Biomedical subjects

T E Milner

Publications and source records attributed to T E Milner.

At least 19 recordsLinked to original sources

PubMed2001

The central nervous system stabilizes unstable dynamics by learning optimal impedance.

To manipulate objects or to use tools we must compensate for any forces arising from interaction with the physical environment. Recent studies indicate that this compensation is achieved by learning an internal model of the dynamics, that is, a neural representation of the relation between motor command and movement. In these studies interaction with the physical environment was stable, but many common tasks are intrinsically unstable. For example, keeping a screwdriver in the slot of a screw is unstable because excessive force parallel to the slot can cause the screwdriver to slip and because misdirected force can cause loss of contact between the screwdriver and the screw. Stability may be dependent on the control of mechanical impedance in the human arm because mechanical impedance can generate forces which resist destabilizing motion. Here we examined arm movements in an unstable dynamic environment created by a robotic interface. Our results show that humans learn to stabilize unstable dynamics using the skillful and energy-efficient strategy of selective control of impedance geometry.

Adaptation, Physiological↗

PubMed2001

Laser-mediated cartilage reshaping with feedback-controlled cryogen spray cooling: biophysical properties and viability.

BACKGROUND AND OBJECTIVE: Recent studies have indicated that chondrocyte viability decreases with prolonged or repeated laser irradiation. To optimize laser-mediated cartilage reshaping, the heating process must be finely controlled. In this study, we use high-power Nd:YAG laser irradiation (lambda = 1.32 microm) combined with cryogen spray cooling (CSC) in an attempt to reshape porcine septal cartilage while enhancing chondrocyte viability. STUDY DESIGN/MATERIALS AND METHODS: Chondrocyte viability was determined after high-power (50 W/cm2) Nd:YAG-mediated cartilage reshaping with and without cryogen spray cooling (CSC) and correlated with dynamic measurements of tissue optical and thermal properties. RESULTS: After 1.5 to 2.0 seconds of laser exposure, characteristic changes in diffuse reflectance (indicating the onset of accelerated stress relaxation) was observed in both laser only and laser with CSC specimens. After 2 seconds of laser exposure, specimens in both groups retained the curved shape for up to 14 days. After one laser exposure, chondrocyte viability was 94.35 +/- 6.1% with CSC and 68.77 +/- 20.1% (P < 0.05) without CSC. After two laser exposures, a similar trend was observed with CSC (70.18 +/- 16.44%) opposed to without CSC (28 +/- 45%; P < 0.05). CONCLUSION: CSC during high-power laser irradiation allows rapid heating while minimizing extreme front surface temperature elevations and axial thermal gradients. Laser irradiation with CSC can be used to effectively reshape cartilage tissue with the additional advantage of increasing chondrocyte viability.

Aerosol Propellants↗

PubMed2001

Theramal, mechanical, optical, and morphologic changes in bovine nucleus pulposus induced by Nd:YAG (lambda = 1.32 microm) laser irradiation.

UNLABELLED: BACKGROUND AND OBJECTIVE To examine the biophysical effects of photothermal heating on herniated intervertebral discs during laser decompression surgery. STUDY DESIGN/MATERIALS AND METHODS: Ex vivo bovine nucleus pulposus specimens were irradiated with a Nd: YAG laser (lambda = 1.32 microm, 100 seconds exposure time, 9-31 W/cm(2), 4.8 mm spot diameter), whereas changes in tissue thermal, mechanical, and optical properties were monitored by using, respectively, infrared radiometry, tissue tension measurements, and diffuse reflectance from a HeNe probe laser. Morphologic changes and mass reduction were monitored by recording shape changes on video and weighing specimens before and after laser exposure. RESULTS: At power densities below 20 W/cm(2), evaporation of water and specimen volume reduction (shrinking) were consistently observed on video during irradiation. In contrast, above 20 W/cm(2), vapor bubbles formed within the specimen matrix and subsequently ruptured (releasing heated vapors). When radiometric surface temperature approaches approximately 60 to 70 degrees C (denaturation threshold for tissue), tissue tension begins to increase, which is consistent with observations of specimen length reduction. The onset of this change in tissue tension is also reflected in characteristic alterations in diffuse reflectance. With cessation of laser irradiation, a sustained increase in tissue tension is observed, which is consistent with changes in specimen length and volume. Higher laser power results in a faster heating rate and subsequently an accelerated tension change. Specimen mass reduction increased with irradiance from 19 to 72% of the initial mass for 9--31 W/cm(2), respectively. Irradiated specimens did not return to their original shape after immersion in saline (48 hours) in contrast to air-dried specimens (24 hours), which returned to their original shape and size. CONCLUSION: These observations suggest that photothermal heating results in irreversible matrix alteration causing shape change and volume reduction (observed on video and evidenced by the increase in tissue tension) taking place at approximately 65 degrees C. Inasmuch as high laser power results in vapor bubble formation and specimen tearing, the heating process must be controlled. Diffuse reflectance measurements provide a noncontact, highly sensitive means to monitor dynamically changes in tension of nucleus purposus.

