Cartilage shaping with the Er:YAG laser: an in vivo experimental study

Bipolar Radiofrequency Plasma-Mediated Ablation of Porcine Nasal Septal Cartilage: A Pilot Investigation

Background

The objectives of this pilot study were to (1) determine whether bipolar radiofrequency plasma-mediated ablation (PMA) can efficiently remove nasal septal cartilage and (2) calculate the ablation rate as a function of device power, probe force, and translation velocity using ex vivo porcine tissue.

Methods

Specimens were secured to a linear translation stage and were subjected to varying translation velocities (4, 7, and 12 cm/s), probe forces (140, 200, and 225 g), and bipolar radiofrequency generator (Coblator ENTec power settings, 38–58, 77–115, and 129–193 Watts root mean squared. Specimen mass loss and depth of ablation were measured using an analytic balance and dissection microscope, respectively.

Results

Visual and microscopic inspection revealed little char. Mass loss increased with decreasing translation velocity and increasing generator setting. Increasing probe force also influenced mass loss and increased crater depth. Depth of ablation did not correlate with translation velocity or generator setting.

Conclusion

PMA effectively ablates nasal septal cartilage and may be able to reduce or contour cartilaginous deformities and framework structures in the head and neck.

Model to Estimate Threshold Mechanical Stability of Lower Lateral Cartilage

IMPORTANCE

In rhinoplasty, techniques used to alter the shape of the nasal tip often compromise the structural stability of the cartilage framework in the nose. Determining the minimum threshold level of cartilage stiffness required to maintain long-term structural stability is a critical aspect in performing these surgical maneuvers.

OBJECTIVE

To quantify the minimum threshold mechanical stability (elastic modulus) of lower lateral cartilage (LLC) according to expert opinion.

METHODS

Five anatomically correct LLC phantoms were made from urethane via a 3-dimensional computer modeling and injection molding process. All 5 had identical geometry but varied in stiffness along the intermediate crural region (0.63-30.6 MPa).

DESIGN, SETTING, AND PARTICIPANTS

A focus group of experienced rhinoplasty surgeons (n = 33) was surveyed at a regional professional meeting on October 25, 2013. Each survey participant was presented the 5 phantoms in a random order and asked to arrange the phantoms in order of increasing stiffness based on their sense of touch. Then, they were asked to select a single phantom out of the set that they believed to have the minimum acceptable mechanical stability for LLC to maintain proper form and function.

MAIN OUTCOMES AND MEASURES

A binary logistic regression was performed to calculate the probability of mechanical acceptability as a function of the elastic modulus of the LLC based on survey data. A Hosmer-Lemeshow test was performed to measure the goodness of fit between the logistic regression and survey data. The minimum threshold mechanical stability for LLC was taken at a 50% acceptability rating.

RESULTS

Phantom 4 was selected most frequently by the participants as having the minimum acceptable stiffness for LLC intermediate care. The minimum threshold mechanical stability for LLC was determined to be 3.65 MPa. The Hosmer-Lemeshow test revealed good fit between the logistic regression and survey data (χ23 = 0.92, P = .82).

CONCLUSIONS AND RELEVANCE

This study presents a novel method of modeling anatomical structures and quantifying the mechanical properties of nasal cartilage. Quantifying these parameters is an important step in guiding surgical maneuvers performed in rhinoplasty.

LEVEL OF EVIDENCE

5.

In vivo laser cartilage reshaping with carbon dioxide spray cooling in a rabbit ear model: a pilot study

BACKGROUND/OBJECTIVES

Similar to conventional cryogen spray cooling, carbon dioxide (CO2) spray may be used in combination with laser cartilage reshaping (LCR) to produce cartilage shape change while minimizing cutaneous thermal injury. Recent ex vivo evaluation of LCR with CO2 cooling in a rabbit model has identified a promising initial parameter space for in vivo safety and efficacy evaluation. This pilot study aimed to evaluate shape change and cutaneous injury following LCR with CO2 cooling in 5 live rabbits.

