Comparison Of The Conventional Method, The Segmental Method, And The Wang Koch Formula For Calculating Axial Length In High Myopia

To compare the axial lengths (AL) obtained using three methods: the conventional method (CM), the segmental method (SM) using an optical biometer and the Wang Koch (WK) formula. In addition, we assessed the differences in postoperative refractive errors.

The study was performed propectively at Hassan II university hospital center of Fez.

This prospective study included highly myopic patients candidates for cataract surgery by phacoemulsification. AL using the CM was measured with an optical biometer, applying a mean refractive index. The AL using the SM was calculated by summing the geometric lengths of the ocular segments (cornea, aqueous humor, lens and vitreous), using specific refractive indices based on data obtained from the optical biometer. In addition, the AL  was estimated using WK formula. The SRK/T formula was used for intraocular lens (IOL) power calculation. Refraction was measured three months after surgery and the mean refractive error (MRE) was calculated and compared.

The study included 80 eyes from 40 patients. The AL obtained using the SM (29.34 ± 1.34 mm) was significantly shorter than that obtained by the CM (30.71 ± 1.39 mm, p < 0.001). However, no statistically significant difference was observed between AL obtained using WK formula and that measured with the SM (29.77 ± 1.56 mm; p = 0.45). The MRE calculated using AL  from the SM was 0.38 ± 0.75 D, which was significantly lower than the MRE using AL  by the CM (1.70 ± 0.76 D, p < 0.001). No significant difference was found between MRE using AL from the SM and that obtained with WK formula (0.45 ± 0.86 D; p = 0.34). 

In highly myopic eyes, using segmented AL or AL obtained with WK formula improves the predictive accuracy of the SRK/T formula compared to AL obtained with the CM.