Unaided vision-

Figure A: Monofocal IOL implanted in-the-bag; Figure B: Symfony IOL;Figure C: Marks on toric IOL lined up with alignment marks on cornea body text;Figure D: Toric IOL needs careful thorough viscoelastic removal to prevent postoperative rotation

The perfect aim of an ideal cataract surgery is to be able to implant the intraocular lens (IOL) in-the-bag and give perfect unaided vision for all distances. Various factors play a role, right from accurate biometry to the type of IOL selected. Setting realistic patient expectations, as well as understanding patient requirements and personality, are also important.

BIOMETRY:

An accurate biometry is key. Scheimpflug topography and posterior corneal curvature are gaining importance to avoid postoperative refractive surprises, especially for toric IOLs. A poor ocular surface may give erroneous keratometric values and this needs treatment prior to surgery. Though axial length measurement is accurate with optical methods (IOLMaster®, LENSTAR®), contact or immersion ultrasound may be advantageous in eyes with media opacities or in those with inability to fix.

IOL power calculation depends on numerous factors, such as the average central corneal power, axial length, effective lens position, desired postoperative refraction, vertex distance, A constant etc. The A-constant should be calculated and fixed according to the IOL type, biometric technique and surgeon factor. Most modern formulae provide reliable results with average keratometry and axial lengths. IOL power calculation is problematic in short and long eyes; eyes with staphyloma or silicone oil-filled eyes; post-refractive surgery eyes; patients with nystagmus and inability to fix; dense cataracts and uncooperative patients such as children.

Various formulae are available such as Holladay, Holladay I and II, Hoffer Q, SRK-T etc, and these should be used in the appropriate situations. For piggyback IOLs, a power equal to myopic and 1.5 times a hypermetropic error is planned. IOL power is underestimated post-myopic LASIK and overestimated post-hyperopic LASIK. The Hill, Wang and Koch ASCRS IOL power calculator for eyes that have undergone previous LASIK/PRK/RK (iolcalc.org) is very useful.

IOL TYPE:

Aspheric IOLs aim at eliminating positive spherical aberration present in traditional IOLs. They are available as either negative spherical aberration IOLs that compensate almost completely (Tecnis®, AMO, with -0.27 microns of spherical aberration) or partially (Acrysof IQ Aspheric®, Alcon, with -0.20 microns of spherical aberration) for the positive aberrations of the average cornea or as zero spherical aberration IOLs (Akreos AO®, B&L).

With smaller pupil size, this may not be a significant advantage. Some residual spherical aberration improves depth-of-field, and this is an advantage of zero spherical aberration IOLs as is decreased impact of IOL decentration and pupil eccentricity.

Hyperopic LASIK induces negative spherical aberration and such patients benefit with traditional spherical IOLs. Negative spherical aberration IOLs benefit patients with previous myopic LASIK. They may also be better in patients with larger mesopic pupils and those with night-time driving.

MONOFOCAL IOLS:

These aim to provide clear focus at a fixed distance, either near, intermediate or far, depending on the patient’s wishes, requirements and the visual acuity and refraction of the other eye. Patients opting for monovision have one eye focussed for distance and one for near/intermediate by aiming for slight myopia. These patients should be given a monovision trial prior to surgery to check suitability.

Modified monovision refers to a similar use of accommodating or multifocal IOLs to combine the expanded range-of-vision offered by different multifocals. Micro-monovision aims at a very low level of myopia in the non-dominant eye.

MULTIFOCAL IOLS:

These aim at decreasing patient dependency on glasses. They work on refractive, diffractive and apodization principles. Patient selection and education is a must. Patients should be warned of the possibility of glare, halos, decreased contrast, difficulty in night-time driving and the necessity to wear glasses for particular viewing conditions. Patients should be warned that it may take some time to get used to the new visual quality. Refractive lens exchange patients used to good quality vision may be less tolerant of multifocal IOLs than cataract patients.

Large or decentered pupil, poor IOL centration, large angle kappa (>5.2 on OrbscanII), irregular astigmatism or a significant amount of regular astigmatism, dry eyes, previous corneal refractive surgery and macular pathology can all contribute to suboptimal results. An IOL explantation may be required in unhappy patients. Astigmatism control is important when implanting a multifocal, and a toric multifocal and/or simultaneous LRI may be required.

ACCOMMODATIVE IOLS:

These attempt to mimic natural accommodation. Various IOLs include the CrystaLens®, Tetraflex®, Synchrony dual optic®, FluidVision® lens, DynaCurve® lens etc. Crystalline lens replacement with polymer as well as the Light Adjustable Lens® are other options. The basis of accommodative IOLs include anterior movement and steepening of the anterior surface of the IOL as a result of ciliary muscle contraction or anterior vaulting and axial movement of the IOL.

These IOLs have less glare, halos and contrast sensitivity loss. Disadvantages include inability to read minute print and changes secondary to bag fibrosis and posterior capsule opacification (PCO). Also, though their distance vision quality may be better than that of multifocals, near correction may not be as good. They may therefore require some micro-monovision or may be used with a multifocal in the other eye in suitable patients.

EXTENDED-RANGE-OF-VISION/ EXTENDED-DEPTH-OF-FOCUS IOLS:

The EDOF IOLs use extended depth-of-focus to achieve near vision. Symfony® (AMO) uses spherical and chromatic aberration control as well as diffractive optic to expand a single focal zone as opposed to creating multiple foci. The EDOF small aperture IOL uses a small aperture principle similar to the Kamra® inlay.

TORIC IOLs:

These correct both sphere and, depending on the manufacturer, variable degrees of astigmatism. In patients with high cylinder, it may need to be combined with LRIs to get full correction. Postoperative LASIK may be needed in some patients. Posterior corneal curvature plays an important role in avoiding post toric IOL surprises. Accurate preoperative marking, correct alignment of IOL and complete removal of viscoelastic including under the IOL are vital.

Intraoperative aberrometer and limbal registration systems can help in making power and axis alignment much more accurate. The right IOL design that prevents postoperative rotation should be chosen. Toric IOLs may be avoided in very large eyes with a higher chance of rotation. Multifocal and accommodative torics are also available.

CONCLUSION:

Ultimately, a satisfied patient is one who is happy with the postoperative quality and range of vision that matches the patient’s expectations set by the surgeon preoperatively. Uncomplicated surgery with a round and appropriately sized rhexis, complete cortical removal and in-the-bag IOL placement should be aimed for. Bioptics in the form of postoperative LASIK/PRK may be needed for any residual refractive error.

Dr Soosan Jacob is Director and Chief of Dr Agarwal’s Refractive and Cornea Foundation at Dr Agarwal’s Eye Hospital, Chennai, India, and can be reached at: dr_soosanj@hotmail.com