Topography-guided ablation for myopia and myopic astigmatism

Topography-based corneal laser surgery can be considered an alternative for wavefrontoptimized and wavefront-guided LASIK surgeries as a primary treatment method. The refractive predictability, efficacy, and safety of topography-based LASIK are comparable to wavefront-optimized and wavefrontguided treatments.

These were the key findings of a retrospective data analysis of topography-based LASIK in 2051 eyes with myopia and myopic astigmatism.¹ All eyes were treated by Dr Jerry Tan, of Jerry Tan Eye Surgery, Singapore.

Study

The refractive and visual outcomes of topographybased LASIK for primary treatments on the Allegretto Wave Eye-Q platform (Alcon Laboratories, Fort Worth, Texas, USA) were analysed retrospectively. 2051 consecutive eyes primarily treated with LASIK for myopia and myopic astigmatism were included. Inclusion criteria were MRSE up to -16.00 D and a cylindrical refractive error up to -6.00 D. The data of the 3 month follow-up were processed by statistical data analysis.

Treatment

For treatment planning, corneal topographies were acquired using an Allegro Topolyzer (Alcon WaveLight, Erlangen, Germany). The first step in the acquisition was to align both eyes horizontally by performing an 'eye-to-eye test'. The topography system was used to ensure a horizontal head position by aligning the corneal apex of both eyes with respect to the horizontal measurement axis of the topography system. At least four, but up to eight good quality images were taken of each eye, while the tear film was maintained by frequent blinking prior to each measurement.

The images to be used for calculation of the ablation profiles were evaluated by Dr Tan. Disparate values in percentage of analysed area, asphericity, keratometric power, and angle between the taken images were considered, with obvious outliers being discarded. The remaining images were used to derive the median of the asphericity value with typical values between Q = 0.0 and -1.0. An empirical Q-adjustment was then made, adding -0.15 to the pre-existing Q-value to compensate for induced spherical aberrations caused by the myopic ablation profiles. The final treatment refraction was adjusted according to the individual nomogram of Dr Tan, based on cases performed prior to the study, to achieve the target postoperative refraction. In calculating the ablation profiles, no attempt was made to compensate for any spherocylindrical effect of treating corneal higher order aberrations in the final treatment.

Before the treatment was transferred to the laser, the astigmatism of the topography was compared to that of the manifest refraction. If the astigmatism differed between the two, treatment was based on the manifest numbers. The optical zone diameter was set at 6.0 to 7.0 mm depending on the mesopic pupil diameter. A residual stromal bed of at least 250 µm was required. The transition zone was chosen for a total treatment zone diameter of 9.0 mm in all cases. The targeted postoperative refraction was plano in 93.03% of eyes and -0.50 to -1.25 D for monovision in 6.97% of eyes.

The surgical procedure was conducted with a femtosecond laser system (IntraLase FS laser, Abbott Medical Optics, Santa Ana, California, USA) to create a defined flap. Excimer laser photoablation with the Allegretto Eye-Q 400 was performed after the flap was lifted and the calculated ablation profile was uploaded to the laser system.