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Figure 1.
Schematic of laser-mediated cartilage reshaping.

Schematic of laser-mediated cartilage reshaping.

Figure 2.
Changes in internal stress radiometric surface temperature and forward-scattered light intensity during laser irradiation.

Changes in internal stress radiometric surface temperature and forward-scattered light intensity during laser irradiation.

Figure 3.
Schematic of cartilage reshaping jig.

Schematic of cartilage reshaping jig.

Figure 4.
Schematic of prototype closed-loop feedback-controlled device for laser-mediated cartilage reshaping.

Schematic of prototype closed-loop feedback-controlled device for laser-mediated cartilage reshaping.

Figure 5.
Reshaped cartilage specimen: A, prior to irradiation; B, specimen in reshaping jig secured to device; C, specimen immediately following reshaping; and D, specimen after 15 minutes of rehydration.

Reshaped cartilage specimen: A, prior to irradiation; B, specimen in reshaping jig secured to device; C, specimen immediately following reshaping; and D, specimen after 15 minutes of rehydration.

1.
Sobol  ENPhase Transformations and Ablation in Laser-Treated Solids New York, NY John Wiley & Sons Inc1995;316- 322
2.
Helidonis  ESobol  ENKavvalos  G  et al.  Laser shaping of composite cartilage grafts Am J Otolaryngol. 1993;14410- 412Article
3.
Helidonis  ESSobol  ENVelegrakis  GBizakis  J Shaping of nasal septal cartilage with the carbon dioxide laser: a preliminary report of an experimental study Lasers Med Sci. 1994;951- 54Article
4.
Sobol  ENBagratashvili  VVOmelchenko  AI  et al.  Laser shaping of cartilage Proc SPIE. 1994;212843- 49
5.
Sobol  ENBagratashvili  VVSviridov  A  et al.  Cartilage reshaping with holmium laser Proc SPIE. 1996;2623556- 559
6.
Sobol  ENBagratashvili  VVSviridov  A  et al.  Phenomenon of cartilage shaping using moderate heating and its application in otorhinolaryngology Proc SPIE. 1996;2623560- 564
7.
Sobol  ENSviridov  ABagratashvili  VV  et al.  Stress relaxation and cartilage shaping under laser radiation Proc SPIE. 1996;2681358- 363
8.
Jacques  SL Role of tissue optics and pulse duration on tissue effects during high-power laser irradiation Appl Optics. 1993;322447- 2454Article
9.
Wong  BJFMilner  TEKim  HKNelson  JSSobol  EN Stress relaxation of porcine septal cartilage during Nd:YAG (λ=1.32 mm) laser irradiation: mechanical, optical, and thermal responses J Biomed Optics. 1998;3409- 414Article
10.
Karamzadeh  AWong  BJFMilner  TEWilson  MLiaw  L-HNelson  JS Angiogenic response in the chick chorioallantoic membrane model to laser-irradiated cartilage Proc SPIE. 1999;3590243- 248
11.
Chao  KWong  BJFKim  HKSun  C-HSobol  ENNelson  JS Viability of porcine nasal septal cartilage grafts following Nd:YAG (λ=1.32 mm) laser radiation Proc SPIE. In press
12.
Wong  BJFMilner  TEAnvari  B  et al.  Thermo-optical response of cartilage during feedback controlled laser-assisted reshaping Proc SPIE. 1997;2970380- 391
13.
Wong  BJFMilner  TEAnvari  B  et al.  Measurement of radiometric surface temperature and integrated back-scattered light intensity during feedback controlled laser-assisted cartilage reshaping Lasers Med Sci. 1998;1366- 72Article
14.
Wong  BJFMilner  TEKim  HK  et al.  Critical temperature transitions in laser mediated cartilage reshaping Proc SPIE. 1998;3245161- 172
15.
Bagratashvili  VVSobol  ENSviridov  APopov  VKOmelchenko  AIHowdle  SM Thermal and diffusion processes in laser-induced stress relaxation and reshaping of cartilage J Biomech. 1997;30813- 817Article
16.
Madsen  SJChu  EWong  BJF The optical properties of porcine nasal cartilage IEEE J Selected Top Quantum Electronics. In press
17.
Beek  JFBlokland  PPosthumus  P  et al.  In vitro double-integrating-sphere optical properties of tissues between 630 nm and 1064 nm Phys Med Biol. 1997;422255- 2261Article
18.
Wong  BJFMilner  TEKim  HK  et al.  Characterization of temperature-dependent biophysical properties during laser-mediated cartilage reshaping IEEE J Selected Top Quantum Electronics. In press
19.
Chew  CWong  BJFMilner  TE  et al.  Feedback-controlled cartilage reshaping with Nd:YAG laser: effect of pH variation Proc SPIE. 1998;3245206- 216
20.
Sobol  ENKitai  MJones  NSviridov  AMilner  TEWong  BJF Heating and structure alterations in cartilage under laser radiation IEEE J Selected Top Quantum Electronics. 1999;35532- 539Article
21.
Wang  ZPankratov  MMPerrault  DFShapshay  SM Endoscopic laser-assisted reshaping of collapsed tracheal cartilage: a laboratory study Ann Otol Rhinol Laryngol. 1996;105176- 181
22.
Wang  ZPankratov  MMPerrault  DFShapshay  SM Laser-assisted cartilage reshaping: in vitro and in vivo animal studies Proc SPIE. 1995;2395296- 302
23.
Sviridov  ASobol  EBagratashvili  V  et al.  In vivo study and histological examination of laser reshaping of cartilage Proc SPIE. 1999;3590222- 228
Citations 0
Original Article
October 1999

