Showing papers in "Annals of Biomedical Engineering in 1989"

Histologic and physiologic evaluation of electrically stimulated peripheral nerve: considerations for the selection of parameters.

Abstract: Helical electrodes were implanted around the left and right common peroneal nerves of cats. Three weeks after implantation one nerve was stimulated for 4–16 hours using charge-balanced, biphasic, constant current pulses. Compound action potentials (CAP) evoked by the stimulus were recorded from over the cauda equina before, during and after the stimulation. Light and electron microscopy evaluations were conducted at various times following the stimulation. The mere presence of the electrode invariably resulted in thickened epineurium and in some cases increased peripheral endoneurial connective tissue beneath the electrodes. Physiologic changes during stimulation included elevation of the electrical threshold of the large axons in the nerve. This was reversed within one week after stimulation at a frequency of 20 Hz, but often was not reversed following stimulation at 50–100 Hz. Continuous stimulation at 50 Hz for 8–16 hours at 400 μA or more resulted in neural damage characterized by endoneurial edema beginning within 48 hours after stimulation, and early axonal degeneration (EAD) of the large myelinated fibers, beginning by 1 week after stimulation. Neural damage due to electrical stimulation was decreased or abolished by reduction of the duration of stimulation, by stimulating at 20 Hz (vs. 50 Hz) or by use of an intermittent duty cycle. These results demonstrate that axons in peripheral nerves can be irreversely damaged by 8–16 hours of continuous stimulation at 50 Hz. However, the extent to which these axons may subsequently regenerate is uncertain. Therefore, protocols for functional electrical stimulation in human patients probably should be evaluated individually in animal studies.

An intrafascicular electrode for recording of action potentials in peripheral nerves

Abstract: We are developing a new type of bipolar recording electrode intended for implantation within individual fascicles of mammalian peripheral nerves. In the experiments reported here we used electrodes fabricated from 25 microns diameter Pt wire, 50 microns 90% Pt-10% Ir wire and 7 microns carbon fibers. The electrodes were implanted in the sciatic nerves of rats and in the ulnar nerves of cats. The signal-to-noise ratio of recorded activity induced by nonnoxious mechanical stimulation of the skin and joints was studied as a function of the type of electrode material used, the amount of insulation removed from the recording zone, and the longitudinal separation of the recording zones of bipolar electrode pairs. Both acute and short term (two day) chronic experiments were performed. The results indicate that a bipolar electrode made from Teflon-insulated, 25 microns diameter, 90% Pt-10% Ir wire, having a 1-2 mm long recording zone, can be used for recording of peripheral nerve activity when implanted with one wire inside the fascicle and the other lead level with the first lead, but outside the fascicle. No insulating cuff needs to be placed around the nerve trunk.

A compartmental model for oxygen transport in brain microcirculation

Abstract: A compartmental model is formulated for oxygen transport in the cerebrovascular bed of the brain. The model considers the arteriolar, capillary and venular vessels. The vascular bed is represented as a series of compartments on the basis of blood vessel diameter. The formulation takes into account such parameters as hematocrit, vascular diameter, blood viscosity, blood flow, metabolic rate, the nonlinear oxygen dissociation curve, arterial PO2, P50 (oxygen tension at 50% hemoglobin saturation with O2) and carbon monoxide concentration. The countercurrent diffusional exchange between paired arterioles and venules is incorporated into the model. The model predicts significant longitudinal PO2 gradients in the precapillary vessels. However, gradients of hemoglobin saturation with oxygen remain fairly small. The longitudinal PO2 gradients in the postcapillary vessels are found to be very small. The effect of the following variables on tissue PO2 is studied: blood flow, PO2 in the arterial blood, hematocrit, P50, concentration of carbon monoxide, metabolic rate, arterial diameter, and the number of perfused capillaries. The qualitative features of PO2 distrbution in the vascular network are not altered with moderate variation of these parameters. Finally, the various types of hypoxia, namely hypoxic, anemic and carbon monoxide hypoxia, are discussed in light of the above sensitivity analysis.

