Effect of Pedicle Screw Fixation on Adjacent Segments in Osteoporotic Spine Following Transforaminal Lumbar Interbody Fusion Under Whole Body Vibration.

Background: Adjacent segmental degeneration (ASD) is the long-term complication of transforaminal lumbar interbody fusion (TLIF) combined with pedicle screw fixation. Both osteoporosis and whole-body vibration (WBV) can alter the biomechanics of adjacent segments. However, the effect of pedicle screw fixation on ASD in an osteoporotic spine after TLIF under WBV was unknown.

Methods: According to a previously validated model of L1-S1, 2 osteoporotic TLIF models with and without pedicle screw fixation were developed. In each model, a 400 N preload was applied and a 5-Hz, 40 N sinusoidal vertical load with a 40-kg mass point was imposed on the superior surface of L1. The parameters of intradiskal pressure, shear stress of annulus fibrosus, disk bulge, superior and inferior end plate stress, and facet joint contact pressure at L3-L4 and L5-S1 levels were evaluated.

Results: At L3-L4, the dynamic responses in intradiskal pressure, shear stress, facet joint contact pressure, superior end plate stress, and inferior end plate stress generated an increase after pedicle screw fixation, and their maximum values increased by 15.1%, 9.5%, 18.6%, 10.6%, and 9.3%, respectively. However, the parameter of disk bulge demonstrated an opposite trend. At L5-S1, the differences in maximum values of the parameters were slight and the corresponding dynamic response curves were close, overlapping, or intersecting.

Conclusions: In an osteoporotic spine after TLIF, removal of pedicle screw fixation can mitigate ASD in the upper adjacent segment but has no apparent influence on the lower adjacent segment under WBV.