The carpus is the anatomical term for the wrist which connects the radius and ulna of the forearm with the metacarpal bones of the hand. It is comprised of eight individual carpal bones that are seated in two neat rows of four. The proximal row of carpal bones, as seen in a lateral to medial direction on the palmar surface, include:
- the scaphoid bone
- the lunate bone
- the triquetral bone
- the pisiform bone
The distal row, as seen from the same viewpoint as above, includes:
- the trapezium bone
- the trapezoid bone
- the capitate bone
- the hamate bone
Each carpal bone has its own unique shape and is multifaceted, giving it the ability to articulate with several bones, both in the same row and in the opposing row as well as with the bones of the hand and the antebrachial region. As a whole, the carpus on its carpal surface is convex proximally and concave distally, whereas on its palmar surface it is simply concave.
The carpal tunnel is formed on the inside of the wrist due to this concavity and is covered by the flexor retinaculum, which is an osteofibrous sheath.
At birth, the carpus is cartilaginous, however during the first year and up, until twelve years of age, it undergoes a slow ossification process. It should be noted that many anatomical variants and accessory bones have been discovered, however only a central bone, a styloid bone, a secondary pisiform bone or a secondary trapezoid bone are generally considered to be so. This article will now discuss the individual carpal bones in detail, as well as highlight the potential pathological conditions that can affect the wrist.
The Proximal Row of Carpal Bones
The scaphoid bone is the largest carpal bones of the proximal row and it lies beneath the anatomical snuff box. From a palmar view, it is surrounded on the proximal side by the radius, on the distolateral side by the trapezium bone and on the distomedial side by the trapezoid bone. Superomedially, it articulates with the lunate bone and inferomedially with the capitate bone. On the palm of the hand, its tubercle is easily palpable as it sits subcutaneously.
The blood vessels that supply the carpal bones enter the wrist along the lateral roughened surface of the scaphoid bone.
The next three proximal carpal bones, namely the lunate bone, the triquetral bone and the pisiform bone all articulare with the head of the radius, which is known as the carpal articular surface.
The lunate bone is a crescent shaped structure that has a large proximal articular surface which relates to the radial bone and its articular disk. It sits medially to the scaphoid bone, superior to the capitate bone and lateral to the triquetral bone. It may also sometimes come into contact with the hamate bone at its inferomedial angle.
The triquetral bone is shaped like a pyramid and, from a palmar perspective, its apex points disto-medially towards the pisiform bone, which is positioned upon its palmar facet. It sits upon the hamate bone, which is in the distal row of carpal bones and its base faces medially and communicates with the lunate bone.
Lastly, the pisiform bone is the most medial proximal carpal bone from a palmar stance. It is also the smallest of all the carpal bones and classified as a sesamoid bone. Its dorsal surface is faceted, allowing it to articulate with the ventral surface of the triquetral bone. It is palpable and lies within the tendon of the flexor carpi ulnaris.
The Distal Row of Carpal Bones
The trapezium is the first and most lateral of the distal row of carpal bones when the hand is viewed from its palmar surface. On the palmar aspect of the bone, there is a palpable tubercle and on its medial side runs a groove that holds the tendon of the flexor carpi radialis. The trapezium bone is bordered medially by the trapezoid bone and superiorly by the scaphoid bone. Inferolaterally, its main articulation is with the first metacarpal bone via a saddle shaped facet. Inferomedially however, it sometimes also articulates with the second metacarpal bone.
The trapezoid bone may look very small in comparison to the other bones from a palmar aspect, however it is much wider on its dorsal side. It communicates via its proximal facet with the scaphoid bone, laterally with the trapezium bone, medially with the capitate bone and its distal facet allows it to articulate with the second metacarpal bone.
The capitate bone is the largest of all the carpal bones, both proximally and distally. It is surrounded by the lunate bone proximally, the third metacarpal bone distally, the trapezoid bone laterally and the hamate bone medially.
The last of the eight carpal bones and the distal row is the hamate bone. It is subcutaneously palpable, due to the hamulus, which is a bony notch on its palmar aspect and curves laterally. It exists in aid of the flexor digiti minimi brevis and the pisohamate ligament.
The hamate bone is surrounded from a proximolateral direction by the lunate bone and a proximomedial direction by the triquetral bone. Laterally, it communicates with the capitate bone and, distally, it articulates with both the fourth and fifth metacarpal bones.
Scaphoid Bone Fracture
The most common fracture that ails one of the carpal bones is that of the scaphoid bone. It is most likely to sustain an injury during a fall where the person lands on their wrist when it is outstretched and trying to prevent the incident. When the wrist is extended, the centre of the bone fractures and clinical symptoms include pain and swelling in the region known as the anatomical snuffbox.
Complications occur due to a lack of blood supply to the area from the palmar carpal branch of the radial artery. This inadequacy of blood and accompanied nutrient deprivation can lead to avascular osteonecrosis.
Carpal Tunnel Syndrome
Carpal tunnel syndrome is the infamous term for a condition known as median nerve compression and is the most common form of compression related neuropathy. It is primarily caused by occupational tendencies, such as repeated wrist flexion and extension, leading to overuse of the anatomical structures that directly work during these movements. As a result, an increase in content mass and pressure within the carpal tunnel push down upon the median nerve leading to sensory disturbances within its areas of innervation.
Chronic compression can lead to lasting nerve damage and thenar atrophy which translates as weakness in the thumb and index fingers. Treatment includes resting the wrist and refraining from using it for a period of time. Bandages and splints may help stabilize the area while anti-inflammatory drugs and painkillers may be prescribed in extreme cases.