Gerdine Meijer

Influence of Interpersonal Geometrical Variation on Spinal Motion Segment Stiffness Implications for Patient-Specific Modeling

Study Design. A validated finite element model of an L3–L4 motion segment is used to analyze the effects of interpersonal differences in geometry on spinal stiffness. Objective. The objective of this study is to determine which of the interpersonal variations of the geometry of the spine have a large effect on spinal stiffness. This will improve patient-specific modeling. Summary of Background Data. The parameters that define the geometry of a motion segment are vertebral height, disc height, endplate width, endplate depth, spinous process length, transverse process width, nucleus size, lordosis angle, facet area, facet orientation, and the cross-sectional areas of the ligaments. All these parameters differ between patients. The influence of each parameter on spinal stiffness is largely unknown and such knowledge would greatly help in patient-specific modeling of the spine. Methods. The range of interpersonal variation of each of the geometric parameters was set at mean ± 2SD (covering 95% of the population). Subsequently, we determined the effect of each of these ranges on the bending stiffness in flexion, extension, axial rotation, and lateral bending. Results. Disc height had the largest influence; a maximal disc height reduced the spinal stiffness to 75–86% of the mean motion segment stiffness, and a minimal disc height increased the spinal stiffness to 154–226% of the mean motion segment stiffness. Lordosis angle, transversal and longitudinal facet angle, endplate depth, and area of the capsular ligament also had a substantial influence (>5%) on the stiffness, but considerable less than the influence of the disc height. Ligament areas, nucleus size, spinous process length, and length of processes are of negligible effect (<2%) on the stiffness. Conclusion. The disc height should be accurately determined in patients to estimate the spinal stiffness. Ligament areas, nucleus size, spinous process length, and transverse process width do not need patient-specific modeling.

The effect of three-dimensional geometrical changes during adolescent growth on the biomechanics of a spinal motion segment.

During adolescent growth, vertebrae and intervertebral discs undergo various geometrical changes. Although such changes in geometry are well known, their effects on spinal stiffness remains poorly understood. However, this understanding is essential in the treatment of spinal abnormalities during growth, such as scoliosis. A finite element model of an L3-L4 motion segment was developed, validated and applied to study the quantitative effects of changing geometry during adolescent growth on spinal stiffness in flexion, extension, lateral bending and axial rotation. Height, width and depth of the vertebrae and intervertebral disc were varied, as were the width of the transverse processes, the length of the spinous process, the size of the nucleus, facet joint areas and ligament size. These variations were based on average growth data for girls, as reported in literature. Overall, adolescent growth increases the stiffness with 36% (lateral bending and extension) to 44% (flexion). Two thirds of this increase occurs between 10 and 14 years of age and the last third between 14 years of age and maturity. Although the height is the largest geometrical change during adolescent growth, its effect on the biomechanics is small. The depth increase of the disc and vertebrae significantly affects the stiffness in all directions, while the width increase mainly affects the lateral bending stiffness. Hence, when analysing the biomechanics of the growing adolescent spine (for instance in scoliosis research), the inclusion of depth and width changes, in addition to the usually implemented height change, is essential.

The effects of creep and recovery on the in vitro biomechanical characteristics of human multi-level thoracolumbar spinal segments

BACKGROUND Several physiological and pathological conditions in daily life cause sustained static bending or torsion loads on the spine resulting in creep of spinal segments. The objective of this study was to determine the effects of creep and recovery on the range of motion, neutral zone, and neutral zone stiffness of thoracolumbar multi-level spinal segments in flexion, extension, lateral bending and axial rotation. METHODS Six human cadaveric spines (age at time of death 55-84 years) were sectioned in T1-T4, T5-T8, T9-T12, and L1-L4 segments and prepared for testing. Moments were applied of +4 to -4 N m in flexion-extension, lateral bending, and axial rotation. This was repeated after 30 min of creep loading at 2 N m in the tested direction and after 30 min of recovery. Displacement of individual motion segments was measured using a 3D optical movement registration system. The range of motion, neutral zone, and neutral zone stiffness of the middle motion segments were calculated from the moment-angular displacement data. FINDINGS The range of motion increased significantly after creep in extension, lateral bending and axial rotation (P<0.05). The range of motion after flexion creep showed an increasing trend as well, and the neutral zone after flexion creep increased by on average 36% (P<0.01). The neutral zone stiffness was significantly lower after creep in axial rotation (P<0.05). INTERPRETATION The overall flexibility of the spinal segments was in general larger after 30 min of creep loading. This higher flexibility of the spinal segments may be a risk factor for potential spinal instability or injury.

