Broad beam laser with Gaussian delivery obviates need for eye tracker in LASEK procedures

Dermot McGrath in Rome

Diffractive microlenses creating a Gaussian repartition of the laser energy.

Gaussian curve: For every pulse, most of the energy reaches the centre and little gets to the periphery.

Electron Microscope: Large and homogeneous ablation zone.

THERE is no compelling reason to use an eye tracker during laser assisted epithelium keratomileusis (LASEK) treatment for myopia using a broad beam excimer laser with a Gaussian delivery system (GDS), according to Patrick Schraepen MD.

"Our clinical experience suggests that poor refractive outcomes in these cases are not due to decentration during the treatment, but other factors which have nothing to do with the presence of an eye tracker or not," said Dr Schraepen at the 7th ESCRS Winter Refractive Surgery Meeting.

He presented the results of a study carried out at the University Hospital of Antwerp, Belgium, co-authored by René Trau MD and Marie-José Tassignon MD, PhD. The team wished to evaluate the actual need for an eye tracker by studying the decentration when using a GDS excimer laser.

The study enrolled 32 myopic patients who were divided into three sub-groups according to visual acuity - low myopia (-3.0 D), moderate myopia (-6.0 D) and high myopia (-10 D). All underwent treatment with the InPro (InPro Intraokulare Prothetik GmbH) excimer laser. Follow-up ranged from six to 12 months and no re-treatments were necessary.

The results showed a mean decentration of 0.14mm (range 0 to 1.0mm). In the low myopia group, the mean decentration was 0.2mm, 0.19mm for the moderate myopic group and 0.07mm for the high myopic group.
"Statistically there was no significant difference among these three groups. Nor did we find a correlation between longer treatment times and increased decentration," Dr Schraepen said.

Postoperative spherical equivalent (SE) in the low myopia group is emmetropia in 100%. In the moderate myopia group, 55% reached emmetropia and 33% achieved -1.0 D. In the high myopia group, 28% achieved emmetropia and 57% achieved -1.0 D.
Postoperative UCVA was 20/20 or better in 80% of patients in the low myopia group, 61% for the second group with moderate myopia and 28% in the third group with high myopia. Two eyes had a postoperative Acanthamoeba-like keratitis and were excluded from the study.

Dr Schraepen noted that in this study, the measurements of the de-focus equivalent gave a more realistic value of the level of astigmatism than the SE. The de-focus equivalent is a technical graph of accuracy made by the numerical addition of the sphere and half the cylinder, without taking into account the sign.
"In our study the de-focus equivalent is less than 1.0 D in 100% in the first group, in 72% in the second and 57% of the third group," Dr Schraepen said. He explained that the term Gaussian relates to a specific profile of how excimer energy is applied to the cornea.

"The Gaussian delivery system consists of a plate containing an array of diffractive micro-lenses, allowing a repartition of the laser energy on the corneal surface, with its peak energy at the centre and a progressively decreasing energy at the periphery. Contrary to a flying spot system, this broad beam excimer laser with a GDS ensures a very large, homogeneous and progressive optical zone," he said.
One of its key advantages is that the system allows for a short treatment time. The broad beam and high frequency result in a treatment time of three seconds to correct 1.0 D and about 18 seconds to correct 10 D, he explained.

Another important point, noted Dr Schraepen, is that the system employs a double fixation control to ensure accuracy. First, the patient has to fixate a green light emitting diode in the centre of the microscope and then the fixation is controlled by the surgeon through a coaxial binocular microscope with a grating system. The surgeon also controls the head position of the patient.

"After we achieve topical anaesthesia, an 8.5mm retaining well is placed on the centre of the cornea and is filled with a 20% alcohol solution. Meanwhile, we use this alcohol well as a trephine by gently pressing it downwards to make the epithelial groove. After 20 to 30 seconds, a sponge absorbs the alcohol and the cornea is irrigated for 15 seconds with a balanced salt solution. The detached flap is then elevated and gathered at its 12 o'clock hinge."

The ablation can then be started. During the ablation, the epithelial flap is protected with a sponge so that it cannot be damaged by accident. While irrigating the cornea, the epithelial flap is repositioned with a replacement spatula. And after treatment, a therapeutic contact lens is always placed on the cornea for three to four days, he said.
Dr Schraepen emphasised that little or no decentration had resulted from the use of the broad beam GDS excimer laser.

"We concluded that the absence of an eye tracker is compensated by a very short treatment time, a double fixation control and a large, homogenous, progressive optical zone. We had expected more decentration in the high myopia group because of the longer treatment time, but this wasn't borne out by the study.

"It seems the lower refractive results in the high myopia group are not correlated to any decentration but to a higher de-focus equivalent. Therefore our visual outcomes would not be improved by an eye tracking system but by lowering the de-focus equivalent. To accomplish that, I think customised ablation would be more useful," he explained.

Patrick Schraepen MD
University Hospital Antwerp, Belgium
Email: patrick.schraepen@uza.uia.ac.be

Top