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INTRODUCTION: Shear lag is a well-known phenomenon in fiber reinforced structures such as carbon fiber golf clubs and reinforced concrete. It governs the interplay between axial and shear stress around fracture points and provides ongoing load transfer after initial compromise of a structure. Uniaxial tests to failure of No.2 FiberWire surgical suture were reported previously and were found to have a fail/reload/fail pattern where load was transferred between suture core and jacket via the shear lag mechanism [1]. However, traditional tensile testing alone cannot fully describe the mechanics of the driving shear lag phenomenon. Finite element modeling is useful for illustrating the governing mechanics of load transfer, but has not previously been applied to multifilament suture such as FiberWire.

PURPOSE: To apply the finite element method to describe the shear lag phenomenon of suture failure by examining the distribution of axial and shear stresses along a suture of partially failed No.2 FiberWire. To describe the clinical implications of the shear lag phenomenon.

MATERIAL & METHODS: Two 3D finite element models of FiberWire suture were created consisting of separate core and jacket. One model included a broken core to investigate its implications. The filaments were assumed to be homogeneous and isotropic elastic.

RESULTS: At the broken core site, the model shows that the core filament locally stops carrying the axial load and sheds it to the surrounding jacket via shear lag, Fig.1. Also, the jacket surrounding the broken core builds load via shear, and transfers that load across the broken site as axial stress. Subsequently, the load transfers back to the core again via shear. The load transfer occurs over an identifiable characteristic length. The models show detailed stress and strain within the suture: the shear stress in core and jacket varies significantly over their cross-section and is highest close to the broken region. The jacket filaments are able to carry load despite complete failure of the core filament, however, the load transfer mechanism is not preserved over subsequent load cycles.

CONCLUSIONS: The consistency of the experimental and numerical results validates of finite element model. The model describes a biomechanical phenomenon by which failed suture can appear to be competent during a surgical procedure. Hence, the clinician should be aware of the possible failure mode and consider remedial actions when such failures are suspected. The described method may also provide insights in describing load transfer to soft tissue, and thus provide opportunity for optimization of suture/tissue interfaces.

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