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Editorial Volume 4 Issue 3
Precise IOL position measurement to improve refractive outcomes in cataract surgery
Filomena Ribeiro,1,2,3
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Miguel Prata,3 João Mendanha Dias3
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1Hospital da Luz, Portugal
2Faculdade Medicina de Lisboa, Universidade de Lisboa, Portugal
3Instituto Superior Técnico, Universidade de Lisboa, Portugal
Correspondence: Filomena Ribeiro, Hospital da Luz, Lisboa, Portugal
Received: May 25, 2016 | Published: May 26, 2016
Citation: Ribeiro F, Prata M, Dias JM. Precise IOL position measurement to improve refractive outcomes in cataract surgery. Adv Ophthalmol Vis Syst. 2016;4(3):84-85. DOI: 10.15406/aovs.2016.04.00111
Abbreviations
IOL: intraocular lens; ELP: effective lens position; ACD: anterior chamber depth
Editorial
Over the last decades cataract surgery techniques have evolved remarkably, culminating in a procedure which aims to restore vision but also to provide spectacle free through the implantation of an Intraocular Lens (IOL). Currently the power of the implanted IOL is calculated with formulas derived by retrospective analysis of a large number of patients who had IOL implantation, and whose data was subjected to regression analysis, resulting in a power calculation formula, characterized by a set of constants which are calculated so that the postoperative refractive error, given by the difference between the target and postoperative refraction, averages zero. These formulas relate several biometric parameters measured preoperatively, such as the corneal power and the axial length of the eye, in order to calculate the power of the IOL. Another parameter taken into consideration when calculating the power is the Effective Lens Position (ELP), which acts as proxy to the physical position of the IOL inside the eye, given by the postoperative anterior chamber depth (ACD).
However, accurately achieving the desired postoperative refraction remains a challenge. According to our data1 there is strong indication that the main underlying cause for this are the difficulties associated with predicting the postoperative position of the lens inside the eye ACD. This estimate is of a major importance to the refractive error. The predictability of ELP has improved over recent years, attributable to enhancements in the formulae used and the accuracy of the preoperative measurements of ocular biometric variables. However, ray tracing evaluation may contribute more to improve ACD estimation. The fact that eye models define the lens physical position2 and not a theoretical position such as the ELP, allows comparisons with post-surgical measurements. Also several authors have explored the possibility of using the symmetry of the eye in order to improve the refractive outcome.3–5 By performing the surgery sequentially the refractive outcome of the first operated eye is assessed and its ACD measured. The power of the IOL to be implanted in the other eye is calculated considering these parameters which results in an improved refractive outcome, compared with calculating the power of each IOL independently. This technique was shown to be more effective when the corneal power of the two eyes was similar (a corneal power difference smaller than 0.6D between the eyes).6
Several authors have explored the possibility of measuring ACD in the past. Methods to verify the IOL position in situ have been based on measurements of the Purkinje-Sanson III + IV images reflected from the anterior and posterior surface of the IOL.7,8 Although such methods have proven highly accurate they are not easy to perform as they require special equipment not within the normal clinical techniques. More recently, the interferometer IOLMaster™ has been used to measure ACD, but Sahin & Hamrah point out that the ACD measuring module of the interferometer IOLMaster™ should not be used to determine the ACD in a pseudophakic eye, as 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 erroneously. Pseudophakic ACD will thus give results which are faulty and unreliable.9 Scheimpflug photography has also been used to take this measurement, but Nemeth et al conclude, from their work, that in pseudophakic eyes, the precise assessment of ACD is dependent on the measuring device, although the exact reason for this difference remains elusive.10 Optical measurements with reflectometry, such as Lenstar™, are not always possible, since in the majority of cases the light beam does not detect both surfaces of the IOL.11
For this reason it is crucial to be able to accurately measure this parameter. Currently there is a need to assess the accuracy of ACD measurement techniques. We assessed the accuracy of a measuring device, the Zeiss Visante™ OCT. A pseudophakic eye phantom was built,11 (Figure 1), using laboratory grade optomechanical components, custom designed components and a 22D SA60AT Alcon AcrySof™ single-pice IOL. The IOL is installed in a custom IOL holder, which is glued onto a custom optical post that is in turn connected to off-shelf optical posts mounted on a translation stage. The stage is actuated via a high precision micrometer head (5 μm/div), allowing for precise axial displacements relatively to the front surface. Calibrations were performed and the span shift error was found to be virtually inexistent, (Figure 2). The thickness of an IOL was measured and a 56 error was found, which, considering this to be representative of the zero shift error, is clinically insignificant. Knowing that OCT based devices perform accurate measurements of ACD will certainly contribute to improve ELP estimation methodologies and continue to push forward ray tracing based methodologies, which make a direct use of the ACD.
Figure 1 Pseudophakic eye phantom and custom IOL holder.
Figure 2 Relation between Visante™ OCT and IOL’s Phantom Eye relative displacement measurements in water in the medium range [3.71, 4.46] mm.
Acknowledgments
None.
Conflicts of interest
The author declares there are no conflicts of interest.
References
- Ribeiro F, Castanheira Dinis A, Dias JM. Refractive error assessment: influence of different optical elements and current limits of biometric techniques. J Refract Surg. 2013;29(3):206–212.
- Ribeiro FJ, Castanheira Dinis A, Dias JM. Personalized pseudophakic model for refractive assessment. PLoS One. 2012;7(10):46780.
- Covert DJ, Henry CR, Koenig SB. Intraocular lens power selection in the second eye of patients undergoing bilateral, sequential cataract extraction. Ophthalmology. 2010;117(1):49–54.
- Olsen T. Use of fellow eye data in the calculation of intraocular lens power for the second eye. Ophthalmology. 2011;118(9):1710–1715.
- Muthappan V, Paskowitz D, Kazimierczak A, et al. Measurement and use of postoperative anterior chamber depth of fellow eye in refractive outcomes. J Cataract Refract Surg. 2015;41(4):778–784.
- Aristodemou P, Knox Cartwright NE, Sparrow JM, et al. First eye prediction error improves second eye refractive outcome: Results in 2129 patients after bilateral sequential cataract surgery. Ophthalmology. 2011;118(9):1701–1709.
- Holladay JT, Long SA, Lewis JW, et al. Determining intraocular lens power within the eye. J Am Intra-Ocular Implant Soc. 1985;11(4):353–363.
- Binkhorst CD, Colenbrander MC, Loones LH. Determination of the power of a convex-plano intraocular lens in situ from the dioptric keratometer reading of its front surface: extension table for the Javal-Schiotz ophthalmometer. Br J Ophthalmol. 1987;71(6):473–476.
- Sahin A, Hamrah P. Clinically relevant biometry. Curr Opin Ophthalmol. 2012;23(1):47–53.
- Nemeth G, Vajas A, Kolozsvari B, et al. Anterior chamber depth measurements in phakic and pseudophakic eyes: Pentacam versus ultrasound device. J cataract Refract Surg. 2006;32(8):1331–1335.
- Miguel Joana de Sousa Prata. Accurate intraocular lens position determination in pseudophakic eye. MSc thesis, Instituto Superior Técnico. 2016.
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