Factors affecting the predictability of SRK II in patients with normal axial length undergoing phacoemulsification surgery

Lim LH, Lee SY, Ang CL
Correspondence: Dr Ang Chong Lye, ang.chong.lye@snec.com.sg

ABSTRACT
Introduction This study aims to evaluate the factors affecting the accuracy of biometry assessment using the SRK II formula in predicting the refractive outcome after uneventful cataract surgery by phacoemulsification for eyes within the normal range of axial lengths.
Methods A retrospective review of 100 consecutive cases of uncomplicated phacoemulsification surgery performed by two surgeons from January to September 2005 in a single centre was done. Eyes with axial length greater than or equal to 22.5 mm and less than 24.5 mm were included in the study. The error of prediction was calculated by actual postoperative refractive error minus the refraction target. A comparison was made between the low predictability (prediction error more than 0.50 dioptres [D]) and high predictability (prediction error less than or equal to 0.50 D) groups of eyes. Statistical analysis was performed.
Results A total of 267 case notes were reviewed, of which 100 met the inclusion criteria. The mean age was 68.3 years, with a mean keratometric reading of 43.94 +/- 1.27 D and mean axial length of 23.38 +/- 0.51 mm. The mean IOL power used was 21.77 +/- 1.50 D. The mean error was +0.25 +/- 0.67 (range -1.58 to +1.80)D, with the standard error of mean 0.669. 45 percent of the patients were within 0.5 D of the predicted refractive error and 83 percent were within 1.0 D. There was no difference in preoperative corneal astigmatism, mean keratometry, axial length, age or gender of the patient, laterality of the operated eye and intraocular lens power between the low and high predictability groups. There was a negative correlation between the axial length and prediction error.
Conclusion SRK II is reliable in the prediction of the refractive outcome in normal axial length eyes. The findings in our study are comparable to those found in previous studies. We found a negative correlation between axial length and prediction error, even within the range of normal axial length eyes.

Keywords: biometry, cataract surgery, eye axial length, phacoemulsification surgery, SRK II formula, target refraction
Singapore Med J 2009; 50(2): 120-125