September 2014

 

CATARACT

 

Capsulotomy centration in laser cataract surgery


by Shamik Bafna, MD

 
 

I believe we are only just beginning to refine our understanding of the factors that influence ELP, determine where and how to best center the capsulotomy, and imagine what other elements of cataract surgery we might be able to improve with the aid of the high-resolution imaging built into these femtosecond lasers.

 

Figure 1: Looking at the capsulotomy options with respect to the IOL implanted here, one can see that the scanned capsule-centered capsulotomy (purple) is better centered over the optic than the pupil-centered capsulotomy (green) would have been.

Source: Shamik Bafna, MD

Study shows that scanned-capsule centration offers the best chance of 100% capsule-optic overlap

The capsulorhexis is a critical element of cataract surgery. Not only does it set the stage for everything that follows, but it plays a key role in IOL positioning. A too-small or too-large capsular opening can potentially, over time, lead to anterior or posterior displacement of the lens optic and a change in the effective lens position (ELP). Even when there is no frank displacement of the lens, uneven capsular contraction around the lens optic can cause it to be slightly tilted, potentially inducing aberrations or reducing the quality of vision, especially with a multifocal IOL.

Warren Hill, MD, has shown that the greatest surgical influence on ELP is the capsule opening, advocating that the capsulotomy be round and slightly smaller than the optic, with 360-degree overlap of the optic edge by the capsule, in order to meet the demands of modern refractive cataract surgery.1

Femtosecond lasers offer a high degree of precision and may make it easier to create a perfect capsular opening. The first step has been to ensure that these lasers can make a complete and free-floating capsulotomy with no capsule tags. Surgeons still report difficulty with this on certain laser platforms. Published analyses have shown greater consistency in size and circularity with femtosecond laser capsulotomies.24 Nagy and colleagues reported fewer cases of incomplete capsule overlap in a group of femto patients (11%) compared to manual capsulorhexis patients (28%)3 and a lower rate of IOL decentration in the laser group.4 Indeed, the use of these lasers completely changes what is possible for us to do, raising a lot of questions about the ideal capsulotomy along the way.

Capsulotomy centration study

The femtosecond laser system that my colleagues and I use, the Catalys system (Abbott Medical Optics, AMO, Santa Ana, Calif.), allows the surgeon to choose among several methods for positioning the capsulotomy, including centering based on the pupil, the limbus, or the scanned capsule; decentering the opening using a custom setting; and maximizing it for the largest possible capsulotomy.

If the pupil center represents the center of the visual axis (an assumption that may not always be correct), then it would be ideal to center the capsulotomyand the IOLon the pupil. Pupil centration also matches how most surgeons create a capsulorhexis manually.

Scanned-capsule centration is a unique capability of the Catalys system. The laser maps the capsular surfaces based on full volume, three-dimensional optical coherence tomography (OCT) imaging of the anterior and posterior capsule. Based on these surface fits, the center of the capsule is identified and then projected onto the anterior capsular surface, and a capsulotomy of the specified diameter is automatically positioned around that centration point. Given that an IOLs spring-like haptics naturally center the implant within the anatomical dimensions of the capsule (without regard to the optical center), it makes sense to me to also create the capsulotomy in the center of the capsule, so that the optic anchored in the capsule will be similarly centered and symmetrically aligned behind that opening.

We decided to compare how 2 of these methods (pupil-centered and scanned capsule) perform for routine use. To test the 2 approaches, we randomly used one method or the other in 50 consecutive eyes. Preoperatively, the surgeon can flip back and forth to view the outlines of both the pupil-centered and scanned capsule-centered capsulotomy. Regardless of which method was used, we programmed the laser to create a 5.1-mm capsulotomy in all cases. Complete capsulotomies with no tags were achieved in all eyes. IOLs were implanted and centered to the best of our abilities in all cases.

Postoperatively, we analyzed the position of each IOL relative to the capsular opening and assessed the degree of capsular overlap. Using the raw video footage from the laser and operating room video, we determined whether the alternate method (e.g., scanned capsule in an eye with a pupil-centered capsulotomy) would have improved or worsened centration of the capsulotomy over the implanted optic. In some eyes, the 2 methods resulted in quite different locations for the capsulotomy. Looking at the capsulotomy options with respect to the implanted IOL in the case in Figure 1, one can see that the scanned capsule-centered capsulotomy (purple) is better centered over the optic than the pupil-centered capsulotomy (green) would have been. Additionally, the pupil-centered opening would not have provided 100% overlap of the optic edge.

Overall, the scanned capsule method offered the better position in 82% of eyes; 9% were in a better position with a pupil-centered capsulotomy; and there was no difference in the other 9%. 100% of the scanned capsule eyes had 360-degree optic overlap by the capsule, compared to only 78% of the eyes with pupil-centered capsulotomies. We found that scanned capsule centration usually results in a slightly more superior and nasal capsulotomy than if it were pupil centered. As a refractive surgeon, I find it interesting that this is similar to the slightly superonasal position of the undilated pupil relative to the limbus. Based on these results, my current practice is to make a 5.0-mm capsulotomy and to always choose the scanned-capsule method if the pupil is well dilated. In an eye with poor dilation, this will sometimes result in a capsulotomy plan that is too close to the iris edge to fit within the safety parameters. In such cases, I will center on the pupil instead. Further research is needed to understand the implications of this small study. I believe we are only just beginning to refine our understanding of the factors that influence ELP, determine where and how to best center the capsulotomy, and imagine what other elements of cataract surgery we might be able to improve with the aid of the high- resolution imaging built into these femtosecond lasers.

References

1. Hill WE. Effective lens position following laser anterior capsulotomy. Paper PA005, presented at the 2011 American Academy of Ophthalmology meeting, Orlando, Fla.

2. Friedman NJ et al. Femtosecond laser capsulotomy. J Cataract Refract Surg 2011;37(7):118998. Erratum in: J Cataract Refract Surg 2011;37(9):1742.

3. Nagy ZZ, Kranitz K, Takacs AI, et al. Comparison of intraocular lens decentration parameters after femtosecond and manual capsulotomies. J Refract Surg 2011;27(8):5649. 4. Kranitz 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(8):55863.

Editors note: Dr. Bafna is in practice at the Cleveland Eye Clinic in Brecksville, Ohio. He has financial interests with AMO.

Contact information

Bafna
: drbafna@clevelandeyeclinic.com

Capsulotomy centration in laser cataract surgery Capsulotomy centration in laser cataract surgery
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