Plastic modification of human cornea in vivo: applications to clinical refractive procedures

PURPOSE: The purpose of this report is to quantify how pressure applied to the human cornea, either physiological or intentional, affects its curvature. In particular, how pneumatic procedures flatten the central cornea and keep it flat over time, thereby decreasing the patient’s myopia. METHODS: A viscoelastic model is developed for plastic deformation which gives us the basic governing equations of the elastic and plastic strain of corneal stroma. The model is applied to data from corneas of six patients who underwent pneumatic keratology (NEumatica Keratologia) to reduce their myopia. RESULTS: The model shows corneal dimensional stability for long periods of time after NEumatica Keratologia that decay with an exponential time constant. Separate equations are developed that relate corneal plastic strain to the pressure applied and its duration ε = σ(0) t(1)/η(1), to change in refraction ε = 2 × ΔRefr, to keratometry radius increase ε = ΔR/R, and to corneal thinning ε = sqr (Δh/h). The average values obtained for ε from the patients’ data are 3%, 3.2%, 3%, and 2.6%, respectively, all in remarkable agreement. The average refraction change is found to remain stable at ΔRefr = +1.67D ± 5.2%. Clinical data yield good agreement of theory and treatment results. CONCLUSIONS: The model proposed is a good description of NEumatica Keratologia outcomes. Practical applications include the long-term stable correction of myopia with refractive procedures. High myopia subjects can benefit from this procedure because NEumatica Keratologia corrects and protects the central cornea radius by stretching the peripheral cornea.