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.

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Keratometry

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.

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Optical Biometry

Optical biometers, such as the Carl Zeiss Meditec IOLMaster^†, perform a new method of preoperative biometry where infrared light is used to calculate axial length, keratometry and anterior chamber depth. This precise, rapid method reduces technical variance and eliminates physical contact with the cornea. However, optical biometry may not perform consistently with patients who have dense cataracts.

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