Electric properties of flowing blood and impedance cardiography

Abstract: An effective resistivity is defined for axisymmetric flow through a circular tube with a uniform electric field in the longitudinal direction. The resistivity of flowing blood is found to be a function of the shear rate profile. Under axisymmetric conditions shear rate profiles are a function of a single parameter: the reduced average velocity, which is the average velocity divided by the radius of the tube. The resistivity of human blood was investigated while the blood was in laminar flow in a circular tube with different constant flow rates. The relative change in resistivity in % is given by: −0.45·H·{1-exp[−0.26·(〈v〉/R)0.39]}; where H is the packed cell volume in % and 〈v〉/R is the reduced average velocity in s−1. In accelerating flow the resistivity change is synchronous with the change in flow rate, but in decelerating flow there is an exponential decay characterized by a relaxation time τ. For packed cell volumes of 36.4% and 47.5% τ was estimated to be 0.21 s, for a packed cell volume of 53.7% τ was estimated to be 0.29 s. The resistivity changes in elastic tubes are influenced by both velocity changes and changes in diameter, but in opposite directions.

Applications of fast orthogonal search: time-series analysis and resolution of signals in noise.

Abstract: In this paper a technique is examined for obtaining accurate and parsimonious sinusoidal series representations of biological time-series data, and for resolving sinusoidal signals in noise. The technique operates via a fast orthogonal search method discussed in the paper, and achieves economy of representation by finding the most significant sinusoidal frequencies first, in a least squares fit sense. Another reason for the parsimony in representation is that the identified sinusoidal series model is not restricted to frequencies which are commensurate or integral multiples of the fundamental frequency corresponding to the record length. Biological applications relate to spectral analysis of noisy time-series data such as EEG, ECG, EMG, EOG, and to speech analysis. Simulations are provided to demonstrate precise detection of component frequencies and weights in short data records, coping with missing or unequally spaced data, and recovery of signals heavily contaminated with noise. The technique is also shown to be capable of higher frequency resolution than is achievable by conventional Fourier series analysis.

The Krotkoff sound

Abstract: As the auscultatory method of blood pressure measurement relies fundamentally on the generation of the Korotkoff sound, identification of the responsible mechanisms has been of interest ever since the introduction of the method, around the turn of the century. In this article, a theory is proposed that identifies the cause of sound generation with the nonlinear properties of the pressure-flow relationship in, and of the volume compliance of the collapsible segment of brachial artery under the cuff. The rising portion of a normal incoming brachial pressure pulse is distorted due to these characteristics, and energy contained in the normal pulse is shifted to the audible range. The pressure transient produced is transmitted to the skin surface and stethoscope through deflection of the arterial wall. A mathematical model is formulated to represent the structures involved and to computer the Korotkoff sound. The model is able to predict quantitatively a range of features of the Korotkoff sound reported in the literature. Several earlier theories are summarized and evaluated.

An impedance cardiography system: a new design.

