IOL Power Calculations
2010; Elsevier BV; Volume: 117; Issue: 2 Linguagem: Inglês
10.1016/j.ophtha.2009.10.036
ISSN1549-4713
Autores Tópico(s)Intraocular Surgery and Lenses
ResumoI read with great interest the article by Chang et al.1Chang S.W. Yu C.Y. Chen D.P. Comparison of intraocular lens power calculation by the IOLMaster in phakic and eyes with hydrophobic acrylic lenses.Ophthalmology. 2009; 116: 1336-1342Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar They used the IOLMaster (Carl Zeiss Meditec, Jena, Germany) to measure axial length, anterior chamber depth (ACD), and corneal curvatures (Ks) prior to surgery and did intraocular lens (IOL) power calculations for 3 lenses using the SRK II (Sanders, Retzlaff, and Kraff), SRK/T, and Haigis formulas. Three months postoperatively they repeated the measurements, re-did the IOL calculations now with the pseudophakic results and calculated the prediction errors based on the preoperative, as well as the postoperative measurement data. While the actual refractive results were good for all IOLs and all formulas, the prediction errors based on pseudophakic measurement data were around 0.2 diopters (D) for SRK II and SRK/T and some 0.7 D for the Haigis formula.What the authors should not have done is (1) to apply the IOLMaster for the measurement of pseudophakic ACDs, and (2) to use the Haigis formula with pseudophakic ACDs.The ACD measuring module of the IOLMaster must not be used to determine the ACD of a pseudophakic eye, because the evaluation software is only designed for phakic eyes. The evaluation algorithms expect scattered light from the crystalline lens, while IOLs essentially produce strong reflections, which will be interpreted wrongly. Pseudophakic ACDs will thus give results, which are, however, faulty and unreliable. This is described in the IOLMaster user manual.2Carl Zeiss Meditec AG. IOLMaster with Advanced Technology Software Release 5.xx User Manual. Publication 000000-1476-299 IOLMaster 27.02.2007.Google ScholarThe Haigis formula3Haigis W. Lege B. Miller N. Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis.Graefes Arch Clin Exp Ophthalmol. 2000; 238: 765-773Crossref PubMed Scopus (505) Google Scholar uses the preoperative anterior chamber depth and the preoperative axial length to predict the effective lens position (ELP). Entering the pseudophakic ACD, even if it was correct, to predict the (pseudophakic) ELP does not make sense. It would make sense if a thick-lens model or a ray-tracing setup would have been applied. The Haigis formula, however, like all currently used “theoretical” formulas, is based on infinitely thin lenses.4Haigis W. Matrix-optical representation of currently used intraocular lens power formulas.J Refract Surg. 2009; 25: 229-234PubMed Google ScholarTheoretically, a decrease of 0.1 mm in axial length would ask for an IOL power change of some 0.3 D. which would be equivalent to a refractive change of some 0.20 to 0.25 D. This is the authors' result for SRK II and SRK/T, which in this context are only sensitive to changes in axial length.Likewise, an increase of 1 mm in ACD will cause a change in the effective lens position in the Haigis formula of some 0.4 mm, which will be reflected in a refractive change of 0.5 to 0.6 D. Both effects together will thus cause a refractive shift of some 0.7 to 0.8 D, which is the authors' result for the Haigis formula.Chang et al correctly point out the increasing importance of pseudophakic measurements. In these cases, IOL power formulas like the Haigis or Holladay-2, which request an ACD-value, cannot be used without any workaround. A solution could be to enter either a typical (average) value or the respective value from the phakic fellow eye–or to use another formula. I read with great interest the article by Chang et al.1Chang S.W. Yu C.Y. Chen D.P. Comparison of intraocular lens power calculation by the IOLMaster in phakic and eyes with hydrophobic acrylic lenses.Ophthalmology. 2009; 116: 1336-1342Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar They used the IOLMaster (Carl Zeiss Meditec, Jena, Germany) to measure axial length, anterior chamber depth (ACD), and corneal curvatures (Ks) prior to surgery and did intraocular lens (IOL) power calculations for 3 lenses using the SRK II (Sanders, Retzlaff, and Kraff), SRK/T, and Haigis formulas. Three months postoperatively they repeated the measurements, re-did the IOL calculations now with the pseudophakic results and calculated the prediction errors based on the preoperative, as well as the postoperative measurement data. While the actual refractive results were good for all IOLs and all formulas, the prediction errors based on pseudophakic measurement data were around 0.2 diopters (D) for SRK II and SRK/T and some 0.7 D for the Haigis formula. What the authors should not have done is (1) to apply the IOLMaster for the measurement of pseudophakic ACDs, and (2) to use the Haigis formula with pseudophakic ACDs. The ACD measuring module of the IOLMaster must not be used to determine the ACD of a pseudophakic eye, because the evaluation software is only designed for phakic eyes. The evaluation algorithms expect scattered light from the crystalline lens, while IOLs essentially produce strong reflections, which will be interpreted wrongly. Pseudophakic ACDs will thus give results, which are, however, faulty and unreliable. This is described in the IOLMaster user manual.2Carl Zeiss Meditec AG. IOLMaster with Advanced Technology Software Release 5.xx User Manual. Publication 000000-1476-299 IOLMaster 27.02.2007.Google Scholar The Haigis formula3Haigis W. Lege B. Miller N. Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis.Graefes Arch Clin Exp Ophthalmol. 2000; 238: 765-773Crossref PubMed Scopus (505) Google Scholar uses the preoperative anterior chamber depth and the preoperative axial length to predict the effective lens position (ELP). Entering the pseudophakic ACD, even if it was correct, to predict the (pseudophakic) ELP does not make sense. It would make sense if a thick-lens model or a ray-tracing setup would have been applied. The Haigis formula, however, like all currently used “theoretical” formulas, is based on infinitely thin lenses.4Haigis W. Matrix-optical representation of currently used intraocular lens power formulas.J Refract Surg. 2009; 25: 229-234PubMed Google Scholar Theoretically, a decrease of 0.1 mm in axial length would ask for an IOL power change of some 0.3 D. which would be equivalent to a refractive change of some 0.20 to 0.25 D. This is the authors' result for SRK II and SRK/T, which in this context are only sensitive to changes in axial length. Likewise, an increase of 1 mm in ACD will cause a change in the effective lens position in the Haigis formula of some 0.4 mm, which will be reflected in a refractive change of 0.5 to 0.6 D. Both effects together will thus cause a refractive shift of some 0.7 to 0.8 D, which is the authors' result for the Haigis formula. Chang et al correctly point out the increasing importance of pseudophakic measurements. In these cases, IOL power formulas like the Haigis or Holladay-2, which request an ACD-value, cannot be used without any workaround. A solution could be to enter either a typical (average) value or the respective value from the phakic fellow eye–or to use another formula. Author replyOphthalmologyVol. 117Issue 2PreviewWe thank Dr. Haigis for his comments. With the increasing use of the IOLMaster (Carl Zeiss Meditec AG, Jena, Germany) for ocular biometry and the high expectations for functional intraocular lens (IOL) outcomes, the purpose of our study was to identify the differences that would be encountered when the IOLMaster is used for biometry in pseudophakic eyes and to provide a solution to fine tune the results. This is especially important for IOL exchanges and for those patients whose biometric data of the pseudophakic eye were used for reference to determine IOL power. Full-Text PDF
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