Jakub Godzik, Bernardo de Andrada Pereira, Anna G. U. Sawa, Jennifer N. Lehrman, Randall J. Hlubek, Brian P. Kelly, and Jay D. Turner
The objective of this study was to evaluate a novel connector design and compare it with traditional side connectors, such as a fixed-angle connector (FAC) and a variable-angle connector (VAC), with respect to lumbosacral stability and instrumentation strain.
Standard nondestructive flexibility tests (7.5 Nm) and compression tests (400 N) were performed using 7 human cadaveric specimens (L1–ilium) to compare range of motion (ROM) stability, posterior rod strain (RS), and sacral screw bending moment (SM). Directions of motion included flexion, extension, left and right lateral bending, left and right axial rotation, and compression. Conditions included 1) the standard 2-rod construct (2R); 2) the dual-tulip head (DTH) with 4-rod construct (4R); 3) FACs with 4R; and 4) VACs with 4R. Data were analyzed using repeated-measures ANOVA.
Overall, there were no statistically significant differences in ROM across the lumbosacral junction among conditions (p > 0.07). Compared with 2R, DTH and FAC significantly reduced RS in extension, left axial rotation, and compression (p ≤ 0.03). VAC significantly decreased RS compared with 2R in flexion, extension, left axial rotation, right axial rotation, and compression (p ≤ 0.03), and significantly decreased RS compared with DTH in extension (p = 0.02). DTH was associated with increased SM in left and right axial rotation compared with 2R (p ≤ 0.003) and in left and right lateral bending and left and right axial rotation compared with FAC and VAC (p ≤ 0.02). FAC and VAC were associated with decreased SM compared with 2R in right and left lateral bending (p ≤ 0.03).
RS across the lumbosacral junction can be 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 a VAC that allowed for straight (uncontoured) accessory rod placement.
Laura A. Snyder, Jennifer N. Lehrman, Ram Kumar Menon, Jakub Godzik, Anna G. U. S. Newcomb, and Brian P. Kelly
Minimally invasive transforaminal interbody fusion techniques vary among surgeons. One decision point is whether to perform a unilateral facetectomy (UF), a unilateral facetectomy plus partial contralateral facetectomy (UF/PF), or a complete bilateral facetectomy (CBF). The authors therefore compared the biomechanical benefits of all 3 types of facetectomies to determine which approach produces improved biomechanical outcomes.
Seven human cadaveric specimens (L3–S1) were potted and prepped for UF, with full facet removal, hemilaminectomy, discectomy, and pedicle screw placement. After distraction, a fixed interbody spacer was placed, and compression was performed. A final fixation configuration was performed by locking the rods across the screws posteriorly with bilateral compression. Final lordosis angle and change and foraminal height were measured, and standard nondestructive flexibility tests were performed to assess intervertebral range of motion (ROM) and compressive stiffness. The same procedure was followed for UF/PF and CBF in all 7 specimens.
All 3 conditions demonstrated similar ROM and compressive stiffness. No statistically significant differences occurred with distraction, but CBF demonstrated significantly greater change than UF in mean foraminal height after bilateral posterior compression (1.90 ± 0.62 vs 1.00 ± 0.45 mm, respectively, p = 0.04). With compression, the CBF demonstrated significantly greater mean ROM than the UF (2.82° ± 0.83° vs 2.170° ± 1.10°, p = 0.007). The final lordosis angle was greatest with CBF (3.74° ± 0.70°) and lowest with UF (2.68° ± 1.28°). This finding was statistically significant across all 3 conditions (p ≤ 0.04).
Although UF/PF and CBF may require slightly more time and effort and incur more risk than UF, the potential improvement in sagittal balance may be worthwhile for select patients.
Jakub Godzik, Jennifer N. Lehrman, Anna G. U. S. Newcomb, Ram Kumar Menon, Alexander C. Whiting, Brian P. Kelly, and Laura A. Snyder
Transforaminal lumbar interbody fusion (TLIF) is commonly used for lumbar fusion, such as for foraminal decompression, stabilization, and improving segmental lordosis. Although many options exist, surgical success is contingent on matching design strengths with surgical goals. The goal in the present study was to investigate the effects of an expandable interbody spacer and 2 traditional static spacer designs in terms of stability, compressive stiffness, foraminal height, and segmental lordosis.
Standard nondestructive flexibility tests (7.5 N⋅m) were performed on 8 cadaveric lumbar specimens (L3–S1) to assess intervertebral stability of 3 types of TLIF spacers at L4–5 with bilateral posterior screw-rod (PSR) fixation. Stability was determined as range of motion (ROM) in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Compressive stiffness was determined with axial compressive loading (300 N). Foraminal height, disc height, and segmental lordosis were evaluated using radiographic analysis after controlled PSR compression (170 N). Four conditions were tested in random order: 1) intact, 2) expandable interbody cage with PSR fixation (EC+PSR), 3) static ovoid cage with PSR fixation (SOC+PSR), and 4) static rectangular cage with PSR fixation (SRC+PSR).
All constructs demonstrated greater stability than the intact condition (p < 0.001). No significant differences existed among constructs in ROM (FE, AR, and LB) or compressive stiffness (p ≥ 0.66). The EC+PSR demonstrated significantly greater foraminal height at L4–5 than SRC+PSR (21.1 ± 2.6 mm vs 18.6 ± 1.7 mm, p = 0.009). EC+PSR demonstrated higher anterior disc height than SOC+PSR (14.9 ± 1.9 mm vs 13.6 ± 2.2 mm, p = 0.04) and higher posterior disc height than the intact condition (9.4 ± 1.5 mm vs 7.1 ± 1.0 mm, p = 0.002), SOC+PSR (6.5 ± 1.8 mm, p < 0.001), and SRC+PSR (7.2 ± 1.2 mm, p < 0.001). There were no significant differences in segmental lordosis among SOC+PSR (10.1° ± 2.2°), EC+PSR (8.1° ± 0.5°), and SRC+PSR (11.1° ± 3.0°) (p ≥ 0.06).
An expandable interbody spacer provided stability, stiffness, and segmental lordosis comparable to those of traditional nonexpandable spacers of different shapes, with increased foraminal height and greater disc height. These results may help inform decisions about which interbody implants will best achieve surgical goals.