Biomechanical simulation of the cornea based on numerical modeling may better predict refractive outcomes than empirically generated nomograms.
by Harald P. Studer, PhD
Refractive corneal surgery corrects optical errors in the visual system by modifying the surface geometry of the cornea. In the case of PRK and LASIK, this is achieved through laser-assisted ablation of corneal tissue. Limbal relaxing incisions—often used in conjunction with cataract surgery—place arcuate cuts, usually close to the limbus, to flatten the cornea in a given axis.
Predicting the effect of such surgical interventions is a major goal of preoperative planning. This is a challenging task: refractive results are influenced by multiple variables, including the cornea’s biomechanical response to surgical intervention, which varies considerably from patient to patient. Studies have shown that deviations from predicted changes are common and, in many cases, significant enough to affect patients’ postoperative vision.1
In recent years, biomechanical modeling of the cornea has emerged as a means to simulate and predict the effects of surgery, and this may offer a reliable means to predict refractive outcomes. Based on finite element analysis, numerical modeling is a potentially more accurate approach than individual surgeon’s nomograms based on their personal experience and outcomes.
A Patient-specific Approach
Optimeyes™, a new software tool that uses numerical modeling, is designed to simulate the patient-specific shape change of the cornea induced by a specific refractive intervention. Geometric data, such as corneal topographic measurements acquired from a Galilei or Pentacam device for an individual patient, is fed into the Optimeyes™ software. Surgeons can then plan the procedure directly on the colored maps displayed on the user interface; by moving sliders, surgeons can see how the corneal shape will change as the result of slightly altering the intervention.
This kind of individualized biomechanical modeling is part of a global megatrend toward personalized medicine. Patients, especially in the western world, are willing to pay additional fees for enhanced cataract and other surgeries, but they expect better outcomes and a more comfortable experience as a result. Doctors need tools such as Optimeyes™ to fulfill these expectations.
Optimeyes™ uses patient data to predict precise outcomes. With the outcome known, the surgeon can tweak the surgical parameters in order to optimize the result. In addition to LASIK and surface ablation, this can be applied to astigmatism treatment/prevention in cataract surgery (by means of clear corneal surgical incision placement and/or limbal relaxing incisions) and used for keratoconus treatment (including collagen cross-linking and insertion of intrastromal corneal ring segments).
Virtual Clinical Trials
Beyond surgical planning and optimization thereof, the Optimeyes™ software is designed to offer virtual clinical studies—simulated clinical trials performed on a computer.
Clinical trials are time-consuming and expensive, and patients involved in clinical trials are at risk of adverse events. Ideally, a product should be perfectly engineered before going to the first clinical trial. When Optimeyes™ is put into clinical use, it will collect a large amount of patient data from which to construct a clinical trial database. This database will allow users to simulate and test the effects of a new surgical product or technique on different corneas in the virtual realm—before testing a device or procedure in patient eyes. Product developers who aren’t happy with the results can change the design and run the simulation loop again until their device or protocol is ready for an actual clinical trial.
Clinical Validation Ongoing
Five or 6 years ago, a tool like Optimeyes™ would not have been possible, because we lacked the computational power for efficient numerical modeling of ocular surgery. Performing the calculations for a single cornea would have taken hours, whereas now it can be done in just minutes.
Optimeyes™ is expected to launch as early as 2016. Right now, clinical validation is ongoing. Retrospective clinical studies have shown that Optimeyes™-predicted effects of cataract surgery incisions, based on patient-specific biomechanical simulation, match up closely with postoperative clinical outcomes.1 Furthermore, a recently published study shows a convincingly close match between predicted- and clinically achieved-outcomes for corneal inlay surgery.2
Studer HP, Riedwyl H, Amstutz CA, et al. Patient-specific finite-element simulation of the human cornea: a clinical validation study on cataract surgery. J Biomech. 2013;46(4):751-8.
Studer HP, KR Pradhan, DZ Reinstein, et al. Biomechanical Modeling of Femtosecond Laser Keyhole Endokeratophakia Surgery. J Refract Surg. 2015:31(7):480-6.