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Information on angle alpha allows more accurate IOL predictions
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Centration of implants — particularly multifocals, extended depth-of-focus lenses or moderate to high toric models — is critical to achieve maximum optical performance. Therefore, when implanting these types of IOLs, proper centration is vital to good visual outcomes. A myriad of issues may result from decentered implants, including glare, halos and a decline of visual acuity. Toric lenses, if not aligned properly, may also result in residual astigmatism and reduced visual quality. The best way to center these IOLs remains, however, a topic of debate.
The terms angle kappa and angle alpha arise in discussions of centering IOLs. Angle kappa, more recently referred to as chord length µ, is the distance between the pupil center and the visual axis. Angle alpha is the difference between the center of the limbus and the visual axis (Figure 1).
Source: Cynthia Matossian, MD
Measuring angle alpha
Measured at the nodal point of the eye, angle alpha is the difference between the center of the limbus (optical center of the cornea) and the visual axis. When the optics of the implant and the optics of the eye align, good outcomes and happy patients are much more likely. To measure angle alpha, I utilize the OPD-Scan III integrated wavefront aberrometer (Marco/Nidek). The OPD-Scan III is an aberrometer and corneal topographer combination unit that performs the functions of an autorefractor, keratometer, pupillometer, corneal topographer and wavefront aberrometer. All of this information is gathered in approximately 15 seconds per eye. Using this information, I can assess the patient’s optical system to determine if he or she has aberrations and, if so, whether those aberrations are lenticular or corneal. This information helps me customize the IOL implant for best results based on the patient’s optical system. Postoperatively, the retroillumination image allows me to see if the implant is properly aligned (Figure 2).
The importance of angle alpha is demonstrated by a recent case in which a patient experienced glare issues. The lens (Tecnis Symfony IOL, Johnson & Johnson Vision) was perfectly centered within the pupil, but the visual center demonstrated by the bright white dot was almost over the first echelette (Figure 3).
Ideal candidates
The presurgical evaluation of angle alpha and angle kappa helps identify patients who may not be ideal candidates after implantation of an EDOF or multifocal IOL (Figure 4).
Using the OPD-Scan III, I assemble a presurgical data set that includes pupil size under different lighting situations, the presence and amount of corneal coma and spherical aberration, the magnitude and pattern of corneal astigmatism, as well as measurements for angles alpha and kappa. This information allows me to select the “best matched” lens for each patient. I can also identify factors that call for exclusion of specific lens types.
The ability to more accurately determine individual ocular parameters and more confidently predict lens alignment by including angle alpha data has helped me achieve greater satisfaction among my cataract surgery patients.
- References:
- Chang DH, et al. Am J Ophthalmol. 2014;doi:10.1016/j.ajo.2014.06.028.
- Holladay JT. Premium IOL centration and patient suitability. https://www.healio.com/ophthalmology/optics/news/print/ocular-surgery-news/%7B68300d9d-e1d9-4aca-b1eb-7fcf3f45248e%7D/premium-iol-centration-and-patient-suitability. Published July 10, 2016. Accessed May 4, 2018.
- Park CY, et al. Curr Opin Ophthalmol. 2012;doi:10.1097/ICU.0b013e3283543c41.
- For more information:
- Cynthia Matossian, MD, can be reached at email: cmatossian@matossianeye.com. Matossian thanks Lisa Baker, PhD, for editorial support in the development of this article.
Disclosure: Matossian reports she is a consultant to Marco and Johnson & Johnson.