IOL Power Calculations & Post-Myopic LASIK Eyes
It is very well known that cataract surgery includes elements of refractive and presbyopic surgery in addition to the removal of the diseased tissue. The implantation of an intraocular lens (IOL) in conjunction with the appropriate power calculation improves patient satisfaction levels and leads to successful outcomes.
In light of the best results and accuracy, every component of the calculation process must be optimized to ensure selection of the correct lens power. IOL power calculation formulas are based on accurate preoperative measurements including axial length, central corneal power, anterior chamber depth, and the individually optimized formula constant of the IOL.
The most important aspect associated with the reduction of the refractive prediction error is the accurate prediction of postoperative optical biometry based on preoperative data.
Biometry is the method of applying mathematics to biology. The term was originally used by Whewell in the 1800s for calculating life expectancy. The refractive power of the eye primarily depends upon the cornea, the lens, ocular media, and the axial length of the eye.
When planning for cataract surgery, in order to achieve the desired post-operative refraction, the required power of the intraocular lens (IOL) implant can be calculated if the corneal refractive power, media type, and axial length are known.
Novel high-resolution imaging-based IOL power calculation formulas that take into account total corneal power measurements and the differential effect of corneal laser refractive surgery on anterior and posterior corneal curvature aim to improve refractive accuracy in eyes with a history of LASI or PRK.
The Latest Research
However, findings from a retrospective clinical cohort study of 170 postmyopic LASIK eyes showed that, when comparing IOL power predictive accuracy, the Pentacam Scheimpflug camera-based Potvin-Hill and spectral-domain optical coherence tomography (OCT)–based True-K formulas did not necessarily outperform other select formulas available on the American Society of Cataract and Refractive Surgery (ASCRS) IOL calculator.
The research was conducted by Neeraj S. Chawla, BS, a medical student at the University of Illinois at Chicago, and Navaneet S. Borisuth, MD, PhD, who is in private practice at Virdi Eye Clinic, with 3 locations in Illinois and Iowa.
Chawla presented the findings at the 2020 ASCRS virtual annual meeting. “Potential limitations of our study included a lack of complete historical LASIK data, as well as sample size,” he said. “Therefore, we believe future studies should continue to explore the Scheimpflug camera-based and OCT-based formulas using larger patient populations [and] tighter experimental conditions, as well as various subgroup analyses.”
Borisuth noted that further investigation may determine the degree and specific conditions in which these imaging-based formulas will improve patient outcomes from cataract surgery. “The Potvin-Hill and OCT True-K formulas confer a theoretical advantage based on their advanced imaging techniques that incorporate total corneal power,” he said.
“For example, the Pentacam rotating Scheimpflug camera may be specifically useful when planning for astigmatic correction in postmyopic LASIK eyes.” Patients included in the study were operated on by Borisuth, with consideration of post-myopic LASIK eyes receiving monofocal, toric, or extended-depth of focus IOL implants.
Eyes with concurrent corneal or retinal pathology were excluded from the study. Postoperatively, 81.6% and 91.1% of eyes fell within ± 0.5 D and ± 1.0 D of target predicted refraction, respectively, therefore indicating “generally favorable” surgical outcomes.
IOL power formula predictive accuracy was determined using postoperative refractive data to back-calculate optimum IOL powers and absolute prediction errors. The formulas analyzed included the Scheimpflug camera-based Potvin-Hill and spectral-domain OCTbased True-K, as well as other conventionally available formulas of the ASCRS IOL calculator.
These included the Barrett True-K, Barrett True-K no-history, Masket, Modified Masket, Shammas, Haigis-L, and ASCRS Average formulas. Mean absolute prediction error ranged from a low of 0.40 ± 0.33 D for the Barrett True-K formula to a high of 0.67 ± 0.51 D for the Shammas formula.
Statistical analyses revealed that the Barrett True-K, ASCRS Average, and Masket formulas were significantly more accurate than the OCT True-K and Shammas formulas only.
No other statistically significant differences were derived. Additional subanalyses assessed directional relationships between predictive error of the imagingbased formulas with axial length and optimum IOL power.
These analyses found that the accuracy of the Potvin-Hill formula improved with increasing axial length and decreasing optimum IOL power, whereas the OCT True-K formula was most accurate for midrange axial length and midrange optimum IOL power. “The Potvin-Hill formula typically underpredicted optimum IOL power, [whereas] the OCT-based formula did not show directionality,” Chawla said.
