Biomedical subjects

N A Langrana

Publications and source records attributed to N A Langrana.

At least 19 recordsLinked to original sources

PubMed2002

Acute thoracolumbar burst fractures: a new view of loading mechanisms.

STUDY DESIGN: An in vitro investigation of loading mechanisms in acute thoracolumbar burst fractures. OBJECTIVES: To assess the validity of the authors' hypothesis that anterior shear forces transmitted by the facet joints are responsible for causing the severe canal compromise associated with acute thoracolumbar burst fractures. SUMMARY OF BACKGROUND DATA: Thoracolumbar burst fractures created in the laboratory rarely match the severity of clinical cases. To date, no studies have examined in great detail the role of facet joint loading in the burst-fracture mechanism. An incomplete understanding of loading mechanisms may contribute to the controversies regarding management. METHODS: Nine human cadaveric motion segments were instrumented with strain gages and subjected to axial compression or axial impact coupled with an extension moment. Failure loads, strain information, and radiographs were collected. RESULTS: Fracture patterns characteristic of acute thoracolumbar burst fractures were observed in the three specimens tested with an extension moment. In this group, high strains were also recorded at the bases of the pedicles, indicating a probable site of fracture initiation. Specimens tested in a neutral orientation experienced crush fractures without an increase in interpedicular distance. Strain patterns were more uniform in this group. CONCLUSIONS: The severity and clinical relevance of the injuries sustained by the specimens tested in extension suggest that facet joint loading plays a critical role in the acute thoracolumbar burst-fracture loading mechanism. Fracture patterns and strain concentrations are in agreement with clinical observations as well as past experimental studies.

Acute Disease↗

PubMed1999

Finite element analysis of vertebral body mechanics with a nonlinear microstructural model for the trabecular core.

In this study, a finite element model of a vertebral body was used to study the load-bearing role of the two components (shell and core) under compression. The model of the vertebral body has the characteristic kidney shape transverse cross section with concave lateral surfaces and flat superior and inferior surfaces. A nonlinear unit cell based foam model was used for the trabecular core, where nonlinearity was introduced as coupled elastoplastic beam behavior of individual trabeculae. The advantage of the foam model is that architecture and material properties are separated, thus facilitating studies of the effects of architecture on the apparent behavior. Age-related changes in the trabecular architecture were considered in order to address the effects of osteoporosis on the load-sharing behavior. Stiffness changes with age (architecture and porosity changes) for the trabecular bone model were shown to follow trends in published experimental results. Elastic analyses showed that the relative contribution of the shell to the load-bearing ability of the vertebra decreases with increasing age and lateral wall curvature. Elasto-plastic (non-linear) analyses showed that failure regions were concentrated in the upper posterior region of the vertebra in both the shell and core components. The ultimate load of the vertebral body model varied from 2800 N to 5600 N, depending on age (architecture and porosity of the trabecular core) and shell thickness. The model predictions lie within the range of experimental results. The results provide an understanding of the relative role of the core and shell in vertebral body mechanics and shed light on the yield and post-yield behavior of the vertebral body.

Aged↗

PubMed1998

Modeling of facet articulation as a nonlinear moving contact problem: sensitivity study on lumbar facet response.

A finite element (FE) based scheme for modeling facet articulation in a spinal motion segment is proposed. The algorithm presented models the facet articulation as a nonlinear progressive contact problem. This algorithm is used to perform a nonlinear FE analysis of a complete L3-L4 motion segment. The role of facets in load transmission through a motion segment and its sensitivity to facet geometric parameters (i.e., spatial orientation of the facets and the gap between the facet articular surfaces) on this load transmission are studied. Compression, flexion, extension, and torsion loads are used in this study. The effect of facetectomy on gross segment response and disk fiber strains is studied by comparing the response of FE models of motion segment with and without facets. Large facet loads are obtained when the motion segment is subjected to torsional and large extension rotations, whereas minimal facet loads are observed under compression and flexion loading. Removal of facets reduces the segment stiffness considerably in torsion and results in higher strain levels in disk fibers. The facet load transmission is sensitive to facet geometric parameters, i.e., spatial orientation and initial facet joint gap. The facet loads increase uniformly with decrease in initial gap between the facet articular surfaces under compression, extension, and torsional loads. The sensitivity to spatial orientation angles of the facet is, however, found to vary with the type of loading. This sensitivity may account for the wide variation in the facet response reported in literature.

