The World Wide Web is filled with a lot of useful, free information—if you know where to look for it. Here are several websites aimed at helping us ophthalmologists do our jobs better, from performing tricky intraocular lens calculations to illustrating our oral presentations.
Free Formulas
• Previze.com. Developed by John R. Moran, PhD, MD this site has been helpful when I have needed extra assistance for difficult intraocular lens implantations, such as piggyback IOLs. Though it's free to use the site, you have to fill out an online registration form before you can start accessing all of the cataract and refractive calculators.
To calculate the proper power for a secondary (piggyback) IOL for example, you go to the site's "Calculation Center" and enter in the patient's current information, such as refraction, keratometry and where you plan to put the second IOL. The online formula will calculate the appropriate power necessary to achieve a particular target postop refraction that you select for the patient.
The site also has a calculator to enable you to determine the central corneal power in a patient who has had LASIK. Since the measured corneal power is not accurate after LASIK, an adjustment to the corneal power is required for IOL calculations to be accurate. The online calculator allows for two methods to determine the adjusted corneal power, either the clinical history method or the contact lens method. Once the data is entered online, the program will calculate the adjusted corneal power. This information can then be used in your own IOL formulas.
The site also has calculators that let you determine a patient's vertex measurement before he undergoes refractive surgery and to quickly translate a patient's refraction from plus- to minus-cylinder notation and vice versa.
• doctor-hill.com. This is the website of Mesa, Ariz., surgeon Warren Hill. Dr. Hill has made preop cataract surgery biometry his focus, and it's evident on this site. Though some of the website's sections are currently under construction, one large section contains many links to separate pages where Dr. Hill provides calculation tips or conversion tables that allow you to hone your axial-length measurements and a-constants. For example, in one section, he provides a table that calculates how much to decrease the power of an IOL placed in the sulcus as opposed to in the capsular bag for a range of IOL powers. It also has a large section with tips for getting the best results with the IOLMaster (Carl Zeiss Meditech), including taking readings through different levels of lens opacities and optimizing your IOL constants when you use the device for preoperative measurements.
In one section, for example, Dr. Hill reviews the current thinking on placing IOLs in eyes that contain silicone oil. By providing a formula and describing the reasoning behind it, the site states that, "for an eye of average dimensions, and with the vitreous cavity filled with silicone oil, the additional power needed for a convex-plano PMMA intraocular lens is typically between +3.0 D to +3.5 D."
Though a free site is often no substitute for a full-featured program you might purchase, these sites can be very handy in a pinch or in situations not covered by the software you currently own.
Dr. Trattler is a refractive surgeon at the Center for Excellence in Eye Care in Miami.
