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RESEARCH PAPERS

J Biomech Eng. 1985;107(3):193-199. doi:10.1115/1.3138543.

Surface damage in polyethylene components for total joint replacement is associated with large contact stresses. An elasticity solution and finite element analyses were used to determine the influence of design parameters on the stresses due to contact in metal-backed components. For nearly conforming contact surfaces, it was found that the stresses in the plastic are very sensitive to clearance, that minimum plastic thickness of 4–6 mm should be maintained for metal-backed components, and that bonding the plastic to the metal backing reduces tensile stresses in the plastic at the edge of the contact zone.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):200-205. doi:10.1115/1.3138544.

The purpose of this study was to measure stresses associated with turbulence (Reynolds stresses), in the region of a 29-mm-dia porcine bioprosthetic valve (Hancock, Model 242). Studies were performed in an in vitro pulse duplicating system with the valve mounted in the aortic position. The Reynolds stresses were calculated from velocities obtained with a two channel laser Doppler anemometer. The largest Reynolds shear stress and normal stress occurred at the highest stroke volume used (80 mL). Averaged over ejection they were 38 dynes/cm2 and 380 dynes/cm2 , respectively. The maximal instantaneous Reynolds shear stress was 2500 dynes/cm2 and the maximal instantaneous Reynolds normal stress was 6800 dynes/cm2 . Stresses of these magnitudes are in the range reported to damage platelets.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):206-218. doi:10.1115/1.3138545.

The dominant mechanism giving rise to the viscoelastic response of articular cartilage during compression is the nonlinear diffusive interaction of the fluid and solid phases of the tissue as they flow relative to one another. The present study is concerned with the role of this interaction under uniaxial stress relaxation in compression. The model is a biphasic mixture of fluid and solid which incorporates the strain-dependent permeability found earlier from permeation experiments. When a ramp-displacement is imposed on the articular surface, simple, but accurate, asymptotic approximations are derived for the deformation and stress fields in the tissue for slow and moderately fast rates of compression. They are shown to agree very well with experiment and they provide a simple means for determining the material parameters. Moreover, they lead to important insights into the role of the flow-dependent viscoelastic nature of articular cartilage and other hydrated biological tissues.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):219-227. doi:10.1115/1.3138546.

Knowledge of tissue thermal transport properties is imperative for any therapeutic medical tool which employs the localized application of heat to perfused biological tissue. In this study, several techniques are proposed to measure local tissue thermal diffusion by heating with a focused ultrasound field. Transient as well as near steady-state heat inputs are discussed and examined for their suitability as a measurement technique for either tissue thermal diffusivity or perfusion rate. It is shown that steady-state methods are better suited for the measurement of perfusion; however the uncertainty in the perfusion measurement is directly related to knowledge of the tissue’s intrinsic thermal diffusivity. Results are presented for a transient thermal pulse technique for the measurement of the thermal diffusivity of perfused and nonperfused tissues, in vitro and in vivo. Measurements conducted in plexiglas, animal muscle, kidney and brain concur with tabulated values and show a scatter from 5–15 percent from the mean; measurements made in perfused muscle and brain compare well with the nonperfused values. An estimate of the error introduced by the effect of perfusion shows that except for highly perfused kidney tissue the effect of perfusion is less than the experimental scatter. This validation of the tissue heat transfer model will allow its eventual extension to the simultaneous measurement of local tissue thermal diffusivity and perfusion.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):228-233. doi:10.1115/1.3138547.

In cancer hyperthermia treatments, it is important to be able to predict complete tissue temperature fields from sampled temperatures taken at the limited number of locations allowed by clinical constraints. An initial attempt to do this automatically using unconstrained optimization techniques to minimize the differences between experimental temperatures and temperatures predicted from treatment simulations has been previously reported [1]. This paper reports on a comparative study which applies a range of different optimization techniques (relaxation, steepest descent, conjugate gradient, Gauss, Box-Kanemasu, and Modified Box-Kanemasu) to this problem. The results show that the Gauss method converges more rapidly than the others, and that it converges to the correct solution regardless of the initial guess for the unknown blood perfusion vector. A sensitivity study of the error space is also performed, and the relationships between the error space characteristics and the comparative speeds of the optimization techniques are discussed.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):234-239. doi:10.1115/1.3138548.

A numerical model is developed to predict zones of thermochemical tooth damage induced by laser radiation. Particular attention is devoted to pulp denaturation, enamel fracture and caries sterilization treatment dependence on laser energy and other beam parameters.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):240-248. doi:10.1115/1.3138549.

An in-vitro, steady flow investigation was conducted in a hollow, transparent vascular replica of the profunda femoris branch of man for a range of physiological flow conditions. The replica casting tested was obtained from a human cadaver and indicated some plaque formation along the main lumen and branch. The flow visualization observations and measured pressure distributions indicated the highly three-dimensional flow characteristics with arterial curvature and branching, and the important role of centrifugal effects in fluid transport mechanisms.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):249-256. doi:10.1115/1.3138550.

