|Year : 1999 | Volume
| Issue : 4 | Page : 215-222
Laser-assisted cataract surgery and other emerging technologies for cataract removal
MK Aasuri, S Basti
L.V. Prasad Eye Institute, Hyderabad, India
M K Aasuri
L.V. Prasad Eye Institute, Hyderabad
Source of Support: None, Conflict of Interest: None
As we near the end of this century, refractive cataract surgery has become a reality through concerted contributions from ultrasonic phacoemulsification, foldable intraocular lens (IOL) implantation technology and keratorefractive surgery. As we enter the new millennium, our sights are set on realizing another dream: accommodative IOL surgery. Towards achieving this goal, many advances have been made in both techniques and technology of cataract removal. Lasers in particular have been under investigation for cataract removal for nearly two decades. The technology has now reached a stage where cataract can indeed be removed entirely with laser alone. Neodymium:YAG and erbium:YAG are the laser sources currently utilized by manufacturers of laser phaco systems. Initial clinical experience reported in the literature has served to highlight the capabilities of lasers and the need for further refinement. Despite the excitement associated with the availability of this alluring new technology for cataract removal, it is necessary to develop more effective laser systems and innovative surgical techniques that optimize its capabilities if laser phaco surgery is to be a genuine improvement over current techniques.
Keywords: Plasma blade, phacotmesis, laser phaco, Nd:YAG laser, erbium:YAG laser
|How to cite this article:|
Aasuri M K, Basti S. Laser-assisted cataract surgery and other emerging technologies for cataract removal. Indian J Ophthalmol 1999;47:215-22
Ultrasonic phacoemulsification, introduced by Kelman1 in 1967, has unquestionably been a major breakthrough in the technique of cataract removal. However, there are two main frontiers that remain to be conquered for optimal results following cataract surgery. These are posterior capsule opacification (PCO) and loss of accommodation. Research continues to be done in both these areas. Alternative techniques for cataract removal and newer implant materials are being investigated to circumvent the second of these drawbacks of modern-day cataract surgery.
Great hope and sometimes unrealistic expectations are evoked among patients and the medical fraternity when considering the use of lasers as a surgical tool. A good example is the common misconception that cataract removal with laser uses a completely noninvasive technique. This belief is so deeply ingrained in some patients that if advised to the contrary, they would rather believe that their physician is not keeping up with the times than correct their own misconception! Nevertheless, lasers are indeed in the forefront of the emerging technologies for cataract removal. This article provides an overview of recent developments in cataract removal technology and in this context discusses in detail the application of lasers for cataract extraction.
[TAG:2}Recently Investigated Cataract-removal Techniques[/TAG:2]
This section discusses cataract extraction methods that have been published in the recent past. Undoubtedly, there are other promising new techniques that have not been included in this discussion due to a paucity of literature on these. Foremost among those not elaborated is the Catarex system pioneered by Richard Kratz. The system utilizes vortex emulsification to remove lens material from the capsular bag using a 1.5 mm opening. (Kratz RP, Sorensen JT, Mittelstein M, Mirhashemi S. Catarex: An endocapsular vortex emulsification. Presented as a video film at the film festival of the annual meeting of the American Society of Cataract and Refractive Society, San Deigo, April, 1998). Further experience with this technology is awaited.
| Focused electromagnetic field technology|| |
Focused electromagnetic field (FEF) technology-based plasma blade is a complex system that vaporizes the surface molecules of a hair-thin plasma probe, thereby creating a microscopic cloud of cutting plasma particles around the probe. According to the proponent Richard J. Fugo, the plasma blade has the ability to create an incision, perform a capsulotomy and fragment the lens. The tip when activated, is said to cut 20-30 times sharper than diamond and is guided by tissue-sensitive incising capability that is designed to "gate off " when approaching structures that should not be cut. While human trials are awaited, initial results of animal studies were presented by Fugo at the American College of Eye Surgeons meeting last year (The never ending quest: Creating a better way to remove the lens, Eyeworld, April 1998). Thus this technology is still in its infancy but holds promise as a possible alternative to ultrasonic phacoemuisification.