Abstract: An IBM compatible impedance cardiac output monitoring prototype system has been developed for use at the bedside on patients in the ICU, CCU, ER, Cath. Lab, and OR, etc. This impedance cardiography (ICG) system, whose operation is completely technician-free, provides a continuous display with digital results and four channel color waveforms on an Enhanced Graphics Display screen. The software is written in C language with several special segments in assembly code where speed is essential. In this prototype system, a real-time algorithm was introduced to modify the ensemble averaging technique so that it averages nonperiodic signals such as: ECG, dZ/dT, ΔZ, etc. Also, a real-time algorithm was developed to adaptively detect R spikes from conventional ECG signals. A signal preprocessor was developed to process signals digitally before any further work is done. This procedure reduces muscle noise, 60 Hz interference, and ventilatory movement. A special digital filter was designed to cope with the cases in which pacemakers are used. A special algorithm was also developed to further reduce the ventilation artifacts so that a period of apnea is unnecessary during the performance of the measurements. An anatomically specified electrode configuration has been defined enabling precise and reproducible positioning of the electrodes—hopefully leading to electrode standardization. At the present time, this prototype system has been compared with standard hand calculation and correlated with the clinical “gold standard,” the Swan-Ganz thermodilution cardiac output. Using 144 sets of data from 10 healthy volunteers, 4 critically ill patients, and 8 healthy exercising volunteers, calculations of cardiac output were made using our system and the standard hand calculation of stroke volume, based upon Kubicek's equation; there was a relatively high and stable correlation: r=0.93, p<0.005 (healthy); r=0.94, p<0.002 (ill); r=0.95, p<0.002 (exercise). From 20 patients at two different hospitals all with Swan-Ganz catheters in their hearts 65 correlation studies between our system and the standard thermodilution technique were performed; the results were encouraging in terms of accuracy and consistency (r1=0.84, p<0.01, n=10 CCU patients), and (r2=0.93, p<0.01, n=ICU, patients). These results along with a growing body of data from other investigators indicate that this noninvasive and technician-free system for measuring cardiac output could have a significant role in patient care.

Impedance-derived cardiac indices in supine and upright exercise.

Abstract: Impedance cardiography was used to determine the classical systolic time intervals (STI's) (i.e., pre-ejection period (PEP), left ventricular ejection time (LVET) and the quotient PEP/LVET), in young, healthy, male subjects during supine and seated exercise. With increasing exercise, there was a tendency toward decreases in PEP, LVET, and PEP/LVET. In the seated position, there was an increase in transthoracic Zo incident to the caudal migration of thoracic blood—a result of the postural change. With seated exercise, there were—in contrast to supine exercise—greater decreases in PEP/LVET and greater increases in the Heather index. Similarly, there was a tendency toward increases in dZ/dtmin and the Rapid Ejection Index. We suggest that these differences are related to increased myocardial contractility resulting from the postural augmentation of cardio-sympathetic activity, added to that of exercise per se. This study, as well as previous ones, indicates that impedance cardiography is reliable, effective, and more practicable than the arteriographic method for monitoring STI's. We also believe that certain impedance-derived indices (i.e., transthoracic Zo, dZ/dtmin and the Heather Index) have considerable potential value for physiologic and clinical investigation.

On the source of peak first time derivative (dZ/dt) during impedance cardiography.

Abstract: This paper deals primarily with the timing and source of peak (dZ/dt). In observations on human volunteers and anesthetized dogs, the peak (dZ/dt) occurred during the rapid ejection phase of the ventricles. Evidence is presented that the ascending aorta is the source of peak (dZ/dt).

New signal processing techniques for improved precision of noninvasive impedance cardiography.

Abstract: Impedance cardiographic determination of clinically important cardiac parameters such as systolic time intervals, stroke volume, and related cardiovascular parameters has not yet found adequate application in clinical practice, since its theoretical basis remains controversial, and the precision of beat-to-beat parameter estimation has until recently suffered under severe shortcomings of available signal processing techniques. High levels of noise and motion artifacts deteriorate signal quality and result in poor event detection. To improve the precision of impedance cardiography, new techniques for event detection and parameter estimation have been developed. Specifically, matched filtering and various signal segmentation and decomposition techniques have been tested on impedance signals with various levels of artificially superimposed noise and on actual recordings from subjects in a laboratory study of cardiovascular response to a cognitive challenge. Substantial improvement in the precision of impedance cardiography was obtained using the newly developed signal processing techniques. In addition, some preliminary evidence from comparisons of the impedance cardiogram with invasive aortic electromagnetic flow measurement in anesthetized rabbits is presented to address questions relating to the origin of the impedance signal.

