[Central placement of screw fixation for scaphoid fracture: a biomechanical study]

To evaluate the effect to the fixation stability of central screw placement during scaphoid fracture surgery.

We designed oblique osteotomies for 32 identical sawbone scaphoids and fixed each specimen with a cannulated screw. Sawbone scaphoids were divided into 4 groups, according to position of the osteotomy (distal waist portion or proximal waist portion) and the position of the screw (central or eccentric). We performed Computed Tomography scanning to one specimen, and then the central zone of the scaphoid was established from volume data by using the preoperative planning system software (VxWork 4.0). The position of the osteotomy plane, the entrance and exit points of the screw guide pin were designed on the software as well. We placed the specimens under the increasing load of a pneumatically driven plunger to compare the load to failure and the distance at failure between the central and eccentric screw groups.

In general, we found the statistical differences of the load to failure and the displacement of fracture between the groups (F=31.485,P=0.001; F=33.328,P=0.018). The average load to failure and fracture displacement was more statistically different in the central group [(80.82 ± 15.63) N, (2.3 ± 0.5) mm] for proximal waist fracture than in the eccentric group [(58.32 ± 17.18) N, (3.1 ± 0.5) mm]. As to the distal waist fracture, the average load to failure and fracture displacement was better in the central group [(76.83 ± 14.54) N, (2.2 ± 0.7) mm] than in the eccentric group [(70.38 ± 13.32) N, (2.5 ± 0.6) mm] without significant difference.

In this biomechanical model of an unstable oblique scaphoid fracture, we find that higher stability of fixation has been achieved with a screw placed centrally in the scaphoid, compared with a screw peripherally placed.