'SIA-Soft' : a new software to calculate surgically induced astigmatism in comparison with manual mathematics by vector method.

Dear Editor,

Surgically induced astigmatism (SIA) is not just the simple subtraction of keratometric or K-values before and after the operation. It involves vector mathematics with algebra and trigonometry.1 For every ophthalmologist, it is not possible to go back to 12 (10 + 2) standard mathematical knowledge and calculate SIA. Even if one were to calculate it correctly, it takes a lot of time and effort for a number of patients. Despite this difficulty, calculating SIA allows the surgeon to have an accurate picture of how his surgery is affecting the corneal curvature and helps improve his surgery.

Computers can be programmed to do all this tedious mathematical calculations for us and that is why this software ′SIA-Soft′ has been developed. The basic mathematics behind this software is the ′gold standard′ vector analysis of SIA.2 We have validated the accuracy and operating feasibility of this Windows-based software for calculating SIA for a number of patients in comparison with gold standard mathematical vector analysis done manually.

The preoperative and postoperative keratometric data (K-values) with the axes of 30 different patients were taken randomly from our cataract clinic either from manual small incision cataract surgery (MSICS) or phacoemulsification category. These 30 sets of data were given to two co-authors for calculation of SIA and the angles of error in a masked way. The first co-author (a student of Class XII) was asked to calculate the SIA on the basis of pure mathematics and submit the data for single and multiple patients and he was also asked to give the mean and standard deviation (SD). Similarly, the second co-author was asked to calculate SIA using the software with the same set of data. Both of them were asked to submit the final data to a third observer for final analysis.

The overall results by two different observers are given in Table 1. Average time taken for each calculation for the first observer by mathematical method was 5.27 ± 0.48 min and for the second observer by using SIA-Soft software was 6.15 ± 0.50 sec (*P<0.001). Up to six-decimal level, there was no difference in SIA and ′angle of error′ values for individual patients or mean of all patients by the two different methods.

Table 1

Overall results by two different observers

Each astigmatism value was considered as a vector quantity, having a magnitude as well as direction. Among all methods of calculating SIA, vector analysis is the most powerful and accurate method.2 Though mathematically tedious, analysis of SIA allows the surgeon to evaluate his own technique and also to maximize his potential for obtaining a consistently good surgical outcome. It further allows the surgeon to choose the right axis, right length, right site and right nature of incision over a period of time. This is especially important while planning for correction of preexisting corneal astigmatism during any type of cataract surgery whether MSICS or phacoemulsification. This software can handle single as well as unlimited number of patients and surgeons with simultaneous calculation of mean and SD.

SIA-Soft is a simple, accurate, off-the-shelf user-friendly statistical software which only requires clicking of the buttons.