The golden mean of capsulotomy

Finding a balance between precision and cost.

By Mark Packer, MD, FACS, CPI

The capsulorhexis can be the most challenging and the most critical step in phacoemulsification.^1,2 A perfectly constructed capsulorhexis provides the foundation for lens extraction and stable in-the-bag IOL fixation, while an errant capsulorhexis, anterior capsule tear or wrap-around tear compounds the difficulty of each subsequent step of the procedure. Anterior capsule tears are particularly significant because they may extend to the equator and beyond. Anterior tears occur on average in 2.3% of manual capsulorhexes,³ with a range as high as 5.1% among trainees.⁴

The ApertureCTC continuous thermal element allows uniform contact with the anterior capsule, and the incision is completed in milliseconds under viscoelastic protection. As the ring is retrieved, it automatically captures and removes the circular capsulotomy button through a 2.2-mm incision.

Overview

An appropriately sized capsulorhexis allows the anterior capsule to completely overlap the optic of the IOL, reducing the incidence of posterior capsular opacification (PCO). As Peng et al. wrote over a decade ago in their classic study of posterior capsule opacification, “Our histopathological observations suggest that creating a CCC [continuous curvilinear capsulorhexis] with a diameter slightly smaller than that of the IOL optic allows the capsule edge to adhere to the anterior surface of the optic, enhancing the efficiency of the barrier effect by creating a closed system.”⁵ However, to make sure that the anterior capsule overlaps the IOL edge in a complete 360-degree fashion after implantation, the surgeon must ensure that a manually constructed capsulorhexis has an average diameter somewhat smaller than ideal, because of unavoidable variation in centration, diameter and circularity.⁶ In real-world clinical practice, where a range of surgical expertise is found, Findl et al. reported 18% of cases with “no rhexis overlap with the IOL.”⁷ While some cases may not create later problems for patients and surgeons, a proportion of them will result in fusion of the anterior and posterior capsular leaflets, with ensuing fibrosis of the bag and IOL subluxation or tilt. YAG capsulotomy may provide a solution in some cases; however, IOL repositioning or exchange can become necessary if warranted by patients’ complaints. These procedures can yield successful outcomes, even years after the original surgery; however, as is true in general, prevention is better than cure.

Thickness

Histology shows that the anterior human lens capsule exhibits a typical variation in thickness with its radius. Barraquer et al. measured the thickness of ex-vivo human lens capsules using digital micrographs. They discovered that the mean capsular thickness increased with age from 11 µm to 15 µm at the anterior pole and from 13.5 µm to 16 µm at the anterior mid-periphery.⁸ Also, they found a local thinning at the pre-equatorial zone, which changed very little with age — the equatorial thickness remained constant at 7 µm. At the posterior periphery, thickness decreased with age from 9 µm to 4 µm. There was no change in thickness at the posterior pole (overall mean of 3.5 µm). As we learned in residency, the capsule is thinnest where it matters most!

This variation in thickness is thought to derive in part from the anchoring of zonular fibers in the peripheral capsule and from development of the capsule for the role it plays in the mechanism of accommodation. Based on these findings, the peak thickness generally occurs at a diameter of 4.9 to 5.5 mm, centered on the anterior pole of the lens. These histological studies therefore suggest that a capsulotomy of 5.25 mm, centered on the anterior pole of the lens capsule, is most likely to intersect the anterior capsule at its thickest (and therefore strongest) point.

Taking into account both prevention of PCO and related IOL complications, and maximizing capsulotomy strength, a diameter of 5.25 mm with centration on the lens apex best serves both purposes. Realistically, surgical manipulation and extraction of lens material, regardless of whether fragmented by a laser or sculpted and chopped manually, is easier through a larger capsulotomy. A wider opening decreases the likelihood that the phaco needle, aspiration port or a second instrument will contact the anterior capsule rim and result in a tear. All these considerations suggest that the ideal capsulotomy diameter is at least greater than 5 mm.

Femto’s role

Centration and circularity of anterior capsulotomies that allowed for reproducible construction of a 5.25-mm diameter while still ensuring 360-degree overlap of the IOL optic became a reality with the advent of femtosecond laser precision. However, beyond the safety implications of basic sizing and location, evidence from the femtosecond laser literature shows that a perfectly centered, round capsulotomy results in less IOL tilt and decentration, as well as a more predictable effective lens position, leading to more accurate refractive results.⁹ While patients today may opt for laser surgery in conjunction with premium IOL upgrades, they need a simpler, safer, cost-effective, predictable solution that allows surgeons to construct the ideal capsulotomy every time.

