Long-term in vivo stability of rabbit nasal septal cartilage following laser cartilage reshaping: a pilot investigation

BACKGROUND AND OBJECTIVES

To evaluate the long-term effect of laser cartilage reshaping on rabbit nasal septal cartilage viability and mechanical integrity in an in vivo model.

STUDY DESIGN/MATERIALS AND METHODS

In vivo animal investigation. Rabbit septal cartilage specimens were laser (Nd:YAG, lambda = 1.32 mum, spot size 5.4-mm diameter, 10 W, 10 seconds, 50 Hz PPR) reshaped and subsequently reimplanted into an interscapular subcutaneous pocket. Specimens were harvested at 8 and 12 months and evaluated using photography, flow cytometry, and histology.

RESULTS

Grossly, specimens showed alteration in the physical integrity with varying degrees of tissue resorption. The non-irradiated control specimens demonstrated significantly increased stiffness. Histologically, there was marked depletion of the extracellular matrix and an overall reduction in tissue mass in laser irradiated tissues. However, flow cytometry data identified viable chondrocytes in laser-irradiated specimens that were identical to those observed in controls.

CONCLUSIONS

Study results demonstrate that the rabbit nasal septal cartilage model can be effectively used to study laser reshaping, however alternative recipient sites with perichondrial lining, such as the pinna, may provide a more realistic physiologic environment for reshaped graft tissue. The dosimetry used in this pilot study likely led to significant thermal injury. Study results underscore the importance of elucidating the optimal laser dosimetry required to initiate permanent shape change while minimizing thermal damage.

Depth-resolved phase retardation measurements for laser-assisted non-ablative cartilage reshaping

Since polarization-sensitive optical coherence tomography (PS-OCT) is emerging as a new technique for determining phase retardation in biological materials, we measured phase retardation changes in cartilage during local laser heating for application to laser-assisted cartilage reshaping. Thermally-induced changes in phase retardation of nasal septal cartilage following Nd:YAG laser irradiation were investigated using a PS-OCT system. A PS-OCT system and infrared imaging radiometer were used to record, respectively, depth-resolved images of the Stokes parameters of light backscattered from ex vivo porcine nasal septal cartilage and radiometric temperature changes following laser irradiation. PS-OCT images of cartilage were recorded before (control), during and after laser irradiation. From the measured Stokes parameters (I, Q, U and V), an estimate of the relative phase retardation between two orthogonal polarizations was computed to determine birefringence in cartilage. Phase retardation images of light backscattered from cartilage show significant changes in retardation following laser irradiation. To investigate the origin of retardation changes in response to local heat generation, we differentiated two possible mechanisms: dehydration and thermal denaturation. PS-OCT images of cartilage were recorded after dehydration in glycerol and thermal denaturation in heated physiological saline. In our experiments, observed retardation changes in cartilage are primarily due to dehydration. Since dehydration is a principal source for retardation changes in cartilage over the range of heating profiles investigated, our studies suggest that the use of PS-OCT as a feedback control methodology for non-ablative cartilage reshaping requires further investigation.

High-sensitivity determination of birefringence in turbid media with enhanced polarization-sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography provides high-resolution cross-sectional characterization of birefringence in turbid media. Weakly birefringent biological tissues such as the retinal nerve fiber layer (RNFL) require advanced speckle noise reduction for high-sensitivity measurement of form birefringence. We present a novel method for high-sensitivity birefringence quantification by using enhanced polarization-sensitive optical coherence tomography (EPS-OCT) and introduce the polarimetric signal-to-noise ratio, a mathematical tool for analyzing speckle noise in polarimetry. Multiple incident polarization states and non-linear fitting of normalized Stokes vectors allow determination of retardation with +/-1 degrees uncertainty with invariance to unknown unitary polarization transformations. Results from a weakly birefringent turbid film and in vivo primate RNFL are presented. In addition, we discuss the potential of EPS-OCT for noninvasive quantification of intracellular filamentous nanostructures, such as neurotubules in the RNFL that are lost during the progression of glaucoma.

