EyeWorld Asia-Pacific September 2012 Issue

34 EWAP rEfrActivE September 2012 D. RAMAMURTHY, MD Chairman, The Eye Foundation D.B. Road, Coimbatore 641002 India Tel. no. +91-422-4242000 Fax no. +91-422-4242099 drramamurthy@theeyefoundation.com I mplantation of toric IOLs offers the best solution to correcting preexisting corneal cylinder in patients undergoing cataract surgery. While toric IOL technology is predictable, there are several different ways of positioning the lens in the eye most appropriately. At present, my standard is to mark the eye at the 3, 6 and 9 o’clock axes with the patient sitting upon the operating table with an ASICO bubble marker (which ensures horizontal orientation of the marking instrument). After draping, under topical anesthesia, I first mark the steep axis of the cornea with a two-handed technique, when the eyeball is firm and before making any incision. The correct toric alignment is always with the steepest meridian of the cornea, because the reference markings on the IOL are always on the lowest power meridian. However, marking of the axis of toric IOLs this way is still not a precise science since there is a potential for 5–10 degrees of deviation in placement of the reference marks at the limbus. Any deviation from precise alignment will result in residual astigmatism. Since nearly one third of the patients presenting for cataract surgery could benefit from toric IOLs, investing in one of the technologies enumerated in this article may be worthwhile. However, it must be kept in mind that suboptimal refractive outcomes with the toric IOL can also arise due to reasons apart from inappropriate positioning of the IOLs. Some of them are: • The manual keratometer measures points 3.2 mm apart, the IOLMaster 2.5 mm apart, and the Lenstar measures at 2.2 mm and 1.7 mm. When the astigmatism is asymmetrical, nonorthogonal or the cornea is irregular or when the astigmatism changes as we move radially, these measurements may not be accurate. The topography and tomography by virtue of the fact that they measure thousands of points instead of four in a 3 mm to 4.5 mm zone may offer a more accurate assessment when there is an irregularity. The limitations of Placido-based topography are central extrapolated zone of 1–2 mm and no measurement of the posterior corneal power. The zonal values from topography & tomography will yield better results for both power and astigmatism of the cornea than the four points from any keratometer. • Web-based calculators incorporate the effect of surgically induced astigmatism (SIA) which could be calculated for each individual surgeon and lens. Although the average SIA is usually between 0.2 D to 0.7 D for small incisions (2.2 to 3.0 mm), the actual magnitude may vary unduly for an individual patient for the same surgeon and differs significantly based on the axis of placement of the incision. • The astigmatic correction achieved by the toric IOL may vary significantly depending on the spherical power of the IOL and the effective lens position (ELP) of the IOL. • The toricity of an IOL is manufactured within a tolerance of 0.25 D, just as the spherical power. Since toric IOLs have come to stay in our quest for emmetropia, any advancement in enhancing the refractive outcomes with these lenses is a welcome step forward. Editors’ note: Dr. Ramamurthy has no financial interests related to his comments. Haike GUO, MD Department of Ophthalmology, Guangdong Eye Institute, Guangdong General Hospital 106 Zhongshan Er Road, Guangzhou, PRChina Tel. no. +86-20-83844380 Fax no. +86-20-83844380 guohaike@medmail.com.cn P erfect axis alignment has always been essential to achieve optimal results for the toric IOL. It is well established that with each degree of off-axis lens rotation, the patient will lose 3% of the power of the toric correction. But perfect axis alignment was not always possible using traditional methods of preoperative astigmatism measurement and ink marking to align the lens. Independent studies from Robert Osher, MD, and Roberto Zaldivar, MD, have found that conventional ink marking induced 13° to 20° of error from the intended target. This approach is sub-optimal for three reasons. First, visual marking of the eye is inherently inaccurate; second, the keratometric cylinder is centered on the corneal apex and not the patient’s visual axis; finally, each cornea will respond differently to the phacoemulsification incision and can deviate from the assumed amount. Experience has shown that these problems may result in some error. Recently, optimal toric IOL alignment has taken a significant leap forward with the development of a new generation of imaging technologies to assist the surgeon in achieving precise toric IOL alignment. The following are the technologies which help to significantly reduce the risk of misalignment. The ORA system is improved from the ORange with a new light source and refined optics that improve the consistency of the readings, and a reticle that provides guidance on the placement of toric IOLs. The system allows real-time analysis during procedures. iTrace is an integrated ray-tracing aberrometer and corneal topographer that features the measurement of angle kappa and alpha as well as the ability to separate corneal and internal optical aberrations. SMI combines two main components: the Reference Unit and the Surgery Pilot. The system can also provide real-time registration and tracking overlay on the microscope camera live image during surgery. Also excellent were the Callisto Eye and Z Align, Holos and so on. The newer generation of astigmatism tools brings a high level of precision and accuracy to toric IOL alignment. A further improvement would give these tools a more enduring vitality. Editors’ note: Prof. Guo is a consultant for Alcon (Fort Worth, Texas, USA/ Hünenberg, Switzerland) and Bausch & Lomb (Rochester, NY, USA), but has no financial interests related to his comments. Views from Asia-Pacific