Magnitude, axis of astigmatism vital to toric IOL alignment, but no consensus on best measure

Take-home message: Though it is generally agreed that accurate identification of the magnitude and axis of astigmatism is essential in order to choose the right toric power and correctly position it, there is no strong consensus about which measurement is best to base magnitude and axis decisions. Results from a recent clinical survey support this trend.

By John A. Vukich, MD, Special to Ophthalmology Times

Madison, WI-When it comes to addressing astigmatism in patients with cataracts with ≥1.25 D of astigmatism, implanting a toric IOL is the most common approach.

Consider that toric IOLs were implanted in 19% of cataract surgery procedures,¹ according to the 2014 American Society of Cataract and Refractive Surgery Clinical Trends Survey-to which more than 1,500 surgeons responded.

In addition, with several new toric IOLs in the marketplace, this category has been growing rapidly and has much room for expansion-given that about 37% of the population has >1 D of astigmatism.

In this surgeon’s hands, toric lenses are more predictable and more consistent at achieving the desired results than incisional correction at 1 D or greater of astigmatism. At lower levels (1 to 1.5 D), it is a reasonable decision to sacrifice some of the predictability of a toric IOL for the convenience and cost-effectiveness of incisional correction, especially if the patient is interested in spectacle independence at near.

Good candidates for a toric IOL include those patients who desire good uncorrected distance vision and are comfortable with wearing spectacles for near. Topography is essential to determining that astigmatism is regular and stable and there is no significant corneal disease affecting the ocular surface or shape.

It is important to ensure that the best astigmatic correction of an irregular cornea-i.e., one with pellucid marginal degeneration, keratoconus, or forme fruste keratoconus-is not substituted for the true axis of astigmatism.

Among good candidates, there is potential for error in selecting the axis or magnitude of astigmatism and in correctly aligning the IOL in the eye. For every degree the lens is rotated off-axis, there is a 3.3% loss of effectiveness of the toric IOL power.²

For best results, the IOL should be within 5˚ of the intended axis, especially when implanting higher-power toric lenses. Despite this, survey respondents indicated that an average 7.2˚ of rotational error would be acceptable, with about one-third of respondents being comfortable with rotational error of 10˚ or more.¹

Determining magnitude, axis

Accurate identification of the magnitude and axis of astigmatism is essential in order to choose the right toric power and correctly position it.

However, there is no strong consensus about which measurement is the best one on which to base magnitude and axis decisions.

In the survey, IOL Master Ks were most commonly used to determine magnitude, followed by topography and manual Ks. Topography is most heavily relied upon for axis determination, with IOL Master and Lenstar following. Other methods- including autokeratometry and intraoperative aberrometry-are also used.

Though these are all good methods, ideally, one should have at least two different measures from two different devices. Because each device measures slightly differently, the Ks are unlikely to be identical, but close agreement should be sought and significant differences among measurements should be reconciled to avoid power calculation or axis errors.

In selecting the best lens power, it is perfectly acceptable to flip the axis, even though nearly half of surgeons say they never do this.¹ The conventional wisdom not to flip the axis is based on incisional techniques, when an axis shift could not be reasonably controlled and might result in a large and poorly tolerated error in the opposite axis.

However, with an optical lens solution that does not involve relaxing the cornea, surgeons have more control. If the choice were between leaving 0.50 D of residual error at the original axis or 0.10 D at the opposite axis, it would be preferable to flip the axis. The amount of residual error is so small that it will be easily tolerated.

Marking the axis

Survey results demonstrate there is also considerable variation in how surgeons mark and align toric IOLs.¹

Thirty-seven percent say they rely on anatomical landmarks without marking the cornea or only mark the major axes preoperatively without using any additional axial marking tools during surgery (Figure 1). For accurate placement, the latter is required, at a minimum.

Ideally, the principal meridians should be marked preoperatively with the patient in a seated position, and the axis marked intraoperatively with the aid of an axial tool. A fine, durable mark that does not wear off or subtend more than a few degrees should be made.

The initial meridian marks can be used as a point of reference intraoperatively, with the patient lying down, to accurately mark the axis of astigmatism even if some cyclorotation has occurred. A Mendez ring or other axial ink marker (one that leaves a fine, durable mark) will be more accurate than surgeons’ estimation of the axis in relation to the principal meridians.

Other marking techniques have been developed and shown to be quite accurate. For example, Robert Osher, MD, has described a thermal device that leaves a fine mark. Others have used a Nd:YAG laser to pit the epithelium at the limbus or a sterile needle to create a linear superficial abrasion that can be stained with fluorescein or another dye to highlight the mark.

Advanced devices-such as intraoperative aberrometry or surgical guidance systems that incorporate digital registration-are not necessary for implantation of toric IOLs, but they do add greater precision for surgeons who want to maximize outcomes.

Aligning the lens

For implantation of the IOL, it is important to choose a rotationally stable toric lens. Unless a very high-powered toric lens is needed, this surgeon’s personal preference is (Tecnis Toric, Abbott Medical Optics). The platform offers predictable, high-quality performance in general, including excellent stability. In clinical studies, the mean change in axis between baseline and 6 months was just 2.74°, indicating good lens stability within the capsular bag.³

Ninety-three percent of eyes with this lens implanted had a change in axis of ≤5° between two consecutive visits about 3 months apart, meeting ANSI criteria for toric lens stability.³ This translates into excellent visual acuity results (Figure 2).

Surgical technique plays a role in the stability and performance of toric IOLs as well. I prefer to use a cohesive viscoelastic. It is important not to over-pressurize the eye. The lens can be injected counterclockwise of its final position and then dialed into correct alignment with the axis marks. Once aligned, I push the IOL gently against the posterior capsule.

Finally, all of the ophthalmic viscosurgical device must be carefully removed from behind the lens.

With this approach of careful preoperative measurement and marking, intraoperative marking and alignment, and good surgical technique, surgeons can achieve success with toric IOLs to the benefit of their practices and their patients with astigmatism.

References

1.     ASCRS Clinical Trends Survey 2014. Survey overview available online at: http://eyeworld.org/supplements/2014_ASCRS_clinical_survey.pdf

2.     Ma JJ, Tseng SS. Effects of steep meridian incision on corneal astigmatism in phacoemulsification cataract surgery. J Cataract Refract Surg. 2012;38:666-671.

3.     TECNIS Toric 1-Piece IOL [package insert]. Santa Ana, CA: Abbott Medical Optics Inc.

John A. Vukich, MD

E: javukich@gmail.com

Dr. Vukich is surgical director, Davis Duehr Dean Center for Refractive Surgery, Madison, WI. He is a consultant to Abbott Medical Optics.