Muscle reflex classification of low-back pain

It has been well documented that low-back pain (LBP) patients have longer muscle response latencies to perturbation than healthy controls. These muscle responses appear to be reflexive and not voluntary in nature, and as a result, might be useful for objectively classifying LBP. The goal of the study was to develop an objective and accurate method for classifying LBP using a sudden load-release protocol. Subjects were divided into two groups: learning group (20 patients and 20 controls), and holdout group (15 patients and 12 controls). Subjects exerted isometric trunk force against a cable in four different directions. Following cable release, the trunk was suddenly displaced eliciting a muscle reflex response. Reflex latencies for muscles switching-on and shutting-off were determined using electromyogram signals from 8 trunk muscles. Independent t tests were performed on the learning group to determine which reflex parameters were to be entered into logistic regression analysis to produce a classification model. The holdout group was used to validate this classification model. The three-parameter model was able to correctly classify 83% of the learning group, and 81% of the holdout group. Using reflex parameters appears to be an accurate and objective method for classifying LBP.

Electrokinetic measurement of cartilage using differential phase optical coherence tomography

When an electric field is applied to cartilage, current-generated stress gradients are produced and stress deformation occurs. Since differential phase optical coherence tomography (DP-OCT) is sensitive to tiny surface displacement, these tiny displacements are induced electrokinetically in cartilage and the electric-current-induced stress gradients were measured with DP-OCT. The electrokinetic surface displacement of cartilage was characterized by applying sinusoidal voltages with two amplitudes (5 and 10 V) and different frequencies (1.0, 0.5 and 0.2 Hz). The results show that by application of DP-OCT the surface displacement increased with increasing applied voltage and decreased with increasing excitation frequency. In the electrokinetic response of cartilage, measured optical phase delay between the surface displacement response and excitation waveform varies inversely with the excitation frequency. Since the streaming potential and other electrokinetic effects in cartilage are directly proportional to proteoglycan density, application of an electric field in cartilage combined with DP-OCT measurements may provide a sensitive indicator of cartilage viability.

Differential phase optical coherence probe for depth-resolved detection of photothermal response in tissue

We describe a differential phase low-coherence interferometric probe for non-invasive, quantitative imaging of photothermal phenomena in biological materials. Our detection method utilizes principles of optical coherence tomography with differential phase measurement of interference fringe signals. A dual-channel optical low-coherence probe is used to analyse laser-induced thermoelastic and thermorefractive effects in tissue with micrometre axial resolution and nanometre sensitivity. We demonstrate an application of the technique using tissue phantoms and ex-vivo tissue specimens of rodent dorsal skin.

Accurate measurement of blood vessel depth in port wine stained human skin in vivo using pulsed photothermal radiometry

We report on application of pulsed photothermal radiometry (PPTR) to determine the depth of port wine stain (PWS) blood vessels in human skin. When blood vessels are deep in the PWS skin (>100 microm), conventional PPTR depth profiling can be used to determine PWS depth with sufficient accuracy. When blood vessels are close or partially overlap the epidermal melanin layer, a modified PPTR technique using two-wavelength (585 and 600 nm) excitation is a superior method to determine PWS depth. A direct difference approach in which PWS depth is determined from a weighted difference of temperature profiles reconstructed independently from two-wavelength excitation is demonstrated to be appropriate for a wider range of PWS patients with various blood volume fractions, blood vessel sizes, and depth distribution. The most superficial PWS depths determined in vivo by PPTR are in good agreement with those measured using optical Doppler tomography (ODT).

Quantitative phase-contrast imaging of cells with phase-sensitive optical coherence microscopy

We describe a method for en face phase-contrast imaging of cells with a fiber-based differential phase-contrast optical coherence microscopy system. Recorded en face images are quantitative phase-contrast maps of cells due to spatial variation of the refractive index and (or) thickness of various cellular components. Quantitative phase-contrast images of human epithelial cheek cells obtained with the fiber-based differential phase-contrast optical coherence microscopy system are presented.

Phase-sensitive optical low-coherence reflectometry for the detection of analyte concentrations

Optical techniques may potentially be used for noninvasive glucose sensing. We investigated the application of phase-sensitive optical low-coherence reflectometry (PS-OLCR) to the measurement of analyte concentrations. The dependence of the PS-OLCR signal on the concentration of various analytes, including aqueous solutions of glucose, calcium chloride, magnesium chloride, sodium chloride, potassium chloride, potassium bicarbonate, urea, bovine serum albumin, and bovine globulin, were determined in clear and turbid media. Obtained results demonstrated (1) a high degree of sensitivity and accuracy of the phase measurements of analyte concentrations with PS-OLCR; (2) a concentration-dependent change in the phase-shift for glucose that is significantly greater than that of other analytes sampled over the same physiological range; and (3) a high submillimolar sensitivity of PS-OLCR for the measurement of glucose concentration. Further exploration of the application of PS-OLCR to the noninvasive, sensitive, and specific monitoring of glucose concentration seems warranted.

