IOLs after LASIK
Formula incorporating posterior cornea data improves cataract visual outcomes
For patients with previous corneal refractive surgery, an intraocular lens (IOL) power calculation formula that incorporates both posterior and anterior cornea measurements can significantly improve cataract surgery visual outcomes over conventional formulae, Graham D Barrett MD told the 2019 American Society of Cataract and Refractive Surgery/American Society of Ophthalmic Administrators Annual Meeting in San Diego, USA.
Posterior corneal measurements obtained using swept-source OCT or Scheimpflug cameras can be added as an option when using the online Barrett True K Formula for prior myopic or hyperopic LASIK/PRK/RK, Dr Barrett said. This is important because traditional keratometry measures only the anterior cornea and accounts for posterior cornea power based on a ratio of the radii of the two surfaces. However, LASIK and other corneal surgeries change this ratio unpredictably, complicating IOL power calculations, Dr Barrett explained.
In a multi-centre retrospective analysis of 60 eyes with previous myopic LASIK that underwent cataract surgery, 70% ended up with refractions within 0.5 dioptre of the spherical power predicted by the Barrett True K TK formula, which incorporates posterior corneal data. This is similar to outcomes in virgin eyes using conventional formulae, and significantly better than the 63.3% recorded in this study for the standard Barrett True K formula that uses a theoretical model to predict posterior lens power (p=0.02).
Other formulae in current use were even less accurate in this challenging post-refractive population, with the Haigis TK at 50.0% (p=0.01), Shammas at 46.7% and Haigis L at 31.7% within 0.5 dioptre (both p=0.0001) of predicted spherical target.
POSTERIOR CORNEAL ASTIGMATISM
Measuring the posterior cornea may also help reduce residual astigmatic error in patients with astigmatism, Dr Barrett said. In a retrospective analysis of 16 eyes, of which two received non-toric IOLs and 14 Alcon AcrySof SN6AT toric IOLs, 75% of toric outcomes were within 0.5 dioptre of target using the Barrett True K Toric with measured posterior cornea astigmatism (PCA), compared with 68.8% for the Barrett formula using predicted PCA.
The Holladay toric calculator without PCA and the Holladay with AK regression analysis achieved 62.5% and 56.3% respectively. However, these differences did not reach statistical significance, possibly due to the small sample size.
“In this most challenging patient subset, post-refractive patients with residual astigmatism, custom posterior corneal data seems to be of some benefit,” Dr Barrett said.
He believes incorporating posterior cornea measures into power calculations can improve visual outcomes for all kinds of cataract surgery.