The human hand can perform a wide range of movements and actions. It achieves this impressive dexterity through the interaction of muscles, joints, tendons, ligaments, nerve fibers, and bones. The 27 small, delicate bones in the human hand account for almost a quarter of the 206 bones in the entire body. Each hand has 14 joints, eight carpal bones, five metacarpal bones, and 14 phalanges that form the underlying structure that supports and works with other tissues.
Synovial joints facilitate movement of the hand and wrist in the same manner as larger joints such as the knee. A layer of articular cartilage covers the ends of long bones. The cartilage and synovial fluid in the joint cavities protect bones during articulation. Metacarpophalangeal or MCP joints include the knuckles. Interphalangeal or IP joints are present between bones that make up the fingers.
Eight carpal bones are held in place by ligaments positioned in two rows of four. They make physical contact with each other and with bones in the wrist, which contains multiple joints between the hand and forearm. Two carpal bones, the scaphoid and lunate, and the radius bone of the forearm, are part of a lower wrist joint. Two carpal bone rows form the other part of this lower joint.
The first row of carpals contains four bones. The scaphoid is shaped like a tiny boat. The lunate is a crescent, and the triquetrum a pyramid. The pisiform is a small, round bone on top of the triquetrum. The second row has four bones as well. The trapezium has four sides, the trapezoid is wedge-shaped, and the capitate is the largest carpal. The hamate is also wedge-shaped but has a bony process in the shape of a hook.
Five metacarpal bones extend from the wrist to form the palm. The bones are numbered from one to five, starting at the thumb and ending with the little finger. Metacarpals are all similar in size and shape. They each form a joint with the wrist on one end and a jointed finger on the other. The round, bulging heads of metacarpals form knuckles.
Phalanges are small long bones in the hands and feet, also numbered one to five. A single bone is a phalanx. Each phalanx's name relates to its position as proximal, middle, or distal. Proximal refers to a position closer to the torso while distal describes the opposite. Phalanges next to the palm are 'promixal' while those in fingertips are 'distal.' The middle is simply the position between proximal and distal bones. Each finger contains three phalanges Thumbs lack a middle phalanx.
The proximal phalanx is the first and largest bone in the finger. It forms a joint with a metacarpal bone and a middle phalanx. The middle phalanx is the second bone and forms a joint at each end with a proximal and distal phalanx. Proximal and middle phalanges are concave on the underside and convex on the upper surface. The middle of each phalanx is the narrowest section of the bone.
Distal phalanges in the fingers are small with a rough, slightly raised surface surrounded by flesh and tissue. The rounded tips of distal phalanges are called apical tufts -- flat and wide to support the pads of tissue and the fingernails. Distal phalanges only form one joint with middle phalanges.
The metacarpal of the thumb is the shortest and has the widest range of movement due to the saddle joint. It is the only metacarpal that can move independently of the others. The specialized saddle joint exists only between the wrist and thumb and enables oppositional movement. An opposable thumb can touch the tips of the fingers on the same hand.
Sesamoid bones form within tendons to provide a smooth surface for tendons to guide muscle movement. They are small, round bones and often develop naturally, but can also form as a response to a strain or injury. The hand contains several sesamoid bones -- the number varies between individuals. The tendons of the metacarpophalangeal joints of the thumb almost always include a pair of sesamoid bones. They can develop at almost every metacarpophalangeal and distal interphalangeal joint, but this is uncommon.
Injuries to bones and joints in the hand are common because so many fragile bones reside under a thin layer of skin and muscle. Hands are constantly in motion, performing the tasks of daily living. There are many more joints in the hand than any other part of the body, so they are also susceptible to arthritis. The most common injuries are sprains affecting the wrist, fractures of individual bones, and dislocation of phalanges.
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