138. Can dual-headed pedicle screws simplify use of dual rods and without sacrificing biomechanical stability?

BACKGROUND CONTEXT Long-segment fixation exposes instrumentation to substantial stresses and rod strain (RS), which result in high fracture rates. Supplemental rods provide greater stability, reduced posterior rod strain, and decreased fracture rates. Use of supplemental rods leads to reduced RS and decreased fracture risk; however the ideal method of supplemental rod attachment is unknown. Traditional methods of attachment include side connectors, which complicate intraoperative technique, and may increase duration of surgery and intraoperative contouring. Novel dual-tulip-headed screws have been developed to facilitate supplemental rod fixation for long segment constructs. PURPOSE The goal of this study was to evaluate a novel connector design and compare with traditional side-connectors such as a fixed angle connector (FAC) or variable angle connector (VAC). We hypothesized that dual-tulip-headed (DTH) screws would provide similar reduction in rod strain compared to fixed (FAC) or variable angled connectors (VAC). STUDY DESIGN/SETTING In vitro biomechanical study using human cadaveric specimens. PATIENT SAMPLE Seven fresh-frozen lumbar spine cadaveric specimens (L1–Ilium) were selected for this study (3F/4M, 48±7yrs, DEXA 0.9±0.1g/cm2). OUTCOME MEASURES Primary outcome measures of interest were range of motion (ROM) stability, posterior rod strain (RS) at L5/S1, and sacral screw bending moments (SM). METHODS Standard nondestructive flexibility tests (7.5 Nm) and compression tests (400 N) were performed (L1-ilium) to compare range of motion stability (ROM), posterior rod strain at L5-S1 (RS), and S1 screw bending moment (SM) between several supplemental 4-rod constructs (4R) vs standard 2-rod construction (2R). Directions of motion included: flexion (FL), extension (EX), left and right lateral bending (LLB, RLB), left and right axial rotation (LAR, RAR), and compression (C). Transforaminal lumbar interbody fusion (TLIF) was performed at L5/S1. Conditions included: (1) 2R, (2) dual tulip head with 4R (DTH), (3) fixed-angle connector with 4R (FAC), and (4) variable-angle connectors with 4R (VAC). Data were analyzed using RM-ANOVA. RESULTS Overall, there were no significant differences in ROM across the lumbosacral junction between conditions (p >.068). Compared to 2R, DTH and FAC significantly reduced RS in extension, LAR, and compression (p ≤.031). VAC significantly decreased RS compared to 2R in flexion, extension, LAR, RAR and compression (p ≤.033), and significantly decreased RS compared to DTH in extension (p=.024). DTH resulted in mildly increased SM compared to 2R in LAR and RAR (p ≤.003), and in LLB, RLB, LAR, RAR compared to FAC and VAC (p ≤.016). FAC and FAC resulted in decreased SM compared to 2R in RLB and LLB (p ≤.025). CONCLUSIONS Strain across the lumbosacral junction is high. Supplemental rod fixation with DTH is an effective strategy for reducing RS across the lumbosacral junction. However, the greatest reduction in RS and SM was achieved with variable connectors, likely due to their ability to allow for differential rod bending. FDA DEVICE/DRUG STATUS Unavailable from authors at time of publication.