Preoperative 3D Planning of Implantation of a Subretinal Prosthesis Using MRI Data

Abstract

Purpose: : To develop a standardized procedure for surgical implantation of a subretinal microphotodiode-array (MPDA) to the preoperatively defined most appropriate location on the fundus.

Methods: : Imaging techniques were used to topographically assess individual eye morphology, retinal structures and functions (photography, angiography, OCT, microperimetry), and geometrical dimensions of the eyeball (ultrasound, interferometry, and 3 Tesla MRI using high resolution measurements) in 5 volunteers and in 1 person scheduled for implantation of a MPDA.The desired location of the MPDA was defined by means of a multimodal fundus-mapping system with the Miller cylindrical projection. Grading of the images considered retinal thickness, presence of small vessels, remaining RPE, and other pathologies (e.g. scars, atrophic degeneration). A score was derived for each location of the fundus-map by weighting the different values of each imaging modality.To create a 3D model of the eyeball we applied the geometrical outcome measures to an ellipsoid eye model. This model was used to calculate the angle and the distance along the surface of the ellipsoid between the scleral point of insertion and the desired location of the MPDA at the posterior pole. A guiding-tool with a mm-scale was used for creating the channel for insertion

Results: : The fundus-mapping system proved to be a valuable adjunct to define a well-suited location on the posterior pole and was applied for preoperative calculations. For surgical planning the distance from incision to posterior pole and the angle of insertion was calculated in one person and provided to the surgeons. The parameters given proved to be precise enough for subretinal placement in the proximity of the desired location. The necessity of repositioning the chip after visual control could be reduced considerably by this procedure, leading to a reduction of mechanical stress on the retina during implantation.

Conclusions: : The grading system is a straightforward and valuable tool to establish a grid that visualizes the results of applying a battery of evaluations to the retina.It is possible to determine the appropriate surgical approach for the optimal MPDA position preoperatively using a combination of MRI data and projection of geometrical measures onto the eye.

Keywords: retina • imaging/image analysis: clinical • computational modeling