How technology aids real-time adjustment of toric IOLs

September 1, 2014

Intraoperative measurement of astigmatism is helpful for accurate positioning of IOLs. Additionally, newly available toric lenses with aspheric optics make it possible to achieve excellent visual outcomes.

 

Take home

Intraoperative measurement of astigmatism is helpful for accurate positioning of IOLs. Additionally, newly available toric lenses with aspheric optics make it possible to achieve excellent visual outcomes.

 

 

Dr. WileyBy William F. Wiley, MD, Special to Ophthalmology Times

Brecksville, OH-Although toric IOL implantation is a growing segment of refractive cataract surgery, there are several hurdles in achieving success with these lenses.

First, the surgeon has to determine the correct magnitude and axis of astigmatism. The IOL must then be accurately positioned on the correct axis during surgery and remain there postoperatively.

More in this issue: New methodology improves accuracy for IOL power selection

Doug Koch, MD, has recently shown that posterior corneal curvature contributes about 0.5 D more against-the-rule (ATR) astigmatism than is measured on the anterior cornea, with considerable variability from one eye to the next.1

This introduces an unpredictable variable when the surgeon is basing the toric IOL power on a preoperative anterior corneal measurement. It is also not unusual for preoperative measurements of the axis to disagree. For example, manual Ks may suggest an axis of 15°, whereas keratometry may suggest it is 10° and the axis is 22° on topography.

These discrepancies are further complicated by surgically induced astigmatism (SIA). The incision at the start of surgery may have adjusted the true power or axis outside one’s expectations.

For all these reasons, intraoperative measurement of astigmatism is enormously helpful. Additionally, newly available toric lenses with aspheric optics make it possible to achieve optimal visual results.

 

NEXT: Aspheric optics and stability + Video

 

Aspheric optics and stability

I recently began implanting a certain toric lens (Tecnis Toric IOL, Abbott Medical Optics). Like other Tecnis one-piece lenses, it has a 6-mm, clear, aspheric optic that fully compensates for the average corneal spherical aberration of +0.27 µm.2,3

As seen with the multifocal version of this platform, the aspheric properties not only provide very high quality of vision, but also offer some forgiveness for minor residual error. This lens-which is available in four different cylinder powers-provides very predictable correction of both sphere and cylinder correction.

The Tecnis toric material is firmer and less “sticky” than other one-piece, acrylic toric lenses, making it easy to perform micro-rotations to adjust the positioning of the lens intraoperatively. However, once the lens is positioned, it is very stable postoperatively, meeting the tough standards set by the American National Standards Institute, which requires that ≥90% of eyes have ≤5° axis change between visits 3 months apart (Figure 1).

 

Lens selection, positioning

The IOL itself is only one component in a successful outcome. Disagreements among preoperative measurements and potential error from posterior corneal curvature or SIA can be resolved with intraoperative aberrometry.

This is particularly so with the latest iteration of this technology, which incorporates streaming refractive data (ORA System with VerifEye, WaveTec Vision). The streaming data makes it possible to visualize the dynamic state of the eye in order to tell when it is stable, adequately pressurized, and ready for measurement.

The aphakic measurement, obtained on the table in the operating room, is used to make a final determination of IOL power and axis-and even to make the final decision of whether to perform limbal relaxing incisions or to implant a toric IOL at all.

Most importantly, for toric IOLs, the streaming data also allow the surgeon to make micro-rotations of the lens after injection so that it can be positioned very precisely to neutralize astigmatism-regardless of how the magnitude or location of that astigmatism may have changed.

 

Objective correction of astigmatism

In a series of the first 36 eyes with the Tecnis toric IOLs that I implanted-all of whom had ORA-guided power selection and lens positioning-the mean cylinder was reduced from 1.36 ± 0.4 D at the aphakic ORA measurement to 0.36 ± 0.24 D at the final pseudophakic measurement on the table (Figures).

This is slightly better than our results with AcrySof toric lenses (Alcon Laboratories) and the previous iteration of the ORA system (without VerifEye).