Biomechanical Phenomena↗

PubMed1997

A biomechanical study of a cervical spine stabilization device: Roy-Camille plates.

STUDY DESIGN: Three-hole Roy-Camille posterior plates (Howmedica, Inc., Rutherford, NJ) were used to fix severely destabilized fresh cadaveric cervical spines. Fixed spine constructs were tested mechanically in flexion-extension and torsion, and the results were compared with the same characteristics in the intact spine before destabilization. Stainless steel and titanium plates and screws were evaluated. OBJECTIVES: To determine if the application of Roy-Camille posterior plates provided suitably strong and rigid fixation of a severe, surgically created three-column instability. SUMMARY OF BACKGROUND DATA: Controversy still remains regarding the exclusive use of posterior cervical plating in the face of three-column instability. Posterior plating has been evaluated biomechanically in severely destabilized calf spines; however, posterior plating of similarly destabilized human cadaveric cervical spines using the Roy-Camille system has not been examined. METHODS: The authors chose to test the main motions of the neck (flexion, extension, and torsion) in the intact and the plated state using a servohydraulic materials testing system. Testing the surgically altered spine before fixation proved to be futile because of drastic instability, which is characteristic of the chosen defect. Once fixed, the spines were tested, and the rigidity of the constructs were compared with that of the intact state. Strength and failure mechanisms were evaluated. RESULTS: The rigidity of the plated spine constructs surpassed that of the intact spines; the stainless steel and titanium systems were mechanically equivalent. Thus, application of the Roy-Camille plates of either type dramatically reduced the motion of the unstable spine. Strength of the fixed spine constructs was limited by screw pull-out at theoretically predictable levels of force. CONCLUSIONS: Posterior application of Roy-Camille plates can fix cervical spines with severe destabilizing defects rigidly. Screw pull-out of the most proximal or distal screw was always the mechanism of failure.

Biomechanical Phenomena↗

PubMed1995

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, retrospondylolisthesis, 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 osteoarthritis 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.

Biomechanical Phenomena↗

PubMed1995

Materials and design concepts for an intervertebral disc spacer. II. Multidurometer composite design.

The main function of the intervertebral disc is to transmit and attenuate compressive and torsional forces, and stabilize the intervertebral joint. Unfortunately, the disc may be displaced or damaged due to trauma or disease causing the nucleus to herniate and protrude into the vertebral canal or intervertebral foramen. Pressure on the spinal nerve may cause pain or paralysis in the area of its distribution. At present, the surgical procedures used to alleviate this condition include disc excision, and/or spinal fusion. A more desirable situation would involve removing the nucleus pulposus and part or all of the annulus fibrosis and implanting a suitable biofunctional equivalent. Such a prosthesis should attenuate stresses and prevent abnormal stress at adjacent intervertebral joints. Maintenance of normal disc height would prevent impingement of the posterior facet joints and facet joint syndrome. In a previous companion paper (J. Applied Biomat. 5:125-132; 1994), the mechanical behavior of disc prostheses manufactured from fiber reinforced, elastomeric thermoset resins were examined. In order to develop devices which were more practical from a manufacturing standpoint and extremely reproducible, the fiber reinforced thermoset resins were replaced by multi-durometer thermoplastic elastomeric materials. In the present paper, the mechanical properties of thermoplastic multicomponent designs have been investigated.

Biocompatible Materials↗

PubMed1995

The application of scanning acoustic microscopy in a bone remodeling study.

Scanning acoustic microscopy (SAM) was used in the evaluation of bone remodeling around a cylindrical unicortical defect. SAM is a technique for the nondestructive evaluation of materials, and has only recently been employed as an orthopaedic research tool. The utility of SAM was demonstrated by using it to measure an elastic property known as acoustic impedance. Specifically, the acoustic impedance of bone formed by remodeling around a cylindrical defect was measured. The defects were filled with either a low modulus "void" or rigid inclusion to create various states of stress in the bone in the vicinity of the defect. After six months of implantation of the inclusions in the sheep metatarsal, new bone formation on periosteal and endosteal surfaces about the defect region was observed. These regions of new bone were less stiff and had 18.0 +/- 6.5% lower acoustic impedance than the pre-existing bone in the intracortical region of the metatarsal. There was no difference in the degree of new bone formation about void and rigid inclusions. Both underwent significant adaptational changes in response to the elevated stress about the defect. These changes affected the basic structure of the bone cross-section at the level of the defect and effectively reduced the stress levels about the defect. By using SAM to measure acoustic impedance, it was seen that little internal remodeling occurred in the intracortical region. Hence, the primary mechanism of strain-induced bone remodeling observed in this experimental model was surface remodeling.

