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RESEARCH PAPERS

J Biomech Eng. 1977;99(4):173-177. doi:10.1115/1.3426286.

Experiments were conducted in a steady flow test apparatus with an axisymmetric aortic shaped test chamber using hot-wire anemometry to obtain local momentum transfer and turbulence data in the vicinity of the model disk, Kay-Shiley disk, Starr-Edwards ball and Bjork-Shiley tilting disk prosthetic heart valve configurations. These data for Reynolds numbers of 2000, 4000, and 6000 were used to locate regions of high stress where erythrocytes may be lysed or severely strained. Nondimensionalized velocity profiles downstream of the valves indicated a similarity of behavior for all Reynolds numbers at an axial location. Velocity gradients of magnitudes potentially damaging to erythrocytes were found. These gradients were largest for disk-type occluders. Large regions of separated flow were found to occur behind the valve sewing ring, distal to the valve occluder, and along the test chamber wall distal to the valve for all valves tested. Relatively high turbulence was found to exist distal to all valves tested. Turbulent stresses of magnitudes potentially hemolytic were measured.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1977;99(4):178-183. doi:10.1115/1.3426287.

Computer model results are reported for the diastolic behavior of blood flow and muscle stress in the left ventricle of the heart. Comparisons are made between results for a rigid-leaflet mitral valve and a caged-ball prosthetic valve. As expected, the model ball valve produces larger pressure drops, a higher wall shear, and slower opening times. In addition, the ball valve creates an abnormal distension of the heart wall. This may affect the long-term behavior of the muscle, particularly during systolic contraction.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1977;99(4):184-188. doi:10.1115/1.3426288.

A mathematical model is developed for calculating the pressures and flows in an artificial heart, its pneumatic drive unit, and a mock circulatory system. The system is divided into convenient subsystems to facilitate the analysis, and each subsystem is then analyzed separately. The set of independent equations developed is solved on a computer and corresponding experimental tests are made on the actual system. A comparison of the experimental and computer results shows good agreement for the mean flow rate through the pump and also for several instantaneous pressures and flow rates in the system.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1977;99(4):189-194. doi:10.1115/1.3426289.

A detailed study is reported of the potential thermal stress effects and surface cracking of human teeth after absorption of laser energy. This study was motivated by the desire to define damage thresholds if lasers are used for preventive dentistry techniques. A large group of extracted teeth was exposed to manually pulsed bursts of energy of varying durations from a CW CO2 laser. The teeth were examined photographically under magnification before and after irradiation, using fluorescent dye to facilitate observation of cracks in the tooth surface. In an attempt to understand the cracking phenomena, predictions of the temperatures and thermal stresses were made. The tooth surface in the vicinity of the focused beam impingement was assumed to behave as a semi-infinite solid for the short periods of time considered. Estimated stresses where cracking occurred are compared in the paper to measured values of the ultimate strength of tooth enamel. Results are shown to be in reasonable agreement with predictions. Based on this work, a criterion is given for minimizing surface damage to the tooth.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1977;99(4):195-202. doi:10.1115/1.3426290.

A technique for post surgical rewarming from clinical hypothermia, which utilizes a thoracic flush with heated saline solution, has been investigated experimentally and theoretically. The rewarming technique was used in conjunction with surface cooling for eighteen separate experiments using mongrel dogs, with the thermal response of the animals determined from thermocouple measurements at twelve strategic locations. Utilizing these measurements, temperature histories were established for the animals, from which representative cooling and rewarming rates have been determined. The rectal temperature was found to provide a suitable clinical reference temperature for monitoring the level of hypothermia, and the rewarming rates (from 10 to 22° C/hr) compare favorably with those obtained by other investigators using surface and extracorporeal rewarming methods. Reasonable agreement was obtained between the measured temperature histories and theoretical predictions obtained using a simple lumped parameter heat transfer model.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1977;99(4):203-208. doi:10.1115/1.3426291.

Uniaxial measurement of stress as a function of strain were performed on strips of sclera and choroid from human eyes. The scleral strips indicated an exponential stress-strain behavior, while the choroidal strips exhibited a power law behavior. Linear elastic models are used to deduce the physiologic range of interest and the relation of the stress-strain behavior to scleral rigidity. Stress changes in the choroid during accommodation are discussed.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1977;99(4):209-214. doi:10.1115/1.3426292.

The vitreous body is a transparent gel occupying the posterior segment of the ocular globe. In diseased states, vision is obscured by haemorrhage or opacity or may be lost because of retinal detachment. The design of instruments for pars plana vitrectomy is based on the therapy of the disease. Conclusions drawn from human and mechanical factor analysis have been grouped into four categories: (a) instrument design parameters; (b) surgical procedure; (c) intraocular illumination; and (d) indications and results. Possible extensions with fibre optic illumination of the infusion system are indicated.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1977;99(4):215-221. doi:10.1115/1.3426293.

The level walking process for an above knee (A/K ) amputee with a conventional prosthesis greatly compromises the amputee’s mobility. The fact that conventional prostheses lock in hyperextension during the stance phase, in contrast to the extend-flexextend pattern during stance for the natural limb, has been suggested as a source of the amputee’s uncosmetic gait and high energy expenditure. While the lock in hyperextension during stance provides stability to prevent buckling, it requires the person to vault over the prosthetic limb. This vaulting during level walking may cause higher vertical displacements of the body center of gravity (c.g.) and accompanying higher energy requirements for the amputee. This investigation employs an amputee-interactive prosthesis simulator system to evaluate the viability of controlling the prosthetic knee joint to follow a normal knee position pattern. In order to insure that the amputee-interactive prosthesis simulator system does not introduce gait anomalies, the system was controlled to simulate a conventional prosthesis. This showed that the simulator system has no undesired side effects since data from walking trials with the simulator system in “conventional prosthesis mode” are very similar to data from conventional prostheses in the literature. Then, an active position control scheme which controls the prosthetic knee joint to follow a normal knee position pattern was tested by two young, active amputees in level walking trials. The subjects experienced very little difficulty in walking with the active control scheme and preferred the simulator with the active control scheme to their conventional prostheses. Measured knee power requirements for the scheme indicate that this type of control is feasible without external power sources. However, measurements of the vertical displacement of the body c.g. show little difference between gait with the active control scheme and gait with a conventional prosthesis. It appears that the increased energy requirements for A/K amputees are not due in total to the lack of the extend-flex-extend position profile at the prosthetic knee joint.

Commentary by Dr. Valentin Fuster

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