INTRODUCTION

Presbyopia surgery, based on the technological growth of intraocular lenses and the vast experience gained from cataract surgery, is one of the great hallmarks of ophthalmology at the beginning of this century.

One of the major challenges of contemporary lens surgery is to achieve the desired postoperative refraction, that is, emmetropia. However, the desired postoperative refraction depends on several factors, some measurable with satisfactory accuracy, such as the refractive power of the cornea and the axial length of the eye.

But there are others that are still susceptible to relative variation, because it is not yet possible to accurately predict the final position that the IOL will occupy within the eye (effective position of the lens), and this interferes with the result of the mathematical calculations that are made for choosing the power of the lens to be implanted1-3.

Unwanted variations in postoperative refraction may occur due to changes in corneal potency, as a result of the healing process of the incisions, and IOL conditioning in the capsular bag, until it rests in its final position.

Similarly, there is no guarantee that the preoperative calculation for the lens to be implanted will give exactly the intended result. In addition to these imponderables, lens manufacturing defects or incorrect preoperative measurements may occur either due to biometric or improper formulation, or even team mistakes during the surgical procedure1,4,5.

Multifocal IOLs, despite constant improvements, have also been associated to a greater or lesser degree with visual aberrations (dysphotopsies)4,6,7.

As a result, one may find dissatisfied patients with a multifocal IOL, despite the fact that the visual results are technically perfect. Surgeons usually measure the success of the procedure based on the medical results achieved. However, for the patient there are additional factors, distinct from the visual result, which may be capital in their final sense of satisfaction. And this is, in their mind, what determines whether they consider the treatment successful or unsuccessful.

Presbyopia patients expect refractive results close to emmetropia with relative independence of glasses, and free of associated visual discomfort4,8. Therefore, the surgeon must be prepared with multiple alternatives and surgical methods, to correct the residual ametropia after lens surgery, and save situations of discomfort related to the new visual condition resulting from the surgery.

In this chapter we will discuss the implantation of Add-On IOLs and the explant of the primary IOL and its replacement.

ADD-ON IOL IMPLANT

Add-On IOL implantation in the ciliary sulcus over the existing implant is a second-lens piggyback implantation technique in the posterior chamber. It was first described by Gayton and Sanders in 19939 and is a relatively simple and atraumatic procedure that has no associated risks and potential complications found in IOL replacement. It is of particular interest in patients with high refractive errors or contraindications for corneal ablation. In addition, the accuracy of the IOL power calculation is theoretically higher than for lens exchange.

Indications of Add-On IOLs:

1 - Refractive surprise

Refractive surprises are much less frequent today than in the past3,4. But despite significant advances in formula measurement technology and effectiveness, all intraocular lens-based presbyopia surgeries can have them.

Refractive surprises are more frequent in the range of high myopia or hyperopia, unusual anterior segment with exceptional depth or very narrow anterior chambers, small or large corneal diameters, extreme central corneal power, previous kerato-refractive surgery or any combination of them.

In the face of one of these refractive surprises, if the solution cannot act on the ocular surface, it can only act on the posterior chamber. Thus, the first option will be the placement of a second Add-On lens in the ciliary sulcus. This option carries a relatively low risk, as it has fewer technical maneuvers and less manipulation than lens exchange.

In addition, determining the IOL power to be added is an exercise independent of axial length and knowledge of IOL power already in place, so it is not necessary to use the same measurements that led to disappointment.

It is also well known that laboratories have margin to manufacture high powers of the order of about 1 D, if in reviewing the calculations one does not find anything concrete to justify the error, and needs to consider whether it is worth replacing the IOL, taking into account it can have a difference of up to almost 2 D from what is already deployed. If possible, it is always best to plan a secondary implant.

Add-On IOLs perform very well in patients with hyperopic postoperative error. LASER correction of this form of ametropia is generally less predictable than residual myopia. And in high hyperopia, corneal correction leaves a very small effective optical zone, which can interfere with the functioning of the implanted multifocal IOL.

Patients with a history of Radial Keratotomy are also good candidates because, being prone to hyperopic surprises after cataract surgery, they are not good candidates for corneal surgery.