A model of force and impedance in human arm movements

This paper describes a simple computational model of joint torque and impedance in human arm movements that can be used to simulate three-dimensional movements of the (redundant) arm or leg and to design the control of robots and human-machine interfaces. This model, based on recent physiological findings, assumes that (1) the central nervous system learns the force and impedance to perform a task successfully in a given stable or unstable dynamic environment and (2) stiffness is linearly related to the magnitude of the joint torque and increased to compensate for environment instability. Comparison with existing data shows that this simple model is able to predict impedance geometry well.

Use of optical coherence tomography to monitor biological tissue freezing during cryosurgery

The use of optical coherence tomography (OCT) for imaging skin during cryosurgery is evaluated. OCT provides high spatial resolution (5-10 microm) images of optical backscattering due to local variations in refractive index, such as the boundary between liquid and frozen water in tissue. Time resolved OCT images were acquired during freezing of water, Intralipid trade mark, and in vivo hamster skin. Subsurface morphological changes were evident only during freezing of Intralipid and skin. A simple thermal model was applied which predicted freezing times on the same order of magnitude as those observed in OCT images. OCT can be used as a feedback tool during cryosurgical procedures to monitor progression of the freezing front.

Influence of interaction force levels on degree of motor adaptation in a stable dynamic force field

Studies have shown that the point-to-point reaching movements of subjects seated in a dark, rotating room demonstrate errors in movement trajectories and endpoints, consistent with the direction of the Coriolis force perturbations created by room rotation. Adaptation of successive reaches and the presence of postrotation aftereffects have indicated that subjects form internal models of the Coriolis field dynamics in order to make appropriate movement corrections. It has been argued that these findings are inconsistent with predictions of peripheral stabilization assumed in equilibrium-point models of motor control. A possibility that has been raised, however, is that the Coriolis field findings may in fact stem from changes in control commands elicited due to the magnitude and destabilizing nature of the Coriolis perturbations. That is, it has been suggested that a perturbation threshold exists, below which central reactions are not necessary in order to maintain movement stability. We tested the existence of a perturbation threshold in normal-speed reaching movements. Twelve normal human subjects performed non-visually guided reaching movements while grasping a robotic manipulandum. The endpoints and trajectory deviations of their movements were measured before, during, and after a position-dependent force field (similar to a Coriolis field in terms of the time history of applied forces) was applied to their movements. We examined the responses to a range of perturbation field strengths from small to considerable. Our experimental results demonstrated a substantial adaptation response over the entire range of perturbation field magnitudes examined. Neither the amount of adaptation after 5 trials nor after 25 trials was found to change as disturbance magnitudes decreased. These findings indicate that there is an adaptive response even for small perturbations; i.e., threshold behavior was not found. This result contradicts the assertion that peripheral stabilization mechanisms enable the central controller to ignore small details of peripheral or environmental dynamics. Our findings instead point to a central dynamic modeler that is both highly sensitive and continually active. The results of our study also showed that subjects were able to maintain baseline pointing accuracies despite exposure to perturbation forces of sizeable magnitude (more than 7 N).

Imaging tissue response to electrical and photothermal stimulation with nanometer sensitivity

BACKGROUND AND OBJECTIVES

Tissue response to thermal, electrical, or chemical stimuli are important in the health and survival of tissue. We report experimental results to assess tissue response to various stimuli using a low coherence differential phase interferometer.

STUDY DESIGN/MATERIALS AND METHODS

The optical system utilized to measure tissue response is a novel fiber-based phase sensitive optical low coherence reflectometer (PS-OLCR). Inasmuch as the PS-OLCR works with back-reflected light, noninvasive sensing of tissue response to stimuli is possible. In addition to high lateral (approximately 10 microm) and longitudinal (approximately 10 microm) resolution, PS-OLCR can measure sub-wavelength changes in optical path-length (Angstrom/nanometer range) by extracting the phase difference between interference fringes in two channels corresponding to orthogonal polarization modes.

RESULTS

When light spatially splits into two polarization states, precise analysis of surface topography or tissue surface response such as swelling or collapse are possible. Time resolved measurements of nanometer-scale path length changes in response to electrical and thermal stimuli are demonstrated using longitudinally delayed polarization channels.

