Carpal bones
The carpus (lat. Ossa carpi) is formed by eight short bones and represents the connection between the forearm bones and the metacarpals. The carpal bones appear in two rows of four small bones each. The rows are called distal and proximal carpal row. These bones form the bony foundation of the carpal.
Anatomy
The carpus is formed by eight short bones. These appear in two rows of four bones each. In contrast to tubular bones such as phalanges, the short bones have no hollow space and no bone shaft. The mobility of the bones to each other is strongly limited, because they are connected by ligaments.
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Characteristic for the arrangement of the carpal bones is a curvature in the direction of the back of the hand which can be seen from the side. This is how the carpal tunnel is formed on the palm of the hand.
Proximal carpal row
Os scaphoideum
The Os scaphoideum is also called scaphoid bone. It is a six-sided bone located on the radial side of the carpus. The scaphoid bone is the second largest bone in the carpal. The bone forms the connection to the distal end of the radius. The distal end of the scaphoid is connected to the Os trapezium, the Os trapezoideum and the Os capitatum. The os lunatum, on the other hand, is connected to the os scaphoideum in the direction of the ulna.
Os lunatum
The os lunatum also refers to the lunatum bone. The name comes from the crescent-shaped structure of the short bone. It creates a connection to the distal surface of the radius. Furthermore, both the os hamatum and the os capitatum are located distally. The os scaphoideum is radially connected to the lunar bone and the os triquetrum to ulnar.
Os triquetrum
The os triquetrum or triquetral bone has a pyramid-like shape. The base of this pyramid is lateral, the tip is medially oriented. The bone is positioned ulnar-sided in the carpus. The triquetral bone is followed by the sesamoid bone, os pisiforme, after ulnar. It is radially bounded by the os lunatum.
Os pisiforme
The os pisiforme or pisiform bone is the smallest of the carpal bones. It has a roundish shape and is located laterally towards the ulnar. The pea bone is completely embedded in the tendon of the Musculus flexor carpi ulnaris. Due to this characteristic, the os pisiforme belongs to the sesamoid bones. Dorsal of the pea leg is the os triquentrium.
Distal carpal row
Os trapezium
The os trapezium is also called trapezium bone. The short bone has an angular shape and is located on the radial side of the carpal. The os trapezoidum is adjacent to the great polygonal bone in the direction of the ulnar. The metacarpal bone I is located on the distal side.
Os trapezoideum
The Os trapezoidum is also called trapezoid bone and is located approximately in the middle of the distal row of the carpus. Radial to this bone is the os trapezium and ulnarly of it the os capitatum. In the proximal direction the distal surface of the scaphoid is connected. The metacarpal bone II articulates distally with the small polygon bone.
Os capitatum
The enlish name of the os capitatum is capitate bone. It is a clearly rounded short bone. It is connected proximally to the distal surface of the scaphoid and the lunate bone. The os metacarpale III connects distally to the capitate bone.
Os hamatum
Because of the hook-shaped bone structure on the palmar side of the os hamatum, the bone is also called the hamate bone. The hamate bone is located on the lateral side of the wrist. It is radially limited by the os capiatatum. The proximal side of the os hamatum is connected to the distal side of the os triquenter. The metacarpal IV and V are located on the distal surface of the hamatum.
Function
Together with the radius and phalanges, the carpal bones form a functional unit. This includes different partial joints, which are formed by different bones. This makes different movements possible.
Movement
The articulated connections between the carpal bones allow different hand movements. Thus, a dorsal extension can be performed through the proximal wrist. This is an extension of up to 70 degrees in the direction of the back of the hand. Bending up to a maximum of 80 degrees in the direction of the palm is also made possible by the joints formed by the metacarpals. The radial abduction, spreading movement in the direction of the thumb is possible in a frame of up to 20 degrees. The opposite movement, ulnar abduction (spreading in the direction of the little finger) is possible up to 40 degrees.
Development
After birth, the core of the bone lying in the cartilage develop. This ossification takes place with a time delay in the various carpal bones and is not completed in the larger bones until around the age of six. The sesamoid bone, Os pisiforme, forms even much later. This ossification actually only takes place between the eighth and twelfth year of life.