Feedback-Controlled Laser-Mediated Cartilage Reshaping

Author Affiliations

From the Beckman Laser Institute and Medical Clinic (Drs Wong, Nelson and Mr Dao) and Department of Otolaryngology–Head and Neck Surgery (Dr Wong), University of California, Irvine; Biomedical Engineering Program, Department of Electrical and Computer Engineering, College of Engineering, University of Texas at Austin (Dr Milner); Department of Engineering, Harvey Mudd College, Claremont, Calif (Messrs Harrington and Ro); and Department of Advanced Laser Technologies, Center for Technological Lasers, Russian Academy of Sciences, Troitsk, Moscow (Dr Sobol).

 

From the Beckman Laser Institute and Medical Clinic (Drs Wong, Nelson and Mr Dao) and Department of Otolaryngology–Head and Neck Surgery (Dr Wong), University of California, Irvine; Biomedical Engineering Program, Department of Electrical and Computer Engineering, College of Engineering, University of Texas at Austin (Dr Milner); Department of Engineering, Harvey Mudd College, Claremont, Calif (Messrs Harrington and Ro); and Department of Advanced Laser Technologies, Center for Technological Lasers, Russian Academy of Sciences, Troitsk, Moscow (Dr Sobol).

Arch Facial Plast Surg. 1999;1(4):282-287. doi:
Abstract

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 (25×10×1.5-2.1 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 (λ=1.32 µm, 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 (λ=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°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.

DURING LASER irradiation, mechanically deformed cartilage undergoes a temperature-dependent phase transformation that results in accelerated stress relaxation.1-7 As a consequence, laser-irradiated cartilage may be molded into complex new shapes that remain stable as the tissue cools without the need for suturing, scoring, or morselization to relieve and/or balance the intrinsic elastic forces that resist deformation. The principal advantages of using laser radiation for the generation of thermal energy in tissue are precise control of both the space-time temperature distribution and time-dependent thermal denaturation kinetics.8

We illustrate the reshaping process schematically in Figure 1. A flat cartilage specimen (Figure 1, A) is maintained in user-defined mechanical configuration by a jig (Figure 1, B) and then irradiated with a laser. Following cessation of laser irradiation, the specimen (and jig) is immersed in saline solution at 20°C for 15 minutes. The jig is removed and a stable shape change is achieved (Figure 1, C).9 The optimization of laser-mediated reshaping requires precise control of the time-dependent heating profile in cartilage during radiation. Cartilage reshaping (stress relaxation) occurs when the tissue reaches a critical temperature transition range and remains within this region for a minimum time interval. If the cartilage tissue remains at elevated temperatures for a prolonged period, cell death and tissue necrosis will result. Effective shape change occurs when the temperature change is adequate to reshape the cartilage without causing nonspecific chondrocyte apoptosis. Our preliminary studies indicated that reshaping can be accomplished without significant loss of chondrocyte viability, but laser dosimetry must be precisely controlled to minimize nonspecific thermal injury.10-11 Feedback control of the reshaping process requires a real-time technique to monitor dynamic changes in the physical properties of laser-irradiated cartilage.

