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Femtosecond laser dominates discussion at ESCRS winter meeting
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ATHENS — From cornea transplant procedures to LASIK flap creation, femtosecond laser technology dominated presentations and discussion here at the European Society of Cataract and Refractive Surgeons Winter Refractive Surgery Meeting.
Also highlighting the meeting were results from the first European patients implanted with the Light Adjustable Lens, which were presented by OSN Europe/Asia-Pacific Edition Editorial Board Member José L. Güell, MD.
“We are now working on the possibility of producing multifocal changes on this lens and on the simultaneous, customized removal of aberrations during the light adjustment procedure,” Dr. Güell said.
Wider channels allow less traumatic Intacs implantation
A study comparing two different channel sizes for Intacs intrastromal corneal ring segment implantation performed with the IntraLase femtosecond laser showed larger channels made insertion easier with fewer complications, according to a speaker here.
“We included in the study 168 keratoconic eyes of 110 patients,” Aylin Ertan, MD, said. “They were divided into two groups, according to IntraLase channel size: wide channels of 6.7 mm by 8.2 mm and narrow channels of 6.6 mm by 7.6 mm. The two groups were compared according to mean changes in uncorrected visual acuity, best corrected visual acuity, manifest spherical equivalent, mean K-value and complications within 6 months after surgery.”
The Intacs were easy to insert in the wide channels, she noted. Narrow channels had to be dilated using confocal microscopy.
After implantation, UCVA and BCVA improved in both groups, and a significant improvement of K-values was observed. Comparing the two groups, visual acuity and refraction did not show a significant difference, and K-values were comparable.
“The narrow channel group had a higher rate of minor complications,” Dr. Ertan said. “Epithelial plug, yellow deposits and tunnel haze around the segments were seen more frequently in these patients. In the same group there was also a tendency of the inferior segment to move upward and slightly come out of the incision, though we didn’t have any case of extrusion. The same complication was not observed in the wide channel group.”
First results with new femtosecond laser promising
The Da Vinci femtosecond laser system produces precise LASIK flap cuts, with predictable thickness and smooth stromal bed surfaces, as shown by the first prospective clinical study with this new instrument, a presenter here said.
The Da Vinci femtosecond laser (Ziemer Ophthalmics) combines pulse energy in the low nanojoule range and frequency in the megahertz range with a high numerical aperture optic.
“This combination results in corneal resections of highly predictable quality,” Bojan Pajic, MD, said.
“The surgical procedure is very fast and easy, with no resistance to flap separation and a cutting time of 20 to 25 seconds, nearly as fast as the mechanical keratome,” Dr. Pajic said. “Small bubbles in the cutting interface disappear immediately after flap lifting, and no opaque bubble layer was observed in the eyes we treated.”
According to Dr. Pajic, the LASIK flaps of the first 20 eyes underwent clinical evaluation and were analyzed using topography, wavefront measurement, corneal OCT and confocal corneal laser-scanning microscopy. The flaps were also compared with an equal number of flaps performed with the Amadeus SIS mechanical keratome. The intended flap thickness was 140 µm in all cases.
“The [Da Vinci] produced smoother interfaces and showed no tissue bridges. Confocal microscopy showed very little edema compared with the SIS keratome. The mean flap thickness was 141 µm with a standard deviation of 8.5 µm. The Amadeus produced slightly thicker flaps, with a mean thickness of 143 µm and a standard deviation of 21.7 µm,” Dr. Pajic said.
A comparison also was made with IntraLase FS laser flaps (IntraLase), which were equally predictable and had a similarly small standard deviation, Dr. Pajic noted.
Femtosecond laser used to recut incomplete LASIK flaps
The IntraLase offers a high degree of precision and safety to incomplete LASIK flaps previously performed with mechanical microkeratomes.
“In three cases of incomplete flaps we created a second flap using a 60 kHz IntraLase femtosecond laser. We aimed at creating flaps that were slightly thicker than the incomplete flaps, which ranged between 100 µm and 126 µm. For each case we calculated a cut that went 20 µm deeper than the scar,” Dr. Pajic said.
Results were good, and a perfect LASIK flap was created even in the area of the scar, Dr. Pajic said.
Femtosecond laser offers advantages in DSEK disc prep
Femtosecond laser preparation of posterior lamellar discs is feasible for performing femtosecond laser-assisted Descemet’s stripping endothelial keratoplasty, or FS-DSEK, according to a study.