In a series of 200 consecutive eyes analyzed with the earlier technologies, mean astigmatism was reduced from 1.74 ± 0.78 D at the aphakic intraoperative measurement to 0.50 ± 0.42 D at the pseudophakic measurement.4

Since this was not a direct comparison, it is difficult to say if the difference is indeed real, and if it was affected more by the new toric IOL or by the streaming refractive data. Both technologies are complementary and move surgeons closer to the goal of eliminating variables and maximizing success in the implantation of toric IOLs.

 

Video case

In a video case, cataract surgery is performed on the left eye of a patient with 1.36 D of ATR astigmatism at 169°, according to preoperative readings (IOLMaster, Carl Zeiss Meditec). The capsulorhexis and cataract incisions are created and pre-segmented and the lens is softened using a femtosecond laser platform (Catalys Precision Laser System, Abbott Medical Optics ([AMO]).

The capsulotomy is centered based on the “scanned capsule” function. This allows for the capsulotomy to be centered on the anatomical center of the capsular bag, resulting in near-perfect, capsule-optic overlap. Symmetric and central overlap of the capsule around the optic can provide for short- and long-term IOL stability and centration, which is critical for toric and/or multifocal IOLs.5,6

Phacoemulsification was uneventful. As the video begins, the eye is centered and inflated. The ORA aphakic screen in the video reflects the changing refraction and axis over a few seconds until it stabilizes and obtains a capture at an aphakic refraction of +10.77 +1.55 × 169. Based on this, the system recommends a 20.0-D ZCT225 IOL, which is predicted to leave the eye with a spherical equivalent of –0.08 D.

The lens is injected through a dedicated injector system (Unfolder Platinum 1 Series Implantation System, AMO), and an ocular reticle is used to center it on axis at 169°.

The IOL unfolds nicely-a little more slowly than silicone lenses, but faster than many other acrylic IOLs. When using intraoperative aberrometry, it is important to know how fast the IOL unfolds. A still-folded lens can induce inaccuracy in the reading, and it is important to be fully aware of whether the lens is in its final position.

While watching the streaming refractions after IOL insertion, micro-adjustments can be made and those adjustments are reflected on the VerifEye screen view. In this case, it was necessary to make a small, counterclockwise rotation to maximize the cylinder correction. A balanced salt solution cannula was used to adjust and re-inflate the eye. The cylinder dropped from 1 D to less than 0.25 D. VerifEye made it possible to act on real-time refractive data to fine-tune the positioning, without having to stop and perform a full capture-somewhat akin to reacting to a video image rather than taking multiple snapshots.

 

At the final pseudophakic capture, the ORA refraction is -0.49 +0.29 × 133, with a predicted spherical equivalent of –0.35 D. No further rotation is recommended.

This case illustrates how controlling the multiple sources of variability can be key to success with a toric IOL. By combining the crisp optics and rotational stability of the Tecnis toric IOL with intraoperative measurement of astigmatism for accurate positioning of the lens, confidence in the results attainable with toric IOL technology is significantly increased.

 

References

1.     Koch DD, Ali SF, Weikert MP, et al. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012;38:2080-2087.

2.     Artal P, Alcon E, Villegas E. Spherical aberration in young subjects with high visual acuity. Presented at: 2006 Congress of European Society of Cataract and Refractive Surgeons, September 2006;London.

3.     Smith G, Cox MJ, Calver R, Garner LF. The spherical aberration of the crystalline lens of the human eye. Vision Res. 2001;41:235-243.

4.     Wiley WF, Bafna S. Objective reduction of cylinder during toric IOL implantation guided by intraoperative wavefront aberrometry. Presented at: 2013 Congress of European Society of Cataract and Refractive Surgeons, October 2013;Amsterdam.

5.     Nagy ZZ, Kranitz K, Takacs AI, et al. Comparison of intraocular lens decentration parameters after femtosecond and manual capsulotomies. J Refract Surg. 2011;27:564-569.

6.     Kránitz K, Takacs A, Mihaltz K, et al. Femtosecond laser capsulotomy and manual continuous curvilinear capsulorrhexis parameters and their effects on intraocular lens centration. J Refract Surg. 2011;27:558-563.

 

William F. Wiley, MD, is in practice at the Cleveland Eye Clinic, Brecksville, OH. Dr. Wiley is a consultant for Abbott Medical Optics and WaveTec Vision. Readers may contact him at drwiley@clevelandeyeclinic.com or 440/922-6722.