STUDY DESIGN/MATERIALS AND METHODS

The midportion of live rabbit ears were irradiated with a 1.45 µm wavelength diode laser (12 J/cm(2)) with simultaneous CO2 spray cooling (85 millisecond duration, 4 alternating heating/cooling cycles per site, 5 to 6 irradiation sites per row for 3 rows per ear). Experimental and control ears (no LCR) were splinted in the flexed position for 30 days following exposure. A total of 5 ears each were allocated to the experimental and control groups.

RESULTS

Shape change was observed in all irradiated ears (mean 70 ± 3°), which was statistically different from control (mean 37 ± 11°, P = 0.009). No significant thermal cutaneous injury was observed, with preservation of the full thickness of skin, microvasculature, and adnexal structures. Confocal microscopy and histology demonstrated an intact and viable chondrocyte population surrounding irradiated sites.

CONCLUSIONS

LCR with CO2 spray cooling can produce clinically significant shape change in the rabbit auricle while minimizing thermal cutaneous and cartilaginous injury and frostbite. This pilot study lends support for the potential use of CO2 spray as an adjunct to existing thermal-based cartilage reshaping modalities. An in vivo systematic evaluation of optimal laser dosimetry and cooling parameters is required.

In-depth analysis of pH-dependent mechanisms of electromechanical reshaping of rabbit nasal septal cartilage

OBJECTIVES/HYPOTHESIS

Electromechanical reshaping (EMR) involves reshaping cartilage by mechanical deformation and delivering electric current to the area around the bend axis, causing local stress relaxation and permanent shape change. The mechanism of EMR is currently unclear, although preliminary studies suggest that voltage and application time are directly related to the concentration and diffusion of acid-base products within the treated tissue with little heat generation. This study aims to characterize local tissue pH changes following EMR and to demonstrate that local tissue pH changes are correlated with tissue damage and shape change.

STUDY DESIGN

Ex vivo animal study involving EMR of rabbit nasal septal cartilage and biochemical estimation of tissue pH changes.

METHODS

The magnitude and diffusion of acid-base chemical products in control (0V, 2 minutes), shape change (4V, 4 minutes; 6V, 1, 2, 4 minutes; 8V, 1, 2 minutes), and tissue damage (8V, 4, 5 minutes; 10V, 4, 5 minutes) parameters following EMR are approximated by analyzing local pH changes after pH indicator application.

RESULTS

There is a direct relationship between total charge transfer and extent of acid-base product diffusion (P <0.05). A "pH transition zone" is seen surrounding the bend apex above 8V, 2 minutes. Colorimetric analysis suggests that small local pH changes (10(-8) hydrogen ions) are at least partly implicated in clinically efficacious EMR.

CONCLUSIONS

These results provide additional insight into the translational applications of EMR, particularly the relationship among pH changes, shape change, and tissue injury, and are integral in optimizing this promising technology for clinical use.

Preliminary investigations on therapy thresholds for laser dosimetry, cryogen spray cooling duration, and treatment cycles for laser cartilage reshaping in the New Zealand white rabbit auricle