Researchers at the University Hospital in Maastricht and the Cornea Bank of Amsterdam evaluated donor grafts for endothelial cell viability both before and after dissection and storage. They also examined the surface and texture of the posterior lamella as well as the depth of the femtosecond laser lamellar cut using scanning electron microscopy.
“Using a 30 kHz IntraLase femtosecond laser, we dissected 400 µm of lamellae from the donor corneas,” Yanny Ying-Yee Cheng, MD, said. “Then we decontaminated and transported them to the Cornea Bank in Amsterdam. There, they prepared the corneoscleral rims and stored them in organ culture. After 1 to 2 weeks, they sent them back to the hospital and the posterior lamellar discs were dissected.”
The IntraLase produced smooth surfaces with little damage to the endothelial cells. The surfaces were also better compared with those produced by the 15 kHz IntraLase model, and the achieved depths correlated well with predictions, Dr. Cheng said.
The FS-prepared endothelial lamellae were implanted in 13 eyes using an IntraLase-assisted DSEK procedure. At 3 and 6 months follow-up, all eyes had a clear graft with functioning endothelium, she said.
At 3 and 6 months follow-up, eyes treated with FS-DSEK showed better results in terms of induced astigmatism compared with 13 eyes treated with penetrating keratoplasty.
Eyes treated with PK also had a lower BCVA, which researchers attributed to a high number of patients with macular diseases and to interface healing problems, Dr. Cheng said.
“We believe that the femtosecond laser has great potential and can give endothelial keratoplasty the advantages of an automated, standardized procedure,” she added.
Zigzag and zigsquare in IntraLase PK
An increasing number of surgeons are converting to PK (penetrating keratoplasty) with the femtosecond laser.
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The cylindrical cut is only used by a minority of surgeons, as it the one that most frequently creates problems of apposition of the anterior surfaces, according to Sheraz Daya, MD.
“You can easily get an override or under ride of the graft,” he said.
The top hat and mushroom seem to be the most popular configurations, but the zigzag, Christmas tree and zigsquare patterns are showing specific advantages.
According to Dr. Daya, the main advantage of the zigzag configuration is the perfect anterior apposition.
“It is like having a LASIK flap,” he said.
One disadvantage is the smaller posterior surface, which may not be suitable when endothelial replacement is needed, like in Fuchs’ dystrophy and bullous keratopathy.
“In addition, the tongues of the zigzag can be quite fragile and tear easily” Dr. Daya said. “However, the large anterior and small posterior surfaces make this configuration ideal for keratoconus.”
The zigsquare takes into consideration the posterior aspects, presenting a larger posterior diameter for endothelial replacement.
“There might be a concern because we are going out further to the limbus with increased risk of rejection,” Dr. Daya said.
Since July 2006 he has performed five IntraLase-assisted keratoplasties. Two were for keratoconus, two for Fuchs’ dystrophy and one for bullous keratopathy.
“In our first case we did a zigzag pattern. Then we decided to get more adventurous and used the zigsquare in four cases,” he said.
When creating the donor graft, it is important to watch the posterior square ledge because it can tear easily, he said.
“Remember that the posterior lamellar tissue is not as strong as the anterior,” he said.
Running sutures are better with this kind of graft because “the graft has to glide into place and find its own level.”
The zigsquare pattern after transplantation was clearly visible with the Visante OCT, according to Dr. Daya.
“It fits in place perfectly,” he said.
The follow-up of four eyes at 6 months showed an endothelial cell count comparable to standard PK and excellent visual outcomes.
“Three of the four eyes are seeing 20/40 or better best corrected,” he said, “and the mean spherical equivalent is just a little on the myopic side.”
Measurement of corneal hysteresis useful
Measurement of corneal hysteresis with the Reichert Ocular Response Analyzer provides useful information on the dynamic biomechanical properties of the cornea before and after refractive surgery, and it may have a role in predicting postoperative keratectasia, according to researchers.
A study carried out in New York demonstrated that corneal hysteresis (CH) values, as well as corneal resistance factor (CRF) measurements, are significantly reduced in keratoconus eyes and in eyes after refractive surgery as compared with normal, non-operated eyes.
“While both CH and CRF were around 10 mm Hg in normal and unoperated eyes, in keratoconus and post-LASIK eyes, CH was about 8 mm Hg and CRF was about 7 mm Hg. These measurements may be able to identify eyes at risk of developing ectasia,” Mitsugu Shimmyo, MD, said.
