The carpal bones are located between the forearm and metacarpal bones. They provide an articulated connection and contribute to the stability and mobility of the hand. There are eight carpal bones in each hand.
What are carpal bones?
The carpal bones (ossa carpi or ossa carpalia) form the connection between the forearm bones and the metacarpal bones (ossa metacarpalia). The carpal bone in humans consists of eight bones arranged in two rows. The proximal (close to the body) row is composed, starting on the thumb side, of the scaphoid bone (Os scaphoideum), the lunate bone (Os lunatum), the triangular bone (Os triquetum) and the pea bone (Os pisiforme). The distal (far from the body) carpal bones, also starting at the thumb, are the greater polygonal bone (Os trapezium), the lesser polygonal bone (Os trapezoideum), the capitate bone (Os capitatum), and the hooked bone (Os hamatum). In order to better remember the names and order of the bones, there is the following mnemonic: “There was a barge in the moonlight, triangular around the pea leg. Polygonal big, polygonal small, the head, it must be by the hook.” The carpal bones belong to the so-called short bones (ossa brevia). They are compact and small and have a cylinder or cube shape.
Anatomy and structure
The eight carpal bones have no cavity inside them, unlike the long bones. Their shape is mostly square or rectangular, they have no bone shaft, and the bone substance is spongy. Externally, they are surrounded by a connective tissue skin (periosteum) and cartilage. The individual bones are stably connected to each other with ligaments, which does not allow too much mobility. Seen from the side, the carpal bones curve in a slight arc towards the back of the hand. This creates the carpal tunnel on the palm side, a kind of groove in which the ulnar and radial nerves, as well as tendons and vessels, run. The scaphoid is the second largest bone in the carpus. It is proximally connected to the radius, as is the crescent moon bone. The triangular bone gets its name from its triangular shape. It is in articulated connection with the pea bone, which is the smallest bone of the carpus. It is one of the sesamoid bones and is attached to a muscular tendon. The large polygonal bone of the distal row forms the thumb joint with the first metacarpal bone. The small polygonal bone is located between the scaphoid and the second metacarpal. The largest bone of the carpus is the capitate bone. The hooked bone gets its name from a small bony hook (hamulus) that points toward the palm and is palpable through the skin.
Function and tasks
The carpal bones form a functional unit of several partial joints proximally with the end of the radius and distally with the metacarpals. The distal row of carpal bones, i.e., the large and small polygonal bones, the capitate and the hook bones, together with the metacarpals, constitute the distal wrist (articulatio mediocarpalis). It is also called the toothed hinge joint (articulatio ginglymus). The articulated connections between the carpal bones are called intercarpal joints (articulationes intercarpales). They are so-called wiggle joints held together by tight ligaments, which causes the rather low mobility. These intercarpal joints ensure that the individual bones can move against each other, allowing movement, albeit slight, in the distal wrist. The proximal wrist (articulatio radiocarpalis), on the other hand, is far more mobile. It is an egg yolk joint formed by the end of the radius and the carpal bones scaphoid, lunate and triangular. Another part of the joint is an intermediate articular disc (discus articularis) consisting of cartilage and connective tissue, which is the connection between the carpal bones and the ulna-socket joint. The movements that can be performed by the proximal wrist are extension in the direction of the back of the hand (dorsal extension) up to 70 degrees and flexion in the direction of the palm up to 80 degrees. In addition, the joint makes abduction movements both in the direction of the thumb (radial abduction) up to 20 degrees and toward the little finger (ulnar abduction) up to 40 degrees.
Diseases and complaints
One of the most common disorders of the wrist is carpal tunnel syndrome.It affects the median nerve, which runs from the arm through the carpal tunnel formed by the carpal bones into the hand. The condition often has no identifiable cause, but it can also result from overuse, from a rheumatic disease or metabolic disorders. Injury to the carpal bones can also cause carpal tunnel syndrome. The symptoms are pain in the wrist, numbness and tingling in the hand. Most often, these symptoms occur at night. Another condition, but much less common, is Loge de Guyon syndrome. Here, the ulnar nerve and the ulnar artery are affected. They both run between the pea bone and the hook bone, from the arm towards the hand, and can be squeezed at this narrow point. This causes paralysis and sensory disturbances. The thumb can no longer be brought to the hand, making grasping and writing difficult, and the little finger feels numb. Injuries can also occur to the carpus. Typically, fractures or torn ligaments occur during a fall when the hand is reflexively extended to soften the impact. This often results in a fracture of the scaphoid or a torn ligament. If the fragments of the bone are not displaced, stabilizing the hand with a cast is sufficient, but surgery is required if the bone is chipped. A torn ligament also requires surgery.
