Lasers in Surgery and Medicine 37:201–209 (2005)

Shape Retention in Porcine and Rabbit Nasal SeptalCartilage Using Saline Bath Immersion andNd:YAG Laser Irradiation

Ryan Wright, BS,1 Dmitry E. Protsenko, PhD,1 Sergio Diaz, PhD,1 Kevin Ho, MD,1 and Brian Wong, MD, PhD1,2,3*

1The Beckman Laser Institute, University of California, Irvine, California2Department of Otolaryngology—Head and Neck Surgery, University of California, Irvine, California3The Department of Biomedical Engineering, University of California Irvine, Irvine, California

Background and Objectives: The process of altering theshape of cartilage using heat has been referred to asthermoforming, and presents certain clinical benefits inreconstructive surgical procedures within the head andneck. Thermoforming allows cartilage in the upper airwayand face to be reshaped without the use of classic surgicalmaneuvers such as carving, morselizing, or suturing. Thegoal of this study was to determine the dependence ofcartilage shape change on both temperature and laserdosimetry using two thermoforming methods: saline bathimmersion and laser irradiation.Study Design/Materials and Methods: Ex-vivo rabbitand porcine nasal septal cartilages were mechanicallydeformed and reshaped using the two thermoformingmethods. With saline bath immersion using rabbit carti-lage, each specimen was deformed by securing it to a smallcopper tube (outer diameter 8 mm) using dental bands. Forporcine cartilage immersed in a saline bath, each samplewas mechanically deformed between two pieces of wiremesh attached to a semicircular acrylic block. With bothporcine and rabbit cartilage, the specimen and apparatuswere then immersed in a hot saline bath for time in-tervals varying from 20 and 320 seconds and at constanttemperatures between 62 and 748C. In laser reshaping,the cartilage specimens were mechanically deformed on ajig and consecutively irradiated with an Nd:YAG laser(l¼ 1.32 mm) in several spots for 6–16 seconds andirradiances of 10.2–40.7 W/cm2 per spot. After either salinebath heating or irradiation, cartilage specimens wereimmersed in room temperature saline for 15 minutes, thenupon removal from the jig the length between the ends ofeach specimen was measured in order to calculate theresulting bend angle.Results: The transition zone for cartilage reshaping wasdefined as where a significant increase in bend anglewas observed between consecutive times of immersion/irradiation at the same temperature/irradiance. For thesaline bath experiments, the transition zone was observedbetween 59–688C and 62–688C for porcine and rabbitcartilage, respectively. Similar transition zones occurredwith laser irradiation below irradiances of 20.4 W/cm2 forboth porcine and rabbit cartilage. In addition, the dosime-try pairs in the transition zones produce peak temperatures

below the thresholds determined from the saline bathimmersion studies.Conclusions: The critical transition temperature regionwas determined by the sharp increase in bend angle atconsecutive times of immersion at the same temperature.This range was determined to be 59–688C and 62–688C forporcine and rabbit cartilage, respectively. Similar transi-tion zones for dosimetry occurred below 20.4 W/cm2 duringcartilage irradiation in both species. Lasers Surg. Med.37:201–209, 2005. � 2005 Wiley-Liss, Inc.

Key words: cartilage; rabbit; reshaping; laser; nasalseptum; plastic surgery; septoplasty

INTRODUCTION

In the past, plastic surgeons have relied upon classicalsurgical techniques such as cutting, scoring, suturing, andmorselization to reshape cartilage tissue. Implementingthese maneuvers generally requires the creation of largeincisions to expose the region of interest where the cartilageis deformed and must be reshaped. Classic open surgeryincreases the risk of complications and requires longerrecovery times. In order to facilitate minimally invasiveoperations to treat cartilaginous deformities and defects inthe head, neck, and airway, our laboratory has engaged inresearch to develop several methods which use heat toreshape cartilage tissue in a generalized process we havetermed thermoforming, which can be accomplished usinglaser irradiation [1], radiofrequency exposure [2], or simple

Sergio Diaz is presently with Fiberblade Elica, Imarcoain(Navarra), Spain.