Caitriona Kirwan, MD, who reported on a study carried out in Dublin, said that CH decreased by variable amounts after refractive surgery, with similar changes observed in both LASIK and LASEK patients.
“We found that postoperative hysteresis correlated strongly with residual stromal bed thickness,” Dr. Kirwan said.
Similar conclusions concerning CH after LASIK or PRK were also reached in a Greek study presented by Gerasimos Kopsinis, MD.
“Corneal hysteresis was significantly lower in eyes after excimer laser surgery, regardless of the type of procedure used,” he said. A moderate regain was observed over time, but it did not achieve preoperative values, however.
Air and liquid provide bifocality in new IOL
A pseudoaccommodative IOL that creates bifocality through air and liquid is currently approaching a phase 3 clinical trial, according to a surgeon speaking here. The new concept in creating multifocality holds “tremendous promise and may prove to be a breakthrough in bifocal IOL technology,” Meenakshi Gupta, MD, said.
The IOL consists of a coaxial plano-convex lens combination. The anterior lens has a convex surface with a refractive power of 20 D within the eye. The posterior lens, which also has a convex anterior surface, has a power of about 3 D in air. “The little space in between these two lenses is sealed around the periphery and filled to about two-thirds with liquid and one-third with air,” Dr. Gupta said.
When the eyes are directed horizontally for distance vision, the liquid occupies the space in between the lenses’ optical zones and neutralizes the 3 D power of the posterior lens. When the eyes swivel down by 30° to 40° for near vision, the fluid level remains horizontal and the air occupies the space in between the optical zones, restoring the additional 3 D power of the posterior lens. The resulting total power of the lens for near is 23 D, Dr. Gupta said.
Relifting preferred over recutting flaps
Lifting rather than recutting the primary LASIK flap seems to be the most popular choice among refractive surgeons when performing enhancement procedures, according to several surgeons.
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“Lifting is safer,” Vikentia Katsanevaki, MD, said. By using a blunt instrument and touching the cornea with a little pressure, the edge of the primary flap can be “easily recognized,” she said.
“In my experience, there is no flap that cannot be lifted. The important thing is that you should not traumatize the epithelium, to avoid the risk of diffuse lamellar keratitis (DLK) and inflammation. The less manipulation of the flap, the better,” she said.
When re-treating, surgeons should make sure the cornea is dry because water changes the ablation rate and may result in undercorrection. Pachymetry should be routinely performed.
Flap thickness and quality are also important in deciding whether to relift or recut the flap.
“I normally relift the flaps of my own patients but prefer to recut a flap that has been made by someone else unless I’m completely sure of its thickness and quality,” Dr. Güell, said. “I feel much more confident recutting than assuming the risk of relifting and finding that it is not the diameter I wanted or the quality I expected.”
Dr. Katsanevaki objected, stating, “There is no such thing as a perfect flap. If you remove the epithelium you find that every flap is wrinkled,” she said, noting that she would only consider recutting a flap if there are obvious problems, such as scarring that suggests a buttonhole, or other flap-related complications. She would also consider recutting in cases in which there was inflammation after the primary LASIK procedure, which increases the risk of epithelial ingrowth.
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A survey of 50 surgeons regarding their preference for lifting vs. cutting the flap during enhancement procedures, conducted between 1998 and 2002, showed that attitudes have changed over time. Initially, almost 20% of surgeons surveyed cut new flaps, but in recent years the number has dropped to less than 1%.
“It is now evident that cutting the flap can create significant complications and jeopardize the ability to enhance again,” Dr. Katsanevaki noted. “Even if I do not have flap thickness information from the previous procedures, I tend to lift because it is safer for the patient and gives me more freedom for enhancement.”
In terms of biomechanical responses, Cynthia Roberts, MD, said that enhancement procedures produce little changes.
“You have already had the majority of the response with the first flap and the first ablation. Whether you relift or recut the flap, since you are just fine-tuning, the biomechanical response from the ablation will be minimal,” she said.
However, cutting a second flap produces a more complicated structure, she noted.
‘Big bubble’ dissection yields smoother graft, less problems
A novel method for endothelial disc dissection that involves the “big bubble” technique may improve the quality of deep lamellar endothelial implants, according to a surgeon.