Posteriorly directed shear loads and disc degeneration affect the torsional stiffness of spinal motion segments; a biomechanical modeling study

Study Design. Finite element study. Objective. To analyze the effects of posterior shear loads, disc degeneration, and the combination of both on spinal torsion stiffness. Summary of Background Data. Scoliosis is a 3-dimensional deformity of the spine that presents itself mainly in adolescent girls and elderly patients. Our concept of its etiopathogenesis is that an excess of posteriorly directed shear loads, relative to the bodys intrinsic stabilizing mechanisms, induces a torsional instability of the spine, making it vulnerable to scoliosis. Our hypothesis for the elderly spine is that disc degeneration compromises the stabilizing mechanisms. Methods. In an adult lumbar motion segment model, the disc properties were varied to simulate different aspects of disc degeneration. These models were then loaded with a pure torsion moment in combination with either a shear load in posterior direction, no shear, or a shear load in anterior direction. Results. Posteriorly directed shear loads reduced torsion stiffness, anteriorly directed shear loads increased torsion stiffness. These effects were mainly caused by a later (respectively earlier) onset of facet joint contact. Disc degeneration cases with a decreased disc height that leads to slackness of the annular fibers and ligaments caused a significantly decreased torsional stiffness. The combination of this stage with posterior shear loading reduced the torsion stiffness to less than half the stiffness of a healthy disc without shear loads. The end stage of disc degeneration increased torsion stiffness again. Conclusion. The combination of a decreased disc height, that leads to slack annular fibers and ligaments, and posterior shear loads very significantly affects torsional stiffness: reduced to less than half the stiffness of a healthy disc without shear loads. Disc degeneration, thus, indeed compromises the stabilizing mechanisms of the elderly spine. A combination with posteriorly directed shear loads could then make it vulnerable to scoliosis. Level of Evidence: N/A

A Possible Role For Posteriorly Directed Shear Loads In The Etiology Of Degenerative Scoliosis

INTRODUCTION: Scoliosis is a three-dimensional deformity of the human spine. Despite more than a century of dedicated research, its etiology remains largely unknown. The results of our past work, however, suggest a possible relation between the existing posteriorly directed shear loads on certain regions of the spine and the development of scoliosis. In modeling studies, we have shown that posteriorly directed shear loads do indeed occur in the human spine. In in vitro studies, we found that these shear loads increase the transverse plane rotational instability of spinal segments. And in in vivo studies we have shown that preexistent vertebral rotations are present and influenced by body position. Based on these results, we believe that the upright spinal stance and the occurrence of posteriorly directed shear loads play a crucial role in the rotational instability of the human spine, and subsequently in scoliosis. Younger spines would then be most vulnerable as they are more rotationally unstable than adult spines. While scoliosis is indeed most seen in adolescent spines, it also occurs in aged spines: degenerative scoliosis is a type of scoliosis that typically occurs in the thoraco-lumbar and lumbar spine of people over 65 years of age. As people in this age group also typically have degeneration of their intervertebral discs, there appears to be a relation between disc degeneration and degenerative scoliosis. Disc degeneration has indeed been shown to cause increased rotational instability, but the effect of additional shear loads on this behavior is yet unknown. In this study we investigated whether the rotational instability of aged spines is related to disc degeneration.