Essentials of Biometry (Part 1): What ocular parameters are important?
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Title: Essentials of Biometry (Part 1): What ocular parameters are important?
Authors: Courtney Goodman, MSIV at University of Miami Miller School of Medicine, Mark Mifflin, MD
Date: 11/14/2022
Keywords/Main Subjects: biometry; intraocular lens; cataract surgery
Review:
Ocular biometry is a crucial step in the preparation for cataract surgery. The process involves measuring the dimensions of the eye, which are plugged into specific formulas (there are many!) to calculate what optimal power of an IOL will give the best refraction.1 In this article, we will define these important dimensions.
The two dimensions with the greatest impact on IOL power calculations are axial length and central corneal power. Other preoperative measurements that also impact calculations, but to a lesser degree, include anterior chamber depth, corneal thickness, lens thickness, and white-to-white distance (Figure 1).
Figure 1 Ocular biometric measurements. (Created with Biorender)
Let’s go over each of these measurements…
Axial Length (AL) = The distance between the anterior corneal surface (anterior pole) and the retinal pigment epithelium near the fovea (posterior pole), measured in millimeters.
Central Corneal Power (K) = The refractive power of the cornea which is calculated based on the radius of curvature of the anterior cornea (ra) and posterior cornea (rp) in the central optical zone, measured in diopters. There are several different formulas for calculating K depending on the keratometry method used for measuring, all based on the assumption that the cornea is shaped like a spherical convex mirror.
Anterior Chamber Depth (ACD) = The distance between the anterior corneal surface and the anterior surface of the lens, and is considered an important determinant of effective lens position (ELP), measured in millimeters.
Lens thickness (LT) = The distance between the anterior and posterior lens surfaces that is also an important determinant of ELP, measured in millimeters.
Corneal thickness (CT) = The distance between the anterior and posterior corneal surfaces, measured in micrometers.
White-to-White distance (WTW) = The distance between the limbal margins along the corneal horizontal meridian, measured in millimeters. This parameter has become increasingly important in newer generation formulas (i.e., the Holladay formula,2 among others), predicting effective lens position (ELP), and selecting appropriate sizes of anterior chamber IOLs and implantable collamer lenses (ICL).3,4 Modern laser interferometer and Scheimpflug techologies accurately obtain this measurement.3
Effective Lens Postion (ELP) = The theoretical distance from the anterior corneal surface to the anterior intraocular lens (IOL) surface, based on an estimate of where the IOL will be placed, such as within the capsular bag or in a secondary location. This dimension tends to be the least accurate preoperative parameter, as it cannot be physically measured and depends on numerous variables including haptic design, IOL shape, and the geometry of the implantation site. Currently, research using artifical intelligence is attempting to improve this important theoretical parameter which can potentially advance the accuracy of IOL power calculations.5,6
Let’s take a moment to define the term “diopter” and discuss its determinants. Diopter is the ophthalmic unit of measurement that quantifies refractive power, or the degree to which a refractive interface converges (positive diopter value) or diverges (negative diopter value) light. In the human eye, the cornea accounts for approximately two-thirds of the total refractive power for converging light, while the lens constitutes the remaining one-third. The anterior and posterior surface curvatures are the most important determinants of refractive power, followed by refractive index of the material (i.e., corneal stroma, acrylic material of an IOL, etc). A general formula that relates these variables is 𝐷=(𝑛−1)/𝑟 , where 𝐷 is the refractive power in diopters, 𝑛 is the refractive index of the interface, and 𝑟 is the radius of curvature measured in meters, assuming that light is initially traveling through air.7 Of note, thickness of the refractive interface also impacts refractive power, but very minimally. Overall, the general principle is that a steeper curvature has a smaller radius, and therefore greater refractive power in diopters.
IOLs are manufactured in half-diopter sizes. There are many different formulas which incorporate the parameters we defined above for calculating and selecting the optimal IOL power.1 The appropriate formula is selected based on the patient’s eye conditions (i.e., a history of refractive surgery, the presence of silicone oil in the posterior segment, etc), or parameters (i.e., range of AL, extreme myopia8, etc). While we won’t cover the long list of formulas and when to use them, we hope this guide serves as an introduction to the different biometric parameters used in these formulas.
References:
1. Kane JX, Chang DF. Intraocular Lens Power Formulas, Biometry, and Intraoperative Aberrometry: A Review. Ophthalmology. Nov 2021;128(11):e94-e114.
2. Hoffer KJ. Clinical results using the Holladay 2 intraocular lens power formula. J Cataract Refract Surg. Aug 2000;26(8):1233-1237.
3. Tana-Rivero P, Aguilar-Corcoles S, Rodriguez-Prats JL, Montes-Mico R, Ruiz-Mesa R. Agreement of white-to-white measurements with swept-source OCT, Scheimpflug and color LED devices. Int Ophthalmol. Jan 2021;41(1):57-65.
4. Chen X, Han T, Zhao W, et al. Effect of the Difference Between the White-to-White and Sulcus-to-Sulcus on Vault and the Related Factors After ICL Implantation. Ophthalmol Ther. Dec 2021;10(4):947-955.
5. Gatinel D, Debellemaniere G, Saad A, Dubois M, Rampat R. Determining the Theoretical Effective Lens Position of Thick Intraocular Lenses for Machine Learning-Based IOL Power Calculation and Simulation. Transl Vis Sci Technol. Apr 1 2021;10(4):27.
6. Chung J, Bu JJ, Afshari NA. Advancements in intraocular lens power calculation formulas. Curr Opin Ophthalmol. Jan 1 2022;33(1):35-40.
7. Olsen T. On the calculation of power from curvature of the cornea. Br J Ophthalmol. Feb 1986;70(2):152-154.
8. Li C, Wang M, Feng R, et al. Comparison of Formula-Specific Factors and Artificial Intelligence Formulas with Axial Length Adjustments in Bilateral Cataract Patients with Long Axial Length. Ophthalmol Ther. Oct 2022;11(5):1869-1881.
Identifier: Moran_CORE_126570
Faculty Approval by: Dr. Mark Mifflin
Copyright statement: Copyright Courtney Goodman, ©2022. For further information regarding the rights to this collection, please visit: http://morancore.utah.edu/terms-of-use/