“One of the main disadvantages of standard DLEK is the irregularity of the endothelial disc surfaces,” Pavel Studeny, MD, said during Cornea Day. “To obtain smoother implant surfaces, we decided to remove the stromal layer from the center of the donor’s graft, thus exposing the smooth Descemet’s membrane surface.”
Dr. Studeny calls the graft preparation method Descemet’s membrane with stromal hem transplantation. It involves using the big bubble technique to separate the stroma from Descemet’s membrane at the center of the graft, leaving a peripheral rim of stromal tissue for better manipulation.
“The result is an 8-mm diameter disc with a 6-mm thinner central zone. Once the disc is implanted with the usual DLEK procedure, we obtain a perfect adhesion between donor and recipient interfaces, with full contact and no space between Descemet’s membrane and the stroma,” he said.
Descemet’s membrane may be layered in structure
Small bubbles detected in some cases within Descemet’s tissue after deep anterior lamellar keratoplasty with the big bubble technique may reveal some unknown anatomical aspects of the Descemet’s structure, according to one surgeon.
“The Descemet’s membrane may not be a uniform structure but be internally divided in layers,” Jörg Krumeich, MD, said during Cornea Day.
In more than one case, after performing DALK with the big bubble technique, he observed bubbles that were not part of the anterior chamber bubble, but were inside Descemet’s membrane.
“At first I thought they were inside the anterior chamber, but they did not move, and slit lamp examination showed that there were not one but two lines at the Descemet’s level,” Dr. Krumeich said. “In other words, some air had penetrated inside the Descemet’s and split it into two layers.”
Promising preliminary results for Light Adjustable Lens
The Calhoun Vision Light Adjustable Lens is showing high predictability, stable results and optimal capability for myopic, hyperopic and astigmatic adjustment, as shown in a study presented here.
The surgical procedure is no different from standard IOL implantation, although the present model requires a 2.2-mm to 2.5-mm incision and the use of forceps. The only special requirement is the use of a light filter during implantation to protect the light-sensitive surface of the lens.
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“After 1 week, during which the patient should wear protective glasses, the lens can undergo refractive adjustment. If the patient is satisfied, the ‘lock-in’ procedure can be performed 2 days later, otherwise two or three further adjustments can be attempted,” Dr. Güell said.
According to the preliminary results of the first four patients implanted with the Light Adjustable Lens in Barcelona, an improvement in UCVA was obtained in all cases, with a precise adjustment of the spherical error.
“More than 90% of the eyes were within 0.25 D of intended correction, and all of them were within 0.5 D,” he said. No loss of BCVA was reported.
Early results of astigmatic correction in five patients are equally promising, according to Dr. Güell.
“We are now working on the possibility of producing multifocal changes on this lens and on the simultaneous, customized removal of aberrations during the light adjustment procedure,” Dr. Güell said.
PK unnecessary for macroperforations
Macroperforations as a complication of deep anterior lamellar keratoplasty can be easily handled without converting the procedure to penetrating keratoplasty, according to a surgeon speaking here.
“With the big bubble technique, which is safer, Descemet’s [membrane] ruptures occur in about 4% of cases,” Vincenzo Sarnicola, MD, said at Cornea Day. Conversion to penetrating keratoplasty (PK) is easy but no longer necessary, he added.
“I’m not converting anymore to PKP. I place the graft over Descemet’s [membrane], suture it, fill the anterior chamber with air and remove the fluid between Descemet’s [membrane] and the stroma,” Dr. Sarnicola said. “If these maneuvers are performed correctly, the Descemet’s membrane will reattach to the stroma in cases you would never expect.”
“If you are used to doing DALK, you do not want to do PK because you know the postoperative course is completely different,” he added.
Reducing the number of PK procedures means “avoiding the risk of so many severe intraoperative complications and the high rejection rate that this procedure entails,” Dr. Sarnicola said.
Posterior disc replacement for post-DSAEK rejection
Posterior disc replacement is safe, easy and well tolerated in eyes with allograft rejection following Descemet stripping automated endothelial keratoplasty, according to a surgeon speaking here.
“Recovery is fast, and the cornea is perfectly clear within 1 week after the operation,” Mohamed Alaa El Danasoury, MD, said at Cornea Day.
After his first case of graft rejection after Descemet stripping automated endothelial keratoplasty (DSAEK), Dr. El Danasoury was initially inclined to do PK, but after discussing it with his patient, they finally agreed to try disc exchange, he said.