Acoustics↗

PubMed1994

Materials and design concepts for an intervertebral disc spacer. I. fiber-reinforced composite design.

The intervertebral disc is a complex joint anatomically and functionally. It may be displaced or damaged due to trauma or a disease process. To alleviate this condition, it may be necessary to remove the involved disc surgically and fuse the two adjacent vertebrae. Fusion is one option; however, replacing the damaged disc (or part thereof) with a suitable synthetic equivalent to allow near normal joint motion is more desirable. Unfortunately, the complex mechanical properties of the lumbar disc cannot be duplicated with homogeneous synthetic materials (polymers). To overcome this fundamental problem we have developed rational designs utilizing biocompatible thermoplastic elastomers of various stiffnesses (durometers) with and without fiber reinforcements. Our design consisted of three components analogous to the natural end plates, annulus, and nucleus. In this study only the fiber-reinforced design is considered. The variables examined in the present study included orientation of the fiber layers, number of fiber layers, and order of the reinforcing layers. The results of mechanical testing of the fiber reinforced disc spacer indicate that the range of compressive and torsional properties can be achieved. The results further demonstrate that properly developed, this design results in properties similar to the natural disc. Designs developed provided adequate compression and compression torsion properties for a synthetic spine disc spacer.

Biocompatible Materials↗

PubMed1994

Mechanical evaluation of a canine intervertebral disc spacer: in situ and in vivo studies.

An elastomeric intervertebral disc spacer with hydroxyapatite ingrowth surfaces was implanted in a canine model. We studied (a) the mechanical behavior of motion segments at time 0 and at 3, 6, and 12 months and (b) the effect of the interface between the spacer and vertebral bone on implant stability and bone ingrowth. A polymeric spacer was designed with compressive and torsional properties similar to those of the isolated canine lumbar disc. Implantation of the spacer in canine cadaver motion segments permitted in situ biomechanical evaluation at time 0. An in vivo study permitted continuous neurological monitoring of animals, with evaluation of mechanical behavior, stability, and ingrowth at 3, 6, and 12 months. Mechanical testing of cadaver motion segments with the spacer in situ resulted in decreased compressive and torsional stiffnesses, averaging 25 and 42%, respectively. This decrease was due to a combination of the surgical insult to the annulus and decortication of adjacent vertebral endplates. In the in vivo study, all 12 animals tolerated the surgery well and none had permanent neurological impairment. The measured parameters indicated that behavior of the spacer-motion segment composite appeared to return to normal within 3-6 months. However, despite use of a porous hydroxyapatite on the implant surface, there was no significant bone ingrowth. Instead, a layer of dense fibrous connective tissue was formed at the spacer-vertebral bone interface. Early migration of five of the 12 spacers resulted in eccentric loading patterns with consistent reactive osteophyte formation.

Animals↗

PubMed1994

Dynamic force feedback in a virtual knee palpation.

A virtual model of a knee joint with muscles, ligaments and bones has been developed. This model includes realistic 3-D surface deformation and tissue stiffnesses. Tissue and bone deformation (palpation) produces real time force feedback to the user hand wearing a DataGlove and Rutgers Master. Collision detection algorithms determine when and where the virtual hand palpates the surface model. This user interaction with the muscles and bones of a human knee model may be used as a training and planning tool for knee surgery.

Algorithms↗

PubMed1992

The mechanical properties of the canine lumbar disc and motion segment.

A study was initiated to measure the mechanical properties of the canine lumbar spine disc and motion segment at two specific levels. Compressive stiffness was determined to be 717.8 N/mm at L2-3 and 949.0 N/mm at L5-6. Torsional stiffness was found to be 1.04 Nm/deg at L2-3 and 1.72 Nm/deg at L5-6. These data were then compared to human lumbar spine disc and motion segment properties that have been reported in the literature. After normalizing for size differences, the canine lumbar disc showed a similar axial modulus (14.03 MPa for L2-3 and 16.30 MPa for L5-6) and a significantly higher torsional modulus (30.80 MPa for L2-3 and 26.17 MPa for L5-6) when compared to human values. The relative contributions of ligaments, posterior elements, and intervertebral disc to overall stability of the motion segment was found to be similar in canines and humans. As has been shown in human spine research, the posterior elements including the facet joints were found to be significant structures in providing torsional rigidity of the canine spine.