Contract grant sponsor: The National Institutes of Health;Contract grant numbers: DC00170-01, DC005572, RR01192;Contract grant sponsor: The Air Force Office of ScientificResearch; Contract grant number: FA9550-04-1-0101; Contractgrant sponsor: The Beckman Foundation; Contract grant sponsor:The Undergraduate Research Opportunities Program at theUniversity of California, Irvine (Mr. Wright).

*Correspondence to: Brian Wong, MD, PhD, The BeckmanLaser Institute, University of Calfornia Irvine, 1002 HealthSciences Road, East, Irvine, CA 92612. E-mail: bjwong@uci.edu

Accepted 3 June 2005Published online 26 August 2005 in Wiley InterScience(www.interscience.wiley.com).DOI 10.1002/lsm.20212

� 2005 Wiley-Liss, Inc.

contact heating with materials such as hot saline or heatconducting materials.

Laser reshaping is a thermoforming technique originallydeveloped by Helidonis et al. [3]. They proposed using laserirradiation to reshape cartilage using non-ablative laserfluences. In this process, heating mechanically deformedcartilage specimens accelerates the process of stress re-laxation and produces a significant shape change. Oneadvantage to laser reshaping is that no suturing, carving,or morselization is required, and cartilage conceivablymay be reshaped into new geometric configurations thatwould not otherwise be possible using classic maneuvers.Laser thermoforming may allow surgeons to create uniqueshapes to better match defects that occur as a consequenceof trauma, congenital malformation, or cancer. In additionto being amenable to minimally invasive delivery techni-ques, the process of laser thermoforming is potentiallyreversible and can provide accurate control of the tempera-ture distribution and evolution of heat.

Cartilage is composed of proteoglycans, type II collagen,and water. The proteoglycans contain a core proteinlinked to a glycosaminoglycan, which contains negativelycharged carboxy and sulfate groups. Because of their nega-tive charge, polar water molecules surround the carboxyand sulfate groups, and innumerable hydrogen bonds existbetween various matrix macromolecules, collectively en-hancing the retention of shape. According to Helidonis [3],Sobol [4,5], and Bagratashvili [6], when heating cartilagetissue, the water molecules may detach from the carboxyand sulfate groups, reducing steric hindrance and inter-molecular forces allowing the proteoglycans to slide pasteach other. This movement along with partial collagendenaturation may be responsible for the permanent shapechange observed in both animal [7–9] and human studies[10].

The two thermoforming techniques used in this study arelaser irradiation and saline bath immersion. The primaryreason for using a saline bath is that cartilage specimensreach thermal equilibrium with the bath rapidly, and thetemperature profile is uniform for the duration of heatingunder most conditions, particularly when the specimenis immersed for a period of time significantly longer thanthe characteristic time of thermal equilibration. In con-trast, with a laser, the temperature gradually increases

until irradiation is terminated, and a spatially inhomoge-neous temperature profile develops and evolves over time.Regardless, laser irradiation is more feasible in a clinicalsetting because of the obvious impracticalities of usingan immersion method in minimally invasive procedures.However, by using a saline bath, we can better examine thetemperature and time-dependence of the shape changeprocess in cartilage tissue and then identify which laserirradiation parameters (and corresponding temperature-time profiles) roughly mimic these conditions. Using bothlagamorph (rabbit) and porcine nasal septal cartilage, theobjectives of this experiment are to measure shape changefollowing: (1) saline bath immersion as a function of tem-perature and immersion time and (2) laser heating as afunction of irradiance (W/cm2) and exposure time with anNd:YAG laser (l¼ 1.32 mm).

MATERIALS AND METHODS

Porcine Cartilage Thermoforming

Porcine nasal septal cartilage was extracted from thecrania of domestic pigs obtained from a local abattoir(Clougherty Packing Company, Vernon, CA) as previouslydescribed [11]. The porcine nasal septum varies in thick-ness and mechanical behavior along the cranial–caudallength of the septum [12]. The cephalic region of the septumis very thin and flexible while the caudal area is denser andmore rigid. To minimize experimental variation due tothese differences, the samples chosen for this study weretaken only from the middle region along the length of theseptum. The mucosa and perichondrial tissues were thenremoved from the underlying tissue. The harvested cartil-age was then cut into slabs (25� 5� 1.5 mm) using acustom-built guillotine microtome [13].

