Optical vs Ultrasonic Biometry: Differences May Impact Lens Thickness Measurements

At the optician Ophthalmology Eye Ultrasound Optometrist medical eye examination
Lens thickness measurement is essential in finding effective lens position when using a newer generation IOL calculation formula.

With every cataract surgery, ophthalmologists strive for perfect estimation of postoperative refractive correction. The pre-procedure biometry clinicians choose — whether optical low-coherence interferometry or ultrasonic biometry — is key in reaching precise measurements, especially now that effective lens position (ELP) is determined with lens thickness (LT) in newer generation intraocular lens (IOL) calculation formulas.

A retrospective cross-sectional study of 1247 preoperative cataract patients takes a deep dive into occasional inconsistencies between the 2 technologies when evaluating axial length (AL), anterior chamber depth (ACD), and how these differences impact measurement of LT. Researchers found in some instances there was inadequate consistency in measuring AL and ACD between optical biometry and A-scan ultrasonography.

“The results showed that when the difference of AL and ACD measured by the 2 techniques was large, the consistency of LT was poor,” the study explains. “This difference has a certain influence on the calculation of IOLs because it would have led to a different IOL power selection when using a newer formula, which predicts the ELP based on LT, such as Olsen or Barrett Universal II formulas.”

Prior research shows that both technologies can reliably determine AL and ACD, but less work has been presented regarding the instances when inconsistencies occur; variations that may impact measurement of lens thickness. In the current study population, the mean age was approximately 67 years, and 60.5% of participants were female. Only 1 eye, undilated, served as the test eye for each patient. Investigators conducted an analysis comparing an average of 10 A-scan ultrasonography assessments with the average of 3 optical biometry tests. 

A “strong linear (Pearson) correlation” was found between the 2 instruments for AL, ACD, and lens thickness measurements. Alternatively, a Bland-Altman consistency analysis showed a mean difference in AL of 0.27 mm between the technologies. A mean difference of 0.15 mm resulted when gauging ACD with the 2 instruments. Despite lower agreement in determining AL and ACD, the average difference for lens thickness measurements was only 0.01 mm.

“Although there was good consistency overall between the 2 methods in LT measurements, there was also poor consistency in LT measurements in patients with significant differences in AL or ACD measurements,” the study authors explained. Consequently, investigators performed further data analysis to find root causes for the differences. Researchers did uncover an issue related to ultrasonography probe technique. When the testing axis deviated from the eye axis, AL and ACD recorded longer than with laser-based optical biometry. Also, the pressure of the ultrasonography probe on the globe resulted in a shorter-than-actual reading of AL and ACD, although it did not significantly affect LT measurement.

A limitation of the study was that the population included only cataract patients, and opacification can impact biometric testing. Overall, investigators concluded that during contact A-scan ultrasonography, probe tilt should be carefully prevented because it may result in an imprecise determination of lens thickness, and therefore inaccuracies, when applying a more recently developed IOL calculation formula. “If this error can be avoided, A-scan ultrasonography can replace [optical biometry] in LT measurement in cataract patients,” according to the study.


Xu J, Li C, Wang L, Li C, Li X, Lu P. Influence of measurement differences of anterior chamber depth and axial length on lens thickness evaluation in cataract patients: a comparison of two testsBMC Ophthalmol. 2020;20(1):481. doi:10.1186/s12886-020-01754-w