The donor disc was prepared in an artificial chamber with the Moria ALTK keratome and a Hanna trephine. Surgery was performed under local peribulbar anesthesia through a 4.5-mm incision, and the epithelium was removed for better visualization.
No viscoelastic was used, but an anterior chamber maintainer and balanced salt solution were used to maintain the anterior chamber throughout the procedure. The donor disc was folded with one drop of viscoelastic on the endothelial side and inserted through the incision, Dr. El Danasoury said.
“I let the disc unfold and guided its positioning on the back of the cornea using a reversed Sinskey hook,” he said. “I removed the anterior chamber maintainer and secured the disc in place by injecting a large air bubble through a 30-gauge needle. No sutures were used to fix the disc.”
The procedure lasted “no more than 20 minutes,” Dr. El Danasoury said. The air bubble started to clear 1 day postop, and after 1 week the cornea was perfectly transparent, he said.
Corneal specialists warn of systemic immunosuppression
Systemic immunosuppression following corneal transplantation should be administered under strict medical surveillance, according to several corneal specialists speaking here.
“It cannot be just in the hands of ophthalmologists, but needs the cooperation of different specialists,” Dr. Güell, said at Cornea Day. “The lungs, liver and kidneys should be screened preoperatively but also checked periodically after surgery.”
“Patients should know that lifelong immunosuppression is unfortunately unavoidable in some cases, but they should also be aware of the consequences it may produce,” noted Jorge L. Alió, MD, PhD, Ocular Surgery News Europe/Asia-Pacific Edition Chairman of the Editorial Board. “Elderly patients are more at risk of severe complications, and women in particular should know that they are going to have fertility problems.”
According to Nikolaos Papadopoulos, MD, the main danger is lung aspergillosis.
In one patient, a 65-year-old woman treated with oral cyclosporine at the U.S. Food and Drug Administration-recommended dosage, lung aspergillosis was accidentally discovered after a second graft rejection.
“For the needs of general anesthesia, we performed chest X-rays, which showed multiple round shadows in the lungs. Further examination with high-resolution CT scan revealed the typical emphysematous cysts of aspergillosis,” Dr. Papadopoulos said.
After a third regrafting, the patient was treated with topical cyclosporine eye drops and showed no sign of rejection at 3 months.
“In this case, we managed to treat the lung condition, but aspergillosis is an extremely severe, life-threatening disease,” he said.
Previously, Dr. Papadopoulos said he experienced the extreme side effects of systemic immunosuppressive drugs in three patients. One patient died of lung aspergillosis caused by systemic steroids, another developed septic arthritis of the hip joint following systemic cyclosporine treatment, and the third patient contracted a severe lung virus induced by systemic steroids.
Dr. Papadopoulos said topical immunosuppression might be a viable alternative, especially in the elderly.
“Topical cyclosporine can be as effective as systemic,” he said.
Dr. Alió objected, saying that topical cyclosporine cannot be effective for long-term immunosuppression. “It doesn’t reach the anterior chamber,” Dr. Alió said, noting he prefers to use tacrolimus in a 1% concentration.
Advances in presby-LASIK technique
Presby-LASIK is an excellent alternative for initial and intermediate presbyopia in patients between ±4 D of spherical equivalent, according to Dr. Alió.
“The results of our pilot study on hyperopic patients are stable after 5 years. More than 80% of the patients have a distance vision of 20/40 and 90% can read J4 or better, which means newspaper articles, without spectacles,” he said.
Using the PresbyMax model, a new optical analysis system based on a light propagation algorithm, the image quality offered by the presby-LASIK treatment at all distances could also be analyzed and new presby-LASIK methods could be designed.
“We can create a model of how the patient is going to see after surgery,” Dr. Alió explained. “A high correlation has been found between this model and the results, which shows that our predictive method does work in real life. We can now select the best profile depending on patient refraction, lifestyle and expectation.”
Of the two principle multifocal ablation patterns currently used, central presby-LASIK is a near-dominant procedure, best in hyperopic patients and good in emmetropes and myopes, Dr. Alió said. Visual rehabilitation is fast.
Peripheral presby-LASIK, on the other hand, is a distance-dominant procedure, excellent in emmetropes and myopes and good in hyperopes. The recovery of distance vision may need some time because it requires some neuroadaptation.
