January-February 2020

CATARACT

Research Highlight
Possible new parameters to improve effective lens position estimates


by Liz Hillman Editorial Co-Director

“Finding ocular parameters that help us estimate the final position of the
intraocular lens after phaco refractive surgery is fundamental in the IOL power calculation.”

—Francisco Castro-Alonso, MD

Estimated effective lens position (ELP) after IOL implantation remains a source for IOL power calculation errors because these predictions are not always accurate. There are efforts, however, to improve accuracy of this estimate, which approximates the IOL’s final position in the eye. Recent research proposes two new parameters to improve ELP estimates: the intracrystalline interphase point (ICIP) and lens equatorial plane (LEP).
Francisco Castro-Alonso, MD, said that “finding ocular parameters that help us estimate the final position of the intraocular lens after phaco refractive surgery is fundamental in the IOL power calculation.”
ICIP, defined by Dr. Castro-Alonso, is the point representing the interphase between the cortical-epinucleus complex and the nucleus.
While most current formulas estimate effective lens position through different variables, Dr. Castro-Alonso said any parameter that can improve this estimation optimizes power calculations and can improve refractive results. For Dr. Castro-Alonso, the research stemmed from an observation.
“For awhile, I was observing the final position of the lens with respect to the preoperative image of the echogram. I superimposed the echogram in pseudophakic mode (where the actual position of the lens can be seen) on the preoperative echogram and checked that the anterior side of the lens was postoperatively located in a position close to the point corresponding to the interface between the cortical-epinucleus complex and the crystalline nucleus in those patients who underwent cataract surgery,” he explained. “This point (ICIP) served as a reference to increase or decrease the power of the IOL as it was greater or lower than the average of the observed population. With this modification of the power, I obtained a better postoperative refractive result than strictly applying the SRK/T formula.”
Dr. Castro-Alonso and coinvestigators used optical low-coherence reflectometry (LENSTAR LS 900, Haag-Streit) to determine the ICIP preoperatively. IOL powers were calculated with the LENSTAR system’s EyeSuite, and implant power was selected according to SRK/T formula guidelines.1 Patients received an AcrySof IQ aspheric SN60WF
IOL (Alcon), enVista MX60 IOL (Bausch + Lomb), or CT Asphina 409M IOL (Carl Zeiss Meditec). Postoperatively, final lens position, IOL position error, postoperative anterior chamber depth, and thickness of the IOL were calculated.
In total, 192 eyes from 174 patients were included in the study. Most eyes (88.5%) were 0.5 D or less from target. Final lens position was significantly correlated with preoperative measurements of axial length, anterior chamber depth, anterior segment depth, and ICIP.
“In the current study, the potential predictive value to estimate the final position of the IOL in eyes undergoing cataract surgery was investigated for a new variable, the preoperative intracrystalline interphase point (ICIP) measured with OCLR,” Castro-Alonso et al. wrote.
The ICIP was significantly correlated with final lens position (FLP, the distance between the corneal epithelium and anterior IOL surface) and FLP2 (the distance between the corneal epithelium and IOL’s central plane)
in the three groups. FLP and FLP2 were also correlated with anterior chamber and anterior segment depths, though less so than ICIP.
Dr. Castro-Alonso noted that this work was performed in mostly medium eyes and added that the description of this new variable opens the door to new studies that assess its usefulness in short and long eyes. He said he hopes other research teams will validate ICIP with future studies. He envisioned that ICIP could be incorporated into multiple regression analysis-based formulas to estimate final lens position, which could subsequently be used in a vergence formula for calculating IOL power.
The report by Castro-Alonso et al. is not the first in recent years to put forth a new parameter to potentially improve ELP estimates. In 2018, Yoo et al. described the lens equatorial plane (LEP)—the distance from the front surface of the corneal to the crystalline lens equatorial plane—as a “promising measure” that could “serve as a new parameter to improve refractive outcomes in cataract surgery.”2 Castro-Alonso et al., comparing the mean value of ICIP (3.63 ± 0.35 mm) in their study group to the LEP described by Yoo et al., wrote that it was lower than the value obtained for LEP.
“Therefore, the point corresponding to the intracrystalline lens interphase is located anterior to the LEP,” Castro-Alonso et al. wrote.
Another study published in 2018 described how information from the 3-D shape of the crystalline lens (obtained via OCT) was incorporated into IOL power calculation formulas as a mode of improving effective lens position estimates as well.3

About the doctor

Francisco Castro-Alonso, MD
Head of ophthalmology
Hospital of Alcañiz
Alcañiz, Spain

References

1. Castro-Alonso FJ, et al. Predictive value of intracrystalline interphase point measured by optical low-coherence reflectometry for the estimation of the anatomical position of an intraocular lens after cataract surgery. J Cataract Refract Surg. 2019;45:1294–1304.
2. Yoo YS, et al. Use of the crystalline lens equatorial plane as a new parameter for predicting postoperative intraocular lens position. Am J Ophthalmol. 2019;198:17–24.
3. Martinez-Enriquez E, et al. Estimation of intraocular lens position from full crystalline lens geometry: towards a new generation of intraocular lens power calculation formulas. Sci Rep. 2018;8:9829.

Relevant disclosures

Castro-Alonso: None

Contact

Castro-Alonso:
franciscojcastroalonso@gmail.com

Possible new parameters to improve effective lens position estimates Possible new parameters to improve effective lens position estimates
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