Changes in optical, thermal, and mechanical properties of cartilage accompanying the laser-mediated reshaping may be dynamically monitored by measuring scattered-light intensity (from a probe laser), radiometric surface temperature (Sc[t]), heat capacity, and internal stress in real time.9, 12-15 Infrared radiometry relies on the measurement of blackbody radiative emissions, resulting from temperature elevations created by absorption of laser radiation. Spectral radiance of blackbody emission from the cartilage surface is a function of tissue emissivity and temperature and is described by the Planck radiation law; the detected radiometric signal contains information concerning the optical and thermal properties of the cartilage. Thermopiles or solid-state infrared detectors can be used to measure the increased blackbody emission and, when properly configured and calibrated, surface temperature can be estimated. As photothermal heating produces alterations in the fine structure of the specimen (eg, protein structure, water content), tissue optical properties also undergo dynamic change that can be measured using a second probe laser beam (at a visible wavelength) centered on the heated region. Intensity of diffusely scattered light (forward scattered or backscattered) results from real-time changes in tissue optical properties (primarily tissue-scattering coefficients for visible laser wavelengths in cartilage).16-17

When a mechanically deformed cartilage specimen is irradiated with laser energy, internal stress initially increases, plateaus, and then rapidly decreases (stress relaxation) (Figure 2, A). Temperature-sensitive tensiometric measurements of cartilage internal stress during laser irradiation suggest that marked stress relaxation occurs when tissue temperature reaches approximately 60°C to 70°C.9, 13 In this temperature range, a slope change (arrow) in the heating curve is also observed, suggesting a change in tissue thermal properties (Figure 2, B). As the tissue is heated, the same molecular alterations in tissue matrix structure that result in reshaping also cause changes in tissue optical properties and may be determined by measuring either forward-scattered or backscattered light from a second probe laser (at a visible wavelength such as 650 nm) (Figure 2, C). The onset of stress relaxation, a local extremum in the light-scattering signal, and slope change in the radiometric temperature occur simultaneously. Noncontact optical and radiometric measurements may be used to infer changes in cartilage mechanical properties, and hence be used to modulate laser energy and feedback control the reshaping process,9 and form the basis for our design and construction of a prototype closed-loop feedback-controlled device to reshape cartilage using real-time measurements of tissue temperature and alterations in optical properties.

MATERIAL AND METHODS

Nasal septal cartilage was extracted from freshly euthanized pigs obtained from a regional abattoir (Clougherty Packing Company, Vernon, Calif) and cut into slabs (25×10 mm) of uniform thickness varying from 1.5 to 2.1 mm using a custom guillotine microtome. The cartilage specimens were secured to an inner cylindrical wire-frame jig (2.5 cm in diameter) with an outer hemicylindrical retaining frame and held in mechanical deformation (Figure 3). The jig was constructed from fencing wire (0.6 cm [0.25 in] square size), which allowed maximal exposure of the cartilage specimen to incident laser irradiation while still maintaining secure and stable mechanical deformation. Light from an Nd:YAG laser (λ=1.32 µm, 50 Hz pulse repetition rate) (New Star Lasers, Auburn, Calif) was delivered via a multimode low hydroxide silica optical fiber terminated with an anti–reflection-coated collimating lens. The fiber delivery system was combined with a thermopile detector (response time of 120 milliseconds [95%]; spectral sensitivity, 7.6-18 µm) in a single unit (New Star Lasers) (Figure 4). The thermopile detected infrared emissions from a source area on the cartilage surface 3 mm in diameter. Calibration was performed as previously described and Sc(t) was calculated.13 Laser spot size (5.4 mm in diameter) was measured with thermal paper (Zap-It; Kentek, Pittsfield, NH). Laser power density (25 W/cm2) was measured with a pyroelectric meter (Model 10A-P; Ophir Optronics, Jerusalem, Israel).

