The hand and wrist represent one of the most mechanically intricate structures in the human body, allowing for both powerful grip and refined dexterity. This skeletal framework provides the necessary support and articulation for delicate movements, such as writing or using tools, while managing extreme forces. The structure combines small, irregularly shaped bones that form a flexible base with longer bones that create the palm and fingers.
The 27 Bones of the Hand and Wrist
The hand and wrist skeleton is composed of 27 individual bones, divided into three groups. The most proximal group is the carpus, or wrist, which consists of eight small, irregularly shaped carpal bones arranged into two rows. This cluster forms a flexible bridge connecting the forearm bones—the radius and ulna—to the rest of the hand.
The proximal row includes the scaphoid, lunate, triquetrum, and pisiform, which articulate directly with the forearm to allow for wrist movement. The distal row consists of the trapezium, trapezoid, capitate, and hamate, serving as the anchor for the palm bones. The capitate is the largest carpal bone, while the scaphoid acts as a structural link between the two carpal rows, making it vulnerable to injury.
Extending from the carpal bones are the five metacarpals, which form the long bones of the palm. These bones are numbered one through five, starting with the thumb, and their length provides the structural framework for the hand’s arch and overall size. Each metacarpal has a base connecting to the wrist, a shaft forming the palm, and a head that creates the prominent knuckles of the hand.
The final group consists of the 14 phalanges, or finger bones, for all five digits. Each of the four fingers contains three phalanges: the proximal, middle, and distal phalanx. The thumb is unique, containing only two phalanges (proximal and distal), which allows it to remain shorter and more robust. The distal phalanges support the fingernails and the sensitive pads of the fingertips.
How Skeletal Structure Enables Movement
The carpal bones in the wrist are plane joints, which primarily permit a subtle gliding motion between their flattened surfaces rather than large rotational movement. This collective gliding movement across the multiple intercarpal and midcarpal joints contributes to the overall flexion, extension, and side-to-side deviation of the wrist. This multi-joint connection helps absorb shock and fine-tune the hand’s position relative to the forearm.
The most unique articulation is the trapeziometacarpal joint, the connection between the thumb’s first metacarpal and the trapezium carpal bone. This joint is a specialized saddle joint, named because the concave and convex surfaces fit together like a rider in a saddle. This biconvex shape permits movement in three planes: flexion-extension, abduction-adduction, and rotation. This combination of movements enables opposition, the ability of the thumb to touch the tips of the other four fingers, which is essential for precision grip.
In the fingers, the joints between the phalanges are primarily hinge joints, allowing movement predominantly in one plane, like the hinge of a door. The metacarpophalangeal joints (the main knuckles) are condyloid joints, permitting a wider range of movement including flexion, extension, and side-to-side spreading. The relative immobility of the second and third metacarpals provides a stable central pillar for the hand. The greater mobility of the first, fourth, and fifth metacarpals allows the hand to cup and conform around objects for a secure grasp.
Common Acute Hand Bone Injuries
The highly mobile and exposed nature of the hand skeleton makes its bones vulnerable to traumatic injuries, most commonly fractures. One frequent injury is the Boxer’s fracture, which involves a break in the neck of the fifth metacarpal bone (leading to the little finger). This injury typically occurs when a person strikes a hard object with a clenched fist, causing the concentrated force to fracture the bone just below the knuckle.
Another common, yet often subtle, injury is a fracture of the scaphoid bone, the most frequently broken carpal bone. Scaphoid fractures often result from falling onto an outstretched hand, where the force travels up the arm into the wrist. Symptoms can initially be mild, leading to missed diagnoses and long-term consequences.
The scaphoid is prone to poor healing because of its unusual blood supply, which enters the bone from the distal end and flows backward toward the proximal end. A fracture across the middle can interrupt this retrograde blood flow, starving the proximal fragment. This disruption can lead to avascular necrosis, where a portion of the bone dies, potentially resulting in long-term wrist instability and early-onset arthritis. Fractures of the proximal phalanges and metacarpal shafts are also common, usually resulting from crushing injuries or direct impact.
Long-Term Conditions Affecting Hand Bones
Hand bones are frequently affected by degenerative conditions that cause gradual changes to the joints and bone density. Osteoarthritis is the most prevalent chronic condition, characterized by the progressive breakdown of the smooth cartilage cushioning the ends of the bones. As the protective cartilage wears away, the bones begin to rub directly against each other, leading to pain, stiffness, and the formation of bony growths called osteophytes.
In the hands, osteoarthritis commonly affects the joint at the base of the thumb and the small interphalangeal joints of the fingers. These bony spurs can present as noticeable hard lumps on the finger joints, sometimes referred to as Heberden’s nodes or Bouchard’s nodes. This joint deterioration alters the shape of the joints and limits the range of motion, making pinch and grip activities increasingly difficult.
Conditions characterized by low bone mineral density, such as osteopenia and its more severe form, osteoporosis, also impact the hand skeleton. While these conditions are often associated with the hip and spine, the distal forearm and wrist are common sites for fragility fractures. Osteoporosis weakens the internal structure of the bones, making them susceptible to breaking from a low-impact fall. A wrist fracture is often one of the first indicators of underlying osteoporosis, reflecting a systemic loss of bone strength.