the currently prescribed concentration of riboflavin , the wavelength of uva radiation and duration of treatment were established after a series of time and dose - response assays in animal models over several years of studies . these would need to be repeated of any alteration is envisaged , before human application . transepithelial cxl was introduced to prevent the adverse events associated with epithelial debridement ( postoperative pain , infectious keratitis , stromal haze , etc . ) as well as for its possible role in treating thinner corneas . initial experimental studies in porcine corneas demonstrated that complete epithelial removal was necessary for riboflavin permeation . superficial epithelial trauma or tetracaine administration or grid - like epithelial removal were not sufficient to achieve adequate stromal riboflavin concentration and may impair the efficacy of cross - linking . wollensak and iomdina , showed in rabbit models that corneal cross - linking without epithelial debridement ( using benzalkonium chloride containing proxymetacaine eyedrops ) reduced the biomechanical effect by approximately one - fifth compared with standard cross - linking . the cytotoxic damage was restricted to 200 m stromal depth , which can be beneficial in corneas < 400 m . transepithelial cxl in 20 patients with bilateral progressive keratoconus using enhanced riboflavin solution , containing trometamol and ethylenediaminetetraacetic acid ( edta ) sodium salt , was undertaken by filippello et al . they reported a statistically significant improvement in visual and topographic parameters and concluded that the treatment appeared to halt keratoconus progression . trometamol is a biologically inert low - toxicity amino alcohol used as buffering solution and sodium edta is a well - known chelator of calcium and magnesium ions . their combination breaks intercellular bonds , thus facilitating the penetration of riboflavin through the intact epithelium . applying a similar technique of transepithelial cxl in ultrathin corneas ( thinnest pachymetry 331 - 389 m ) moderate efficacy was reported by spadea et al . though both these studies reported no endothelial toxicity , it still remains a concern because improper stromal concentration of riboflavin may not be effective in absorbing all the uv radiation . another method for cross - linking in thin corneas , proposed by kymionis et al . , is customized pachymetric guided epithelial debridement . the technique involves mechanically removing 8.0 mm diameter of corneal epithelium , while preserving a small localized island corresponding to the thinnest area or the area of maximum topographic steepening . they cross - linked two patients with this technique with thinnest pachymetry of 380 and 375 m . preservation of epithelium over the thinnest area also has possible advantage of prevention of local stromal dehydration apart from blocking excessive uv radiation in this susceptible area . the effect of cross - linking with this technique was studied with anterior segment optical coherence tomography ( as - oct ) and confocal microscopic imaging by kaya et al . , haze formation and demarcation line were present only in the deepithelialized stroma and were not evident under areas which had an intact epithelium . further confocal microscopy revealed keratocyte loss with intense stromal edema only in the deepithelialized areas . for cxl to be effective , it is vital for the thinnest areas to achieve cross - linking . but here this area seems to be spared from the effect of cxl , putting the efficacy of the whole procedure in doubt . even after partial epithelial removal in a grid pattern , the riboflavin uptake is limited and nonhomogenous , which might compromise the efficacy of cross - linking procedure . to treat corneas thinner than 400 m , hafezi et al . , modified the technique of cxl by swelling the corneas to increase stromal thickness before uv irradiation . the deepithelialized cornea can swell to double its normal thickness when irrigated with a hypoosmolar solution , because of the hydrophilic property of the stromal proteoglycans . their technique involved applying isoosmolar riboflavin 0.1% solution with 20% dextran first , every 3 min for 30 min , on 9.0 mm of deepithelialized cornea . this was followed by application of hypoosmolar riboflavin ( riboflavin 0.5% with 0.9% sodium chloride solution ) every 20 s for 5 more min or till the minimum corneal thickness reaches 400 m . during irradiation isotonic riboflavin 0.1% they treated 20 patients with minimum stromal thickness 320 - 400 m post epithelial removal and observed a swelling of between 36 - 110 m after using hypoosmolar riboflavin . their technique differed as they applied