Biometry Pearls: Evaluation of Biometric Data

Educational Objectives

The goal of this program is to improve preoperative planning of cataract surgery using optical biometry. After hearing and assimilating this program, the clinician will be better able to:

1. Interpret measurements on optical biometry printouts to assess measurement quality and guide intraocular lens selection.
2. Obtain approximate measurements of angle alpha to determine candidacy for diffractive optics.
3. Choose an intraocular lens formula when biometric measurements are unavailable or inaccurate.

Summary

Optical biometry: an invaluable and indispensable part of the cataract surgery process; many surgeons look for intraocular lens (IOL) power and choose a closest-to-plano or slightly myopic target; better refractive outcomes lead to happier patients and help build practice

Preoperative preparation: requires time, energy, and dedication; however, preoperative preparation makes the surgery easier; the speaker focuses on 2 common optical biometer devices in the United States (IOLMaster 700, Lenstar)

Key Pearls for Biometry

Review of the biometry printouts: the first sheet on the biometry printout contains a few IOL suggestions and provides IOL power; however, the IOL suggestions may be based on flawed measurements; the device may provide warnings about the measurements (but not always); the first sheet provides a summary (eg, axial length [AL], anterior chamber depth [ACD], lens thickness [LT], white-to-white corneal diameter [WTW], keratometry measurements); the second or third sheet with raw data is important (eg, “Analyze” sheet)

The “Analyze” sheet: the top of the sheet contains warnings (eg, long or short AL, high astigmatism)

Quality of measurements: the standard deviation (SD) for AL, central corneal thickness (CCT), and ACD should be <10 μm; the SD for LT can be 20 to 25 μm; the SD for keratometry measurements should be <0.05 diopter (D); if the SD values are high, consider repeating biometry or postponing biometry after rehabilitation of the ocular surface (especially if planning for premium IOLs)

B scan: shows vertical lines (automated gates) indicating CCT, LT, and retina; in some cases, LT can be improperly gated and inaccurate; newer formulas (eg, Barrett, Kane, Emmetropia Verifying Optical, Hill-Radial Basis Function) rely on LT, whereas older formulas (eg, SRK/T, Hoffer Q, Holladay 1, Haigis) rely less on LT; B scan provides a clue on the fixation of the patient and readings similar to macular optical coherence tomography (OCT); presence of epiretinal membrane or retinal pathology may prompt a formal macular OCT

Central topography: provides information on the central 4 mm of the cornea (eg, regularity of astigmatism); toric IOL may not be an option with irregular astigmatism; studies show that central topography correlates with formal topography (75%-90%); however, central topography cannot detect posterior corneal changes

Information specific for Lenstar: ensure 5 spikes are present (corresponding to the anterior and posterior cornea, anterior and posterior lens, and the retinal pigment epithelium); ensure SD is <10 μm for AL, CCT, and ACD and <0.05 D for keratometry; the device may falsely spike the anterior and posterior cortex instead of the anterior and posterior lens capsules, yielding a falsely short LT and long ACD (can lead to a myopic surprise with the Olsen formula); 4 spikes should be visible in the lens area (corresponds to the anterior capsule, anterior cortex, posterior cortex, and posterior capsule)

Use the device’s full features: use central topography and low-resolution macular OCT to explore the need for formal tests

Importance of angle alpha values: angle kappa is the distance between the pupillary and visual axes measured at the center of the entrance pupil; angle alpha is the distance between the visual and optical axes; avoid IOLs with diffractive optics in patients with high angle alpha and angle kappa values (exact cutoff values uncertain) to avoid unwanted photic phenomenon (eg, glare, halos); high angle alpha and angle kappa values are associated with higher rates of postoperative photic phenomenon and unwanted refractive outcomes; recently, angle kappa has been rebranded as Chang-Waring (CW)-chord or chord mu (important for laser vision correction); angle alpha is important for cataract surgery; the purest way to measure angle kappa is a synoptophore; CW-chord is used clinically and can be measured with biometry or tomography; angle kappa depends on the AL, while CW-chord depends on the anterior segment; technically, IOLMaster measures chord mu and chord alpha (and not angle kappa and angle alpha, respectively)

Angle alpha: reliable benchmark for refractive IOL surgery, as IOL settles on the optical axis; ideally, angle alpha should be zero; with a large angle alpha (>0.5 mm), the visual axis may pass through the rings instead of the center of the optic (optical axis), leading to pseudophakic photic phenomenon; angle alpha is more important than angle kappa for diffractive IOLs; Wallerstein et al (2023) found that preoperative angle kappa does not have a predictive clinical effect on postoperative multifocal IOL visual outcomes, refractive accuracy, or subjective patient satisfaction; angle kappa as a single variable should not be used to determine multifocal IOL candidacy

Measurement of angle alpha: although other devices (eg, Pentacam, iTrace) can measure angle alpha, absolute values of Ix (x coordinate of iris center relative to corneal apex) and Iy (y coordinate of iris center relative to corneal apex) in optical biometry can be summed up to approximate angle alpha (eg, a patient with Ix of -0.6 mm and Iy of +0.2 mm has an angle alpha of 0.8 mm and is not a good candidate for diffractive IOLs); patients with a large angle alpha may be better candidates for monofocal or monofocal plus IOLs (accommodating or pinhole-optics IOLs may also be considered); presbyopia-correcting extended depth of focus IOL (Symfony) tends to be the best tolerated diffractive IOL for patients with a large angle alpha

