 
Optimising excimer corneal ablation results: In my March 1999 Reflections on Refractive Surgery column following the ESCRS Winter Refractive Surgery meeting I wrote that "despite enormous advances…surgeons remain frustrated with complications related to faulty ablation by the laser… at this meeting, as at many meetings prior to this one, surgeons were asking how much longer it will be before laser manufacturers will finally provide them with truly adequate quality controls". More than four years later we are still asking the same question. Even the greatest skill and knowledge of a surgeon cannot always prevent an undesirable LASIK, LASEK or PRK result caused by a laser malfunction. Although infrequent, suboptimal laser performance is still a source of poor visual quality, incorrect refractive correction and increased optical aberrations after laser vision correction - even with the latest laser models. I still have not used or witnessed a commercial excimer laser that has truly adequate monitoring. Currently available commercial excimer laser systems still have only partial monitoring systems. They still lack complete instantaneous, intraoperative monitoring and recording of each individual pulse and cumulative pulse profile relative to energy and spatial distribution. We need to be able to assure –and not just assume- delivery to every cornea of the intended beam energy and dose distribution of each pulse specific to each treatment algorithm and pre-treatment calibration data. I have appreciated the importance of monitoring radiant energy delivery to the cornea ever since my experimental studies in 1979. In fact, it was my focus on the laser delivery and energy monitoring components described in a 1975 near-ultraviolet study of corneal damage that led me to first conceive in 1981 of modifying and incorporating some of those components into a computerised system for far ultra-violet light delivery for corneal removal and recontouring. My focus on the automated scanning device used in that study to provide information regarding energy distribution led me to conceive of incorporating a scanning means to move around a far-ultraviolet laser spot, which could be varied optically (as with the diverging lens in the study) to vary the area of exposure on the cornea. Pitfalls despite progress Technological advances to enable successful and practical application of my original concepts have been developed progressively over the past two decades. Just as more work lies ahead to optimise integration of more accurate refractive, optical and corneal data, more attention must be given to fail safe monitoring systems. Just this past month another incident involving a recent model laser was brought to my attention. Although laser malfunction was not indicated on the laser print out, more than a dozen patients sustained "islands" from "cold spots", which later were discovered to have been due to debris on one of the optics. Patients with these types of islands can be difficult to treat – even with the newest techniques and technologies. As Keith Hamilton M.D. emphasised in his 2000 ASCRS presentation on treatment of islands resulting from beam irregularities from partially blocked mirrors, "The main thing with this problem is that you want to prevent it…I'm absolutely amazed how often this does occur." Despite current widespread awareness of some sources of mirror debris and energy irregularities – such as salt formation on mirrors from spraying of balanced salt solution or misting up into the laser optics during corneal irrigation – we still are learning about other causes. For example, Robert Maloney M.D. discussed the effects of organic vapours on excimer laser beams in a March 2003 article in Ocular Surgery News. Measuring laser beam power with a calorimeter, Maloney and associates demonstrated a significant cumulative decline in energy after perfume exposure and suggested that perfume can degrade laser optics. Even if a surgeon tests fluency prior to every eye, laser performance problems can develop intraoperatively. Laser delivery systems involve many components and are becoming increasingly complex. Partial detection of problems in the laser delivery system, and particularly in high performance components such as the scanner-mirror assembly is not enough. Lasers have been recalled over the past few years because of these types of system malfunctions. Other recalls have been initiated because of software error. Survey confirms need for better monitoring In a survey conducted in 2002 by Calzone Marketing and directly mailed to 600 U.S. members of the ASCRS, ophthalmologists voiced their concerns about laser performance and confirmed the need for improved monitoring. Respondents included users of all laser systems available in the U.S. About a third of the survey respondents reported experiencing an "optics failure" resulting in irregular, non-homogenous ablation (hot spots, cold spots). More than a third indicated that they had had a software failure resulting in the interruption of treatment. More than 96% of respondents had experienced an "unexplained outcome, i.e. overcorrection/undercorrection of spherical and or astigmatic component of refractive error" – with over 50% not sure of the cause and 12.9% sure that improper delivery of laser energy was the cause. After mechanical failure of the laser delivery system or laser computer failure, only 16% and 6% of respondents respectively answered that treatment resumed with absolute confidence that the correct final dose would be delivered. Real time monitoring of energy delivery was very desired by the respondents. More than 87% answered affirmatively when asked whether efficiency, focus and level of confidence during surgery would improve if the laser incorporated a "real time monitoring "failsafe mechanism. Various designs of these systems already exist, but none have been incorporated in commercial excimer lasers. Hopefully laser companies will offer truly adequate monitoring of laser performance soon. |