| Phacotmesis|| |
Phacotmesis, pioneered by Aziz Anis, is a technology for cataract removal that combines high-speed (4,000-5,000 rpm) rotation and ultrasonic linear oscillation at the probe tip. Tmesis is believed to be safer than conventional ultrasonic phacoemuisification in that it requires less phaco power and is associated with a lower risk of posterior capsule dehiscence (PCD). In an editorial review published in 1996, Anis reported no cases of PCD attributable to the Tmesis tip in a series of 120 cases. Standard phaco techniques like divide-and-conquer and phaco-chop can be performed with Tmesis. While phacotmesis is available commercially, and has advantages over standard ultrasonic phaco, the technology has not gained widespread popularity as evidenced by the paucity of reports from other surgeons using it for cataract removal.
| Laser|| |
Among the newer technologies for cataract removal, laser is clearly the most intensely investigated and possibly generates the greatest interest at the present time.
| Why laser for cataract removal?|| |
Ultrasonic phacoemuisification, though a highly sophisticated and popular technique, has certain drawbacks. The salient differences between currently available laser and ultrasonic cataract removal systems are listed in [Table - 1].
Traditionally, the irrigating sleeve that surrounds the ultrasonic probe tip must necessarily be made of a soft plastic or comparable material to allow for the forward-backward oscillating movement of the titanium tip. This soft and pliable material, however, is susceptible to compression. Thus in wounds that are inadvertently created smaller, the pliable sleeve is compressed by the wound. Anterior chamber depth fluctuations can occur in such situations, especially during aspiration of large pieces of nuclear material. Innovations in tip design such as the Mackool tip or the Microflow tip are attempts at improving anterior chamber fluidics. Unlike the ultrasonic handpiece, there is no active movement at their probe tip and hence sleeves for multifunction laser probes can be rigid, thus avoiding the risk of inflow occlusion and chances of chamber shallowing during nucleus removal.
It is well recognized that heat is generated at the tip of an ultrasonic phaco hand piece. The irrigation line hence cannot be separated from the hand piece. Thermal burns at the wound are known to occur with ultrasonic phacoemuisification, especially under certain circumstances. In contrast, laser phacoablation generates less heat. In the experiment of Berger et al, in model systems and human cadaver eyes, ultrafine thermocouples were interfaced to a microcomputer data acquisition system to measure the heat generated. It was noted that the rise in temperature was 10-15 times greater after pulsed application of ultrasound energy than after erbium: YAG laser application. Similar results have been reported with Nd:YAG laser, while comparing the Dodick laser phacolysis and ultrasonic phacoemuisification in-vitro. Therefore aspiration, irrigation, or both can be separated from the laser probe during laser cataract surgery. This permits incisions of 1.5-2 mm in bimanual laser-assisted cataract surgery. Also, since heat generation is minimal in this system, corneal burns are uncommon.
Endothelial trauma is a potential problem with ultrasonic phacoemuisification, particularly in eyes with dense cataracts. A combination of factors, including mechanical trauma from the nuclear fragments, turbulence and free-radical mediated damage, are believed to be responsible for this. Presumably, these factors are not significant with laser phaco surgery, which appears to be safer for the endothelium. Mean endothelial cell loss rate of 7.6%±12.8 has been reported by Neubaur et al in a series of 32 patients who underwent Er:YAG laser cataract extraction. Stevens et al noted a cell loss of 0-10% six weeks postoperatively in 12 eyes of patients who had undergone laser cataract surgery. They did not report on cell morphology in their patients. More recently, endothelial cell loss rates of 2.5 % with the Q-Switched YAG laser (Kanellopoulos AJ, Dodick JM, Brauweiler PH, Alzner EH. Lens laser lysis [LLL] for cataract surgery: Early experience with the Q-switched YAG laser in 100 consecutive patients. Presented at the annual meeting of the American Academy of Ophthalmology, New Orleans, November 1998) and 1.0% with the Erbium laser (Hoeh HR, Fischer E. Clinical study on Erbium laser phacoemulsification[ELP] (Ophthalmology Times, April 15, 1999: 26-27). Presented at the annual meeting of the American Academy of Ophthalmology, New Orleans, November 1998) have been reported. Further clinical experience and detailed endothelial analysis is necessary to determine the degree of safety, if any, conferred by laser phaco techniques.