Based on a regular array of cubic unit cells, each containing a body-centered spherical void, we created an idealized three-dimensional model for both subchondral trabecular bone and a class of porous foams. By considering only face-to-face stacking of unit cells, the inherent symmetry was such that, except at the surface, the displacements and stresses within any one unit cell were representative of the entire porous structure. Using prescribed displacements the model was loaded in both uniaxial compressive strain and uniaxial shear strain. Based on the response to these loads, we found the tensor of elastic constants for an equivalent homogeneous elastic solid with cubic symmetry. We then compared the predicted modulus with our experimental values for bovine trabecular bone and literature values for an open-celled latex rubber foam.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):257-267. doi:10.1115/1.3138551.

An experimental investigation was carried out to acquire an understanding of local pressure changes and flow along the main lumen of arterial branch models similar to the femoral artery of man with three different branch angles (30, 60, and 90 deg) and side branch to the main lumen diameter ratio of 0.4. Effects of branch to main lumen flow rate ratios and physiological Reynolds numbers were found to be significant on the local pressure changes, while that of branch angle was also found to be important. The flow visualization study revealed that the flow separated in the main lumen near the branch junction when the pressure rise coefficient along the main lumen was above a critical value (i.e., 0.35 ∼ 0.46), which was observed to be a function of the Reynolds number. The critical value of the branch to main lumen flow rate ratio was found to be about 0.38 ∼ 0.44 also depending on the Reynolds number. Time averaged pressure distributions for pulsatile flow were similar in trend to steady flow values although they differed somewhat in detail in the main lumen in the branch region.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):268-273. doi:10.1115/1.3138552.

The hypothesis is made that a disturbance in blood flow at one place can be detected in the arterial pulse waves at a distant site. This hypothesis was motivated by the traditional Chinese medicine which uses arterial pulse waves as a principal means of diagnosis. We formulated a test by asking whether a disturbance to the blood flow in a leg can be detected by changes in the pulse waves in the radial arteries. In particular, we ask whether the radial artery can differentiate a disturbance in the right leg from that in the left leg. We put force transducers on the radial arteries, depressed them by a specific amount, and recorded the force waves in response to a 2-min occlusion of the blood flow in the right or left tibial artery. The results show that the radial artery force waves do change in response to the flow disturbance. For a given individual, the force varies with the location of the force transducer on the radial artery, the specific amount of initial depression, and the right or left leg occlusion. Generally, an occlusion in the right leg reduces the force level in both radial arteries, the more so in the right radial artery than in the left. Although the discrimination is not very strong, the phenomenon is novel, and warrants further investigation.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):274-280. doi:10.1115/1.3138553.

Recent applications of 20 MHz pulsed ultrasound Doppler velocimetry (PUDVM) in microsurgical research have necessarily employed piezoelectric crystals whose diameter is not negligible compared to the lumen size (1–2 mm) of many vessels of interest. A three-dimensional numerical model was developed to explore relationships between actual and detected flow field parameters, for (steady) Poiseuille flow, when appreciable velocity gradients exist within the PUDVM sample volume. Validation studies showed that highly accurate velocity profiles could be obtained in the limiting case of a very small sample volume (0.1 mm radius), but that for currently employed crystals ( ≈ 0.5 mm radius) there was appreciable underestimation of the centersteam velocity, and appreciable overestimation of the flow stream diameter. Errors in perceived velocity and flow rate were found to be relatively insensitive to perturbations in the sample volume thickness, in the size of the sampling range increment, or in the angle of insonation beam divergence. By contrast, these apparent flow parameters were found to be very sensitive to perturbations of sample volume diameter or of the Dopper angle. Small variations in the degree of partial sample volume overlap of the flowstream periphery were shown to be capable of causing large fluctuations in apparent flow stream diameter.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):281-285. doi:10.1115/1.3138554.

An upright, muscle-relaxed human spine, suffering from a mild functional scoliosis, caused by a small difference in leg length, is modeled as an anisotropic, elastic beam. The lower end of the beam is built-in in a fixed body, i.e., the laterally tilted pelvis. The upper end is rigidly attached to a rigid body, i.e., the supported upper part of the trunk, which is supposed to move freely in the frontal plane. It is shown that the characteristic scoliotic curvature of the spine, observed on an X-ray picture, can be reproduced by means of buckling analysis of the beam model, using realistic values of geometric and loading parameters and a properly chosen bending stiffness, which is found to be in reasonable agreement with earlier experimental findings. The analysis also shows that the muscle-relaxed upright equilibrium position of the spine is mechanically unstable.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1985;107(3):286-290. doi:10.1115/1.3138555.

The response of living tissue to surgical lasers was studied numerically. An algorithm computed the crater boundaries formed by a single laser pulse and the thermochemical damage around this crater. Heat conduction and beam attenuation by tissue vapors were found to be the major factor in the reduction of cutting efficiency.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J Biomech Eng. 1985;107(3):291. doi:10.1115/1.3138556.

It is theoretically shown that, in the high-stress limit, the exponentially stiffening tendon will have constant strain increment along its length, regardless of changes in cross-sectional area.

Topics: Stress , Tendons
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster

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