Anterior Segment Cataract
Intraocular Lens Calculation After Refractive Surgery Wolfgang Haigis
Professor, Department of Ophthalmology, University of Wuerzburg
Abstract
More and more patients who have had corneo-refractive surgery present for intraocular lens (IOL) implantation. IOL calculation in these patients is still a challenge. After refractive surgery, if eyes are treated as normal eyes, high hyperopic errors can occur in previously myopic eyes and moderate myopic errors in formerly hyperopic eyes. Three main sources for these errors can be identified: the radius measurement error, the keratometer index error and the IOL formula error. The literature presents a confusing variety of procedures and formulas to cope with this situation. An analysis of the available literature reveals the different methods used to address the individual error contributions, the magnitude of which is assessed by model calculations. The most relevant formulas for clinical practice are the no-history procedures, which require no previous patient data. Using these methods to calculate IOL power after refractive surgery makes it possible to obtain clinical outcomes of a similar quality to that for normal eyes.
Keywords intraocular lens (IOL) calculation, refractive surgery, laser vision correction, myopic, hyperopic
Disclosure: The author is a consultant to Carl Zeiss Meditec. Received: 28 October 2011 Accepted: 29 November 2011 Citation: European Ophthalmic Review, 2012;6(1):21–4 Correspondence: Wolfgang Haigis, Universitaet Wuerzburg, Augen- und Poliklinik, 11, Josef-Schneider-Str., D-97080 Wuerzburg, Germany. E:
w.haigis@augenklinik.uni-wuerzburg.de
Six decades have passed since the first implantation of an artificial intraocular lens (IOL) into a human eye. Since then, cataract and refractive surgery have developed into the most frequent and most successful surgical interventions in the history of medicine. Every year more than 11 Mio lenses are implanted worldwide, providing excellent chances for the majority of patients to regain good vision. However, a small, yet increasing, number of eyes require special consideration. Among these problem eyes are those with extreme axial lengths and those after refractive surgery. The number of patients presenting with cataract after refractive corneal surgery has been continuously increasing over the years. IOL calculation in these patients is still a challenge, although the special problems associated with these eyes are well understood. In the literature, there are a considerable number of articles offering solutions or workarounds to the IOL calculation problems after refractive surgery. Several review articles include an in-depth analysis of current algorithms.1–4
Help can also be
obtained via the Internet. A spreadsheet (Hoffer/Savini tool) programmed with virtually all the algorithms published to date can be downloaded from Dr Ken Hoffer’s website5
at no cost. Also, on the
American Society of Cataract and Refractive Surgery (ASCRS) website6 an online calculator offers free use of a variety of published calculation methods. Recently, the Asia-Pacific Association of Cataract and Refractive Surgeons (APACRS) also launched a free online calculator for ‘Biometry calculation post refractive surgery‘,7 which is an implementation of Graham Barrett‘s true-K formula.8
This article does not aim to create another detailed review of the current methods to determine IOL power for eyes after refractive surgery. Excellent articles serving this purpose can be found in the literature. Rather, it intends to give a more generalised characterisation of the
© TOUCH BRIEFINGS 2012
specific problems involved and how these are addressed by the different algorithms available.
Also an update on the clinical performance of the Haigis-L formula9 is
presented. Since its publication in 2008, the outcomes of 91 more eyes have become available, leading to a total of 278 documented cases.
Biometry
After refractive surgery eyes are considered problem eyes because of the specific difficulties associated with IOL calculation (which are discussed below) and the high expectations patients have about the outcomes of surgical procedures. In this situation, all factors potentially threatening the quality of the surgical result have to be under control. Measurement errors, for example, must be minimised and this is especially true for axial length, which has been the largest source of error in IOL calculation. With optical biometry, the quality of axial length measurement is no longer a problem. Consequently, patients who have undergone refractive surgery should have axial length measurement using optical biometry (or immersion ultrasound) rather than contact ultrasound.
Specific Problems for Eyes after Refractive Surgery
Error Sources for Intraocular Lens Calculation Essentially, there are three main sources of errors in IOL calculation after refractive surgery: the radius measurement error, the keratometer index error and the IOL formula error.
The refractive power of the cornea (however it may be defined) is an important input parameter for the calculation of IOL power.
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