A diode probe laser (λ=650 nm, 5 mW) (MWK Industries, Corona, Calif) was directed perpendicularly onto the irradiated surface of the cartilage specimen and centered within the laser spot produced by the Nd:YAG laser.12 A mechanical chopper (Model R540; Stanford Research Systems, Sunnyvale, Calif) was used to amplitude modulate (600 Hz) the intensity of the diode laser. Forward-scattered light from the diode laser was focused by a condenser lens (numerical aperture of 0.5, 86 mm in diameter; MWK Industries) into an integrating sphere (15-cm [6-in] diameter, IS-060; Labsphere, North Sutton, NH) and synchronously detected using a silicon photoreceiver (Model 2001; New Focus, Mountain View, Calif) and a lock-in amplifier (time, 100 milliseconds) (Model SR 830; Stanford Research Systems) to give integrated forward-scattered light intensity (Ifs[t]). Data were acquired using a 16-bit AD converter (AT-MIO-16XE-50; National Instruments, Austin, Tex) and a personal computer (Equicomp Solutions, Westminister, Calif) running software written in LabView (version 5.0; National Instruments).

The cartilage and reshaping jig were secured to a computer-controlled positioning device (Lego Dacta, Pittsburg, Kan), which rotated or linearly translated the specimen relative to the fixed laser beam in stepwise increments, and hence permitted sequential irradiation of the entire specimen outer surface. Laser irradiation was terminated at each site on the specimen when either Sc(t) reached a user-defined end point (70°C) or when Ifs(t) reached an absolute minimum. The positioning device, laser power, and temperature–light-scattering analysis algorithms were controlled by a "virtual instrument" programmed in LabView, with software designed to detect both temperature thresholds and minima in the light-scattering signal. The entire outer surface of the cartilage specimen could be irradiated with multiple laser passes over the same region. In general, repeated laser irradiation (on a given region) results in more pronounced shape changes and sustained stress relaxation; however, repeated irradiation decreases chondrocyte viability. Three laser passes were used, as effective reshaping can be achieved without compromising chondrocyte viability significantly.11

Immediately following laser irradiation, the cartilage specimen was removed from the jig and secured to a smaller frame-wire jig (1-1.5 cm in diameter) of similar construction. The specimen and jig were immersed in saline solution at ambient temperatures for 15 minutes to allow rehydration. The specimen was removed from the jig and stored in saline solution at 4°C for 21 days. Specimens were photographed before, during, and after reshaping at various time intervals.

RESULTS

Figure 5 is a photographic montage of a cartilage specimen (2.1 mm×25 mm×10 mm) undergoing reshaping. Figure 5, A, the cartilage specimen before reshaping; Figure 5, B, during laser irradiation (secured in the reshaping jig); Figure 5, C, immediately after laser radiation; and Figure 5, D, the same specimen following 15 minutes of rehydration in normal saline solution (while wrapped around a jig of smaller diameter). Measurements of Ifs(t) and Sc(t) were used to terminate laser irradiation. The entire outer surface of the specimen (secured within the jig, Figure 3) was irradiated 3 times. Consistent with prior observations,9 specimen morphology did not change during a 3-week time interval. Similar findings were observed in experiments involving thinner cartilage specimens.

COMMENT

The interaction of coherent light with tissue may result in a variety of effects depending on the laser wavelength, pulse duration, irradiance, and tissue thermal and optical properties.8 In industry, photothermal, photomechanical, and photochemical effects are used for ablative and nonablative applications in many areas, such as semiconductor, alloy, ceramic, and polymer processing. While industrial nonablative laser applications are commonplace, lasers have been used predominantly in medicine to ablate or photocoagulate tissue. Few nonablative uses of lasers exist in medicine, and laser-mediated reshaping of cartilage is a novel application. Laser interactions in cartilage result in thermal-mediated alterations in cartilage biophysical properties, strongly suggesting the occurrence of a phase transformation. In cartilage, these energy-dependent changes in molecular structure are manifest by alterations in the tissue matrix. Preliminary investigations have determined the optical, thermodynamic, and mechanical changes in cartilage tissue in response to laser irradiation, and the critical temperature regions in which they occur,4-7,9, 12-15,18-20 although the molecular basis for thermal-mediated stress relaxation remains incompletely understood. We have used these findings to develop a prototype device to reshape cartilage tissue using dynamic measurements of Sc(t) and alterations in tissue optical properties.

