The carpal bones (i.e. carpus) are eight irregularly-shaped bones located in the wrist region. These bones connect the distal aspects of the long bones of the forearm (radius and ulna) to the proximal aspects of the metacarpal bones.
The carpal bones are organized in two rows: proximal and distal. The proximal row of carpal bones (from radial to ulnar) includes the scaphoid, lunate, triquetrum and pisiform bones. The distal row includes the trapezium, trapezoid, capitate, hamate bones.
Each carpal bone has its own unique shape and is multifaceted, meaning that they have the ability to articulate with several surrounding bones, muscles and ligaments of the forearm and hand. This way, the carpal bones provide flexibility and various types of movements to the soft tissues of the hand. They also provide the majority of the skeletal framework of the wrist that allows the passageway for the different neurovascular structures of the hand.
This article will discuss the anatomy and function of the carpal bones.
|Definition||The carpal bones are the eight small bones that make up the wrist (or carpus) and connect the hand to the forearm.|
|Proximal row||Scaphoid bone, lunate bone, triquetrum bone, pisiform bone|
|Distal row||Trapezium bone, trapezoid bone, capitate bone, hamate bone|
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(Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate)
- Proximal row
- Distal row
- Capitate bone
- Hamate bone
- Clinical relations
The scaphoid bone is the largest carpal bone of the proximal row. The scaphoid is a boat-shaped bone (“scaphos” in Greek means boat, hence its name) that lies just inferior to the anatomical snuffbox. It articulates proximally to the radius, and distally with the trapezium and trapezoid. In addition, the scaphoid articulates with the lunate and capitate bones.
The scaphoid has a bony prominence located on its palmar surface known as the scaphoid tubercle; as it sits subcutaneously the tubercle is easily palpable on the palm of the hand. The scaphoid is the most frequently fractured bone of the wrist. Scaphoid fractures mostly occur when a person falls onto the outstretched hand.
The lunate is the next bone of the proximal row, located between the scaphoid and the triquetrum bones. Proximally, the lunate articulates with the head of the radius (carpal articular surface) and the articular disc of the distal radioulnar joint, while distally it articulates with the capitate bone. It gets its name from its moon-shaped appearance.
The triquetrum is a pyramid-shaped bone (its name means three-cornered bone) located on the medial aspect of the carpus. It articulates with the lunate bone laterally and the hamate bone distally. In addition, the triquetrum bears an isolated oval-shaped facet on its distal palmar surface for articulation with the pisiform bone.
The pisiform is a tiny, pea-shaped bone located on the distal palmar surface of the triquetrum bone. It has a dorsal articular facet for the articulation with the triquetrum. The pisiform is a sesamoid bone meaning that it is fully embedded in a tendon, more specifically in the tendon of the flexor carpi ulnaris muscle. This bone also lies superficially in the palm and it is easily palpable.
The trapezium is the first and most lateral of the distal row of carpal bones. It forms articulations with the scaphoid, trapezoid, and 1st and 2nd metacarpals. The trapezium bone has a tubercle and groove on its rough palmar surface. These bony features provide a site for tendons and ligaments to either pass through or attach. The dorsal surface of the bone is closely related to the radial artery.
The trapezoid is a wedge-shaped bone in the distal row of the carpal bones. It may look very small in comparison to the other bones from a palmar aspect, however, it is much wider on its dorsal side. It forms articulations with the scaphoid bone proximally, laterally with the trapezium, medially with the capitate and its distal facet allows it to articulate with the second metacarpal bone.
The capitate bone is the largest of all the carpal bones. It primarily articulates distally with the 3rd metacarpal bone. In addition, it forms articulations with the surrounding carpal bones; with the trapezoid, scaphoid, lunate, and hamate.
The hamate is wedge-shaped and the most medial bone of the distal row. The hamate forms articulations distally with the 4th and 5th metacarpals, with the capitate laterally and triquetrum proximally.
The main anatomical feature of the hamate bone is the hamulus, an elongated bony structure that projects from the distal aspect of its palmar surface. The hamulus contributes to the formation of the medial wall of the carpal tunnel and the lateral wall of the ulnar canal (i.e. Guyon's canal). The hamulus also serves as the attachment point for a number of different muscles and ligaments of the hand and forearm, including the flexor retinaculum.
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A useful mnemonic in order to remember the order of the carpal bones from lateral to medial, in the proximal row and then the distal row is the following:
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Scaphoid bone fracture
The most common fracture of the carpus is the fracture of the scaphoid bone. It usually occurs when a person falls on an outstretched hand while trying to prevent the incident. The symptoms include pain and swelling in the region known as the anatomical snuffbox.
Complications occur due to a lack of blood supply to the area from the palmar carpal branch of the radial artery. This inadequacy of blood and accompanied nutrient deprivation can lead to avascular osteonecrosis.
Carpal tunnel syndrome
Carpal tunnel syndrome is the infamous term for a condition known as the median nervecompression and is the most common form of compression-related neuropathy. It is primarily caused by occupational tendencies, such as repeated wrist flexion and extension, leading to overuse of the anatomical structures that directly work during these movements. As a result, an increase in content mass and pressure within the carpal tunnel push down upon the median nerve leading to sensory and motor disturbances within its areas of innervation.
Chronic compression can lead to lasting nerve damage and atrophy of thenar muscles which translates as weakness in the thumb and index fingers. Treatment includes resting the wrist and refraining from using it for a period of time. Bandages and splints may help stabilize the area while anti-inflammatory drugs and painkillers may be prescribed in extreme cases.
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