EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/357599
72 August 2014 Reporting from the 2014 ASEAN Ophthalmology Society (AOS) Congress Bangkok, Thailand EW MEETING REPORTER Dr. Chansue has been perform- ing the procedure for 4 years now and has used it in 5,029 eyes. About 96% of his patients, he said, see 20/20 or better. Very few of them have dry eye, and most report very good night vision. While the VA achieved by his patients with ReLEx was at par with published results with LASIK, ReLEx does much better at higher correc- tions, he said. In LASIK, he said, higher cor- rections result in increased spherical aberrations, leading to night vision problems. ReLEx avoids this, being a "very neutral surgery" with a wide effective optical zone not seen with LASIK. In a subjective study he con- ducted on his patients comparing night vision between ReLEx and LASIK, Dr. Chansue found that fewer patients complained of night vision problems with ReLEx, while more patients who underwent LASIK re- quired a lot of chair time to counsel them through their night vision problems. Today, Dr. Chansue performs ReLEx on almost all of his myo- pic cases. It is, he said, a "natural move," particularly considering that ReLEx is a less invasive procedure— a fact, he added, that patients will surely identify with. Editors' note: Dr. Chansue is an unpaid investigator for Carl Zeiss Meditec, occasionally receiving travel grants from the company. Dr. Kim has no related financial interests. IOL calculation after laser refractive surgery According to Sabong Srivanna- boon, MD, Thailand, the different forms of refractive surgery may be classified according to how IOL power is to be calculated. Under this classification system, phakic IOL surgery falls under one class, intra- ocular surgery; most surgeons will be performing procedures in the other class, corneal or keratorefractive sur- gery. This class is further subdivided into reversible (intracorneal ring or ICR surgery) and non-reversible (radial keratotomy, PRK, LASIK, and ReLEx). Most problems, said Dr. Srivannaboon, arise from the non-reversible forms of corneal surgery. These procedures mainly change the anterior corneal curva- ture, leaving the posterior curvature as is. This leads to errors in IOL power calculations in otherwise routine cataract surgery. IOL power calculations in routine cataract surgery, said Dr. Srivannaboon, use a Gaussian optics formula, which uses the anterior corneal surface radius, modified by a keratometric index of 1.3375 to incorporate the posterior corneal surface curvature. This approach relies on two as- sumptions: (1) the posterior corneal surface is about 1.2 mm flatter than the anterior corneal surface; and (2) there is a constant relationship between the anterior and posterior corneal surfaces. However, keratorefractive surgery alters the relationship between the anterior and posterior corneal surfaces. Using the kerato- metric index, total corneal power is overestimated in myopic correction and underestimated in hyperopic correction. The solution, then, is to dis- card the keratometric index in these cases. The gold standard at the moment is to take direct mea- surements of both the anterior and posterior corneal powers using a corneal topographer such as Orbscan (Bausch + Lomb, Bridgewater, N.J.) or a Scheimpflug camera such as the Pentacam (Oculus, Wetzlar, Germa- ny). Unfortunately, not all clinics will have this kind of equipment on hand. In these clinics, ophthalmic surgeons will have to fall back on history—which isn't always avail- able—or find a substitute refractive index. Dr. Srivannaboon indicated IOL formulas such as the Haigis L and Shammas as options for these surgeons. However, he noted that these formulas provide modifica- tions to IOL power calculations that are based on the respective authors' particular series of patients—none of which consisted of Asian eyes. Dr. Srivannaboon and his col- leagues at the Siriraj Hospital thus formulated the Siriraj formula for Asian eyes, published in a 2008 issue of the Journal of Cataract & Refractive Surgery. The formula, he said, does not differ much from other IOL calculation formulas, but has been refined for Asian eyes using their own series of patients. This applies to post-LASIK and, theoretically, post-ReLEx eyes, although the latter procedure is still too young for Dr. Srivannaboon and his colleagues to have experience with patients developing cataract. In any case, Dr. Srivannaboon recommended using more than one method. In his practice, he uses sev- eral, checking for consistency and performing frequency analysis after four or five different methods. Editors' note: Dr. Srivannaboon has no related financial interests. Femto laser 'like magic' "Do we really need femto laser for cataract surgery?" wondered Pornchai Simaroj, MD, Thailand. Most ophthalmic surgeons make the case for femtosecond lasers in cataract surgery by focusing on the accurate, predictable, and fast cap- sulotomies made with these devic- es. For instance, using the Catalys Precision Laser System (Abbott Medical Optics, Santa Ana, Calif.), Dr. Simaroj said that he completes capsulotomies in about 1/4 the time it takes to make a manual capsu- lorhexis, creating a complete, often free-floating capsulotomy in under 2 seconds. A manual rhexis generally takes him almost 10 seconds. However, what really sold Dr. Simaroj on the femtosecond laser is the use of the laser for lens fragmen- tation, he said. Dr. Simaroj compared femtosec- ond laser lens fragmentation to put- ting a bubble in an ice cube, making the cube easier to break. In his experience, lens fragmen- tation is effective in all grades of cataract, and is of benefit in com- plicated cases such as those with shallow anterior chambers, Marfan syndrome, small pupils, and even cataracts with fibrotic lens capsules that make manual capsulorhexis difficult.