Previous studies have demonstrated the feasibility of laser irradiation (λ = 1.45 μm) in tandem with cryogen spray cooling (CSC) to reshape rabbit auricular cartilage using a total energy density of 14 J/cm(2). The aim of this study was to further explore and identify the dosimetry parameter space for laser output energy, CSC duration, and treatment cycles required to achieve shape change while limiting skin and cartilage injury. Ten New Zealand white rabbits were treated with the 1.45 μm diode laser combined with cryogen spray cooling (Candela Smoothbeam™, Candela Co., Wayland, MA, USA). The ear's central portion was bent around a cylindrical jig and irradiated in consecutive spots of 6 mm diameter (13 or 14 J/cm(2) per spot) along three rows encompassing the bend. CSC was delivered during irradiation in cycles consisting of 25-35 ms. At thin and thick portions of the ear, 4-7 and 6-10 treatment cycles were delivered, respectively. After surgery, ears were examined and splinted for 6 weeks. Treatment parameters resulting in acceptable (grades 1 and 2) and unacceptable (grade 3) skin injuries for thick and thin regions were identified, and shape change was observed. Confocal and histological analysis of cartilage tissue revealed several outcomes correlating to laser dosimetry, CSC duration, and treatment cycles. These outcomes included expansion of cartilage layers (thickening), partial cartilage injuries, and full-thickness cartilage injuries. We determined therapy thresholds for laser output energy, cryogen spray cooling duration, and treatment cycles in the rabbit auricular model. These parameters are a starting point for future clinical procedures aimed at correcting external ear deformities.

Ex vivo investigations of laser auricular cartilage reshaping with carbon dioxide spray cooling in a rabbit model

Laser cartilage reshaping (LCR) with cryogen spray cooling is a promising modality for producing cartilage shape change while reducing cutaneous thermal injury. However, LCR in thicker tissues, such as auricular cartilage, requires higher laser power, thus increasing cooling requirements. To eliminate the risks of freeze injury characteristic of high cryogen spray pulse rates, a carbon dioxide (CO2) spray, which evaporates rapidly from the skin, has been proposed as the cooling medium. This study aims to identify parameter sets which produce clinically significant reshaping while producing minimal skin thermal injury in LCR with CO2 spray cooling in ex vivo rabbit auricular cartilage. Excised whole rabbit ears were mechanically deformed around a cylindrical jig and irradiated with a 1.45-μm wavelength diode laser (fluence 12-14 J/cm(2) per pulse, four to six pulse cycles per irradiation site, five to six irradiation sites per row for four rows on each sample) with concomitant application of CO2 spray (pulse duration 33-85 ms) to the skin surface. Bend angle measurements were performed before and after irradiation, and the change quantified. Surface temperature distributions were measured during irradiation/cooling. Maximum skin surface temperature ranged between 49.0 to 97.6 °C following four heating/cooling cycles. Significant reshaping was achieved with all laser dosimetry values with a 50-70 °C difference noted between controls (no cooling) and irradiated ears. Increasing cooling pulse duration yielded progressively improved gross skin protection during irradiation. CO2 spray cooling may potentially serve as an alternative to traditional cryogen spray cooling in LCR and may be the preferred cooling medium for thicker tissues. Future studies evaluating preclinical efficacy in an in vivo rabbit model are in progress.

Changes in the tangent modulus of rabbit septal and auricular cartilage following electromechanical reshaping

Transforming decades' old methodology, electromechanical reshaping (EMR) may someday replace traditionally destructive surgical techniques with a less invasive means of cartilage reshaping for reconstructive and esthetic facial surgery. Electromechanical reshaping is essentially accomplished through the application of voltage to a mechanically deformed cartilage specimen. While the capacity of the method for effective reshaping has been consistently shown, its associated effects on cartilage mechanical properties are not fully comprehended. To begin to explore the mechanical effect of EMR on cartilage, the tangent moduli of EMR-treated rabbit septal and auricular cartilage were calculated and compared to matched control values. Between the two main EMR parameters, voltage and application time, the former was varied from 2-8 V and the latter held constant at 2 min for septal cartilage, 3 min for auricular cartilage. Flat platinum electrodes were used to apply voltage, maintaining the flatness of the specimens for more precise mechanical testing through a uniaxial tension test of constant strain rate 0.01 mm/s. Above 2 V, both septal and auricular cartilage demonstrated a slight reduction in stiffness, quantified by the tangent modulus. A thermal effect was observed above 5 V, a newly identified EMR application threshold to avoid the dangers associated with thermoforming cartilage. Optimizing EMR application parameters and understanding various side effects bridge the gap between EMR laboratory research and clinical use, and the knowledge acquired through this mechanical study may be one additional support for that bridge.

