[Back-calculation of the keratometer index-Which value would have been correct in cataract surgery?]

In the clinical routine the conversion of corneal radii into corneal refractive power using a keratometer index is rarely discussed. The purpose of this study was to back-calculate the keratometer index in pseudophakic eyes based on the refractive power of the lens, biometric measurements and refraction, and to compare it to clinically established values.

In this retrospective case series 99 eyes of 99 patients without pathological alterations, previous diseases, comorbidities or history of ocular surgery apart from the uneventful cataract surgery were enrolled. In all eyes a CT Asphina 409M(P) (Carl-Zeiss Meditec, Berlin, Germany) had been implanted by two surgeons (EF and PE). For calculation we used shape and power data of the intraocular lens and data from optical biometry (axial length, pseudophakic anterior chamber depth, lens thickness, corneal radius; IOLMaster 700, Carl-Zeiss Meditec, Jena, Germany). The refraction was derived manually with a trial frame (measurement distance 5 m) and autorefractometry (iProfiler, Carl-Zeiss, Jena, Germany). For this three model eyes were used: a thin lens with the nominal refractive power positioned in the equatorial plane (model A) or in the secondary principal plane of the thick lens (model B) as well as a model considering the intraocular lens as a thick lens located at its measured position (model C).

Back-calculation of the keratometer index using vergence formulas resulted in a keratometer index based on subjective refraction measurements considering lane distance correction of 1.3307 ± 0.0026/1.3312 ± 0.0026/1.332 ± 0.0027 for model A/model B/model C, respectively. Based on objective refraction measurements (autorefraction calibrated to infinity object distances) resulted in a keratometer index of 1.3301 ± 0.0021/1.3306 ± 0.0021/1.3315 ± 0.0021, for model A/model B/model C, respectively. The keratometer index did not show any trend in linear regression for axial length or corneal radius for any of the three models or for any refraction method.

The keratometer index derived from back-calculation matched with the Zeiss index (1.332) but was much lower compared to other established indexes, e.g. the Javal index (1.3375). The missing trend for axial length or corneal radius implies that simple vergence formulas for intraocular lens refractive power calculation without correction terms or fudge factors perform best with a keratometer index slightly below 1.332, if the biometrically measured position of the intraocular lens is used as the effective lens position.