Resistances and compliances of a compartmental model of the cerebrovascular system.

Abstract: A lumped parameter compartmental model for the nonsteady flow of the cerebrovascular fluid is constructed. The model assumes constant resistances that relate fluid flux to pressure gradients, and compliances between compartments that relate fluid accumulation to rate of pressure changes. Resistances are evaluated by using mean values of artery and cerebrospinal fluid (CSF) fluxes and mean compartmental pressures. Compliances are then evaluated from clinical data of simultaneous pulse wave recordings in the different compartments. Estimate of the average CSF compartmental deformation, based on the compliance between the CSF and brain tissue compartments, proves to be of the order of magnitude of actual experimental measurements.

Estimation of peripheral chemoreflex gain from spontaneous sigh responses

Abstract: A method is proposed for quantifying the responsiveness of the peripheral chemoreflex loop to CO2 by utilizing the natural fluctuations in ventilation and endtidal PCO2 which occur subsequent to the appearance of spontaneous sighs. The advantage of this method lies in its simplicity and noninvasiveness: the need for administering inhaled mixtures with high CO2 content is eliminated. Using autoregressive moving-average (ARMA) analysis, we demonstrate that post-sigh responses can be adequately described by a simple chemoreflex model that contains first order dynamics and a pure time delay. The effective gain of this model is shown to reflect peripheral chemosensitivity closely when the estimation procedure is applied to ‘data’ obtained from computer simulations of the respiratory control system. Although central chemosensitivity affects the absolute values of effective gain, the slope of the linear correlation between effective and peripheral gains remains unchanged. Application of the procedure to spontaneously breathing anesthetized dogs shows that, in every case, effective gain increased with the induction of hypoxia, which is known to enhance peripheral chemosensitivity.

Electric and dielectric properties of wet human cancellous bone as a function of frequency.

Abstract: In this study the electrical and dielectric properties of wet human cancellous bone from distal tibiae were examined as a function of frequency and direction. The resistance and capacitance of the cancellous bone specimens were measured at near 100% relative humidity. The measurements were made in all three orthogonal directions at discrete frequencies ranging from 120 Hz to 10 MHz using an LCR meter. At a frequency of 100 kHz, the mean resistivity and specific capacitance for the thirty cancellous bone specimens were 500 ohm-cm and 8.64 pF/cm in the longitudinal direction, 613 ohm-cm and 15.25 pF/cm in the anterior-posterior direction, and 609 ohm-cm and 14.64 pF/cm in the lateral-medial direction. All electrical and dielectric properties except the resistivity and the impedance were highly frequency dependent for the frequency range tested. All electrical and dielectric properties were transversely isotropic as the values for the longitudinal direction were different from values obtained for the two transverse directions and properties in the two transverse directions were approximately similar.

Simultaneous measurement of stroke volume by impedance cardiography and nuclear ventriculography: comparisons at rest and exercise.

Abstract: We have conducted four major impedance cardiography (ZCG) studies to provide data on validity, reproducibility, and sensitivity of response to interventions. The reference technique was quantitative nuclear ventriculography (NVG). Subjects were healthy young men in situations where minimally invasive and unobtrusive techniques were preferred. Interventions used included caffeine ingestion and exercise. Validity of ZCG estimates of stroke volume index (SVI, ml/m2) was tested in 35 men at rest. SVI was 49±9 by ZCG and 46±7 by NVG (r=0.82). Measurements of SVI during bicycle exercise showed no overall difference by the two methods (F=0.26, P=NS). Vascular resistance index (mean BP/CI) increased comparably by both techniques (+9.6% by ZCG and +9.7% by NVG) following caffeine (3.3 mg/kg). The reproducibility of ZCG was demonstrated in the day-to-day consistency of caffeine's effect on vascular resistance in 3 other studies (+11.9%, 12%, and 8.9%). Reliability across and within days was shown by repeated tests in the same subjects (SVI, r's=.96, .92). Conclusion: Impedance cardiography was shown to be a useful noninvasive technique for evaluation of cardiodynamics in biomedical research since it was highly reproducible and yielded equivalent results of relative changes produced by pharmacological and physical challenge. In addition, there was good agreement with NVG in absolute values for grouped data.

