Measurement and Selection of Degree of IOL Implantation

1) The standard degree of orthokeratology: the posterior chamber type needs to be +19D.

2) Extrapolation of IOL degree according to the refractive error: the estimation formula is P=19+(R×1.25), in which P is the degree of IOL degree, and R is the patient's preoperative refractive state. For example, the patient's preoperative refractive error is -2.0D, substituting the formula P = 19 + (-2.0 × 1.25) = 16.5D, then the patient's required IOL is 16.5 D. Calculation with this method is relatively simple, but its accuracy is far less than that of biometric calculations, which sometimes cause large errors, the reason is that it is difficult to obtain the accurate parameters of the original refraction. However, for those who do not have the conditions to carry out the measurement, it is still desirable.

3) Biometric calculation of IOL size: Preoperative measurement of the optical parameters of the eye, including corneal curvature and axial length, is required. The corneal curvature meter was used to measure the curvature of the cornea in the vertical and horizontal axes, i.e. corneal K value. The length of the ocular axis was measured with an A-ultrasound. The SPK regression formula was used to calculate the number of degrees of the proposed IOL. SPK regression formula: P=A-2.5L-0.9K. P is the number of degrees of the proposed IOL, A is a constant (which varies with the type of IOL), L is the length of the ocular axis, and K corneal curvature.