Segmented refractive index may improve axial length measurement accuracy

Data from a theoretical modeling study showed that individual variation in the crystalline lens refractive index may introduce clinically relevant errors into optical measurements of axial length (AL) and lens thickness (LT). Using a segmented refractive index (SRI) approach reduced the modeled AL error compared with conversion based on a weighted mean ocular refractive index (WMORI).¹

The findings, published August 1, 2026, in Ophthalmic and Physiological Optics, may be relevant to clinicians interpreting small longitudinal biometric changes in myopia management and to surgeons relying on AL measurements for IOL power calculations. However, the analysis was theoretical and did not evaluate refractive outcomes in patients.

“The SRI approach provides an improvement over the WMORI in AL measurement because the influence of the VCD was eliminated. Use of a model eye parameter for [optical path-length, or] OPL conversion always causes biometric measurement errors in individual eyes due to the variations in lens refractive index. The newly developed in vivo measurement of lens refractive index needs to be applied in optical biometry to improve AL and LT measurements for individual eyes in the areas of myopia research and cataract surgery,” study author Ji C. He, PhD, stated.

Study overview

He, of the New England College of Optometry in Boston, Massachusetts, modeled biometric measurement errors while varying the crystalline lens refractive index and associated ocular dimensions from those of a 30-year-old model eye.¹ No patient cohort or clinical intervention was included.

The study examined how optical path length is converted into geometric measurements. Two conversion methods were compared. The WMORI approach applies a composite refractive index to the eye, whereas the SRI method uses different refractive indices for individual ocular segments.

The principal outcomes were modeled errors in AL and LT as the lens refractive index, LT, and vitreous chamber depth (VCD) changed.

Key findings

With WMORI conversion, simultaneous variation in lens refractive index, LT, and VCD produced AL errors ranging from an underestimation of 0.209 mm to an overestimation of 0.390 mm.¹ The direction and magnitude of error, therefore, depended on the modeled ocular anatomy rather than following a consistent offset.

VCD was identified as a major contributor to error with the WMORI method. By comparison, the SRI approach eliminated VCD’s influence in the model and reduced the AL measurement error to 0.077 mm.¹ The study reported an LT measurement error of the same magnitude as the AL error with the segmented approach.

No statistical significance testing was reported in the abstract, and safety outcomes were not applicable because the investigation did not involve human participants or an intervention.

Clinical context

Optical biometry relies on assumptions about refractive indices to convert measured optical path lengths into anatomic distances. Earlier work has shown that biometers may use specific refractive indices to derive axial dimensions, whereas subsequent studies have examined segment-based alternatives to single-index AL calculations.^2,3

Accurate AL measurement is particularly important in cataract surgery because biometry is a central input for IOL power formulas.⁴ In myopia care and research, clinicians also use repeated AL measurements to quantify ocular growth. The present model suggests that some apparent differences, especially small changes over time or differences between eyes, could partly reflect refractive-index assumptions rather than true anatomy.

The crystalline lens is not optically uniform, and its refractive properties may vary among individuals. He previously described an in vivo method for measuring the refractive index of the crystalline lens, which the current study proposes to incorporate into individualized optical biometry.⁵

Interpretation and limitations

The results favor segment-specific conversion over a weighted mean ocular index under the modeled conditions. Still, the remaining 0.077-mm error indicates that segmenting the eye does not fully resolve errors arising from individual variation in lens refractive index.

The principal limitation is the theoretical design. Results derived from a 30-year-old model eye may not generalize across ages, refractive states, cataract severities, accommodative conditions, or biometric devices. The study also did not establish whether individualized lens refractive-index measurements improve postoperative refractive accuracy or longitudinal assessment of myopia progression.

Future prospective studies should compare modeled and measured errors in diverse eyes and determine whether incorporating in vivo lens refractive index meaningfully changes clinical decisions or outcomes.

References

1. He JC. Influence of refractive index of the crystalline lens on optical biometric measurements. Ophthalmic Physiol Opt. Published online August 1, 2026. doi:10.1007/s44402-026-00146-9

2. Suheimat M, Verkicharla PK, Mallen EA, Rozema JJ, Atchison DA. Refractive indices used by the Haag-Streit Lenstar to calculate axial biometric dimensions. Ophthalmic Physiol Opt. 2015;35:90-96. doi:10.1111/opo.12182

3. Wang L, Cao D, Weikert MP, Koch DD. Calculation of axial length using a single group refractive index versus using different refractive indices for each ocular segment: theoretical study and refractive outcomes. Ophthalmology. 2019;126:663-670. doi:10.1016/j.ophtha.2018.12.046

4. Kane JX, Chang DF. Intraocular lens power formulas, biometry, and intraoperative aberrometry: a review. Ophthalmology. 2021;128:e94-e114. doi:10.1016/j.ophtha.2020.08.010

5. He JC. Refractive index measurement of the crystalline lens in vivo. Optom Vis Sci. 2023;100:823-832. doi:10.1097/OPX.0000000000002081.