Laser phaco is believed to decrease the risk of PCD. Snyder et al in an in-vitro study have demonstrated the relative safety of erbium:YAG laser to the posterior capsule at low energy levels. At higher energy levels, erbium:YAG laser was comparable to ultrasonic phacoemulsification in its ability to damage the posterior capsule. However, scanning electron microscopy of laser-induced capsular tears revealed more localized breaks that are less likely to result in vitreous loss, as against tears caused by ultrasonic phacoemulsification, which were rough, irregular, and extended to the periphery. PCD has been noted to occur clinically with laser phaco surgery (Pita-Salorio D, Simon-Castellvi GL. Er: YAG phacolaser soon to be new tool for cataract surgery. Ocular Surgery News, International edition, Slack Inc, Thorofare, NJ, February 1996). In a clinical study using ELP, Hoeh and Fischer reported 4 PCDs, 3 of which occurred during the learning period. Whether or not incidence of PCD will decrease significantly with laser phaco will be known once more clinical experience is gained.
It has been recognized for over 3 decades that removal of the cataractous lens through a small anterior capsulotomy followed by injection of a malleable, clear polymer into the capsular bag, might simulate a virgin lens and facilitate accomodation. Such a procedure was indeed performed in monkeys by Haefliger et al, where accomodation was restored after the evacuated capsular bag was refilled with a silicone lens. The process was termed phaco-ersatz. Recently, Nishi et al have demonstrated the feasibility of refilling the lens capsular bag with silicone in rabbit eyes. The refractive changes achieved suggested a possible accommodative yield in primates. In humans however, a surgical technique to achieve similar results has been elusive. For cataract removal to be possible through a minute capsulotomy, the tip of the probe should function efficiently even when bent and manipulated through such a capsulotomy. Laser technology has the potential to work this way since laser, unlike ultrasound, is nonlinear. What is available today, however, is laser phaco technology in a relatively early stage of evolution, and currently available laser cataract systems are well below ultrasonic phacoemulsification in terms of efficacy. Nevertheless, the fact that laser energy delivery is nonlinear will continue to make it an attractive tool for investigations aiming to make phaco-ersatz possible in humans.
| Evolution of laser cataract surgery|| |
In addition to the above-mentioned advantages of lasers over ultrasound, the ability of lasers to remove tissue with a high degree of precision and the availability of a wide variety of laser wavelengths, each with different absorption spectra, makes them attractive as tools for lens removal. During the last 2 decades, both ultraviolet and infrared lasers have been investigated,, for their effects on the lens [Table - 2].
Radiation from ultraviolet lasers causes photochemical decomposition of proteins and other organic molecules, resulting in precise removal of tissue with minimal surrounding thermal damage. Although ultraviolet lasers have had a major impact on keratorefractive surgery, they have not been found suitable for cataract ablation. Four wavelengths have been studied (193 nm, 248 nm, 308 nm, and 351 nm) but the 308 nm wavelength alone has been found capable of both ablating the lens and lending itself to fibre-optic delivery for intraocular use. However, this wavelength is also cataractogenic making it hazardous to the surgical team. Solid-state ultraviolet lasers, including the 5th harmonic neodymium:YAG (213 nm), tunable laser (near 1200 nm), and ultraviolet diode lasers, have all shown initial promise but problems with fibre-optic delivery and risk of carcinogenesishave limited their use.
