Multipoint pelvic fixation with multirod constructs is increasingly used for long-segment deformity constructs to reduce rates of distal failure. However, more robust distal fixation may negatively impact proximal junction biomechanics, and this potential relationship has not been extensively studied. Standard nondestructive flexibility tests (7.5 Nm) were performed on 7 cadaveric specimens (L1-pelvis) to assess intervertebral flexibility (range of motion ROM), rod strain, and screw bending moments along a posterior fusion construct (pedicle screw and rod PSR) spanning L2-S1, supplemented by bilateral primary S2 alar-iliac (S2AI) fixation (2 S2AI screws and 2 rods), followed by additional S2AI screw placement and bilateral accessory rod placement spanning L4-S2AI (4 S2AI screws and 4 rods). Four conditions were tested for each specimen: 1) intact; 2) L2-S1 PSR; 3) L2-S2AI PSR; and 4) L2-S2AI plus L4-S2AI. Data were analyzed using repeated-measures ANOVA. Seven cadaveric specimens were included. Proximal rod strain at the L2-3 level did not change across the varying test conditions in the 7 specimens tested (p > 0.05 for all conditions). There was no significant difference detected in proximal screw strain across conditions (p > 0.05). Finally, no significant difference was found in L2-3 ROM (p > 0.05) across instrumented variations, all of which were more rigid than intact specimens. Pelvic fixation with 2 or 4 screws and 2 or 4 rods, respectively, did not significantly alter proximal junction screw or rod strain in a cadaveric model. Robust pelvic fixation might protect against distal failure without deleterious effects on the proximal junction.
Rudy et al. (Fri,) studied this question.