Saline Bath Immersion

The harvested porcine nasal septal cartilage speci-mens were mechanically deformed in a semi-circular jig(diameter 13 mm) (schematically illustrated in Fig. 1).During bending, force is distributed uniformly across thelength of the specimen so the mechanical stress in eachcross-sectional area is the same. The jig is composed of twopieces of wire mesh, which are used to deform the specimenwhile exposing the saline bath to as much of the tissue as

Fig. 1. Mechanical deformation apparatus for porcine cartilage immersed in saline bath.

202 WRIGHT ET AL.

possible. Once the cartilage is deformed, both meshes aresecured to an acrylic platform by an elastic band to pro-vide adequate force to maintain a specimen in deformation.The entire assembly was placed in a room temperaturesaline bath, and allowed to equilibrate for five minutes. Theassembly was then removed and rapidly placed into a salinebath at temperatures between 56–748C and for immer-sion times between 5–320 seconds. To minimize thermalgradients within the bath, the water was constantly stirred.A thermocouple placed into the water bath monitored thetemperature. Once the designated time interval elapsed,the jig was removed from the hot saline bath andimmediately re-immersed in the room temperature salinebath for 15 minutes. Then the cartilage specimen wasremoved form the jig and the distance between the ends ofthe slab was measured to determine bend angle (see below).In control experiments cartilage specimens were mechani-cally deformed in the jig and then submerged in a roomtemperature saline bath for duration of 5–320 seconds.

Laser Irradiation

Harvested porcine nasal septal cartilage specimens weremechanically deformed on a custom jig assembly, con-structed from aluminum tubing and high tension wires[14]. Rectangular cartilage specimens secured in the jigassumed a semicylindrical shape 13 mm in diameter. Thejig was secured to a motorized mechanical stage, whichrotated the cylindrical jig relative to the laser (Fig. 2). Thestage is controlled using software written in LabView(National Instruments, Austin, TX). Light from an Nd:YAGlaser (l¼ 1.32 mm, 50 Hz PPR, New Stars Lasers, Auburn,CA) was directed perpendicularly at the cartilage specimenusing a multimode optical fiber, coupled in a collimator witha thermopile detector (New Star Lasers). Thus real timemeasurements of surface temperature could be measuredduring the irradiation sequences. The laser spot size wasmeasured using burn paper (Kentak, Pittsfield, NH). Sincethe cartilage specimen had dimensions of 5� 25 mm andthe laser spot size was approximately 5 mm, the Nd:YAGlaser was programmed to irradiate five positions in a non-overlapping vertical sequence. A 15 second delay wasprogrammed between each irradiation spot to minimizeoverheating due to heat conduction. The porcine nasal

septal cartilage specimens were irradiated at irradiancesof 10.2, 20.4, 30.6, and 40.7 (W/cm2) for irradiation times of4–16 seconds per spot. In control trials, the cartilage wassecured in the jig for the same amount of time as theexperimental trials, and not irradiated. Once the irradia-tion sequence was complete, the reshaped cartilage speci-men was removed from the motorized jig assembly andquickly wrapped around a copper tube with the samediameter as the jig, immediately secured with dental elasticbands, immersed in room temperature saline, and allowedto rehydrate for 15 minutes. The distance between the endsof the slab was measured after rehydration.

Lagamorph Cartilage Thermoforming

Rabbit nasal septal cartilage was extracted from thecrania of freshly euthanized New Zealand White Rabbits aspreviously described [12]. The mucosal and perichondrialtissues were removed from the underlying cartilage, whichwas then cut into uniform slabs (5�15 mm) with thicknessvarying from 0.70 to 0.80 mm. Because rabbit nasal septalcartilage is already thin, no attempt at cutting them to auniform thickness was made. The thickness of the septumdoes not vary considerably in the rabbit, unlike the pig [12].

