sutureless clear corneal incisions are now arguably the most common incisions made to perform cataract surgery with phacoemulsification , replacing scleral tunnel and limbal incisions . there are significant differences in the effects of clear corneal and scleral or limbal incisions , related to anatomical and physiological differences in the respective structures where the incisions are made . the cornea is comprised of a regular lamellar structure of collagen fibrils that stretch from limbus to limbus , arranged in a lattice formation ; this is what provides the primary structural support of the cornea and accounts for its transparency . vascular arcades are present , providing a potential source of fibroblasts . the differences in the healing effects of incisions at the limbus and the cornea have been previously discussed in the literature [ 24 ] . limbal incisions appear to heal more quickly and are more resistant to deformation pressure than those in the cornea . clear corneal incisions also appear to increase the likelihood of endophthalmitis , potentially for the reasons above . in short , there are no disadvantages to a limbal incision in terms of surgical safety . from a structural point of view it is known that incisions for cataract surgery will induce a flattening effect when made on ( or near ) the steep axis of the cornea . this effect is positively correlated with incision size ( larger incisions generating more astigmatism , all other things being equal ) and location ( scleral or limbal incision inducing less astigmatism than clear corneal ) , though for small incisions the effect of location appears less critical [ 6 , 7 ] . wound construction also appears to have an effect , with square incisions reported to affect astigmatism the least . data related to surgically induced astigmatism in the recent literature is primarily related to clear corneal incisions . the data reported here summarize the astigmatic changes produced with a small ( 2.2  mm ) square posterior limbal incision . a retrospective patient chart review was conducted at one site ( pe ) for a study of toric and spherical iols . those eyes with preoperative and postoperative keratometry results eyes were excluded if they had irregular ( nonorthogonal ) corneal astigmatism or if they had previous corneal surgery . this obviated the need for a specific informed consent or irb approval for the data collection undertaken here . in addition , all patients sign an acknowledgement that their deidentified phi data may be used for research purposes when they are seen in the practice . comparative surgically induced astigmatism data were obtained from one of the authors ( w. hill ) from a website specifically designed to collect preoperative and postoperative keratometry data for the purposes of calculating surgically induced astigmatism . this site provides the necessary spreadsheet and instructions to surgeons for the calculation of sia . an option allows any surgeon who uses the spreadsheet to upload their data to the website for the purposes of aggregate analysis . again , no phi was collected so informed consent was not required for this data set . results from ph and the aggregate data from wh were tabulated in microsoft excel and analyzed using statistica statistical software . differences between groups were calculated with t - tests and analysis of variance ( anova ) tests , with significance at p < 0.05 . for the patients at one site ( p. ernest ) the data were corroborated with an automated keratometry reading and the automated keratometry feature of the iolmaster ( carl zeiss meditec , jena , germany ) . the automated keratometry readings were repeated on all patients postoperatively , and the comparison to the preoperative automated keratometry was used to calculate sia . the surgical technique employed in the time period in which the retrospective data were collected was constant for all patients . all incisions were temporal , 2.2  mm square posterior limbal incisions , using a technique previously described in the literature . surgical technique was not described in the aggregate sia data collected off the web , but incision type ( e.g. , clear corneal , limbal ) was generally indicated , and the size of the incision was noted . these variables were collected so that surgeons could submit different preoperative and postoperative data sets for different incision sizes and locations . the review of available clinical records from pe yielded a total of 38 eyes that both met the criteria for inclusion and had available postoperative keratometry data for analysis . surgically induced astigmatism was calculated as the vector difference between preoperative and postoperative anterior corneal astigmatism as measured with automated keratometry . the sia in this cohort averaged 0.25  d with a standard deviation of 0.14  d.   a total of 1,712 eyes were available in the aggregate sia data from w. hill , from 51 different surgeons . the data were filtered to remove duplicates , include only incision sizes of 2.2  mm , and exclude those records where the incision type was not indicated . where there were not at least 20 surgeries in any incision category ( size and location ) for a surgeon , those data were also deleted . after this data - screening step , 246 surgeries from 5 surgeons remained for comparative analysis . average preoperative keratometry was not statistically significantly different between surgeons ( p = 0.41 ) ; means ranged from 43.8  d to 44.5  d. the magnitude of preoperative corneal cylinder was also not statistically significantly different between surgeons ( p = 0.13 ) ; means ranged from 0.79  d to 1.04  d. 
