General Session: Biomechanics
Presented by: L. Ferrara - View Audio/Video Presentation (Members Only)
C. Sutterlin(1), A. Field(2,3), A. Freeman(4), L. Ferrara(5)
(1) LifeHealthCare, Sydney, NSW, Australia
(2) Auckland City Hospital, Auckland, New Zealand
(3) Starship Children's Hospital, Auckland, New Zealand
(4) Fortus Medical, Minneapolis, MN, United States
(5) OrthoKinetic Technologies, Southport, NC, United States
Summary: The biomechanical performance of long posterior fusion constructs was assessed comparing L5/S1 support with axial interbody fusion (AIF), TLIF and S2 alar-iliac (S2AI) screws, +/- crosslinks, and measuring L5/S1 ROM and rod strain and S1 screw strain. S2AI constructs were stiffer than other constructs and provided greater offloading of S1 screws, especially in lateral bending and axial torsion. Crosslinks improved performance in axial torsion.
Hypothesis: S2AI screws improve stiffness across L5/S1 and offload S1 screws in long posterior constructs, compared with pedicle screws alone and with TLIF or AIF. Design: Cadaveric biomechanical analysis.
Introduction: S1 screw failure and L5/S1 non-union are issues with long fusions to S1. Improved construct stiffness and S1 screw offloading can help avoid this. S2AI screws have been shown to provide similar stiffness to iliac screws when added to L3- S1 constructs. Comparison with L5/S1 interbody support, and analysis of S2AI effect on rod and S1 screw strain, have not previously been performed.
Methods: Two S1 screws and one rod with strain gages (at L5/S1) were used in L2-S1 screw-rod constructs in 7 L1-pelvis specimens (two with low BMD). ROM, S1 screw and rod strain were assessed using a pure-moment flexibility testing protocol. Specimens were tested intact, and then in five instrumentation states consisting of 1) Pedicle Screws alone from L2-S1, 2) Pedicle screws L2-S1 and S2AI screws, 3) Pedicle screws L2-S1 and TLIF L5/S1, 4) Pedicle screws L2-S1 and AIF L5/S1, 5) Pedicle screws L2-S1 and S2AI screws and AIF L5/S1. The five instrumentation conditions were also tested with crosslinks at L2/3 and S1/2. Tests were conducted in flexion-extension, lateral bending and axial torsion with no compressive preload. Motion was tracked using a non-contact camera-based motion measurement system with simultaneous acquisition of strain data.
Results: Range of motion: Flexion/extension: all constructs were stiffer than PS; those constructs with S2AI, TLIF and AIF were equivalent; combined S2AI/AIF was stiffest. Lateral bending: S2AI constructs were stiffer than all others. Axial torsion: crosslinks had the greatest effect on stiffness, also some effect from S2AI screws. Strain: S1 offloading was greatest with S2AI and AIF in flexion/extension. Lateral bending: S2AI constructs were only ones that offloaded S1 screws. Axial torsion: greatest offloading with crosslink and with S2AI. Rod strain in flexion increased at L5/S1 with S2AI screws compared to other constructs.
Conclusion: S2AI screws provide improved stiffness and S1 screw offloading in all planes compared with constructs without S2AI screws. Adding crosslinks increases axial torsion stiffness. Addition of S2AI screws to a long posterior construct increases stiffness and offloads S1 screws compared with constructs without S2AI screws +/- interbody support.