Purchase this article with an account.
Michael K Smolek, Neil Notaroberto; A Physics-Based Catenary Model that Defines Corneal Biomechanical Stability for Refractive Surgery Applications. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4321.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Despite an excellent success rate for refractive surgery, postoperative corneal ectasia remains a major complication. A recent meta-analysis estimated that 1 in 200 surgical cases (0.5% incidence) will experience postoperative ectasia. The current study tests the hypothesis that a physics-based catenary model of tensile stress equilibrium can distinguish between normal and non-normal corneas based on static topography data. Non-normal refers to a biomechanically unstable shape or an anisotropic arrangement of tensile strength properties, which are attributes of diseased or distorted corneas.
A numerical equation model of biomechanical shape stability was derived from digitized photographs of physical exemplars of catenary curves. This model was then validated with de-identified (IRB exempt) corneal topography exams from Normals (N=48), Keratoconus (N=28), KC Suspects (N=38), and Post-Myopic LASIK (N=34). Exams were processed to extract elevation data along the flattest and steepest meridians, from which sagitta, chord and arc-length were interpolated at regular intervals. All screening evaluations were performed with a 10 mm chord.
100% of the Normal cases were within the model's mean ±95% CI for predicted normal catenaries. 100% of the KC Suspects had one or more semi-meridians that were biomechanically non-normal. 100% of Keratoconus cases were non-normal, and 25% of these cases exceeded the reversible sagitta limit. 86% of the LASIK cases had at least one non-normal semi-meridian, suggesting the corneas were unstable, or that ablations were decentered and resulted in asymmetrical tissue properties (e.g., residual bed thickness).
Further study is needed, but the physics-based corneal catenary model appears to correctly distinguish between normal and non-normal corneal topography using only 2 physical law-dependent inputs at a fixed chord width. Screening refractive surgery candidates with the physics-based catenary model may enhance preoperative ectasia detection. It is a simpler initial screening approach compared to highly complex screening algorithms that use multiple indices that do not take into account the physics of shape. Furthermore, post-operative ectasia incidence may be reduced if ablation profiles are designed with physics principles in mind in order to improve postoperative stress equilibrium.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
This PDF is available to Subscribers Only