Emerging, cost-effective technology

One new technology offering a capsulotomy solution is CAPSULaser’s capsulotomy laser, which attaches beneath the operating microscope. Once a routine paracentesis incision is made, the capsule, stained with trypan blue, creates a photosensitive target. Placement of a surgical contact lens stabilizes the eye. Then, in approximately three seconds’ time, the laser makes a continuous curvilinear capsulotomy by being continuously rotated in a circular pattern. The molecular basis of the incision is a phase change, converting type 4 collagen into amorphous collagen.

Preclinical studies have demonstrated consistent sizing, with a standard deviation of 27 μm in the diameter of the capsulotomy, which can be varied from 4.5 to 7 mm. Initial clinical results were presented at the XXXIII Congress of the ESCRS in Barcelona.¹⁰ In those results, complete circular capsulotomies were completed in 10 eyes. No tears or tags occurred at the capsulotomy edge. At one month postoperatively, eight of 10 eyes achieved visual acuity of 20/20 or better. All IOLs were well centered with no adverse events reported.

Another entrant into the field of cost-effective capsulotomy, Zepto (Mynosys Cellular Devices), consists of a disposable hand piece and capsulotomy tip that connects to a control console. The tip includes a nitinol ring and surrounding silicone suction tube that delivers direct current when placed on the anterior capsule to create the incision, also known as precision pulse capsulotomy (PPC). Preclinical study authors demonstrated that the system produces consistent, round anterior capsulotomies.¹¹ “Overall, Miyake-Apple imaging showed that the suction activated PPC device seemed to stabilize the lens during performance of the capsulotomy.” The device was inserted and removed through a 3.0-mm corneal incision.

A strain gauge study in paired human cadaver eyes comparing the strength of the capsulotomy to femtosecond laser capsulotomy (FSLC) and manual capsulorhexis concluded that Zepto demonstrated the most strength of the three. The authors reported that, “In the first study arm PPC capsulotomies were substantially stronger than FSLC in the fellow eye by an average of 3.1-fold. The PPC capsulotomies were also consistently and substantially stronger than manual CCC on average by 4.1-fold. No significant difference was found between the tear strength of manual CCC and FSLC.”¹² Of note, imaging demonstrates that the cut-edge of the capsulotomy produced by PPC curls up and over towards the periphery, presenting a smooth, uncut surface to the interior of the capsulotomy opening, which may contribute to its strength.

Continuous thermal capsulotomy (CTC) offers another solution for consistent capsulotomy construction. With ApertureCTC (International Biomedical Devices), thermal generation powers a microincision-compatible ring, which extends from an ergonomic hand piece. The continuous 360-degree thermal element allows uniform contact with the anterior capsule without the need for vacuum suction, and the incision is completed in milliseconds under viscoelastic protection. As the ring is retrieved, it automatically captures and removes the circular capsulotomy button through a 2.2-mm incision. The tip is disposed of after the case and the reusable hand piece is sterilized in standard fashion.

CTC creates the same type of upturned capsulotomy edge seen with Zepto, and an initial study shows a similarly increased resistance to tears. Currently in preclinical development, CTC is expected to initiate clinical trials later this year.

Conclusion

The goal of these innovative technologies is to ensure safer, more effective cataract surgeries and provide more predictable outcomes for all patients, without incurring the expense of femtosecond laser technology. While the advantages of laser surgery go well beyond the capsulotomy, extending to arcuate incisions for the correction of astigmatism, improved construction of clear corneal incisions and reduced morbidity related to high levels of ultrasound energy required by standard phacoemulsification, these innovations have, for the vast majority, been reserved for patients electing refractive cataract surgery. The new generation of cost-effective capsulotomy devices strives to bridge the gap between accuracy and affordability. By simplifying the current manual procedure, and avoiding the expenses of a femtosecond laser, a broader range of surgeons throughout the world can achieve surgical incisions with precise placement and a high degree of consistency for all patients. OM

REFERENCES

About the Author
	Dr. Packer is president of Mark Packer MD Consulting. [Relevant financial disclosures: Dr. Packer is a consultant to Alcon (Novartis AG), Bausch & Lomb (Valeant Pharmaceuticals International), International Biomedical Devices and Lensar (Alphaeon)].