Noshir A. Langrana

Lumbosacral spinal fusion. A biomechanical study.

The effects of spinal fusion on fused segment and the adjacent, unfused segments play a significant role in the clinical effectiveness of spinal fusion for low-back pain with or without sciatica. Much of the information on this important subject is derived from clinical impressions. The purpose of this biomechanical study is to investigate the altered kinematics and biomechanics of the three different types of spinal fusion (posterior, bilateral-lateral, and anterior) on the adjacent, unfused segment as well as within the fused segment and to investigate their clinical implications. Sixteen fresh human cadaver lumbosacral spines were tested under a simulated physiologic loading condition. The test specimens included three motion segments, L3-4, L4-5, and L5-S1. To study the mechanics of the lumbar spine under combined compression and bending loads, a special apparatus was designed. These loads were applied by an MTS machine through two sets of pulley systems. The loads, as well as displacement data from both actuators, were recorded. A video camera system was utilized to record the kinematics of the spinal motion segment. The unfused specimen was tested first, and the fused specimen then was retested under the identical loading conditions. A total of 16 spine specimens were tested and evaluated--five posterior, four bilateral-lateral, and seven anterior fusions. All types of fusion resulted in increased bending and axial stiffnesses. Overall, anterior fusion provided the largest increase in stiffness, followed by bilateral-lateral fusion and posterior fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of ligaments and facets in lumbar spinal stability.

Study Design The issue of segmental stability using finite element analysis was studied. Effect of ligament and facet (total and partial) removal and their geometry on segment response were studied from the viewpoint of stability. Objectives To predict factors that may be linked to the cause of rotational instabilities, spondylolisthesis, retropondylolisthesis, and stenosis. Summary of Background Data The study provides a comprehensive study on the role of facets and ligaments and their geometry in preserving segmental stability. No previous biomechanical study has explored these issues in detail. Methods Three-dimensional nonlinear finite element analysis was performed on L3-L4 motion segments, with and without posterior elements (Ligaments and facets), subjected to sagittal moments. Effects of ligament and facet (partial and total) removal and their orientations on segment response are examined from the viewpoint of stability. Results Ligaments play an important role in resisting flexion rotation and posterior shear whereas facets are mainly responsible for preventing large extension rotation and anterior displacement. Facet loads and stresses are high under large extension and anterior shear loading. Unlike total facetectomy, selective removal of facets does not compromise segmental stability. Facet loads are dependent on spatial orientation. Conclusions Rotational instability in flexion or posterior displacement [retrospondylolisthesis) is unlikely without prior damage of ligaments, whereas instability in extension rotation or forward displacement (spondylolisthesis) is unlikely before facet degeneration or removal. The facet stress and displacement distribution predicts that facet osteoerthritis or hypertrophy leading to spinal stenosis is most likely under flexion-anterior shear loading,Selective facetectomy may restore spinal canal size without compromising the stability of the segment. A facet that is more sagittally oriented may be linked to the cause of spondylolisthesis, whereas a less transversely oriented facet joint may be linked to rotational instabilities in extension.

Structural quality of parts processed by fused deposition

Commercial solid freeform fabrication (SFF) systems, which have been developed for fabrication of wax and polymer parts for form and fit and secondary applications, such as moulds for casting, etc., require further improvements for use in direct processing of structural ceramic and metal parts. Defects, both surface as well as internal, are undesirable in SFF processed ceramic and metal parts for structural and functional applications. Process improvements are needed before any SFF technique can successfully be commercialized for structural ceramic and metal processing. Describes process improvements made in new SFF techniques, called fused deposition of ceramics (FDC) and metals (FDMet), for fabrication of structural and functional ceramic and metal parts. They are based on an existing SFF technique, fused deposition modelling (FDM) and use commercial FDM systems. The current state of SFF technology and commercial FDM systems results in parts with several surface and internal defects which, if not eliminated, severely limit the structural properties of ceramic and metal parts thus produced. Describes systematically, in detail, the nature of these defects and their origins. Discusses several novel strategies for elimination of most of these defects. Shows how some of these strategies have successfully been implemented to result in ceramic parts with structural properties comparable to those obtained in conventionally processed ceramics.

Mechanical Properties of a Reversible, DNA-Crosslinked Polyacrylamide Hydrogel

Mechanical properties of a polyacrylamide gel with reversible DNA crosslinks are presented. In this system, three DNA strands replace traditional chemical crosslinkers. In contrast to thermoset chemically crosslinked polyacrylamide, the new hydrogel is thermoreversible; crosslink dissociation without the addition of heat is also feasible by introducing a specific removal DNA strand. This hydrogel is characterized by a critical crosslink concentration at which gelation occurs. Below the critical point, a characteristic temperature exists at which a transition in viscosity is observed. Both temperature-dependent viscosity and elastic modulus of the material are functions of crosslink density.

