Prediction of Mechanical Behaviors at Interfaces Between Bone and Two Interbody Cages of Lumbar Spine Segments

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Study Design. The effects of mechanical parameters at bone–implant interfaces of the lumbar spine segments were investigated under various combined loadings using the finite element method.Objective.To investigate the mechanical behaviors at bone–cage interfaces of lumbar spine segments with two interbody cages (two thread inserts).Summary of Background Data. It is known that among many factors, relative micromotion at bone–implant interfaces can hinder bone growth into the surface pores of an implant. Loading conditions, mechanical properties of the materials, friction coefficients at the interfaces, and geometry of spinal segments would affect relative micromotion and spinal stability. In particular, relative micromotion is related closely to friction at bone–implant interfaces after arthroplasty.Methods.A finite element model of human L3–L4 lumbar segments with two titanium interbody cages was constructed. This finite element model was used to investigate mechanical behavior at the bone–cage interface. Relative micromotion (slip distance on the contact surfaces), posterior axial displacement, and stress were predicted for changes of friction coefficients, loading conditions, and age-related material–geometric properties of the spinal segments.Results.Relative micromotion (slip distance) at the interfaces was obvious at their edges under axial compression. The slip occurred primarily at the anterior edges under torsion with preload, whereas it occurred primarily at the edges of the left cage under lateral bending with preload. Relative micromotion at the interfaces increased significantly as the apparent density of cancellous bone or the friction coefficient of the interfaces decreased. A significant increase in slip distance at the anterior anulus occurred with an addition of torsion to the compressive preload.Conclusions.Relative micromotion is sensitive to the friction coefficient of the interfaces, the bone density, and the loading conditions. A reduction in age-related bone density is less likely to allow bone growth into surface pores of the cage. However, it is likely that the larger the disc area or pedicle diameter, the more stable the interbody fusion of the spinal segments.

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