Presbyopia / SPO - 3.2.6.3. Evaluation and management of rotation of multifocal intraocular to

3.2.6.3. Evaluation and management of rotation of multifocal intraocular toric lenses

Ramón Ruiz Mesa, Antonio Abengózar Vela y María Ruiz Santos

Oftalvist CIO Jerez, Jerez de la Frontera, Cádiz, España.

INTRODUCTION

As described in previous chapters, current multifocal intraocular lenses (IOLs) are an alternative for the treatment of presbyopia, offering good intermediate and near visual acuity, as well as good vision quality at all distances.

The development of new materials and geometries has allowed the implementation of the optical qualities of multifocal lenses with toric geometries, which has led in recent years to the appearance on the market of different types of toric multifocal lenses. The behavior of the toric monofocal and toric multifocal lenses, in terms of stability and rotation, is very similar, however, visual acuity is more compromised in the multifocal lens1.

Although current models of toric multifocal IOLs show good rotational stability2,3, they may not maintain their position after surgery. Small rotations do not affect the power of astigmatism. However, large rotations reduce the power of the toric IOL by disappearing the astigmatic correction of the IOL if it rotates 30º4. In addition, a rotation of 5º in the axis produces a significant decrease in the modulation transfer function (MTF)5, and the residual refractive effect induced by the toric lens will not be pure astigmatism but will generate a certain degree of hyperopia6. In general, 10º is the limit from which the need and effectiveness of a reintervention to reposition the lens should be assessed7.

The main causes of IOL rotation are (8-11):

Large axial lengths and wide bags.
Fluctuations in intraocular pressure.
Poor lens positioning.
Lens design.
Remains of viscoelastic.
Compression of haptics.
Capsular contraction due to fibrosis.
Pupillary size.

It is important to keep in mind, that toric lenses have the highest rotation in the first two weeks after surgery, stabilizing their position after that1,12,13. Therefore, the evaluation and decision for a reintervention, either by rotating the lens or opting for other surgical techniques, should be done in the short term.

In this chapter we will discuss the current methods for the evaluation of the position of the axis of the toric lenses in general and the alternatives to compensate the rotation of the toric IOL.

EVALUATION OF THE POST-SURGICAL POSITION OF THE AXIS

The exact alignment of the axis in the toric IOLs in the pre-surgical planned meridian is crucial to achieve the desired correction of corneal astigmatism, since 1° rotation outside the calculated axis results in a loss of up to 3.3% of the correction of the cylinder power14. Therefore, if there is a suspicion of a loss of visual acuity related to the rotation of the toric IOL, a correct evaluation of the axis position must be carried out to confirm that the decrease in vision is due to the rotation of the lens.

Before evaluating the position of the axis, it must be ensured that the lens is well centered, even more so in multifocal toric lenses, where multifocality causes decentration to produce a decrease in visual quality15. In addition, if the IOL is not centered, an error will be made in the measurement of the axis, which will be independent of the method used.

Measurement with slit lamp

The most widely used and easily implemented method is the alignment of the IOL axis with the lamp slit. Some manufacturers incorporate numbered shaft marks to measure the angle. However, sometimes the scale is represented by 10º spaced points (e.g. Haag-Streit BM900 lamp), which makes the measurement difficult and inaccurate. In these cases, an axis measuring strip can be used to measure the angles spaced at 1º, which provides a precise and clinically valid measurement of the position of the IOL16.

Image analysis

The image obtained with a slit lamp can be processed and analyzed once it has been obtained and saved in digital image format. The capture can be done with a digital camera or even with a smartphone17. Once the image is captured, it can be processed and analyzed with free or paid software17,18. The measurement of the postoperative axis of the toric lens by software is as reliable and predictable as that measured with a slit lamp19.

In the case of smartphones, there are currently applications developed in Android and iOS for measuring the axis of the toric IOL such as "Toreasy/Toraxis" (Android/iOS) developed by Dr. Gatinel, or toriCAM (iOS) developed by Dr. Barrett. However, there are no studies that describe the accuracy and effectiveness of these applications.

Measurements by internal aberrometry

The measurement of the lens axis by internal aberrometry is a reliable and reproducible objective method20. In these instruments, the alignment of the lens is determined objectively by evaluating the astigmatic axis on the internal optical path difference map of the system (internal aberrometric map)21. In these systems, corneal aberration and total eye aberration are determined.

By subtracting the total ocular aberrations of the corneal aberrations, the amount and orientation of the astigmatism of the lens and therefore its orientation can be determined. The drawback of these systems is that the measure provides information on postoperative lenticular astigmatism rather than the visualization of the alignment of the IOL.

