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Increased vertical spacing reduces femtosecond laser capsulotomy irregularities
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Increased vertical spacing of laser pulses and shallow incision depth reduced the rate of femtosecond anterior capsulotomy irregularities, according to a study.
At the American Society of Cataract and Refractive Surgery meeting in San Diego, Wendell Scott, MD, showed that sub-1-second femtosecond laser capsulotomy with the Catalys Precision Laser System (Abbott Medical Optics) minimized the incidence of slivers, or tags, along the capsulotomy edge that can spur the formation of anterior capsule tears.
“This is where there’s a lot of interest in trying to perfect the capsulotomy and trying to avoid anterior capsule tears,” Scott said in a subsequent interview with Ocular Surgery News.
Increased vertical spacing reduced the number of laser pulses and capsulotomy treatment time, minimizing the effects of eye movement, capsule elasticity and subcapsular gas development on capsulotomy quality and integrity.
Spacing and incision depth
Scott said that standard settings used on the Catalys laser were based on test results obtained with porcine capsules, which are markedly thicker than human capsules. Standard vertical spacing for the Catalys laser is 10 µm.
“Porcine capsules are three to four times thicker than human capsules. Because of that, more than one pulse from the laser is required to cut a porcine capsule. That’s not true for the human capsule. For the human capsule, only one pulse is necessary to cut the capsule,” Scott said.
Vertical laser pulses that are too close together can lead to unnecessary laser bursts, resulting in longer capsulotomy surgery times. Vertical pulses that are too far apart can lead to gaps in treatment and miss the capsule entirely, so optimizing the vertical spacing is important, Scott said.
The shape of the laser pulse is vertically elongated, so its effect is much greater vertically than it is horizontally, according to Scott. The porcine capsule is 50 µm thick, so to cut through it cleanly, it takes more than one pulse, and these pulses, stacked one on top of the other, need to be close enough together to overlap to completely cut the capsule.
“The 10 µm spacing was developed from these studies. The human capsule is 15 µm. The vertical effect of the laser pulse is probably 30 µm to 40 µm or greater, so clearly, only one pulse is needed to cut through the human capsule,” he said. “In fact, placing the vertical pulses too close together can lead to one pulse cutting the capsule and the next pulse also having cutting force at the level of the capsule. If there is a slight horizontal shift of the capsule, such as that caused by elasticity movement, patient movement or displacement from subcapsular gas, the capsule can be hit again. This causes a radial point of weakness and may lead to a radial defect or tear.”
This has been demonstrated on electron microscopy as “aberrant laser marks” on the capsule and has been seen with all lasers, he said. However, the cause was not known.
“The vertical setting is too close and is a probable cause for these findings,” Scott said. “When the capsule is cut cleanly, it is very strong. Optimizing the settings and controlling the variables can result in perfect capsulotomies with no radial tears. This clinical research needs to be completed for each laser system.”
Patients and methods
In a retrospective study that included 2,466 femtosecond laser capsulotomies, Scott and colleagues evaluated the frequency of anterior capsule irregularities when using the Catalys laser to create an anterior capsulotomy. They also compared the effect of different vertical spacing and incision depth settings on the quality and speed of laser capsulotomy.
“This study was retrospective, and the reason it was this way is that we were using the standard settings. Then, after we had been doing that for several months, we were having these little slivers. I thought the thing wasn’t cutting well enough, so I needed to make the spacing closer. At the time this seemed logical, but I was wrong. It was exactly the opposite. I dug deeper, talked to the engineers and understood why,” Scott said.
Cases were assigned to three groups: 6-µm vertical spacing with an incision depth of 600 µm, 10-µm vertical spacing with an incision depth of 600 µm, and 15-µm vertical spacing with an incision depth of 400 µm.
The researchers then recorded rates of capsulotomy irregularities.
Results and conclusions
Sub-1-second femtosecond laser capsulotomy involved vertical spacing of 15 µm, incision depth of 400 µm, capsulotomy diameter of 4.9 mm, laser power of 4 mJ, horizontal spacing of 5 µm and total laser firing time of 0.710 second. Sub-1-second capsulotomy involved 82,068 laser bursts compared with 123,080 bursts with the standard setting (vertical spacing of 10 µm and incision depth of 600 µm).
Of 2,009 cases, 88 cases with 6-µm vertical spacing had 15 slivers (17.1%); 1,035 cases with 10-µm vertical spacing had 96 slivers (9.3%); and 886 cases with 15-µm vertical spacing had 12 slivers (1.4%).
The relative risk of developing slivers was reduced 84.9% between 10-µm vertical spacing and 15-µm vertical spacing.
Scott recommended further changes to the Catalys laser’s capsulotomy parameters.
“I think the horizontal spacing being slightly less, like 4 µm, is probably a little better,” he said. “I make the vertical spacing 15 µm and the incision depth of 500 µm, and the power is still at 4 mJ for a 4.9-mm capsulotomy.” – by Matt Hasson
- For more information:
- Wendell Scott, MD, can be reached at Mercy Eye Specialists, 1229 E. Seminole St., Suite 430, Springfield, MO 65804; email: wendell.scott@mercy.net.
Disclosure: Scott reports he is a consultant for Abbott Medical Optics.