Scaphoid anatomy and mechanics: Update and review

This review has been an attempt to collate previous research on scaphoid anatomy, load mechanics and kinematics into one article so that the reader can assimilate the information in a convenient way. The scaphoid is a unique carpal bone in shape and function. It has a three-dimensionally oblique orientation and performs a unique function as a mechanical link between the distal and proximal carpal rows on the radial aspect. The load mechanics of the scaphoid play an important role in wrist function and kinematics. Studies of the normal biomechanics of the proximal wrist joint have determined that the scaphoid and lunate bones have separate, distinct areas of contact on the distal radius surface. Kinematics of the scaphoid can be viewed either from proximal or distal. From a proximal view, during radial/ulnar deviation of the wrist, the scaphoid exhibits both a flexion/extension and radial/ulnar deviation motion. Distally, kinematic studies revealed that the scaphoid-trapezoid motion during wrist extension is the same as in wrist radial deviation and the scaphoid-trapezoid motion in wrist flexion is the same as in wrist ulnar deviation. Knowledge of the anatomy of the dorsal carpal ligaments has also provided a wealth of information on their role in carpal motion. The lateral V construct of the DRC (Dorsal Radiocarpal), DIC (Dorsal Intercarpal), and dorsal component of the SLIO (Scapho-lunate interosseous) ligament delivers indirect dorsal stability, yet allows relative free motion of the scaphoid. Injuries that involve the scaphoid can be ligamentous or can involve fracture of the scaphoid with or without ligamentous injury. Because of its complex anatomy, treating scaphoid fractures and nonunions can be challenging and difficult to detect true displacement patterns from plain X-ray films due to the overlapping of the other carpal bones. Degenerative arthritis of the STT (scapho-trapezium-trapezoid) joint is the second most common site of degenerative changes in the wrist. Plain radiographs can be used to measure TT (trapezium-trapezoid) inclination, which has been shown to be correlated with degenerative changes in the STT joint. Research in our lab has demonstrated that wrists with a higher TT inclination had a higher incidence of degenerative changes. This new perspective taking into account anatomy, mechanics and kinematics begins to offer a more global view of how form and function are related and what part the scaphoid plays in carpal function and stability.