Biomedical subjects

Yener N Yeni

Publications and source records attributed to Yener N Yeni.

12 recordsLinked to original sources

PubMed2007

Biomechanical analysis of differing pedicle screw insertion angles.

BACKGROUND: Pedicle screw fixation to stabilize lumbar spinal fusion has become the gold standard for posterior stabilization. A significant percentage of surgical candidates are classified as obese or morbidly obese. For these patients, the depth of the incisions and soft tissue makes it extremely difficult to insert pedicle screws along the pedicle axis. As such, the pedicle screws can only be inserted in a much more sagittal axis. However, biomechanical stability of the angled screw insertion has been controversial. We hypothesized that the straight or parallel screw was a more stable construct compared to the angled or axially inserted screw when subjected to caudal cyclic loading. METHODS: We obtained 12 fresh frozen lumbar vertebrae from L3 to L5 from five cadavers. Schantz screws (6.0 mm) were inserted into each pedicle, one angled and along the axis of the pedicle and the other parallel to the spinous process. Fluoroscopic imaging was used to guide insertion. Each screw was then subjected to caudal cyclic loads of 50 N for 2000 cycles at 2 Hz. Analysis of initial damage, initial rate of damage, and total damage during cyclic loading was undertaken. FINDINGS: Average total fatigue damage for straight screws measured 0.398+/-0.38 mm, and 0.689+/-0.96 mm for angled screws. Statistical analysis for total fatigue damage ratio of angled to straight screws revealed that a significant stability was achieved in straight-screw construct (P<0.03). INTERPRETATION: This study showed that straight screw insertion results in a more stable pedicle-screw construct. The angled screw insertion technique resulted in more scattered values of damage indicating that the outcome from the angled screw fixation is less predictable. This validates the use of this technique to implant pedicle screws across the axis of the pedicle (parallel to the mid sagittal line) rather than along the axis, and has broad implications in instrumented posterior lumbar spinal surgery.

Adult↗

PubMed2006

Do sacrificial bonds affect the viscoelastic and fracture properties of bone?

Sacrificial bonds have been suggested as a toughening mechanism for bone tissue. Ionic bridges formed by divalent calcium ions between collagen molecules have been proposed as candidates for sacrificial bonds. If this mechanism is active at the macroscopic level, we should observe changes in mechanical properties of bone when calcium ions are maintained or removed from the tissue. To test this hypothesis, we measured viscoelastic and monotonic mechanical properties of cortical bone subjected to differing ionic environments. Storage modulus of bone could be changed up to 3.8% by the presence or absence of Na+ or Ca++ in the environment in a reversible fashion when bones were monitored continuously during treatments. A long-term one-time treatment increased the viscoelastic properties of bone soaked in Na+ solutions whereas the viscoelastic properties of bones soaked in Ca++ solutions were maintained. However, the strength and toughness of bone specimens soaked and fractured in treatment solutions were not improved. The presence of Ca++ affected the mechanical behavior of mineralized bone tissue at the macro scale. These effects were reversible, consistent with the original proposal. However, these effects may not necessarily indicate an increase in strength or toughness of the tissue at the macro scale.

Aged, 80 and over↗

PubMed2006

Evaluation of filler materials used for uniform load distribution at boundaries during structural biomechanical testing of whole vertebrae.

This study was designed to compare the compressive mechanical properties of filler materials, Wood's metal, dental stone, and polymethylmethacrylate (PMMA), which are widely used for performing structural testing of whole vertebrae. The effect of strain rate and specimen size on the mechanical properties of the filler materials was examined using standardized specimens and mechanical testing. Because Wood's metal can be reused after remelting, the effect of remelting on the mechanical properties was tested by comparing them before and after remelting. Finite element (FE) models were built to simulate the effect of filler material size and properties on the stiffness of vertebral body construct in compression. Modulus, yield strain, and yield strength were not different between batches (melt-remelt) of Wood's metal. Strain rate had no effect on the modulus of Wood's metal, however, Young's modulus decreased with increasing strain rate in dental stone whereas increased in PMMA. Both Wood's metal and dental stone were significantly stiffer than PMMA (12.7 +/- 1.8 GPa, 10.4 +/- 3.4 GPa, and 2.9 +/- 0.4 GPa, respectively). PMMA had greater yield strength than Wood's metal (62.9 +/- 8.7 MPa and 26.2 +/- 2.6 MPa). All materials exhibited size-dependent modulus values. The FE results indicated that filler materials, if not accounted for, could cause more than 9% variation in vertebral body stiffness. We conclude that Wood's metal is a superior moldable bonding material for biomechanical testing of whole bones, especially whole vertebrae, compared to the other candidate materials.

Biocompatible Materials↗

PubMed2005

Effect of microcomputed tomography voxel size on the finite element model accuracy for human cancellous bone.

