Metacarpal Bones

Definition of Metacarpal Phalanges

Metacarpal phalanges refer to a group of bones in the human hand, situated between the wrist and the finger phalanges [1]. These bones are categorized as elongated bones and collectively they constitute the metacarpus, forming the palm of the hand [2]

Quantity of Metacarpal Phalanges in the Palm of Hand

The palm of the hand contains five metacarpal phalanges wherein each bone is associated with a specific finger [1]:

1. First Metacarpal Phalange – for the thumb
2. Second Metacarpal Phalange – for the index finger
3. Third Metacarpal Phalange – for the middle finger
4. Fourth Metacarpal Phalange – for the ring finger
5. Fifth Metacarpal Phalange – for the little finger

Location of the Metacarpal Phalanges

The metacarpal phalanges lie amid the distal carpal bones (trapezoid, trapezium, capitate, hamate) and the proximal phalanges (the lowest of the three bones making up each finger) [2]. They are palpable on the back of the palm, beneath each finger [2].

Metacarpal Phalanges

Formation and Ossification

Metacarpal phalanges ossify from two centers; the first is for the shaft, and the second for the base in the first metacarpal phalange and the head in the remaining four [17]. The ossification centers of the second and third metacarpal phalanges appear first, around the 8th-9th week of fetal development, while the first or thumb metacarpal phalange is the final to ossify. Ossification of all the metacarpal phalanges completes approximately around the 20th year of life [16].

Metacarpal Phalanges X-Ray Image

Structure of the Metacarpal Phalanges

Segments of a Metacarpal Phalange

Each of these elongated bones can be segregated into three segments:

Metacarpal Phalange Head: The rounded distal end (the upper end on the side of the fingers) [3]. The area just below the head is termed as the neck of a metacarpal phalange.

Metacarpal Phalange Body/Shaft: The elongated portion between the head and the base; the metacarpal phalange shaft has a concave palmar aspect and sides. On its dorsal aspect, there is a triangular area on the distal side [3, 4].

Metacarpal Phalange Base: The enlarged proximal end (the lower end on the side of the wrist) [3]; the third metacarpal phalange has a styloid process projecting dorsally, extending behind the capitate [5].

Articulations of Metacarpal Phalanges

Metacarpal phalanges create crucial joints and articulations on both ends:

Articulations Between the Medial Metacarpal Phalanges

The four medial (excluding the thumb) metacarpal phalanges are interconnected through articular surfaces at the base, while their distal ends are connected by ligaments. This configuration shapes the hollow of the palm, granting flexibility to the fingers [18].

Metacarpophalangeal Joints (Metacarpal-Phalangeal Joints)

As implied, these joints are between proximal phalanges and metacarpal phalanges. Each metacarpal phalange forms a smooth articular facet [6] on its distal end or head for articulation with the corresponding proximal phalanx. The first metacarpal phalange articulates with the proximal phalanx of the thumb, the second metacarpal phalange with the proximal phalanx of the index finger, and so on [4]. These joints form the most prominent knuckles of the hand [7].

Carpometacarpal Joints (Carpal-Metacarpal Joints)

The connections between the metacarpal and carpal bones are all planar synovial joints, except the thumb, which is a saddle joint (another type of synovial joint) [8].

The five metacarpal phalanges establish notable articulations on their base or proximal end, with one or more of the four distal carpal bones [4]:

1. Metacarpal phalange of the thumb: With the trapezium
2. Metacarpal phalange of the index finger: With both the trapezoid and the trapezium
3. Metacarpal phalange of the middle finger: With the Capitate
4. Metacarpal phalange of the ring finger: With the hamate, as well as the capitate
5. Metacarpal phalange of the little finger: With the hamate

Vascular Supply

These bones are nourished by the three metacarpal arteries that originate from the deep volar arch and anastomose with the three common digital branches of the superficial palmar arch at the head of the metacarpal phalanges. The metacarpal arteries also join the dorsal metacarpal arteries [4].

Primary Ligament Connections

Dorsal Metacarpal Ligaments: Unify the metacarpal phalanges together [9].

Palmar Metacarpal Ligaments: Link the metacarpal phalanges and the phalanges [9].

Pisometacarpal Ligament: Binds the pisiform bone to the base of the fifth metacarpal phalange, which links with the little finger [9].