Animals↗

PubMed2001

Motor impairment in the human hand following eccentric exercise.

Motor impairment was induced by having subjects perform two sets of 50 maximal contractions, using the first dorsal interosseus (FDI) muscle to abduct the index finger, while the muscle was being stretched. Tests were conducted prior to the exercise (pre-exercise) and 24 h following the exercise (post-exercise). There were declines of 19% in maximal abduction torque and 15% in maximal flexion torque at the metacarpaphalangeal joint, during isometric contraction post-exercise compared to pre-exercise. The ability to stabilize the metacarpophalangeal joint about the abduction/adduction axis was reduced by 14% post-exercise, and the variability in tracking an isometric torque target increased by 30%. There was a decrement of 7%-10% in the median frequency of the power density spectrum of FDI electromyogram (EMG) throughout a 60 s maintained abduction at 50% maximal voluntary contraction. The mean rectified EMG, on the other hand, increased by 100%-175% for torque levels below 40% of maximal voluntary contraction, post-exercise. The results were consistent with preferential injury of type II muscle fibres in FDI. Although non-exercised synergist muscles appeared to be inhibited during maximal voluntary flexion, there was evidence that they compensated for injured FDI muscle fibres during maintained contraction at sub-maximal flexion torque.

Adult↗

PubMed2001

Non-contact measurement of thermal diffusivity in tissue.

We demonstrate the application of an infrared (IR) imaging technique for non-contact determination of thermal diffusivity in biological materials. The proposed method utilizes pulsed laser excitation to produce an initial three-dimensional temperature distribution in tissue, and records IR images of subsequent heat diffusion. The theoretical model assumes that the time-dependent temperature increase following pulsed laser exposure is due to independent heat diffusion in longitudinal and lateral directions. A nonlinear least-squares algorithm is used to compute the lateral thermal point spread function from a pair of recorded IR images and to determine the thermal diffusivity of a test specimen. The recorded time-sequence of IR images is used to compute thermal diffusivity as a function of increasing time interval between two IR emission images. Experimental application of the method was demonstrated using tissue phantoms, ex vivo samples of hydrated cartilage and in vivo epidermis.

Adult↗

PubMed2001

Muscle function at the wrist after eccentric exercise.

PURPOSE: The purpose of this experiment was to investigate the effects of eccentric exercise by the wrist extensor muscles on the function and motor control of synergist wrist extensor muscles and the antagonist wrist flexor muscles. METHODS: Ten subjects were tested repeatedly over a period of 11 d, once before and four times after a bout of strenuous eccentric exercise with the wrist extensor muscles. Tests performed as indicators of muscle injury were wrist extension MVC, ROM, and soreness. Tests performed as measures of function and motor control were maximum joint velocity, ability to sustain a constant torque, and the ability to track a changing torque. RESULTS: Indicators of muscle injury: subjects exhibited a decline in wrist extension MVC and ROM, which peaked on day 1, and reported that muscle soreness was greatest on day 2. All measures returned to baseline values by day 10. Measures of function and motor control: subjects exhibited a greater difficulty sustaining a submaximal contraction and tracking torque after eccentric exercise. Greater torque variances in these tests were most evident at high torque levels. Subjects exhibited the greatest difficulty 24 h after eccentric exercise and had recovered by day 10. There was no change in maximal wrist extension velocity. CONCLUSIONS: Strenuous eccentric exercise by wrist extensors had an effect on function and motor control of the wrist extensor muscles. The effect was most evident during contractions in which high torque was required. The response of all of the wrist extensors after the exercise bout was similar, suggesting that they operated in a synergistic manner. The antagonists wrist flexors showed increased coactivation after eccentric exercise.

