Accurate preoperative or intraoperative measurements of the eye are essential to achieving the desired postoperative refractive result. Modern intraocular lens (IOL) power formulas incorporate the measurements for axial length (AL), cornea power, and effective lens position (ELP). The formulas have evolved over time, becoming increasingly more complex and theoretical in an effort to continually improve accuracy. The modalities used to obtain the measurements are also continually improving, providing better data to use when calculating IOL power calculations. Intraoperative aberrometry may also be used to directly determine the IOL power without needing to measure the patient’s corneal power or AL. This technique may be particularly useful for children or patients who are unable to cooperate with office-based testing.
Ocular AL is a key component of IOL power calculations. An error as small as 1 mm in the AL measurement can lead to significant postoperative refractive error, especially in shorter eyes. AL can be measured with several techniques; no matter which is used, it is helpful to obtain data for both eyes, even if surgery is planned for only 1 eye. The difference in AL between the 2 eyes is typically no greater than 0.3 mm, unless there is a refractive difference or there are other relevant ocular findings. It is important to document and explain any significant disparity in AL.
Optical biometers are noncontact instruments that use infrared laser light (780 nm) and partial coherence interferometry to measure multiple parameters, such as AL, corneal curvature, anterior chamber depth, lens thickness, and horizontal white-to-white distance (corneal diameter). These devices require the patient to fixate on a target, which gives an AL along the visual axis. This technique is beneficial for patients with posterior staphyloma. Because the ocular media must be clear enough to allow voluntary fixation and light transmission, this technique is not ideal for taking measurements through dense or opacified media such as cornea scars, mature or posterior subcapsular cataracts, or vitreous hemorrhage. In some cases, measurements cannot be obtained with this modality, and ultrasound is required.
In A-scan ultrasonography, the transit time of the ultrasound pulse is measured. Using an estimated average velocity through the various ocular media (cornea, aqueous, lens, and vitreous), the biometric software calculates the AL. This value should be altered when velocities differ from the norm (see the Clinical Pearl sidebar). Measurements are obtained either via immersion (Fig 7-1) or contact applanation. With the immersion technique, a shell is placed on the eye between the eyelids to provide a watertight seal over the cornea, and an ultrasound transducer is mounted in the shell. This technique minimizes measurement errors when compared with contact applanation. With the contact applanation method, the examiner must be careful not to indent/compress the cornea, which results in an artificially shortened AL measurement, or to take the measurement through a tear meniscus, which results in an artificially lengthened AL. Both types of false measurement can result in postoperative refractive errors, particularly in short eyes.
Bjeloš Rončević M, Bušić M, Cima I, Kuzmanović Elabjer B, Bosnar D, Miletić D. Intra-observer and interobserver repeatability of ocular components measurement in cataract eyes using a new optical low coherence reflectometer. Graefes Arch Clin Exp Ophthalmol. 2011;249(1):83–87.
Ianchulev T, Hoffer KJ, Yoo SH, et al. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology. 2014;121(1):56–60.
Roessler GF, Huth JK, Dietlein TS, et al. Accuracy and reproducibility of axial length measurement in eyes with silicone oil endotamponade. Br J Ophthalmol. 2009;93(11): 1492–1494.
Figure 7-1 Immersion shells. Although there are other immersion shells, they are now rarely used in the United States, as visual axis alignment is more easily achieved with infusion shells than with cup shells. Infusion shells are also easier to use. A, In immersion ultrasonography, the probe is immersed in the solution, placing it away from the cornea. B, Prager shell for immersion A-scan. C, Ultrasound probe and Kohn shell.
(Courtesy of Kenneth J. Hoffer, MD.)
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.