lasers have been used in laryngeal surgeries since 1972 , when strong and jako first reported the use of carbon dioxide laser ( co2 ) in the human larynx.1 despite initial criticism , endoscopic laser treatments have achieved universal acceptance and been successfully used in endolaryngeal microsurgery ( ems).2 most of the laser procedures involve either excision or tissue vaporization , depending on the interaction between the wavelength (  ) and the chromophores of the tissue . for years , the co2 (   =  10600 nm ) seemed to be the preferred laser for transoral laser microsurgery due to its precise cut,3 given that it is better absorbed by water . however , there are some disadvantages of the co2 device , namely , its straight line beam delivery from surgical microscope , narrow penetration depth to tissue , difficult transportation , and increased cost .
these drawbacks favored the study and application of other wavelengths in larynx , such as thulium yag (   =  2000 nm ) , nd : yag ( neodimium : yttrium - aluminum - garnet -   =  1064 nm ) , diode (   =  805 - 980 nm ) , pdl ( pulsed dye laser -   =  585 nm ) and ktp ( potassium titanyl phosphate -   =  532 nm).3 
 4 
  the diode laser has excellent hemostatic properties as a result of high absorption by hemoglobin and particularly by oxyhemoglobin .
it is also absorbed by water , but less so than the co2 laser.5 furthermore it is portable , small , relatively inexpensive , and simple to use .
it also has stable power output , long lifetime , and low installation and maintenance costs.2 
 6 moreover , the diode laser is delivered down by a fine glass fiber that allows the surgeon to hold it in a pencil - like holder for accurate manipulation.2 the fiber guidance system tip can be angled , which allows access to areas that are difficult to handle with the co2 laser.7 as a result , the use of diode laser in ems has increased in recent years.8 nonetheless , the ideal parameters of diode laser in ems and long - term results remain unclear .
therefore , we conducted a systematic review to clarify the use of diode laser in laryngeal diseases and outcomes shown so far .
a systematic search was performed in pubmed , cochrane library , ovid , web of science , and scielo databases .
airway  or  laryngeal surgery  or  vocal fold  and  diode laser . 
we considered all studies relating to human laryngeal pathologies ( from superior edge of epiglottis to inferior edge of cricoid cartilage ) .
we only included studies that provided information on patient and lesion characteristics , treatment ( diode laser 's parameters used in surgery 
at least two items described among wavelength , fiber diameter , output power , frequency , wave radiation , and contact mode ) , and outcomes related to the laser surgery performed . outcomes
could be  cure of the initial pathology ,   need of complementary operation ,   recurrence of the lesion ,  or even  complications or adverse / side effects .  studies were excluded if they consisted of less than two subjects or case report .
finally , studies that described the use of diode laser with non - surgical intent were excluded .
we collated data from included studies using an excel ( microsoft , redmond , washington ) spreadsheet .
specifically , data on sample demographics and clinical characteristics , sample size , diode laser parameters used , duration of follow - up , main outcome findings , and complications were extracted from each study .
article authors were contacted directly to obtain further information in cases of incomplete reported data .
we identified no randomized clinical trial and only two experimental controlled studies ( not randomized ) were included .
our search strategy yielded a total of 284 articles ( including duplicates ) . by screening the titles alone ,
234 articles could be excluded , as they were either review articles , obviously not relating to the use of diode laser in larynx , or clearly not eligible for inclusion ( for example , studies looking at diode laser in animals ) .
after reading the abstracts , one was removed due to its focus on endobronchial diseases .
then , the 13 eligible articles were full - text assessed , but two more were excluded due to failure to meet eligibility criteria ( one that did not describe the laser parameters used at one single surgery and another that included patients with lesions outside the larynx ) .
we manually searched reference lists for further relevant articles , but none was eligible ( fig .
the 11 studies selected for inclusion were published between 2003 and 2014 and originated from 5 different countries .
two studies were experimental controlled but not randomized8 
 9 and another one was a prospective experimental uncontrolled study.7 the other studies comprised of 8 retrospective reviews.2 
 3 
 5 
 6 
 10 
 11 
 12 
 13 we found no randomized controlled trial or multicenter trials . a total of 357 subjects made up the study population from the 11 included studies .
the laryngeal pathologies included were suprastomal granuloma , laryngomalacia , ventricular dysphonia , vocal polyp , glottic web , papillomas , glottic carcinoma , bilateral cord palsy , subglottic cysts , subglottic stenosis , and subglottic hemangioma / lynfangioma .
abbreviations : m , months ; y , years f  =  female m  =  male .
intervention included diode laser surgery performed in various combinations of parameters between wavelength , fiber diameter , output power , frequency , wave radiation and contact mode , as illustrated in table 2 .
the minor fiber diameter described was 300 m and the isolated major one was 1400 m .
the output power varied from 3  w to 60 w. all authors with available data opted for the direct contact of the fiber to the tissue as well as the continuous mode of energy .
length of follow - up was variable among the studies , ranging from two to 86 months .
five of 11 authors ( 46% ) declared to have experienced no complications or side effects by the use of the diode laser .
granulation and posterior synechia were described related to treatment of vocal fold paralysis.10 tracheal posterolateral bridge synechia and laser - induced subglottic granuloma related to treatment of subglottic hemangioma / lynfangioma.5 regarding glottic cancer surgeries , no authors reported edema requiring tracheostomy .
intraoperatively , thyroid cartilage exposure was described and laser - induced fire occurred due to thermal injury of the laser on the intubation tube .
two patients developed cutaneous emphysema that resolved in one day without intervention.12 in the postoperative period , synechia and granuloma formation was reported.7 some rare side effects were infection , foul smelling halitosis ( in whom relatively more extended laser surgery was performed ) , laryngeal stenosis,12 hemorrhage , and fistula formation.7 