CONCLUSIONS

Since PS-OLCR is a useful tool to detect ultra-small path length changes, the system has potential to aid scientists in investigating important phenomena in biomaterials and developing useful diagnostic and therapeutic imaging modalities. Applications include tissue surface profilometry, measurement of tissue, and cell response to various stimuli, high-resolution intensity and phase imaging.

Polarization-maintaining fiber-based optical low-coherence reflectometer for characterization and ranging of birefringence

We describe a polarization-maintaining fiber-based polarization-sensitive optical low-coherence reflectometer for measurement of depth-resolved birefringence. Unlike for other fiber-based polarization-sensitive optical low-coherence reflectometers, here the linear birefringence of a sample can be measured from data recorded in a single A scan. Simultaneous measurement of retardation and orientation of birefringent axes with mica wave plates is demonstrated. The measured retardation is insensitive to sample rotation in the plane perpendicular to ranging.

Effect of lithotripsy on holmium:YAG optical beam profile

PURPOSE

To determine the effect of holmium:YAG lithotripsy on the optical beam profile.

MATERIALS AND METHODS

Beam profiles of the laser light from holmium:YAG optical fiber systems were characterized with a pyroelectric camera. Beam profiles were measured with 272-microm and 365-microm optical fibers both straight and bent to simulate lower-pole ureteronephroscopy. Struvite calculi were irradiated. Beam profiles and energy outputs were characterized for the fibers before and after ablation. Ablation crater geometry was characterized with optical coherence tomography.

RESULTS

Undamaged, straight fibers produced a near-Gaussian beam profile. Craters showed a similar near-Gaussian shape. Undamaged, bent 272-microm fibers produced a near-Gaussian beam but slightly flatter profile than the straight fiber. The bent 272-microm fiber transmitted 99% to 100% of the energy, similar to the 100% transmission of the straight fibers. After ablation, measured energy output dropped by 30% within 50 pulses at 0.2 J pulse energy. The damaged fibers produced irregular beam profiles with hot spots. Craters showed irregular contours.

CONCLUSIONS

During Ho:YAG lithotripsy, the beam profile at the optical fiber tip approaches a Gaussian distribution. This shape corresponds to the crater produced on the stone surface. With further ablation, the beam profile becomes erratic and unpredictable, with loss of lithotripsy efficiency. The findings provide further insight into the photothermal mechanism of Ho:YAG lithotripsy.

Radiofrequency cartilage reshaping: efficacy, biophysical measurements, and tissue viability

OBJECTIVES

To assess the feasibility of reshaping cartilage using radiofrequency (RF) heating, and to examine the effects of this process on tissue biophysical properties (optical and thermal) and cellular viability.

METHODS

Mechanically deformed porcine septal cartilage was reshaped using 2 RF-generating devices. We performed dynamic measurements of tissue thermal and optical properties while heating cartilage with one of these devices. Cellular viability was assessed immediately and 7 days after treatment.

RESULTS

A characteristic change in the diffuse transmittance of light through the cartilage occurred during heating. Change in transmittance has been shown to accompany the onset of stress relaxation in cartilage. Peak radiometric surface temperature during heating was 88.6 degrees C. Specimens retained their user-specified curved shape for the observed period of 14 days. Chondrocyte viability in RF-heated tissue was 19% and 14% of that in untreated control specimens at days 0 and 7 after treatment, respectively.

CONCLUSIONS

Radiofrequency heating has been shown to effectively reshape cartilage while maintaining cellular viability, illustrating a novel application for a widely used technology.

Review of polarization sensitive optical coherence tomography and Stokes vector determination

Polarization sensitive optical coherence tomography (PS-OCT) provides depth resolved measurements of the polarization state of light reflected from turbid media such as tissue. The theory and calculation of the Stokes vector of light reflected from turbid media is described and application of PS-OCT to contemporary biomedical imaging problems is given. Measurement of the depth resolved Stokes parameters allows determination of the degree of polarization and optical axis orientation in turbid media that can be modeled as a linear retarder. Effect of multiple scattering and speckle on the accuracy and noise of the computed Stokes parameters is discussed. Future directions for development of PS-OCT instrumentation for biological and medical applications is given.

