Lasers in small-incision cataract surgery

Introduction

When Kelman1 first introduced the concept of ultrasound phacoemulsification, a radically different technique at the time, few could envision the smallincision cataract surgery revolution that would then begin. The development of small-incision cataract surgery stimulated the development of new lens technology (the foldable intraocular lens (IOL)), and the development of the newer lens technology, in turn, spurred greater acceptance of the phaco-emul- sification technique. With the greater refinement of surgical techniques, clear corneal phacoemulsification has become the procedure of choice among cataract surgeons.2 Clear corneal cataract phacoemulsification has the advantage of enhancing patient outcomes by decreasing wound complications, minimizing induced astigmatism, and enabling quicker patient rehabilitation.

However, phacoemulsification is associated with unique risks not seen in other types of cataract surgery. There are issues of safety related to the release of energy at the tip of the probe, and concerns regarding inadvertent effects of this energy upon nontarget tissues, such as the cornea, iris, and posterior capsule. The excessive heat that the phacoemulsification probe tip generates requires a cooling irrigation sleeve to reduce the risk of corneal burn and/ or wound distortion. The current technology necessitates a 2.2-3.2 mm incision size. Just as the development of phacoemulsification permitted the concept of small-incision cataract surgery, the development of newer technology and techniques may foster the growth of micro-incision cataract surgery, which can overcome current technological limitations.

The history of lasers in cataract surgery

The first reported laser cataract removal was described in Krasnov in 1975, and consisted of phacopuncture.3 With this technique, a Q-switched ruby laser was used to create microperforations in the anterior capsule, which allowed for the gradual release and absorption of the lens material over time. This technique was very limited, as it was only effective for very soft cataracts (such as congenital cataracts) and patients required chronic treatment with both dilating drops (to prevent the closure of the puncture sites) and chronic steroids (to limit the uveitic response).

In the following years, interest shifted to four different ultraviolet wavelengths: argon fluoride (193 nm), krypton fluoride (248 nm), xenon chloride (308 nm), and xenon fluoride (351 nm).4-6 Of these wavelengths, xenon chloride held the most promise, as it had superior transmissibility through fiber optics and maintained a favorable ablation profile for endocapsular ablation: the threshold for ablation of the lens nuclei and cortex was significantly lower than that of the lens capsule.4 How ever, despite this early promise, concerns remained regarding possible side-effects, especially regarding its cataractogenic, carcinogenic, and retinotoxic ef- fects.7-10 These persistent questions directed further research towards the area of the infrared wavelengths, the Er:YAG11-14 (erbium:yt-trium-alumi- num-garnet) and the Nd:YAG15-17 (neo-dymium: yttrium-aluminum-garnet) lasers.

By the early 1980s, the Nd:YAG laser (1064 nm) had been described for posterior capsulotomy in pseudophakic patients,18-20 peripheral iridoto- my,20-22 and lysing of the pupillary membranes,20-23 and anterior capsulotomy.24 Anterior capsulotomy

prior to cataract surgery never gained widespread popularity due to the problems with inflammation, rises in intraocular pressure (IOP), and poor mydriasis post-laser, all of which necessitated the surgery to be performed promptly post-laser and also converted a one-step procedure into a two-step

one.25,26

The next major development in the use of cataracts in laser surgery was laser phacofragmenta- tion.27-31 With this technique, an Nd:YAG laser was used to photodisrupt and soften the lens nucleus prior to phacoemulsification. Although this procedure had the advantages of shorter phaco time and decreased phaco power, it carried the increased risk of inadvertent perforations of the anterior and posterior capsules, as well as having the disadvantage of converting a one-stage operation into two-stages.

Er:YAG systems

Another attractive area of research of lasers in cataract surgery has been the Er:YAG laser. This laser, first investigated by Peyman and Katoh13 and Tsubota,14 also has the advantages of lower energy requirements and lack of heat production.32 It works by being focused directly on to the lens nucleus, and the ensuing optical breakdown causes microfractures of the lens material. An additional advantage of the Er:YAG system is that its wavelength (2940 nm) corresponds to the maximum peak of water absorption which leads to minimal penetration (approximately 1 µm). Thus, the surrounding water absorbs the excess energy without dispersion to nearby non-target tissues. Higher pulse frequencies can lead to the creation of longitudinal chains of cavitation bubbles forming at the tip of the probe, according to Lin.33 These cavitation bubbles have the advantage of allowing the laser energy to travel farther and increase the depth of penetration than if fired in water (thus emulsifying denser nuclei), but have the disadvantage of risking damage to nearby adjacent non-target tissues.

