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Striving to achieve the perfect sphere
Although there is truly no perfect visual outcome, today's technology can help bring the surgeon that much closer to a premium result.
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In this era of refractive cataract surgery, our cataract patients have the same postoperative visual expectations as the “wow factor” typically experienced by LASIK patients. Managing astigmatism intraoperatively and/or postoperatively is critical to meeting this expectation.
When addressing astigmatism, it is crucial to differentiate between corneal astigmatism and lenticular astigmatism. As part of the premium preoperative evaluation, corneal topography is essential to make this distinction. Other innovative devices that can aid in separating corneal from lenticular astigmatism include the OPD-Scan III (Marco/Nidek) or iTrace Surgical Workstation (Hoya).
Obviously if there is little to no corneal astigmatism preoperatively, all or most of the refractive cylinder will be treated with the removal of the cataract. Managing corneal astigmatism or achieving the perfect sphere then becomes the real driver to a successful visual outcome.
Treating astigmatism
Astigmatism treatment options are increasing, so deciphering which option will achieve the most premium outcome for our patients is the real challenge. Established methods include simply performing an on-axis, or steep axis, corneal incisions at the time of cataract surgery, or performing limbal relaxing incisions (LRIs).
My advice for manual LRIs is staying inside the limbal vascular arcade to avoid vascularization of the incisions, which can cause regression of effect. The more advanced femtosecond laser-created LRIs are achievable with all four U.S. Food and Drug Administration-approved platforms, allowing for more precise placement and the ability to titrate their effect postoperatively as needed by opening the incisions at the slit lamp to gain more astigmatic effect.
Whether a manual or femtosecond laser technique is employed, vector analysis must be utilized with help from websites such as www.lricalculator.com (Abbott Medical Optics).
Toric IOLs
The toric IOL armamentarium is increasing, as well, with many established and newer options available to the U.S. surgeon. The STAAR toric IOL can correct up to 3.5 D of astigmatism at the IOL plane and up to 2.3 D at the corneal plane. The newer STAAR toric TL model, with a horizontal diameter of 11.2 mm, offers improved rotational stability. The Alcon AcrySof toric IOL can provide up to 6 D of astigmatism correction at the IOL plane, which translates to 4.11 D at the corneal plane.
The newest additions include the Abbott Medical Optics Tecnis toric IOL, covering up to 4 D correction at the IOL plane and 2.74 D at the corneal plane, and the Bausch + Lomb Trulign toric IOL, offering up to 2.75 D of IOL plane astigmatism correction and up to 1.83 D corneal astigmatism correction, with the additional benefit of a broader range of vision correction similar to its Crystalens AO platform.
No different than LRIs, toric IOLs must undergo vector analysis using the appropriate toric IOL calculator websites from each manufacturer. Simplified applications even exist for toric IOL calculation and are easily downloadable, such as the Express toric calculator.
The major concern with toric IOLs has always been misalignment from the intended axis of correction. One study showed that out of 240 eyes, there was an average cyclorotation from upright to supine position causing 4.1±3.7° of misalignment. It is well established that for every 1° of misalignment, toric IOLs lose 3% of their corrective effect, for every 4° they lose 14% of their effect, and for every 10° they lose 34% of their effect. Utilizing new alignment devices such as the AXsys electronic toric marking device (ASICO) can minimize this error. Intraoperative wavefront aberrometry with ORA (WaveTec) allows for real-time adjustment of axis alignment with its newest upgrade, known as VerifEYE, and even improves IOL power selection in post-refractive surgery cases such as LASIK/PRK/RK.
Posterior corneal astigmatism
The most recent advances in managing astigmatism are based on the excellent analysis by Douglas Koch, MD, and colleagues regarding how posterior corneal astigmatism can influence visual outcomes. Ignoring posterior corneal astigmatism may yield incorrect estimation of total corneal astigmatism. Selecting toric IOLs based on anterior corneal astigmatism alone could lead to overcorrection in eyes that have with-the-rule astigmatism and undercorrection in eyes that have against-the-rule astigmatism.
I have adjusted my toric IOL decision process based on this study such that I tend to choose a toric IOL slightly undercorrecting a patient who has with-the-rule astigmatism and slightly overcorrecting a patient who has against-the-rule astigmatism after toric calculator vector analysis.
In this era of refractive cataract surgery, there is truly no perfect visual outcome, but striving to achieve the perfect sphere only brings the premium surgeon that much closer to a premium outcome.
Watch for my next premium channel column on hitting the post-refractive surgery IOL bulls-eye.