Resumen de Theoretical considerations on the hyperopic shift effect observed when treating negative cylinder in laser refractive surgery

Diego de Ortueta, Samuel Arba Mosquera, Christoph Haecker

• PURPOSE: To provide a theoretical background to explain the sources of «Hyperopic shift» when applying negative cylinder corrections to the cornea. The well-known effect of loss of efficiency when ablating tissue at non-normal incidence is used to get a modelling for the «Coupling Factor.» METHODS: Microsoft™ Excel was used to calculate intended, achieved and required volumes based only on spherocylindrical refraction and loss of efficiency. The differences between each pair of ablation volumes are displayed 2D and 3D to a better understanding.

  RESULTS: Theoretically, the so-called «Hyperopic shift» and «Coupling Factor» observed by applying negative cylinder correction in LASER refractive surgery can be mainly explained by the geometrical effect of the LASER applied onto an aspherical surface.

  Moreover, knowing some basic figures from specific LASER devices for refractive surgery the amount of this effect can be predicted.

  CONCLUSIONS: The «Coupling Factor» is a nomogram-like «adjustment» introduced by surgeons to achieve the intended result. With the introduction of Wavefront guided ablation volumes and loss of efficiency compensation factors; these effects should be mainly compensated in the devices by refined algorithms instead of nomogrammed by the surgeon.