Project hyperopic power prediction: accuracy of 13 different concepts for intraocular lens calculation in short eyes

Abstract

PurposeTo evaluate the accuracy of intraocular lens (IOL) power calculation in a patient cohort with short axial eye length to assess the performance of IOL power calculation schemes in strong hyperopes.MethodologyThe study was a single centre, single surgeon retrospective consecutive case series at the Augen- und Laserklinik, Castrop-Rauxel, Germany. Inclusion of patients after uneventful cataract surgery implanting either spherical (SA60AT) or aspheric (ZCB00) IOLs. Inclusion criteria were axial eye length <21.5 mm and/or emmetropising IOL power >28.5 D. Lens constants were optimised on a separate patient cohort considering the full bandwidth of axial eye length. Data of one single eye per patient were randomly included. The outcome measures were: mean absolute prediction error (MAE), median absolute prediction error, mean prediction error with SD and median prediction error and the percentage of eyes with an MAE within 0.25 D, 0.5 D, 0.75 D and 1.0 D.ResultsA total of 150 eyes from 150 patients were assessed. Okulix, PEARL-DGS, Kane and Castrop provided a statistically significantly smaller MAE compared with the Hoffer Q and SRK/T formulae.ConclusionIn our patient cohort with short axial eye length, the use of PEARL-DGS, Okulix, Kane or Castrop formulae showed the lowest MAE. The Castrop formula has not been published before, but will be disclosed with a ready-to-use Excel sheet as an addendum to this paper.

Considerations on the Castrop formula for calculation of intraocular lens power

“…Such software tools do not allow batch calculations on a large set of patient data. Today, in many countries of the world classical formulae are increasingly being replaced by ‘modern’ calculation strategies such as the Barrett Universal II, Kane, Pearl, EVO, VRF or T2 formula [ 10 , 11 , 26 ]. To compare the prediction performance with other formulae it is necessary to enter the data manually, introducing a large risk of transcription errors.…”

“…In general, it is difficult to check the performance of different formulae, as it is mostly dependent on the composition of the dataset, and there are no common rules on how to evaluate the performance. Therefore, some formulae are superior in long or short eyes [ 11 , 12 ], or in eyes with a steeper or flatter cornea, or selected combinations of axial length and corneal curvature. Additionally, the performance of a formula mostly depends on the dataset used for constant optimization as well as the technique applied for optimizing the constants, and it can also depend on the biometer settings, quality of the clinical measurement data (e.g.…”

The Castrop formula for calculation of toric intraocular lenses

“…A correction for corneal back surface astigmatism is not necessary in such cases. If no data are available from corneal back surface curvature or thickness, the Castrop formula uses a preset value for the corneal thickness (500 μm) and derives corneal back surface curvature in both meridians from the respective corneal front surface curvature and a fixed proportion of front to back surface radius of 7.77/6.4 as derived from the Liou-Brennan schematic model eye [35]. In this case a correction for the corneal back surface curvature CPA should be applied at the corneal front apex plane.…”

“…The refractive indices of the optical model as shown in Fig. 1 are derived from the Liou-Brennan schematic model eye [35] (cornea: n C = 1.376, aqueous humor: n A = 1.336, vitreous: n V = 1.336). The vertex distance (VD) as the distance of the back vertex of the spectacle correction to the corneal front apex is assumed to be 12 mm.…”

“…For the corneal back surface, the respective parameters are the radius RCP 1 in an axis of ACP 1 and the radius RCP 2 in an axis orthogonal to ACP 1 (ACP 2 = ACP 1 + 90°). If corneal back surface data are not available, the corneal back surface radius is calculated from the respective corneal front surface radius assuming a fixed proportion of front to back surface radius (derived from the Liou-Brennan schematic model eye [35]) with RCP 1 = RCA 1 •6.4/7.77 (in an axis ACP 1 = ACA 1 ) and RCP 2 = RCA 2 •6.4/7.77 (in an axis ACP 2 = ACA 2 ). If CCT is not available, we use CCT = 500 μm as extracted from the Liou-Brennan schematic model eye.…”