P13. Biomechanical effects of proximal PEEK rod extension on the upper instrumented and adjacent levels in a human long segment construct: A cadaveric model

Abstract

<h3>BACKGROUND CONTEXT</h3> Proximal junctional kyphosis (PJK) is a sequela of spinal fusion that is potentially related to mechanical failure and revision surgery. The high mechanical stress zone, created by the sudden transition from a rigid to a flexible region, is involved in PJK physiopathology. <h3>PURPOSE</h3> We evaluated the biomechanical performance of polyetheretherketone (PEEK) rods used as a semi-rigid transition from a long-segment metallic rod construct to a non-fused thoracic spine. <h3>STUDY DESIGN/SETTING</h3> Cadaveric biomechanical study. <h3>PATIENT SAMPLE</h3> Seven human cadaveric specimens. <h3>METHODS</h3> Seven cadaveric specimens (T2-pelvis with intact rib cage) were used for this study. Standard pure moment range of motion (ROM) tests (7.5 Nm) were performed in flexion, extension, lateral bending (LB), and axial rotation (AR) followed by compression (200 N). Specimens were tested 1) intact, 2) after instrumentation with T10-S1 pedicle screws and S2 alar-iliac screws and metallic rods (PSR), 3) and after extending the proximal construct to T6 using PEEK rods (PSR+PEEK). T10-T11 rod strain, T9 anterolateral bone strain, and T10 screw bending moments were analyzed using paired t test or repeated measures analysis of variance. <h3>RESULTS</h3> PSR decreased T10-T11 ROM in all loading directions (mean 80%), but the decrease was statistically significant only during flexion (70%, P=0.003). PSR+PEEK significantly decreased ROM during flexion (68%, P=0.04) compared with PSR. Compared with PSR, PSR+PEEK significantly decreased T9-T10 ROM in flexion (115%, P=0.003), extension (104%, P=0.006), left LB (46%, P=0.01), and right LB (63%, P=0.02). Compared to the intact condition, PSR+PEEK significantly decreased T9-T10 ROM in flexion (111%, P=0.01) and extension (105%, P=0.003). For PSR+PEEK versus PSR, T5-T6 ROM decreased in all directions (mean 27%) but only significantly during extension (44%, P=0.03) and left AR (40%, P=0.048). The reduction in T9 anterior column bone strain was statistically significant with PSR+PEEK versus PSR during right LB (P=0.02). Compared with PSR, PSR+PEEK significantly reduced the contralateral T10 screw bending moment by 37% (left AR, P=0.03), while the mean ipsilateral decrease was not significant at 14% (right AR, P=0.53). Rod strain polarities reversed with PEEK rods in all loading directions except compression. <h3>CONCLUSIONS</h3> Extending a long-segment construct using PEEK rods redistributed the strain on the upper instrumented and adjacent levels, and caused a decrease in adjacent-level hypermobility, which might contribute to the physiopathology of PJK. Further studies are necessary to observe the clinical outcome of this technique. <h3>FDA DEVICE/DRUG STATUS</h3> PEEK rods (Approved for this indication).