Scaphoid bone

The scaphoid bone is one of the carpal bones of the wrist. It is situated between the hand and forearm on the thumb side of the wrist (also called the lateral or radial side). It forms the radial border of the carpal tunnel. The scaphoid bone is the largest bone of the proximal row of wrist bones, its long axis being from above downward, lateralward, and forward. It is approximately the size and shape of a medium cashew nut.

Structure

The scaphoid is situated between the proximal and distal rows of carpal bones. It is located on the radial side of the wrist,<ref name="BEASLEY2003" />Template:Rp adjacent to the styloid process of the radius.<ref name=":02">Template:Cite book</ref> It articulates with the radius, lunate, trapezoid, trapezium, and capitate.<ref name=BEASLEY2003>Template:Cite book</ref>Template:Rp Over 80% of the bone is covered in articular cartilage.<ref name=EATHORNE2005 />

Bone

The palmar surface of the scaphoid is concave, and forming a distal tubercle, giving attachment to the transverse carpal ligament. The proximal surface is triangular, smooth and convex.<ref name=EATHORNE2005>Template:Cite journal</ref> The lateral surface is narrow and gives attachment to the radial collateral ligament. The medial surface has two facets, a flattened semi-lunar facet articulating with the lunate bone, and an inferior concave facet, articulating alongside the lunate with the head of the capitate bone.<ref name="Gray-6b1-4" />

The dorsal surface of the bone is narrow, with a groove running the length of the bone and allowing ligaments to attach, and the surface facing the fingers (anatomically inferior) is smooth and convex, also triangular, and divided into two parts by a slight ridge.<ref name="Gray-6b1-4" />

Blood supply

It receives its blood supply primarily from lateral and distal branches of the radial artery, via palmar and dorsal branches. These provide an "abundant" supply to middle and distal portions of the bone, but neglect the proximal portion, which relies on retrograde flow.<ref name=BEASLEY2003 />Template:Rp The dorsal branch supplies the majority of the middle and distal portions, with the palmar branch supplying only the distal third of the bone.<ref name=EATHORNE2005 />

Variation

The dorsal blood supply, particularly of the proximal portion, is highly variable.<ref name=BEASLEY2003 />Template:Rp Sometimes the fibers of the abductor pollicis brevis emerge from the tubercle.<ref name="Gray-6b1-4" />

In animals

In reptiles, birds, and amphibians, the scaphoid is instead commonly referred to as the radiale because of its articulation with the radius.

Function

The carpal bones function as a unit to provide a bony superstructure for the hand.<ref name=GRAYS2005>Template:Cite book</ref>Template:Rp The scaphoid is also involved in movement of the wrist.<ref name=BEASLEY2003 />Template:Rp It, along with the lunate bone, articulates with the radius and ulna to form the major bones involved in movement of the wrist.<ref name=GRAYS2005 /> The scaphoid serves as a link between the two rows of carpal bones. With wrist movement, the scaphoid may flex from its position in the same plane as the forearm to perpendicular.<ref name=BEASLEY2003 />Template:Rp

Clinical significance

Fracture

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Fractures of the scaphoid are the most common of the carpal bone injuries, because of its connections with the two rows of carpal bones.<ref name=BEASLEY2003 />Template:Rp

The scaphoid can be slow to heal because of the limited circulation to the bone. Fractures of the scaphoid must be recognized and treated quickly, as prompt treatment by immobilization or surgical fixation increases the likelihood of the bone healing in anatomic alignment, thus avoiding mal-union or non-union.<ref name=":0">Template:Cite journal</ref> Delays may compromise healing. Failure of the fracture to heal ("non-union") will lead to post-traumatic osteoarthritis of the carpus.<ref name=BEASLEY2003 />Template:Rp One reason for this is because of the "tenuous" blood supply to the proximal segment.<ref name=EATHORNE2005 /> Even rapidly immobilized fractures may require surgical treatment, including use of a headless compression screw such as the Herbert screw to bind the two halves together.

Healing of the fracture with a non-anatomic deformity (frequently, a volar flexed "humpback") can also lead to post-traumatic arthritis. Non-unions can result in loss of blood supply to the proximal pole, which can result in avascular necrosis of the proximal segment.

Scaphoid fractures may be difficult to diagnose via plain x-ray. A repeat x-ray may be required at a later date, as might cross-sectional imaging via MRI or CT scan.<ref name=":0" />

Other diseases

A condition called scapholunate instability can occur when the scapholunate ligament (connecting the scaphoid to the lunate bone) and other surrounding ligaments are disrupted. In this state, the distance between the scaphoid and lunate bones is increased.<ref name=BEASLEY2003 />Template:Rp

One rare disease of the scaphoid is called Preiser's Disease.

Palpation

The scaphoid can be palpated at the base of the anatomical snuff box. It can also be palpated in the volar (palmar) hand/wrist. Its position is the intersections of the long axes of the four fingers while in a fist, or the base of the thenar eminence. When palpated in this position, the bone will be felt to slide forward during radial deviation (wrist abduction) and flexion.

Clicking of the scaphoid or no anterior translation can indicate scapholunate instability.

Etymology

The word scaphoid (Template:Langx) is derived from the Greek skaphos, which means "a boat", and the Greek eidos, which means "kind".<ref>Template:Cite book</ref> The name refers to the shape of the bone, supposedly reminiscent of a boat. In older literature about human anatomy,<ref name="Gray-6b1-4">Template:Cite book</ref> the scaphoid is referred to as the navicular bone of the hand (this time from the Latin navis for boat); there is also a bone in a similar position in the foot, which is called the navicular. The modern term for the bone in the hand is scaphoid; in human anatomy the term navicular is reserved for the bone in the foot.

Additional images

• Scaphoid bone (left hand) - animation01.gif

  Scaphoid bone of the left hand (shown in red). Animation.

• Scaphoid bone (left hand) - animation02.gif

  Scaphoid bone of the left hand. Close up. Animation.

• Scaphoid bone1.jpg

  Scaphoid bone.

• Gray219 - Scaphoid bone.png

  Scaphoid shown in yellow. Left hand. Palmar surface.

• Gray220 - Scaphoid bone.png

  Scaphoid shown in yellow. Left hand. Dorsal surface.

• Ospoignet - Scaphoid bone.png

  Cross section of wrist (thumb on left). Scaphoid (labelled as "Navicular") shown in red.

• Dissection of the human hand - 01 - scaphoid bone.png

  Wrist joint. Deep dissection. Posterior view.

• Slide1dsds - Scaphoid bone.png

  Scaphoid forms the radial (thumb-side) border of the carpal tunnel. Wrist joint. Deep dissection. Anterior (palmar) view.