Natatory Ligament/Superficial Transverse Metacarpal Ligament: Unite the metacarpal phalange heads together, extending between the fingers, just beneath the skin, creating the web spaces of the palm [10].

Deep Transverse Metacarpal Ligament: These traverse across the palmar surface of the hand, from the second to fifth metacarpal phalange head, reinforcing the palmar ligaments and stabilizing the metacarpal phalange joints of the index, middle, ring, and little fingers [11].

Muscular Connections in the Metacarpal Phalange Region

Principal muscular connections in this region encompass the abductor pollicis longus, opponens pollicis, opponens digiti minimi, extensor carpi radialis longus & brevis, and the extensor carpi ulnaris [4, 12]. These muscles govern the mobility of the palm, fingers, and also the flexion of the wrist in the radial and ulnar sides.

Metacarpal Phalanges Function

The fundamental role of the metacarpal phalanges is to serve as the linkage between the wrist and fingers, framing the structure of the hand. Together, as the carpus, they are the crucial part of the framework that connects the small and large bones in the human hand, steadying its dorsal and palmar sides [6]. Consequently, they play a pivotal role in the proper development, movement, and operation of the hand.

Common Injuries and Related Conditions

Fractures and Dislocations: These are among the most commonly fractured bones, with car accidents being one of the common causes behind a broken or injured metacarpal phalange [2]. Fractures in particular locations may have specific names, for example, that of the fourth and fifth metacarpal phalanges is known as the boxer’s fracture [19]. Dislocations of the carpometacarpal joints are fairly rare, with few medical cases reported. Symptoms of an injury usually include pain, swelling, and inability to move the wrist or fingers [13]. Treatment varies depending on which metacarpal phalange is affected and the nature and location (head/shaft/base) of the fracture [12].

Metacarpal Boss: Occasionally, there may be a bony mass or bump on the back of the hand around the carpometacarpal joints. This is referred to as a metacarpal or carpometacarpal boss and is painless unless associated with some other condition. Such bony prominence most commonly occurs at the base of the second or third metacarpal phalange. Treatment depends on the presence of any underlying causes [14].

Carpometacarpal Arthritis (First Carpometacarpal Joint Arthritis): Arthritis may affect any of the five carpometacarpal joints but is more prevalent in the thumb joint between the first metacarpal and trapezium bones. It is characterized by pain at the base of the thumb when gripping or picking something up or during twisting movements like opening a jar. Treatment may include immobilizing the thumb with a splint along with oral medication and injection. Severe cases may even necessitate surgical intervention [15].

References

1. http://teachmeanatomy.info/upper-limb/bones/bones-of-the-hand-carpals-metacarpals-and-phalanges/
2. https://www.healthline.com/human-body-maps/metacarpals
3. https://www.earthslab.com/anatomy/bones-of-the-hand-carpals-metacarpals-and-phalanges/
4. https://radiopaedia.org/articles/metacarpal-bones-1
5. https://www.imaios.com/en/e-Anatomy/Anatomical-Parts/Styloid-process-of-third-metacarpal-III
6. https://www.kenhub.com/en/library/anatomy/the-metacarpal-bones
7. https://www.medicinenet.com/image-collection/finger_anatomy_picture/picture.htm
8. https://www.earthslab.com/anatomy/carpometacarpal-joints/
9. http://www.innerbody.com/image_skelfov/skel22_new.html
10. http://www.wheelessonline.com/ortho/natatory_ligament
11. http://www.anatomyexpert.com/app/structure/15491/98/
12. https://www.orthobullets.com/hand/6037/metacarpal-fractures
13. https://emedicine.medscape.com/article/1287549-overview?pa=SD9xYECMl4hVnkhiH9PJsdbdWTj7FC52217t3WaBYjX%2BVcs%2BUbpoFAdt5NROHwzwNFsYxDuz%2Fz2hge3aAwEFsw%3D%3D#showall
14. https://www.handandwristinstitute.com/metacarpal-boss/
15. http://www.kleinertkutz.com/common-conditions-carpal-metacarpal-arthritis.html
16. https://prohealthsys.com/central/anatomy/grays-anatomy/index-10/index-10-2/index-10-2/ossificationhand/
17. https://www.bartleby.com/107/56.html
18. https://www.mananatomy.com/body-systems/skeletal-system/metacarpal-bones
19. https://www.webmd.com/a-to-z-guides/boxers-fracture