Incisional techniques in FS laser-assisted cataract surgery

Femtosecond incisions deliver advantages, but surgeons should expect a learning curve.

By Lisa B. Arbisser, MD

Cataract surgery incision size, style, location and mode have evolved for centuries. In my 35 years in ophthalmology I have fashioned 15-mm limbal intracapsular, 10-mm limbal extracapsular and 5-mm scleral tunnel incisions, as well as ever-shrinking clear-corneal incisions — first 3.2, then on to 1.8 mm. I “smiled,” “frowned,” funneled, grooved and hinged. I’ve cauterized, tickled and subsequently avoided vessels. I experimented with superior to on-axis and finally moved to temporal incisions.

The long journey reminds me of the children of Israel wandering in the desert for 40 years after leaving bondage. A single-plane parabolic clear corneal incision (Fine) of 2.2 mm as square as possible (Ernest) with a Triamond diamond trapezoidal blade (Mastel) served me well now for years. I began with silk sutures, progressed to nylon to sutureless incisions, to absorbable sutures and returned to self-sealing. I’ve witnessed the first FDA-approved sealant for cataract incisions come to market. I employed metal, ruby and diamond scalpels. The parameters were few.

When femtosecond (FS) laser-assisted cataract surgery arrived, myriad ways to give cataract patients clear vision arrived — but with a few caveats. While FS delivers significant, exciting advantages in incision creation, it also presents new challenges that require a careful, researched approach.

FEMTOSECOND LASER INCISIONS

How they work

We have four FDA-approved systems in the United States: LenSx (Alcon), Catalys (AMO), LensAR (LensAR) and Victus (B&L). All FS laser technology systems cut with photodisruption: optical breakdown occurs due to generation of a microplasma as well as cavitation bubble expansion. Current technologies use neodymium: glass 1053 μm (near-infrared) wavelength light, focused at a 3-μm spot size accurate to within 5 μm in the anterior segment. The quadrillionth-of-a-second focused pulse reduces collateral damage but this technology minimizes thermal damage to surrounding tissue compared to longer-duration laser frequencies.

Corneal incisions, whether manual- or laser-created, elicit a healing response. In vitro studies of incised cornea with the LenSx show a more saw-toothed pattern at the cut edge than with manually cut incisions.¹ Research shows no statistically significant difference in inflammatory cell response, but apoptotic cell death is more up-regulated in FS cut tissue.² The significance of these findings is not entirely obvious, though the need for optimization of parameters is.

Incision pointers

Since many rows of laser pulses are placed close to each other to create a continuous cut, the surgeon can alter many parameters to achieve a desired result. Energy of laser pulses, diameter of laser spots, distance between adjacent laser spots and distance between rows (line-spot separation) determine if continuous tissue dissection planes will be created.

For example, to create one FS laser clear corneal incision (CCI), the surgeon could specify numerous parameters in addition to determining the width, length and location: any uncut region, the anterior and posterior plane depth, the anterior and posterior side cut angle, anterior, posterior and central line density, anterior and posterior line distance, the vertical spot spacing and pulse energy. Despite the complexity, however, reproducible and standardized incisions of virtually any shape are now within our grasp.

By the same token, it is hard to interpret and compare FS studies with different parameters. Most surgeons today cut a three-plane incision for clear corneal cataract surgery and create paired arcuates for astigmatism correction. They can adjust line density parameters, for example, to allow titration of astigmatism incisions. Patients can be moved from laser to operating suite because CCIs do not open spontaneously. Under the microscope they require the surgeon to find the plane with a spatula or hook and bluntly dissect to complete the opening into the anterior chamber. Standard settings are machine specific and ever evolving.

The OCT factor

What is truly remarkable about femtosecond laser technology is that most systems are OCT guided. As a result, in-situ, almost real time, tissue specific, reproducible incisions of any complexity can be programmed at precise depths. This is especially important for the creation of refractive corneal incisions.

Evidence for superiority

In vivo studies show greater architectural stability and reproducibility with FS incisions compared to metal keratome-made incisions in cadaver eyes without discrete opacity.³ In vivo routine human cataract surgery using default parameters for FS laser-assisted clear corneal incisions have significantly lower endothelial gaping, endothelial misalignment, Descemet’s membrane detachment, and posterior wound retraction than metal keratome-created CCIs.

All incisions in one study were predictably within 10% of the intended length, depth and angle measurements. All incisions were water tight with counter pressure.¹ My experience with the Catalys laser showed more tendencies for unintentional intraoperative stromal hydration and a more visible healing line at the internal Descemet’s entrance visible at the slit lamp postoperatively compared to diamond scalpel incisions ( click here to view video 1). I experienced no postoperative wound leaks with FS technology.

AVOIDING COMPLICATIONS

Influences on incision design

Subtle variance in tissue lamellae and photo-disrupted tissue behavior affects laser incision design. For example, the surgeon may need to modify parameters for a superior vs. inferior femtosecond paracentesis. The consistency of tissue sitting in the tear film vs. that under the upper lid differs. Though subtle, these differences may require different energy parameters to achieve the same result.

Many surgeons at this stage of development still create a manual paracentesis even when using laser for arcuate and CCIs. They note small paracenteses may be difficult to find, open or seal. The surgeon must fashion FS incisions slightly larger than keratome incisions for the same size phaco tips or insertion cartridges; the tissue edges seem less resilient and, like all incisions, are most secure when not stretched.

