Bernardo de Andrada Pereira, Jennifer N. Lehrman, Anna G.U. Sawa, Piyanat Wangsawatwong, Jakub Godzik, David S. Xu, Jay D. Turner, Brian P. Kelly, Juan S. Uribe
Neurospine 2022;19(3):828-837. Published online September 30, 2022
Objective The high mechanical stress zone at the sudden transition from a rigid to flexible region is involved in proximal junctional kyphosis (PJK) physiopathology. We evaluated the biomechanical performance of polyetheretherketone (PEEK) rods used as a nontraditional long semirigid transition phase from a long-segment metallic rod construct to the nonfused thoracic spine.
Methods Pure moment range of motion (ROM) tests (7.5 Nm) were performed on 7 cadaveric spine segments followed by compression (200 N). Specimens were tested in the following conditions: (1) intact; (2) T10-pelvis pedicle screws and rods (PSRs); and (3) extending the proximal construct to T6 using PEEK rods (PSR+PEEK). T10–11 rod strain, T9 anterolateral bone strain, and T10 screw bending moments were analyzed.
Results At the upper instrumented vertebra (UIV)+1, PSR+PEEK versus PSR significantly decreased ROM in flexion (115%, p = 0.02), extension (104%, p = 0.003), left lateral bending (46%, p = 0.02), and right lateral bending (63%, p = 0.008). Also, at UIV+1, PSR+PEEK versus intact significantly decreased ROM in flexion (111%, p = 0.01) and extension (105%, p = 0.003). The UIV+1 anterior column bone strain was significantly reduced with PSR+PEEK versus PSR during right lateral bending (p = 0.02). Rod strain polarities reversed with PEEK rods in all loading directions except compression.
Conclusion Extending a long-segment construct using PEEK rods caused a decrease in adjacent-level hypermobility as a consequence of long-segment immobilization and also redistributed the strain on the UIV and adjacent levels, which might contribute to PJK physiopathology. Further studies are necessary to observe the clinical outcomes of this technique.
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Objective The lordotic shape of the lumbar spine differs substantially between individuals. Measuring and recording strain during spinal biomechanical tests is an effective method to infer stresses on spinal implants and predict failure mechanisms. The geometry of the spine may have a significant effect on the resultant force distribution, thereby directly affecting rod strain.
Methods Seven fresh-frozen cadaveric specimens (T12-sacrum) underwent standard (7.5 Nm) nondestructive sagittal plane tests: flexion and extension. The conditions tested were intact and pedicle screws and rods (PSR) at L1-sacrum. The posterior right rod was instrumented with strain gauges between L3–4 (index level) and the L5–S1 pedicle screw. All specimens underwent lateral radiographs before testing. Lordotic angles encompassing different levels (L5–S1, L4–S1, L3–S1, L2–S1, and L1–S1) were measured and compared with rod strain. Data were analyzed using Pearson correlation analyses.
Results Strong positive correlations were observed between lordosis and posterior rod strain across different conditions. The L3–S1 lordotic angle in the unloaded intact condition correlated with peak rod strain at L3–4 with PSR during flexion (R = 0.76, p = 0.04). The same angle in the unloaded PSR condition correlated with peak strain in the PSR condition during extension (R = -0.79, p = 0.04). The unloaded intact L2–S1 lordotic angle was significantly correlated with rod strain at L3–4 in the PSR condition during flexion (R = 0.85, p = 0.02) and extension (R = -0.85, p = 0.02) and with rod strain at L5–S1 in the PSR condition during flexion (R = 0.84, p = 0.04).
Conclusion Lordosis measured on intact and instrumented conditions has strong positive correlations with posterior rod strain in cadaveric testing.
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