It is one of the bones of the wrist carpus.
The scaphoid lies between the proximal and distal rows of the carpal bones. It is located on the radial side of the wrist and articulates with the radius, lunate, trapezius and capitation. More than 80% of the bone is covered with articular cartilage.
The scaphoid is one of eight small bones that make up the “carpal bones” of the wrist. Connect two rows of these bones: the proximal row (closer to the forearm) and the distal row (closer to the hand). It forms the radial edge of the carpal tunnel.
The scaphoid bone is the largest bone in the proximal row of bones of the wrist, its longitudinal axis is from top to bottom, sideways and forward. It is about the size and shape of a medium cashew nut.
The palmar surface of the scaphoid is concave and forms a tubercle that gives rise to the transverse carpal ligament. The proximal surface is triangular, smooth and convex, and articulates with the radius and adjacent carpal bones, such as the lunate, the large bone, the trapezius and the trapezium.
The lateral surface is narrow and gives fixation to the radial collateral ligament. The medial surface has two facets, a flattened semilunar facet that articulates with the lunate bone, and a lower concave facet, which articulates along the lunate with the head of the large bone.
The dorsal surface of the bone is narrow, with a groove that extends along the bone and allows the union of the ligaments, and the surface facing the fingers (anatomically inferior) is smooth and convex, also triangular, and divided into two parts by a slight ridge.
It receives its blood supply mainly from the lateral and distal branches of the radial artery, through the palmar and dorsal branches.
These provide an “abundant” supply to the middle and distal bone, but neglect the proximal portion, which is based on retrograde flow. The dorsal branch supplies most of the middle and distal portions, with the palmar branch supplying only the distal third of the bone.
The supply of dorsal blood, particularly of the proximal part, is very variable. Sometimes the abductor pollicis brevis fibers emerge from the tubercle.
The scaphoid can be felt at the base of the anatomical snuff box. Palmar wrist can also be felt in the hand.
Its position is the intersection of the long axes of the four fingers in a fist, or the base of the thenar eminence. When palpated in this position, you will feel that the bone slides forward during radial deviation (wrist abduction) and flexion.
The carpal bones function as a unit to provide a bony superstructure for the hand. The scaphoid is also involved in the movement of the wrist. This, together with the lunate, is articulated with the radius and the ulna to form the main bones involved in the movement of the wrist.
The scaphoid serves as a link between the two rows of carpal bones. With the movement of the wrist, the scaphoid can flex from its position in the same plane as the forearm to perpendicular.
Scaphoid fractures are the most common lesions of the carpal bone, due to its connections to the two rows of carpal bones. A fracture of the scaphoid bone is usually caused by a fall on the outstretched hand.
The scaphoid may be slow to heal due to limited circulation to the bone.
Scaphoid fractures should be recognized and treated quickly, since immediate treatment by immobilization or surgical fixation increases the likelihood that the bone will heal in anatomical alignment, thus preventing defective union or lack of attachment.
Delays can compromise healing. If the fracture does not heal, it will lead to post-traumatic osteoarthritis of the carpus. One reason for this is due to the “tenuous” blood supply to the proximal segment.
Even rapidly immobilized fractures may require surgical treatment, including the use of a headless compression screw, such as the Herbert screw, to join the two halves together.
Healing the fracture with a non-anatomical deformity (often a “hunchback” in flexion to fly) can also lead to post-traumatic arthritis. The lack of union can cause the loss of blood supply to the proximal pole, which can cause avascular necrosis of the proximal segment.
Other scaphoid bone diseases:
A condition called scapholunate instability can occur when the scapholunate ligament (which connects the scaphoid to the lunate bone) and other surrounding ligaments rupture. In this state, the distance between the scaphoid and lunate bones increases.
This rare disease, also called avascular necrosis (idiopathic) of the scaphoid, is a rare condition in which ischemia and necrosis of the scaphoid bone occurs without a previous fracture.
It is thought to be caused by repetitive microtrauma or medication side effects (eg, steroids or chemotherapy) along with a defective vascular supply existing at the proximal pole of the scaphoid. MRI along with CT and X-rays are the methods of choice for diagnosis.