FEMTOSECOND CATARACT SURGERY

Why do I need another €500,000 femtosecond laser for cataract surgery when I already have one for LASIK flaps?†It’s a question physicist Holger Lubatschowski PhD hears often from ophthalmic surgeons.

The answer stems from different and somewhat incompatible technical requirements for corneal procedures compared with those deeper in the eye, said Dr Lubatschowski, founder and CEO of Rowiak GmbH, Hannover, Germany, and consultant to Ziemer Group, Port, Switzerland.

“In principle we could combine them doing their jobs rather poorly. But with new systems and designs we could do both very well,†he told the XXIX Congress of the ESCRS.

Precision vs. volume

Current corneal and cataract lasers share many operating principles and parameters, Dr Lubatschowski noted. Both rely on photodisruption to cleave tissue without thermal damage to surrounding tissues. Both use lasers of about 1,000 nm wavelength because they are economical to produce and penetrate clearer ocular tissues well. And both are capable of pulse duration in the femtosecond range with nanojoule to microjoule energy.

Corneal and cataract laser systems are also similar in patient application. Both require fixation of the eye, and the patient is either moved in and out on a movable bed, or the laser mirror arm is moved to the patients’ eye.

“The applications are not only similar, they are identical,†Dr Lubatschowski said.

Where they differ is in depth and ablation precision requirements. For corneal flaps, intrastromal refractive procedures or lamellar transplant cuts, the laser must operate over an area of up to 10mm wide, ablating tissue in essentially a single planar cut within less than 1.0mm of the corneal surface. The laser cut must be very smooth to avoid inducing “rainbows†or other corneal aberrations, particularly after refractive surgery.

Cataract surgery, on the other hand, requires multiple ablation planes to disrupt a much larger tissue volume, measuring about 7.0mm diameter by 4.0mm deep and located about 5.0mm to 10.0mm below the corneal surface. The cuts do not have to be as smooth since the ablated lens tissue is removed, but the cutting must be quick.

Achieving these divergent clinical objectives requires lasers of different power and, perhaps more important, different numerical apertures result in different cavitation profiles in tissue, Dr Lubatschowski said. Numerical aperture is a function of the width and focal length of the laser beam. Cavitation is a function of numerical aperture and laser energy.

For a given focal length, a wider lens creates a higher numerical aperture, which results in a wide cone of energy producing a very small focal spot. A numerical aperture of around 0.2 is typically used in corneal lasers. This creates a wide cone of energy focusing into a very small focal spot. With low pulse energy, say some hundreds of nanojoules in a 200 femtosecond pulse, this creates a nearly spherical cavitation bubble in the 10 micron range that does not disrupt surrounding tissue.

This level of precision is required for corneal surgery because a three-micron irregularity produces about one micron of wavefront error, but it cannot be achieved in the crystalline lens, Dr Lubatschowski said.

“For corneal applications we have a high focus quality, but because of the high aperture we have more aberrations when there are more refractive surfaces, which limits focus quality deeper in the eye. Moreover, we have scattering loss in the crystalline lens. Both aberrations and scattering loss inhibit the laserpulse to reach the threshold for disruption.â€

By contrast, a numerical aperture closer to 0.1 is typical in cataract lasers. This creates a narrower cone of energy that produces a more elongated focal spot. At higher energy levels, say 1.35 microjoules in a 200 femtosecond pulse, this results in a more or less elliptical cavitation profile in the 100 x 20 micron range that also disrupts surrounding tissue.

“To target volume you need higher pulse energy and smaller numerical aperture with a weaker focus,†Dr Lubatschowski explained.

The more-parallel, higher energy beam produced by a lower numerical aperture aberrates less as it moves across several refractive surfaces and through layers of tissue with different refractive indices, including cloudy lens material. This allows the beam to retain enough energy to achieve photodisruption deep in the eye, but also inherently reduces focus quality below that required for corneal treatment, he added.

Navigation required

For cataract surgery, the laser also must operate at a greater range of depths with slightly different anatomy for each patient, Dr Lubatschowski said. For any patient, there is a different depth of anterior chamber and size of crystalline lens. Thus, cataract lasers tend to employ more complex mechanisms to adjust the focal spot to the target.

With corneal lasers, the laser is docked directly to the tissue target, applying a flat cut or another distinct geometrical pattern into the cornea and making imaging during surgery less necessary. However, the greater distance of the lens capsule from the corneal surface, where the laser is docked, to the target in the capsular bag is variable depending on patient anatomy. As a result, real time imaging is required to keep cataract lasers on target, Dr Lubatschowski noted.

“In my opinion OCT is the most powerful technology for this application. It quite easily delivers the 3-D data to access the lens and capsule.â€

The imaging system and laser share a common optical system to ensure they are in synch, which further complicates design to accommodate multiple wavelengths.

Dr Lubatschowski noted that cataract laser is in its early stages of development, and he believes technical challenges can be overcome to produce lasers that will do a good job of both corneal and cataract work. These might include lasers operating at 1,400 to 1,700 nanometer wavelengths capable of penetrating sclera, though producing these wavelengths is currently prohibitively expensive, he said.