IOL power calculation

Using a ray tracing approach for IOL power calculation offers several important potential advantages over conventional approaches, reported Nino Hirnschall MD, PhD, Vienna Institute for Research in Ocular Surgery, Austria, at the XXXV Congress of the ESCRS in Lisbon, Portugal. “There are two potential advantages of a ray tracing-based IOL power calculation. First, more individual IOL power calculation is possible, for example in eyes with irregular cornea or those eyes who have previously undergone refractive surgery. Second, because it is a purely physical approach, no empirical ‘optimisation’ is required,” said Dr Hirnschall, referring to formula adjustments required for each surgeon. He presented the results of a study comparing the spherical equivalent outcomes of ray tracing-based IOL power calculation, using exact IOL design information, with the outcomes of a triple-optimised Haigis formula. The prospective study of 49 eyes of 49 patients used the IOLMaster 700 individualised eye model data, a physical lens position predictor and exact CT Asphina 409 MP IOL design information. “Ray tracing offers more detailed information of the cornea, as well as anatomical data instead of estimated lens position, both of which lead to a better refractive outcome,” said Dr Hirnschall. A potential limitation of the method is that exact IOL design information is needed Ray tracing resulted in a 2.5% increase in outcomes within 0.5D and 0.75D of target refraction, and a 10% increase in those coming within 1.00D. Prediction errors larger than 1.00D away from target refraction were lower in the ray tracing group (n=2) than in the Haigis group (n=13). “This shows promise not only for outcome performance, but also for surgeon and patient education, as well as management of patient expectations,” he concluded. Ray tracing involves calculating the path of a single “ray” of light passing through an optical system. In this case, retrospective ray tracing-based IOL power calculation was performed using retinal image quality metric (RIQM) criteria in an iterative procedure, and retinal image simulations were performed. In the ray tracing-based method, IOL selection is based on predicted visual acuity and a neuronal weighted RIQM. It involves ssOCT-based biometry and keratometry, using anterior and posterior corneal surfaces, corneal thickness, axial length, anterior chamber depth and white-to-white distance. Nino Hirnschall: nino.hirnschall@gmail.com