A continuous cardiac output computer based on thermodilution principles

Abstract: A totally self-contained instrument for the measurement of cardiac output is described. The microcomputer controlled instrument is based upon the principles of thermodilution and is capable of making cardiac output determinations on a minute by minute basis. A bolus of heat is delivered to the blood via a resistive heating element wound on the surface of a conventional thermodilution catheter, and the resulting transient pulmonary artery blood temperature increase is monitored with the thermistor located near the tip of the catheter. The performance of the instrument was tested in a mock circulatory loop and in dogs for periods of up to 13 hours. The accuracy and reproducibility of flow determinations made with the system compare favorably with those made with a conventional cardiac output monitor. This study demonstrates the feasibility of a stand-alone cardiac output computer that can provide virtually continuous measurements of blood flow without the intervention of a technician.

Factors affecting respiratory system stability.

Abstract: Periodic breathing (recurrent central apneas) occurs frequently during sleep. Periodic breathing can arise as a result of unstable behavior of the respiratory control system. A mathematical model of the respiratory control system was used to investigate, systematically, the effect of severity of disturbances to respiration and certain system parameters on periodic breathing occurring during sleep. The model consisted of multi-compartment representation of O2 and CO2 stores, a peripheral controller sensitive to O2 and CO2, and a central controller sensitive to CO2. The effects of hypoxia and hypercapnia on the upper airway muscles were not considered in the model. Episodes of hyperventilation or asphyxia were used to disturb the control system and explore the boundaries of stable breathing. Circulation time and metabolic rate were also varied. Simulations with the model produced the following findings: The number of central apneas associated with periodic breathing were greater as circulation time increased; controller gain increases also made the number of apneas greater, although periodic breathing occurs with lower controller gains as circulation time increases. At each level of circulation time there was a range of controller gain changes which caused little change in the number of apneas. There were more apneas with hypoxia; also the number of apneas increased with sleep-associated reductions in metabolic rate. The more rapidly resting PCO2 rose at sleep onset, the greater the likelihood of recurrent apneas. Finally, the more intense the disturbance, the more apneas there were.

A programmable system for acquisition and reduction of respiratory physiological data

Abstract: A portable software package (TIDAL—Tools for Intelligent Data Acqusition in the Laboratory) for acquiring and analyzing respiratory physiological data is described. TIDAL supports flow-, volume-, and concentration-measuring devices, and any other instruments that produce linear analog outputs. The system allows users to specify the names and types of channels to be sampled, and the calculations involved in reducing samples to breath-to-breath values. The specification of channels and calculations is given in EDL, and Experiment Description Language designed for respiratory physiology. EDL comprises a set of internal functions (primitives) which can be combined into arbitrarily complex expressions. To simplify EDL programming, TIDAL includes a macro processor and a standard macro library; the library contains definitions for a wide variety of respiratory variables. Examples are inspiratory and expiratory times and total volumes, mean inspired and expired gas volumes and concentrations, and end-tidal concentrations. Variables that are derived from these primary data, such as respiratory quotient, are also easily specified. TIDAL is written entirely in the C programming language, with special attention to portable coding practices. The code is organized in a modular structure that eases porting to multiple hardware/compiler/operating system environments.

A dynamic nonlinear lumped parameter model for skeletal muscle circulation.

