Biomedical subjects

Do-Gyoon Kim

Publications and source records attributed to Do-Gyoon Kim.

8 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

Fatigue debonding of the roughened stem-cement interface: effects of surface roughness and stem heating conditions.

The aim of this study was to determine the effects of cyclic loading on the debond process of a roughened stem-cement interface used in total hip arthroplasty. The specific goals were to assess the effects of two surgeon-controlled variables (stem heating and degree of stem surface roughness) and to determine if an independent finite element-based fracture mechanics model could be used to predict the debond response. A clamped cantilever beam geometry was used to determine the fatigue debond response of the stem-cement interface and was created using an experimental mold that simulated in vivo cementing conditions. A second experiment was performed using a torsion-loading model representative of the stem-cement-bone composite. For both experiments, two stem heating (room temperature and 50 degrees C) and surface roughness conditions (grit blasted: Ra = 2.3 and 5.1 microm) were used. Finally, a finite element model of the torsion experiment with provision for crack growth was developed and compared with the experimental results. Results from both experiments revealed that neither stem preheating nor use of a stem with a greater surface roughness had a marked effect on the fatigue debond response. There was substantial variability in the debond response for all cases; this may be due to microscopic gaps at the interface for all interface conditions. The debond rate from the finite element simulation (10(-7.31) m/cycle) had a magnitude similar to the experimental torsion model (10(-(6.77 +/- 1.25)) m/cycle). This suggests that within the context of the experimental conditions studied here that the debond response could be assessed using a linear elastic fracture mechanics-type approach.

Biocompatible Materials↗

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

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

A fatigue damage model for the cement-bone interface.

Loss of fixation at the cement-bone interface can contribute to clinical loosening of cemented total hip replacements. In this study, the fatigue damage response was determined for cement-bone constructs subjected to shear fatigue loading. A typical three-phase fatigue response was observed with substantial early damage, followed by a long constant damage rate region and a final abrupt increase in damage to fracture. All of the damage resulted from creep (permanent) deformation during fatigue loading and there was no loss in cyclic stiffness. Using a Von Mises equivalent stress/strain concept, a general damage model was developed to describe the fatigue creep response of the cement-bone interface under either shear or tensile fatigue loading. Time to failure was highly correlated (r2=0.971) with equivalent creep strain rate and moderately related (r2=0.428) with equivalent initial strain for the two loading regimes. The equivalent creep strain at failure (0.052+/-0.018) was found to be independent of the applied equivalent stress. A combination of the creep damage model (to describe the damage process) with a constant final equivalent strain (as a failure criteria) could be used to assess the cement-bone failure response of cemented implant systems.

Bone Cements↗

PubMed2004

Creep dominates tensile fatigue damage of the cement-bone interface.

Fatigue damage from activities of daily living has been considered to be a major cause of aseptic loosening in cemented total hip arthroplasty. The cement-bone interface is one region where loosening could occur, but to date the fatigue response of the interface has not been examined. Cement-bone specimens were prepared from fresh frozen human cadaver tissue using simulated in vivo conditions. Tensile fatigue tests to failure were performed in an environmental chamber. Loss of specimen stiffness (stiffness damage) and permanent displacement after unloading (creep damage) were found in all specimens. At failure, creep damage accounted for the majority (79.9+/-10.6%) of the total strain damage accumulation at failure (apparent strain, epsilon=0.0114+/-0.00488). A power law relationship between strain-damage rate and time-to-failure showed that the strain-damage rate was an excellent predictor of the fatigue life of the cement-bone interface. The S-N response of the interface was obtained as a function of the applied stress ratio and the initial apparent strain. The total motion between cement and bone (72.2+/-29.8 microm) prior to incipient failure due to both stiffness and creep fatigue damage may be sufficient to result in fibrous tissue formation and contribute to eventual clinical loosening.

Aged↗