Does Lumbar Interbody Fusion Modality Affect the Occurrence of Complications in an Osteoporotic Spine Under Whole-Body Vibration? A Finite Element Study.

OBJECTIVE: To evaluate the effects of 3 lumbar interbody fusion techniques on the occurrence of complications in an osteoporotic spine under whole-body vibration. METHODS: A previously developed and validated nonlinear finite element model of L1-S1was modified to develop anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), and transforaminal lumbar interbody fusion (TLIF) models with osteoporosis. In each model, the lower surface of the sacrum was absolutely fixed, a follower load of 400N was applied through the axis of the lumbar spine, and an axial sinusoidal vertical load of ±40N (5 Hz) was imposed on the superior surface of L1, to perform a transient dynamic analysis. The maximal values of intradiscal pressure, shear stress on annulus substance, disc bulge, facet joint stress, and screw and rod stress, along with their dynamic response curves, were collected. RESULTS: Among these 3 models, the TLIF model generated the greatest screw and rod stress, and the PLIF model generated the greatest cage-bone interface stress. At the L3-L4 level, compared with the other 2 models, the maximal values and dynamic response curves of intradiscal pressure, shear stress of annulus ground substance, and disc bulge were all lower in the ALIF model. However, the facet contact stress at the adjacent segment in the ALIF model was higher than that in the other 2 models. CONCLUSIONS: In an osteoporotic spine under whole-body vibration, TLIF has the highest risk of screw and rod breakage, PLIF has the highest risk of cage subsidence, and ALIF has the lowest risk of upper adjacent disc degeneration, but the highest risk of adjacent facet joint degeneration.

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.