 table 1 contains a summary of the surgically induced astigmatism by surgeon . looking at the aggregate clear corneal incision data versus the ernest posterior limbal data , there was a statistically significant difference in the surgically induced astigmatism by incision type ( p < 0.001 ) . inside the clear corneal group as a post hoc test , the ernest data were compared to the surgeon with the lowest average sia from the clear corneal incision group ( surgeon 5 ) ; there was a statistically significantly lower mean sia in the ernest cohort ( p = 0.001 ) . note , too , that the standard deviation of the ernest cohort is less than half that of the cohorts of 4 of the 5 other surgeons , and 40% lower than surgeon 5 despite having less than half the sample size . surgeons 1 to 5 used 2.2  mm clear corneal incisions , while pe used a 2.2  mm square posterior limbal incision . there was a statistically significant difference in sia by surgeon ( p < 0.001 ) . post hoc testing showed that the ernest data yielded a statistically significantly lower sia than the other surgeons ' data . the results suggest that the magnitude and the variability of surgically induced astigmatism with small incision surgery ( 2.2  mm ) is significantly lower if a posterior limbal incision is used instead of a clear corneal incision . results for the clear corneal incisions used here as comparative data are consistent with those recently reported in the literature . wilczynski et al . reported sia values of 0.50  0.25  d with 1.7  mm and 1.8  mm surgical incisions . holland reported sia values of 0.6  0.4  d for 2.4  mm clear corneal incisions . masket reported that sia results were somewhat lower with his 2.2  mm clear corneal incisions ( 0.35  0.21  d ) but it is worth noting that his incision was started with a groove at the temporal limbus . some of the variability of sia data from surgeons other than ernest is likely related to differences in technique between surgeons . in addition , intrasurgeon variability may be slightly higher if surgeons did not measure their postoperative keratometry 1 month or more after surgery , as there is some change in keratometry expected in that first month . the minimization of the magnitude ( and variability ) of surgically induced astigmatism is important for modern day cataract surgery , particularly with the use of toric and multifocal intraocular lenses residual astigmatism after surgery will reduce the likelihood of spectacle independence for distance vision for these patients . minimizing astigmatism for multifocal iols is equally important , as even small amounts of residual astigmatism can compromise the outcome with regard to uncorrected visual acuity at distance . surgeons interested in reducing the magnitude and variability of induced astigmatism at the time of cataract surgery may want to consider the use of a 2.2  mm square posterior limbal incision .
purpose . to compare the surgically induced astigmatism from clear corneal and square posterior limbal incisions at the time of cataract surgery 
 . methods . 
 surgically induced astigmatism was calculated for a set of eyes after cataract surgery using a temporal 2.2  mm square posterior limbal incision . 
 results were compared to similar available data from surgeons using clear corneal incisions of similar size . 
 results . 
 preoperative corneal astigmatism averaged 1.0  d and was not significantly different between the incision types . surgically induced astigmatism with 
 the 2.2  mm posterior limbal incision averaged 0.25  0.14  d , significantly lower in magnitude than the aggregate 
 surgically induced astigmatism produced by the 2.2  mm clear corneal 
 incision ( 0.68  0.49  d ) . conclusion . the 2.2  mm 
 square posterior limbal incision induced significantly less , and 
 significantly less variable , surgically induced astigmatism 
 relative to a similar - sized clear corneal incision . 
 this is likely 
 to improve refractive outcomes , particularly important with regard 
 to premium intraocular lenses .