Regression Simulation

What is surgical regression, and why does it happen?

Regression of the planned correction is a major concern for the refractive surgeon, limiting the predictability of both PRK and LASIK. Typically refractive regression occurs proportionally to the intended correction with higher correction tending to regress more. Together with regression of the correction, aberrations and vision disturbances can occur. It is known that the corneal epithelium has the ability to alter its thickness in order to re-establish a smooth, symmetrical optical surface and either partially or totally mask the presence of an irregular stromal surface from the front surface.

In the field of refractive surgery, epithelial thickness changes have been associated with regression, suggesting that alterations of the thickness of these layers can replace part of the ablated tissue. 

In particular, the epithelium tends to become thinner over the peaks of curvature and tends to thicken in the area of relatively low curvature. In myopic ablations, the epithelium thickens centrally and thins in the periphery; in contrast, in hyperopic ablation, the epithelium thickens peripherally and thins in the center. This behavior leads to regression, and the edges of the OZ are the areas where the rate of change of curvature is higher (particularly with myopic ablations).

Regression modelling

Together with the world-famous surgeons Dr. Riccardo Vinciguerra and Dr. Paolo Vinciguerra we have developed a numerical model, capable of predicting corneal surface changes after surgery. This model is called the Vinciguerra Regression Module (VRM), which, in combination with biomechanical simulations and predictions, is a very powerful tool for anterior segment eye surgery. 

The VRM in the Optimeyes™ Software was used to predict surgery regression after PRK. The software calculates local changes in epithelial thickness, based on curvature and curvature gradient of the immediate postop topography, and predicts the follow-up.

VRM regression modeling shows very good predictive quality in two study populations: 1) Standard PRK cases 2) Gradient Guided Ablation (GGA)







Source: ARVO 2016 poster by R. Vinciguerra, H. Studer, C. Azzolini, P Vinciguerra