Impact of multipoint pelvic fixation and multirod distal constructs on proximal junction biomechanics in cadaveric specimens.

Objective: 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.

Methods: 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.

Results: 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.

Conclusions: 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.