Lunate Bone

Meaning: What Exactly is the Half-Moon Shaped Bone?

The half-moon shaped bone, also known as the semilunar bone, is one of the eight carpal or wrist bones in humans [1]. Resembling a crescent moon in shape, the bone gets its name from the Latin term ‘luna’, which means ‘moon’ [2].

Where Can the Half-Moon Shaped Bone Be Found?

The half-moon shaped bone is located between the scaphoid bone and the distal carpal bone triquetrum [3]. It is the second bone from the thumb in the proximal carpal row, after the scaphoid [4].

Half-Moon Shaped Bone

Evolvement and Ossification

It is cartilaginous at birth, starting to ossify between the second and fourth year of life [5]. Ossification usually completes in girls around the age of 6, while in boys it occurs when they are around 7 years old [1, 6].

Half-Moon Shaped Bone on X-Ray

Structure and Anatomy of the Half-Moon Shaped Bone

Surfaces and Connections

The distal surface of the half-moon shaped bone articulates with the capitate, and hamate bones, while the scaphoid bone forms an articulation with its lateral surface [3]. The triquetrum, the fourth carpal bone connected with the half-moon shaped bone [4], articulates with its medial surface [1]. The proximal surface of the half-moon shaped bone articulates with the radius, while, in conjunction with the triquetrum and scaphoid, it forms the distal surface of articulation of the wrist or radiocarpal joint [7].

The proximal facet (the one articulating with radius) is convex, while the distal facet (articulating with the capitate) is concave, giving the half-moon shaped bone its characteristic half moon shape [1].

Ligament Attachments

The half-moon shaped bone is connected with the bones around it by the scapholunate, lunotriquetral, radiolunotriquetral, and ulnolunate ligaments [1]. However, this bone has no muscular attachments [8].

Blood Supply

The half-moon shaped bone has a rich blood supply from dorsal radiocarpal and intercarpal arch branches, via its palmar and dorsal surfaces, or sometimes only the palmar surface [1, 9].

Roles: What are the Functions of the Half-Moon Shaped Bone?

Similar to all the other wrist bones, the half-moon shaped bone is responsible for shaping the wrist and maintaining its flexibility. As part of the proximal carpal row, it is involved in the joint between the carpal bones and the lower arm bones radius and ulna (the radiocarpal joint) [8].

Related Conditions and Common Injuries

Dislocation: Due to only a few ligaments, and no muscular attachments holding the half-moon shaped bone in place, it is the most commonly dislocated of all the wrist bones [8].

Kienböck’s Disease (avascular necrosis of the half-moon shaped bone): Occasionally, the blood supply to a bone may be hindered (due to an accident or trauma), making the bone weak or brittle, ultimately leading to necrosis. The half-moon shaped bone, with its rich blood supply and only ligament attachments, is believed to be more susceptible to this condition. In severe cases, Kienböck’s disease may require surgery to regain use of the hand and wrist [4, 9].

References

1. https://radiopaedia.org/articles/lunate-1
2. http://www.anatomyexpert.com/app/structure/65/96/
3. http://anatomyzone.com/anatomy-feed/lunate-bone/
4. https://www.healthline.com/human-body-maps/lunate-bone
5. http://sketchymedicine.com/2016/01/carpal-bone-ossification/
6. https://www.researchgate.net/publication/308941525_Osseous_Anatomy_and_Microanatomy_of_the_Lunate
7. https://www.kenhub.com/en/library/anatomy/carpal-bones#section3
8. https://musculoskeletalkey.com/structure-and-function-of-the-wrist/
9. https://www.ncbi.nlm.nih.gov/pubmed/21348025