Publicação
Improving strategies on toric intraocular lens power calculation
| Resumo: | Cataract surgery is the most frequent surgical procedure in developed countries. In recent years, while having increasingly high success and patient satisfaction rates, a steady fusion of cataract and refractive surgery occurred. Nevertheless, one of the factors limiting visual acuity and spectacle independence after cataract surgery is astigmatism. Corneal astigmatism over 1.25 diopters (D) is present in up to 29% of patients submitted to cataract surgery. Intraocular lens (IOL) power calculation is primarily based on formulas derived from normative ocular biometric parameters. Therefore, knowledge of these parameters is essential. In the Portuguese population, there was no published data on the ocular biometric parameters or their associations, so we described the mean ocular biometric parameters and the prevalence of corneal astigmatism in cataract surgery candidates in Portugal. We found that the mean axial length, anterior chamber depth, and mean keratometry values were closer to those published for the United States population than most series in different European Caucasian populations, with and the disparities representing potential differences of 1 D or more in both refractive error and IOL power evaluation. Corneal astigmatism was higher than that in most published series, which may affect the type of IOL to be implanted. Among the various techniques for correcting astigmatism during cataract surgery, toric IOLs are the most effective and predictable. When calculating the cylindrical power of these lenses, there are, however, multiple sources of error. Preoperatively, precise evaluation of the astigmatism to be corrected is mandatory. In such a way, there is an ongoing debate on which instrument is most accurate for evaluating total corneal astigmatism (TCA). Accounting the limited precision, several promising technologies were recently developed, such as color-light emitting diode (LED) topography. This technology is commercially available in the Cassini (i-Optics, The Hague, the Netherlands) topographer, which uses ray tracing algorithms to provide a complete analysis of the cornea. To contribute to the scientific knowledge on this new topographer, we have studied which method evaluates corneal astigmatism with higher precision by comparing color-LED topography with two other established astigmatism measurement methods (a slit-scanning topographer and an automated keratometer). We investigated the comparability and repeatability of these three methods. In a first study, it was shown that the evaluation of corneal astigmatism by color-LED topography was more precise than the other technologies. A subsequent study demonstrated that all measurement techniques show comparable keratometry and astigmatism axis values. However, the wide data spread found suggests these devices should not be used interchangeably. Besides the limited precision in the preoperative evaluation of astigmatism, other limitations exist in the power calculation of toric IOLs. One of these limitations is the assumption, by most toric IOL calculators, of a fixed ratio between the cylindrical power at the IOL and corneal planes. This results in undercorrections in long and overcorrections in short eyes. Moreover, although scientific literature is scarce on the subject, the cylindrical power of the IOL at the corneal plane also depends on the IOL’s spherical power, due to the different vergence of the light rays. Also, knowledge and consideration of total corneal astigmatism is mandatory for precise toric IOL calculation. To overcome these known limitations, several new calculation strategies were recently developed. To investigate the most precise of the novel calculation methods, we have calculated the prediction error for each of them in a group of patients submitted to cataract surgery with toric IOL implantation and investigated whether it would be better to directly evaluate total corneal astigmatism or use the current nomograms that estimate its value. Overall, the Barrett toric calculator and the Abulafia-Koch formula yielded the lowest astigmatic prediction errors. Findings from the consecutive study demonstrated that, at present, directly measuring the posterior corneal surface is not superior to predicting its power with theoretical models. We suggest that the clinical results of toric IOL implantation may be improved by using Barrett toric calculator and the Abulafia-Koch. Another source of error in cataract surgery with toric IOL implantation is arising from the surgically induced astigmatism (SIA), which must considered for toric IOL calculation. SIA depends on numerous factors related to the individual, the incision, and the type of surgery. How these factors interplay to determine SIA had not been studied. To improve knowledge of SIA, using the same clear cornea incision (CCI) size, we compared its value after phacoemulsification with femtosecond laser and manually-created CCIs and investigated the influence of individual factors and incision characteristics on SIA. It was found that femtosecond laser-created CCIs resulted in more reproducible wound architecture and lower SIA values, although the difference in SIA did not reach statistical significance and the dispersion of SIA magnitudes was high. Association of SIA with specific individual features remains highly variable. Thus, for toric IOL calculation, a mean value should be considered. In summary, we demonstrated that the prevalence of corneal astigmatism in the Portuguese population is high. To evaluate preoperative total corneal astigmatism, color-LED topography is a precise new technology. We showed the most precise of the recently developed toric IOL calculators are the Barrett toric calculator and the Abulafia-Koch formula. Also, that estimating the total corneal astigmatism with mathematical models revealed to be superior to measuring it directly with Scheimpflug-based tomography. Thus, this should be, at present, the preferred calculation method for toric IOLs. While we contributed to improve knowledge of SIA with a large series in a prospective randomized clinical study, its predictability is still low for both manual and femtosecond laser assisted cataract surgery. In conclusion, correctly evaluating preoperative total corneal astigmatism, using the most precise toric IOL calculator and precisely predicting SIA will ultimately improve the clinical results of cataract surgery with toric IOLs. Future research topics to further refine calculation of these IOLs are suggested. |
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| Autores principais: | Ferreira, Tiago Luís do Carmo Bravo |
| Assunto: | Toric Intraocular Lens Surgery Ophthalmology Cirurgia Oftalmologia |
