Superior Sclera Versus Temporal Corneal on Steep Axis Incision to Correct Pre-Existing Corneal Astigmatism Less Than 1.5 D

Purpose: Cataract is the leading cause of blindness worldwide. Even in an uneventful cataract surgery, surgically induced astigmatism remains the major hurdle in attaining good unaided visual acuity post surgery. If pre-existing corneal astigmatism is not corrected at the time of surgery, it will result in more postoperative astigmatism; therefore, it is important to correct pre-existing corneal astigmatism. Material & Methods: In this prospective study, 100 eyes of 100 age and sex matched patient’s with comparable preoperative astigmatism and visual acuity were divided in two groups according to pre operative corneal astigmatism. SIG (superior scleral incision group) included 50 eyes with steeper vertical axis and TIG (temporal corneal incision group) included 50 eyes with steeper horizontal axis. SIG and TIG were subjected to superior scleral tunnel and temporal corneal tunnel incisions respectively. Corneal astigmatism was measured preoperatively and at 1, 2 and 6 weeks postoperatively. Results: The mean postoperative astigmatism in SIG and TIG after 6 weeks was 0.545 ± 0.51D and 0.59 ± 0.48 D in comparison to preoperative astigmatism of 1.105 ± 0.54 D and 1.120 ± 0.49 D respectively. The difference was statistically significant in both the groups with P< 0.001 for both groups. The difference in surgically induced astigmatism between the two groups was not significant at any point of time after surgery (p>0.05). Conclusion: Incision site on steeper axis helps in reduction of pre-existing corneal astigmatism in cataract surgery. Both superior scleral and temporal corneal incisions help in neutralization of approx. 0.4-0.7 D of astigmatism. DOI : 10.14302/issn.2470-0436.jos-14-572 Corresponding author: Kavita Bhatnagar, Dr D Y Patil Medical College, Hospital & Research Center, Pune, India , Email: rajankavita12@rediffmail.com


Introduction
Cataract is the leading cause of blindness worldwide.
Even in an uneventful cataract surgery, surgically induced astigmatism remains the major hurdle in attaining good unaided visual acuity post surgery. If preexisting corneal astigmatism is not corrected at the time of surgery, it will result in more postoperative astigmatism; therefore, it is important to correct preexisting corneal astigmatism. Surgically induced astigmatism (SIA) depends on the type, length and position of incision and also the method of wound closure. [1] The purpose of this study was to find out if it was possible to achieve correction of preoperative astigmatism by planning incision on steeper axis during cataract surgery and to assess surgically induced astigmatism and any shift in astigmatism from 'with the rule astigmatism' (WTR) with steep axis in vertical meridian (axis 0-30 or 150-180 degrees) to 'against the rule astigmatism' (ATR) with steep axis in horizontal meridian (axis 60-120 degrees) and vice-versa.

Material and Methods
This hospital based prospective clinical study was conducted in the Department of Ophthalmology of a tertiary care teaching hospital from Nov-2010 to Nov-2012. The sample size was 100 eyes of 100 patient's with uncomplicated senile cataracts with pre operative astigmatism between 0.25-1. 5  A p value of < 0.05 was considered significant.

Results
The mean age in superior incision group (SIG) was    pre-existing corneal astigmatism less than 1.5 D. [2,3] Astigmatic keratotomy may give rise to glare sensation, diplopia and fluctuation of refractive error due to proximity of the incisions to the centre cornea. [4] Corneal relaxing incisions are technically easy, producing less symptoms, early wound stabilization but requires diamond knife and preoperative pachymetry in addition to controversies in application of the nomogram. [5,6] Choosing incision on steep corneal axis is the simplest method for lower degree of astigmatism.
In this prospective study on 100 eyes of 100 age and sex matched patients, we tried to evaluate effectiveness of on steep axis incisions in reducing pre-existing corneal astigmatism keeping all the other parameters same in both the groups except site and type of incision.
Even pre-existing astigmatism and visual acuities were comparable in the two groups. We used superior scleral incision for pre-existing with the rule astigmatism and temporal corneal incision for pre-existing against the rule astigmatism.
Studies suggest that for patients with less than 1 D of pre-existing corneal astigmatism, placing the phacoemulsification incision on the steep meridian of corneal astigmatism is the ideal approach. [7] Nielsen PJ [8] found that the refractive effect of the clear corneal incisions did not change significantly from day one to week six. Preoperative against-the-rule astigmatism was reduced significantly by temporally placed clear corneal incisions and preoperative with-the-rule astigmatism, by superiorly placed clear corneal incisions. Our results correspond with these findings. The only difference is that we used superior sclera incision and temporal corneal incision. In this study, the difference in mean preoperative and postoperative astigmatism 6 weeks after surgery was statistically significant in both superior sclera and temporal corneal incision groups (P < 0.001).
This proves that there was a significant reduction in preexisting astigmatism. There are reports that sclera incisions are less effective in reducing astigmatism than corneal incisions while our study did not find any such difference. Both superior sclera and temporal corneal incisions were effective in reducing pre-existing astigmatism and could neutralize approx. 0.4 D to 0.7 D astigmatism so our first objective was achieved.
Our second objective was to compare surgically induced astigmatism between the two groups. The difference between SIA in both the groups was not significant at any point of time postoperatively (p>0.05). These findings corroborate with other similar studies. [9,10] Our third objective was to assess any shift from preoperative 'with the rule astigmatism' (astigmatism on