Technical notes on prism-dioptre and Prentice's rule.

PURPOSE: To show an alternative interpretation for prism-dioptre and access Prentice's rule accuracy. METHODS: Algebraic calculation. RESULTS: The prism-dioptre can be expressed as a percentage of the distance between the prism and the plane of prismatic dislocation. Prentice's rule is an imprecise mathematical expression prone to expressive errors. CONCLUSIONS: The prism-dioptre seems to be more informative when interpreted as a percentage. Prentice's rule is more effective as a pedagogical approach for introducing the concept of spherical lenses' prismatic image dislocation rather than a method for calculating its actual value.

Predictibilidad en la inducción de aberración esférica basada en la asfericidad corneal post LASIK en miopes / Predictability of spherical aberration based on corneal asphericity after myopic laser customized corneal ablation

Purpose: To determine the predictability of postoperative ocular spherical aberration (Z 4,0) based on postoperative corneal asphericity (Q value) in patients with myopic astigmatism. Design: Case series, retrospective. Methods: Wavefront-guided laser in situ keratomileusis (LASIK) was performed in 40 eyes of 23 patients using the Schwind Esiris Excimer Laser. Patients were evaluated preoperatively and 3 months postoperatively. Spherical aberration was measured with a COAS Shack-Hartmann wavefront sensor, and determined for a 6 mm analysis diameter zone. Corneal asphericity was measured with the Orbscan II topographer. Correlation and linear regression analysis was performed. A P value less than 0.05 was considered statistically significant. Results: Treated eyes had a mean manifest spherical error of ­2.98 + 1.36 Diopters (D), and a mean manifest astigmatic error of 0.52 + 0.46 D. The mean Q was -0.30 + 0.13 preoperatively and +0.03 + 0.22 postoperatively. The mean spherical aberration was 0.054 + 0.109 microns preoperatively and 0.262 + 0.135 microns postoperatively. We found a positive statistically signifi cant correlation between postoperative Q value and postoperative spherical aberration (r=0.367) (P=0.02). The postoperative Q value showed a low correlation with postoperative spherical aberration (R2=0.14). Conclusions: There was an increase in corneal asphericity and spherical aberration after surgery. Postoperative corneal asphericity is mildly associated with postoperative spherical aberration. Predictability of postoperative spherical aberration in terms of correlation with postoperative Q value was poor, suggesting that corneal asphericity is not a major predictor of spherical aberration induction.