Potential problems

The FS laser cannot cut opaque tissue. When too peripheral, the scleral edge may be difficult to detect and causes an imperforate wound. Opacity negatively affects OCT imaging and creates scatter of the laser energy. Rarely, gas can track along tissue planes or even vertically break through epithelium or endothelium. Abnormalities in Bowman’s membrane can potentially cause unwanted migration of bubbles to the surface.

Intraoperative view of femtolaser-created CCI and astigmatic keratotomy incisions with capsulotomy and lens fragmentation

Planned CCI, paracentesis and astigmatic keratotomy incisions on the Catalys (AMO) femtosecond laser.

False tissue planes can develop if the surgeon does not properly use the laser to create the incision or if the surgeon begins to dissect into the incision to open it. Surgeons should perform FS laser CCI creation with caution or avoid it entirely in patients with corneal scars, dense arcus senilis, dystrophies and previous corneal surgery such as LASIK, RK or CK.

Microscopic fixational eye movements can cause skip lesions as well as regular lines of aberrant misfired pits and increase the risk for vertical gas breakthrough. Some surgeons cover the fellow eye; others prefer to give the patient a fellow eye fixation target to minimize movement despite adequate docking and suction. To date, none of the systems have real-time adjustment or eye tracking.

An air bubble in the anterior chamber can be evidence of an unintended full thickness arcuate incision. If an epithelial break is noted during the CCI creation or after the FS laser part of the procedure, the surgeon should use a keratome to create an alternate incision and place a suture in the false femto incision to ensure that the area of breakthrough is closed.⁴

ASTIGMATISM TREATMENT

Arcuate incisions

FS-assisted astigmatic keratotomy incisions are potentially far more reproducible and predictable than manual incisions. OCT imaging just before laser application facilitates a consistent depth and angle at the chosen location. Paired incisions are precisely alike. With available options of limbal or pupil centration, the surgeon can achieve an exact optical zone (OZ).

I have found that arcuate incisions at 9 mm, OZ centered on the limbus at 85% depth are both predictable and effective. It is still necessary to mark the 90-degree and steep axis by standard means. Surgeons should use nomograms and enter their predictive calculations into the laser data base plan preoperatively (Figure 1). The surgeon can best view marks under the infrared camera when using a black marker rather than the typical violet marker.

Figure 1. LenSx FS procedure with planned incisions.

We await a laser-specific nomogram as currently most surgeons adapt manual limbal-relaxing incision nomograms such as the Donnenfeld and NAPA by reducing their magnitudes by 30% and 20%, respectively. Naturally the number of incisions, incision length, patient age and gender still influence the amount of astigmatism reduction achieved, but the consistent depth and uniformity of the cut improve affectivity.

Theoretically, it is patient movement that leads to rare complications, including unintended full thickness perforation or imperforate incisions. I find that the vast majority are, as advertised, for both depth and placement. I prefer to open them with a blunt spatula or hook at the end of my cataract procedure since they often open spontaneously, at least in part not requiring much instrumentation at this stage. I have intraoperative keratoscopy to confirm their effect. (See Figures on page 32, and accompanying video at ophthalmologymanagement.com). Some surgeons titrate the incisions’ effect by opening them in the postoperative period.

Intrastromal incisions

Intrastromal incisions, unique to FS laser technology, are under investigation. The goal is to leave Bowman’s membrane, epithelium and endothelium intact. As no external opening exists, the chance of incision infection is nil. Research shows intrastromal incisions are feasible, stable and reproducible in one paper with an N of 16.⁴ One case of subsequent LASIK inadvertently crossing an intrastromal incision and violating Bowman’s caused a sudden, gross change in the magnitude and axis of astigmatism.⁵

Surgeons should enhance with caution since these intrastromal incisions can be invisible postoperatively.⁶ Another reason for caution: In one case, perforation occurred post DSEK; donor lenticules lack any limbal attachment and should not have been included in the thickness calculation.⁷ Who knew?...

Conclusion

FS laser-assisted cataract surgery is still in its infancy and slowly growing in adoption worldwide. The future is exciting.

Not that cataract surgeons have everything on our wish lists. We have yet to see technology that finds the steep axis on a real-time basis, calculates the appropriate nomogram and automatically inputs it. Nor do we have the means of tracking the eye adjusting for micro-saccades to increase safety and accuracy by prompting an immediate adjustment or pause in the procedure. No doubt, however, this level of integration of diagnostic and treatment modalities will evolve.

What we do have now in FS technology, though, is the capacity to customize and standardize the size and shape of capsulotomy. Ultimately this will influence implant design. Further, laser disruption of the nucleus reduces or eliminates ultrasound for lens removal, theoretically protecting endothelial health. FS technology may lead to a future where posterior capsule management will change dramatically, eliminating the need for a secondary YAG laser capsulotomy procedure.

With regard to incision architecture, standard settings are machine-specific and ever evolving. Incisions made by any means require meticulous assessment for wound sealing, but burgeoning evidence shows improved refractive outcome and wound integrity with the FS laser. Unlike Moses, I fortunately entered the Promised Land of FS; however, the next generation will settle and develop that land. OM

REFERENCES

About the Author
	Lisa B. Arbisser, MD, is an anterior segment surgeon. She is an adjuct associate professor at the Moran Eye center, Univ of Utah, a co-founder of Eye Surgeons Asociates, Bettendorf Iowa where she is now emeritus and former president of the American College of Eye Surgeons Contact her at: drlisa@arbisser.com or 563-343-8896.