And in such cases, as in others where corrective surgery should be postponed until confirmation of refractive stability, it is preferable to use an Add-On lens than to explant.

2 - Negative dysphotopsies

In negative dysphotopsies, knowing that the space between the implanted IOL in the bag and the iris must be narrowed, one of two actions can be taken:

1. If accepted and if a small myopia is useful, the original IOL body can be forwarded to the front of the anterior capsule – a maneuver with few dangerous steps, that will not alter astigmatism.
2. If there is also a refractive error, this can be corrected, together with the correction of negative dysphotopsies, by implanting an appropriate Add-On IOL in sulcus.

PREPARATION FOR SURGERY

The implantation of Add-On lenses in the ciliary sulcus requires some characteristics to be successful: the primary IOL must be stable and fully contained in the capsular bag, preferably with the border overlapped by the anterior capsule in 360 degrees; there must be adequate space between the iris and the implanted IOL; the anterior chamber must be normal or deep; the endothelium must be in good condition; and there has to be no evidence of pigmentary dispersion syndrome. Therefore, before making a decision, the anatomical characterization of the chamber should be carried out by means of the anatomical characterization of the posterior chamber, with slit lamp, and possibly ultrasonic biomicroscopy (UBM).

Contraindications to Add-On IOL implantation are all those cases that compete with glaucoma, narrow anterior chamber, pigment dispersion, zonular weakness (secondary to trauma or pseudo-exfoliation) and low endothelial count.

Also at high risk, are those who needed a capsular tension ring in primary surgery, and those who have posterior synechiae with the capsular bag.

Lens Features

Add-On lenses are thin and flexible (they pass well through incisions from 2.6 to 2.75 mm). The most used ones have an appropriate profile to be installed in the ciliary sulcus.

They have an optical diameter between 6.5 and 7.0 mm and an overall diameter of 14 mm. They are concave on the face facing the primary IOL so as not to touch it (theoretically they are placed 0.5 mm away from the optics of the original lens). They also have 10-degree angled haptics to move the optics away from the back surface of the iris (Figure 1). They are available between -10.0 and +10.0 D (Sulcoflex Aspherica – Reyner) and between -6.0 and + 6.0 D (HumanOptics monofocal).

CALCULATION OF ADD-ON LENS POWER

The calculation of the power of the add-on IOL is based on a simple rule. If the spherical equivalent is between + and - 7 diopters and Ks being normal, it is necessary to know the manifest refraction to plan the power of the new implant.

For hyperopia with spherical equivalent below +7.0 D, the spherical equivalent is multiplied by 1.5 to result plano; while for myopic refractive errors below -7.0 D the spherical equivalent is multiplied by 1.2 for the same purpose.

For myopia and hyperopia with more than 7 D, non-normal Ks and in previous refractive surgery, it is mandatory to use refractive vergence formulas or Barret's Rx formula.

However, it is always best to consult the online calculator for Add-On IOLs. It is very simple to apply. A good explanation of how refractive formulas work and with examples is on the Internet at www.doctor-hill.com/iol-main/piggiback.htm. At www.doctor-hill.com/physicians/download.htm one can freely obtain a calculator of the refractive vergence formula in a Microsoft Excel format based on Holladay’s description10.

The most sophisticated, commercially available refractive vergence formula is the Holladay R included in Holladay IOL consultant software (Holladay Consulting). The Rx formula, recently introduced by Graham Barret (available at www.apacrs.org), points out solutions in three scenarios: piggyback IOL, IOL exchange, and existing IOL rotation for a new alignment meridian.

ADD-ON SURGICAL TECHNIQUE

Placing an Add-On lens differs from primary implantation only because the lens is larger and there is less work space in relation to the endothelium.

We begin with dilatation with tropicamide and topical and intracamerular anesthesia with lidocaine diluted in physiological saline at 1:3. The original incision is used or if a small cylinder is corrected, a new 2.6 mm corneal tunnel is made in the steepest meridian. The anterior chamber is filled with cohesive viscoelastic to protect the endothelium and distend the space of the ciliary sulcus.

The IOL is mounted and folded into the injector under the microscope to ensure that its position is correct. The implant is slowly injected, directing the first haptic directly into the ciliary sulcus. Then the second haptic is driven with an IOL, or Sinskey’s rotator, and released under the iris under the incision. The IOL is carefully positioned and centered until it is in a stable position.

