Use of corneal power-specific constants to improve the accuracy of the SRK/T formula.
OBJECTIVE
To evaluate the effect of average corneal power (K) and axial length (AL) in a data-adjusted A-constant for improving the refractive outcome in the Sanders-Retzlaff-Kraff (SRK)/T formula.


METHODS
Retrospective, consecutive, case series.


METHODS
A total of 637 patients (637 eyes) who underwent uncomplicated phacoemulsification with implantation of the Acrysof IQ (IQ; Alcon, Fort Worth, TX; 314 eyes) or Akreos AO (AO; Bausch & Lomb, Rochester, NY; 323 eyes) intraocular lens (IOL) by a single surgeon.


METHODS
The correlation among the K, AL, and predicted refractive error in the SRK/T formula was analyzed. Patients were divided into 2 subgroups, the first to calculate the different data-adjusted A-constants based on the K and the second to compare the median absolute error (MedAE) based on different A-constants with the traditional A-constant in the SRK/T formula.


METHODS
The data-adjusted A-constant and the MedAE (diopters [D]).


RESULTS
The data-adjusted A-constant showed a decreasing trend as K increased. The data-adjusted A-constant was 119.04 in the IQ group and 118.27 in the AO group. The calculated A-constant was 119.33 in the IQ group and 118.57 in the AO group when the cornea was flatter than 43.0 D and 43.2 D, respectively. The A-constant was 118.71 in the IQ group and 117.96 in the AO group when the cornea was steeper than or equal to 44.7 D and 45.0 D, respectively. The MedAE decreased from 0.29 D to 0.23 D in the IQ group (P = 0.001) and from 0.44 D to 0.38 D in the AO group (P < 0.001) when different A-constants were used. The MedAE further decreased from 0.36 D to 0.24 D in the IQ group (P = 0.005) and from 0.58 D to 0.37 D in the AO group (P < 0.001) when subjects with K 1.00 D or more above or 1.00 D below the most accurate K in each group were compared.


CONCLUSIONS
For a steep cornea, the calculated A-constant was smaller than that of the entire K, but for a flat cornea, a larger A-constant was calculated. Using different A-constants based on the K improved the refraction outcomes relying on the SRK/T formula.
Department of Ophthalmology, Korea University College of Medicine, Seoul, South Korea.
Youngsub
Y
Eom
Department of Ophthalmology, Korea University College of Medicine, Seoul, South Korea.
Su-Yeon
SY
Kang
Department of Ophthalmology, Korea University College of Medicine, Seoul, South Korea.
Jong-Suk
JS
Song
Department of Ophthalmology, Korea University College of Medicine, Seoul, South Korea. Electronic address: hyomkim@hanmail.net.
Hyo Myung
HM
Kim
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D058452:Axial Length, Eye / Q000033:anatomy & histology; D001699:Biometry / Q000379:methods*; D003315:Cornea / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D019654:Lens Implantation, Intraocular; D007910:Lenses, Intraocular; D008297:Male; D008875:Middle Aged; D055095:Optics and Photonics; D018918:Phacoemulsification; D011183:Postoperative Complications / Q000503:physiopathology; D012029:Refraction, Ocular / Q000502:physiology; D012030:Refractive Errors / Q000503:physiopathology; D015203:Reproducibility of Results; D012189:Retrospective Studies; D014792:Visual Acuity / Q000502:physiology