Biomechanical Comparison of Lumbar, Sacral and Iliac Screw Strain with the Novel Eiffel Tower Configuration Versus Conventional Pelvic Fixation in Adult Spinal Deformity Surgery

A common problem in long spinal fusions is caudal end fixation failure. To address sacral screw loosening, fixation may be extended to the pelvis using iliac screws. Iliac screws provide strong anchor strength, but loosening is common due to high strain. A novel ‘Eiffel Tower’ (E-T) construct is introduced to reduce iliac screw strain. A saw-bone spine model was instrumented with pedicle screws and rods from T9 to S1 as native fixation. L5 and S1 screw strain was measured with application of 25Nm loading in flexion, extension, lateral bending and rotation. Strain was measured for L5, S1 and iliac screws after fixation was extended to the pelvis. The conventional model used a bridge connecting a screw in the medial iliac crest to the main rod superior to L5. The E-T fixation used two rods connected to the main rods at L1 and to two screws in the superolateral iliac crest. S1 strain is reduced by 65% by conventional fixation and 77% by E-T fixation. S1 screw strain reduction was greater with E-T fixation compared to conventional fixation in all loading directions. Iliac screw strain was decreased with E-T fixation compared to conventional fixation in all loading directions. Conventional fixation increased L5 screw strain compared to native fixation, but E-T fixation did not. Both conventional and E-T fixation reduce S1 screw strain, but E-T fixation did so to a greater extent. Terminal fixation at the iliac screw suffered a higher load with conventional fixation relative to E-T fixation. (Journal of Surgical Orthopaedic Advances 35(2):072–076, 2026)

Key words: proximal junction failure, dynamic stabilization system, screw pull out