Carpal Bones (Wrist Bones)

What Are the Wrist Bones

The wrist bones are a collection of abbreviated bones [24] in the human hand that constitute the wrist along with the final parts of the radius and ulna [1]. Therefore, they are also referred to as wrist bones. Together known as the carpus, they individually connect with the elongated bones in the lower arm radius and ulna and the metacarpals to form the wrist joint.

How Many Wrist Bones are there in the Hand

There are 8 bones in the human wrist, with each one being named based on its shape:

1. Scaphoid (boat-shaped)
2. Lunate (crescent moon-shaped)
3. Triquetrum (pyramidal)
4. Pisiform (pea-shaped)
5. Trapezium (irregular trapezium-shaped)
6. Trapezoid (wedge-shaped)
7. Capitate (head-shaped)
8. Hamate (wedge-shaped with a bony extension or ‘hook’)

The capitate is the largest of all carpal bones [2].

Wrist Bones X-Ray Anatomy

Functions

Each wrist bone is crucial in composing the carpus or wrist joint, which is pivotal for hand movement [14], enabling activities such as writing, typing, eating, and grasping objects. The wrist bones serve as the linkage between the forearm and hand and are fundamental for generating torque, providing humans with grip strength [15].

Arrangement of the Wrist Bones

How Are They Positioned

The wrist bones in the human hand are organized into two rows – the proximal carpal row, connecting with the lower arm bones radius and ulna, and the distal carpal row, connecting with the metacarpals.

Wrist Bones

Understanding their Location

The radial side means on the side of the radius bone, with the easiest way of remembering this being that the radius is located on the same side as the thumb [6]. Thus, the scaphoid, being the initial bone in the proximal row, connects with the radius. The trapezium, the first bone in the distal row, connects with the first metacarpal (metacarpal of the thumb), while the trapezoid (second bone in the distal row) connects with the second metacarpal and so on [7].

Simple way to remember the wrist bones (Mnemonic)

Memorize the following sentences as they serve as an acronym for the wrist bone names:

Wrist Bones Mnemonic

Wrist Bone Joints

All the joints involving the wrist bones are synovial joints, where the articulation surface has a flexible cartilage layer, along with a fluid lining to allow for better freedom of movement [22].

The Radiocarpal Joint: Those between the radius and the proximal carpal bones (except pisiform) [8].

Intercarpal Joints: Connections between the wrist bones in the hand are an example of gliding joints [9] (a type of synovial joint). The bones meet and articulate on a nearly flat surface, and they need to glide past the adjacent bones in different directions during movement [10].

Carpometacarpal Joints: Those connecting the metacarpals and the distal carpal bones. The carpometacarpal joint of the thumb has a characteristic saddle shape, which makes the thumb much more flexible than the rest of the fingers [8]

The Wrist Tunnel

The wrist tunnel is a passageway for the medial nerve, as well as nine tendons passing from the wrist into the hand and fingers [11]. It is located on the palmar side of the wrist, with its boundaries formed by the wrist bones and the flexor retinaculum (a fibrous band arching over the wrist bones on the palmar side) [12].

Blood Supply

The wrist bones receive their primary blood supply through the radial, ulnar, and anterior interosseous arteries, as well as the deep palmar arch [13].

Ligament Attachments

The ligaments in this area can be classified into separate groups depending on the bones involved in their attachment. The radioscaphocapitate and the long and short radiolunate ligaments joint the radius with various wrist bones. Similarly, the ulnolunate and ulnocapitate ligaments attach the ulna with the lunate and capitate bones respectively [14].

Additionally, multiple ligaments keep the wrist bones in place by attaching them with each other. These include the lunotriquetral, trapeziotrapezoid, scaphotrapezial, scaphotrapezoidal, scaphocapitate, capitotrapezoid, capitohamate, and triquetrohamate ligaments [14].

Development and Ossification

All the wrist bones are cartilaginous at birth, starting to ossify one by one within 1-2 months of age [3]. Seven of these eight bones become fully developed by the time a child is 6-7 years old, with the order of ossification being the capitate at first, followed by the hamate, triquetrum, lunate, scaphoid, trapezium, and trapezoid. The pisiform is the last to develop, ossifying fully by 12 years [4].

Common Injuries and Associated Conditions

Fracture and Dislocation: The wrist is most frequently injured among all joints in the human body [16]. Due to their position in the hand, the wrist bones often get fractured or dislocated as a result of accidents, like falling on an outstretched hand [17], and sports injuries, especially when playing sports like hockey and tennis. One characteristic symptom of a broken or dislocated wrist bone is that the pain gets worse with movement [18]. The scaphoid is the most commonly fractured wrist bone, while the most common forms of dislocations in this area involve the lunate [16].

Wrist Tunnel Syndrome: Another common condition involving the wrist, the wrist tunnel syndrome occurs when the medial nerve gets compressed in its passage through the wrist. It usually causes a characteristic pain, numbness, and tingling sensation in the fingers (may not be as prominent in the little finger) [19].

Wrist Avascular Necrosis: A condition where a lack of blood supply to the wrist bone cells causes serious damage, finally resulting in their death. The lunate and scaphoid are most prone to this degenerating disorder [20].

Other conditions that may involve the wrist include torn ligaments, arthritis, overuse injuries, and joint infections [21].

References

1. https://www.mayoclinic.org/carpal-bones/img-20007898
2. http://www.mananatomy.com/body-systems/skeletal-system/carpal-bones-wrist
3. https://radiopaedia.org/articles/ossification-centres-of-the-wrist
4. http://sketchymedicine.com/2016/01/carpal-bone-ossification/
5. https://www.kenhub.com/en/library/anatomy/carpal-bones
6. http://sketchymedicine.com/2015/04/describing-where-things-are-on-the-hand/
7. http://teachmeanatomy.info/upper-limb/bones/bones-of-the-hand-carpals-metacarpals-and-phalanges/
8. https://www.dummies.com/education/science/anatomy/joints-of-the-wrist-hand-and-fingers/
9. http://www.innerbody.com/image_skel07/skel32.html
10. http://www.bbc.co.uk/science/humanbody/body/factfiles/joints/gliding_joint.shtml
11. http://teachmeanatomy.info/upper-limb/areas/carpal-tunnel/
12. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3558235/
13. http://www.wheelessonline.com/ortho/blood_supply_to_the_wrist
14. https://emedicine.medscape.com/article/1899456
15. https://www.kenhub.com/en/library/anatomy/the-wrist-joint
16. https://patient.info/doctor/carpal-fractures-and-dislocations
17. https://emedicine.medscape.com/article/97565-overview
18. http://www.nsmi.org.uk/articles/wrist-injuries/fractured-carpal-bones.html
19. https://orthoinfo.aaos.org/en/diseases–conditions/carpal-tunnel-syndrome/
20. http://www.davidhildrethmd.com/carpal-avascular-necrosis.html
21. http://baylorarlington.com/hand-and-wrist-pain/wrist-conditions/
22. https://opentextbc.ca/anatomyandphysiology/chapter/9-4-synovial-joints/
23. https://emedicine.medscape.com/article/1254517-overview?pa=gCCnT9WoQtvHVbIRiKc1dD%2FEuWoLmcgPcQJlQvLUG0Q9hvpv8mBToC%2B8%2BRE9%2BGgs56MI7dGTgNawPfsOtJla9Q%3D%3D