| Ano: | 2019 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade Nova de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório Institucional da UNL |
| Resumo: | Cataract surgery is the most frequent surgical procedure in developed countries. In recent years, while having increasingly high success and patient satisfaction rates, a steady fusion of cataract and refractive surgery occurred. Nevertheless, one of the factors limiting visual acuity and spectacle independence after cataract surgery is astigmatism. Corneal astigmatism over 1.25 diopters (D) is present in up to 29% of patients submitted to cataract surgery. Intraocular lens (IOL) power calculation is primarily based on formulas derived from normative ocular biometric parameters. Therefore, knowledge of these parameters is essential. In the Portuguese population, there was no published data on the ocular biometric parameters or their associations, so we described the mean ocular biometric parameters and the prevalence of corneal astigmatism in cataract surgery candidates in Portugal. We found that the mean axial length, anterior chamber depth, and mean keratometry values were closer to those published for the United States population than most series in different European Caucasian populations, with and the disparities representing potential differences of 1 D or more in both refractive error and IOL power evaluation. Corneal astigmatism was higher than that in most published series, which may affect the type of IOL to be implanted. Among the various techniques for correcting astigmatism during cataract surgery, toric IOLs are the most effective and predictable. When calculating the cylindrical power of these lenses, there are, however, multiple sources of error. Preoperatively, precise evaluation of the astigmatism to be corrected is mandatory. In such a way, there is an ongoing debate on which instrument is most accurate for evaluating total corneal astigmatism (TCA). Accounting the limited precision, several promising technologies were recently developed, such as color-light emitting diode (LED) topography. This technology is commercially available in the Cassini (i-Optics, The Hague, the Netherlands) topographer, which uses ray tracing algorithms to provide a complete analysis of the cornea. To contribute to the scientific knowledge on this new topographer, we have studied which method evaluates corneal astigmatism with higher precision by comparing color-LED topography with two other established astigmatism measurement methods (a slit-scanning topographer and an automated keratometer). We investigated the comparability and repeatability of these three methods. In a first study, it was shown that the evaluation of corneal astigmatism by color-LED topography was more precise than the other technologies. A subsequent study demonstrated that all measurement techniques show comparable keratometry and astigmatism axis values. However, the wide data spread found suggests these devices should not be used interchangeably. Besides the limited precision in the preoperative evaluation of astigmatism, other limitations exist in the power calculation of toric IOLs. One of these limitations is the assumption, by most toric IOL calculators, of a fixed ratio between the cylindrical power at the IOL and corneal planes. This results in undercorrections in long and overcorrections in short eyes. Moreover, although scientific literature is scarce on the subject, the cylindrical power of the IOL at the corneal plane also depends on the IOL’s spherical power, due to the different vergence of the light rays. Also, knowledge and consideration of total corneal astigmatism is mandatory for precise toric IOL calculation. To overcome these known limitations, several new calculation strategies were recently developed. To investigate the most precise of the novel calculation methods, we have calculated the prediction error for each of them in a group of patients submitted to cataract surgery with toric IOL implantation and investigated whether it would be better to directly evaluate total corneal astigmatism or use the current nomograms that estimate its value. Overall, the Barrett toric calculator and the Abulafia-Koch formula yielded the lowest astigmatic prediction errors. Findings from the consecutive study demonstrated that, at present, directly measuring the posterior corneal surface is not superior to predicting its power with theoretical models. We suggest that the clinical results of toric IOL implantation may be improved by using Barrett toric calculator and the Abulafia-Koch. Another source of error in cataract surgery with toric IOL implantation is arising from the surgically induced astigmatism (SIA), which must considered for toric IOL calculation. SIA depends on numerous factors related to the individual, the incision, and the type of surgery. How these factors interplay to determine SIA had not been studied. To improve knowledge of SIA, using the same clear cornea incision (CCI) size, we compared its value after phacoemulsification with femtosecond laser and manually-created CCIs and investigated the influence of individual factors and incision characteristics on SIA. It was found that femtosecond laser-created CCIs resulted in more reproducible wound architecture and lower SIA values, although the difference in SIA did not reach statistical significance and the dispersion of SIA magnitudes was high. Association of SIA with specific individual features remains highly variable. Thus, for toric IOL calculation, a mean value should be considered. In summary, we demonstrated that the prevalence of corneal astigmatism in the Portuguese population is high. To evaluate preoperative total corneal astigmatism, color-LED topography is a precise new technology. We showed the most precise of the recently developed toric IOL calculators are the Barrett toric calculator and the Abulafia-Koch formula. Also, that estimating the total corneal astigmatism with mathematical models revealed to be superior to measuring it directly with Scheimpflug-based tomography. Thus, this should be, at present, the preferred calculation method for toric IOLs. While we contributed to improve knowledge of SIA with a large series in a prospective randomized clinical study, its predictability is still low for both manual and femtosecond laser assisted cataract surgery. In conclusion, correctly evaluating preoperative total corneal astigmatism, using the most precise toric IOL calculator and precisely predicting SIA will ultimately improve the clinical results of cataract surgery with toric IOLs. Future research topics to further refine calculation of these IOLs are suggested. |
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