however , it has some limitations , such as increased cost and difficulty in the management of deep and curved areas .
in addition , it has short penetration in tissues ( 0.10.3  mm on average ) , which reduces effective sealing of vessels ( it coagulates vessels of calibers smaller than 0.5 mm ) , thus , increasing bleeding during surgery.3 
 4 the diode laser emits wavelengths that are absorbed predominantly by hemoglobin and melanin ( near infrared spectrum ) .
the penetration depth depends on the concentration of these chromophores and , generally , it reaches 0.31.0  mm in diode laser application , which makes it ideal for photocoagulation.4 
 6 
 14 in conformity to that , some authors advocate that the diode laser 35w allows thermal coagulation that had been unimaginable with the co2 laser.10 all of these characteristics , according to some authors , confer on diode laser a surgical precision only slightly inferior to the co2 laser one , but with a superior coagulation capability for treating laryngeal vascular lesions.5 compared with other devices , diode laser also appears to be more precise and less invasive .
for instance , the cutting precision of the ndyag ( 1064 nm ) is considerably lower , and the instrument also led to greater immediate and deferred inflammatory damage due to thermal diffusion ( high penetration capacity of up to 4 mm).3 
 10 
  furthermore , the diode laser 's beam , in contrast to the carbon dioxide laser , is carried by flexible optic fibers that can be coupled with telescopes permitting access to sites that are difficult to explore with other techniques , such as the subglottic region.5 recently , a co2 laser beam delivered through a flexible hollow tube has become available that delivers the beam close to the target.15 however , the use of diode laser by contact ( or extremely close distance ) makes it much safer than other laser sources by avoiding damage due to  beam scape  in an open field.5 
  as this review shows , the diode laser can be a useful tool for treatment of different laryngeal pathologies , such as suprastomal granuloma , laryngomalacia , ventricular dysphonia , vocal polyp , glottic web , papilloma , glottic carcinoma , bilateral cord palsy , subglottic cyst , subglottic stenosis , subglottic hemangioma and lymphangioma . in this sense , the laser parameters must be set depending on the goal ( vaporization , section or coagulation ) and the clinical problem .
with respect to output power , diameter of fiber and wavelength , the articles selected in this review presented different arrangements for diode laser .
this reveals the lack of standardization in setting the best parameters of diode laser for laryngeal surgery .
although these parameters may influence collateral tissue damage , the studies are difficult to compare , based on the following concepts of laser 's physics .
power , given in watts ( w ) , measures the rate that the laser beam transmits .
beam energy , measured in joules ( j ) , can be found by multiplying the power ( w ) by exposure time ( in seconds ) .
power density , also called irradiance or spot brightness , determines the rate at which tissue is removed at the surgical site , measured in units of watts per square centimeter ( w / cm2 ) .
fluence is a key parameter once it combines previously mentioned parameters of power density and dosage , and is measured in units of joules per square centimeter ( j / cm2 ) .
it is important to understand this to provide minimal damage to tissues adjacent to the incision site , as using a higher pulse power for a shorter period of time results in less tissue damage than using lower power for a longer period of time .
tissue damage is dependent upon the tissue absorption coefficient , the wavelength of the laser , power density , and the length of time over which the energy is delivered , which is largely technique dependent.16 these data were not provided by any of the included studies . an additional property that can affect the severity of tissue damage is thermal relaxation time , the time required for tissue to lose 50% of its heat through diffusion.17 one can decrease tissue damage by allowing heated tissue to cool during a procedure
. however , it could be accomplished through the use of a pulsed laser , spacing out laser impact , even for a continuous incision , that decreases thermal damage by allowing time for the tissue to cool between impacts.16 in contrast , the authors of studies included in this review opted for the continuous wave radiation , some followed by manual tissue cooling .
special attention must be given to laser surgery for the treatment of glottic cancer , once it can be more cost - effective than  cold surgery when managing laryngeal tumors , as they allow briefer hospital stays and shorter wound recovery periods.18 the co2 laser surgery was presented as one of the most accepted treatment options of early glottic carcinoma due to its high local control rates , low morbidity , and good postoperative voice quality.19 however , if the surgeon has difficulty in managing laryngeal anterior commissure tumors , he or she is encouraged by some authors to avoid co2 laser microsurgery .
alternatively , the 810  nm diode laser has been recently used for the treatment of glottic tumors and may allow better exposure and resection of tumors in the anterior commissure . the first to establish long - term results for glottic cancer treatment were ferri et al , in 2008 . however , it seems that diode laser has not been well accepted worldwide for laryngeal surgery , as our review shows a gap of publications between 2008 and 2013 .
according to that , we speculate that most institutions continued using the co2 laser , due to doctors ' familiarity with its use , capabilities , and limitations.9 
 16 on the other hand , the 980  nm diode laser is a new technology , and has been reported for the treatment of early glottic tumors and vocal fold polyps since 2013.7 
 8 
 9 the main difference between these two diode laser wavelengths is their distinct absorption by the vocal fold , as the 980  nm is slightly more absorbed by water than the 810  nm one . the three studies that used the 980  nm diode laser were among those with a better design in our review .
two referred to controlled trials8 
 9 and the other was an uncontrolled trial.7 these studies showed good efficacy results and low rates of major complications .
concerning functional outcomes , as measured by the voice handicap index ( vhi ) , karasu et al showed good results of 980  nm diode laser in treating vocal fold polyps.8 a comparison between diode and co2 laser could be quite useful to highlight the differences and compare the results , especially the functional ones .
the ideal comparison to assess both types of lasers would involve a randomized clinical trial with these two techniques , which is nonexistent in the literature .
even though the comparison of non - controlled studies of two different techniques can involve bias , in table 4 we show some results of some authoritative works related to the use of co2 laser for larynx surgery in the literature in similar conditions to those with diode laser on our review , so that the reader can grossly compare with table 3.17 
 20 
 21 
 22 
 23 
  abbreviations : m , months ; y , years . regarding safety issues , five authors reported no complications or side effects by the use of the diode laser , but none of these studies was related to glottic cancer or extensive procedures .
the major complications were related to more extensive surgeries and with anterior commissure involvement.7 possibly the most feared complication is the endotracheal tube airway fire , which can be catastrophic or even fatal.24 to avoid that , some authors use reinforced tubes for laser surgery6 while others described the tube being inflated using saline with methylene blue dye , so that a puncture could be immediately diagnosed.7 
  this review highlights the lack of good evidence for the use of diode laser in laryngeal surgery .
furthermore , it is clear that there is wide variation in how it is used around the world .
the 980  nm diode laser seems to be a promising laser device , so we would expect that , as experience increases , it will be more extensively used as well as new lasers wavelengths can emerge.8 