Adult↗

PubMed2001

Soleus stretch reflex during cycling.

The modulation and strength of the human soleus short latency stretch reflex was investigated by mechanically perturbing the ankle during an unconstrained pedaling task. Eight subjects pedaled at 60 rpm against a preload of 10 Nm. A torque pulse was applied to the crank at various positions during the crank cycle, producing ankle dorsiflexion perturbations of similar trajectory. The stretch reflex was greatest during the power phase of the crank cycle and was decreased to the level of background EMG during recovery. Matched perturbations were induced under static conditions at the same crank angle and background soleus EMG as recorded during the power phase of active pedaling. The magnitude of the stretch reflex was not statistically different from that during the static condition throughout the power phase of the movement. The results of this study indicate that the stretch reflex is not depressed during active cycling as has been shown with the H-reflex. This lack of depression may reflect a decreased susceptibility of the stretch reflex to inhibition, possibly originating from presynaptic mechanisms.

Adult↗

PubMed2000

Bioheat transfer analysis of cryogen spray cooling during laser treatment of port wine stains.

BACKGROUND AND OBJECTIVE: The thermal response of port wine stain (PWS) skin to a combined treatment of pulsed laser irradiation and cryogen spray cooling (CSC) was analyzed through a series of simulations performed with a novel optical-thermal model that incorporates realistic tissue morphology. STUDY DESIGN/MATERIALS AND METHODS: The model consisted of (1) a three-dimensional reconstruction of a PWS biopsy, (2) a Monte Carlo optical model, (3) a finite difference heat transfer model, and (4) an Arrhenius thermal damage calculation. Simulations were performed for laser pulses of 0.5, 2, and 10 ms and a wavelength of 585 nm. Simulated cryogen precooling spurts had durations of 0, 20, or 60 ms and terminated at laser onset. Continuous spray cooling, which commenced 60 ms before laser onset and continued through the heating and relaxation phases, was also investigated. RESULTS: The predicted response to CSC included maximal pre-irradiation temperature reductions of 27 degrees C at the superficial surface and 12 degrees C at the dermoepidermal junction. For shorter laser pulses (0.5, 2 ms), precooling significantly reduced temperatures in superficial regions, yet did not effect superficial vessel coagulation. Continuous cooling was required to reduce significantly thermal effects for the 10-ms laser pulse. CONCLUSIONS: For the PWS morphology and treatment parameters studied, optimal damage distributions were obtained for a 2-ms laser pulse with a 60-ms precooling spurt. Epidermal and vascular morphology as well as laser pulse duration should be taken into account when planning CSC/laser treatment of PWS. Our novel, realistic-morphology modeling technique has significant potential as a tool for optimizing PWS treatment parameters.

Biopsy↗

PubMed2000

Optical and thermal properties of nasal septal cartilage.

