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Roibeard O’hÉineacháin
in Nice

EARLY reports of better results obtained with customised PRK and Lasek, and increasing concern about flap-related complications, are presenting a challenge to Lasik’s position as the predominant form of excimer laser refractive surgery.
Speaking in defence of Lasik at an symposium of the XX ESCRS Congress, German ophthalmologist Michael Knorz MD said that while the refractive results of the three excimer laser procedures are very similar, Lasik is quicker and easier for both surgeon and patient. Underlining his commitment to the procedure is the fact that he himself underwent Lasik.

“In the long term, results with the three techniques are more or less the same, but Lasik deserves its popularity and will most likely maintain it. Current technology provides a very easy and extremely safe way of performing Lasik,” Dr Knorz said.
He noted that in most of the published studies with the more modern scanning and flying spot excimer lasers, 70% to 80% of myopic patients achieve an uncorrected visual acuity of 20/20 with all three refractive techniques.
Regarding microkeratome-related complications, the results of the largest published study (Jacobs et al, Journal of Cataract and Refractive Surgery, 2002; 28:23-28) indicate that newer microkeratomes have greatly reduced their incidence.
The study reviewed 84,711 Lasik treated eyes and showed that microkeratome complications as low as 0.16% with the Hansatome, compared to 6.38% with the older Automated Corneal Shaper.

In fact, from a surgeon’s perspective, the microkeratome adds to the ease of the procedure and makes it faster and more reliable than PRK and Lasek, Dr Knorz continued.
Furthermore, re-treatments after Lasik are easy to perform and offer almost instant visual rehabilitation. In contrast, PRK and Lasek re-treatments are more difficult to perform than the initial treatment, and healing seems to be slower.
But PRK may be the most comfortable of the three procedures from a patient’s perspective. Lasik comes second in that respect because of the pressure induced during the microkeratome cut. Lasek comes last because of the longer duration of surgery, Dr Knorz said.

Postoperatively, Lasik has the clear advantage because any discomfort lasts for only minutes or hours, whereas Lasek and PRK cause at least some discomfort for a few days, and in a significant number of cases they also cause pain.
“From the patients’ perspective, the most important issues in refractive surgery are pain and visual rehabilitation. A procedure which causes more pain and a slower visual rehabilitation must therefore provide considerable advantages over Lasik to be considered an alternative,” Dr Knorz said.
Niels Ehlers MD, taking up the case for PRK, noted that the surface ablation procedure’s longer period of visual rehabilitation might actually be to its advantage, as it appears to result in the correction of optical aberrations.

In a randomised study involving 45 patients who underwent Lasik or PRK, the two techniques produced similar long-term visual acuity and refraction results but those undergoing PRK had more coma but less spherical aberration after one year than those undergoing Lasik. The patients in the study had a preoperative refraction between of 6.0 D and 8.0 D and no astigmatism. All underwent Lasik with a Mel 70 flying spot laser.
Optical analysis from corneal topography using a C max computer programme before, and after one, three, six and 12 months showed that PRK had a longer lasting, more variable course of visual recovery than Lasik.
With a 4.0 mm pupil, optical errors including astigmatism, coma and spherical aberration all changed much more over a year’s follow-up in the PRK group. However, at one year, astigmatism in the two groups was roughly the same, while coma was greater with PRK and spherical aberration was greater with Lasik.

“Can we explain it? I would hardly say so. Maybe the irregular wound healing after PRK might explain why the PRK results changed more over the 12 months compared to the faster healing after Lasik, and maybe with Lasik the corneal biomechanics have been altered more extensively,” Dr Ehlers said.
Daniel Durrie MD told the symposium that the smoothness of the surface ablated in Lasek makes the technique inherently better than Lasik. Moreover, advances in both excimer laser and wavefront technology enable the surface ablation approaches to achieve results that are at least as good as anything Lasik can achieve.
“The Bowman’s membrane is very smooth and this is the surface where the laser is doing the ablation with Lasek. That is one of the reasons why I think the vision and optical quality I’m getting with this technique is better than with standard Lasik techniques,” he said.

Dr Durrie noted that he has recently begun performing a procedure that is a hybrid of Lasek and PRK. The new technique involves creating an epithelial flap with a sharp trephine and a 20% alcohol solution and then removing the flap prior to applying the laser ablation.
His procedure differs from PRK in that edges of the remaining epithelium on the perimeters of the ablation zone are very clean and even. The benefit to patients is a faster re-epithelialisation time, he said.
“The patient’s re-epithelialisation time is at least 24 hours sooner. That is, I can remove the bandage contact lens in three days whereas it was usually four days if I left the epithelium on. Also the patient’s visual recovery is faster with this technique.”

To illustrate the visual outcome the technique can achieve, Dr Durrie presented results from a study involving 21 patients who underwent the hybrid procedure with a conventional laser algorithm in one eye and a custom algorithm in the other to correct –1.0 D to –7.0 D of myopia.
At three month’s follow-up, 90% of patients had a UCVA of 20/20 or better in both eyes, 70% were 20/16 in both eyes, and 100% had a BCVA of 20/20 or better. Furthermore, 80% were within 0.5 D of emmetropia. While spherical aberration increased by a mean of 60% in conventionally treated eyes, it decreased by a mean of 40% in eyes which underwent custom ablation. The decreased aberrations occurred in 60% of custom treated eyes.
“These results are excellent and certainly comparable to any Lasik results for this level of myopia,” Dr Durrie said.
Cynthia Roberts PhD noted that all of the types of excimer laser refractive surgery are bound to be unpredictable until the laser algorithms take corneal mechanics into account.

All three procedures cut through the collagen fibres of the stromal tissue, she said. The fibres are like rubber bands that maintain a tension on the corneal lamellae and prevent them from absorbing excess fluid.
However, during ablative procedures the laser, and the microkeratome in the case of Lasik, cuts through the collagen fibres, releasing the tension and allowing the sponge-like lamellae to soak up moisture. The resultant flattening of the cornea makes the correction slightly more hyperopic than the laser algorithm predicted.
With Lasik the effect is particularly unpredictable because the flaps themselves are unpredictable. Several prospective studies have shown that flaps induce aberrations, but the studies are inconsistent on how much aberrations the flaps induce, she pointed out.

Furthermore, different flaps cut by the same microkeratome have different thickness and aberration profiles. As a result any algorithm based on preoperative measurements will not be able to properly take the effect of the flap into account.
“It doesn’t look like we can predict the effect of the flap unless we can get identical flaps. We won’t be able say whether it can be measured and accounted for in ablation algorithms until we get the results from trials using a two-step approach, where the algorithm is based on the cornea after flap creation,” Dr Roberts said.

Michael Knorz MD
University Medical Centre, Mannheim, Germany
Email: knorz@eyes.de

Cynthia Roberts PhD
Ohio State University, US
Email: roberts.8@osu.edu

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