Kinematics of Disc Degeneration - Computer Analysis of Center of Rotation
1Yildiz Technical University, Istanbul, Turkey, 2Medeniyet University, Istanbul, Turkey, 3Loma Linda University, Loma Linda, CA, USA
Introduction: Disc degeneration is a known cause of low back pain and instability in the spine. Extensive in-vitro studies investigated the quantitative changes (ie, range of motion (ROM)) in the spinal segment with disc degeneration. However, studies focused on qualitative changes (ie, instantaneous center of rotation (ICR)) in the spinal motion are scarce and incomplete. A better explanation of biomechanical changes in the degenerated spinal segment can assist the diagnosis of the dysfunctional spinal level when imaging modalities alone are inadequate. This project aimed to describe the three-dimensional characteristics of the ICR in a degenerated segment during various loading levels and directions. Therefore, the hypothesis of the present study was that isolated simulation of age-related changes in the disc in the lower lumbar spine segment with normal facets would not cause significant shift in the ICR.
Methods: A three-dimensional finite element (FE) model of the L4-5 segment (Fig. 1) with three levels of disc degeneration (normal, mild, and moderate) was modeled and validated. Cortical and trabecular bone modeled by using solid wedge, tetrahedral and pyramid elements whereas facet cartilage layers, annulus ground substance, nucleus pulposus and cartilage endplates were modeled by using solid hexahedral elements. The annulus fibers and ligaments and were modeled with tension-only link elements. The orientation the annulus fibers were 23° at the anterior side and changed continuously up to the 58° through the posterior side. The nucleus pulposus composed 43% of the whole volume of the disc. Facet joints had frictionless contact. The model consisted of 1575 link elements and 270324 solid elements in total.
Disc degeneration was simulated by changing disc height (20 and 40% loss for mild and moderate degeneration, respectively), material properties of nucleus pulposus (increased near to that of the annulus) and adding buckling effect in ligaments, which was caused by the decreased disc height. A pure bending moment of 7.5 Nm was applied to the model in all three planes of motion. The ICR of different degeneration conditions, loading levels and directions were evaluated by using the Reuleaux method (the intersection of the perpendicular bisectors of displacement vectors of two nodes of the L4 inferior endplate). The characteristics of the centrode, i.e., the path that ICR followed during a full loading cycle, were investigated. Facet joint forces and annulus stresses were calculated.
Results: Validation: The assessment of the FE model showed an agreement with the previous computational and in-vitro studies [1-3] regarding the ROM, facet forces, and annulus stresses of the L4-L5 segment for various loading conditions and degeneration scenarios.
Effects of Disc Degeneration: As a result of simulated mild degeneration, a laxity occurred in the motion segment and ROM increased in all anatomic planes. However, further degeneration caused a decrease in ROM. Facet forces decreased with mild degeneration but there was no significant change with further degeneration in extension. No facet forces occurred in flexion in any degeneration level. Annulus stresses increased constantly with each step of simulated disc degeneration.
ICR Analysis: In the intact segment, the ICR was located close to the superior endplate of the L5 vertebra in sagittal motion, close to the disc center in lateral bending and at the postero-lateral periphery of the L5 vertebra in axial rotation. The maximum change in the position of the centrode due to degeneration was below 5 mm in sagittal plane, 6 mm in axial plane, and 3 mm in lateral plane (Table 1, for brevity only the details of the sagittal plane was listed.).
Discussion: This study investigated the behavior of the instantaneous center of rotation of a degenerated lower lumbar spine during various loading levels and directions and for various degeneration levels using finite element method. The results of the study showed that ICR did not shift significantly in the isolated disc degeneration model. Such small change in the ICR might be difficult to determine using X-ray films. Thus, the ICR alone might not be a reliable tool to detect degenerative segments. However, when considered as adjunct to MRI and discography, the changes in the ICR could be used to confirm the clinical evaluations.
This study investigated the early-mid stage, isolated degeneration and did not include biomechanical and structural changes in the ligaments and facets. It might be possible that changes in the spinal motion would reach a clinically detectable level only after certain point in the degeneration process. Therefore, kinematics-based diagnosis of degeneration-related instability can be safely applied in the later stages of the degeneration with the current imaging modalities.
Significance: This study investigated, for the first time, three dimensional qualitative changes in the motion of the lower lumbar spine after disc degeneration. Our study estimated the changes in the ICR in the degenerated spinal segment as an assistive biomechanical guideline for clinicians in accurately determining the degenerated segment.
References: 1- Schilling C, Kruger S, Grupp TM, et al. The effect of design parameters of dynamic pedicle screw systems on kinematics and load bearing: an in vitro study. European Spine Journal 2011;20:297-307.
2- Rohlmann A, Zander T, Schmidt H, et al. Analysis of the influence of disc degeneration on the mechanical behaviour of a lumbar motion segment using the finite element method. Journal of Biomechanics 2006;39:2484-90.
3- Ruberte LM, Natarajan RN, Andersson GBJ. Influence of single-level lumbar degenerative disc disease on the behavior of the adjacent segments-A finite element model study. Journal of Biomechanics 2009;42:341-8.
Figure 1. Section view of the finite element model of the segment (left) and intervertebral disc structure with annulus fibers (right). (O: origin, C: 16.5 mm, A: 33.7 mm, E: 7.6 mm)
Table 1. Mean ICR locations in sagittal plane for different conditions. (x,y coordinates measured from postero-superior corner of L5.) (for brevity, only the details of the sagittal plane was listed)