A portable dextrous master with force feedback

Dextrous masters control robots and artificial environments through hand gestures. Commercial products have open-loop control, without force feedback to the operator. There is a need for portable systems that have force feedback, but are still sufficiently compact to be desktop. In this paper we discuss a prototype master providing force feedback for the Dataglove. The master structure and its actuator characteristics are presented first. Then a control model is given based on finger parameters and joint coupling. The glove calibration is subsequently discussed, taking into account the influence of the feedback structure. The experimental setup and initial results are presented last.

Development of a prosthetic intervertebral disc.

This article is a preliminary report of a 10-year investigation of the development of an intervertebral disc prosthesis. Spinal fusion is a method for the treatment of chronic, disabling low-back pain that does not respond to nonoperative treatments. Spinal fusion, however, has various adverse effects, and the results of spinal fusion are often unpredictable. The goal of this research project was to develop disc prostheses that have mechanical properties very similar to those of natural, normal discs. Two types of disc prosthesis, one with fiber-reinforced polyurethane and the other with multicomponent, non-fiber-reinforced polymers (C-Flex), have been designed and manufactured. The fiber-reinforced disc was made of polyurethane end-plates with A100 hardness, a homogenous nucleus with A40, and 12 layers of multidirectional (0, +45°), fiber-reinforced anulus with A40 polyurethane. The design and modeling of the multicomponent polymers (non-fiber-reinforced) was made of C-Flex endplates with A90 hardness, a nucleus with A35 occupying 35% of the volume, and an anulus with 70A. Mechanical testing of these disc prostheses demonstrated similar mechanical properties to those of natural, normal discs.

Biomechanics of lumbosacral spinal fusion in combined compression-torsion loads.

The current study investigates the stabilizing effects of three different types of spinal fusion to the juxta-free motion segments and to the fused segment of the lumbosacral spine under combined compression-torsion loads. Sixteen fresh human cadaver lumbosacral spines were tested under a simulated physiologic loading condition. The relative movements of the motion segments, as well as the angular rotations and the center of rotation were then computed and analyzed. The average torsional stiffness of the unfused three-motion segment was found to be 2.35 nm/ degree. After fusion, the torsional stiffness did not increase significantly. Under the compression-torsional load, the anterior and bilateral-lateral fusions provided adequate stabilizing effect on the fused segment. The posterior fusion provided the least amount of stabilizing effect. These findings are similar to the results of the compression- bending experiment. Whereas the compressionbending loads produced significantly increased stress at the juxta-free segments, the compression-torsional loads did not produce any significant amount of increase in torsional stress at the juxta-free segments.

Quantitative assessment of back strength using isokinetic testing.

The overall objective of this research is to relate disorders of the lumbar spine to the mechanics of the system. An isokinetic-isometric testing procedure was designed, and groups of normal subjects and patients with back pain were tested. The procedure allows sensitive detection of muscle weakness specific to some part of the range of motion or some functional contraction speed. A biomechanical analysis was performed on several parameters of back strength assessment to develop performance indexes that can be used in establishing screening modalities. Maximum torque and trunk angles are different in normal and patient populations.

Neurite Outgrowth on a DNA Crosslinked Hydrogel with Tunable Stiffnesses

Mechanical cues arising from extracellular matrices greatly affect cellular properties, and hence, are of significance in designing biomaterials. In this study, a DNA crosslinked hydrogel was employed to examine cellular responses of spinal cord neurons to substrate compliances. Using DNA as crosslinkers in polymeric hydrogel formation has given rise to a new class of hydrogels with a number of attractive properties (e.g., reversible gelation and controlled crosslinking). Here, it was demonstrated that by varying length of crosslinker, monomer concentration, and level of crosslinking, DNA gel stiffnesses span from ∼100 Pa to 30 kPa. Assessment of neurite outgrowth on functionalized DNA gels showed that although primary dendrite length is not significantly affected, spinal cord neurons extend more primary dendrites and shorter axons on stiffer gels. Additionally, a greater proportion of neurons have more primary dendrites and shorter axons on stiffer gels. There is a pronounced reduction in focal adhesion kinase (FAK) when neurons are exposed to stiffer substrates, suggesting its involvement in neuronal mechanosensing and neuritogenesis in response to stiffness. These results demonstrate the importance of mechanical aspects of the cell–ECM interactions, and provide guidance for the design of mechanical properties of bio-scaffolds for neural tissue engineering applications.

A novel system for fused deposition of advanced multiple ceramics

In this article we present the system that we have developed at Rutgers University for the solid freeform fabrication of multiple ceramic actuators and sensors. With solid free form fabrication, a part is built layer by layer, with each layer composed of roads of material forming the boundary and the interior of the layer. With our system, up to four different types of materials can be deposited in a given layer with any geometry. This system is intended for fabrication of functional parts; therefore the accuracy and precision of the fabrication process are of extreme importance.