More success has been achieved with use of infrared lasers for cataract ablation. Several such lasers tested for cataract extraction are mentioned in [Table - 2]. A 2-step technique using picosecond pulse Nd:YLF laser has previously been described. This utilizes an initial slitlamp delivery system, creating emulsification and softening of the lens. The softened lens is subsequently aspirated in the operating room. At present however, only the erbium:YAG (Er:YAG) and the neodymium:YAG (Nd:YAG) lasers are manufactured and marketed commercially (Safety, marketability of laser phaco makes it attractive, Ocular Surgery News, International Edition, Slack Inc, Thorofare, NJ, April 1998). These are discussed in detail below.
| Recently Developed Laser Phaco Systems|| |
The two lasers that are in the process of being clinically investigated for laser-assisted cataract surgery, namely Nd:YAG and Er:YAG, have several distinct differences. These are mentioned in [Table - 3]. The six commercially manufactured laser phaco systems are listed in [Table - 4].
| Systems using Nd:YAG laser for lens removal|| |
These may be broadly categorized into direct and indirect acting systems based on whether or not laser energy directly makes contact with lenticular tissue. Each of these will be discussed separately.
| Direct acting systems|| |
The Photon (Paradigm Medical Industries, Salt Lake City, Utah, USA) uses the 1064 nm Nd: YAG laser which is broadcast through a unique 1.8 mm diameter tip designed like a ski tip. It has three functions: fragmentation, aspiration, and irrigation. Laser energy travels along a fibre and across an open area called the photofragmentation zone. The latter is a 2.5 mm zone into which nuclear material is aspirated. The resulting nuclear fragments are removed from the eye by aspiration and irrigation. At the distal end of the photofragmentation zone is the tip which is bent upwards, providing a backstop for the laser energy.
It must be noted that Nd:YAG laser-based systems for phaco have the potential for deep penetration. For this reason, the Photon system is designed to deliver laser energy which has an absorption coeffiecient lower than that required for phacovaporization. It thus acts by fragmentation and aspiration of fragmented material. The backstop design of the tip of this machine prevents the beam from damaging non-target tissues. A report based on data from 10 cataract extractions performed with this laser has been submitted to the United States Food and Drug Administration (FDA) and is pending approval.
| Indirect acting systems|| |
In 1989, Jack M. Dodick pioneered the development of an indirect method of cataract removal using the Nd: YAG laser.[13-15] Using this system, termed the Dodick laser lens ablation device, pulsed Nd:YAG laser energy transmitted from the source by a quartz fibre strikes a titanium target on the surface of which occur optical breakdown and plasma formation [Figure - 1]a. The optical breakdown causes shock waves to emanate toward the distal opening of the probe. It is at this opening that the shock waves make contact with the lens material which is held in apposition with the tip of the probe by suction. The shock waves disrupt the lens material at the mouth of the probe and the fragmented material is then aspirated out. Thus with this system, there is no direct exposure of the Nd: YAG laser to either the lens or surrounding tissues. Dodick has been using the device for cataract removal under the Investigational Device Exemption from the FDA. He recently reported on the feasibility of removing cataracts through incisions of 1.0-1.3 mm using a bimanual system and described a 6% decrease in endothelial cell counts with no change in corneal thickness following the procedure (Presented at the International Congress of Ophthalmology meeting, Amsterdam, The Netherlands, June 1998).
| Systems using Er:YAG laser for lens removal|| |
There are currently 4 systems using Er: YAG laser that are commercially marketed [Table - 4]. The mechanism of action of the laser in removing lens tissue is as follows: Er:YAG is a mid-infrared laser with a wavelength of 2.94 microns. There is a strong water absorption peak at this wavelength., Since the crystalline lens is composed of about 63% water, the Er: YAG laser is believed to be well suited to achieve phacovaporization of the lens. The mechanism by which EnYAG laser acts in fluid has been studied by Lin et al using high-speed photography and based on their findings, it is believed that explosive evaporation forms a cavitation bubble. Microspikes within the laser pulse traverse this cavitation bubble and generate energy a little beyond it [Figure - 1]. The propagation of energy increases when higher energy is used. This has been observed clinically by Stevens et al and also using a model by Berger et al. The cavitation bubbles cause microfractures and breakup of lens material. The loose lens material is then aspirated.
