The wrist links the hand to the arm. The wrist is a complex mechanical system of 8 small bones known as the carpal bones. The carpal bones are arranged in 2 interrelated rows. One row connects with the ends of the bones in the forearm—radius and ulna. If you hold your hand in the thumbs-up position, the bone on the top of your forearm is the radius; the one on the bottom is the ulna. The other row of carpal bones connects with the bones of the palm of the hand. There are synovial joints between the carpal bones and the wrist. The joint surfaces, where the bones meet, are covered with articular cartilage which helps movement.
Knuckle cracking does not serve any beneficial purpose and may be harmful to the fingers due to the stretching of the joint capsule. The hand has 19 bones: 5 elongated metacarpal bones, which are next to the wrist and make up the palm; 14 phalanges which make up the fingers. Each finger has 3 phalanges, the thumb has 2. These 19 bones collectively form 14 separate joints. The knuckle joints, metacarpophalangeal (MCP) joints, join the fingers to the palm. The interphalangeal (IP) joints are the finger joints. There are synovial joints between the metacarpals and phalanges—these bones are also covered with articular cartilage.
The adult skeleton is mainly made of bone and a little cartilage in places. Bone and cartilage are both connective tissues, with specialized cells called chondrocytes embedded in a gel-like matrix of collagen and elastin fibers. Cartilage can be hyaline, fibrocartilage and elastic and differ based on the proportions of collagen and elastin. Cartilage is a stiff but flexible tissue that is good with weight bearing which is why it is found in our joints. Cartilage has almost no blood vessels and is very bad at repairing itself. Bone is full of blood vessels and is very good at self repair. It is the high water content that makes cartilage flexible.
Hand Muscles and Hand Tendons
The muscles in the forearm and palm (thenar muscles) all work together to keep the wrist and hand moving, stable, and aligned. The image below shows the bones from the back side of the hand. The red lines show where the tendons attach the muscles to the bones.
The muscles that move the fingers and thumb are above the wrist in the forearm. Long flexor tendons extend from the forearm muscles through the wrist and attach to the small bones of the fingers and thumb. When you bend or straighten your finger, these flexor tendon slide through a snug tunnel, called the tendon sheath, that keeps the tendon in place next to the bones.
Tendons are white, flexible fibrous cords at the ends of muscles that attach the muscles to the radius, ulna, carpals, metacarpals and phalanges. When the muscles contract, they pull on the tendons to move the bone. The tendons that run down our fingers are held in place by a series of ligaments, called pulleys, that arch over the tendons forming a “tunnel-like” sheath. Normally, the tendons glide easily through the tunnel. Some tendons also serve as stabilizers.
A series of ligaments in a tunnel-like arrangement hold the tendons in place on the bones. A slippery coating, called tenosynovium, surrounds the tendons and keeps the tendons moving smoothly under the ligaments when the hand grasps objects.
Hand Ligaments are tough bands of fibrous tissue that join bones together. Six major ligaments give stability to the wrist by joining the radius to the carpal bones and binding the two rows of carpal bones together. These ligaments joint with others to link the wrist to the hand.
Other stabilizers in the hand include joint capsules, which are made of fibrous connective tissue that surrounds the joints. A synovial membrane inside the joint capsules provides synovial fluid to lubricate all the joints.
The median, radial and ulnar nerves are the three major nerves that run the length of the arm through the wrist and down into the hand. These nerves contract specific muscles and gives us sensations of touch, and to feel hot, cold, and pain.