Scaphoid Bone – Anatomy, Biomechanics and Mechanism of Fracture

Fractures of the scaphoid make about 50% to 80% of carpal injuries and occur in young, active individuals. The position of the scaphoid on the radial side of the wrist, as the proximal extension of the thumb ray, makes it vulnerable to injury.

The peak incidence of scaphoid fractures occur in the second and third decades of life.

Anatomy of The scaphoid

Biomechanics of Scaphoid Bone

Intrinsic ligaments  that attach scaphoid  the lunate and distally to the trapezium and trapezoid stabilize the scaphoid.

These ligaments restrict the motion permitting a degree of rotation proximally and a degree of gliding distally .

The scaphoid acts as a link across the midcarpal joint, connecting the proximal and distal carpal rows.

Any shear strain that occurs across the midcarpal joint is transferred through the scaphoid, and may cause fractures and dislocations.

The scaphoid also serves to coordinate and smooth the motions of the proximal and distal rows.

An unstable scaphoid fracture results in  a dissociation of the proximal and distal carpal rows and tendency to collapse.

Mechanism of Fractures

Most of the patients have this fracture following fall on outstretched hand (FOOSH).  These fractures are common in young men  following falls, athletic injuries or motor vehicle accidents.

There is a bending and compression forces on dorsal aspect whereas on palmar surface there are tensile forces.

This results in forced dorsiflexion of the wrist. On geting dorsiflexed >95 degrees, the proximal pole of the scaphoid is tightly held between the capitate, the dorsal lip of the radius, and the taut palmar capsule resulting in fracture  at the waist. Several other mechanisms are responsible for other types of fractures of scaphoid. For example fractures of the tubercle can  be caused by either compression or avulsion.

Proximal pole fractures can be caused by an avulsion of the scapholunate ligament.

Scaphoid fractures in children and elderly  are uncommon. In children, the force causes distal radial physis to fail first and in elderly  the elderly, the distal radial metaphysis usually fractures before the scaphoid.