18. Do differences in segmental angle of single level posterior lumbar fixation affect adjacent level biomechanics: cadaveric study of range of motion and optical disc strain

Abstract

BACKGROUND CONTEXT Single-level lumbar fusion had been performed for years with little concern with final segmental angle. However, subtle changes on a segmental lordosis may mechanically impact adjacent level motion and stress triggering a domino effect of adult spinal deformity with sagittal imbalance and failed back fusion. The objective of this study was to quantify adjacent segment motion and disc surface strain changes with altered segmental angles. PURPOSE Study adjacent-level motion and strain distribution in cadaveric specimens with a lumbar spine single segment fusion in an offset angle. STUDY DESIGN/SETTING Biomechanical cadaveric study. PATIENT SAMPLE Seven human cadaveric specimens. OUTCOME MEASURES Range of motion (ROM) and optic strain. METHODS Seven human specimens (L2-Sacrum) underwent L4-L5 pedicle screws and rods fixation and were tested in neutral angle (NEU), imposed +5° lordosis (LOR) and -5° kyphosis (KYP). Pure moments (7.5 Nm) were applied in flexion; extension; lateral bending (LB); axial rotation (AR) followed by 400 N of compression (C) alone, and combined with pure moments. Range of motion (ROM) and strain using digital image correlation (DIC) system were tracked. Principle maximum (E1) and minimum (E2) strains were analyzed within four quarters on the lateral disc surface antero-posteriorly (Q1; Q2; Q3 and Q4). Data were analyzed using one-way RM ANOVA. RESULTS At the upper adjacent level, significant increase in ROM was observed in both conditions KYP and LOR compared to NEU in flexion (p=0.001; p=<0.001) and extension (p=0.02; p=0.009). Increased ROM was also observed in LOR compared to NEU (p=0.026) and compared to KYP (p=0.004) during compression. KYP had increased ROM compared to NEU and LOR (p=0.031; p=0.025) in C+EX. LOR had increased E1 in Q3 compared to NEU in RLB (p=0.041); LOR and KYP had decreased E1 in Q3 compared to NEU in C (p=0.002; p=0.03). LOR had decreased E1 in Q3 compared to NEU (p=0.013) while KYP had increased E1 in all quartiles and increased E2 in Q2 compared to LOR in C+FL (p≤0.047). KYP decreased E1 in Q3 (p=0.021) and E2 in Q1 (p=0.006) compared to NEU while LOR had decreased E1 in Q3 (p=0.008) compared to NEU in C+EX. CONCLUSIONS Lumbar spine mono-segmental fixation with 5° offset from neutral/native individual segmental angle increases the motion at adjacent level and can also induces disc strain in most direction of loads with final angle in kyphosis being worse than in lordosis. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs. Single-level lumbar fusion had been performed for years with little concern with final segmental angle. However, subtle changes on a segmental lordosis may mechanically impact adjacent level motion and stress triggering a domino effect of adult spinal deformity with sagittal imbalance and failed back fusion. The objective of this study was to quantify adjacent segment motion and disc surface strain changes with altered segmental angles. Study adjacent-level motion and strain distribution in cadaveric specimens with a lumbar spine single segment fusion in an offset angle. Biomechanical cadaveric study. Seven human cadaveric specimens. Range of motion (ROM) and optic strain. Seven human specimens (L2-Sacrum) underwent L4-L5 pedicle screws and rods fixation and were tested in neutral angle (NEU), imposed +5° lordosis (LOR) and -5° kyphosis (KYP). Pure moments (7.5 Nm) were applied in flexion; extension; lateral bending (LB); axial rotation (AR) followed by 400 N of compression (C) alone, and combined with pure moments. Range of motion (ROM) and strain using digital image correlation (DIC) system were tracked. Principle maximum (E1) and minimum (E2) strains were analyzed within four quarters on the lateral disc surface antero-posteriorly (Q1; Q2; Q3 and Q4). Data were analyzed using one-way RM ANOVA. At the upper adjacent level, significant increase in ROM was observed in both conditions KYP and LOR compared to NEU in flexion (p=0.001; p=<0.001) and extension (p=0.02; p=0.009). Increased ROM was also observed in LOR compared to NEU (p=0.026) and compared to KYP (p=0.004) during compression. KYP had increased ROM compared to NEU and LOR (p=0.031; p=0.025) in C+EX. LOR had increased E1 in Q3 compared to NEU in RLB (p=0.041); LOR and KYP had decreased E1 in Q3 compared to NEU in C (p=0.002; p=0.03). LOR had decreased E1 in Q3 compared to NEU (p=0.013) while KYP had increased E1 in all quartiles and increased E2 in Q2 compared to LOR in C+FL (p≤0.047). KYP decreased E1 in Q3 (p=0.021) and E2 in Q1 (p=0.006) compared to NEU while LOR had decreased E1 in Q3 (p=0.008) compared to NEU in C+EX. Lumbar spine mono-segmental fixation with 5° offset from neutral/native individual segmental angle increases the motion at adjacent level and can also induces disc strain in most direction of loads with final angle in kyphosis being worse than in lordosis.