Abstract: A dynamic nonlinear lumped parameter model of the circulation of skeletal muscle for constant vasoactive state is presented. This model consists of four compartments that represent the large arteries, the arterioles, the capillaries and venules, and the veins, respectively. The first compartment consists of a linear compliance (C1) and resistance (R1). The third compartment possesses no compliance and is represented by a linear resistance (R3). The second and fourth compartments each consist of a nonlinear pressure-volume relation, resulting in a pressure dependent compliance (C2, C4, respectively) and nonlinear resistance (R2, R4, respectively). The eleven model parameters were collected in a complementary way: they were partly obtained from a priori knowledge including, information at the microscopic level, and partly determined by means of an estimation algorithm. Estimated values of the compliances (in cm3·kPa−1·100 g−1, 1kPa=7.5 mmHg) and resistances (in kPa·s·cm−3·100 g) at an (arterial) inflow pressure of 10 kPa and a (venous) outflow pressure of 0 kPa were: C1: 0.014; R1: 6.6; C2: 0.565; R2: 84.6; R3: 37.9; C4: 1.044; R4: 24.5. The model (with the nonlinear pressure-volume relations) is able to predict the static and dynamic instantaneous (i.e., for constant vasomotor tone) pressure-flow relation and the instantaneous zero flow pressure intercept. These phenomena are therefore not necessarily the result of the rheological properties of blood. The secondary or delayed dilatation upon a positive inflow pressure step (or negative step in venous pressure) is predicted by the model implying that delayed dilatation is not necessarily related to changes in vasomotor tone. Venous outflow delay, upon a positive inflow pressure step (starting from zero flow), is also predicted by the model.

Comparison of Two Impedance Cardiographic Techniques for Measuring Cardiac Output

Abstract: The purpose of the present study was to compare cardiac outputs obtained by both the Kubicek (MIC) and Sramek (NCCOM3) impedance cardiographic techniques with thermodilution (TD) in critically ill patients. The two impedance techniques were also compared in normal subjects. Seven healthy subjects and ten patients in the intensive care unit were enlisted in the study. Only those subjects with successful measurements by all three methods were used in the data analysis. Three measurements of cardiac output were made in each subject. In patients, there were no significant differences in cardiac outputs as measured by TD (6.61/min), MIC (6.3 1/min), NCCOM3 (6.4 1/min). MIC and NCCOM3 cardiac outputs were correlated and approximated the line of identify when compared to TD. In normals, however, the NCCOM3 overestimated the cardiac output (NCCOM3, 9.2 1/min; MIC, 6.2 1/min). Because of these inconsistent results, caution is urged when interpreting the values obtained by the NCCOM3. In contrast, the use of the MIC in both populations has been reaffirmed.

Analysis of the longitudinal and radial resistivity measurements of the nerve trunk.

Abstract: Of the many models of peripheral nerve in the literature, essentially all have relied on experimentally derived values for the anisotropic resistivity of the nerve bundle interstitial space. This paper is a comprehensive mathematical analysis of the most referenced experiment for the longitudinal and radial resistivity measurements of the nerve trunk, namely that of Tasaki. For the analysis of the longitudinal measurements we introduce intracellular as well as interstitial current pathways (not considered by Tasaki). For the radial measurement the resistivity is expected to depend on the geometric packing of fibers within the trunk. Since Tasaki's paper did not include a histological examination of the nerve trunk these measurements are difficult to evaluate. However, our analysis indicates the importance of including the above factors, in addition to the epineurial (and perineurial) sheath as a resistive pathway. Our mathematical analysis supports the experimental measurements and confirms an assumed nerve trunk composition with theoretically derived values for the interstitial resistivities. It is therefore concluded that, at present, an appropriate procedure in determining resistivity values for use in modeling is to derive these values for an idealized nerve bundle based on the microscopic (electrolytic) resistivity of the interstitial medium.

Effect of tactile stimulation pulse characteristics on sensation threshold and power consumption.