Although some of these systems have been reported to have been widely used outside the United States (Safety marketability of laser phaco makes it attractive. Ocular Surgery News, Slack Inc, Thorofare, NJ, April 1998) there have been no reports in the peer-reviewed literature on this experience, to our knowledge. Stevens and co-authors reported the first published series on Er:YAG laser-assisted cataract surgery performed in the United States. They used the Centauri system (Premier Laser Systems, Irvine, CA) for cataract removal in a prospective nonrandomized case series of 15 patients. The authors achieved complete lens removal with laser alone in 9 of the 15 patients. In the rest, ultrasonic phacoemulsification was required for completion because of defective capsulorhexis (one patient), loss of power through the fibre optic (two cases), and surgeon caution (three cases). The postoperative best corrected visual acuity was 20/30 or better in all eyes treated. No complications or adverse effects attributable to the laser were seen in any of the patients. The only drawback of the procedure was a prolonged laser time in all cases (maximum 20 minutes, minimum of 3 minutes). Stevens et al found that usage of a unimanual multifunction probe improved the efficiency of the surgery. Recently, Neubaur et al reported their clinical experience with Er:YAG lasers. In their series of 32 patients, conversion to ultrasonic phacoemulsification was required in 13 cases. Laser time, surgeon experience, and other subjective factors determined the need for conversion. While 3 PCDs occurred in this series, only one was attributable to high laser energy cutting the capsule during nuclear fracture.
Hoeh and Fischer in a prospective, multicenter trial with erbium laser (MCL 29, Aesculap Meditec Co., Jena, Germany), achieved nucleus removal in 37 of 40 eyes with laser phaco alone (Hoeh HR, Fischer E. Clinical study on Erbium Laser Phacoemulsification[ELP] (Opthalmology Times, April 15, 1999;26-27). Presented at the annual meeting of the American Academy of Ophthalmology, New Orleans, November 1998). The authors suggest the effective use of laserphaco up to a nuclear hardness of 2 (0-4 scale).
In our preliminary experience with the Centauri system, we have been able to remove moderate-density nuclei with the laser alone with an uneventful intra- and postoperative course. We have also noted prolonged laser time in our cases. However, we strongly believe that the multifunction probe is suboptimal for efficient removal of lens material [Figure - 2]a. We conclude so because the presence of the 600 micron laser fibre and its 800 micron olive tip occupies a large part of the cavity of the probe tip, leaving very little space for aspiration [Figure - 2]b. For this reason, we have noted very slow aspiration of the nuclear material from the anterior chamber, often causing clouding of the anterior chamber and a positive lens-milk sign. This occurs even when aspiration is at the maximum permissible level, the wound is adequate, the bottle height is proper and there is no kinking of the aspiration line. This fact, and the facility of using small incisions with bimanual probes, has indeed prompted Er: YAG manufacturers recently to renew manufacture of probes, for bimanual laser cataract surgery (Safety, marketability of laser phaco makes it attractive, Ocular Surgery News, International edition, Slack Inc, Thorofare, NJ, April 1998). We concur with Stevens et al that the laser energy acts 1-2 mm beyond the tip, especially at higher energy levels.
As was recommended based on a model of Er:YAG laser vaporization it might be prudent to use low pulse energy at high repetition rates, especially when deeper parts of the nucleus are being removed. Such an approach is likely to decrease the risk of PCD during the procedure. One such system developed by Coherent Lasers (Palo Alto, CA) operates at higher repetition rates of 200 Hz and can efficiently ablate at relatively low pulse energies of 6-8 mj/pulse. However, experience in human eyes is yet to be reported.
| Limitations with presently available laser phaco systems|| |
In the last few years, considerable progress has been made in the development and refinement of laser technology for cataract surgery. However, there still remain many areas of concern which need to be addressed in the near future.