Ensure high-quality corneal measurements: the Analyze sheet provides information about the corneal dots; Lenstar provides 4 sets of pictures of 32 dots each; the dots should be of equal intensity and spacing; blurry or dim dots may indicate ocular surface disease (OSD) or dry eye; repeat the test after rehabilitation of ocular surface for a few weeks; IOLMaster 700 has 18 infrared light-emitting diodes; treatment of OSD prior to surgery is important for premium and toric IOLs (astigmatism may turn out to be OSD)

Use the built-in macular OCT: provides B scan; low-resolution macular OCT is primarily used to confirm fixation through identification of foveal contour; patient looking away from the center can change AL and corneal power measurements and provide false astigmatism values; pathology identified on the lower-resolution OCT can be used to obtain formal macular OCT or referral to a retina specialist and counseling about appropriate IOL options

Strategies if LT measurements are poor: lens gates on IOLMaster 700 and lens spikes on Lenstar are important because the biometer may provide inaccurate LT measurements; lens calculations may not be performed on the day of evaluation for cataract surgery and the patient may not always be available for repeat measurements; therefore, formulas that do not require LT or ACD can be used (especially in eyes with normal AL or keratometry measurements); use classical vergence formulas (eg, SRK/T, Hoffer Q, Holladay 1 [the Ladas Super Surface Formula combines these formulas]); avoid multivariable formulas (eg, Kane, Olsen); the Barrett formula provides acceptable results even with inaccurate LT measurement

Predictive value of optional parameters in new formulas: Wendelstein et al (2023) evaluated new formulas with and without optional biometric parameters (eg, ACD, LT, CCT, WTW) and found that inaccurate LT values affect new formulas more than classical formulas; inaccurate CCT or WTW measurements had a minimal effect on the results; the Barrett formula was the least affected when LT values were inaccurate or excluded

Use optical biometry to assess history of laser vision correction (LVC): applicable to IOLMaster 700; optical biometry can be used to determine whether myopic LVC (M-LVC) or hyperopic LVC (H-LVC) was performed (eg, when records are not available); Cooke et al (2023) developed the Cooke−Riaz−Wendelstein (CRW1) index to identify myopic LVC (M-LVC) eyes using IOLMaster 700 values; total keratometry (TK) and anterior keratometry (K) values can also be used to determine history of LVC; Riaz et al (2023) found that M-LVC was performed in 99% of cases in which TK minus K was <0 (negative); M-LVC was highly likely if TK minus K was between 0 and 0.06,; H-LVC was most likely performed if TK minus K was >0.07; if in doubt, formal topography or tomography can provide additional information

Assess pupil size: optical biometry provides pupil size in scotopic or mesopic lighting conditions; small pupils (<3 mm) are more likely to have myopic shift, especially when using extended depth of focus IOLs (eg, Vivity IOL); in patients with small pupils, aim for slightly hyperopic (0.25-0.5 D) optics to compensate for the myopic shift; small pupil size may also be more forgiving for borderline angle alpha and angle kappa values (the patient has to look through the center of the IOL)

Readings

Akman A, Asena L, Güngör SG. Evaluation and comparison of the new swept source OCT-based IOLMaster 700 with the IOLMaster 500. Br J Ophthalmol. 2016;100(9):1201-1205. doi:10.1136/bjophthalmol-2015-307779; Cervantes-Coste G, Tapia A, Corredor-Ortega C, et al. The influence of angle alpha, angle kappa, and optical aberrations on visual outcomes after the implantation of a high-addition trifocal IOL. J Clin Med. 2022;11(3):896. Published 2022 Feb 8. doi:10.3390/jcm11030896; Cooke DL, Riaz KM, Murphy DA, et al. The CRW1 index: identification of eyes with previous myopic laser vision correction using only a swept-source OCT biometer. Am J Ophthalmol. 2023;247:79-87. doi:10.1016/j.ajo.2022.10.024; Riaz KM, Cooke DL, Wendelstein JA. Determining the type of previous laser vision correction using keratometry measurements obtained from an SS-OCT biometer. J Cataract Refract Surg. 2023;49(4):438-439. doi:10.1097/j.jcrs.0000000000001143; Wallerstein A, Ridgway C, Gatinel D, et al. Angle kappa influence on multifocal IOL outcomes. J Refract Surg. 2023;39(12):840-849. doi:10.3928/1081597X-20231101-01; Wendelstein JA, Rothbächer J, Heath M, et al. Influence and predictive value of optional parameters in new-generation intraocular lens formulas. J Cataract Refract Surg. 2023;49(8):795-803. doi:10.1097/j.jcrs.0000000000001207.

Disclosures

For this program, the following relevant financial relationships were disclosed and mitigated to ensure that no commercial bias has been inserted into this content: Dr. Riaz has been a consultant for Ambrx, Bausch and Lomb, Exelixis, ImmunoGen, and Neumora Therapeutics; received speaking fees from CorneaGen, Bausch and Lomb, and PrecisionLens; and received personal/travel fees from Aurion Therapeutics and Epion Apricity Therapeutics. Members of the planning committee reported nothing relevant to disclose.

Acknowledgements

Dr. Riaz was recorded exclusively for Audio Digest on June 4, 2024. Audio Digest thanks the speaker for their cooperation in the production of this program.

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 1.25 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 1.25 CE contact hours.

Lecture ID:

OP621602

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.