Abstract: The psychophysical responses of human subjects to vibratory tactile stimulation of the skin were investigated experimentally. The parameters, of the waveform important to the minimization of power consumed by the tactile array of electromechanical vibrators and the maximization of the skin sensitivity to the stimulus were explored to develop optimum stimulation. Parameters investigated included the amplitude, frequency, and duty cycle of the current waveform used to drive the vibrators as well as the number of pulses per stimulating burst and the recovery time between bursts. Graphical techniques were used to determine, the optimal combination of the parameters which gave a stimulus that excited the skin to above tactile threshold while maintaining at a relative minimum the power required for the stimulus. The optimal stimulation waveform contains a burst of 10 rectangular pulses of 4% duty cycle separated by a period of nonstimulation of 2 s. Such a waveform can elicit a sensitivity of 29.4 mA−1 consuming only 55 μW of power.

An orthogonal ARMA identifier with automatic order estimation for biological modeling.

Abstract: In this paper an ARMA identification algorithm is developed for modeling biological time series data. The algorithm, is based on Gram-Schmidt orthogonalization of automatically selected basis functions from a specified function space. The selection criterion is based on recursive testing of potential benefit to the model of candidate functions. The candidate functions, AR and MA terms, are tested in a pairwise search direction until a least-squares criterion is satisfied, thereby estimating the order. Additive noise is considered and the basic algorithm extended to improve performance in noise. The algorithm is also extended to systems with inaccessible inputs (signal modeling). Modeling of biological data from speech is included, and indicates good performance. The algorithm is derived from earlier work on nonlinear systems identification.

Mechanics of a thin walled collapsible microtube.

Abstract: The purpose of this study is to measure the transmural pressure-cross sectional area relation of micro tubes (240 μm diameter) and to compare the measured perfusion pressure-flow relation with the pressure-flow relation calculated from the experimental pressure-cross sectional area relation. The microtubes are made by dipping a glass mould in a latex solution and glueing their outside ends to the inside of glass pipettes. The pressure-cross sectional area relation is determined both with a microplethysmograph (pressure-volume relation) and the microscope (pressure-diameter relations). Heparinized blood is used to include the rheological properties of blood as a perfusion medium. Static pressure-flow relations are obtained with a constant velocity piston pump for two values of external pressure (0 and 10 kPa) and with two downstream resistor settings (0 and 380 kPa cm−3 sec). The calculated pressure-flow relations using length and the experimental pressure-cross sectional area relation, Poiseuille's law, and accounting for the diameter-and shear-dependent viscosity compared well with the relations obtained from the experiments. It is also found that the pressure-flow relation shows an apparent zero flow pressure axis intercept (the extrapolation of the pressure-flow relation to the pressure axis), which can therefore be explained on the basis of the shape of the pressure-area relations.

Evaluation of effectiveness of a facet wiring technique: an in vitro biomechanical investigation.

Abstract: The effects of facet wiring procedure commonly used for stabilizing cervical spines after laminectomy or bilateral facet dislocation on the motion behavior of whole cervical spines are investigated using a Selspot II system. A fresh human ligamentous intact specimen was potted at T1/T2 vertebra and clinically relevant loads applied to the topmost vertebra (C2) of the specimen. The resulting three rotational components of each of the five vertebral bodies (C3-C7) were recorded. Specimen was injured to mimic total laminectomies at C5 and C6 vertebral levels and tested again. The injured specimen was stabilized, using a facet wiring construct, across C4-C7 segment before testing for the final time. The injured specimens, compared to the intact specimens, demonstrated an increase in flexion-extension of about 10%. Facet wiring imparted stability to the cervical spine by stiffening segments up to roughly four times intact values.

Factors which influence the accuracy of corrosion rate determination of implant materials

Abstract: This study has revealed several factors which influence the accuracy and interpretation of electrochemical test results with surgical implant materials. These include variation between corrosion rate determination methods, IR drop, diffusional effects, and inherent statistical variations. The AC impedance method (ACI) is particularly useful for studying electrochemical mechanisms, measuring IR drop, and separating stress-enhanced ion release effects into components related to current density and surface area changes. This method can also measure true surface area even of irregular objects and can detect changes in area caused by cracking or surface plastic deformation. The corrosion rate method which is most accurate, in an absolute sense, must be determined by chemical analysis of the electrolyte.