Saline Bath Immersion

The specimens were wrapped around a cylindrical jig(5.5 mm) fashioned from a copper dowel, which was per-forated with a 1 mm drill numerous times to increasecontact between the saline and the specimen. The cartilagewas secured to the dowel using dental elastic bands. The jigalong with the attached cartilage was immersed in a hotsaline bath at temperatures between 62–748C and for timesbetween 20 and 320 seconds. Thermoforming was accom-plished using the same general methodology discussedabove for porcine tissues. After thermoforming and equili-bration with an ambient temperature saline bath for15 minutes, the cartilage specimen was then removed formthe jig and the distance between the ends of the slab wasmeasured. With control trials, the cartilage was mechani-cally deformed and immersed in room temperature salinefor times between 20 and 320 seconds.

Laser Irradiation

Rabbit nasal septal cartilage specimens were mechani-cally deformed on the same motorized assembly as theporcine cartilage slabs, but using a jig with a diameter of5.5 mm. Since the spot size of the laser is approximately5 mm, the irradiation sequence for the rabbit cartilagespecimens consisted of three vertical non-overlappingirradiation spots (Fig. 3). The specimens were irradiatedwith the Nd:YAG laser (l¼ 1.32 mm) at irradiances of 10.2,15.3, 20.4, 30.6, and 40.7 W/cm2 for 6–16 seconds per spot.Once the irradiation was complete, the reshaped cartilagespecimen was removed from the motorized jig assembly andattached to a dowel of the same diameter as the jig, securedwith dental elastic bands, and then allowed to rehydratein a room temperature saline bath for 15 minutes. Thedistance between the ends of the slab was measured afterFig. 2. Laser irradiation of nasal septal cartilage.

SHAPE RETENTION IN CARTILAGE 203

rehydration. Again, in control trials the cartilage wassecured in the jig for the same amount of time as theexperimental trials and not irradiated.

Bend Angle Determination

The distance between the ends of each specimen wasmeasured before reshaping and at 15 minutes after im-mersion in order to calculate the bend angle [15]. In order tocompare the shape change between the various specimens,we assumed each specimen was reshaped into an arcsegment of a circle. Using this model, the distance betweenthe ends of a cartilage slab is equal to the length of the chordjoining the ends of the arc, where:

L ¼ 2Li sinðy=2Þy

L is the distance between the ends of the bent sample, Li

is the initial length of the sample before clamping, and y isthe bend angle in radians. The bend angle correspondingto each set of measurements was numerically determinedby solving the above equation in Excel (Microsoft Corpora-tion, Redmond, WA). The bend angle in degrees was usedfor analysis. Digital photographs were also taken of thecartilage slabs 15 minutes after laser irradiation or salinebath immersion to visually compare the two thermoform-

ing methods. At least five experiments were repeated foreach species, heating method, and heating setting.

RESULTS

Porcine and Lagamorph Saline Bath Immersion

Figure 4a is a montage of digital images of water bathreshaped specimens illustrating the effect of immersiontime (5, 20 80, 320 seconds) on shape change at 718C. Like-wise, Figure 4b depicts similar images where the immer-sion time was constant (80 seconds) while the temperatureof the saline bath (65, 68, 71, and 748C) was varied.

The bend angle as a function of five distinct immersiontimes (5, 20, 80, 160, 320 seconds) is shown in Figure 5 forporcine cartilage immersed at eight different saline bathtemperatures. For clarity, the error bars have been omittedin this figure, but are identical to those illustrated insubsequent figures. The resultant bend angle after thermo-forming gradually increases with immersion time for eachtemperature between 59 and 658C. Yet after 658C, the bendangles do not significantly increase with consecutive timesof immersion. For example, the bend angles produced at718C remain fairly constant for immersion times between20 and 320 seconds. In control trials, only the porcine tissueproduced a measurable bend angle during saline bathimmersion. By keeping the time of immersion constant, the

Fig. 3. Laser irradation pattern for rabbit nasal septal cartilage.

204 WRIGHT ET AL.

bend angles begin to plateau, approaching the maximumtheoretical bend angle (2128). The bend angle of rabbitnasal septal cartilage following saline bath immersion(Fig. 6) follows a similar trend to its porcine counterpart.The bend angle also shows a similar plateau effect between

68 and 748C for the given immersion times, and approachesa maximum theoretical bend angle of 2448. These resultswill provide clues as to which laser parameters to useduring laser reshaping, since we now have an estimate ofshape change as a function of temperature.

