title: Range of motion, sacral screw and rod strain in long posterior spinal constructs: a biomechanical comparison between S2 alar iliac screws with traditional fixation strategies.
abstract: BACKGROUND
  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 shown to provide similar stiffness to iliac screws when added to L3-S1 constructs. We sought to examine and compare the biomechanical effects on an L2-S1 pedicle screw construct of adding S2AI screws, AxiaLIF, L5-S1 interbody support via transforaminal lumbar interbody fusion (TLIF), and to examine the effect of the addition of cross connectors to each of these constructs.
  
  
  METHODS
  Two S1 screws and one rod with strain gauges (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: (I) Pedicle screws (PS) L2-S1; (II) PS + S2AI screws; (III) PS + TLIF L5/S1; (IV) PS + AxiaLIF L5/S1; (V) PS + S2AI + AxiaLIF 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.
  
  
  RESULTS
  S2A1 produces reduced S1 screw strain for flexion-extension, lateral bending and axial torsion, as well as reduced rod strain in lateral bending and axial torsion in comparison to AxiaLIF and interbody instrumentation, at the expense of increased rod flexion-extension strain. Cross-connectors may have a role in further reduction of S1 screw and rod strain.
  
  
  CONCLUSIONS
  From a biomechanical standpoint, the use of the S2AI technique is at least equivalent to traditional iliac screws, but offers lower prominence and ease of assembly compared to conventional sacroiliac stabilization.
authors.affiliation: Department of Neurosurgery, University of Florida, Gainesville, Florida, USA;; Spinal Health International, Longboat Key, Florida, USA.
authors.forename: Chester E
authors.initials: CE
authors.lastname: Sutterlin
authors.affiliation: Auckland City Hospital and Starship Children's Hospital, Grafton, Auckland 1023, New Zealand.
authors.forename: Antony
authors.initials: A
authors.lastname: Field
authors.affiliation: Orthokinetic Testing Technologies, Southport, NC, USA.
authors.forename: Lisa A
authors.initials: LA
authors.lastname: Ferrara
authors.affiliation: Excelen Center for Bone and Joint Research and Education, Minneapolis, USA.
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Freeman
authors.affiliation: NeuroSpine Research Group (NSURG), Prince of Wales Private Hospital, Sydney, Australia;; University of New South Wales (UNSW), Sydney, Australia.
authors.forename: Kevin
authors.initials: K
authors.lastname: Phan
keywords: Biomechanics; S2AI; alar iliac screws; fixation; sacral fusion; sacral screw; sacroiliac fusion; spine surgery