Advances in noninvasive bone measurement.

Abstract: Several noninvasive measurement methods are used for evaluation of metabolic disease. Single-photon (125I) scans of the peripheral skeleton are useful in some diseases but are ineffective in osteoporosis (even on the distal radius or os calcis) because they cannot predict spinal or femoral density. Also, peripheral measurements show high percentages of false negatives, that is many patients with fractures have normal peripheral density. Dual-photon (153Gd) scans of the spine, femur, and total skeleton are precise and accurate (2% error) and provide direct measurements of bone strength at fracture sites. This gives the best discrimination of abnormality and the most sensitive monitoring. Quantitative computed computed tomography (QCT) allows measurement of the spine but not the critical proximal femur area. QCT has a large accuracy error because (a) the limited are measured (under 5 cm3) fails to represent the total vertebral body, (b) technical errors and (c) variable fat and osteoid influence the results.

A parametric axisymmetric model study on the interface motions in porous-surfaced tibial implants

Abstract: The effects of a number of variables on the interface relative motions in poroussurfaced tibial implants are investigated using a simplified axisymmetric finite element model. The parameters considered are contact or link spacing, height of the central metal stem, presence of a circumferential metal flange, presence of a UHMWP articular plate resting freely on or fixed to the metal base, resting of the prosthesis edge on the cortical shell, and type of the metal alloy. In order to represent the immediate post-surgical situation with no bone ingrowth, the interface between the bone and porous-surfaced metal is modelled by frictionless rigid links oriented normal to the interface. Cases are also studied in which the horizontal interface is assumed to be fixed while the vertical interface remains frictionless. The magnitudes of the interface motion are negligibly affected by the variation in the link spacing from 0.3 mm to about 3.0 mm. The interface relative motion is predicted to decrease in cases with a shorter central metal stem, with the addition of a circumferential metal flange, with the use of more rigid prosthesis, and with the addition of a UHMWP articular plate.

Structural stiffness of the hoffmann simple anterior tibial external fixation frame

Abstract: Tibial external fixation frames were constructed on aluminum tube simulating tibia bone. A 20-mm gap was left at the fracture site in order to measure the structural stiffness of the frame rather than the aluminum tube. The performance of the frames were experimentally evaluated and quantified using tests which simulated the loading conditions encountered in normal walking. These included axial compression, anteroposterior (AP) bending, lateral bending and torsional loading of the frame. The parameters studied were (a) number of fixation pins, (b) number of connecting rods and (c) location of clamps on the pins. Four constants were evaluated from these tests using various structural configurations of the frames; these resulted in four stiffness coefficients in compression, AP bending, lateral bending and torsion. Stiffnesses of various frames with different geometric configurations were compared by comparing their appropriate stiffness coefficients.

Analysis of human torso motion with muscle actuators.

Abstract: A biomechanical model is devised to analyze the motion of the human torso and estimate the load on the lumbar spine, the contraction forces in the trunk muscles, and the sensory signals between the muscle and the nervous system. A state space formulation of three-dimensional (3D) equations of motion of the human torso is presented with muscle forces as input to the system. At least 3 pairs of skeletal muscles are considered to be necessary for 3D motion of human torso. Functional anatomy of these major muscles of the human trunk are discussed. These muscles as well as their feedback and feedforward sensory paths are modeled by linear viscoelastic components and force generators. Stability of the torso with three pairs of muscles is studied both with muscle spindles inactive (open loop) and active (closed loop). Voluntary point-to-point motion of the torso in three-dimensional space is simulated on a digital computer, employing a dynamic controller, where feedback gains are tuned (programmed) by higher centers of nervous system.