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Biomedical subjects

Raghu N Natarajan

Publications and source records attributed to Raghu N Natarajan.

7 recordsLinked to original sources
PubMed2006

Inclusion of regional poroelastic material properties better predicts biomechanical behavior of lumbar discs subjected to dynamic loading.

Understanding the relationship between repetitive lifting and the breakdown of disc tissue over several years of exposure is difficult to study in vivo and in vitro. The aim of this investigation was to develop a three-dimensional poroelastic finite element model of a lumbar motion segment that reflects the biological properties and behaviors of in vivo disc tissues including swelling pressure due to the proteoglycans and strain-dependent permeability and porosity. It was hypothesized that when modeling the annulus, prescribing tissue specific material properties will not be adequate for studying the in vivo loading and unloading behavior of the disc. Rather, regional variations of these properties, which are known to exist within the annulus, must also be included. Finite element predictions were compared to in vivo measurements published by Tyrrell et al. (1985) of percent change in total stature for two loading protocols, short-term creep loading and standing recovery and short-term cyclic loading with standing recovery. The model in which the regional variations of material properties in the annulus had been included provided an overall better prediction of the in vivo behavior as compared to the model in which the annulus properties were assumed to be homogenous. This model will now be used to study the relationship between repetitive lifting and disc degeneration.

Biomechanical Phenomena↗
PubMed2006

Novel model to analyze the effect of a large compressive follower pre-load on range of motions in a lumbar spine.

A 3-D finite element model (FEM) of the lumbar spine (L1-S1) was used to determine the effect of a large compressive follower pre-load on range of motions (ROM) in all three planes. The follower load modeled in the FEM produced minimal vertebral rotations in all the three planes. The model was validated by comparing the disc compression at all levels in the lumbar spine with the corresponding results obtained by compressing 10 cadevaric lumbar spines (L1-S1) using the follower load technique described by Patwardhan et al. [1999. A follower load increases the load-carrying capacity of the lumbar spine in compression. Spine 24(10), 1003-1009]. Further validation of the model was performed by comparing the lateral bending and torsion response without pre-load and the flexion-extension response without pre-load and with an 800 N follower pre-load with those obtained using cadaver lumbar spines. Following validation, the FEM was subjected to bending moments in all three planes with and without compressive follower pre-loads of up to 1200 N. Disc compression values and the flexion-extension range of motion under 800 N follower pre-load predicted by the FEM compared well with in vitro results. The current model showed that compressive follower pre-load decreased total as well as segmental ROM in flexion-extension by up to 18%, lateral bending by up to 42%, and torsion by up to 26%.

Adaptation, Physiological↗
PubMed2006

Modeling changes in intervertebral disc mechanics with degeneration.

Mechanical response of the spine to various dynamic loading conditions can be analyzed by way of in vitro and in vivo studies. Ethical concerns, interpretation of conclusions reached in animal studies, and lack of detailed stress distributions in the disc components are the major disadvantages of relying solely on in vivo studies. Intraspecimen variability, difficulty in including muscle activity, and inability to mimic fluid exchange into the disc during unloading are some of the disadvantages of in vitro models. The poroelastic finite element models can provide a method of understanding the relationship between biomechanical performance of the disc due to cyclic loading and disc degeneration. A poroelastic finite element model, including regional variation of strain-dependent permeability and osmotic pressure, was used to study the effect of disc degeneration on biomechanical properties as well as propagation of failure in the disc components when cyclic loading was applied to the lumbar disc. The results predicted that healthy discs were much more flexible than degenerated discs, and the disc stiffness decreased with increasing the number of load cycles independent of degenerative condition. Failure was found to progress as the drained elastic properties of the disc components decreased due to the presence of failure. Poroelastic finite element modeling, including strain-dependent permeability and osmotic pressure, is the most advanced analytical tool currently available that can be used to understand how cyclic loading affects the biomechanical characteristics of a degenerated lumbar disc. However, a complete understanding of behavior of the intervertebral disc will ultimately be achieved only with use of a combination of computational models together with in vitro and in vivo experimental methods. Finite element models of discs with varying degrees of disc degeneration will help clinicians understand the initiation and progression of disc failure and degeneration and will assist in the development of approaches to stimulate the regeneration of disc tissues.

Animals↗
PubMed2004

Recent advances in analytical modeling of lumbar disc degeneration.