BACKGROUNDS AND OBJECTIVES: The aim of the study was to measure the spectral dependence of optical absorption and reduced scattering coefficients and thermal conductivity and diffusivity of porcine nasal septal cartilage. Values of optical and thermal properties determined in this study may aid in determining laser dosimetry and allow selection of an optical source wavelength for noninvasive diagnostics for laser-assisted reshaping of cartilage. MATERIALS AND METHODS: The diffuse reflectance and transmittance of ex vivo porcine nasal septal cartilage were measured in the 400- to 1,400-nm spectral range by using a spectrophotometer. The reflectance and transmittance data were analyzed by using an inverse adding-doubling algorithm to obtain the absorption (mu(a)) and reduced scattering (mu(a)') coefficients. A multichannel thermal probe controller system and infrared imaging radiometer methods were applied to measure the thermal properties of cartilage. The multichannel thermal probe controller system was used as an invasive technique to measure thermal conductivity and diffusivity of cartilage at three temperatures (27, 37, 50 degrees C). An infrared imaging radiometer was used as a noninvasive method to measure the thermal diffusivity of cartilage by using a CO(2) laser source (lambda = 10.6 microm) and an infrared focal plane array (IR-FPA) camera. RESULTS: The optical absorption peaks at 980 nm and 1,180 nm in cartilage were observed and corresponded to known absorption bands of water. The determined reduced scattering coefficient gradually decreased at longer wavelengths. The thermal conductivity values of cartilage measured by using an invasive probe at 27, 37, and 50 degrees C were 4.78, 5.18, and 5.76 mW/cm degrees C, respectively. The corresponding thermal diffusivity values were 1.28, 1.31, and 1.40x 10(-3) cm(2)/sec. Because no statistically significant difference in thermal diffusivity values with increasing temperature is found, the average thermal diffusivity is 1.32 x 10(-3) cm(2)/sec. The numerical estimate for thermal diffusivity obtained from infrared radiometry measurements was 1.38 x 10(-3) cm(2)/sec. CONCLUSION: Values of the spectral dependence of the optical absorption and reduced scattering coefficients, and thermal conductivity and diffusivity of cartilage were measured. The invasive and noninvasive diffusivity measurements were consistent and concluded that the infrared imaging radiometric technique has an advantage to determine thermal properties, because damage to the cartilage sample may be avoided. The measured values of absorption and reduced scattering coefficients can be used for predicting the optical fluence distribution in cartilage and determining optical source wavelengths for the laser-assisted cartilage reshaping studies. The thermal conductivity and diffusivity values can play role in understanding thermal-dependent phenomenon in cartilage during laser irradiation and determining laser dosimetry for the laser-assisted cartilage reshaping studies.

Animals↗

PubMed2000

Passive and active wrist joint stiffness following eccentric exercise.

The purpose of this study was to investigate the effects of exercise-induced muscle injury on passive and active wrist joint stiffness. Ten male subjects were repeatedly tested over a period of 11 days, once prior to, and four times following a bout of eccentric exercise with the wrist extensor muscles. Static wrist stiffness was measured by applying a 3 degrees ramp and hold displacement of the manipulandum, which stretched the wrist extensor muscles. Wrist extension maximum voluntary contraction (MVC) declined by 24.5% from pre-exercise to 24 h after the exercise bout (P < 0.001). There was a reduced passive range of motion (ROM) from 82.8 degrees pre-exercise to 70.2 degrees on day 1 (P < 0.01), but no change in the passive joint stiffness at the neutral joint position, suggesting mechanical changes in the non-contractile tissues, or swelling that only resisted movement at the extremes of the ROM. Active joint stiffness at 50% pre-exercise MVC declined from 0.299 Nm deg(-1) pre-exercise to 0.254 Nm deg(-1) on day 1 (P < 0.025). Active joint stiffness at 10% pre-exercise MVC did not change on any of the days of testing compared to pre-exercise. These findings may indicate that large muscle fibers were more affected by the injury than small muscle fibers.

Adult↗

PubMed2000

A method for measuring endpoint stiffness during multi-joint arm movements.

Current methods for measuring stiffness during human arm movements are either limited to one-joint motions, or lead to systematic errors. The technique presented here enables a simple, accurate and unbiased measurement of endpoint stiffness during multi-joint movements. Using a computer-controlled mechanical interface, the hand is displaced relative to a prediction of the undisturbed trajectory. Stiffness is then computed as the ratio of restoring force to displacement amplitude. Because of the accuracy of the prediction (< 1 cm error after 200 ms) and the quality of the implementation, the movement is not disrupted by the perturbation. This technique requires only 13 as many trials to identify stiffness as the method of Gomi and Kawato (1997, Biological Cybernetics 76, 163-171) and may, therefore, be used to investigate the evolution of stiffness during motor adaptation.

Arm↗

PubMed2000

The effect of eccentric exercise on intrinsic and reflex stiffness in the human hand.

