Present your astigmatic patients with the latest IOL options

Marc R. Bloomenstein

The advent of new technology has given rise to a new set of standards in cataract surgery. Patients are no longer satisfied with just seeing better, they want to see without the need for spectacles or contacts. Thus, cataract surgeons have to adopt a new philosophy when talking with patients, a way of thinking that is more in line with a refractive surgeon. No more is this evident than when dealing with IOLs and astigmatism.

About 15% to 29% of cataract patients have more than 1 D of corneal or refractive astigmatism, which can prevent them from achieving the 20/20 visual acuity and spectacle freedom they desire. Surgical options need to be discussed with these patients.

Importance of topography

In the aphakic state, the cornea alone determines the magnitude and axis of refractive astigmatism. Therefore, the preoperative and postoperative evaluation must include a topographical map of the corneal surface.

Topography is also necessary to root out any corneal pathology that may rear its ugly head after lens insertion. It may be obvious that in today’s efforts to achieve pseudophakic emmetropia it is critical to reduce higher magnitudes of natural corneal astigmatism. However, it is the smallest amount of astigmatism that will leave patients with less than desirable visual results. Residual astigmatism is the cause of glare, halos, shadowing and a decrease in visual quality, none of which makes for a great refractive result.

The origin of the astigmatism is the first place to start when evaluating a patient for surgical intervention. The topography will elicit the magnitude and direction of corneal astigmatism. With these parameters established, a plan should be set in motion before IOL implantation. All of the presbyopic correcting lenses are affected by residual refractive error and, more importantly, astigmatism, and none of them are toric lenses.

Limbal relaxation incisions up to 4 D

Nichamin demonstrated that limbal relaxation incisions (LRIs) are capable of correcting up to 4 D of astigmatism. A key advantage of LRIs is that overcorrection is a less likely complication. Because the incisions are made close to the limbus, they heal faster, and, thus, the refractive effect stabilizes more quickly. Furthermore, patients experience less irregular astigmatism, glare and foreign body sensation compared with more central incisions.

IOL calculations and the lens implant’s power remain unchanged, because LRIs do not affect the postoperative spherical equivalent. Regardless of which LRI nomogram is ultimately used, the single or paired incisions will be placed at the most peripheral extent of the clear corneal tissue, just inside the true surgical limbus.

If the primary surgical wound coincides with the LRI, the surgeon should create the cataract wound within the LRI. If an LRI is placed where the paracentesis would have been otherwise, the surgeon should create the paracentesis just central to the location of the LRI.

On-axis corneal incision can flatten cornea

Modern cataract surgery with incisions of 3 mm or shorter induces little-to-no change in the preexisting cylinder. In the appropriate patient, an on-axis incision and its manipulation can be quite beneficial. As a guide, longer incisions induce greater amounts of corneal flattening and can reduce higher amounts of astigmatism without overcorrection.

The results from a study by Rao and colleagues of 21 eyes with at least 1.75 D of against-the-rule astigmatism demonstrated a flattening effect of approximately 1.00 D to 1.50 D by lengthening the cataract incision to 4.5 mm and 5.5 mm, respectively, at the end of the cataract surgery.

Regarding on-axis incisions, it is recommended that the closer the incision is to the 90· axis, the greater the flattening potential is with at least 1.50 D of with-the-rule astigmatism.

A caveat to on-axis incisional surgery is the advantageous placement of an opposite clear-corneal incision 180· away from the main wound after the IOL has been implanted and before the viscoelastic has been removed. Khokhar and colleagues reported that this technique can effectively and safely reduce cylinder by 1.00 D to 1.50 D in patients with at least 1.50 D of preexisting astigmatism.

The overall advantage of the on-axis technique is that all surgeons are familiar with the creation of a cataract incision. The disadvantages lie in technically awkward locations and large, leaky wounds that will not self-seal and, therefore, require sutures. Once the sutures are removed or dissolve, the amount of astigmatism may change.

LASIK/surface ablation to treat astigmatism

The use of LASIK/epi-LASIK to treat the astigmatism should be carefully discussed with the patient before any surgery. In fact, in a preplanned LASIK surgery, creating the flaps prior to the insertion of the lens will facilitate a speedier recovery.

Because the pressure of creating the flap can cause lens displacement before the fibrosing in the capsule, patients could have to wait months after lens insertion to have LASIK. However, if the flap has already been created, a lift can be accomplished within a month, when refractive stabilization is reached.

The two-stage approach to astigmatism correction with an IOL, such as the presbyopic lenses, is a lengthier process; however, the advantages lie in the ability to titrate the surgery and ultimately shorten the time from start to finish. The absence of the flap or use of PRK could be accomplished in a shorter time, with a longer recovery period.

Pseudophakic correction

What about patients who are not ideal corneal refractive patients? As more is learned of posterior floats, early signs of pellucid marginal degeneration, forme fruste keratoconus and unusual topographies, surgical centers are opting to avoid the cornea. The corneal topography should be carefully evaluated in any patient who is considering cataract surgery as well.

The alternative of correcting astigmatism with an IOL has distinct advantages compared with treatments directed at corneal tissue involving incisions or laser ablation. Implanting a toric IOL is a single-step, reliable, small-incision approach with a result that is independent of the postoperative corneal healing response. Toric IOLs can also correct higher degrees of cylinder than corneal procedures.

