P18. Characterization of sacropelvic fixation using novel porous fusion/fixation implants in combination with thoracolumbar instrumentation

Abstract

<h3>BACKGROUND CONTEXT</h3> Sacropelvic fixation is often used at the distal end of long constructs in adult deformity surgery in order to provide biomechanical support to the base of the constructs, with the aim of reducing complications such as pseudarthrosis of the L5-S1 joint and hardware failure or loosening. <h3>PURPOSE</h3> The objective of this study was to investigate configurations in which a porous fusion/fixation (PFF) implant was employed to enhance sacropelvic fixation. <h3>STUDY DESIGN/SETTING</h3> Finite element study. <h3>OUTCOME MEASURES</h3> The outputs analyzed in this comparison were: (a) range of motion (ROM) of L5-S1 and sacroiliac joint (SIJ) with respect to PED; (b) the maximal von Mises stresses in S1 pedicle screws; (c) the maximal von Mises stresses in either the S2 alar-iliac screws or in PFF implants in a S2 alar-iliac trajectory; (d) the maximal stresses in the posterior rods between the pedicle screws in L5 and S1 (ie, lumbosacral junction). <h3>METHODS</h3> Three instrumented T10-pelvis models were created from an intact validated model: (1) pedicle screws and rods in T10-S1, and PFF implants inserted bilaterally in a S2 alar-iliac trajectory (PFFI); (2) pedicle screws and rods in T10-S1, PFF implants inserted bilaterally in a S2 alar-iliac trajectory, and triangular implants inserted bilaterally above the PFFI in a sacro-alar-iliac trajectory (PFFI-IFSAI); and (3) pedicle screws and rods in T10-S1, PFF implants inserted bilaterally in a S2 alar-iliac trajectory, and PFF implants in a SAI trajectory stacked cephalad to those in the S2 alar-iliac position (2-PFFI). These configurations were compared against two baseline models: (1) pedicle screws and rods in T10-S1 (PED); and (2) pedicle screws and rods in T10-S1, and S2 alar-iliac screws (S2AI). All the instrumented models were used to simulate pure moments of 7.5 Nm to the upper endplate of the T10 vertebra in flexion, extension, left and right lateral bending, and left and right axial rotation. Double leg stance was simulated by fixing both acetabula of the models. <h3>RESULTS</h3> Sacropelvic fixation demonstrated a greater decrease in ROM of both L5-S1 and SIJ with respect PED. S2AI and PFFI had similar results in terms of L5-S1 and SIJ motion. PFFI-IFSAI did not affect the ROM between L5 and S1 but did reduce that of the SIJ. 2-PFFI further decreased L5-S1 motion and demonstrated a behavior similar to PFFI – IFSAI in terms of SIJ motion. S2AI fixation showed a slight increase of the stresses on S1 pedicle screws and on implants in the S2AI position with respect to PFFI. PFFI-IFSAI slightly decreased the stresses in S1 pedicle screws and on implants in the S2AI position with respect to PFFI. 2-PFFI significantly decreased the stresses on these implants (28-62 MPa for S1 pedicle screws and up to 66% for implants in the S2AI position) with respect to other configurations. In the posterior rods, the von Mises stresses were found to be highest when PFF implants were used with respect to S2AI, though all stresses were below those found in S1 pedicle screws in PED. <h3>CONCLUSIONS</h3> This study confirmed that sacropelvic fixation is able to reduce ROM at L5S1 and lower instrumentation stresses suggesting a reduced risk of pseudarthrosis at the joint and screw breakage, respectively. Moreover, outcomes suggest that PFF implants increase stability of the L5-S1 and SIJ, and reduce the risk of screw breakage. Clinical evaluation should be performed to confirm the applicability of results to patient outcomes. <h3>FDA DEVICE/DRUG STATUS</h3> iFuse Implant System (SI-BONE, Santa Clara, CA) (Approved for this indication), iFuse Bedrock Granite Implant System (SI-BONE, Santa Clara, CA) (Investigational/Not Approved).