Spectral variation of the infrared absorption coefficient in pulsed photothermal profiling of biological samples

Pulsed photothermal radiometry can be used for non-invasive depth profiling of optically scattering samples, including biological tissues such as human skin. Computational reconstruction of the laser-induced temperature profile from recorded radiometric signals is sensitive to the value of the tissue absorption coefficient in the infrared detection band (muIR). While assumed constant in reported reconstruction algorithms, muIR of human skin varies by two orders of magnitude in the commonly used 3-5 microm detection band. We analyse the problem of selecting the effective absorption coefficient value to be used with such algorithms. In a numerical simulation of photothermal profiling we demonstrate that results can be markedly impaired, unless the reconstruction algorithm is augmented by accounting for spectral variation muIR(lambda). Alternatively, narrowing the detection band to 4.5-5 microm reduces the spectral variation muIR(lambda) to a level that permits the use of the simpler, unaugmented algorithm. Implementation of the latter approach for depth profiling of port wine stain birthmarks in vivo is presented and discussed.

Investigation of the transduction mechanism of infrared detection in Melanophila acuminata: photo-thermal-mechanical hypothesis

Differential phase optical low coherence reflectometry (OLCR) was used to detect sub-wavelength displacements in the infrared-sensitive thoracic pit organ of Melanophila acuminata (Coleoptera: Buprestidae) upon absorption of infrared radiation at 3.39 microm. The displacement had more complex morphology but similar amplitude ( approximately 100 nm at 1 W cm(-2)) when compared to the displacement measured from the exocuticle in an alternate region on the beetle's body. In addition, a simplified finite difference model was developed to predict the temperature distribution and resultant thermal expansion in the pit organ tissue. The experimental and model results were interpreted to help clarify the mechanism by which the sensilla in the pit organ convert infrared radiation to neural signals. The results of this paper are discussed in relation to the photo-thermal-mechanical transduction hypothesis. This is the first experimental examination of the transduction mechanism in Melanophila acuminata.

Use of a blood substitute to determine instantaneous murine right ventricular thickening with optical coherence tomography

BACKGROUND

A satisfactory imaging technique to determine regional wall thickening of the murine myocardium is not available. Although cardiovascular imaging with light offers a novel solution, application is problematic because scattering by erythrocytes causes significant optical attenuation.

METHODS AND RESULTS

Optical coherence tomography (OCT) is a technique for detailed resolution imaging of highly scattering biological tissues. To reduce the high level of blood scattering, a method was devised whereby murine blood was replaced with a hemoglobin-based blood substitute. The scattering and absorption properties of in vitro preparations of whole blood and dilutions of blood with a blood substitute were determined with a spectrophotometer and an inverse-adding doubling algorithm. OCT imaging of the same dilutions demonstrated a significant reduction in scattering at a hematocrit <5%. A fiber-optic OCT imaging system was used to image the murine right midventricular free wall before and after isovolumic replacement with blood substitute. Strong light attenuation prevented full thickness imaging before replacement, whereas visualization of the full ventricular thickness was possible after replacement. Baseline and imaging hematocrits were 52.4+/-3.8% and 3.7+/-1.2%, respectively. End-systolic and end-diastolic thickness values were 0.458+/-0.051 mm and 0.352+/-0.047 mm. Percent thickening fraction was 30.8+/- 7.5%.

CONCLUSION

Optical imaging of the intact beating murine right ventricle was substantially improved by isovolumic blood replacement with a hemoglobin-based blood substitute. Although the current study has been directed toward imaging the murine heart, a blood substitute may be applied to various optical diagnostic and therapeutic techniques under investigation in cardiovascular medicine.

Refractive-index profiling of embedded microstructures in optical materials

We describe use of a phase-sensitive low-coherence reflectometer to measure spatial variation of refractive index in optical materials. The described interferometric technique is demonstrated to be a valuable tool to profile the refractive index of optical elements such as integrated waveguides and photowritten optical microstructures. As an example, a refractive-index profile is mapped of a microstructure written in a microscope glass slide with an ultrashort-pulse laser.

