Ray Tracing Profiles

Abstract

Ablation profiles are generally derived from diagnostic data including the manifest refraction and then applied to calculations that generate the ablation profile and shot pattern. To date however, whether doing a wavefront optimized (WFO), wavefront-guided, asphericity-guided or topography-guided procedure, the calculations assume that the eye to be treated looks like the Gullstrand eye model. This has an axial length of 24 mm and a 43D corneal curvature while the eyes that undergo refractive surgery have a wide range of axial length and corneal curvature variations. With a ray tracing (RT) procedure, the Gullstrand eye model is replaced with the virtual eye model using the patient’s exact data for axial length, corneal radii and crystalline lens position. The ablation profile is obtained by ray-tracing this virtual model, generating an ablation profile and then doing a virtual procedure using this RT profile. Once the virtual procedure has been done, the virtual eye is examined again (using ray tracing) and if the light rays are all meeting on the macula, the profile is ready to be used. If the rays are not all converging on the macula, the aberrant rays are ray-traced backwards, and the necessary adjustments are made on the cornea. This is incorporated into the new RT profile and this iteration is repeated until all the light rays converge at the same point within the eye. Now the RT profile is ready to be used. Results show that RT outcomes perform all other profiles in all respects. More corneal tissue is used than with some other profiles however. The value of RT profiles is especially marked in those eyes that differ from the Gullstrand eye mode and they specifically benefit from the personal measurements. The RT profile is not commercially available at this juncture but will hopefully become available in the foreseeable future.