ROTATING SCHEIMPFLUG CAMERA TOTAL CORNEAL REFRACTIVE POWER VARIATION BETWEEN 3.0- AND 4.0-MM ZONES

Analyse the variation of total corneal refractive power (TCRP) from a 3.0 mm and a 4.0 mm diameter ring centered on the pupil axis and its influence on the prediction of residual refractive astigmatism.

Group for Lasers and Plasmas, Institute for Plasmas and Nuclear Fusion, Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal; Hospital da Luz Lisbon, Lisbon, Portugal.

Corneal astigmatism and axis measurements calculated via ray-tracing by the rotating Scheimpflug camera (Pentacam, Oculus), for a 3.0- and 4.0- mm zone, were considered for 500 eyes from 376 patients.  Astigmatism between zones was assessed for all eyes and for the with-the-rule (WTR) and against-the-rule (ATR) subgroups. Using the Næser-Savini Toric Calculator, the toric intraocular lens (IOL) power suggested to be implanted based on the 3.0- and 4.0-mm TCRP values was determined for 125 eyes, randomly chosen for astigmatism differences between zones lower than 0.50 D and including all eyes for higher differences.  The predicted residual refractive astigmatism correspondent to the toric IOL power determined to be implanted with TCRP values from the 3.0 mm zone was compared to the prediction for the same toric IOL power but with TCRP values from the 4.0 mm zone.

Mean and standard deviation (SD) astigmatism at the 3.0 mm zone and the 4.0 mm zone for all eyes was, respectively, 1.15±0.83 D and 1.14±0.84 D. For the WTR subgroup these were, respectively, 1.23±0.80 D and 1.25±0.84 D, and for the ATR subgroup 1.19±0.88 D and 1.12±0.84 D. The power of the toric IOL to be implanted changed between zones in 76.80% of the eyes, and from these 52.08% had lower toric IOL powers in the 4.0 mm zone than in the 3.0 mm zone. Considering the toric IOL power determined for the TCRP of the 3.0 mm zone, for astigmatism differences below 0.25 D, between 0.25-0.50 D, 0.50-0.75 D, and 0.75-1.0 D, and higher than 1.0 D, the mean absolute error (MAE) between the correspondent predicted residual refractive astigmatism and the one for the 4.0 mm zone were, respectively, 0.08, 0.26, 0.41, 0.75, 1.09.

Astigmatism changed between the 3.0- and the 4.0-mm zone with an average increase for the WTR subgroup and decrease for the ATR subgroup. The toric IOL power determination changed based on the TCRP of the different zones, mainly decreasing towards the periphery. With the increasing astigmatism difference between zones, also the predicted residual refractive astigmatism difference increased for the toric IOL power determined based on the 3.0 mm zone astigmatism.