Journal of Clinical Ophthalmology and Research

COMMISSIONED ARTICLE
Year
: 2017  |  Volume : 5  |  Issue : 1  |  Page : 45--50

Prevention and correction of residual refractive errors after cataract surgery


Jeevan S Ladi 
 Department of Cataract and Refractive Surgery, Dada Laser Eye Institute, Pune, Maharashtra, India

Correspondence Address:
Jeevan S Ladi
Dada Laser Eye Institute, Office No. 202, Gulmohar Apartments, 2nd Floor, East Street, Camp, Pune - 411 001, Maharashtra
India

Abstract

Cataract surgery today has become more of a refractive surgery with high patient expectations and demand for independence from glasses. With advances in biometry, precise intraocular lens (IOL) power calculation is the most important step in achieving desired target refraction. Noncontact optical biometry has become the gold standard worldwide with a few exceptions where contact ultrasound biometry still needs to be performed. Although there is no single formula for IOL power calculation suitable for all eyes, our understanding of which formulae to use in specific situations is much better. Despite advances in biometry and cataract surgery techniques, unsatisfactory visual and refractive outcomes occur occasionally. Enhancement after cataract surgery is required in these cases. The common methods to treat such cases are corneal-based laser refractive surgery or lens-based procedures such as piggyback IOL and lens exchange.



How to cite this article:
Ladi JS. Prevention and correction of residual refractive errors after cataract surgery.J Clin Ophthalmol Res 2017;5:45-50


How to cite this URL:
Ladi JS. Prevention and correction of residual refractive errors after cataract surgery. J Clin Ophthalmol Res [serial online] 2017 [cited 2022 Aug 12 ];5:45-50
Available from: https://www.jcor.in/text.asp?2017/5/1/45/195311


Full Text

Modern cataract surgery is characterized by obtaining precise postoperative target refraction as the number of patients opting for premium intraocular lenses (IOLs) is growing and patient expectations to get independence from glasses are very high. It may be not out of place to state that cataract surgery has become more of refractive surgery.

 Biometry



Precise IOL power calculation is the most important step in achieving desired target refraction. Biometry is the process of measuring the power of cornea (keratometry) and the length of the eye and using this data to determine the ideal IOL power. [1],[2]

Contact ultrasound biometry: A (amplitude) mode contact ultrasound biometry has been the gold standard for many decades. It is of 2 types - Contact applanation and immersion.

Contact type requires a probe placed perpendicularly on the cornea, ideally in the sitting position. Errors are possible due to the excess indentation of the cornea (compression of 0.5 mm can cause error of −1.25D.). Off-axis measurement, if the probe is not perpendicular also causes errors (0.5 mm off axis gives +1.25 D error). The risk of transmission of infections between patients is there with contact method.

Steps for accurate measurement of axial length using applanation [3]

Machine should be calibrated and set for the correct velocity setting or mode (phakic cataract, aphakic or pseudophakic)Echoes from the cornea, anterior lens surface, posterior lens surface, and retina should be present and of good amplitude. 5 th spike is from the sclera (misalignment along the optic nerve is recognized by an absent scleral spike)Set the gain at the lowest level at which a good reading is obtainedAvoid excess corneal compressionAverage the 10 most consistent readings giving the lowest standard deviation (<0.06 mm)Always measure both eyes and repeat if axial length difference between two eyes is >0.3 mm or if consecutive measurements differ by more than 0.2 mmBe extra careful in short eyes (<22 mm) and long eyes (>25 mm). Axial length errors get amplified in short eyes, and posterior staphyloma may be present in a long eyeHigher gain setting needed for dense cataractsPosterior staphyloma in myopic eyes causes an elongated globe and macula may be on the slope of the staphyloma. The axial length may get measured longer leading to postoperative hyperopia. In these cases, vitreous depth is taken from the B-Scan and added to anterior chamber depth (ACD) and lens thickness.

Immersion biometry involves placing a scleral shell (Prager) filled with saline between the probe and the eye. Corneal indentation is avoided as no pressure is applied on the eye.

Noncontact optical biometry: Accurate assessment of axial length by contact-free optical biometry has become popular and is now the current gold standard due to ease of use, accuracy, and reproducibility.

