Femtosecond to fully replace microkeratome

September 16, 2008

The age of microkeratomes is coming to an end as femtosecond lasers come of age. That's the chief conclusion drawn by Dr Gunther Grabner, professor of ophthalmology at the Paracelsus University of Salzburg, in a survey on the advantages and limitations of femtosecond lasers.

The age of microkeratomes is coming to an end as femtosecond lasers come of age. That's the chief conclusion drawn by Dr Gunther Grabner, professor of ophthalmology at the Paracelsus University of Salzburg, in a survey on the advantages and limitations of femtosecond lasers.

"We here at the university clinic have not used microkeratomes for the last three years. We have the Intralase (AMO) and the VisuMax (Carl Zeiss Meditec) and we use those," Dr Grabner said in an interview. "We are not a big clinic like some of the US institutions, but I believe all medical centres will go the same way eventually."

Such was the conclusion drawn from a review of current laser systems. Dr Grabner examined in some detail the Intralase, the Da Vinci (Ziemer), the Femtec (20/10 Perfect Vision) and the VisuMax.

Dr Grabner noted that modern surgical lasers were amazingly complex machines and maintenance runs to the 10s of thousands of dollars annually. "It is an important factor to bear in mind, these systems are expensive to acquire and maintain," he said.

He noted that technically, the systems were similar in many ways. Central wavelength is in the infrared around 1043 ? 53 nm, and the pulse duration ranges from 200 to 800 femtoseconds. All use amplifiers, except the Da Vinci, which uses an oscillator only.

Again, repetition rate was similar for the Intralase, Femtec and VisuMax systems, which all use amplifiers, with a range of 60 kHz to 200 kHz, while the Da Vinci operates at a much higher frequency of several 10 Mhz, but this has a lower maximum laser pulse energy. "It is 250 to 500 times lower than the Intralase and the Femtec lasers, whereas the VisuMax is in-between; it's at 50 to 400 nJ per pulse.

Again, in a comparison of operating performance for a LASIK flap procedure, Dr Grabner noted that the most of the exceptions related to the Da Vinci. It uses a "ploughed field" cutting pattern of long, narrow meanders and offers suction ring controlled flap diameters ranging from 8.5 to 10 mm, and uses a distance foil to control cutting depth between 90 and 135 microns.

The IntraLase system uses a raster cutting pattern, and Femtec a spiral out-in cutting pattern. The VisuMax currently uses a spiral in-out pattern and is moving to out-in as well in the near future.

Otherwise there were many common points between the IntraLase, Femtec and VisuMax lasers, according to Dr Grabner. They are all computer controlled and offer selectable cutting depth.

Treatment times, spot size and surface quality are similar for all four machines. "The surface quality is similar to that of the microkeratome, while treatment time ranges from 15 to 40 seconds, and spot size is a few microns," Dr Grabner remarked.

Dr Grabner revealed an interesting finding that IOP increases during the procedure probably differ significantly between applanating and non-applanating docking systems. He noted that AMO with the plano patient interface claims an intraoperative IOP rise of about 35 mmHg, whereas the Femtec and VisuMax system use a spherical corneal interface concept to reduce this assumed pressure rise.

On IOP, Dr Grabner tested the IntraLase, VisuMax and Da Vinci using a standard and highly accurate test apparatus on unoperated human donor eyes. The research team performed a standard procedure, tracking IOP throughout, and starting from a stable pressurization of 18.0 to 22.0 mmHg.

The results showed very high intraoperative IOP increases across all systems, with the IntraLase displaying an average cut IOP of 180.6 ± 21.6 mmHg for the minimal applanation pressure. The Da Vinci recorded average cut IOP of 150.9 ±17.2 mmHg while the VisuMax showed the best performance with an average cut IOP of 84.9 ±7.3 mmHg.

Dr Graber noted that no increase in adverse clinical incidences has been reported yet, but that the differences might have implications for ocular blood flow and structural stress during the procedure.

Dr Grabner summarized his survey by saying that the options of cutting geometry and flexibility were the strengths of the IntraLase, Femtec and VisuMax systems, while mobility was the strong point of the Da Vinci.

Dr Grabner also looked at the potential for Femtosecond Lens Extraction (FLEx) and noted that it offered the advantage of doing away with an excimer laser, and that early results are promising but had not quite reached the excimer laser "gold-standard".

Taking a broader look at the future, Dr Grabner said that all femtosecond laser systems would need to offer a wider range of cuts for PKP and LKP, channels, (for intacs and similar devices) along with pockets and options for astigmatic KT. A challenge still to be met for AKT is a better understanding of biomechanics and noted that creating lamellar cuts parallel to Descemet's membrane was a problem that remained to be solved.

Optics will continue to improve, offering tighter beams at higher power, and the upshot is that the age of the microkeratome is over because laser development will continue to be very rapid.