The level of structural detail that can be acquired and incorporated in a finite element (FE) analysis might greatly influence the results of microcomputed tomography (microCT)-based FE simulations, especially when relatively large bones, such as whole vertebrae, are of concern. We evaluated the effect of scanning and reconstruction voxel size on the microCT-based FE analyses of human cancellous tissue samples for fixed- and free-end boundary conditions using different combinations of scan/reconstruction voxel size. We found that the bone volume fraction (BV/TV) did not differ considerably between images scanned at 21 and 50 microm and reconstructed at 21, 50, or 110 microm (-0.5% to 7.8% change from the 21/21 microm case). For the images scanned and reconstructed at 110 microm, however, there was a large increase in BV/TV compared to the 21/21 microm case (58.7%). Fixed-end boundary conditions resulted in 1.8% [coefficient of variation (COV)] to 14.6% (E) difference from the free-end case. Dependence of model output parameters on scanning and reconstruction voxel size was similar between free- and fixed-end simulations. Up to 26%, 30%, 17.8%, and 32.3% difference in modulus (E), and average (VMExp), standard deviation (VMSD) and coefficient of variation (COV) of von Mises stresses, respectively, was observed between the 21/21 microm case and other scan/reconstruction combinations within the same (free or fixed) simulation group. Observed differences were largely attributable to scanning resolution, although reconstruction resolution also contributed significantly at the largest voxel sizes. All 21/21 microm results (taken as the gold standard) could be predicted from the 21/50 (r2adj= 0.91-0.99;p<0.001), 21/110 (r2adj =0.58-0.99;p<0.02) and 50/50 results (r2adj=0.61-0.97;p<0.02). While BV/TV, VMSD, and VMExp/sigma(z) from the 21/21 could be predicted by those from the 50/110 (r2adj =0.63-0.93;p<0.02) and 110/110 (r2adj =0.41-0.77;p<0.05) simulations as well, prediction of E, VMExp, and COV became marginally significant (0.04<p<0.13) at 50/110 and nonsignificant at 110/110 (0.21<p<0.70). In conclusion, calculation of cancellous bone modulus, mean trabecular stress, and other parameters are subject to large errors at 110/110 microm voxel size. However, enough microstructural details for studying bone volume fraction, trabecular shear stress scatter, and trabecular shear stress amplification (VMExp/sigma(z)) can be resolved using a 21/110 microm, 50/110 microm, and 110/110 microm voxels for both free- and fixed-end constraints.

Absorptiometry, Photon↗

PubMed2004

Apparent viscoelastic anisotropy as measured from nondestructive oscillatory tests can reflect the presence of a flaw in cortical bone.

There is evidence that damage, viscoelastic stiffness properties, and postyield mechanical properties are related in bone tissue. Our objective was to test whether presence of a flaw would have an influence on the apparent viscoelastic properties of bone. Examining the effect of flaw orientation on apparent viscoelastic properties and utilization of dynamic mechanical analysis (DMA) as a nondestructive means for detection of damage were our secondary objectives. Cortical bone beams (2 x 2 x 19 mm) machined from the cranial cortex of the radii of six Warhill sheep were used. The specimens were placed in a DMA machine and baseline measurements of storage modulus (E1) and loss factor (tan delta), once for loads in the craniocaudal and once in the mediolateral directions, were performed using a three-point bending configuration for a frequency range of 1-10 Hz. Craniocaudal/mediolateral measurement ratio was calculated as a measure of anisotropy for tan delta and E1. After cutting a thin through-thickness macroscopic notch on the caudal surface at the center of each beam, oscillatory tests were repeated. Two-way repeated measures analysis of variance followed by Tukey's test was used with group (craniocaudal, mediolateral, notched craniocaudal, and notched mediolateral measurements) and frequency as factors. Regression analysis and analysis of covariance were used for examining the relationship between viscoelastic parameters and frequency. Tan delta and E1 were not different between craniocaudal and mediolateral measurements before the flaw was introduced (p > 0.8 and p = 1, respectively). In the presence of the flaw, tan delta was significantly increased (p < 0.003) whereas E1 was significantly reduced (p < 0.001) for craniocaudal measurements. Tan delta and E1 were nearly isotropic in the tested directions before the introduction of a flaw into the bone tissue. Introduction of a flaw resulted in increased tan delta and E1 anisotropy. Presence of a notch resulted in a significant increase in tan delta anisotropy with increasing frequency. In conclusion, we have demonstrated that cortical bone tissue exhibits a different apparent viscoelastic behavior in the presence of a flaw and depending on the flaw's orientation. Our finding that the presence of a notch and its orientation can be detected by nondestructive DMA suggests that in vivo techniques may be developed for detection of cortical bone damage.

Animals↗

PubMed2004

Effects of end boundary conditions and specimen geometry on the viscoelastic properties of cancellous bone measured by dynamic mechanical analysis.