The IOL Master (Carl Zeiss Meditec AG, Jena, Germany) was introduced in 1999 and is based on laser partial coherence interferometry (PCI). It uses a 780 nm laser diode infrared light to measure axial length. The anterior corneal curvature is calculated at 6 reference points in a hexagonal pattern at the 2.3 mm optical zone. [4]

A novel biometry device the Lenstar LS 900 (Haag-Streit AG, Bern, Switzerland) was introduced in 2008. It is based on the low-coherence optical reflectometry and uses an 820 nm superluminescent diode. It can also measure the crystalline lens thickness, retinal thickness, central corneal thickness (CCT), pupil size, and pupil centricity. K readings are calculated by analyzing the anterior corneal curvature at 32 reference points oriented in two circles at 2.3 and 1.65 mm optical zones.

Differences between IOL Master and lenstar [1]

ACD: Lenstar measures ACD from corneal endothelium to anterior lens surface while IOL Master measures it from corneal epithelium to anterior lens surfaceLenstar measures more parameters such as CCT, retinal thickness, and pupil size/centricityLenstar takes twice as much time as IOL Master for measurements.

No statistically significant difference was found in axial length measurements between the two devices. [1]

Advantages of optical biometry devices:

Operator independent as it is noncontactNo risk of transmission of infections between patientsRapid and ease of use.

Optical and immersion biometry (± 0.02 mm) is five times more accurate than applanation contact A-scan (± 0.1 mm).

Disadvantages of optical biometry devices: Axial length measurements are not possible in mature or dense subcapsular cataracts as the laser beam gets blocked by a dense cataract. In these cases, contact or immersion A-scan biometry has to be used.

Corneal refractive power: Keratometry can be performed by manual or automated methods. The error of 0.5D in corneal refractive power leads to an error of 0.5D in postoperative refraction.

Manual keratometry is still practiced at a few places. To ensure good results [3]

Use single dedicated and calibrated instrumentAdjust the eyepiece and bring central cross hairs into focusTake an average of 3 readings including the axesIf K readings are <40 or >48, or if difference between the 2 eyes is >1.5D, repeat the readings by another observerIn a scarred cornea use readings of the fellow eye.

Manual keratometry measures corneal power using 4 points 3.2 mm from the corneal center. Power is thus derived from the paracentral area while true corneal power is in the central 1-1.5 mm.

Automated keratometry measures central 4-6 points 2.6 mm from the center of the cornea.

Topography measures entire cornea (>5000 points) and can pick up irregular astigmatism.

 Intraocular Lens Power Calculation and Formulae



There is no single formula which is suitable for all eyes. [1] The third-generation formulas such as Hoffer Q, Holladay 1, and SRK/T formulae are commonly used. SRK I and II regression formulae are obsolete [Table 1].{Table 1}

ACD: Modern formulas for IOL power calculation depend on the AC depth measurement to increase the accuracy of IOL power prediction curve. All modern biometry optical devices such as IOL Master and Lenstar, Slit scanning Videokeratography (Orbscan), Scheimpflug imaging (Pentacam and Galilei), and anterior segment optical coherence tomogram are capable of accurately measuring ACD. The ACD measuring module of IOL Master should not be used to determine ACD in a pseudophakic eye as the evaluation software is only designed for phakic eyes.

A constant: Depends on IOL position, diameter, design, angulation, convexity, and manufacturer. It is considered as a fudge factor that adjusts IOL predictions for systematic errors arising from the clinical environment, biometry measurement devices, patient population, and surgical technique. The estimated IOL constants published by manufacturers is typically intended for use with contact ultrasound biometry, though recently constants related to optical biometry are also being increasingly provided.

Common causes of mistakes in biometry [3]

Biometry performed in a hurry or delegated to untrained staff and not cross-checked by the surgeonWrong selection of formula or A constantWrong entry of K readings while typing.

Refractive needs of the patient: Emmetropia will be the goal for most patients. Some patients may be intentionally left myopic postoperative to aid reading. Anisometropia more than three-dimensional may cause diplopia. Monovision with a difference of more than −1.50 D between the two eyes may lead to disphotopsia such as glare, haloes, etc. Patients may not tolerate these leading to unhappiness about the outcome.

Common causes or errors in achieving target refraction: The Royal College of Ophthalmologist guidelines highlights the need for achieving postoperative target refraction of ± 1D in 90% cases and ± 0.5 D in 60% patients [2] [Table 2].{Table 2}

Errors which can be avoided to achieve these targets are: [3]

Biometry printout attached to wrong patients notesIncorrectly labeled IOLWrong eye or wrong patient in theaterReversed IOL opticNot discussing target refraction with the patientNot taking account of the other eyeUsing other eye K readings or axial length in unilateral high myopes/hyperopes or anisometropesInadvertent placement in sulcus instead of "in the bag."