It is important to remember that special attention should be given to the deployment of the IOL at the exit of the injector, so that the endothelium does not get injured. And when the optic is made of silicone it is prudent to use a thick viscoelastic, like Healon GV (Abbott Medical Optics), so that it contains its force when unfolding, avoiding uncontrolled movements in the anterior chamber. If the IOL is toric, attention should be paid to its position and alignment, according to the axis to be corrected.

At this point, if the operating microscope is not assisted in marking the desired axis (e.g. Verion System), it will be necessary to mark the vertical and horizontal axes before the patient lies down, to avoid positioning errors due to decubitus cyclotorsion.

After removal of the viscoelastic, it should be noted that adequate space exists between the posterior face of the iris and the anterior face of the lens. The surgery ends with the hydration of the stroma in the corneal incisions and injection of intracamerular antibiotic.

The surgical technique is familiar to all cataract surgeons and requires no additional equipment or learning curve.

In the author's experience the implantation of a second lens for the correction of refractive defects or negative dysphotopsies has been shown to be safe, effective and stable. The results are usually very predictable. The Add-On also has the advantage of having a quick recovery.

Like all surgeries, intraocular Add-On lens placement can cause some complications, which are the same as any intraocular procedure plus others unique to Add-On IOLs. The most common, although rare, is inter-lenticular opacity. The best way to prevent membrane formation between the two lenses is to ensure that they do not touch each other and to choose the Add-On of a material other than the original IOL – if the one in the bag is acrylic, the one in sulcus should be silicone, or vice versa. Another complication is postoperative pigment dispersion, which occurs when the sulcus lens touches the posterior face of the iris. This risk is reduced by using thin profile Add-On lenses with smooth, rounded, angled haptic edges.

EXPLANTATION AND IOL EXCHANGE

Patients with significant postoperative refractive error and those with decentered or damaged IOLs have as a first repair option the lens exchange. This procedure is best done early in the postoperative period before the capsule has formed adhesions that lock the IOL in its bed. For this reason, this option works best when the surgeons feel that they can safely remove the original IOL, while still preserving the integrity of the capsular bag.

When it is thought that the lens should be explanted, one must take into account several factors. The patient is the first consideration: age, occupation, refraction of the eye in question, refraction of the fellow eye, and thorough review of the patient's own expectations and needs are required11.

The anatomical condition of the eye is the second consideration, and includes: IOL centering, VA, aberrometric analysis, endothelial cell count, capsular bag integrity, how long the IOL has been implanted, IOL position in the eye and retinal state. It is also worth remembering, that the potential complications of this option include posterior capsular rupture and zonular dialysis, that may destabilize the new implant.

INDICATIONS FOR IOL EXCHANGE

IOL exchange is the preferred option for refractive surprise, early in the postoperative course, when the power of the existing IOL is known and Add-On placement is not feasible. The most common causes for IOL exchange are refractive surprise (inability of the formula, difficulties in preoperative measures), problems with the lens (decentration or optic damage) and the adverse effects of multifocality (dysphotopsies)12,13.

Decentration

Cases of multifocal IOL or bag-multifocal IOL complex malposition will, as a rule, be resolved by explanting. Very exceptionally, with great conviction that it will be well centered and with good endothelium, it may be possible to suture the IOL, or the complex, to the sclera.

Poorly positioned IOL usually originates from an incident during primary surgery, due to anterior or posterior capsular rupture, where its importance was underestimated, resulting in lens off-center positioning or even subluxation. It is not uncommon to find offsets in the high myopes implanted with fixed diameter dish lenses (because they have large capsular bags)13,14.

Late dislocation or decentration appears in capsular sac phimoses and progressive zonulopathies, such as pseudo-exfoliation and Marfan’s syndrome, or eye trauma.

When it is necessary to be more rigorous in the evaluation of these cases, it is possible to resort to wavefront aberrometry and/or UBM. UBM is very useful, mainly in cases where it is planned to place the IOL with the haptics in the sulcus, with or without optical capture. The UBM reveals the amount of capsular support and shows whether or not there is a capsular tension ring at the equator. Aberrometry that measures the internal aberrations of the eye is of great value, especially when the patients are dissatisfied with their vision and no clear reason is found.

