Metacarpals are long bones in the human hand, forming the main structure of the palm. They connect the wrist bones (carpals) to the finger bones (phalanges). These bones provide the hand’s shape and enable its versatility.
Anatomy and Structure of Metacarpals
Each hand contains five metacarpal bones, numbered one to five. The first metacarpal connects to the thumb, the second to the index finger, the third to the middle finger, the fourth to the ring finger, and the fifth to the little finger. This numbering progresses from the radial (thumb) side to the ulnar (pinky) side of the hand.
Each metacarpal consists of three parts: a base, a shaft, and a head. The base is the proximal end, closest to the wrist, articulating with the carpal bones. The shaft, or body, is the long, central portion, providing structural support to the palm. The head is the distal end, near the fingers, forming the knuckles and articulating with the proximal phalanges (the first set of finger bones).
The metacarpals connect at the carpometacarpal (CMC) joints with the distal row of carpal bones. The first metacarpal articulates with the trapezium, while the second connects with the trapezoid and trapezium. The third articulates with the capitate, and the fourth and fifth connect with the hamate bone. Distally, the metacarpal heads form the metacarpophalangeal (MCP) joints with the proximal phalanges of each finger.
Key Functions of Metacarpals
Metacarpals establish the skeletal framework of the palm, providing stability and shape. This stable foundation distributes forces across the hand during activities, helping prevent injury.
These bones also contribute to the hand’s dexterity and gripping strength. The unique saddle joint of the first metacarpal with the trapezium allows for the thumb’s wide range of motion and its ability to oppose other fingers. This opposition is fundamental for tasks requiring a firm grasp and fine motor control, such as holding objects or writing.
Metacarpals serve as attachment points for numerous hand and forearm muscles. Muscles like the interossei and lumbricals originate or insert on the metacarpals, enabling precise finger movements, including flexion, extension, abduction, and adduction. The metacarpals act as levers, amplifying muscle force to facilitate a wide array of hand motions and complex manipulative tasks.
Common Metacarpal Injuries
Metacarpal fractures are common, often resulting from direct impact, falls onto an outstretched hand, or crushing injuries. Symptoms include localized pain, swelling, and bruising in the affected area.
A common metacarpal fracture is a Boxer’s fracture, involving a break in the neck of the fifth metacarpal (little finger bone). It often results from punching a hard object with a closed fist, especially with improper form. Though named for boxers, it is more prevalent in individuals not trained in punching techniques.
A Boxer’s fracture can cause a noticeable knuckle depression and may lead to a bent or misaligned pinky finger. Other metacarpal fractures can involve the shaft, head, or base of any of the five metacarpals, with varying severity. Medical evaluation, often including X-rays, is important for accurate diagnosis and to determine appropriate treatment, ranging from immobilization with a splint or cast to surgical intervention for complex cases.