OBJECTIVE: The purpose of this work was to determine the effect of strenuous eccentric exercise on joint stiffness and to separate joint stiffness into components due to intrinsic muscle mechanics and delayed reflex muscle activation. DESIGN: Subjects performed 100 maximal eccentric contractions, using the first dorsal interosseus muscle to abduct the index finger while undergoing a 20 degrees displacement of the metacarpophalangeal joint. Joint stiffness was measured 24 h later during 15% and 65% maximal voluntary contraction and during electrical muscle stimulation at 15% of maximal voluntary contraction torque. BACKGROUND: Joint stiffness can be varied by changing voluntary muscle activation and thereby serves an important role in joint stabilization. Eccentric exercise has been shown to result in muscle fiber injury, reducing maximal muscle force. However, it is not known whether intrinsic muscle stiffness or reflex stiffness is also affected. METHODS: Displacements of 3 degrees amplitude were used to estimate joint stiffness about the neutral angle of the index finger. The difference between measurements made during voluntary muscle activation and electrical muscle stimulation was used to obtain reflex stiffness. RESULTS: There was no change in the passive joint stiffness nor was there any change in either the intrinsic or reflex stiffness at 15% maximal voluntary contraction. However, intrinsic stiffness for the electrically stimulated muscle was higher post-exercise than pre-exercise, while active joint stiffness at 65% maximal voluntary contraction (comprising intrinsic and reflex stiffness) was lower. CONCLUSION: The observed mechanical changes are compatible with the hypothesis that type II muscle fibers are more susceptible to injury than type I muscle fibers, which have higher intrinsic stiffness. RELEVANCE: Muscle stiffness is important for maintaining mechanical stability of a joint. The effects of eccentric exercise on muscle stiffness are likely analogous to the effects of muscle injury, making this a good model for biomechanical changes associated with muscle injury.

Adult↗

PubMed2000

Combining two excitation wavelengths for pulsed photothermal profiling of hypervascular lesions in human skin.

When pulsed photothermal radiometry (PPTR) is used for depth profiling of hypervascular lesions in human skin, melanin absorption also heats the most superficial skin layer (epidermis). Determination of lesion depth may be difficult when it lies close to the epidermal dermal junction, due to PPTR's limited spatial resolution. To overcome this problem, we have developed an approximation technique, which uses two excitation wavelengths (585 and 600 nm) to separate the vascular and epidermal components of the PPTR signal. This technique permits a noninvasive determination of lesion depth and epidermal thickness in vivo, even when the two layers are in close physical proximity to each other. Such information provides the physician with guidance in selecting the optimal parameters for laser therapy on an individual patient basis.

Blood↗

PubMed2000

Proteoglycan synthesis in porcine nasal cartilage grafts following Nd:YAG (lambda = 1.32 microns) laser-mediated reshaping.

Mechanically deformed morphologic cartilage grafts undergo temperature-dependent stress relaxation during sustained laser irradiation resulting in stable shape changes. In this study, porcine nasal septal cartilage specimens were evaluated for viability by measuring the incorporation of Na2(35)SO4 into proteoglycan (PTG) macromolecules in whole tissue culture following laser-mediated reshaping. Synthesis rates of PTG were determined by scintillation counting lyophilized specimens and normalizing these values by total protein content. Positive controls were established by inducing chondrocyte apoptosis using prolonged exposure to nitric oxide (NO). In chondrocytes, apoptosis induced using NO resulted in significantly lower PTG synthesis rates compared to untreated native specimens. Cartilage specimens were irradiated with light emitted from a Nd:YAG laser (25 W/cm2, lambda = 1.32 microns) while recording simultaneously radiometric surface temperature, internal stress and back-scattered light intensity from a probe laser. Each specimen received one, two or three sequential laser exposures. The duration of each exposure was determined from real-time measurements of characteristic changes in back-scattered light intensity that correlate with accelerated stress relaxation. A 5 min time interval between each laser exposures allowed the cartilage specimen to return to thermal equilibrium. Average PTG synthesis rates decreased with successive laser exposures, though these were always higher than baseline rates established for NO-treated tissues, suggesting that laser-mediated cartilage reshaping acutely does not eliminate the entire population of viable chondrocytes. The reduction in PTG synthesis is correlated with the time-temperature-dependent heating profile created during laser irradiation, supporting our hypothesis that careful monitoring of laser dosimetry is required to ensure chondrocyte viability.

Animals↗

PubMed1999

Modeling laser treatment of port wine stains with a computer-reconstructed biopsy.