DYSPHOTOPSIES

In cases of intolerance to repeatedly debilitating positive dysphotopsies and after all attempts to avoid them, the solution will be to explant and then implant monofocal IOLs. The mental preparation of the patient for the final decision should include a confirmation of monofocal vision, showing how they will see for near without multifocality (putting on -3.0 D glasses). This may eventually lead the patient to rethink the drawbacks of dysphotopsies and not proceed with the explant. Patients’ subjective complaints are higher in the early postoperative period and decrease as the brain adapts to the new vision of various foci, selecting the focused image for each distance2,4,15.

Multifocal IOLs require a degree of cerebral adaptation. The patient's brain needs to adjust to a new visual system by suppressing undesired, out-of-focus images, and learning to ignore glare and halos.

The patient is usually encouraged to wait 6 to 12 months for neuroadaptation to occur. However, it should not take much longer for the decision to be made because IOL exchange will become increasingly difficult. And one must be very attentive to the evolution of the emotional reaction of the patient. The final decision on resolving their dissatisfaction will have to be consistent or they will lose their confidence.

ALTERATION OF IOL TRANSPARENCY

Deformity of the optic or lens haptics is rare3,16. It usually occurs during insertion and may be underestimated during surgery, potentially having an effect as early as the first postoperative days. Like damage caused during YAG capsulotomy, it may produce visual interference or abnormal light phenomena.

Although it is a very rare phenomenon, intraocular lenses can also lose transparency over time16. The constituent elements of current foldable lenses are silicone or various ranges of acrylics. Depending on the nature of their composition, each one can change with time, basically with the formation of calcium deposits or micro vacuoles, which interfere with transparency. The mechanism of the changes is very diverse and difficult to interpret but can lead to progressive decrease of vision due to the loss of transparency of the optics material.

In all these cases, it is necessary to think about explanting the damaged IOL and implanting a multifocal lens again, if the capsular bag can be dissected harmoniously to ensure a perfect centering.

REFRACTIVE SURPRISE

In cases of myopic or hyperopic refractive surprise1,3,17, IOL exchange may also be a solution3,8,17,18, especially if there is not enough room to implant another IOL in the sulcus (Add-On).

In patients who are dissatisfied with their postoperative cylinder after receiving a non-toric IOL, and if they are not eligible for kerato-refractive surgery, it may be a solution to proceed to the exchange of the non-toric IOL by a toric one inside the bag.

In refractive surprise, IOL exchange is contraindicated in all cases of capsular or zonular instability, low endothelial cell count and retinal susceptibility (propensity for cystoid macular edema).

PREOPERATIVE PREPARATION

Except for recently operated cases where operative data and characteristics of the implanted IOL exist, often in the referred patients, there is no detailed previous clinical history or reliable intraoperative data, although usually a good initial functional outcome can be reported. In these cases it is desirable to obtain at least reliable information on the model and power of the implanted lens.

Patients scheduled for inappropriate lens extraction and replacement with a secondary implant, need a detailed explanation of the nature and source of their problem. Any technical or other occurrences of the conventional lens surgery should be revisited and explained, and it should be stressed that this surgery, now proposed, has different potential risks and possible complications.

It should be noted that there are several modalities of secondary insertion, depending on the eye condition during the intervention, with a relative unpredictability of the result. Once known and understood these details should be written and included in the consent form.

Calculating the power of the IOL to exchange

If the characteristics of the implanted lens are known, the power of the new lens is calculated as for the Add-On, simply by adding power or subtracting from an existing optical system. The recommended formulas for exchange are those of refractive vergence, Holladay R or Barrett Rx, available online. To work with refractive vergence formulas, one must know the spherical equivalent of manifest refraction, the current Ks, and the effective position of the lens to be implanted.

Once again it is an exercise independent of axial length. An approximation to the effective position of the lens to be implanted is tabulated based on the Optimized SRK/T A-Constant, and is for example 5.61 for an A-constant of 119.1 (if you want to use multifocal), or 5.20 for an A-constant of 118.4 (if you want to use non-aspherical monofocal IOL in cases of poor centering).