Animals↗

PubMed1992

Shear stability of an elastomeric disk spacer within an intervertebral joint: a parametric study.

A parametric study was conducted to compare the resistance to shear force provided by three surgical implantation techniques and three endplate designs for use with a polymeric lumbar intervertebral disk spacer. While under an axial load, the implant was pushed out laterally through the surgical window created for implantation. Force at 0.3 mm displacement, slope of the initial portion of the force-displacement graph, and maximum force were measured. The results indicate that implants with pegs, inclined planes, or domes on the surfaces of the disk will add significantly to the shear stability of the implant while maintaining the simplicity of a single part device.

Animals↗

PubMed1991

Finite-element modeling of the synthetic intervertebral disc.

An ideal and realistic finite-element analysis should include several factors, such as geometry (shape and size), the relative volumes of the nucleus to the anulus fibrosus, the fiber volume fraction within the anulus, and material properties. In this study, a finite-element analysis model for the disc that included these factors was developed. In addition, synthetic disc prostheses with the same specifications of geometry and material properties were manufactured. The developed model was found to be reliable and accurate when finite-element analysis results were compared with experimental synthetic disc data. The model also matched the mechanical behavior of the natural disc. This finite-element analysis model provides a tool to study the mechanical behavior of discs with varying degrees and types of injury and degeneration.

Biomechanical Phenomena↗

PubMed1991

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 degrees), 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.

Biocompatible Materials↗

PubMed1989

The biomechanical evaluation of a new fixation technique for spondylolysis using single and double tension-band wiring.

Patients with spondylolysis or spondylolisthesis with persistent symptoms are often marked for surgical treatment. This paper presents the results of a biomechanical study which evaluated the effects of two tension-band wiring methods for this clinical anomaly. Experimental spondylolytic defects were created in canine cadaver lumbar spines, and both wiring techniques were evaluated in cantilever bending. The results demonstrated that experimentally created spondylolytic defects produce a significant decrease in bending stiffness (flexion/extension), and that both the intra- and intersegmental wiring techniques increase the bending stiffness to that of the normal intact spinal segment.

Animals↗

PubMed1987

Contralateral compensation with knee impairment.

Knee motion of four healthy teenagers was unilaterally impaired by means of cast braces. Computerized analysis from video recording of walking was used to study the compensatory effects and to compare them with six patients. Restricted knee flexion caused little change in stance-phase knee motion on the restricted side. The unimpaired knee displayed exaggerated stance phase flexion and phase shifts, which in turn produced pelvic vaulting. The forces on the braces were high. Impairments to extension produced bilateral crouch without loss of flexion extension patterns within the limits of the impairment. Fatigue was more prominent than with blocks to flexion. Circumduction was found to be overrated as a compensation for stiff-leggedness. Lateral shift to the well side, combined with freezing of the well-side stance adduction, was a frequently used effective clearance mechanism. Phasic changes in motion of many body parts may combine to produce low-level pelvic displacement, especially when clinical weakness is present. Shortened stride length is the most sensitive indicator of this phenomenon. Graphs of individual joint motion do not easily convey the important phasic relationships that are fundamental to that motion and to the interpretation of its effects. Stick figures were better for analysis of this aspect of motion analysis, even though they are more subjective.

Adolescent↗

PubMed1986

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 compression-bending 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.

Biomechanical Phenomena↗

PubMed1985

Computer-assisted analysis of ligament constraints in the knee.

A computer graphics technique was employed to investigate the ligament constraints in the knee joint. A computer-assisted tomography scan system was employed to obtain the geometry of the knee and the ligament insertion locations. A cadaver knee was mounted in a special rig that kept the femur and the tibia in a specified position. The scans of the knee at 5-mm distances were recorded. These scans were projected in the computer to generate a data base for the tibia, the femur, and the tibial and femoral insertion locations for the various ligaments. The computer graphics analysis provides realistic views of the bone structures. The views compare favorably with the original CT scans. Interestingly, the scans near the ligament insertion site were clearly observed to have a higher bone density. Higher density was also observed at the intercondylar notch below the patella. The analysis was used to simulate ACL and PCL insufficiencies. A Lachman test at 25 degrees of flexion is a sensitive test for ACL insufficiency, whereas the 90 degrees drawer test may not be a sensitive indicator of PCL insufficiency. This computer graphics technique may prove useful in the design and development of artificial ligaments as well as in planning surgical procedures.

Biomechanical Phenomena↗