BACKGROUND AND OBJECTIVE: The efficacy of laser treatment of port wine stains (PWS) has been shown to be highly dependent on patient-specific vasculature. The effect of tissue structure on optical and thermal mechanisms was investigated for different pulse durations by using a novel theoretical model that incorporates tissue morphology reconstructed tomographically from a PWS biopsy. STUDY DESIGN/MATERIALS AND METHODS: An optical-thermal numerical model capable of simulating arbitrarily complex, three-dimensional tissue geometries was developed. The model is comprised of (1) a voxel-based Monte Carlo optical model, (2) a finite difference thermal model, and (3) an Arrhenius rate process calculation to predict the distribution of thermal damage. Simulations based on previous computer-based reconstruction of a series of 6 microm sections from a PWS biopsy were performed for laser pulse durations (taup) of 0.5, 5.0, and 10.0 ms at a wavelength of 585 nm. RESULTS: Energy deposition rate in the blood vessels was primarily a function of vessel depth in skin, although shading effects were evident. Thermal confinement and selectivity of damage were seen to be inversely proportional to pulse duration. The model predicted blood-specific damage for taup = 0.5 ms, vascular and perivascular damage for taup = 5 ms, and widespread damage in superficial regions for taup = 10 ms. The effect of energy deposition in the epidermis was most pronounced for longer pulse durations, resulting in increased temperature and extent of damage. CONCLUSION: Pulse durations between 0.5 and 5 ms are likely optimal for the PWS analyzed. The incorporation of a tomographically reconstructed PWS biopsy into an optical-thermal model represents a significant advance in numerical modeling of laser-tissue interaction.

Adult↗

PubMed1998

Damping of the wrist joint during voluntary movement.

Damping characteristics of the musculoskeletal system were investigated during rapid voluntary wrist flexion movements. Oscillations about the final position were induced by introducing a load with the characteristics of negative damping, which artificially reduced the damping of the wrist. Subjects responded to increases in the negatively damped load by stronger cocontraction of wrist flexor and extensor muscles during the stabilization phase of the movement. However, their ability to counteract the effects of the negatively damped load diminished as the negative damping increased. Consequently, the number and frequency of oscillations increased. The oscillations were accompanied by phase-locked muscle activity superimposed on underlying tonic muscle activation. The wrist stiffness and damping coefficient increased with the increased cocontraction that accompanied more negatively damped loads, although changes in the damping coefficient were less systematic than the stiffness. Analysis of successive half-cycles of the oscillation revealed that the wrist stiffness and damping coefficient increased, despite decreasing muscle activation, as oscillation amplitude and velocity declined. This indicates that the inverse dependence of the damping coefficient on oscillation velocity contributes significantly to damping of joint motion. It is suggested that this property helps to offset a negative contribution to damping from the stretch reflex.

Adult↗

PubMed1998

Quantization of human motions and learning of accurate movements.

This paper presents a mathematical model for the learning of accurate human arm movements. Its main features are that the movement is the superposition of smooth submovements, the intrinsic deviation of arm movements is considered, visual and kinesthetic feed-back are integrated in the motion control, and the movement duration and accuracy are optimized with practice. This model is consistent with the jerky arm movements of infants, and may explain how the adult motion behavior emerges from the infant behavior. Comparison with measurements of adult movements shows that the kinematics of accurate movements are well predicted by the model.

Adult↗

PubMed1998

Signal attenuation and localization in optical coherence tomography studied by Monte Carlo simulation.

A Monte Carlo model has been developed for optical coherence tomography (OCT). A geometrical optics implementation of the OCT probe with low-coherence interferometric detection was combined with three-dimensional stochastic Monte Carlo modelling of photon propagation in the homogeneous sample medium. Optical properties of the sample were selected to simulate intralipid and blood, representing moderately (g = 0.7) and highly (g = 0.99) anisotropic scattering respectively. For shallow optical depths in simulated intralipid (<3 scattering mean free path (mfp) units), the number of detected backscattered photons followed the extinction-single-backscatter model, and OCT was found to detect only minimally scattered photons. Within this depth range the backscatter positions of detected photons corresponded well with the nominal focus position of the probe. For propagation to deeper positions in intralipid, localization of backscattering was quickly lost due to detection of stray photons, and the number of detected photons remained constant with increasing depth in the non-absorbing medium. For strongly forward-directed scattering in simulated blood, the number of detected photons approached the extinction-single-backscatter model only for very shallow depths (<2 mfp units). However, backscattering positions for detected photons correlated well with the nominal focus position of the probe even for optical depths greater than 40 mfp units.

Blood↗