The viscoelastic properties of cancellous bone can be measured nondestructively in compression testing using a dynamic mechanical analyzer. In this study, we examined the effects of end boundary conditions and specimen geometry on the viscoelastic properties of cancellous bone measured by dynamic mechanical analysis. During dynamic compression testing, the cancellous bone specimens may be mechanically fixed (e.g., glued) to the loading platens or they may be free to expand across the platen surface. When specimens of cancellous bone were tested between platens with gluing, the dependence of loss tangent on frequency was not consistent with previously observed strain-rate-dependent mechanical behavior of cancellous bone. When long specimens of cancellous bone (length = 10 mm, diameter = 8 mm) were tested without gluing, the relationship between loss tangent and frequency depended on the level of load applied. For short specimens (length = 5 mm, diameter = 8 mm) tested without gluing, however, the frequency dependence of loss tangent agreed with existing data reported for the strain-rate-dependent behavior of cancellous bone and also with the frequency dependence of cortical bone viscoelasticity. Therefore, we recommend that short cancellous bone cylinders with a length of 5 mm and a diameter of 8 mm should be used without gluing in the dynamic mechanical analysis of cancellous bone. This is consistent with the American Society for Testing and Materials testing recommendations for plastics, but different from current practice for unimodal mechanical testing of cancellous bone.

Animals↗

PubMed2004

The effect of microcomputed tomography scanning and reconstruction voxel size on the accuracy of stereological measurements in human cancellous bone.

Stereological parameters have been used as an approximation for the architecture of trabecular bone. Structural indices such as bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), bone surface-to-volume ratio (BS/BV), degree of anisotropy (MIL1/MIL3), and connectivity density (-Euler/Vol) have been widely studied to investigate pathological conditions in bone. Due to its high resolution and nondestructiveness, microcomputed tomography (micro-CT) has been utilized to take precise three-dimensional (3D) images of trabecular microstructures. However, spatial limitations for applying micro-CT-based analyses to large specimens, such as whole vertebral bodies, require using larger scanning and reconstruction voxel sizes. In this study, combinations of three different scanning and reconstruction voxel size were used to represent best possible voxel size (21 microm; best in our scanner for the specimen size used) relative to other voxel sizes used in this study, commonly used intermediate voxel sizes (50 microm), and those applicable to scans of whole human vertebral bodies (110 microm) in order to examine the effect of scanning and reconstruction voxel size on stereological measures for human cancellous bone. The error in stereological parameters calculated using combinations of large voxel sizes compared to the gold standard (best possible case) ranged from 0.1% to 102%. The signed magnitude of the error in other cases relative to the gold standard was a function of either scanning or reconstruction voxel size or both (r2=0.55-0.95). For most of the structural indices, the results from analysis of images with larger voxel sizes were correlated with those from the gold standard (r2=0.55-0.99) except for Tb.N at 110/110 microm, MIL1/MIL3 at larger than 110 microm reconstruction voxel size, and -Euler/Vol at any combination of voxel sizes. Overall, it was observed that resampling a high resolution image at lower resolutions (corresponding to increasing reconstruction voxel size in this study) had different effects on the calculated parameters than scanning at the same low resolution (corresponding to increasing scanning voxel size in this study). Our results show that investigations of image resolution should include actual scans at the resolution of interest rather than simply coarsening of high-resolution images as is customarily done.

Analysis of Variance↗

PubMed2004

Fracture mechanics of cortical bone tissue: a hierarchical perspective.

The performance of bone tissue in the presence of flaws is a highly remarkable one. Bone tissue is the outcome of an adaptive evolutionary process; thus, insight into the mechanisms by which it fails would provide valuable information not only for development of mechanically superior biomimetic materials but also for development of treatment modalities to prevent debilitating bone fractures. Clinically, fractures of skeletal organs occur as a result of aging, disease, overuse, and trauma. Fracture mechanics, a sub-discipline of solid mechanics that investigates the performance of cracked materials, has been employed extensively in characterizing the mechanisms by which bone tissue fractures. At present the fracture mechanisms at the macroscale are better characterized than at the microscale. On the other hand, a mechanistic understanding of damage evolution at the submicroscopic scale is largely limited to postulations with little experimental insight. The challenge of skeletal fragility will be dealt with more efficiently with deeper understanding of the fracture process at each hierarchical size scale. The most recent review on this subject matter was a decade ago, and there have been numerous developments in the fracture mechanics of bone since then. This review recaps the existing literature with an emphasis on the hierarchical nature of the fracture process in bone, entailing the supramolecular, microscopic, and macroscopic scales.

Animals↗

PubMed2004

The dependence between the strength and stiffness of cancellous and cortical bone tissue for tension and compression: extension of a unifying principle.

