Biomechanical Advantages of Novel Duet Screws Plus Bilateral Satellite Rods Fixation in the Correction Surgery for Adult Spinal Deformity.

Abstract

Purpose: Adult spinal deformity (ASD) patients undergoing pedicle subtraction osteotomy (PSO) with long fusion to the pelvis have a high risk of rod failure at the PSO level or adjacent areas. This study aimed to investigate the biomechanical advantages of novel duet screws (multiaxial screws with dual heads) plus satellite rods in ASD correction using finite element models and cadaveric specimens.

Methods: A lumbar-pelvis finite element model was constructed, and Von Mises equivalent stress was used to analyze the structural stress under different fixation constructs. Six human cadaveric spine segments (T11-S2) were selected to validate the finite element results. L3 PSO was performed, with posterior fixation from L1 to pelvis using S2-alar-iliac (S2AI) screws, and satellite rods (L2-L4) were connected to the primary rods using duet screws. Three fixation constructs were evaluated: two rods (Group A), three rods (Group B), and four rods (Group C). In Group B, a biomechanical test was performed on two sides: single rod side (Group B-S) and double rods side (Group B-D). In vitro motion tests were performed under pure moments in lateral bending (LB), flexion/extension (FE), and axial rotation (AR) to measure the range of motion. Rod strain was measured at L3 and S2.

Results: Finite element analysis revealed stress concentration near the PSO site, the screw bases, and the L5-S1 region. The use of duet screws with satellite rods significantly reduced primary rod stress. Cadaveric tests showed that all fixation constructs significantly reduced motion in LB, FE, and AR compared to the intact condition (p < 0.001). No significant differences in total motion or motion at the upper lumbar segments (L1-L3) were observed among the three groups, but differences were found in the lower lumbar segments (L4 and L5) and the sacrum (p < 0.007). Increasing the number of satellite rods significantly decreased rod strain (p < 0.004). Group B-D showed significantly lower rod strain compared to Group B-S (p < 0.042), indicating that satellite rods combined with duet screws were highly effective in reducing primary rod strain at L3. Satellite rods also reduced primary rod strain at the sacrum.

Conclusions: This study supports clinical practice, providing biomechanical evidence for using four-rod constructs with duet screw-based satellite rods in L3 PSO and long fusion. Satellite rods dispersed rod strain, potentially reducing pseudarthrosis and rod breakage at PSO and sacrum without compromising spinal motion.

Trial registration: Registered in ClinicalTrials (NCT06144879).