Showing papers by "Francis E. Kennedy published in 2006"

Elastic and Rupture Properties of Porcine Aortic Tissue Measured Using Inflation Testing

Abstract: A new inflation test device was developed to study the mechanical properties of aortic tissue. The device was used to measure failure (rupture) strength and to determine the nonlinear, anisotropic elastic properties of porcine thoracic aorta. The tester was designed to stretch initially flat, circular tissue specimens to rupture under uniform biaxial loading. Water was chosen as the pressurizing fluid. Mechanical stretch and radius of curvature during inflation were measured optically in two orthogonal directions, and the Cauchy stress components were calculated from the deformation and the applied pressure. All porcine samples that ruptured successfully did so via a tear in the circumferential direction. Thus, the failure strength was taken to be the stress in the axial direction immediately prior to rupture. The mean failure strength was 1.75 MPa and mean axial stretch at failure was 1.52. These values agree well with published data for other arterial tissues. The nonlinearly elastic deformation behavior was modeled using a hyperelastic constitutive law of the type proposed by Holzapfel et al. [Holzapfel GA, Gasser TC, Ogden RW. J Elasticity 2000;61:1-48]. The results showed that the dominant directions of anisotropy in the porcine aortas were approximately 45 degrees to the axial and circumferential directions, and that the isotropic contribution to the constitutive model was insignificant.

Data-Guided Brain Deformation Modeling: Evaluation of a 3-D Adjoint Inversion Method in Porcine Studies

Abstract: Biomechanical models of brain deformation are useful tools for estimating parenchymal shift that results during open cranial procedures. Intraoperative data is likely to improve model estimates, but incorporation of such data into the model is not trivial. This study tests the adjoint equations method (AEM) for data assimilation as a viable approach for integrating displacement data into a brain deformation model. AEM was applied to two porcine experiments. AEM-based estimates were compared both to measured displacement data [from computed tomography (CT) scans] and to model solutions obtained without the guidance of sparse data, which we term the best prior estimate (BPE). Additionally, the sensitivity of the AEM solution to inverse parameter selection was investigated. The results suggest that it is most important to estimate the size of the variance in the measurement error correctly, make the correlation length long and estimate displacement (over stress) boundary conditions. Application of AEM shows an average 33% improvement over BPE. This paper represents the first evidence of successful use of the AEM technique in three dimensions with experimental data validation. The guidelines established for selection of model parameters are starting points for further optimization of the method under clinical conditions

Magnetic resonance elastography of the plantar fat pads: Preliminary study in diabetic patients and asymptomatic volunteers.

Abstract: Objective To test the feasibility of applying the magnetic resonance elastography (MRE) technique to map the elastic modulus of the plantar fat pads in diabetic and nondiabetic subjects. Methods A prototype MRE imaging apparatus was used to produce quantitative maps of the heel fat pad in a pilot study of 12 volunteers and 4 patients with diabetes with neuropathy. Anatomic images corresponding to MRE maps allowed precise selection of regions of interest in the fat. Results Magnetic resonance elastograms of the heel fat pads were successfully created; mean measurements in the volunteers and the diabetic patients were 4.85 and 5.26 kPa, respectively. Conclusion It is feasible to perform MRE on the plantar fat pads and to produce elasticity maps. The trend toward stiffer fat pads, as demonstrated in patients with diabetes, suggests that the fat pads were qualitatively different. Magnetic resonance elastography offers great potential to investigate the mechanical properties of soft tissues in vivo noninvasively.

Lubrication and Wear of Artificial Knee Joint Materials in a Rolling/Sliding Tribotester

Abstract: This paper describes the influence of lubrication on wear during testing of materials for artificial knee joints in a rolling/sliding tribotester built to simulate contact conditions in a total knee replacement. The test configuration consists of parallel cylinders (pucks) of ultrahigh molecular weight polyethylene (UHMWPE) and polished cobalt-chrome alloy in oscillatory rolling/sliding contact in a bath of dilute (25%) bovine serum. Wear tests of three different UHMWPE materials were run under constant load at 40% sliding for 1.5 million oscillation cycles at 1.5 cycles/s. Wear of the UHMWPE was determined by measuring the profile of the cylindrical contact surface of the puck before and after each test. Profile measurements were repeated after at least 53 days to eliminate the contribution from creep. Differences between initial and final profiles were attributed to wear of the UHMWPE. It was found that the largest wear depth in the lubricated tests occurred near the ends of the oscillatory contact area, while dry (unlubricated) tests of the same materials showed a peak wear depth near the center of the contact area. In the lubricated tests, the worn depth was lowest for the most heavily irradiated material. Analysis of the elastohydrodynamic lubrication in the rolling/sliding contact was carried out assuming a line-contact situation with smooth cylindrical surfaces. The time-dependent modified Reynolds equation and the elasticity equation with initial conditions were solved numerically using a multigrid technique with full approximation scheme, and using a Newton Raphson method to solve the highly nonlinear system of equations. The thickness of the lubricating film of bovine serum was determined for points along the length of the wear track. It was found that the smallest film thickness (h min ) occurs very close to the location in the oscillating contact where the greatest wear occurs, owing to the very low entraining velocity near the ends of the oscillation cycle. The coefficient (K) for wear of the UHMWPE was found to be relatively constant over the central section of the oscillatory motion, but increased to a higher value where h min decreased to near zero. Thus, the important influence of lubrication on wear of artificial knee bearings was demonstrated.

