Investigation of Mechanical Behavior coupled with Optical Behavior for Keratoplasty using FEM Analysis

Abstract

The cornea is located in the outermost part of the eyeball and is a transparent tissue with no blood vessels. If corneal opacity or disease such as corneal degeneration occurs, full or superficial keratoplasty will be performed. Nowadays, results of corneal transplantation highly depends on physicians' proficiency. In particular, it is difficult to maintain various factors such as suture tension and suture depth during the surgery, resulting in non-uniform suture tension and suture shape. This has a great effect on the patient's visual recovery after surgery. Recently, robots for corneal transplantation have been developed to maintain a constant tension during suture, which results in improved accuracy compared to conventional surgical methods. However, there is still a need to rely on physician proficiency without the accurate surgical information. Therefore, in order to improve the accuracy and precision of the operation, it is required to provide a method of providing the surgical information considering various factors. In this study, simulation system that can analyze the mechanical behavior such as the change of the curvature of the cornea using the finite element analysis (FEM) was developed. The cornea consists of five thin layers, requiring a large number of elements near the boundary layer. This leads to a problem that takes a high computational cost. Therefore, a Representative Volume Element (RVE) method was used. RVE method is a technique used to replace a layer composed of several different material properties with equivalent material properties. Finally, it was confirmed that the error within 3% compared with the experimental results. Python script was used to develop an optical simulation system for predicting optical behavior (wavefront aberration and corresponding Zernike coefficients) with the mechanical behavior changes derived from the finite element analysis. Using the developed system, parameter studies were performed according to the change of four surgical factors (suture tension, suture position, number of sutures, and suture depth) during corneal transplantation. The maximum curvature of the cornea was increased with increasing suture tension and suture location. As the curvature changes, the aberrations also increased. However, it was confirmed that the curvature change decreased with increasing the number of sutures. Moreover, the change of curvature according to the suture depth was not significant. As the suture depth increases, the gap between the tissues becomes small, and thus the aberration becomes smaller in terms of optics. Finally, sensitivity analysis was carried out. It showed that the position and number of suture is the most important factor of optics. Developed system is an important technology that can contribute to improve the accuracy and precision of surgery by optimizing the factors of the robot corneal transplant surgery.