Preoperative Biometry Basics

Understanding IOL Calculations

Accurate preoperative biometry is essential to attaining optimal results with the AcrySof® ReSTOR® IOL. These precise measurements of the shape and size of the eye help ensure that proper IOL power is implanted, potentially providing freedom from glasses. Preoperative biometry measurements include axial length and corneal curvature.

Axial Length

Axial length is the distance from the cornea to the retina. Use of a standard immersion A-scan is recommended during preoperative biometry because it involves no direct corneal contact or compression. An A-scan uses ultrasound echolocation to locate the internal structures of the eye, which are then mapped as a one-dimensional sonogram. This method gives a true axial length and is generally considered operator-independent.


Another stage of preoperative biometry is the measurement of corneal curvature and power, also called keratometry. This plays a critical role in the accuracy of the IOL power calculation. Keratometry detects and measures astigmatism by determining the steepest and flattest meridians of the corneal surface. K readings may be obtained manually or by an automated method. These readings are typically expressed in diopters, while millimeters (mm) serve as an alternate unit of measurement.


As with any refractive procedure, it is important to recognize the impact of contact lenses on the corneal surface. This is why it is important to educate the patient of the importance of contact lens removal prior to A-scan and pre operative measurements related to surgical calculations and surgical outcomes.

The rule of thumb is basically the same as with any refractive procedure, soft contacts should be removed 7-10 days prior to pre operative evaluation, and RGP’s approximately 3 weeks but may take up to 1 week of removal for every year of contact lens wear. Topographies should be performed at cataract evaluation, if lenses are worn, for a baseline reading and then repeated prior to A-scan. Several topographies may be required for these patients if changes of 1 diopter or more occur in the topographies.

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