If you find that the error occurred when entering any of the data (axial length or Ks) into the lens calculation formula, simply enter the true data and get the power of the IOL as if you were planning the original surgery.

In cases where the type and power of the original IOL is unknown, the decision is subjected to new pseudophakic biometrics.

Surgical Technique of IOL Exchange

There is a very great difference in the degree of technical difficulty associated with the explantation, depending on the time that has elapsed since implantation.

Removing an intraocular lens from the capsular bag is a relatively simple maneuver when performed early in the postoperative course5,13,18. But if it is an IOL implanted several years ago, with intra-bag fibrosis that tightly surrounds the haptics and the optic, the difficulty in its release is noticeably greater5,16-18.

The technique is generally as follows:

By means of two paracenteses the capsular edge is dissected from the anterior surface of the IOL in all quadrants with dispersive viscoelastic, and a 30G needle with the bevel upwards, may be used in the viscoelastic syringe. Its pointed and sharp end allows easy penetration between the anterior capsule and the optic, releasing the adhesions throughout the border of the circular capsulotomy and creating a plane of cleavage.

The viscoelastic is then injected between the anterior and posterior capsule leaflets in the haptic joint zones. In this location, generally, the capsules are not firmly bonded. Sometimes the viscoelastic wave injected alongside the haptics already extends by dissecting the capsular bag to the equator.

It is at this stage that the most peripheral capsules of the IOL haptics are also carefully separated and viscoelastic passes onto their posterior surface. The ideal viscoelastic for this maneuver is a dispersive type, e.g. Viscoat (Alcon).

The bulb tips of the single-piece acrylic IOL haptics may require additional visco-dissection, in the sense of releasing them from their housing, in the fibrous equator of the capsular bag. When it is considered that the haptics are sufficiently freed, the ability to mobilize the lens in the capsular bag is carefully tested. If you are not sure, you should continue the visco-dissection until you enjoy good mobility.

The mobilization of the haptics must always be done by rotating the lens and never by centripetal traction. Once untied, one of the haptics is carefully rotated with a manipulator hook out of the capsule bag, and the rotation is continued until the opposite haptic is released, completing this way the luxation of the lens into the anterior chamber. If the haptics cannot be removed safely, they can be amputated and left in place without problems.

Occasionally, it is also necessary to inject viscoelastic between the posterior capsule and the posterior surface of the IOL, so that it can be mobilized more easily.

Now with the lens completely movable and in the anterior chamber, the optic can be folded or cut, so that it can be extracted by a small incision.

To fold it, the anterior chamber is extensively reformed with dense viscoelastic. A flat manipulator or cyclodialysis spatula is placed 180° from the inlet under the lens body. Then, with a pair of tweezers (Mc Pherson, Buratto, among others) placed on the IOL, the optics are pressed firmly against the spatula (at its bisector) until folded (Figure 2). Being folded in half, the lens is rotated 90 degrees and drawn through the slightly enlarged corneal incision. Given the abundance of manipulations it is prudent to suture the corneal incision so that it closes hermetically.

Alternatively, and in the author's less traumatic experience, the partial or full section of the optics may facilitate passage through the incision without enlarging it. Bisection or trisection of the IOL body will leave individual fragments easily removable with forceps without forcing the small corneal incision (Figures 3 and 4).

There are several instruments available to cut lenses safely and relatively easily. The Mackool system for removing foldable IOLs (Ambler Surgical) and the Packer/Change IOL Cutters (MicroSurgical Technology) are good for cutting acrylic and silicone lenses. But if unprepared, any strong scissors that can perform through a small incision will be useful if used cautiously.

With few exceptions, it is usually possible to preserve the capsular bag, placing a new IOL in the same location from which the other was explanted.

Keeping the effective position of the lens for the new implant can achieve the expected refractive result. If the posterior capsule is compromised it is a good alternative to implant a 3-piece IOL in the sulcus and stabilize the lens by capturing the optic at the leading edge of the anterior capsule, of course, with proper power adjustments for this position.

The most frequently reported intraoperative complications are capsular rupture and zonular dehiscence. The most frequent postoperative complications are posterior capsule opacification, cystoid macular edema and epiretinal membrane 5,18.