STUDY DESIGN: Review of the most recent advances in the development of poro-elastic analytical models that include physiologic parameters used for understanding lumbar disc degeneration due to repetitive loading. OBJECTIVES: To discuss how poro-elastic finite element models that include physiologic parameters such as strain-dependent permeability and porosity and regional variation of poro-elastic material properties of a motion segment can be used to understand the effect of disc degeneration due to cyclic loading on the disc biomechanical properties. SUMMARY OF BACKGROUND DATA: Mechanical response of the spine to various dynamic loading conditions can be analyzed using in vitro and in vivo studies. Ethical concerns, interpretation of conclusions reached using animal studies, and lack of detailed stress distributions in the disc components are the major disadvantages in using in vivo studies for understanding disc degeneration process. Intraspecimen variability, noninclusion of muscle activity, and difficulty of influx of fluid into the disc during unloading are some of the disadvantages while using in vitro models to understand disc degeneration. The poro-elastic finite element models can provide a method that can circumvent the disadvantages mentioned above and allow a way to understand the relationship between biomechanical performance of the disc due to cyclic loading and disc degeneration. METHODS: Several types of finite element model were developed in understanding relationship between disc degeneration and associated changes in biomechanical properties. Simplest among them include material and geometric nonlinearity of the disc and was used to predict failure in the disc anulus and endplates under static loading conditions. Response of the lumbar disc under creep loading was studied using poro-elastic models. These models were further refined by including swelling pressure and variable permeability due to change in porosity under load to understand time-dependent deformation of a lumbar disc in a multiple creep compression expansion loading. Regional variation of strain dependent permeability and osmotic pressure was included to further refine the poro-elastic finite element model. This refined model was used to study the effect of disc degeneration on biomechanical properties when cyclic loading was applied to the lumbar disc. RESULTS: The refined model that included regional variation of strain-dependent permeability and osmotic pressure was validated by comparing diurnal change in total stature measured in vivo. The study showed that disc height loss was larger after considerable number of cyclic loadings both in normal and degenerated discs. Cyclic loading also showed that Grade I discs were much more flexible than Grade IV discs. The disc stiffness also decreased as the load cycle increased. CONCLUSIONS: A number of different approaches have been used to address the issue of disc degeneration. Poro-elastic finite element model including strain-dependent permeability and osmotic pressure is the most popular analytical tool currently available that can be used to understand how cyclic loading affects the biomechanical characteristics of a degenerated lumbar disc. However, it is important to note that a complete understanding of the behavior of the intervertebral disc will ultimately be arrived using a combination of analytical models, such as the models presented here, in addition to in vitro and in vivo experimental methods.

Animals↗
PubMed2003

Effects of slip severity and loading directions on the stability of isthmic spondylolisthesis: a finite element model study.

STUDY DESIGN: Using a validated finite element model, the biomechanical effects of pars defect in a lumbar segment with and without different degrees of slip (up to 50% slip) were studied. OBJECTIVES: To study the effects of slip severity and loading parameters on the stability of the lytic and adjacent motion segments. Better knowledge of the biomechanics of spondylolisthesis may help formulate treatment strategies such as bracing or spinal implants. SUMMARY OF THE BACKGROUND DATA: Clinically, spondylolisthesis exists in varying grades of anterior slip, and the biomechanical stability of the motion segments at the lytic defect and adjacent level probably varies as well. In vitro studies of L4-L5 and L5-S1 isthmic spondylolisthesis slips have concluded that an L4-L5 pars defect is more unstable than an L5-S1 pars defect. Comparing the stability of lytic motion segments with different grades of spondylolisthesis is difficult to do experimentally and therefore has not been done. Further assessing the stresses in the bone and intervertebral discs at or adjacent to a lytic defect is also difficult to study experimentally, so no data are available. METHODS: A finite element model of L4-S1 was validated with and without a pars defect at L5. The model was then revised to represent different degrees of slip at L5, and six different moment loadings were applied. RESULTS: The current study showed larger decrease in stiffness with increasing percent slip. The decrease in disc stiffness and increase in disc stresses with increasing percent slip were larger at the level of spondylolisthesis as compared to the changes in the adjacent segment. Lateral bending moment and torsion load showed the largest decrease in stiffness due to slip. At 50% slip, the maximum increase in motion (as compared to motion in an intact segment) was seen under lateral bending moment load (about 55% at L4/L5 and 250% at L5/S1). Lateral bending also produced the largest increase in stresses due to 50% slip in the anulus and endplates (300% increase in anular stress and 190% increase in endplate stress) at L5/S1. CONCLUSIONS: The stiffness of a spondylolisthetic motion segment decreases as the slip increases. Lateral bending and torsion are moment directions causing the greatest resulting motions.

Biomechanical Phenomena↗
PubMed2002

Coventry Award paper. Backsurface wear and deformation in polyethylene tibial inserts retrieved postmortem.

Wear and deformation were characterized at the backsurface of 25 posterior cruciate-retaining total knee arthroplasty polyethylene inserts retrieved postmortem from 20 subjects. The mean implantation time was 64.1 months (range, 4-156 months). The backsurface of the inserts was inspected using a stereomicroscope with a digital optical system. Coronal histologic sections of 13 proximal tibias were inspected for the presence and extent of penetration of granuloma. Damage to the backsurface was limited. Polishing was recorded on 21 (84%) of the inserts and abrasive wear on five (20%) inserts. Pitting was present in 21 (84%) components, but involved less than 1% of the area in all but one of these components. Delamination and cracking were not observed. Extrusions were seen in all 10 of the components that had screw holes in the tibial tray. A correlation was found between the depth of penetration of the granuloma along the posteromedial screw and the height of the corresponding extrusion. The anteroposterior profiles showed a concave deformation of the backsurface in 24 (96 %) of the cases. The concave deformation of tibial inserts may facilitate accumulation and transportation of wear debris to the tibial bone-implant interface through the screw holes in implants designed for cementless fixation.

Aged↗