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Refractive Surgery | Sep 2010

i-ASSORT Provides Astigmatic Analyses Using Topography

Surgeons can compare vector magnitudes and axes separately when planning astigmatic changes.

Ideveloped the Alpins method of astigmatism analysis to assist surgeons with providing more effective astigmatic treatments. Because these techniques involve complex mathematics, a computer system is required. The Alpins Statistical System for Ophthalmic Refractive Surgery Techniques (ASSORT) is a total ophthalmic surgical analysis system that integrates and analyzes all measurable ophthalmic parameters including visual acuities and personalized A-constants. A major feature of the ASSORT system is its use in astigmatic surgery planning and analysis. By using the software's reporting capabilities, surgical techniques can be compared in different patient groups, and pre- and postoperative events can be documented and the changes quantified.

ASSORT provides an innovative way of analyzing vectors for changes in astigmatism. The Alpins method of astigmatism analysis is displayed numerically and graphically in a clear format.

The newly introduced i-ASSORT program performs astigmatic analyses using the measured corneal topography provided by the diagnostic instrument into which the software has been installed. On combined topography/wavefront aberrometry systems, refractive analysis is also available. The i-ASSORT will import the required parameters—ie, simulated keratometry (K) readings (sim K) from topography and second- order astigmatism values from aberrometry— and display analyses comparing preand postoperative maps. Using these data, the surgeon can determine success and refine his surgical nomograms.

Differences between refractive and corneal values are quantified by the program. When these differences are significant, this can be a red flag for a possible adverse visual outcome. The program also calculates topographic disparity—a dioptric measure of corneal irregularity—as well as visual indication of the success of the astigmatic treatment using familiar symbols of ticks and crosses (Figure 1). Additionally, graphic analyses display the calculated parameters using double-angle and polarvector diagrams (Figure 2).

The i-ASSORT software is available for use as third-party software with topographers including the Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany), the Keratron Scout (Optikon 2000 Industrie, Rome), the CSO corneal topographer (Costruzione Strumenti Oftalmici, Florence, Italy), the Magellan (Nidek, Gamagori, Japan), and the Medmont Studio (Medmont International Pty Ltd, Vermont, Australia). Currently, the software is interfaced with these six topography systems, and it should work with several more as we complete the interfaces.

Surgeons who use a topographer will find the i-ASSORT useful. Comparing pre- and postoperative magnitudes (sim K) of astigmatism is not enough information for an effective surgical selfevaluation. With the i-ASSORT software, topographers can quantify the change after any corneal surgical procedure.

Surgeons can obtain a display and printout of information before surgery that provides indicators regarding the patient's risk for having a less-thansatisfactory astigmatic outcome. Astigmatism emanates from the cornea, and we perform spectacle correction for it. In the real world, however, astigmatism correction in the spectacles does not always match the magnitude or orientation of the astigmatic error in the cornea. When a patient's examination reveals a difference between refraction and corneal topographic values, that patient's eye cannot be treated according to the spectacle refraction alone and also have a spherical cornea after treatment. In this case, there will be remaining astigmatism, which will increase as the difference between the spectacle and the corneal astigmatism increases—either in magnitude, axis, or both.

Ocular residual astigmatism is a vectorial value calculated by the i-ASSORT to quantify the differences of magnitude and axis together for corneal and refractive astigmatism. If this value is greater than 1.00 D, as happens in approximately 25% of eyes, it raises a red flag. The surgeon can advise these patients that, because of limitations or imperfections in the optical systems of their eyes, they cannot be corrected completely for astigmatism, and there will be some left over, as quantified by the ocular residual astigmatism.

Surgeons can now know the ocular residual astigmatism and the irregularity quantified by topographic disparity before a refractive procedure. The surgeon also has a consistent gauge of irregularity that can be used universally among devices to give the benefit of standardized topographic irregularity assessments.

The software system is installed onto the topographer and appears in the user interface. The surgeon chooses what information is to be exported. The technician inputs the amount of astigmatism the procedure is attempting to reduce and what meridian is being flattened; the analysis is performed using the treatment data. The technician can enter the spectacle refraction and record the treatment while the sim K readings are imported.

Surgeons have asked for simple software systems to interface with topographers, and the i-ASSORT offers this. Surgeons also have the ability to perform sophisticated analyses with the technology if they desire by using the more advanced functions available with the program.

The i-ASSORT program provides a useful way for surgeons to use corneal topography data to assess eyes for surgery preoperatively. Additionally, astigmatic outcomes can be appraised and nomograms can be optimized by comparing values from postoperative and preoperative maps. For surgeons requiring a total outcomes analysis program, the i-ASSORT software offers numerical and graphical displays for all parameters measurable on the eye.

Noel Alpins, FRANZCO, FRCOphth, FACS, is an Associate Fellow at the University of Melbourne, Victoria, Australia. Dr. Alpins states that he has financial interest in ASSORT and i-ASSORT. He may be reached at tel: +61 3 9584 6122; fax: +61 3 9585 0995; e-mail: alpins@newvisionclinics.com.au.