The STAAR lens

Without the opportunity to manipulate the cornea, the toric IOLs may be viewed as the only good option for patients who want to correct their refractive error. The STAAR Toric IOL (a silicone toric plate haptic design, with scored markings at the haptic’s edges 180· apart) comes in two astigmatism corrections at the corneal plane. The optics of the 2.00-D and 3.50-D lenses correct 1.4 D and 2.3 D of astigmatism, respectively, with proper implantation and orientation. Thus, the best patients are those with corneal astigmatism of 1.50 D to 3.50 D.

Chang’s study clearly demonstrates that the STAAR Toric produces a predictable and beneficial clinical reduction in preoperative refractive cylinder, but also that it has a tendency toward occasional, early, off-axis rotation, the incidence and severity of which vary.

Recent advances have decreased the frequency of early misalignment with the STAAR lens. Foremost is the availability of the longer TL model for use in myopic eyes. Another is the reversed positioning in which the optic’s toric surface faces the posterior rather than the anterior capsule. This implantation technique improves uncorrected visual acuity and increases the refractive effect, despite a theoretical 8% reduction of toric power that occurs when the toric surface resides closer to the nodal point. The more precise rotational alignment is more important than the mild theoretical reduction of toric power. The IOL’s reverse positioning, though an off-label use, is now recommended and widely practiced in the United States.

Fortunately, the infrequent misalignments can be fully corrected. Because these rotations occur in the early postoperative period, before capsular fibrosis or contraction has begun, repositioning the implant will restore its beneficial effect.

Other IOL designs that rotate months after capsular fibrosis and contraction are difficult to reposition. The optimal time for repositioning the STAAR Toric IOL is 2 weeks postoperatively. Therefore, it is critical that these patients are dilated within 2 weeks postoperatively. If manipulated earlier, the problem may recur. Patients need to be counseled that there is a good chance they may still need some form of corrective eye wear to compensate for the residual astigmatism.

The AcrySof

The AcrySof Toric IOL (Alcon Laboratories) is composed of an acrylic polymer that has UV and blue-light absorbers. The lens is built on the same platform as the AcrySof Natural Single-Piece IOL. The toric lens has an overall length of 13 mm with an optic of 6 mm in diameter. Etched marks on the peripheral aspect of the optic are coincident with the origin of the haptics that delineate the steep axis.

Three available models allow for the correction of 1.50 D, 2.25 D and 3.00 D of astigmatism. The toric lens easily folds in half and may be inserted through an incision measuring less than 3 mm using the Monarch II injector (Alcon).

Two major features of the lens design limit posterior capsular opacification. First, its biomaterial adheres to the capsular bag via a single layer of lens epithelial cells. The resulting lack of space through which essential, life-sustaining nutrients can pass ultimately leads to the cells’ death. Subsequently, the AcrySof material adheres directly to the lens capsule via common extracellular proteins such as fibronectin and collagen IV. This adhesive property also minimizes the lens’ rotation, which is crucial to success with a toric IOL.

The design of the AcrySof Toric IOL’s posterior optic edge also increases its ability to maintain a clear posterior capsule and ultimately reduces the need for an Nd:YAG capsulotomy. The lens’ Stableforce haptic design (Alcon) provides maximum conformance to the capsular bag and thus offers the greatest possible surface area for adherence between the IOL and the capsular tissue. This quality, in turn, enhances the stability of the IOL and leads to a pronounced wrapping effect during the early postoperative period that locks the lens into place within the capsule.

Bioptics: combining procedures

Many of these procedures can be combined to correct astigmatism. The use of a toric IOL with a presbyopic lens that sits in the sulcus (piggyback), an LRI with a toric IOL, taking two toric IOLs together – the possibilities are endless. However, it all starts in your chair and with a good game plan.

Approach each cataract patient as an opportunity to create and treat uniquely and individually. Investing the time to talk about the options and deciding on the best course of action prior to surgery will save you time and effort afterwards. Interview doctors in your area to find the surgeons who share in your philosophy. Understand what it will take to minimize astigmatism, and both you and your patient will benefit.

For more information:

• Marc R. Bloomenstein, OD, FAAO, is a member of the Editorial Board of Primary Care Optometry News and director of optometric services at Schwartz Laser Eye Center, 8416 E. Shea Blvd., Ste., C-101, Scottsdale, AZ 85260; (480) 483-3937; e-mail: drbloomenstein@schwartzlaser.com. Dr. Bloomenstein has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

• The Staar Toric IOL is available from STAAR Surgical Company, 1911 Walker Ave., Monrovia, CA 91016; (626) 303-7902; fax: (626) 359-8402; www.staar.com. The AcrySof Toric IOL is available from Alcon Laboratories, 6201 South Fwy, Ft. Worth, TX 76134-2099; www.alconlabs.com.

References:

• Chang DF. Early rotational stability of the longer Staar toric intraocular lens: Fifty consecutive cases. J Cataract Refract Surg. 2003;29:935-940.
• Khokhar S. Lohiya P, Murugiesan V, Panda A. Corneal astigmatism correction wtih opposite clear corneal incisions or single clear corneal incision: Comparative analysis. J Cataract Refract Surg. 2006;32:1432-1437.
• Nichamin LD. Astigmatism management for the modern phaco surgeon. Int Ophthalmol Clin. 2003;43:53-63.
• Rao SN, Konowal A, Murchison AE, Epstein RJ. Enlargement of the temporal clear corneal cataract incision to treat preexisting astigmatism. J Refract Surg. 2002;18:463-467.