The Cardiff Eye Shape Analysis Protocol (CESAP): Producing a digital representation of the anterior ocular surface

To develop a method for accurate 3-dimensional (3D) representation of the anterior ocular surface (AOS) based on ocular impression and reverse engineering of an AOS plaster biomodel.

An AOS plaster biomodel was fabricated using ocular impression and a developed casting support device (CSD) and landmark registration element (LRE) to stabilise and consistently fix the impression casting tray within the casting support during the casting process. The touch-trigger probe on a co-ordinate measurement machine (CMM) digitised the AOS plaster biomodel to represent the AOS shape. The CSD and LRE were manufactured using an Additive Manufacturing selective laser sintering (SLS) process. A single stainless-steel ball (diameter: 22 mm) was cast as a surrogate AOS biomodel using polyvinylsiloxane impression material. The surrogate biomodel which was used to evaluate material selection and stability of the AOS biomodels fabricated using the CSD and LRE, and to evaluate repeatability and reproducibility of the point cloud data collection methods. The points of circular profiles were measured at different Z values in mm: z = -1 mm, z = -2 mm, z = - 3 mm, z = -4 mm and z = -5 mm.

The measurements were highly repeatable with an acceptable tolerance. For the typical case of the surrogate AOS biomodel, the average distance of the digitised points to the best-fit sphere of all the digitised points from four measurements ranges from 0.002 to 0.010 mm. The shrinkage study of the surrogate AOS biomodels was conducted, with measurements taken one month apart for comparison. The analysis results showed that most of the surrogate AOS biomodels reduced in size but within an acceptable tolerance, in which the mean error is from 0.005 to 0.010 mm for the 2D circular profiles measured at Z = -4 mm.

The Cardiff Eye Shape Analysis Protocol (CESAP) provides a repeatable and consistent method for producing solid, white-plaster, representations of a plaster cast AOS biomodel. Casting an impression in white plaster (Novadur™) produces a consistent surrogate AOS biomodel of a single stainless-steel 22 mm diameter ball. CESAP can be used as a framework for consistently converting an ocular impression into a 'real' AOS model that can be reverse-engineered to create 3D CAD models of the AOS shape for potential applications in optical image (topographer) calibration, prosthetic shell and scleral lens design, and AOS database development.