Coherent thermal wave imaging of subsurface chromophores in biological materials

Thermal wave imaging of discrete subsurface chromophores in biological materials is reported using a phase sensitive coherent detection technique applied to recorded infrared (IR) images. We demonstrate that utilization of a periodically modulated laser source for thermal wave excitation and coherent detection applied to each pixel may be used to compute images of thermal wave amplitude and phase at the laser modulation frequency. In comparison to recorded IR images, the narrow-band detection technique significantly improves the quality of thermal wave amplitude images of subsurface chromophores in biological materials. Additionally, the technique provides phase information, which may be used to estimate chromophore depth in tissue. Application of the technique is demonstrated using tissue phantoms and in vivo biological models. We present a theoretical analysis and computer simulations that demonstrate the effect of tissue optical and thermal properties on thermal wave amplitude and phase. In comparison to the pulsed photothermal technique, coherent thermal wave imaging of subsurface chromophores in tissue for diagnostic applications allows reduction of peak incident laser fluence by as much as four orders of magnitude and is safer and more amenable to in vivo imaging.

Primate retina imaging with polarization-sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography (PSOCT) is applied to determine the depth-resolved polarization state of light backreflected from the eye. The birefringence of the retinal nerve fiber layer (RNFL) was observed and measured from PSOCT images recorded postmortem in a Rhesus monkey. An image-processing algorithm was developed to identify birefringent regions in acquired PSOCT retinal images and automatically determine the thickness of the RNFL. Values of the RNFL thickness determined from histology and PSOCT were compared. PSOCT may provide a new method to determine RNFL thickness and birefringence for glaucoma diagnostics.

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.

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.

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

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.

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.

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.

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.

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.

Angioresistance of thermally modified cartilage grafts in the chick chorioallantoic membrane model

BACKGROUND

The chick chorioallantoic membrane (CAM) model allows direct observation of vascularization acutely in explanted or cultured tissues in an immunologically isolated environment. In vivo, angioinvasion of the tissue matrix does not occur in viable cartilage tissue, whereas denatured or nonviable grafts are readily vascularized and/or resorbed.

OBJECTIVE

To determine, using the CAM model, whether angioinvasion of thermally altered cartilage explants occurs acutely.

MATERIALS AND METHODS

Porcine septal cartilage specimens were removed from freshly killed animals and divided into 3 groups (n = 10): an untreated control group, a group in which cartilage was boiled in isotonic sodium chloride solution (normal saline) for 1 hour, and a laser-irradiated group (Nd:YAG, lambda = 1.32 microm, 30.8 W/cm2, irradiation time = 10 seconds). Tissue specimens were then washed in antibiotic solutions, cut into small cubes (approximately 1.5 mm3), placed on the surface of 30 CAMs (7 days after fertilization), and allowed to incubate for an additional 7 days. After incubation, the membranes and specimens were fixed in situ with formaldehyde and then photographed using a dissection microscope.

RESULTS

Examination with a dissecting microscope showed no obvious vascular invasion of the cartilage or loss of gross tissue integrity in any of the 3 experimental groups, although all specimens were completely enveloped by the CAM vascular network. No vascular invasion of the tissue matrix was observed histologically.

CONCLUSION

These experiments demonstrate that cartilage specimens remain acutely resistant to angioinvasion or metabolism by the immunologically immature CAM whether native unmodified tissue, completely denatured (boiled), or thermally modified following laser irradiation.

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.

Doppler-angle measurement in highly scattering media

We describe a dual-channel optical low-coherence reflectometer for accurate measurement of Doppler angles in highly scattering media. Accurate fluid-flow velocity estimation requires measurement of the Doppler shift and angle. Estimated values of the Doppler angle and average fluid-flow velocity from experimental data are in good agreement with preset values.

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.

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.

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.

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.

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.

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.

Optical low-coherence reflectometer for differential phase measurement

A birefringent-fiber-based dual-channel optical low-coherence reflectometer capable of differential phase measurement is described. Phase noise owing to environmental perturbations that is common to both channels is canceled, resulting in accurate measurement of the phase difference between light backscattered from two spatially separated sites. Results are presented that demonstrate the accuracy and sensitivity of the system.

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.

Feedback-controlled laser-mediated cartilage reshaping

OBJECTIVE

To demonstrate feedback-controlled laser-mediated cartilage reshaping using dynamic measurements of tissue optical properties and radiometric surface temperatures.

DESIGN

Flat cartilage specimens were reshaped into curved configurations using a feedback-controlled laser device.

MATERIALS

Fresh porcine nasal septum, stripped of perichondrium and cut into uniform strips (25 x 10 x 1.5-2.1 mm) with a custom guillotine microtome.

