Researchers analyze femtosecond laser bubble morphology

The size and shape of femtosecond laser bubbles do not correlate with the energy applied, a study in eye bank eyes found.

Issue: March 1, 2006

ByJared Schultz

LISBON – The size and shape of bubbles produced in the corneal stroma by femtosecond laser did not correlate closely with the amount of laser pulse energy used, a histology study in eye bank eyes found. Current levels of energy used with femtosecond lasers appear to be well above the threshold for producing intrastromal optical breakdown, one of the researchers said.

Investigators at the Mannheim Medical School of Heidelberg University in Germany have been studying laser pulse disruption patterns in corneal tissue in an effort to determine how to achieve a continuous dissection line with femtosecond lasers. In the hope of performing intrastromal ablation and other procedures with femtosecond lasers in the future, it is important to understand how the laser and ocular tissue react to one another, said Urs Vossmerbaeumer, MD, MSc. He presented some of the results of recent experiments at the European Society of Cataract and Refractive Surgeons meeting in Lisbon.

This line of research will be important in the potential use of femtosecond lasers for, for instance, continuous linear intrastromal dissection and ablation in procedures such as lamellar keratoplasty, Dr. Vossmerbaeumer said at the meeting.

“At this point, it is crucial to understand, or try to understand, the laser-tissue interaction,” he said.

In a small experimental study in about 20 human donor bank eyes, Dr. Vossmerbaeumer and colleagues evaluated the relationship between focus depth, energy level, spatial distribution and the morphology of femtosecond laser pulse effects in corneal tissue.

They used pulse energy levels between 4 µJ and 8.5 µJ to create models of LASIK flaps of various thicknesses. Specimens were prepared for histological evaluation, and a total of 790 effects were classified. The researchers then analyzed the resultant 3,160 data sets to establish correlations between the micromechanical features of the corneal layers and the pulse energy.

Initial findings

Among other findings, Dr. Vossmerbaeumer and colleagues noted that the laser reacts differently in different layers of the cornea.

“It appears that the laser effect has a different morphology in the anterior, the median and posterior corneal stroma,” he said (Figure 1).

The researchers classified the laser effects into three categories of bubbles.

“Single form bubbles result probably from the single pulse shot (Figure 2), then there are bubbles where two in fact seem to be confluent to form another greater bubble (Figure 3), and then finally the bubbles, which I would term amorphic burst bubbles, formed by three or more regular forces confluencing against each other (Figure 4),” he said.

However, the researchers found only a weak correlation between the size and shape of the bubble effects and the pulse energy, leading them to hypothesize regarding what other factors might be involved in the bubble morphology, Dr. Vossmerbaeumer said.

“Since size and shape of the bubbles did not closely correlate with the pulse energy applied, other factors such as intercellular structures may be important,” he said.

He also said that perpendicular and oblique incisions were more challenging to obtain than parallel incisions.

In this oblique dissection line, circles mark three areas of differing morphology in the layers between the epithelium on top to the endothelium at bottom.

Bubbles resulting from a single shot from a femtosecond laser are shown.

Bubbles are merged from overlapping shots. Notice the disruption in the bubble from the merged bubbles.

Cavity resulting from multiple femtosecond laser shots.

Images: Vossmerbaeumer U

Varying energy levels

Dr. Vossmerbaeumer explained that there are a number of parameters that can be varied in this type of study, but he and his colleagues chose to focus on the energy levels of the laser.

He said that varying the energy levels within a rather broad range did not have a major effect on the bubble morphologies, which suggested that the levels were well above the determining threshold for microdisruptive effects.

“Probably you can work with energy levels much lower than you do currently, because the levels we are using right now just cause punctual disruption of the stroma, with tissue bridges remaining in between,” he told Ocular Surgery News in an interview. “To open the dissection line, we still need to tear apart these structures.”

He said the researchers did an experimental series using power as high as 15 µJ, which created larger disruption bubbles but still did not yield a clear dissection line.

“You don’t gain better results by just going to a higher energy,” he said.

Dr. Vossmerbaeumer said that one shortcoming of the study was that it is difficult to measure the actual energy output of the laser source.

“On a research basis, it will be perhaps a question to know whether the machine really emits a 4 µJ pulse. It might be that one is a 3 µJ pulse and one is 5 µJ,” he said in the interview. “The femtosecond laser pulses are known not to be perfectly stable, so they oscillate in the energy they emit. That’s important to keep in mind when looking at all these results.”

Remaining tissue bridges

Dr. Vossmerbaeumer and his colleagues hypothesized that at certain energy levels the remaining tissue bridges that make flap lifting difficult might be gone. However, in their experiments, the bridges remained with regular pulse focus spacing at all energy levels.

“If you have an ideal combination of temporal and spatial pulse display modes and optimal energy levels, you might expect to at least minimize the tissue bridges,” Dr. Vossmerbaeumer said. “I don’t think they will be totally gone, as the corneal stroma is not a homogeneous material.”

Dr. Vossmerbaeumer said in vivo models will be the next stage in this research, which will address questions regarding deeper intrastromal procedures.

“Further work has to be done to more thoroughly understand what’s going on there in the tissue.”

For Your Information:

Urs Vossmerbaeumer, MD, MSc, can be reached at Mannheim Medical School, University of Heidelberg, Universitäts-Augenklinik, Klinikum Mannheim, Theodor Kutzer Ufer 1-3, D-68167 Mannheim, Germany; 49-621-383-2242; fax: 49-621-383-3803; e-mail: urs.vossmerbaeumer@augen.ma.uni-heidelberg.de.

Jared Schultz is an OSN Staff Writer who covers all aspects of ophthalmology. He focuses geographically on Europe and the Asia-Pacific region.