FEB 2014

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

Issue link: https://digital.eyeworld.org/i/274531

Contents of this Issue


Page 54 of 114

Boston Fun Fact E W CORNEA 52 February 2014 New research suggests high-speed, low-energy laser passes can improve reliability of both DALK and DSEK procedures F emtosecond lasers have made major contributions to the field of corneal ker- a toplasty. Thanks to customized trephination patterns, such as the zigzag tech- nique, that can only be accom- plished with a femtosecond laser, we have been able to dramatically improve visual recovery time and visual acuity after penetrating keratoplasty (PK). 1 The zigzag fem- tosecond incisions have also made big bubble deep anterior lamellar keratoplasty (DALK) much easier to perform, with results comparable to PK while preserving the endothe- lium. 2 However, we have thus far only been able to make the peripheral incisions for DALK with the laser, relying on Anwar's big bubble or manual dissection to separate the diseased anterior corneal stroma from Descemet's membrane. In as much as 30% of the cases the bubble ruptures without separating the tis- sue, forcing the surgeon to convert to full thickness PK. No matter what we have tried in the past, attempts to make a lamellar laser pass in the deep stroma to re- place the big bubble have not been successful. The same gas bubbles that cause the harmless opaque bubble layer (OBL) in LASIK flap creation can more easily migrate vertically in the posterior cornea, and high energy pulses therefore create an irregular dissection plane. The ridged, irregular interface scatters light, degrading optical performance and visual acuity. Laser evolution Advances in femtosecond laser technology have expanded how we think about performing DALK, though. We are now using an iFS laser (Abbott Medical Optics, Santa Ana, Calif.), a 150 kHz laser that is 2.5 times faster than its predecessor, the FS60, and faster still compared to the FS30 laser that we used for most of our femtosecond laser keratoplasty procedures we have reported. Not only does the faster laser make it more practical to lay down a large number of spots, but it also has significantly better optics. That means that we can create much smaller spots, achieving optical breakdown in confined, contained areas at much lower energy levels, which reduces the explosive power that causes tissue damage. Mini-bubble technique With this combination of faster speed and better optics, we have developed a new, mini-bubble technique that promises to be more r eliable than big bubble DALK, re- ducing the rate of conversion to PK. It is based on work by Jaime Martiz, MD, which demonstrated that a smoother deep lamellar fem- tosecond laser dissection is possible with multiple passes of widely spaced, very low-energy passes. At very low energy, one typically d oesn't get a clean separation of tis- sue. But experience with LASIK flaps in complex, post-RK eyes had previ- ously shown that with a double pass, it was possible to deal with the RK scars and get a cleaner separation of the flap without tissue damage. Dr. Martiz found that with the iFS it was possible to make six or eight l amellar passes from different angles at very low energy so that the cumu- lative effect of the bubbles was con- fined to a single layer and produced a clean separation with no ridges in the Descemet's-stromal interface. Since that insight, we've been investigating this further to define the ideal laser parameters and validate the safety of the technique, particularly with respect to the toler- ance of the endothelium to the laser pulses. Animal models are not help- ful because the collagen structure is so different, so the research to date has been done in human donor tis- sue. In a study presented at the 2013 Association for Research in Vision and Ophthalmology meeting, 10 eye bank corneal-scleral rims were mounted on an artificial anterior chamber. 3 The iFS laser, used in engi- neering mode, was programmed to cut at a depth of 30 to 50 µm ante- rior to Descemet's membrane, based on ultrasonic pachymetry. We looked at the ease of tissue separa- tion, quality of the tissue bed, spectral domain OCT, and staining of the endothelium for a variety of laser variables. Tissue separation was easiest with close spot separation (4 x 4 µm) and high energy (3.6 µJ). The smoothest bed was obtained with wide pulse spacing (10 x10 µm), en- ergy just above threshold for optical breakdown (0.45 to 0.6 µJ), and multiple passes. We used alternating raster and spiral passes, rotated 90 degrees after each pair of passes, for a total of eight passes. The interface with this technique is much by Roger F. Steinert, MD Femtosecond mini-bubble technique The smooth interface achieved with a multi-pass, low-energy mini-bubble technique for deep lamellar dissection Source: Roger F. Steinert, MD continued on page 54 The Boston University Bridge on Commonwealth Avenue is one of the only places in the world where a boat can sail under a train passing under a car driving under an airplane. Source: www.cityofboston.gov 50-55 Cornea_EW February 2014-DL2_Layout 1 1/30/14 10:41 AM Page 52

Articles in this issue

Links on this page

Archives of this issue

view archives of Eyeworld - FEB 2014