despite the heterogeneous populations , varied inclusion criteria , and retrospective designs of most the studies considered in this review , they do provide useful information to the surgeon who intends to use diode laser for transoral laser microsurgery .
the evidence from recent studies suggests an improvement in diode laser technology ( especially the 980  nm wavelength ) . further long - term multicenter prospective research is needed , although the findings of this review suggest that diode laser is a useful tool that should be considered in laryngeal surgeries .
 
 introduction  the diode laser has been frequently used in the management of laryngeal disorders . 
 the portability and functional diversity of this tool make it a reasonable alternative to conventional lasers . 
 however , whether diode laser has been applied in transoral laser microsurgery , the ideal parameters , outcomes , and adverse effects remain unclear . 
 
 objective  the main objective of this systematic review is to provide a reliable evaluation of the use of diode laser in laryngeal diseases , trying to clarify its ideal parameters in the larynx , as well as its outcomes and complications . 
 
 data synthesis  we included eleven studies in the final analysis . from the included articles , we collected data on patient and lesion characteristics , treatment ( diode laser 's parameters used in surgery ) , and outcomes related to the laser surgery performed . 
 only two studies were prospective and there were no randomized controlled trials . 
 most of the evidence suggests that the diode laser can be a useful tool for treatment of different pathologies in the larynx . in this sense 
 , the parameters must be set depending on the goal ( vaporization , section , or coagulation ) and the clinical problem . 
 
 conclusion : the literature lacks studies on the ideal parameters of the diode laser in laryngeal surgery . 
 the available data indicate that diode laser is a useful tool that should be considered in laryngeal surgeries . 
 thus , large , well - designed studies correlated with diode compared with other lasers are needed to better estimate its effects . 