Measurement by optical coherence tomography

Although it is not strictly measured by optical coherence tomography (OCT), the software implemented in some anterior segment OCT instruments allows the determination of the lens axis. This type of software provides an objective method to evaluate the alignment of the toric IOL that is not affected by head tilt or cyclotorsion22.

MANAGEMENT OF LENS ROTATION

Once it is identified that the source of the residual refractive error is given by the inadequate position of the lens axis, the action plan for its treatment can be initiated. The procedures for the compensation of residual astigmatism are the following:

Rotation of the lens

The rotation of the IOL should be the preferred choice within the first 2 months after surgery, provided that the rotation reduces the refractive cylinder and there has not been a modification in corneal astigmatism due to surgery23. The rotation of the IOL is sufficient to reduce residual refractive astigmatism below 0.50 D in more than 40% of cases24. Therefore, it is necessary to know the direction of rotation and the amount of power compensated with the rotation.

The following tools can be used to determine the direction of rotation and the amount of compensation:

Online calculators (Internet)

There are a variety of calculators available on the network that allow to determine the amount of rotation and the compensated dioptric power. As of the publication of this monograph, some of the available web pages are:

recalculatortoric.com: software developed by the Institute of Advanced Ophthalmology (IOA, Madrid, Spain). The calculation of the amount of rotation is based on the differential vector analysis of the pre- and post-surgical refractive defect and the lens position.
ascrs.org/toric-results-analyzeroastigmatismfix.com (figure 1): calculator developed by doctors John Berdahl and David Hardten available on the ASCRS website. To determine the expected residual rotation and refraction, compare the current location of the toric lens with the current manifest refraction of the patient²⁴.

Figure 1: ASCRS calculator results screen.

Calculators implemented in optical instrumentation

Some measuring instruments incorporate specific software for the calculation of the variation of the dioptric power with the rotation of the lens. An example is the iTrace ray tracing aberrometer (Figure 2). In this software you get the amount of power of the cylinder that can be reached by rotating the lens based on the data obtained by the wavefront ray tracing.

Figura 2: Análisis postoperatorio con iTrace

Surgical considerations

In the surgical procedure to rotate the lens, the incision initially made on the clear cornea can be opened. The viscoelastic used for the anterior chamber is cohesive and is injected below the capsulorhexis, to separate the lens from the anterior capsule, and then injected between the lens and the posterior capsule to completely separate the lens and perform the rotation. Once the compensatory rotation of the lens has been carried out, it is appropriate to know that a subsequent application of YAG laser for the posterior capsulotomy does not induce a significant rotation in the toric IOL¹³.

Refractive surgery

In some cases, the rotation of the IOL will not result in a significant reduction of the refractive cylinder, in which case it is likely that the residual ocular astigmatism is not due to the lens, unless there has been an incorrect selection of the toric IOL. In this case, the most appropriate option may be to perform PRK or LASIK with a target of plano and correct the refractive surprise²⁵. There is no data that demonstrates a difference between treatment with PRK or LASIK after the implantation of a toric lens. Laser treatments guided by wavefront may be beneficial after implantation of aspheric toric IOL, since this can minimize high-order aberrations²⁶. However, the treatment of refractive errors by wavefront in patients with diffractive multifocal lenses is effective, even if high-order aberrations do not improve²⁷.

Lens exchange

If the residual refractive error after surgery is high (> 3D), the exchange of the IOL is recommended within the first 6-8 weeks after the primary surgery. In this case, it is advisable to repeat all the calculations to ensure accuracy in the new implementation.

Limbal relaxing incisions

In patients with a medium residual astigmatism who do not wish to undergo surface refractive surgery, performing limbal relaxing incisions (LRI) is a safe and effective way to treat residual astigmatism²⁸. Currently, LRIs can be performed with femtosecond laser. Femtosecond lasers create precise incisions in predictable depth and length, however, there are no nomograms for this treatment modality.

Eyeglass or contact lens correction

The correction of residual astigmatism with glasses or contact lenses is an alternative when the secondary surgical process is contraindicated, or the patient opts for this alternative.

CONCLUSION

Presbyopic patients with astigmatism, undergoing surgery with toric mutifocal lenses, expect optimal refractive results at all distances with minimal astigmatism after surgery. The rotation or displacement of the lens can make the visual results differ from the expected.

Therefore, the precise post-surgical evaluation of the position, and the visual quality of multifocal toric IOLs, are important points in the postoperative evaluation of the phako-refractive surgeon, who must know the procedures for evaluation and treatment of the toric lens rotation.

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