concave shape on the palmar side, contributing to the carpal tunnel.
  • Articulations
    • Proximally, the carpal bones articulate with the radius and ulna.
    • Distally, they articulate with the bases of the metacarpal bones.
    • Numerous intercarpal joints exist between the carpal bones themselves.[8]

Anatomy of the Proximal Row

Scaphoid

  • Shape and Location: Boat-shaped bone located on the lateral side of the proximal row.
  • Articulations: Articulates with the radius, lunate, trapezium, and trapezoid.
  • Notable Features: Has a tubercle on its palmar surface, which provides attachment for the flexor retinaculum.

Lunate

  • Shape and Location: Crescent-shaped bone located medial to the scaphoid.
  • Articulations: Articulates with the radius, scaphoid, triquetrum, and capitate.
  • Notable Features: Its smooth proximal surface contributes to the radiocarpal joint.[1]

Triquetrum

  • Shape and Location: Pyramid-shaped bone located medial to the lunate.
  • Articulations: Articulates with the pisiform, lunate, and hamate, as well as the triangular fibrocartilage complex (TFCC).
  • Notable Features: Visible primarily on the dorsal aspect of the wrist.

Pisiform

  • Shape and Location: Small, pea-shaped bone located anteriorly to the triquetrum.
  • Articulations: Articulates only with the triquetrum.
  • Notable Features: Serves as a sesamoid bone embedded within the tendon of the flexor carpi ulnaris.

Anatomy of the Distal Row

Trapezium

  • Shape and Location: Irregularly shaped bone located on the lateral side of the distal row.
  • Articulations: Articulates with the scaphoid, trapezoid, and the first metacarpal (thumb).
  • Notable Features: Has a groove for the tendon of the flexor carpi radialis.[4]

Trapezoid

  • Shape and Location: Small, wedge-shaped bone medial to the trapezium.
  • Articulations: Articulates with the scaphoid, trapezium, capitate, and second metacarpal.

Capitate

  • Shape and Location: Largest carpal bone located centrally in the distal row.
  • Articulations: Articulates with the scaphoid, lunate, trapezoid, hamate, and the third metacarpal.
  • Notable Features: Forms the keystone of the carpal arch.

Hamate

  • Shape and Location: Hook-shaped bone located on the medial side of the distal row.
  • Articulations: Articulates with the triquetrum, capitate, and the fourth and fifth metacarpals.
  • Notable Features: The hook (hamulus) on its palmar surface serves as an attachment point for the flexor retinaculum and other structures.

Arrangement and Orientation

  • Carpal Arch
    • The carpal bones form an arched structure, concave on the palmar side. This arch is bridged by the flexor retinaculum to form the carpal tunnel, through which the median nerve and flexor tendons pass.[6]
  • Interosseous Ligaments
    • The carpal bones are connected by strong interosseous ligaments, providing stability while allowing small movements between the bones.

Vascular Supply

  • Proximal Row: Supplied by branches of the radial and ulnar arteries.
  • Distal Row: Primarily supplied by the deep palmar arch.
  • The scaphoid has a retrograde blood supply, making it prone to avascular necrosis in case of fractures.

Notable Features

  • Complex Articulations
    • The carpal bones form multiple joints, including the radiocarpal, midcarpal, and carpometacarpal joints, contributing to wrist flexibility.
  • Structural Variability
    • The shape and arrangement of the carpal bones vary slightly between individuals but maintain their overall function and alignment.
  • Muscle and Ligament Attachments
    • The carpal bones serve as attachment sites for several muscles and ligaments that stabilize the wrist and facilitate hand movements.

Function

Support and Stability
The carpal bones form the structural base of the wrist, supporting the hand and enabling its attachment to the forearm.

Facilitation of Movement
The carpal bones allow a wide range of wrist movements, including flexion, extension, abduction, adduction, and circumduction, by forming articulations with the radius, ulna, and metacarpal bones.

Shock Absorption
The arrangement of carpal bones helps absorb and distribute forces transmitted through the hand and wrist during activities like gripping, pushing, or lifting.

Formation of the Carpal Tunnel
The carpal bones create a concave structure (carpal arch), which, along with the flexor retinaculum, forms the carpal tunnel. This tunnel protects the median nerve and tendons of the forearm muscles as they pass into the hand.

Muscle and Ligament Attachment
The carpal bones provide anchoring points for various muscles and ligaments, contributing to wrist stability and facilitating hand and finger movements.

Clinical Significance

The carpal bones play a critical role in wrist function and are prone to various injuries and disorders:

  • Fractures:
    • Scaphoid Fracture: The most commonly fractured carpal bone, often caused by a fall on an outstretched hand. Poor blood supply increases the risk of avascular necrosis.
    • Hamate Fracture: Involves the hook of the hamate, commonly seen in sports injuries involving racquets or clubs.
  • Carpal Tunnel Syndrome:
    • Compression of the median nerve within the carpal tunnel, caused by swelling of the tendons or inflammation, leads to pain, numbness, and weakness in the hand.
  • Dislocations:
    • Lunate Dislocation: A severe wrist injury that can lead to nerve compression and chronic instability.
    • Perilunate Dislocation: Involves dislocation of other carpal bones relative to the lunate.
  • Arthritis:
    • Degenerative changes in the carpal bones can lead to wrist stiffness, pain, and reduced mobility, particularly in rheumatoid arthritis or osteoarthritis.
  • Congenital Anomalies:
    • Variations like carpal coalition (fusion of carpal bones) can restrict wrist motion and cause discomfort.

References

  1. Palastanga, N., Soames, R., & Williams, P. L. (2018). Anatomy and Human Movement: Structure and Function (7th ed.). Elsevier Health Sciences. ISBN 9780702072260.
  2. Hollinshead, W. H. (1982). Hollinshead’s Textbook of Anatomy (5th ed.). Harper & Row. ISBN 9780061422888.
  3. Romanes, G. J. (1981). Cunningham’s Manual of Practical Anatomy, Volume 1: Upper and Lower Limbs (15th ed.). Oxford University Press. ISBN 9780192631387.
  4. Flatt, A. E. (2007). The Care of the Skeletal System (2nd ed.). Charles C Thomas. ISBN 9780398077428.
  5. Berkovitz, B. K. B., Holland, G. R., & Moxham, B. J. (2017). Color Atlas and Textbook of Human Anatomy (6th ed.). Thieme. ISBN 9783135334097.
  6. Healy, J. C., & Hodge, J. C. (2020). Anderson’s Atlas of Functional Anatomy (11th ed.). CRC Press. ISBN 9780367330506.
  7. Martini, F. H., & Nath, J. L. (2020). Fundamentals of Anatomy & Physiology (11th ed.). Pearson. ISBN 9780134810423.
  8. Woodburne, R. T., & Burkel, W. E. (1994). Essentials of Human Anatomy (9th ed.). Oxford University Press. ISBN 9780195045044.