Today’s PracTice refracTive fundamenTals

MyOPiC LASer COrreCtiOn The most popular type of laser correction for myopia is LASIK. Myopic laser correction is achieved by central corneal ablation, which reduces the refractive power of the anterior corneal surface. Myopic LASIK has been shown to be effective in correcting low to moderate myopia.1,2 LASIK correction of high myopia has been fraught with quality issues until recently. Factors that have enhanced outcomes include increasing the effective optical zone, decreasing the quantity of tissue ablated, and reducing higher-order aberrations (HOAs), especially spherical aberration. Before myopic laser correction, the cornea is prolate in shape, meaning that corneal power is stronger in the center than the periphery. A prolate cornea reduces spherical aberration. In myopic correction, the central cornea is ablated, resulting in a flatter anterior corneal surface. Previousgeneration lasers resulted in an oblate cornea after myopic correction due to several factors. First, there is less laser energy power when the laser strikes the peripheral area, even though it is still within the optical zone. This is partly because this area is slightly out of focus relative to the center of the cornea and partly because of the increase in the angle of incidence and the ovalization and consequent enlargement of the laser spot. Part of the laser energy is also reflected. As a result, the actual power delivered to the periphery of the cornea is lower than the intended power, causing underablation (Figure 1). The end result is a cornea flatter in the center and less flat in the periphery of the optical zone. In other words, the resultant cornea is oblate and has a smaller effective optical zone. This is the reason for the nighttime myopia experienced by many highly myopic patients after LASIK correction. As the pupil gets larger at night, more light passes through the periphery of the optical zone and, because this area is steeper, more light is focused anterior to the retina. This is also part of the reason why patients experience glare and halos at nighttime. In order to solve this problem, wavefront-optimized ablation profiles have been developed.3 Optimized ablation