Biometry in silicone oil-filled eyes: [5] Patients who have undergone vitrectomy with silicone oil surgery for retinal detachment often develop cataracts. Cataract removal with IOL implantation and oil removal can be done at the same time for visual improvement. Biometry in silicone oil-filled eyes is difficult to perform due to sound attenuation property of oil and readings may be unobtainable. Conventional ultrasound biometry with immersion technique is prone to errors such as a false longer eye due to slow sound speed, poor penetration from sound absorption by oil, and multiple fluid interfaces causing errors in measurement. Optical biometry with the IOL Master overcomes these hurdles as it is based on laser PCI and axial length can be measured based on the reflection of interference signal from the retinal pigment epithelium.

Velocity conversion: Standard A-scan contact biometry can be used to calculate axial length.

Axial length = (980/1532 × AL of vitreous) + AC depth + lens thickness

Pediatric cataract: Current IOL power calculation formulas are accurate in adults but not in children. High prediction errors are encountered in shorter eyes with all currently available formulas. Careful preoperative measurements reduce errors. Immersion A-scan biometry is superior in measuring axial length in children. [6],[7]

At <1 year age, ignore keratometry readings, use standard K reading of 44D [Table 3] and [Table 4]. {Table 3}{Table 4}

 Intraocular Lens Power Calculation after Refractive Surgery [1],[8]



In the normal human cornea, the center is steep while there is peripheral flattening. After refractive surgery such as laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), the center becomes flat and peripheral steepening is seen.

The most accurate method to calculate IOL power after refractive surgery is a combination of clinical history and contact lens method. The data required for this is preoperative keratometry and refraction, postoperative refraction, and there should be no lenticular myopia.

If history is not available and refraction is not possible due to a dense cataract, the following formulae can be used.

Shammas equation K = 1.14× (current average K) − 6.8Maloney method K = 1.1× (current average K) − 6.1Topography method K = 1.114× (current average sim K) − 6.1

 Postphotorefractive Keratectomy/Laser in situ Keratomileusis Nomograms

[9]

Feiz nomogram for myopia: Myopia reduced in D, manual/Auto K, and SRK/T.

[INLINE:1]

Feiz nomogram for hyperopia

[INLINE:2]

 Estimation of Corneal Refractive Power after Radial Keratotomy

[10],[11]

Pearls for post-RK IOL power calculation:

Corneal thickness unchangedCentral flattening and peripheral steepening seen after RKAutokeratoetry preferredUse 3 rd generation formula like SRK/TTarget refraction is −1.5D myopiaThe frequency of hyperopia reduced by 60%.

 Intraocular Lens Power Calculation Postradial Keratotomy



Calculate IOL power by contact A-scan biometryAdd + 1.5D for 4-6 incision RKAdd + 2 D for 8 incision RKAdd + 2.5D for >8 incision RK.

 Treatment of Residual Refractive Error after Cataract Surgery



Modern lens removal techniques and advanced preoperative diagnostic methods allow most cataract patients to be spectacle-independent for distance and sometimes also for near. Refractive considerations are integrated into modern cataract surgery and these days; patients expect an adequately predicted refractive outcome. Despite such advances in cataract surgery, unsatisfactory refractive and visual outcomes occasionally occur and in these cases, the enhancement after cataract surgery is required to achieve the best final visual outcome.

Emmetropia is our main target today in modern cataract surgery. Accurate biometric analysis, selection and calculation of the adequate IOL, and modern techniques for cataract surgery all help surgeons to move toward the goal of cataract surgery as a refractive procedure free from refractive error. However, in spite of all these inputs, residual refractive error still occasionally occurs after cataract surgery. The causes for these can be divided into preoperative, operative, and postoperative.

Preoperative causes

Incorrect estimation of postoperative IOL position and faulty measurement of preoperative axial lengthInadequate selection of IOL power. There are limitations of calculation formula, especially in extreme ametropia. Potential error in estimation of postoperative ACD by current IOL power formulaLack of precision in IOL manufacturingPreoperative corneal astigmatism (which occurs in up to one-third of cataract cases) may lead to residual refractive errorsPrevious LASIK or PRK done for myopia may lead to undesirable hyperopia postcataract surgery due to an error in calculating corneal power and effective lens position.

 Operative Causes



Surgical variation in size and central position of capsulorrhexisSurgically induced astigmatismResidual astigmatism after toric IOL implantation may occur due to the effect of spherical power and ACD in toric IOL calculation, the effect of posterior corneal astigmatism, and effect of large pupil size.

 Postoperative Causes



Anterior movement of IOL may occur postoperatively due to capsular bag fibrosis and contraction.