A strong positive correlation between the apparent ultimate strength and stiffness of bone tissue that can be expressed by a unified relationship has been observed for cortical bone in tension and low-density cancellous bone in compression. For practical purposes, the existence of a relationship between strength and stiffness is significant in that bone stiffness can be measured in vivo using non-invasive methods. It is generally accepted that bone strength is greater in compression than in tension whereas there is no substantial evidence that bone stiffness in compression is different from that in tension. This might suggest that compressive strength would relate to the stiffness, if at all, in a way that is different from tensile strength. In order to examine similarities and differences in the way strength is associated with stiffness between modes of loading and tissue type, we tested equine cortical bone and bovine cancellous bone in compression and examined these data together with previously reported data from compression testing of human cancellous bone as well as tensile testing of cortical bone from various sources. We have found for cortical bone that (i) the sensitivity of strength to stiffness is the same for tension and compression (p>0.75, ANCOVA), and (ii) the difference between the magnitudes of compressive and tensile strength for cortical bone is the result of an additive, rather than a multiplicative factor (52.1 MPa after adjusting to 1 microstrain/s, p<0.0001, ANOVA). High-density bovine tibial cancellous bone, on the other hand, has a steeper slope for its compressive strength-stiffness relationship than that for cortical bone and human cancellous bone, resulting in a transitional relationship between compressive strength and stiffness for a range of bone types and densities. Based on the current results and previous work, it is suggested that the offset strength in the compressive strength-stiffness relationship may be a direct manifestation of the difference between the compressive and tensile strengths of the bone material that constitutes the building blocks of the bone structure. Deviation of high-density cancellous bone compressive behavior from the other bone types and densities is attributed to stress distribution differences between the bone types.

Adaptation, Physiological↗

PubMed2003

A rate-dependent microcrack-bridging model that can explain the strain rate dependency of cortical bone apparent yield strength.

Although there are empirical correlations between strain rate, cortical and cancellous bone apparent stiffness, apparent yield strength, apparent ultimate strength and cortical bone fracture toughness, a mechanistic description for these phenomena is lacking. Microcracking is a major mechanism in cortical and cancellous bone failure, however, microdamage content alone cannot explain the strain rate dependence of bone strength without considering time-dependent behavior of the crack. Using a rate-dependent model of a fiber-bridged microcrack and data from the literature, we demonstrate that the experimental apparent yield strength of bone can be predicted directly from measurements of apparent moduli of elasticity of bone constituents and failure strain of the collagenous matrix. Yield strength predictions for estrogen depleted bone were made using the model and data from ovariectomized sheep. It was predicted that the yield strength of estrogen-deficient bone is comparable to that of normal bone within strain rates associated with physiological activities. For high strain rates, however, the strength of estrogen-depleted bone was predicted to be much weaker than normals suggesting a higher fracture risk due to impact from falls, for individuals with estrogen-depleted bones such as in post-menopausal osteoporosis.

Bone and Bones↗

PubMed2003

Trabecular shear stresses predict in vivo linear microcrack density but not diffuse damage in human vertebral cancellous bone.

Linear microcracks and diffuse damage (staining over a broad region) are two types of microscopic damage known to occur in vivo in human vertebral trabecular bone. These damage types might be associated with vertebral failure. Using microcomputed tomography and finite element analysis for specimens of cancellous bone, we estimated the stresses in the trabeculae of human vertebral tissue for inferosuperior loading. Microdamage was quantified histologically. The density of in vivo linear microcracks was, but the diffuse damage area was not, related to the estimates of von Mises stress distribution in the tissue. In vivo linear microcrack density increased with increasing coefficient of variation of the trabecular von Mises stress and with increasing average trabecular von Mises stress generated per superoinferior apparent axial stress. Nonlinear increase in linear crack density, similar to the increase of the coefficient of variation of trabecular shear stresses, with decreasing bone stiffness and bone volume fraction suggests that damage may accumulate rather rapidly in diseases associated with low bone density due to the dramatic increase of shear stresses in the tissue.

Adult↗

PubMed2002

Prestress due to dimensional changes caused by demineralization: a potential mechanism for microcracking in bone.

Microcracking in bone due to internal strains caused by mineralization is a possible mechanism of damage. Similar damage can be seen in other biological composites such as trees experiencing growth-related prestresses. Dimensional changes in cortical bone due to demineralization and experimental glycation were studied to test whether mineralization-related prestrains are consistent with observed microcracking patterns in bone. A microscopy technique that enables wet measurements of length and angle of milled bone specimens was used. Demineralization of bovine and human bones caused significant anisotropic changes in tissue size. Dimensional changes due to demineralization in bovine bone were prevented or reduced when collagen cross linking was increased by glycation. The dimensional changes of bone caused by demineralization are consistent with the hypothesis that mineralization-caused stresses in remodeling tissue can cause microcracks.

Animals↗