Figure 5 illustrates the modification of a thick (2.1 mm) cartilage specimen from a flat to a curved shape. Specimen shape was determined by the reshaping jig, which provides sustained mechanical deformation during laser exposure. A simple curve was selected for these initial studies, though other geometries can be attained by altering the jig shape and modifying the software-controlling specimen translation. The device used dynamic measurements of tissue light-scattering properties and surface temperature to control the reshaping process.

The changes in forward-scattered light signal Ifs(t) are small (on the order of 5%), and, without a lock-in detection method, difficult to observe in the presence of noise from ambient lighting and display panel sources. With appropriate amplification technique, changes in tissue optical properties are quite dramatic and easily identified.13 However, detecting absolute minima in Ifs(t) is challenging when the specimen and/or jig moves or shifts during laser irradiation, or when the probe laser beam is blocked or partially reflected by the wire frame of the reshaping jig. As a fail-safe measure, Sc(t) measurements were used to prevent overheating of the specimen, and laser irradiation was terminated if surface temperature reached 70°C. Cartilage undergoes accelerated stress relaxation near 70°C, though the critical temperature is dependent on the rate of temperature change.14 The requirement for a fail-safe approach is extremely critical for specimens irradiated with multiple passes of the laser, as the characteristic minimum in Ifs(t) becomes increasingly difficult to observe.

Three laser-mediated cartilage reshaping studies have been performed in animals, though none of these investigations monitored changes in tissue temperature, optical properties, or internal stress or was a feedback control system used to modulate laser dosimetry.21-23 Laser-mediated cartilage reshaping is undergoing clinical trial in septoplasty operations in Russia; to date, 33 patients have undergone laser surgery (without feedback control) using a Ho:YAG laser (λ=2.12 µm) without near-term morbidity (E. Sobol, PhD, oral communication, 1999). Furthermore, this technology may be adapted for use with minimally invasive techniques to reshape cartilage in regions difficult to access such as the trachea.21-22 While laser reshaping of relatively thick cartilage specimens was demonstrated in this study, the device works equally well with thinner specimens, which would likely be encountered in the nasal tip, septum, and trachea.

CONCLUSION

Laser-mediated cartilage reshaping relies on temperature-dependent accelerated stress relaxation. The therapeutic interval between adequate shape change and overt chondrocyte death is narrow, and a real-time method to access alterations in tissue physical properties during laser irradiation is required. We have constructed a device using noncontact optical and radiometric techniques to monitor changes in tissue biophysical properties and used these to feedback control the reshaping process and minimize uncontrolled heating. Cartilage may be heated in a controlled manner to the point where accelerated stress relaxation occurs, well below the threshold for thermal necrosis. If properly developed and carefully implemented, feedback-controlled laser-mediated cartilage reshaping may radically alter the practice of head and neck reconstructive surgery. The treatment of protuberant lop ears, nasal deformities, laryngeal crush injuries, and tracheal deformity could be improved using laser-based devices, without the attendant donor site morbidity or irreversibility associated with traditional reconstructive techniques. We plan to pursue animal studies and clinical trials using further modifications of the prototype device described in this study.

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Article Information

Accepted for publication June 30, 1999.

This work was supported in part by grants N00014-94-0874, the Office of Naval Research, Arlington, Va; 1 K08 DC 00170-01, AR-43419, RR-01192, HL-59472, and 59472-03, National Institutes of Health, Bethesda, Md; and 95-3800459, the Department of Energy, Washington, DC.

The authors would like to acknowledge the software expertise of Simon Evans Fraser and Kevin Mee and the comments and suggestions of Christopher Saxer, PhD, Johannes F. de Boer, PhD, and B. Samuel Tanenbaum, PhD.

Reprints: Brian J. F. Wong, MD, Beckman Laser Institute and Medical Clinic, University of California, Irvine, 1002 Health Sciences Rd E, Irvine, CA 92612 (e-mail: bjfwong@bli.uci.edu).

