Biomechanical Analysis of an Interspinous Process Fixation Device with In Situ Shortening Capabilities: Does Spinous Process Compression Improve Segmental Stability?

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The objective of this study was to characterize the biomechanical implications of spinous process compression, via in situ shortening of a next-generation interspinous process fixation (ISPF) device, in the context of segmental fusion. Seven lumbar cadaveric spines (L1-L4) were tested. Specimens were first tested in an intact state, followed by iterative instrumentation at L2-3 and subsequent testing. The order followed was 1) stand-alone ISPF (neutral height); 2) stand-alone ISPF (shortened in situ from neutral height; shortened); 3) lateral lumbar interbody fusion (LLIF)+ ISPF (neutral); 4) LLIF+ ISPF (shortened); 5) LLIF+ unilateral pedicle screw fixation; 6) LLIF+ bilateral pedicle screw fixation. A 7.5-Nm moment was applied in flexion/extension, lateral bending, and axial rotation via a kinematic test frame. Segmental range of motion (ROM) and lordosis were measured for all constructs. Comparative analysis was performed. Statistically significant flexion/extension ROM reductions: all constructs versus intact condition (P < 0.01); LLIF+ ISPF (neutral and shortened) versus stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF+ USPF versus ISPF (neutral) (P= 0.049); bilateral pedicle screw fixation (BPSF) versus stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF+ BPSF versus LLIF+ unilateral pedicle screw fixation (UPSF) (P < 0.01). Significant lateral bending ROM reductions: LLIF+ ISPF (neutral and shortened) versus intact condition and stand-alone ISPF (neutral) (P < 0.01); LLIF+ UPSF versus intact condition and stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF+ BPSF versus intact condition and all constructs (P < 0.01). Significant axial rotation ROM reductions: LLIF+ ISPF (shortened) and LLIF+ UPSF versus intact condition and stand-alone ISPF (neutral) (P ≤ 0.01); LLIF+ BPSF versus intact condition and all constructs (P ≤ 0.04). In situ shortening of an adjustable ISPF device may support increased segmental stabilization compared with static ISPF.