 Prevention of Residual Refractive Error after Cataract Surgery



Surgically induced Astigmatism: To prevent this - microincision cataract surgery using corneal topography data and standard formulas for calculation of IOL power gives stable refractive results. Preoperative study of corneal hysteresis and biomechanical properties of cornea will predict better the postoperative outcome. [12]

Preoperative corneal astigmatism can be corrected by the placement of incision on the steeper axis, peripheral corneal relaxing incisions, and use of toric IOL. [13]

Patients who have undergone previous corneal refractive procedure such as LASIK or PRK - the surgeon should rely more on nonhistory methods. For postmyopic excimer laser refractive surgery patients, the Holladay 2 flat K method has been found to give the most accurate IOL power. [8]

Patients who undergo bilateral sequential cataract surgery: The surgeon can study if refractive error in the first eye exceeds 0.5D - if so refractive error in the second eye can be improved by modifying the IOL power. [14]

Options for the treatment of small residual refractive error include glasses and contact lenses. However, refractive surprise after cataract surgery involving large errors in spherical or cylindrical power is an unpleasant and frustrating situation for both the surgeon and the patient. The methods to treat these situations are either corneal-based laser refractive surgery or lens-based procedures, namely, Piggyback IOL and IOL exchange. Rotation of toric IOL may be needed for residual cylinder postoperatively.

Indications and advantages of lens-based procedures (piggyback IOL and IOL exchange)

Correction of large residual refractive spherical errorsLens-based procedures do not change the corneal refractive power as they do not alter the corneal surfaceThe original cataract wound can be reopened and used to implant the IOL soon after initial surgeryUseful for surgeons who do not have an excimer laser in their setup.

If the IOL to be removed is foldable, it can be removed after cutting it through 2.8-3 mm incision. [15]

It is preferable to do IOL exchange early within 2 weeks before the capsular bag has fibrosed.

Piggyback technique involves the implantation of the second IOL in the posterior chamber (sulcus), over the first IOL (in the bag). It is an easier procedure than IOL exchange, especially if the capsular bag has fibrosed as the risks associated with explantation - namely, capsular rupture, vitreous loss, zonular dialysis, macular edema, and retinal tears are avoided. [16]

Apart from the ease of technique with piggyback IOL, reversibility is feasible (the IOL can be easily removed if correction is not accurate). Due to its peculiar design of large optic size and rounded anterior optic edge, side effects such as optic capture, glare, and iris trauma are reduced.

Sulcoflex (Rayner, UK): IOL Powers available from −10 to +10D.

An alternate option is a three piece hydrophobic acylic IOL in sulcus.

Sulcoflex piggyback multifocal IOLs are also available for a hyperopic or myopic surprise from −3 to +3D. Pseudophakic emmetropic patients with monofocal IOL can also get their presbyopia corrected with sulcoflex multifocal piggyback IOL. Sulcoflex toric and toric multifocal are also available for the correction of associated astigmatism.

One of the simplest formulas to calculate for piggyback IOL is:

Postoperative myopia = spherical equivalent (sphere + half cylinder)

Postoperative hyperopia = spherical equivalent × 1.5

Complications: Interface membrane can develop between the two IOLs. Treatment involves a surgical wash or yttrium-aluminum-garnet (YAG).

Lens exchange to correct residual astigmatism by exchanging a monofocal IOL with a toric IOL is not a good option due to surgically induced astigmatism caused by wound enlargement while explanting the original IOL.

El Awady and Ghanem have found piggyback IOL to be preferable to IOL exchange for treatment of residual refractive error after cataract surgery as it is easier, well tolerated, reversible and accurate. [17]

 Corneal-Based Laser Refractive Surgery



Advantages [16]

Avoids additional intraocular surgical proceduresBetter accuracy and higher predictability of results than piggyback IOL or IOL exchange, especially for the correction of residual cylindersProvides greater flexibility and achieves better final refraction than lens-based procedures. A recent study using LASIK to correct residual refractive error after cataract surgery found 92.85% eyes achieved a final spherical equivalent within ± 0.5D and 100% eyes within ± 1DLASIK is easier in eyes with previous YAG capsulotomy done; where lens exchange is risky and difficult.

Limitations of laser in situ keratomileusis

Correction of high residual refractive errors will depend on corneal thicknessAvailability of excimer laser with the cataract surgeonHigher cost.

 Conclusions



LASIK is the most accurate procedure to correct residual refractive error after cataract surgery. Lens-based procedures, such as IOL exchange or piggyback lens implantation, are also possible alternatives in cases with extreme ametropia, corneal abnormalities, or in situations where excimer laser is not available. Piggyback IOL is a safer and more accurate method than IOL exchange. [17],[18],[19]

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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