Phacolase MCL-29 (Asclepion-Meditec)

The MCL-20, an Er:YAG-based laser system using a zirconium-fluoride based optical fiber, has been approved for cataract surgery in Europe since 1998. Due to toxic properties of the material when degraded, it needs to be coupled to a nontoxic silica tip.34 In addition, the far end of the fiber within the laser handpiece is made of quartz, which minimally attenuates the energy,35 and must be replaced after every four to five surgeries. As this is a unimanual system (coupling both infusion and aspiration), the MCL-29 handpiece requires a 3.2-mm incision.

In a study of 40 cases using the MCL-29, Hoh and Fischer36 found that four of the cases (10%) could not be completed with the laser and required

conversion to an alternate type of surgery. This conversion led to an increase in the surgical time in three of the cases, but a rupture of the posterior capsule in only one. The mean reported laser time was three minutes, and the average total energy was 38.5 J. Endothelial cell count decline was only 0.96%, well below that of phacoemulsification.

In an investigation of 35 cataracts treated with the MCL-29 Er:YAG laser system, Francini and Galarati37 found the average laser time to be nine minutes 23 seconds, with a total average energy of 62.74 J. Of note, the cataracts ranged in density from 1-4+ in density, and two additional cases of dense lenses required conversion to phacoemulsification and were excluded from the study results. At six months’ follow-up, they found the average endothelial cell loss to be 5.15%, with the average endothelial cell loss with phacoemulsification averaging 6.19%. Interestingly, on the first postoperative day corneal pachymetry was increased only 6.1%, as opposed to that of the average phacoemulsification patient, whose pachymetry increased 9.101%.

The phacolase MCL-29 is based on a peristaltic system with a Venturi-like effect (the Megatron irrigation/aspiration pump).38 The phacolase MCL29 delivers pulse energy of 5-50 mJ at a frequency of 10-100 Hz. Each pulse duration lasts for 200 msec. The surgeon can work in one of two modes: the energy mode (in which the fixed frequency allows for variable energy delivery) or the frequency mode (in which the fixed energy per pulse permits variability of the frequency of the pulses).

More recently, Francini35 has been conducting a prospective study using the Phacolase MCL-29 with a bimanual technique, separating the laser/aspiration probe from the irrigation probe (resulting in two paracentesis wounds that range in size from 1.0-1.5 mm). The average energy used was 31.1865 J (the nuclear density ranged from 1-4+) and the average laser time was four minutes 48 seconds. Endothelial cell loss one year postoperatively was 2.123%. Corneal pachymetry performed on the first postoperative day revealed an increase in thickness of 5.82%, which is comparable to that reported with the unimanual technique.47

Current Nd:YAG laser systems

Photon (Paradigm Medical Industries)

The Paradigm system consists of an Nd:YAG system coupled with a conventional ultrasound phacoemulsification system. This is a uni-manual unit, in which the irrigation/aspiration system is combined with the laser probe into one handpiece. The probe tip has a diameter that ranges from 1.2-1.7 mm and passes through a 3.0-3.5 mm incision. The unit is based on a peristaltic system with up to 500 mmHg vacuum, and currently uses a repetition rate of 10-

50 Hz. The fluidics of this system are reported to allow for surge control at all vacuum levels up to 500 mmHg. Over 100 cases have been performed with this system to date, and the results demonstrate ‘quieter’ postoperative eyes compared with phacoemulsification. The reported endothelial cell loss is 7.6% at three months’ postoperatively.

Dodick photolysis (ARC lasers)

The Dodick photolysis system, currently the only laser approved for cataract extraction by the Food and Drug Administration, transfers the laser energy in a Q-switched mode from the laser source to a laser probe via a 300-µm quartz-clad fiber optic (Fig. 1). The laser probe produces no significant heat,39 and thus requires no irrigation-cooling sleeve, unlike phacoemulsification, which generates heat by the transformation of electrical energy into mechanical energy with the subsequent generation of emulsifying shock waves.1 The lack of heat production with Dodick photolysis offers two clear advantages: it eliminates the risk of corneal burn, which can lead to wound dehiscence, astigmatism, and endothelial cell compromise; and, it allows for the separation of the irrigation port from the emulsifying needle, which permits a reduced diameter of the separated handpieces and allows for smaller wound sizes.

The combination laser/aspiration handpiece (Fig. 2), which is similar to the aspiration probe in a set of standard bimanual irrigation/aspiration hand-

Fig. 1. The Dodick photolysis unit.

