Biomechanical Effects of Proximal Polyetheretherketone Rod Extension on the Upper Instrumented and Adjacent Levels in a Human Long-Segment Construct: A Cadaveric Model

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 ca-daveric spine segments followed by compression (200 N). Specimens were tested in the fol-lowing conditions: (1) intact; (2) T10-pelvis pedicle screws and rods (PSRs); and (3) ex-tending 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 bend-ing (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 ad-jacent-level hypermobility as a consequence of long-segment immobilization and also redis-tributed the strain on the UIV and adjacent levels, which might contribute to PJK physio-pathology. Further studies are necessary to observe the clinical outcomes of this technique.

Automated summary

The use of PEEK rods as a transition material in long-segment metal rod constructs can reduce excessive mobility at adjacent levels and redistribute stress on the spine, which is significant in preventing proximal junctional kyphosis.