“Using the PresbyMax we have been investigating the possibility of mixing and matching the two procedures, creating an asymmetrical induction of negative and positive spherical aberration with a binocular neutral effect,” he said.
“In the dominant eye we create a prolate aspherical profile with negative spherical aberration while in the non-dominant eye we create a prolate aspherical profile plus positive spherical aberration,” Dr. Alió explained.
PRK feasible in keratoconus after cross-linking
Riboflavin-UVA corneal cross-linking followed by topography-guided PRK can effectively improve vision in patients with keratoconus and post-LASIK ectasia, according to one surgeon.
“We now have the experience of a considerable number of cases of both keratoconus and corneal ectasia that were effectively treated with this modality,” John Kanellopoulos, MD, said.
At 16 months the ectasia was stabilized in all cases. Corneal flattening was obtained in proportion to the size of the cone, for an average of 3 D.
A reduction in spherical equivalent was obtained in some of the eyes, and no endothelial cell change was caused by the procedure.
In 27 eyes with keratoconus the UVA collagen cross-linking treatment was followed by limited topography- guided ablation in order to achieve better visual rehabilitation.
Several eyes with post lasik ectasia were treated in a similar fashion. In both conditions, a considerable improvement of vision was obtained.
“In one patient with severe post-LASIK ectasia UCVA was 20/400 and BCVA was 20/200. We applied 3 mW/cm2 of UVA light for 30 minutes combined with 0.1% riboflavin ophthalmic solution as a pre-treatment to stabilize the ectasia,” Dr. Kanellopoulos said.
After the treatment UCVA had improved to 20/70 and BCVA to 20/40. Three months later a topography-guided PRK using the Wavelight 400Hz Eye-Q excimer platform was performed to treat the irregular refraction of –4.50 D –4.00 D × 115.
“At 18 months of follow up the patient is 20/20,” Dr. Kanellopoulos said. “Refraction is +0.50 D –0.50 D × 160, and is stable since the first postoperative months.”
This and other cases demonstrate that UVA corneal cross-linking can be used as a pre-treatment to enable ectatic corneas to undergo a limited 50 µm treatment with topography-guided PRK, which would be otherwise contraindicated.
“Our therapeutic goal was not emmetropia, but improved best spectacle corrected visual acuity. Some of these patients now wear soft contact lenses or glasses,” Dr. Kanellopoulos explained.
Safe RLE surgery can be performed with Healon 5
The new ophthalmic viscosurgical devices (OVD) are “an instrument that we can use to make refractive lens exchange (RLE) safer, with an extremely low complication rate,” according to Manfred Tetz, MD.
“Many surgeons take OVDs for granted, without realizing that they can play an extremely important role and be the key to safe, uncomplicated surgery,” he said.
There are characteristics in the latest viscoelastic substances, like Healon 5, which can be exploited to provide complete endothelial protection, prevent capsular breaks and even help in the performance of a perfect, pre-designed rhexis, which is a prerequisite for perfect IOL centration, Dr. Tetz said.
The viscoadaptive properties of Healon 5, which changes from cohesive to dispersive according to the different physical situations in the eye during surgery, can be exploited to “compartmentalize the eye,” creating a protective barrier in some areas and allowing at the same time complete freedom of movement in other areas.
A pre-designed capsulorrhexis can be performed by filling the chamber up to 80% with Healon 5, and then slowly injecting an additional small quantity of the substance underneath, toward the center of the lens. This creates a ditch in the center of the capsule with a ring of higher elevation around it.
“When you start pulling the capsule, all the forces are immediately directed along the highest elevation, so you can pre-design your rhexis,” Dr. Tetz said.
During phaco, the surgeon can compartmentalize the anterior chamber in an upper area and in a lower area, and only work in the lower area.
The fluid will only enter in the compartment where the surgical maneuvers are performed, and “by using very low irrigation and low ultrasound power, and creating very little turbulence, the surgeon can work free and undisturbed underneath the protective cushion of Healon 5.”
Healon 5 is like “a ball of wool, tightly wrapped,” Dr. Tetz said.
“If you pull the thread very slowly you get your wool, but if you pull it rapidly it ruptures. In the same way, the binding forces keep the molecules of Healon 5 together if you pull very slowly, but if you move very rapidly the molecules break,” Dr. Tetz said.
“By using the changing physical properties of this substance we can create rupture chains toward the center of the eye and a cushion of Healon 5 above,” he explained.