hypoosmolar riboflavin 0.1% solution every 2 min for 30 min . during irradiation also they reported stabilization of ectasia in terms of mean k - value and best corrected visual acuity ( bcva ) . our own experience with hypoosmolar riboflavin followed by cxl also seems to indicate that while stabilization of the ectatic process is most often achieved , the flattening of the steepest keratometry is more modest than what is seen with isoosmolar riboflavin in the standard cxl . fifty eyes that underwent hypoosmolar cxl were compared with 50 eyes that underwent isoosmolar ( standard ) cxl in the same period . at 1 year follow - up , the steepest keratometry ( pentacam ) had decreased by an average of 0.88  2.26 d in the isoosmolar group and 0.18  3.23 d in the hypoosmolar group . this poses a doubt whether artificially swollen corneas behave in the same way as nonswollen keratoconus corneas . cxl might be expected to have a smaller effect on the biomechanics of an artificially swollen cornea because of lower relative concentration of collagen in the hydrated stroma . small angle x - ray scattering study , post cxl ( iso- and hypotonic ) in postmortem and and posttransplant corneal button has been done to evaluate the stromal collagen ultrastructure . it showed that some fluid enters the collagen fibrils to increase the fibril diameter , but majority of swelling occurs between the fibrils . , when they demonstrated through their study that the artificial swelling effect of hypoosmolar riboflavin is transient . they followed the technique described by hafezi et al . , and found that thinnest pachymetric readings decreased significantly after 10 and 30 min compared with the readings at the end of hypoosmolar riboflavin application . this can probably be avoided by repeated application of hypoosmolar riboflavin during the irradiation process also , rather than isoosmolar riboflavin . optical pachymetry does not provide reliable data in swollen corneas because of excessive light scattering and absorption . identifying the thinnest area is neither easy nor accurate while the procedure is going on , even if it has been marked previously on the basis of corneal topography . thus , there is still some ambiguity regarding the correct way of measuring intraoperative corneal thickness . the hypoosmolar riboflavin film is very unstable with a breakup time of 90 s as compared to isotonic ( with dextran ) riboflavin film which has a breakup time of 22 min . the corneal absorption coefficient of the combined stroma - riboflavin film system was 56.36 cm using dextran - riboflavin and 48.19 cm using hypoosmolar riboflavin . this poor shielding effect and lower absorption of hypoosmolar riboflavin film can cause irradiance levels at endothelial level to reach the toxicity threshold of 0.36 mw / cm . thus the safety and efficacy of hypoosmolar cxl still needs to be established by detailed studies . failure of hypoosmolar riboflavin in a case with an extremely thin cornea has been reported . the patient with a post epithelial removal corneal thickness of 268 m still showed progression of 1.9 d after 3 months of cxl , though no endothelial adverse events were noted . the author concluded that in order to prevent ectasia , a minimum stromal thickness of 330 m should be present so as to achieve a minimum cross - linked stromal thickness of 250 m ( 75% of 330 m ) . clinically , there are distinct interindividual variations in the stromal swelling response of different corneas , making it difficult to formulate a fixed protocol for the treatment of such eyes . in summary
collagen cross - linking ( cxl ) has become the standard of care for progressive keratoconus , after numerous clinical studies have established its efficacy and safety in suitably selected eyes . 
 the standard protocol is applicable in eyes which have a minimum corneal thickness of 400 m after epithelial debridement . 
 this prerequisite was stipulated to protect the corneal endothelium and intraocular tissues from the deleterious effect of ultraviolet - a ( uva ) radiation . 
 however , patients with keratoconus often present with corneal thickness of less than 400 m and could have otherwise benefited from this procedure . 
 a few modifications of the standard procedure have been suggested to benefit these patients without a compromise in safety . 
 transepithelial cross - linking , pachymetry - guided epithelial debridement before cross - linking , and the use of hypoosmolar riboflavin are some of the techniques that have been attempted . although clinical data is limited at the present time , these techniques are worth considering in patients with thin corneas . 
 further studies are needed to scientifically establish their efficacy and safety .