Scoliosis is a musculoskeletal abnormality causing complex three dimensional curvatures in the spine. Current surgical treatments for this adolescent spinal deformity are successful but invasive. Potential new treatments that are less invasive are being developed based on altering growth by mechanically redistributing stresses across the vertebral growth plates. In the literature, in vivo and in vitro tests have shown biomechanical changes in the disc and growth plates due to insertion of staple like implants used in these new methods. In order to understand the biomechanics behind these potential new methods, a nonlinear finite element analysis (FEA) is performed and various biomechanical properties of the spinal segment with and without the implant are determinedA three-dimensional FE model of T7-T8 motion segment was developed from a CT scan of a porcine spine and imported to ABAQUS (an FEA software). Various material properties and contact interactions were used from the literature in determining the model that best predicted the available experimental load-displacement curve and the compressive properties of the disc. Bending loads were applied to this FE model to determine the reduction in the motion of the spinal segment. Sensitivity of the implant features were examined against the compressive properties of the disc. Mechanobiological growth models have been partially developed to study various biomechanical factors causing deformities in spine. This available model was utilized in understanding how growth in a normal spine could be influenced due to the presence of these implants.
Digital models based on data from medical images have recently become widespread in the field of biomechanics. This book summarizes medical imaging techniques and processing procedures, both of which are necessary for creating bone models with finite element methods. Chapter 1 introduces the main principles and the application of the most commonly used medical imaging techniques. Chapter 2 describes the major methods and steps of medical image analysis and processing. Chapter 3 presents a brief review of recent studies on reconstructed finite element bone models, based on medical images. Finally, Chapter 4 reveals the digital results obtained for the main bone sites that have been targeted by finite element modeling in recent years.
The aim of the research was to develop a three-dimensional finite element model to study the biomechanical and kinematic characteristics of the human lumbar spine in flexion. An analytical model of the lumbar spine capable of taking into consideration the actual geometry, non-linear material properties and realistic loading would be of benefit in studying normal biomechanics, as well as in-vivo behavior in injured and surgically altered spines. Fundamental to this approach is an accurate model of the spine. This was achieved by modeling the lumbar segments L2-L4 from Computed Tomography (CT) data and analyzing them under loading conditions that best approximated the human lumbar segments in flexion. An in-vitro study was performed for validation of the finite element model. Human lumbar cadaveric spinal segments (L2-L4) were loaded based on test conditions similar to those defined in the finite element analysis. The results of the cadaver biomechanical study and finite element analysis were compared. The results suggest that the model is a valid approach to assessing the range of motion of the L3 segment under flexion. Rotation under lateral bending moments was additionally investigated to provide a thorough validation of the model.
Lumbar interspinous spacers have recently become popular as an alternative treatment for low back pain. These devices are primarily used to treat spinal stenosis and facet arthritis and are intended to unload the facet joints, restore foraminal height and provide stability mainly in extension while allowing normal range of motion in other loading modes at the index level and also at the adjacent segments. The goals of this study were three fold: (i) to evaluate the biomechanical stability provided by the Superion Interspinous Spacer (ISS) (Vertiflex®, San Clemente, CA) implanted in the lumbar spine; (ii) to study the effect of transection of supraspinous ligament (SSL) on the lumbar spine implanted with ISS; and (iii) to investigate the effect of graded facetectomies (50%, 75%, 100% facetectomies for both unilateral and bilateral cases) following the placement of ISS in the lumbar spine. This study is basically divided into two parts: an in vitro investigation and finite element analysis. The in vitro biomechanical study was conducted on six human lumbar motion segments (3 L2-L3 and 3 L4-L5) in the following test sequence: (i) intact, (ii) implanted (SSL intact), (iii) SSL dilated longitudinally at the center (with ISS), (iv) 50% resection of SSL (with ISS), (v) 100% resection of SSL (with ISS) (vi) injured (ISS removed). A finite element (FE) analysis was performed for the same test cases and also to investigate the effect of graded facetectomies using an experimentally validated 3D L3-S1 model. A bending moment of 10 Nm was applied in all loading modes (flexion-extension, right/left lateral bending, and right/left axial rotation) while a bending moment of 10 Nm with 400N compressive follower load was applied only in flexion-extension. Range of motion (ROM) was recorded for each of the test constructs. In addition to ROM, intradiscal pressure (IDP), facet loads and stress contour plots for these test constructs were obtained from the FE model. Repeated measures one way ANOVA was used to perform statistical analysis to determine the statistically significant differences between different test constructs for the in vitro data. The in vitro results showed that the mean ROM was significantly (p0.05) reduced in extension post-implantation. There was a minimal decrease in mean ROM in flexion as well, but it was not significant (p0.05). ROM in lateral bending and axial rotation were not affected. Also, there was no significant difference in ROM in any of the loading modes for the instrumented cases with and without SSL. The FE results were in agreement with the in vitro results except in flexion. Unlike the in vitro results, the flexion ROM increased slightly with progressive transection of SSL. From the FE data, it was observed that there was a significant reduction in IDP and facet loads in extension following the placement of ISS. However, there was no considerable difference in IDP and facet loads for the implanted cases with partial or complete transection of SSL. ROM, IDP and facet loads were not affected at the adjacent levels in any of the loading modes for any of the test constructs. It was also observed that there was an effect of graded facetectomies (50%, 75% and 100%) on the instrumented spine with a significant increase in ROM in flexion in case of both unilateral and bilateral facetectomies. The ISS provided an increased stability in extension while preserving motion comparable to intact in the other loading modes. Also, it can be inferred that SSL plays an insignificant role in segmental stability in extension, lateral bending and axial rotation and has nominal effect in flexion. In addition, the results suggest that ISS may not be used in combination with graded facetectomies for both unilateral and bilateral cases.