Fig. 4. a: Acute porcine cartilage reshaping after immersion in 718C saline at different time

intervals. b: Acute porcine cartilage reshaping after immersion for 80 seconds at different

saline bath temperatures. c: Acute porcine cartilage reshaping after irradiation at 10.2 W/cm2

for varying irradiation times. d: Acute rabbit cartilage reshaping after irradiation at

10.2 W/cm2 for varying irradiation times.

Fig. 5. Resulting bend angles for porcine nasal septal cartilage

15 minutes after immersion in hot saline bath.

Fig. 6. Rabbit nasal septal cartilage immersed in hot saline

bath.

SHAPE RETENTION IN CARTILAGE 205

Porcine and Lagamorph Laser Irradiation

The bend angles produced using a laser to reshape bothrabbit and porcine nasal septal cartilage (Figs. 7–10) de-monstrate some unique features that differ from what isobserved in saline bath heating. As expected, however,increasing either the irradiance or the irradiation timecauses an increase in the bend angle, until the irradianceexceeds 20.4 w/cm2. After this cutoff, the bend angle doesnot significantly increase as the time of irradiation in-creases. The nasal septal cartilage in the control trialsproduced no measurable bend angle. The bend angle de-creases at irradiances above 40.7 W/cm2. At these irradi-ance levels, very visible reactions, including pyroloysis,

vaporization of water, and the appearance of cracks alongthe surface of the cartilage were observed.

The thermopile used to record temperature in the laser-irradiated regions provided estimates of the cartilagesurface temperature during the irradiation sequence. Foreach individual laser exposure, the peak temperature wasrecorded. Tables 1 and 2 list these peak temperature mea-surements for laser irradiated rabbit and porcine nasalseptal cartilage. The temperatures highlighted in boldfacetype correspond to the threshold temperature range inwhich we observe a saturation effect in the saline bathimmersion experiments. Bend angles observed in salinebath experiments at temperatures higher than the thresh-old did not drastically increase with immersion time.

Fig. 7. Resulting bend angles for laser irradiated rabbit nasal

septal cartilage as a function of time.

Fig. 8. Resulting bend angles for laser irradiated porcine nasal

septal cartilage as a function of time.

Fig. 9. Resulting bend angles for laser irradiated rabbit nasal

septal cartilage as a function of irradiance.

Fig. 10. Resulting bend angles for laser irradiated porcine

nasal septal cartilage as a function of irradiance.

206 WRIGHT ET AL.

DISCUSSION

Heated saline bath studies determined that significantshape change is first observed for immersion times greaterthan 80 seconds for 568C and this shape change exists for alltimes above this threshold temperature in porcine tissues(Fig. 5). A similar threshold for shape change was observedat the lower immersion times of 628C in rabbit cartilage(Fig. 6). These temperature and time combinations can bedesignated as the initial stage of the transition zone where alarge increase in bend angle is observed between con-secutive immersion times at the same temperature. Forporcine and lagomorph nasal septal cartilage, the transi-tion zone signifies the existence of a biophysical changewithin the cartilage tissue which is manifest by an increasein the rate of stress relaxation. The heat deposited intothe cartilage induces the necessary alteration in matrixcomponents resulting in a shape change. This process maybe caused by several complimentary processes such as atransition between bound and unbound states of watermolecules [3–6], protein denaturation in the extracellularmatrix, or some other factors. Regardless of the mechanismunderlying this biophysical alteration, the significantincrease in bend angle reveals what specific time andtemperature combination will result in the process of stressrelaxation. In Figures 5 and 6, the transition zone forporcine and rabbit nasal septal cartilage occurs between59–688C and 62–688C (respectively) for the given immer-sion times, and above these temperatures a clearly observ-ed saturation effect suggests completion of the cartilagealteration process.

