Optimized keratometry and total corneal astigmatism for toric intraocular lens calculation

J Cataract Refract Surg. 2017 Sep;43(9):1140-1148. doi: 10.1016/j.jcrs.2017.06.040.


Purpose: To compare keratometric astigmatism (KA) and different modalities of measuring total corneal astigmatism (TCA) for toric intraocular lens (IOL) calculation and optimize corneal measurements to eliminate the residual refractive astigmatism.

Setting: G.B. Bietti Foundation IRCCS, Rome, Italy.

Design: Prospective case series.

Methods: Patients who had a toric IOL were enrolled. Preoperatively, a Scheimpflug camera (Pentacam HR) was used to measure TCA through ray tracing. Different combinations of measurements at a 3.0 mm diameter, centered on the pupil or the corneal vertex and performed along a ring or within it, were compared. Keratometric astigmatism was measured using the same Scheimpflug camera and a corneal topographer (Keratron). Astigmatism was analyzed with Næser's polar value method. The optimized preoperative corneal astigmatism was back-calculated from the postoperative refractive astigmatism.

Results: The study comprised 62 patients (64 eyes). With both devices, KA produced an overcorrection of with-the-rule (WTR) astigmatism by 0.6 diopter (D) and an undercorrection of against-the-rule (ATR) astigmatism by 0.3 D. The lowest meridional error in refractive astigmatism was achieved by the TCA pupil/zone measurement in WTR eyes (0.27 D overcorrection) and the TCA apex/zone measurement in ATR eyes (0.07 D undercorrection). In the whole sample, no measurement allowed more than 43.75% of eyes to yield an absolute error in astigmatism magnitude lower than 0.5 D. Optimized astigmatism values increased the percentage of eyes with this error up to 57.81%, with no difference compared with the Barrett calculator and the Abulafia-Koch calculator.

Conclusion: Compared with KA, TCA improved calculations for toric IOLs; however, optimization of corneal astigmatism measurements led to more accurate results.

MeSH terms

  • Astigmatism* / diagnosis
  • Astigmatism* / surgery
  • Corneal Topography
  • Humans
  • Lens Implantation, Intraocular
  • Lenses, Intraocular*
  • Postoperative Period
  • Prospective Studies