Central placement of screw fixation in scaphoid fracture—a computational approach and biomechanical study

Yang Guo MD, FHKCOSAssociate ConsultantDepartment of Hand Surgery, Ji Shui Tan hospital, beijing, China

OPTIMAL position of fixation for scaphoid fractures continues to be debatedCentral placement : supported

by(Tremble,Mccallister,2000)biomechanical studies /clinical evaluations Has been accepted as a prefered method of fixationRecently, some studies :(Luria 2010,2012) biomechanical studies /finite element analysissuggested 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 systemScaphoid surface model was extracted.One pixel space was allowed by one time of shrinkage in the programScaphoid 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 Plane Location of Screw

Group 1 Proximal Proximal Central Zone

Group 2 Proximal Distal Central Zone(eccentric)

Group 3 Distal Distal Central Zone

Group 4 Distal Proximal Central Zone(eccentric)

Biomechanical study ： osteotomy design

Group 1 Group 2

Group 3Group 4

Screw axisthread

Fracture plane

Materials and Test machine

Estimation of sample size and power10 of scaphoids ， consistent with a previous dataData 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 fixtureNone of screw was bent

Stroke-Force curve

ResultsLoad to failure (N)

Dispalcement of fracture （ mm） Significance

Group 1 80.82±15.63 2.3±0.5 0.001

Group 2 58.32±17.18 3.1±0.5

Group 3 76.83±14.54 2.2±0.7 0.166

Group 4 73.38±13.32 2.5±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

LimitationsSame with any cadaveric biomechanical studyThe 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!