Stabilization of costal cartilage graft warping using infrared laser irradiation in a porcine model

OBJECTIVE

To develop a method to rapidly stabilize the shape change process in peripheral slices of costal cartilage by using infrared laser irradiation in a porcine model.

METHODS

Forty peripheral porcine costal cartilage specimens (40 × 10 × 2 mm) were harvested. Thirty of these specimens were immediately irradiated with an Nd:YAG laser (λ = 1.32 μm; spot size, 2-mm diameter) using 1 of 3 exposure treatments: 6 W, 2 seconds, and 4 spots; 8 W, 3 seconds, and 4 spots; or 6 W, 2 seconds, and 8 spots. Ten control specimens were only immersed in 0.9% saline solution. Angle of curvature was measured from photographs taken at 0 minutes, immediately after irradiation, and at 30 minutes, 1 hour, 5 hours, and 24 hours. Infrared imaging was used to measure surface temperatures during irradiation. Cell viability after irradiation was determined using a live/dead assay in conjunction with fluorescent confocal microscopy.

RESULTS

Compared with the untreated controls, the irradiated grafts underwent accelerated shape change within the first 30 minutes to reach a stable geometry. Thereafter, irradiated grafts underwent little or no shape change, whereas the control group exhibited significant change in curvature from 30 minutes to 24 hours (P < .001). The average peak irradiated spot temperatures ranged from 76°C to 82°C. Cell viability measurements at the laser spot sites demonstrated a hemispherically shaped region of dead cells with a depth of 0.8 to 1.2 mm and a surface diameter of 1.9 to 2.7 mm.

CONCLUSIONS

Laser irradiation of peripheral costal cartilage slices provides an effective method for rapidly stabilizing acute shape change by accelerating the warping process. The temperature elevations necessary to achieve this are spatially limited and well within the limits of tolerable tissue injury.

Minimally invasive ear reshaping with a 1450-nm diode laser using cryogen spray cooling in New Zealand white rabbits

BACKGROUND

Otoplasty is the current standard of care for treating prominent ears, a psychologically and sometimes functionally disabling disorder. The technically demanding procedure carries many risks such as poor aesthetic outcome, need for revision surgery, and need for general anesthesia. This study investigates the use of laser irradiation combined with cryogen skin cooling and stenting to reshape cartilage in the ears of New Zealand white rabbits.

METHODS

In this prospective, randomized, internally controlled animal study, the right ears of 9 rabbits were mechanically deformed with a jig and then irradiated with a 1450-nm diode laser combined with cryogen skin cooling (14 J/pulse with cryogen spray for 33 milliseconds per cycle and a 6-mm spot size). The left ear served as the control. The ears were splinted for 1, 3, or 4 weeks. The rabbits were then given a lethal dose of intravenous pentobarbital, and the splints were removed and ears examined and photographed. Light and confocal microscopy were performed on the specimens.

RESULTS

Shape change was observed in all 9 treated rabbit ears, while none of the control ears (stenting alone) showed significant change. Qualitatively, reshaped ears were stiffer after 4 weeks of splinting than after 1 or 3 weeks. None of the rabbits showed evidence of skin injury nor did they show signs of postprocedural pain. Findings from histologic analysis in the treated areas showed evidence of an expanded chondrocyte population in the region of laser irradiation, along with some perichondrial thickening and some fibrosis of the deep dermis. Confocal microscopy revealed minimal cellular death at 1 week and none thereafter.

CONCLUSIONS

Cartilage reshaping using laser energy can be performed safely transcutaneously using cryogen spray cooling in rabbits. This animal model has similarity to human ears with regard to skin and cartilage thickness and is a stepping stone toward developing minimally invasive laser auricle reshaping in humans.

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.