In order to effectively communicate the inner workings of the hand, it is important to understand some of the commonly used references. When referring to an aspect of the anatomy that is located in closer proximity to the body, the term proximal is used. And distal refers to that which is more distant from the body’s center. Something that is close to the surface is said to be superficial. Palmer refers to that which is located on the palm side of the hand, while dorsal is at the back of the hand. And the joining or moving of bones against one another – facilitated at the joints by the presence of articular cartilage – is the manner in which they articulate.
About the Hand
The hand is one of the most fascinating features of the human body – and the one that most distinctly separates homo sapiens from other primates within the animal kingdom.
The unique ability humans have to rotate the small (fifth metacarpal) and ring fingers (fourth metacarpal) across the palm to meet the thumb (first metacarpal) is the result of a special flexibility of the carpometacarpal joints located in the middle of the palm.
Approximately a quarter of the motor cortex in the human brain (the part controlling movement in the body) is devoted to the hand muscles. Three main nerves beginning at the shoulder and traveling to the hand and fingers carry the signals from the brain to the muscles to prompt movement throughout. These nerves also transmit back to the brain sensations resulting from touch.
The hand has approximately 50 nerves, 34 muscles that move the fingers and thumb, over 120 known ligaments, 30 major joints, and nearly 30 bones. The thumb alone is controlled by nine distinct muscles, which require three major hand nerves to function, and can attain a range of movement in a variety of directions – easily rotated 90 degrees compared to the approximate 45 degrees attained by the other fingers.
The Anatomy of the Hand
The hand consists of a palm, or metacarpus, and five digits attached to the forearm by a joint called the wrist, or carpus. There are five bones in the palm – one for each of the five digits (fingers and thumb). The bones of the fingers are called phalanges – each one is a phalanx. Of the 14 phalanx bones in the hand, two are in the thumb (distal and middle phalanx) and three are in each of the fingers (distal, middle and proximal phalanx).
Carpal bones located in the wrist connect to the metacarpals in the palm of the hand – five in all. One metacarpal connects to each finger and the thumb. The primary knuckle joints are the result of the connection between the phalanges to the metacarpals and are called metacarpophalangeal joints (MCP joints). These joints facilitate the bending and straightening of the fingers and thumb. The three phalanges in each finger are separated by two interphalangeal (IP) joints – the proximal (PIP) and the distal interphalangeal (DIP) joints, which also facilitate this type of movement. The end of each joint is covered with articular cartilage, which serves as a shock absorber as the bones move against one another, or articulate.
The collateral ligaments are located on both sides of each finger and thumb joint, providing joint stabilization. The strongest of these is the volar plate, which joins the proximal phalanx with the middle phalanx on the palmar side of the joint. The extensor tendons allow the finger joints to straighten.
The muscles that control finger function are located in the middle of the palm and extend to the mid forearm or elbow. These muscles are connected to the finger bones by the tendons – serving as a type of pulley in moving the fingers. The movements of the hand are achieved by extrinsic muscle groups – the long flexors and extensors, and intrinsic muscle groups – the thenar and hypothenar (the thumb and little finger).
The primary nerves affecting hand, finger and thumb movement are the radial, median and ulnar nerves. The radial nerve runs along the outer thumb-side edge of the forearm and stops at the back of the hand, resting on the end of the radius bone. The median nerve runs through the carpal tunnel within the wrist and the ulnar nerve runs through the Guyon’s canal, which is a tunnel between two of the carpal bones (pisiform and hamate) and the connecting ligament. These nerves are accompanied by large blood vessels – the largest of which is the radial artery, which runs across the front of the wrist near the thumb and is used to take a patient’s pulse. The ulnar artery is found along the ulnar nerve through the Guyon’s canal. These two arteries supply blood to the front of the hand, while other smaller arteries supply blood to the back of the hand, fingers and thumb.
The Thumb
Worthy of its own paragraph, the thumb’s unique ability to rotate with far greater range than the fingers is a result of the surface shapes of the two bones that comprise it, the phalanges. These are connected to the longer first metacarpal bone in the hand. Utilizing the muscles arranged around the first metacarpal, the joint at the base of the thumb is able to rotate and oppose the fingers – enabling such action as grasping and pinching. Also present and essential in thumb function is the ulnar collateral ligament, which is a band of fibrous tissue connecting the bones at the base of the thumb – preventing it from pointing excessively away from the hand.
Vulnerabilities
The hand is vulnerable to a variety of fractures and sports-related sprains because of its role in most daily and athletic activities. For menopausal woman and those having sustained trauma from serious accidents, arthritis of the hand is also common – particularly at the base of the thumb.
The constant movement of the wrist and fingers in daily life also places a strain on the nerves running through the tunnels, prompting a number of nerve-related.