Biomechanical assessment of proximal junctional semi-rigid fixation in long-segment thoracolumbar constructs

OBJECTIVE Proximal junctional kyphosis (PJK) and failure (PJF) are potentially catastrophic complications that result from abrupt changes in stress across rigid instrumented and mobile non-fused segments of the spine (transition zone) after adult spinal deformity surgery. Recently, data have indicated that extension (widening) of the transitional zone via use of proximal junctional (PJ) semi-rigid fixation can mitigate this complication. To assess the biomechanical effectiveness of 3 semi-rigid fixation constructs (compared to pedicle screw fixation alone), the authors performed cadaveric studies that measured the extent of PJ motion and intradiscal pressure changes (Delta IDP). METHODS To measure flexibility and Delta IDP at the PJ segments, moments in flexion, extension, lateral bending (LB), and torsion were conducted in 13 fresh-frozen human cadaveric specimens. Five testing cycles were conducted, including intact (INT), T10-L2 pedicle screw-rod fixation alone (PSF), supplemental hybrid T9 Mersilene tape insertion (MT), hybrid T9 sublaminar band insertion (SLB1), and hybrid T8/T9 sublaminar band insertion (SLB2). RESULTS Compared to PSF, SLB1 significantly reduced flexibility at the level rostral to the upper-instrumented vertebral level (UIV+1) under moments in 3 directions (flexion, LB, and torsion, p <= 0.01). SLB2 significantly reduced motion in all directions at UIV+1 (flexion, extension, LB, torsion, p < 0.05) and at UIV+2 (LB, torsion, p <= 0.03). MT only reduced flexibility in extension at UIV+1 (p = 0.02). All 3 constructs revealed significant reductions in Delta IDP at UIV+1 in flexion (MT, SLB1, SLB2, p <= 0.02) and torsion (MT, SLB1, SLB2, p <== 0.05), while SLB1 and SLB2 significantly reduced Delta IDP in extension (SLB1, SLB2, p = 0.02) and SLB2 reduced Delta IDP in LB (p = 0.05). At UIV+2, SLB2 similarly significantly reduced Delta IDP in extension, LB, and torsion (p <= 0.05). CONCLUSIONS Compared to MT, the SLB1 and SLB2 constructs significantly reduced flexibility and Delta IDP in various directions through the application of robust anteroposterior force vectors at UIV+1 and UIV+2. These findings indicate that semi-rigid sublaminar banding can most effectively expand the transition zone and mitigate stresses at the PJ levels of long-segment thoracolumbar constructs.