Even though the saline bath experiments provide evid-ence for the successful reshaping of cartilage, laser reshap-ing is the more clinically feasible option for reconstructivesurgery, particularly when considering minimally in-

vasive operations. In laser reshaping experiments, transi-tion zone behavior is related to laser dosimetry. Dosimetryalong with tissue optical and thermal properties deter-mines the time-dependent temperature change. For simpli-city, we limit the discussion to dosimetry, and similartransition zone behavior is observed at or below irradiancesof 20.4 W/cm2 for both porcine and rabbit septal cartilage.According to previous studies, changes in the internalstress during the laser irradiation of porcine cartilage occurwhen the surface temperature reaches 60–708C [11]. Table2 can be used to identify the laser parameters producingpeak surface temperatures that correspond to the upperlimits of the transition zone (�688C). These findings are inagreement with previous studies that identified that thecritical temperature for stress relaxation in septal cartilageoccurs between 60 and 708C [11].

In both saline bath and laser heating, the plateau in bendangle reached with higher temperatures or greater energydeposition respectively reveals the existence of a saturationeffect. The endpoints of the transition zone (e.g., whereprolonged exposure does not produce an appreciable in-crease in bend angle) indicate the completion of allprocesses needed for the biophysical change to take placeand the beginning of the saturation point. Regardless ofhow long the cartilage is immersed in the saline bath orirradiated, at some point the bend angle of the reshapedcartilage cannot physically exceed a certain level. Sinceall possible biophysical reactions have been completed athigher immersion temperatures and irradiances, there is asmall difference between the bend angles of consecutivetimes of immersion and irradiation. This saturation effect isclearly observed in Figures 9 and 10 where the data hasbeen reformatted to reflect the dependence of bend angle onirradiance for specific irradiation times; the plateau occursat irradiance levels above 20 w/cm2. When the data isreformatted in terms of total energy delivery (Figures 11and 12), the plateau region is observed to begin just above70 J. With any laser parameters that exceed these condi-tions, the bend angle does not incrementally increase. Incontrast, increasing the irradiation time for the irradiancesin the transition zone (e.g., 10.2 and 15.3 W/cm2) willproduce significantly larger bend angles.

At a laser irradiance of 40.7 W/ cm2 for rabbit cartilage, adecrease in bend angle was observed as the time of irradia-tion increased. This decrease is associated with obviousphysical damage in the sample due to the appearance ofcracks, tears, and ablation craters on its surface. Thedamage originates at and around the points of contactbetween cartilage and metal wires. The increased density ofcartilage and the concentration of stress at these sites leadto an increased absorption of laser light, an excessivetemperature rise, and the fracturing and softening of thecartilage tissue matrix. A further increase in laser irradi-ance will undoubtedly result in greater injury andadditional loss of structural integrity.

Tables 1 and 2 provide the peak temperature measure-ments for different combinations of irradiance andexposure time for the laser reshaping of porcine andrabbit nasal septal cartilage, respectively. The boldfaced

TABLE 1. Peak Temperature Measurements for Laser

Irradiated Rabbit Cartilage

Irradiance

Seconds

6 8 10 12 16

10.2 59 64 70 72 76

15.3 64 68 74 76 93

20.4 72 77 81 93 103

30.6 87 91 105 113 123

40.7 115 127 137

TABLE 2. Peak Temperature Measurements for

Porcine Nasal Septal Cartilage

Irradiance

Seconds

4 6 8 10 12 16

10.2 34 41 44 47 51 53

20.4 43 52 59 60 65 74

30.6 51 69 80 83 88 86

40.7 59 70 82

SHAPE RETENTION IN CARTILAGE 207

temperatures correspond to the values at or above thesaturation temperatures identified in the series of salinebath experiments. The laser parameters, which reproducethese temperatures, are identical to the dosimetry pairsthat result in saturation effects as well. Thus it can beinferred that similar biophysical changes occur during boththermoforming methods.

When calculating the bend angle, we assumed that thereshaped cartilage specimen could be perfectly representedas an arc segment of a circle, which is a fairly simple andpractical way to gauge shape change using linear dimen-sions to represent curved shapes. However, in laser re-shaping experiments, the mechanical forces transmitted bythe tension wires were not uniformly distributed across thespecimen to exactly match the circular geometry of the jig.These deviations in specimen shape may be better repre-

sented by a model that incorporates the concentration offorce at discrete locations along the specimen length; such aformulation is beyond the scope of the present study.

