Thermoforming of tracheal cartilage: viability, shape change, and mechanical behavior.

BACKGROUND AND OBJECTIVES: Trauma, emergent tracheostomy, and prolonged intubation are common causes of severe deformation and narrowing of the trachea. Laser technology may be used to reshape tracheal cartilage using minimally invasive methods. The objectives of this study were to determine: (1) the dependence of tracheal cartilage shape change on temperature and laser dosimetry using heated saline bath immersion and laser irradiation, respectively, (2) the effect of temperature on the mechanical behavior of cartilage, and (3) tissue viability as a function of laser dosimetry. MATERIALS AND METHODS: Ex vivo rabbit trachea cartilage specimens were bent and secured around a cylinder (6 mm), and then immersed in a saline bath (45 and 72 degrees C) for 5-100 seconds. In separate experiments, tracheal specimens were irradiated with a diode laser (lambda = 1.45 microm, 220-400 J/cm(2)). Mechanical analysis was then used to determine the elastic modulus in tension after irradiation. Fluorescent viability assays combined with laser scanning confocal microscopy (LSCM) were employed to image and identify thermal injury regions. RESULTS: Shape change transition zones, between 62 and 66 degrees C in the saline heating bath and above power densities of 350 J/cm(2) (peak temperatures 65+/-10 degrees C) for laser irradiation were identified. Above these zones, the elastic moduli were higher (8.2+/-4 MPa) than at lower temperatures (4.5+/-3 MPa). LSCM identified significant loss of viable chondrocytes within the laser-irradiation zones. CONCLUSION: Our results indicate a change in mechanical properties occurs with laser irradiation and further demonstrates that significant thermal damage is concurrent with clinically relevant shape change in the elastic cartilage tissues of the rabbit trachea using the present laser and dosimetry parameters.

Viability of human septal cartilage after 1.45 microm diode laser irradiation.

BACKGROUND AND OBJECTIVES: Chondrocyte viability following laser irradiation and reshaping has not been established for human nasal septal cartilage. Knowledge of the relationship between thermal injury and laser dosimetry is needed in order to optimize septal laser cartilage reshaping. The objective of this study was to determine the depth and width of thermal injury in human septal cartilage following laser irradiation. STUDY DESIGN/MATERIALS AND METHODS: Excess fresh nasal septal cartilage sections from rhinoplasty or septoplasty operations were irradiated using a 1.45 microm diode laser 1.25-3.6 W (2.8 mm spot diameter) with 1 second fixed exposure time, and then at exposure times of 1-4 seconds for a fixed power of 1.25 W. An infrared camera recorded surface temperature profiles during irradiation, and the temperature data were incorporated into a rate process model to numerically estimate thermal damage. Calcein AM and ethidium homodimer-1 fluorescent dyes combined with confocal laser microscopy (CLM) were used to measure thermal damage. RESULTS: CLM demonstrated clear demarcation between dead and living cells following irradiation. The extent of non-viable chondrocyte distributions increased with power and exposure time. The maximum depths of injury were 1,012 and 1,372 microm after 3.6 W 1 second and 1.25 W 4 seconds irradiation respectively. The damage predictions made by the rate process model underestimated thermal injury when compared with CLM measurements. CONCLUSIONS: The assay system identified regions of non-viable chondrocytes in human septal cartilage and defined how thermal injury varies with dosimetry when using a 1.45 microm diode laser.

Analysis of Nd:YAG laser-mediated thermal damage in rabbit nasal septal cartilage.

BACKGROUND AND OBJECTIVES: Laser cartilage reshaping (LCR) involves the use of photo-thermal heating to reshape cartilage. Its clinical relevance depends on the ability to minimize thermal injury in irradiated regions. The present study seeks to understand the safety of LCR by determining shape change and resultant tissue viability as a function of laser dosimetry. STUDY DESIGN/MATERIALS AND METHODS: Rabbit nasal septal cartilage were irradiated using a Nd:YAG laser (lambda = 1.32 microm, 5.4 mm spot diameter) with different exposure times of 4, 6, 8, 10, 12, and 16 seconds and powers of 4, 6, and 8 W. Temperature on the cartilage surface in the laser-irradiated region was collected using infrared thermography, this data was then used to predict tissue damage via a rate process model. A Live/Dead viability assay combined with fluorescent confocal microscopy was used to measure the amount of thermal damage generated in the irradiated specimens. RESULTS: Considerable thermal injury occurred at and below the laser-reshaping parameters that produced clinically relevant shape change using the present Nd:YAG laser. Confocal microscopy identified dead cells spanning the entire cross-sectional thickness of the cartilage specimen (about 500 microm thick) at laser power density and exposure times above 4 W and 6 seconds; damage increased with time and irradiance. The damage predictions made by the rate process model compared favorably with measured data. CONCLUSIONS: These results demonstrate that significant thermal damage is concurrent with clinically relevant shape change. This contradicts previous notions that there is a privileged laser dosimetry parameter where clinically relevant shape change and tissue viability coexist.

Characterization of temperature dependent mechanical behavior of cartilage.

BACKGROUND AND OBJECTIVES: Few quantitative studies have investigated the temperature dependent viscoelastic properties of cartilage tissue. Cartilage softens and can be reshaped when heated using laser, RF, or contact heating sources. The objectives of this study were to: (1) measure temperature dependent flexural storage moduli and mechanical relaxation in cartilage, (2) determine the impact of tissue water content and orientation on these mechanical properties, and (3) use these measurements to estimate the activation energy associated with the mechanical relaxation process. STUDY DESIGN/MATERIALS AND METHODS: Porcine nasal septal cartilage specimens (30 x 10 x 2 mm) were deformed using a single cantilever arrangement in a dynamic thermomechanical analyzer. Stress relaxation measurements were made at discrete temperatures ranging from 25 to 70 degrees C in response to cyclic deformation (within the linear viscoelastic region). The time and temperature dependent behavior of cartilage was measured using frequency multiplexing techniques (10-64 Hz), and these results were used to estimate the activation energy for the phase change using the Williams-Landel-Ferry (WLF) equation and the Arrhenius kinetic equation. In addition, the effect of tissue orientation was examined with specimens oriented in both transverse and longitudinal directions at room temperature. RESULTS: The storage moduli of porcine cartilage decreased with increasing temperature, and a critical change in mechanical properties was observed between 58 and 60 degrees C with a reduction in the storage modulus by 85-90%. The shift of the stress relaxation behavior from viscoelastic solid to viscoelastic liquid was observed between 50 and 57 degrees C and likely corresponds to the transition temperature region in which structural changes in the tissue occur. The storage moduli for transverse and longitudinally oriented specimens were 19-22 and 14-16 MPa, respectively at ambient temperature. Reducing the water content (<10% mass loss) by allowing it to dry under ambient conditions resulted in reduction in the storage modulus by 31-36%. The activation energy associated with the mechanical relaxation of cartilage was 147 kJ/mole at 60 degrees C. This value was calculated by measuring stress-strain relationship under conditions where linear viscoelastic behavior was observed (0.09-0.15% of strain) within the transition temperature region (58-60 degrees C). CONCLUSIONS: The anisotropic mechanical behavior of cartilage was quantitatively analyzed in the transversely and longitudinally oriented specimens. Viscoelastic behavior appeared to be strongly dependent on the water content. Using empirically determined estimates of the transition zone temperature range accompanying stress relaxation, the activation energy for stress relaxation was calculated using time and temperature superposition theory and WLF equation. Further investigation of the molecular changes, which occur during laser irradiation, may assist in understanding the thermal and mechanical behavior of cartilage and how the reshaping process might to be optimized.