Photorefractive keratectomy continues to be the hottest topic in the ophthalmic community, even though FDA marketing clearance for this procedure may take another 3 years. As VISX and Summit researchers complete Phase HI clinical studies of photorefractive keratectomy involving hundreds of patients, results from Phase HB and Phase III clinical studies in the United States indicate that a 1.00-diopter level of accuracy is certainly attainable in eyes with less than - 6.00 D of myopia. Summarizing recent results that have been reported, George O. Waring IH, MD, FACS, of Emory University in Atlanta, Ga, stated that photorefractive keratectomy was accurate to within 1.00 D in 90% of patients by 6 months postoperatively. Other US researchers supported this assessment, reporting that from 80% to 90% of their patients achieved this level of final correction in follow-up studies.
During a pro and con session attended by at least 2000 anxious attendees of the American Academy of Ophthalmology Annual Meeting (October 12-17, Anaheim, Calif), Alan Stern, MD, summarized the published results of photorefractive keratectomy on 1400 patients in the US by saying, "Photorefractive keratectomy has a greater risk of loss of visual acuity than radial keratotomy. But if photorefractive keratectomy could be properly centered, it would be better than radial keratotomy." Stern noted significant decentration of the beam during the ablation process which led to unpredictable results. This problem is primarily due to the manual and visual methods currently employed for centration of the optical axis. Similarly, Dr Waring pointed to the need for "better control of patient eye movement - or possibly the addition of an eye-tracker to the excimer system."
With the Summit system, the patient simply fixates on a light for approximately 15 seconds while the ablation process takes piace. VTSX uses a more complex approach involving a suction ring held in place by the surgeon, a blinking fight that the patient stares at, and a reticle on the microscope that the surgeon aligns with the center of the patient's pupil. Once the surgeon has the ring held in place on the patient's eye, patient movement will not alter centration, according to a spokesman for VISX. At this point, continuous centration throughout the 30-second ablation process depends on physician hand control.
Aside from decentration during the procedure, a more basic problem for VISX researchers involved agreeing on exactly how to locate the center of the optical zone. In the beginning, some researchers were using the optical axis as a reference point for centration. Yet, better results have been obtained since these researchers converted to using the center of the pupil as a centration reference point. At the International Society of Refractive Keratoplasty pre-academy meeting, VISX users Stephen Klyce, PhD, and Marguerite McDonald, MD, of New Orleans, La, reported that significantly improved results were obtained once the optimum alignment was understood.
Commenting on the centration problem during the ISRK meeting, VISX researchers called for an improved technical approach. According to Michael Dietz, MD, of Kansas City, Kan, if the "ablation zone isn't centered, you create irregular astigmatism. This occurred on some of my patients and we're waiting to see how they turn out." VISX researcher Olivia Sederavic, MD, of New York, NY, suggested that the addition of a tracking system would solve the problem of centration. However, changing the equipment at this point would complicate the regulatory process.
The VISX system utilizes a nitrogen air-flow system that blows nitrogen across one side of the eye and aspirates it from the other. Its purpose is to blow away tissue debris ejected from the ablated surface of the eye, causing opacification during ablation. This change in the surface may change the rate of ablation and the laser-tissue interaction. According to VISX chairman Charles Munnerlyn, PhD, opacification at the interface does not affect the fixation or centration since the surgeon holds the eye stable with the suction ring. Furthermore, Munnerlyn states that preoperative calculations used by VISX researchers include the effect of the drying on the ablation rate. However, the question has arisen that the nitrogen blow-by may affect the severity and duration of anterior stromal haze in the postoperative period, and many VISX centers now use a low flow of the gas.
Control of the ablation rate was another problem identified during the ISRK meeting. "The excimer promised to give us sub-micron accuracy and predictability," said Dr Dietz. "But so far we are just guessing at the ablation rate. I don't know exactly how much tissue each pulse is taking off" Some of this variability is based on operating conditions and differences in tissue effects as the ablation process moves down through the epithelium into the stroma. But another factor is related to fluctuation in energy fluences within the laser beam due to the complex physics of the excimer laser. Because of this energy fluence factor, both Summit and VISX use extra beam sampling and diagnostic steps. VISX users have always performed a sample ablation on a piece of plastic (PMMA) prior to operating on the patient. This sample is then tested with a lensometer to see if the refraction is correct. If it is incorrect, the excimer is adjusted before the surgical procedure is performed. "We have always emphasized the importance of calibration to the outcome of the procedure," Munnerlyn said. "However, our Taunton system users did not implement the PMMA sampling step until recently." One Taunton user noted that his clinical results improved from only 33% of eyes within 1.00 D at 6 months without the PMMA calibration step to 70% of eyes within 1.00 D at 6 months after this step was implemented. (Note that Summit uses PMMA only as a system maintenance check on a weekly basis.)
Although nearly 15,000 patients throughout the world have undergone photorefractive keratectomy, none of the radial keratotomy surgeons attending would undergo photorefractive keratectomy at this time or recommend it for members of their families. Nevertheless, a VISX employee stood up and announced that two people in his company, including company president Alan McMillan, had undergone the procedure themselves. Aside from this show of good faith, the general opinion among US researchers is that low myopes should wait for further development, while clinical results on high myopes have not been encouraging. At this point, photorefractive keratectomy seems to be a painful procedure with many unresolved postoperative complications. However, the technical expertise and ingenuity of the researchers should allow them to refine the techniques and hardware sufficiently to fulfill the promise of laser corneal sculpting.