References
1.
Sobol  ENPhase Transformations and Ablation in Laser-Treated Solids New York, NY John Wiley & Sons Inc1995;316- 322
2.
Helidonis  ESobol  ENKavvalos  G  et al.  Laser shaping of composite cartilage grafts Am J Otolaryngol. 1993;14410- 412Article
3.
Helidonis  ESSobol  ENVelegrakis  GBizakis  J Shaping of nasal septal cartilage with the carbon dioxide laser: a preliminary report of an experimental study Lasers Med Sci. 1994;951- 54Article
4.
Sobol  ENBagratashvili  VVOmelchenko  AI  et al.  Laser shaping of cartilage Proc SPIE. 1994;212843- 49
5.
Sobol  ENBagratashvili  VVSviridov  A  et al.  Cartilage reshaping with holmium laser Proc SPIE. 1996;2623556- 559
6.
Sobol  ENBagratashvili  VVSviridov  A  et al.  Phenomenon of cartilage shaping using moderate heating and its application in otorhinolaryngology Proc SPIE. 1996;2623560- 564
7.
Sobol  ENSviridov  ABagratashvili  VV  et al.  Stress relaxation and cartilage shaping under laser radiation Proc SPIE. 1996;2681358- 363
8.
Jacques  SL Role of tissue optics and pulse duration on tissue effects during high-power laser irradiation Appl Optics. 1993;322447- 2454Article
9.
Wong  BJFMilner  TEKim  HKNelson  JSSobol  EN Stress relaxation of porcine septal cartilage during Nd:YAG (λ=1.32 mm) laser irradiation: mechanical, optical, and thermal responses J Biomed Optics. 1998;3409- 414Article
10.
Karamzadeh  AWong  BJFMilner  TEWilson  MLiaw  L-HNelson  JS Angiogenic response in the chick chorioallantoic membrane model to laser-irradiated cartilage Proc SPIE. 1999;3590243- 248
11.
Chao  KWong  BJFKim  HKSun  C-HSobol  ENNelson  JS Viability of porcine nasal septal cartilage grafts following Nd:YAG (λ=1.32 mm) laser radiation Proc SPIE. In press
12.
Wong  BJFMilner  TEAnvari  B  et al.  Thermo-optical response of cartilage during feedback controlled laser-assisted reshaping Proc SPIE. 1997;2970380- 391
13.
Wong  BJFMilner  TEAnvari  B  et al.  Measurement of radiometric surface temperature and integrated back-scattered light intensity during feedback controlled laser-assisted cartilage reshaping Lasers Med Sci. 1998;1366- 72Article
14.
Wong  BJFMilner  TEKim  HK  et al.  Critical temperature transitions in laser mediated cartilage reshaping Proc SPIE. 1998;3245161- 172
15.
Bagratashvili  VVSobol  ENSviridov  APopov  VKOmelchenko  AIHowdle  SM Thermal and diffusion processes in laser-induced stress relaxation and reshaping of cartilage J Biomech. 1997;30813- 817Article
16.
Madsen  SJChu  EWong  BJF The optical properties of porcine nasal cartilage IEEE J Selected Top Quantum Electronics. In press
17.
Beek  JFBlokland  PPosthumus  P  et al.  In vitro double-integrating-sphere optical properties of tissues between 630 nm and 1064 nm Phys Med Biol. 1997;422255- 2261Article
18.
Wong  BJFMilner  TEKim  HK  et al.  Characterization of temperature-dependent biophysical properties during laser-mediated cartilage reshaping IEEE J Selected Top Quantum Electronics. In press
19.
Chew  CWong  BJFMilner  TE  et al.  Feedback-controlled cartilage reshaping with Nd:YAG laser: effect of pH variation Proc SPIE. 1998;3245206- 216
20.
Sobol  ENKitai  MJones  NSviridov  AMilner  TEWong  BJF Heating and structure alterations in cartilage under laser radiation IEEE J Selected Top Quantum Electronics. 1999;35532- 539Article
21.
Wang  ZPankratov  MMPerrault  DFShapshay  SM Endoscopic laser-assisted reshaping of collapsed tracheal cartilage: a laboratory study Ann Otol Rhinol Laryngol. 1996;105176- 181
22.
Wang  ZPankratov  MMPerrault  DFShapshay  SM Laser-assisted cartilage reshaping: in vitro and in vivo animal studies Proc SPIE. 1995;2395296- 302
23.
Sviridov  ASobol  EBagratashvili  V  et al.  In vivo study and histological examination of laser reshaping of cartilage Proc SPIE. 1999;3590222- 228
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