INTERVENTIONS

Cartilage specimens secured in a cylindrical reshaping jig (2.5 cm in diameter) and irradiated with an Nd:YAG laser (lambda = 1.32 microns, 25 W/cm2, 50-Hz pulse repetition rate). During laser irradiation, radiometric surface temperature was measured along with changes in forward-scattered light from a diode probe laser (lambda = 650 nm, 5 mW), using a lock-in detection technique. Sequential irradiation of the specimen outer surface was made (3 laser passes). Characteristic changes in tissue temperature and light-scattering signals were used to terminate laser irradiation.

RESULTS

Effective reshaping was accomplished for both thin (1.5-mm) and thick (2.1-mm) specimens. Following reshaping, specimens were stored in saline solution at 4 degrees C for 21 days. No return to the original flat configuration was noted during this period.

CONCLUSIONS

The prototype device effectively reshapes flat native porcine cartilage into curve configurations. The use of optical and thermal signals provides effective feedback control for optimizing the reshaping process.

Polarization Effects in Optical Coherence Tomography of Various Biological Tissues

Polarization sensitive optical coherence tomography (PS-OCT) was used to obtain spatially resolved ex vivo images of polarization changes in skeletal muscle, bone, skin and brain. Through coherent detection of two orthogonal polarization states of the signal formed by interference of light reflected from the biological sample and a mirror in the reference arm of a Michelson interferometer, the depth resolved change in polarization was measured. Inasmuch as any fibrous structure will influence the polarization of light, PS-OCT is a potentially powerful technique investigating tissue structural properties. In addition, the effects of single polarization state detection on OCT image formation is demonstrated.

Polarization Effects in Optical Coherence Tomography of Various Biological Tissues

Polarization sensitive optical coherence tomography (PS-OCT) was used to obtain spatially resolved ex vivo images of polarization changes in skeletal muscle, bone, skin and brain. Through coherent detection of two orthogonal polarization states of the signal formed by interference of light reflected from the biological sample and a mirror in the reference arm of a Michelson interferometer, the depth resolved change in polarization was measured. Inasmuch as any fibrous structure will influence the polarization of light, PS-OCT is a potentially powerful technique investigating tissue structural properties. In addition, the effects of single polarization state detection on OCT image formation is demonstrated.

Characterization of dentin and enamel by use of optical coherence tomography

Optical coherence tomographic images of human dentin and enamel are obtained by use of polarization-sensitive optical coherence tomography. A birefringence effect in enamel (lambda = 856 nm) and light propagation along dentinal tubules are observed. The group index of refraction for both dentin and enamel was measured at 1.50 +/- 0.02 and 1.62 +/- 0.02, respectively.

Determination of the depth-resolved Stokes parameters of light backscattered from turbid media by use of polarization-sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography (PS-OCT) was used to characterize completely the polarization state of light backscattered from turbid media. Using a low-coherence light source, one can determine the Stokes parameters of backscattered light as a function of optical path in turbid media. To demonstrate the application of this technique we determined the birefringence and the optical axis in fibrous tissue (rodent muscle) and in vivo rodent skin. PS-OCT has potentially useful applications in biomedical optics by imaging simultaneously the structural properties of turbid biological materials and their effects on the polarization state of backscattered light. This method may also find applications in material science for investigation of polarization properties (e.g., birefringence) in opaque media such as ceramics and crystals.

Characterization of multijoint finger stiffness: dependence on finger posture and force direction

The two-dimensional static stiffness of the index finger was measured with the interphalangeal joints in flexed and extended postures. The stiffness of the relaxed finger was compared with the stiffness when voluntary force was exerted in different directions. The finger stiffness was found to be anisotropic, with the direction of greatest stiffness being approximately parallel to the proximal phalange of the finger. This direction was relatively unaffected by finger posture or direction of finger force. Finger stiffness was more anisotropic when the interphalangeal joints were extended than flexed. The stiffness was most anisotropic when the interphalangeal joints were extended and force was being exerted in the direction of pointing, while it was least anisotropic when the interphalangeal joints were flexed and force was being exerted in directions normally associated with pinching and tapping actions. The stiffness of the individual finger joints was computed and the relation between stiffness and joint torque was examined. Previous studies, which examined single finger joints in isolation, had found that joint stiffness varied in a linear fashion with net joint torque. In contrast, we did not find a monotonic relation between joint stiffness and net joint torque, which we attributed to the need to vary the amount of cocontraction of antagonistic muscles when controlling the direction of finger force.