John W. Lewis

A three-part system for refining intraocular lens power calculations

ABSTRACT A three‐part system that determines the correct power for an intraocular lens (IOL) to achieve a desired postoperative refraction is presented. The three components are (1) data screening criteria to identify improbable axial length and keratometry measurements, (2) a new IOL calculation formula that exceeds the current accuracy of other formulas for short, medium, and long eyes, and (3) a personalized “surgeon factor” that adjusts for any consistent bias in the surgeons results, from any source, based on a reverse solution of the new formula; the reverse solution uses the postoperative stabilized refraction, the dioptric power of the implanted IOL, and the preoperative corneal and axial length measurements to calculate the personalized surgeon factor. The improved accuracy of the new formula was proven by performing IOL power calculations on 2,000 eyes from 12 surgeons and comparing the results to seven other currently used formulas.

The optimal size of a posterior capsulotomy

The optimal posterior capsulotomy diameter should be equal to the normal pupillary diameter in dark conditions to avoid glare and other undesirable optical aberrations. We studied 14 patients whose average pupillary diameter following cataract surgery was approximately 3.9 mm in scotopic conditions.

Diagnosis and treatment of mysterious light streaks seen by patients following extracapsular cataract extraction

Complaints of light streaks, such as might be seen with a Maddox rod or Bagolini lens, are becoming more common with the trend toward extracapsular cataract surgery. The light streaks are usually a result of high plus cylinders somewhere in the patients optical system. Windshields, spectacles, contact lenses, lashes, an excessive tear meniscus, intraocular lens scratches, and posterior capsular opacification are possible causes that can be easily identified and treated.

Determining intraocular lens power within the eye

A method is described for determining the power of an intraocular lens (IOL) within the eye by measuring the horizontal dimension of the corneal reflected image (Purkinje-Sanson I) and the anterior IOL reflected image (Purkinje-Sanson III) as seen through a standard slitlamp with a target positioned 68 mm anterior to the focal plane of the biomicroscope. The horizontal K-reading (at 180 degrees) and the anterior chamber depth are the two other parameters necessary to calculate the exact power of the IOL. Seven tables that use these four measurements have been provided, eliminating the need for complex calculations. To determine the accuracy of this technique, ten implanted IOLs ranging from 9 diopters (D) to 27 D were chosen and their powers calculated; these calculated values were then compared to the actual IOL powers. The largest error was 0.5 D and the average error was 0.17 D.

Pterygia as cause of post-cataract with-the-rule astigmatism.

Pterygia often induce with-the-rule astigmatism as they invade the cornea. Significant amounts of astigmatism occur long before a pterygium encroaches on the visual axis. We present an example in which a 3.3-mm pterygium resulted in 9 diopters (D) of with-the-rule astigmatism. Six weeks after the pterygium removal, the patients cornea became spherical, but by three months the pterygium had recurred 2.2 mm onto the cornea and induced 4 D of with-the-rule astigmatism. Since the second removal and the application of beta irradiation, there has been no recurrence. The final correction was -1.00 + 1.00 X 127 degrees with a visual acuity of 20/20 +3. Characteristic keratoscopic photographs are shown and contrasted with surgically induced with-the-rule astigmatism.