Optimal fixation of acute scaphoid fractures: finite element analysis.

PURPOSE: The hypothesis of this study was that more stable fixation of acute scaphoid fractures may be achieved by a screw placed perpendicular to the fracture plane than along the long axis of the scaphoid, as previously suggested. We examined this assumption on different fracture patterns using a finite element analysis model.

METHODS

A computed tomography scan of an intact scaphoid of a young man provided the data set for all fracture models. We used semiautomatic segmentation to create 3-dimensional computer models of the 3 simple fracture configurations: oblique, transverse waist, and proximal fractures, according to the Herbert classification. Each fracture type was analyzed, using finite elements, for its biomechanical response to 2 types of virtual fixation: a screw placed either perpendicular to the fracture plane or centrally along the long axis of the scaphoid. We measured motion at the fracture plane (in millimeters) and strain in the screw threads (in millipascals).

RESULTS

Considerably less motion was measured at the fracture plane with the perpendicular screw compared with the long axis screw, especially in the oblique-type fractures: (1) Herbert-type B1 oblique fracture mean motion of 0.05 mm (+/-0.03) for the perpendicular screw versus 0.28 mm (+/-0.05) for the long axis screw; (2) B2 transverse waist fracture mean motion of 0.06 mm (+/-0.03) for the perpendicular screw versus 0.18 mm (+/-0.06) for the long axis screw; and (3) B3 proximal fracture mean motion of 0.07 mm (+/-0.01) for the perpendicular screw versus 0.28 mm (+/-0.011) for the long axis screw. Higher strains were measured on the screw placed perpendicular to the fracture.

CONCLUSIONS

According to this model, higher fixation stability is achieved when the scaphoid is fixated perpendicular to the fracture. In transverse waist fractures, a centrally placed screw will also be perpendicular to the fracture, which explains the results of previous models.