Central placement of screw fixation in scaphoid fracture—a computational approach and biomechanical study Yang Guo MD, FHKCOS Associate Consultant Department of Hand Surgery, Ji Shui Tan hospital, beijing, China

OPTIMAL position of fixation for scaphoid fractures continues to be debated  Central placement : supported by(Tremble,Mccallister,2000) biomechanical studies /clinical evaluations Has been accepted as a prefered method of fixation  Recently, some studies :(Luria 2010,2012) biomechanical studies /finite element analysis suggested no difference in stability between central and eccentric placement The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits.

To be central or Not to be? Our study consisted of 2 parts  Establishment “central” concept computational approach  Biomechanical test central Vs peripheral

1 Establishing “Central”

Shrinkage in a uniform manner in software-- pre-op planning system Scaphoid surface model was extracted. One pixel space was allowed by one time of shrinkage in the program Scaphoid model: smaller and smaller

Scahpoid distal pole Scahpoid proximal pole Waist portion Central one third Further shrinkage of 1.5mm Central zone Central one-third zone and central zone

Central zone-geometry Screw axis

Part 2: Biomechanical Study Design ： Types of fixation Fracture PlaneLocation of Screw Group 1Proximal Proximal Central Zone Group 2ProximalDistal Central Zone(eccentric) Group 3DistalDistal Central Zone Group 4DistalProximal Central Zone(eccentric)

Biomechanical study ： osteotomy design

Group 1Group 2 Group 3 Group 4 Screw axis thread Fracture plane

Materials and Test machine

 Estimation of sample size and power 10 of scaphoids ， consistent with a previous data  Data analysis: Wilcoxon nonparametric analysis Failure load of the specimens (P <0.05).  The mode of failure :Screw migration fracture at the screw–bone interface, sliding along the fracture.  None of the specimens dislodged from the potting fixture  None of screw was bent

Stroke-Force curve

Results Load to failure (N) Dispalcement of fracture （ mm ） Significance Group ± ± Group ± ±0.5 Group ± ± Group ± ±0.6

Brief Disccussion  1. Concept of Central need to be clarified Straight central axis does not exist  2. Central placement : important as long as possible

Limitations  Same with any cadaveric biomechanical study  The screw itself may contributed resistance,when it was placed in a more horizontal direction  Saw bone material: Failure load lower than previous tests  We simplified the forces into a single vector and did not perform cyclic loading  Fx plane differed slightly, which affects its stability.

Thanks!