In Vitro Study of Backside Wear Mechanisms on Mobile Knee-Bearing Components

Abstract: The long-term success of a total knee replacement depends on the wear performance of a polyethylene bearing that separates a metal femoral component from a metal tibial tray. Although fixed bearing designs secure the polyethylene bearing to the tibial tray. mobile bearing knees allow the polyethylene to move relative to the tibial tray. This study has evaluated the wear performance of an intended articulation on the inferior surface of the LCS@-Rotating Platform mobile bearing by conducting clinically relevant tribological testing and comparing results to retrieved knee bearings. A retrieval analysis leads to the conclusion that third-body particles in the contact produce curvilinear scratches longer than the expected rotation of the knee on both the polyethylene bearing and the CoCr tibial tray. Tribological testing shows that polymethylmethacrylate (PMMA) bone cement particles produce worn surfaces most similar to retrievals. Porous-coating beads and bone debris also have the ability to damage both surfaces. Worn polyethylene surfaces from pin-on-flat tests show scratches longer than the excursion length, and "skipping marks"-pits spaced at smaller rotation intervals along a scratch-as observed in retrievals. These wear features suggest that a ratcheting mechanism, which moves the third-body particles further along the scratch with each cycle, may be responsible for the observed wear.

MR elastographic methods for the evaluation of plantar fat pads: Preliminary comparison of the shear modulus for shearing deformation and compressive deformation in normal subjects

Abstract: MR elastography (MRE) images the intrinsic mechanical properties of soft tissues; e.g., the shear modulus, μ. The μ of the plantar soft tissues is important in understanding the mechanisms whereby the forces induced during normal motion produce ulcers that lead to amputation in diabetic feet. We compared the compliance of the heel fat pad to compressive forces and to shearing forces. The design of prosthetics to protect the foot depends on the proper understanding of the mechanisms inducing damage. In the heel fat pads of six normal subjects, between 25 and 65 years of age, the μ for deformation perpendicular to the direction of weight bearing is similar but not identical to that determined for deformation along the weight bearing axis. The average difference between μ along the weight bearing axis and μ perpendicular to the weight bearing axis, is well correlated with age (Correlation Coefficient = 0.789). The p-value for the data being random was 0.0347 indicating that the observed difference is not likely to be random. The p-value for control points is 0.8989, indicating a random process. The results are suggestive that the high compressive forces imposed during walking damage the heel fat pads over time resulting in softening to compression preferentially over shearing. It is important to validate the observed effect with larger numbers of subjects, and better controls including measures of activity, and to understand if diseases like diabetes increase the observed damage.

Optimized motion estimation for MRE data with reduced motion encodes

Abstract: Motion estimation is an essential processing step common to all Magnetic Resonance Elastography (MRE) methods. For dynamic techniques, the motion is obtained from a sinusoidal fit of the image phase at multiple, uniformly spaced relative phase offsets, φ , between the motion and the motion encoding gradients (MEGs). Generally, 4 to 8 uniformly spaced values of φ are used. We introduce a method, termed RME (reduced motion encodes), of reducing the number of relative phases required, thereby reducing the imaging time for an MRE acquisition. A frequency-domain algorithm was implemented using the Discrete Fourier Transform (DFT) to derive the general least-squares solution for the motion amplitude and phase given an arbitrary number of phase offsets. Simulation result shows that the noise level decreases as the number of φ increases. The decrease is largest when smaller numbers of φ are used and becomes less significant as the number increases. The minimum noise is obtained for a specific number, n, of φ when the phase is evenly distributed with interval π/n. Phantom studies show a similar trend with noise level. The resulting displacement images from different numbers of phase offsets are compared.