The fibre material currently in use in Er:YAG laser systems is zirconium-fluoride and sapphire. Zirconium-fluoride has a variable life span and is sensitive to hygroscopic changes. Therefore, the fibre cannot be autoclaved and sometimes loses power during the course of surgery, requiring replacement., We have noted a loss of efficacy following gas sterilization of the fibres. The fibres are also expensive (approximately US$ 500 each). Sapphire is an alternative material. Fibres made of this can be autoclaved and last longer, but are more brittle and have relatively low transmission power. The fibre material used in Nd:YAG systems is quartz, which is relatively inexpensive and can be flash sterilized several times (The never ending quest: Creating a better way to remove the lens, Eyeworld, April, 1998). Quartz, however has a high water content and since Er:YAG energy is highly absorbed by water, it is not well suited to such systems. In a recent publication, Neubaur et al discussed in detail the role of fibre-optic delivery systems in Er:YAG cataract surgery. Clearly, fibre technology is an important area in Er:YAG laser phaco systems that merits further investigation.
The other major drawback of almost all the laser phaco systems is their inability to remove dense nuclei. Even in cases of nuclei of moderate density, the laser time is prolonged at least 3-5 times more than what might be expected with ultrasonic phacoemulsification. It must be conceded however, that laser phaco technology, the surgical techniques and machine settings suited for them are still being refined; with further improvements, this drawback is likely to be alleviated. If laser cataract systems are to be used for phaco-ersatz procedures, the probe will have to be significantly more efficient than it is now. It is conceivable that if the probe is extremely efficient in removing lens tissue, it can literally act like an etching pencil and remove lens material through even a minute capsulotomy. The "divide and conquer" techniques are probably less well suited to accomplish the latter.
The commercially available laser phaco systems cost between US $ 80,000 and 1, 30, 000 (Safety, marketability of laser phaco makes it attractive, Ocular Surgery News, International edition, Slack Inc, Thorofare, NJ, April 1998). Thus they are priced higher than even the higher-end ultrasonic phaco machines. One of the reasons given to justify this cost is that the lasers are potentially applicable in multiple specialities such as glaucoma, vitreoretinal surgery, plastic surgery, etc. However, modules for many of these functions are still being designed for some of the machines. At their present state of utility it is therefore difficult to justify the cost of these systems.
| Future Directions|| |
Unlike the time when ultrasonic phacoemulsification was developed, when the only other alternative was intracapsular cataract extraction, laserphaco is being compared with a procedure which is itself highly successful. Thus to supercede ultrasonic phaco-emulsification, laser cataract surgery has to offer significantly better results and be able to remove the lens in a manner that ultrasonic phaco cannot. As has been discussed in the earlier part of this article, laserphaco has the potential to do just this. However, we have yet to progress to the stage when we can answer in the affirmative, that perpetual, distressing question "Will you be using the laser to remove my cataract?". This is because considerable improvements in the present state of laser cataract removal technology are awaited. However, if indeed these changes do occur, they will still not suffice. Simultaneous developments in polymer technology with the availablity of an injectable, inert polymer that mimics the human lens will be the perfect partner for laserphaco surgery through a mini capsulotomy. If this combination permits accomodation, it will be a true improvement rather than being just an alternative to ultrasonic phaco-emulsification and foldable IOL implantation. As in the case of this last mentioned duo, it seems very likely that once again, advances occurring in complementary pairs will be required to compel surgeons to make the transition from their existing technique. The recent progress seen in laser cataract surgery and polymer technology provides hope that history might well be repeating itself.
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[Figure - 1], [Figure - 2], [Figure - 3]
[Table - 1], [Table - 2], [Table - 3], [Table - 4]