Metacarpal bones are a group of five long bones in the hand, located between the wrist (carpal bones) and the fingers (phalanges).[6] These bones form the palm and contribute to the overall structure and movement of the hand. Each metacarpal is named and numbered from the thumb (first metacarpal) to the little finger (fifth metacarpal). They consist of a base, shaft, and head, with the base articulating with the carpal bones and the head forming joints with the proximal phalanges.
Location
The metacarpal bones are located in the hand, specifically in the palm region. They extend from the distal row of carpal bones in the wrist to the base of the fingers.
Structure and Anatomy
The metacarpal bones are five long bones forming the framework of the palm. They are numbered from the thumb (first metacarpal) to the little finger (fifth metacarpal). Each metacarpal consists of three main parts: the base, shaft, and head.[4]
General Features
Base
- Shape: The proximal end of each metacarpal is broad and flat.
- Articulations:
- The base articulates with the carpal bones at the carpometacarpal joints.
- Adjacent metacarpals articulate with each other through intermetacarpal joints.
Shaft
- Shape: The shaft is long and slightly curved, being concave on the palmar surface and convex on the dorsal surface.
- Features:
- Provides attachment sites for muscles and ligaments.
- Nutrient foramen is located on the shaft, allowing entry for blood vessels.
Head
- Shape: The distal end of each metacarpal is rounded and forms the knuckle.[1]
- Articulations: Articulates with the proximal phalanges at the metacarpophalangeal joints.
- Features: The head is smooth and covered with articular cartilage for joint movement.
Individual Metacarpals
First Metacarpal (Thumb)
- Size and Shape: The shortest and thickest of the metacarpals.
- Base: Articulates with the trapezium at the first carpometacarpal joint, allowing for significant mobility (saddle joint).
- Head: Forms the joint with the proximal phalanx of the thumb.
Second Metacarpal
- Size and Shape: The longest metacarpal.
- Base: Articulates with the trapezoid primarily, and also with the trapezium, capitate, and third metacarpal.
- Head: Connects with the proximal phalanx of the index finger.
Third Metacarpal
- Size and Shape: Slightly shorter than the second metacarpal.[8]
- Base: Articulates with the capitate and the second and fourth metacarpals.
- Notable Feature: Has a prominent styloid process on its dorsal surface for ligament attachment.
- Head: Forms a joint with the proximal phalanx of the middle finger.
Fourth Metacarpal
- Size and Shape: Shorter and narrower than the third metacarpal.
- Base: Articulates with the hamate and the third and fifth metacarpals.
- Head: Connects with the proximal phalanx of the ring finger.
Fifth Metacarpal (Little Finger)
- Size and Shape: The smallest and thinnest metacarpal.
- Base: Articulates with the hamate and the fourth metacarpal.
- Head: Forms the joint with the proximal phalanx of the little finger.
Articulations
- Carpometacarpal Joints
- Proximal articulation between the bases of the metacarpals and the distal carpal bones.
- The first carpometacarpal joint (thumb) is a saddle joint, providing a wide range of motion.
- Intermetacarpal Joints
- The bases of adjacent metacarpals articulate with one another, stabilized by interosseous ligaments.
- Metacarpophalangeal Joints
- Distal articulation between the heads of the metacarpals and the proximal phalanges. These are condyloid joints, enabling flexion, extension, abduction, and adduction.[7]
Vascular Supply
- Arteries: Supplied by branches of the radial and ulnar arteries.
- Nutrient Foramina: Located on the shafts, allowing blood vessels to enter and nourish the bone.
Muscular Attachments
- Extrinsic Muscles:
- Tendons of muscles like the flexor digitorum superficialis and profundus, as well as the extensor digitorum, attach to the bases and shafts.
- Intrinsic Muscles:
- Interossei muscles attach to the sides of the shafts.
Function
Support and Stability
The metacarpal bones provide the structural framework of the hand, forming the palm and connecting the wrist to the fingers.[5] They maintain the hand’s shape and stability during rest and movement.
Facilitation of Movement
The metacarpals enable movements of the fingers and thumb through their articulations with the phalanges at the metacarpophalangeal joints.
The first metacarpal allows the thumb’s opposable movements, which are essential for gripping and precision tasks.
Force Transmission
The metacarpals transfer forces from the fingers to the wrist during activities such as gripping, pushing, or lifting.
Muscle Attachment
The metacarpals serve as sites for attachment of intrinsic and extrinsic hand muscles, facilitating fine motor skills and coordinated hand movements.[3]
Formation of Arches
The longitudinal and transverse arches of the hand, supported by the metacarpals, enhance the hand’s ability to grip objects and absorb mechanical forces.
Clinical Significance
The metacarpal bones are essential for hand function, and injuries or disorders affecting them can significantly impact daily activities:
- Fractures:
- Boxer’s Fracture: A common fracture of the fifth metacarpal, often caused by punching a hard surface.
- Shaft Fractures: Can result from trauma or falls, leading to hand deformity or functional impairment.
- Arthritis:
- Degenerative changes in the carpometacarpal or metacarpophalangeal joints can cause pain, stiffness, and reduced hand mobility, particularly in the thumb.
- Dislocations:
- Dislocations at the carpometacarpal or metacarpophalangeal joints can result from severe hand injuries and may require surgical intervention.
- Congenital Deformities:
- Conditions like brachydactyly or syndactyly may affect the length or alignment of the metacarpal bones, impacting hand functionality.[2]
- Tendon Injuries:
- Tendons attached to the metacarpals can be strained or ruptured due to overuse or trauma, leading to compromised hand movements.