S2AI Screws and iFuse (Casaroli - Eur Spine J 2019)
Evaluation of Iliac Screw, S2 Alar-iliac Screw and Laterally Placed Triangular Titanium Implants for Sacropelvic Fixation in Combination with Posterior Lumbar Instrumentation: A Finite Element Study
Purpose: This study aimed to implement laterally placed triangular titanium implants as a technique of sacropelvic fixation
in long posterior lumbar instrumentation and to characterize the effects of iliac screws, S2 alar-iliac screws and of triangular
implants on rod and S1 pedicle screw stresses.
Methods: Four female models of the lumbopelvic spine were created. For each of them, five finite element models replicating
the following configurations were generated: intact, posterior fixation with pedicle screws to S1 (PED), with PED and
iliac screws (IL), with PED and S2 alar-iliac (S2AI) screws, and with PED and bilateral triangular titanium implants (SI).
Simulations were conducted in compression, flexion–extension, lateral bending and axial rotation. Rod stresses in the L5-S1
segment as well as in the S1 pedicle screws were compared.
Results: One anatomical model was not simulated due to dysmorphia of the sacroiliac joints. PED resulted in the highest
implant stresses. Values up to 337 MPa in lateral bending were noted, which were more than double than the other configurations.
When compared with IL, S2AI and SI resulted in lower stresses in both screws and rods (on average 33% and 41%
for S2AI and 17% and 50% for SI).
Conclusions: Implant stresses after S2AI and SI fixations were lower than those attributable to IL. Therefore, pedicle screws
and rods may have a lower risk of mechanical failure when coupled with sacropelvic fixation via S2AI or triangular titanium
implants, although the risk of clinical loosening remains an area of further investigation.