Mahdi Ebrahimkhani, Navid Arjmand, Aboulfazl Shirazi-Adl


May 2022, pp 1 - 10 Original Article Read Full Article 10.1007/s00586-022-07262-3

First Online: 28 May 2022

Purpose

This study exploits a novel musculoskeletal finite element (MS-FE) spine model to evaluate the post-fusion (L4–L5) alterations in adjacent segment kinetics.

Methods

Unlike the existing MS models with idealized representation of spinal joints, this model predicts stress/strain distributions in all passive tissues while organically coupled to a MS model. This generic (in terms of musculature and material properties) model uses population-based in vivo vertebral sagittal rotations, gravity loads, and an optimization algorithm to calculate muscle forces. Simulations represent individuals with an intact L4–L5, a preoperative severely degenerated L4–L5 (by reducing the disc height by ~ 60% and removing the nucleus incompressibility), and a postoperative fused L4–L5 segment with either a fixed or an altered lumbopelvic rhythm with respect to the intact condition (based on clinical observations). Changes in spine kinematics and back muscle cross-sectional areas (due to intraoperative injuries) are considered based on in vivo data while simulating three activities in upright/flexed postures.

Results

Postoperative changes in some adjacent segment kinetics were found considerable (i.e., larger than 25%) that depended on the postoperative lumbopelvic kinematics and preoperative L4–L5 disc condition. Postoperative alterations in adjacent disc shear, facet/ligament forces, and annulus stresses/strains were greater (> 25%) than those found in intradiscal pressure and compression (< 25%). Kinetics of the lower (L5–S1) and upper (L3–L4) adjacent segments were altered to different degrees.

Conclusion

Alterations in segmental rotations mainly affected adjacent disc shear forces, facet/ligament forces, and annulus/collagen fibers stresses/strains. An altered lumbopelvic rhythm (increased pelvis rotation) tends to mitigate some of these surgically induced changes.


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