Are There Categories of Corneal Shapes?

This study investigates the possible existence of different natural corneal shape categories. This is important to better describe cornea for both diagnostic and therapeutic assessments. We started by describing corneal shape of different populations as a function of influencing clinical data i.e. age, ametropia and gender. This was done by averaging Zernike polynomial (ZP) decomposition of the anterior surfaces in each subgroup. The results showed small but significant differences of shape that are supported by the literature. This motivated us to examine the feasibility of characterizing the normal corneal shape with an automatic method of clustering independent of any clinical a priori knowledge. Since we did not know beforehand the number of corneal categories, agglomerative hierarchical clustering was applied on ZP coefficients for a large database. The dendrogram based on the Ward's distance was evaluated with two different clustering validity indexes (coefficient of determination R² and semi partial R² (SPR²). The optimal number of categories was around four showing corneal shapes ranging from flatter to steeper.