General Session: Biomechanics - Hall F
Presented by: J. Godzik
J. Godzik(1), J. Lehrman(2), A. Newcomb(2), M. Ramkumar(2), A. Whiting(1), B. Kelly(2), L. Snyder(1)
(1) Barrow Neurological Institute, Phoenix, AZ, United States
(2) Barrow Neurological Institute, Spinal Biomechanics Laboratory, Phoenix, AZ, United States
Introduction: Transforaminal lumbar interbody fusion (TLIF) has emerged as a commonly used method for lumbar fusion, with many indications, including foraminal decompression, stabilization, and improvement of segmental lordosis. While many TLIF interbody spacer options exist, attaining surgical success may be contingent upon matching design strengths with surgical goals. In this cadaveric biomechanical study, we investigated the effects of a novel expandable interbody spacer and two traditional spacer designs with posterior augmentation in regards to stability, compressive stiffness, foraminal height, and segmental lordosis. To evaluate the biomechanical stability, compressive stiffness, foraminal height, and segmental lordosis achievable with a novel expandable lumbar interbody spacer and traditional fixed spacers.
Methods: Standard nondestructive flexibility tests (7.5 Nm) were performed on 7 cadaveric lumbar specimens (L3-S1) to assess intervertebral stability of three types of posterior interbody spacer constructs with posterior screw rod fixation (PSR) bilaterally at L4/L5. Stability was determined as mean range of motion (ROM) in flexion/extension (F/E), right and left lateral bending (LB), and right and left axial rotation (AR). Axial compressive loading (300 N) was performed to determine compressive stiffness. Additionally, foraminal height and segmental lordosis were evaluated using x-ray interpretation and image analysis following controlled posterior rod compression (170 N each side). Comparisons were made for 4 conditions: 1) intact; 2) 28 mm length expandable spacer (ES+PSR); 3) 26mm length fixed ovoid spacer (FOS+PSR); and, 4) 24 mm fixed rectangular spacer (FRS+PSR). Testing order was randomized between conditions 2-4. Data were analyzed using RM-ANOVA (P< 0.05).
Results: All constructs demonstrated greater stability at L4/L5 than intact (p< 0.001). There were no significant differences in ROM during F/E, AR, or LB between constructs. Similarly, the compressive stiffness was significantly greater than intact for all cases (p< 0.010), but there were no significant differences between constructs (p>0.75). ES+PSR demonstrated significantly increased mean foraminal height at L4-5 than FRS+PSR (p=0.026). ES+PSR demonstrated higher anterior disc height than FOS+PSR (14.9mm vs. 13.6mm, p=0.04), and higher posterior disc height than intact (p=0.002), FOS+PSR (p< 0.001), and FRS+PSR (p< 0.001). There were no significant differences in segmental lordosis between FOS+PSR (10.0±2.3°), ES+PSR (7.9±0.3°), and FRS+PSR (10.1±1.3°).
Conclusion: While the expandable interbody spacer provided similar segmental lordosis compared to traditional spacers of different shapes, it did so with the benefit of increased foraminal decompression and greater disc height compared to traditional spacers. If an expandable interbody spacer provides the same lordosis with a slightly longer length and thus a greater footprint, it may also contribute to less subsidence, although subsidence was not measured in this study. All these factors must be taken into account when choosing an interbody spacer during the TLIF approach, and thus the interbody spacer can be tailored to match surgical goals and indications.