The scaphoid bone is one of the carpal bones of the wrist on the lateral side of the wrist. Carpal bones are arranged in two rows and scaphoid is most lateral of the proximal row.
The carpus is made up of 8 carpal bones, which are arranged in two rows.
Proximal row
It contains (from lateral to medial side)
- Scaphoid
- Lunate
- Triquetral
- Pisiform
Distal row
From lateral to medial side
- Trapezium
- Trapezoid
- Capitate
- Hamate
The scaphoid bone is the largest bone of the proximal row of wrist bones, its long axis being from above downward, lateral, and forward.
It is approximately the size and shape of a medium-sized cashew.
It is also called os scaphoideum and is the largest of the proximal row of carpal bones.
Skaphe means “a boat,” and eidos means “form” in Greek. The name is derived by combining two words signifying its shape.
It is important for stability and movement at the wrist.
The bone has a tubercle on its lateral side directed laterally, laterally, forwards and downwards.
Surfaces & Articulations
The superior surface is convex, smooth, of triangular shape, and articulates with the lower end of the radius.
The inferior surface, directed downward, lateralward, and backward, is also smooth, convex, and triangular, and is divided by a slight ridge into two parts, the lateral articulating with the trapezium, the medial with the trapezoid.
On the dorsal surface is a narrow, rough groove, which runs the entire length of the bone, and serves for the attachment of ligaments.
The volar surface is concave above and has a tubercle, which is directed forward and gives attachment to the transverse carpal ligament and sometimes origin to a few fibers of the abductor pollicis brevis.
The lateral surface is rough and narrow and gives attachment to the radial collateral ligament of the wrist.
The medial surface presents two articular facets; of these, the superior articulates with the lunate bone the inferior for the head of the capitate bone.
The distal convex surface articulates with trapezium and trapezoid.
The scaphoid articulates with five bones: the radius, trapezoid, trapezium, lunate and capitate.
- proximal surface: radius
- distal surface: laterally with the trapezoid and trapezium; medially with the capitate
- ulnar surface: lunate
Proximally, the smooth convex surface of the scaphoid articulates with the distal radius.
The distal surface is split into two separate articular surfaces by a bony ridge. Radially, it articulates with the trapezoid and trapezium while on the ulnar side, it articulates with the capitate.
The medial surface has a concave appearance and articulates with the lunate.
Attachments
There are no musculotendinous attachments to the scaphoid bone. Following ligaments attach with the scaphoid bone
Ligaments
- Dorsal surface: dorsal radiocarpal ligament
- Radial surface: radial collateral ligament
- Scapholunate ligament
- Radioscapholunate ligament
- Scaphocapitate ligament
Relations
The radial artery crosses the dorsal surface.
The scaphoid forms the radial portion of the carpal tunnel and is therefore related to the structures that pass through it, namely fibers from flexor digitorum profundus and superficialis, the median nerve, flexor pollicis longus and flexor carpi radialis which lie media to it.
Muscles of the thumb and associated tendons are also close.
Variations in Anatomy
- Bipartite scaphoid
- A coalition with neighboring bones
- Isolated scaphoid hypoplasia
Ossification
The scaphoid has a single ossification center, as do the trapezoid and trapezium. Ossification begins around the 4th year, and as with most ossification in the hand and wrist, it tends to occur earlier in females.
Blood Supply
Scaphoid receives the majority of its blood supply via dorsal vessels at or just distal to waist area passing through perforations on the dorsal surface.
Approximately 75% of the arterial supply is from branches of the radial artery.
Rest of the blood supply is from palmar & superficial palmar branches of radial artery & enter carpal scaphoid in the region of its distal tubercle perfusing 20-30 % of bone, including tuberosity.
No anastomoses have been found between the dorsal and palmar vessels. The vascular supply to the proximal pole is mainly retrograde, a fracture through the tubercle or the waist places the proximal pole at risk of avascular necrosis.
Surface Anatomy and 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 at the intersections of the long axes of the four fingers while in a fist, or the base of the thenar eminence.
Clinical significance of Scaphoid
Fractures
Scaphoid fractures are the most common fractures of carpal bones. They occur due to falling on an outstretched hand. These fractures must be recognized and treated quickly.
Delays may complicate healing leading to increased the likelihood of osteoarthritis and avascular necrosis of the proximal segment.
Scapholunate Instability
Scapholunate instability can occur when the scapholunate ligament is disrupted.