In the saline bath studies, we assumed that the immer-sion technique produces instantaneous step-wise tempera-ture changes that occur uniformly through the entirespecimen [1]. In reality, non-uniform heating due to heatconduction at a finite rate through the thickness of thesespecimens occurs. Each specimen was unique with regardsto its thermal profile because slight differences in immer-sion time produced by the manual removal and replace-ment of a specimen from the heated saline bath intoambient temperature saline. These effects have minimalimpact for the longer immersion times, but do play duringthe shorter immersion times. The data’s systematic error,error bars in Figures 5 and 6, demonstrate that theseeffects have minimal impact for the longer immersiontimes. Also, because of differences in both geometry andsize, the thinner rabbit cartilage reaches a thermal equi-librium during thermoforming much more quickly than thethicker pig cartilage.

Since thermoforming cartilage is a time and temperaturedependent process, we can conclude that the underlyingmolecular mechanisms may conform to the characteristicsof a rate process [16]. If cartilage reshaping is in fact a rateprocess, the results of this study may be used to charac-terize the shape change mechanism by developing rateprocess models. With such a model, then it may be possibleto implement numerical simulations that can predictretained bend angle knowing both the time of immersionand the temperature, or in the case of laser irradiation,the time and space dependent evolution of temperature.These rate process models can be of great use to develop amore general theory about how shape change depends uponboth time and temperature. Previously, our group hasperformed rate process model analysis focused on chon-drocyte viability using the water bath immersion techniquedescribed in this study [1]. The exact biophysical changesthat occur at different temperatures must be furtherstudied in order to fully understand the mechanism behindcartilage reshaping.

One important future goal is to further characterizechondrocyte viability following thermoforming. Tissue thathas been exposed to extreme environmental conditionssuch as high temperatures will either undergo necrosis orcellular apoptosis. During necrosis, cells within the damag-ed area will swell and burst, causing inflammation. Withapoptosis, cells undergo programmed cell death. They willshrink and ultimately be recycled by macrophages, causinglittle injury to surrounding tissue. In both laser irradiationand saline bath immersion, the extent of necrosis andapoptosis will depend upon the amount of heat producedduring the process of thermoforming. It is clear fromnumerous previous investigations that laser cartilagereshaping is a process that must balance the competingdemands of producing effective shape change and preser-ving tissue viability [9,17–19]. This is the first study thathas systematically studied the effect of time and tempera-ture on shape change in cartilage using two different

Fig. 11. Resulting bend angles for laser irradiated rabbit nasal

septal cartilage as a function of energy.

Fig. 12. Resulting bend angles for laser irradiated porcine

nasal septal cartilage as a function of energy.

208 WRIGHT ET AL.

methods of tissue thermoforming. The second key issuerelated to cartilage reshaping, namely, tissue viability willbe addressed in a forthcoming companion paper.

CONCLUSIONS

The critical transition temperatures for cartilage ther-moforming were determined to be between 59–688C and62–688C for porcine and rabbit cartilage, respectively.Likewise in laser reshaping, the transition irradianceswere observed below 20.4 W/cm2. Peak temperaturesobserved during laser irradiation at and above transitionirradiances were equal or higher than transition tempera-tures in the saline bath heating. Above the transition zone,higher temperatures and irradiances will not produce in-crementally greater bend angles, and may lead to increasein thermal injury.

Laser cartilage reshaping may replace classic surgicaltechniques, which treat deformities and defects that resultfrom cancer surgery, trauma, or congenital malformation.Optimization of this technology will depend upon under-standing the mechanisms of thermoforming.

ACKNOWLEDGMENTS

This work was presented at the American Society forLasers in Medicine and Surgery in Orlando, Florida inMarch 2004 and received the best student research paperaward. Ryan Wright received a travel award to attend thismeeting from the United States Air Force. The authorsappreciate the technical contributions of Chao Li andWalter Tsang. The views and conclusions contained hereinare those of the authors and should not be interpreted asnecessarily representing the official policies or endorse-ments, either expressed or implied, of the Air ForceResearch Laboratory or the U.S. Government.

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