Fig. 2. The Dodick photolysis laser/aspiration probe for ablation and aspiration of cataract material.

pieces, transmits the laser beam to a titanium plate within the tip of the probe (Fig. 3). The titanium target allows plasma formation to occur at a much less lower energy level, since it permits the optical breakdown of the laser energy to occur at a greatly reduced threshold. The plasma formation causes shock waves to be formed from the tip of the laser probe, which can then emulsify and aspirate the cataractous material (Fig. 4).40 The titanium target also has the advantage of shielding non-target tissues from direct laser light, such as endothelium, retina, and the surgeon’s eyes.41,42

The Dodick photolysis system is a Venturi-pump system that generates aspiration with a range of 0- 650 mmHg. The continuous pressurized infusion system ranges from 0-200 mmHg. The laser output can be used with a pulse rate of 1-20 Hz, and an output of 8 or 10 mJ per pulse. The current probes in use have an external diameter of 1.2 mm and an internal diameter of 0.75 mm, although smaller probe designs (0.9-mm external diameter) are currently being investigated. The current technique involves the creation of two 1.4-mm paracentesis ports, one for the irrigation infusion and the other for the combination laser/aspiration probe. The handpieces are disposable, given the relative inexpensiveness of the quartz-clad fiber and the titanium targets, and the same handpieces can be used

Fig. 4. Dodick photolysis cataract surgery. Left: the laser/ aspiration probe disrupts and removes lens material presented to it by the irrigation probe (below). Both probes currently fit through 1.4-mm incisions.

for the removal of cortical material, if so desired. The photolysis unit comes with a Venturi phacoemulsification unit that uses 2.75-mm phacoemulsification needles and either one-piece or bimanual irrigation/aspiration handpieces for cortical remo-

val. It also contains a superior high-speed vitrectomy unity that can use a one-piece or split vitrector.

Studies conducted with Dodick photolysis

Since the early 1990s, Dodick has been studying the use of the Q-switched pulsed 1064-nm Nd:YAG laser for a single-stage photolysis of cataractous lenses.17 In 1991, Dodick and Christiansen developed a method of high-speed photography to study the formation and propagation of shock waves created after the Nd:YAG laser pulses struck the titanium target. During these studies, it was successfully shown that, within 205 nsec after the laser fired, a shock wave was formed on the inside surface of the inside target, with a weaker shock wave formed on the outside surface. As the shock wave propagated towards the mouth of the aspiration port, only the attenuated shock waves radiated out of the probe. Using the high-speed photography method, they were successfully able to modify the configuration of the target to maximize shock waves within the probe while minimizing their propagation outside of the tip.

Animal studies were performed after the successful completion of studies on cadaveric lenses.15,16

Nd:YAG laser energy levels far in excess of those required for cataracts were used on one eye each of 14 New Zealand white rabbits, with the contralateral eye being used as a control.43 The rabbits’ eyes were subsequently examined using ultrasonic pachymetry, specular microscopy, and light microscopy, none of which revealed any damage to the corneal endothelium, trabecular meshwork, or retina. Using this system, the first laser ablation of a nuclear sclerotic cataract was performed in a 60-year old woman in 1991, without incident (after receipt of an Investigation Device Exemption from the FDA). The cataract required less than four minutes of laser time and, postoperatively, there was minimal inflammation and endothelial cell loss.

Subsequently, 50 patients were treated with this system, the results of which were presented at the 1999 ASCRS meeting. In the treatment of cataracts that ranged in density from 1 to 3+, an average of 4.5 J per case was used (with an average of 450 shots at 10 mJ per shot). This is significantly less than the energy produced by phacoemulsification, which has been reported as a mean of 3264 ± 1218 J for ‘divide and conquer’ phacoemulsification and 782 ± 446 J for ‘phaco-chop’.44 Other techniques have been described recently in order to further reduce the amount of energy used, such as ‘choochoo chop’ and ‘flip phacoemulsification’.45

In 1999, Kanellopoulos et al.46 published the results of a multicenter series of 100 Nd:YAG laser cataract patients, measuring outcomes by visual acuity, endothelial cell loss, change in IOP, total energy, and complications. Lens density ranged from 1 to 4+ in density. The mean visual acuity improvement was from 20/46.5 (0.43) to 20/26.6 (0.75), the average endothelial cell loss was 7.55% (similar to phacoemulsification), and there was no change in IOP. The average energy used during the cases was a total of 6.7 J. Three of the cases were associated with posterior capsule rupture, one of which occurred during photolysis, another when a 4+ cataract was converted from photolysis to phacoemulsification, and one tear took place when the intraocular implant was being placed. The conclusion of this pilot study was that Dodick photolysis resulted in comparable surgical times compared with phacoemulsification when used for 1-2+ nuclear sclerotic cataracts.

In the subsequent multicenter study of 1000 cases, Kanellopoulos et al.36 measured photolysis patients in terms of improvement in visual acuity, total energy, mean operative time, and intraand postoperative complications. The mean visual acuity improvement was from 20/70.2 to 20/24.4, and the mean energy used was 5.65 J per case. Average photolysis time was comparable to phacoemulsification times for the cataracts from 1-2+ in density, but the average time for 3+ lenses was 9.8 minutes. There were 16 cases of capsular ruptures and two of intraoperative hyphemae. In the postoperative period, there was one case of cystoid macular ede-

ma (partial anterior vitrectomy with sulcus IOL), one of pseudophakic bullous keratopathy (PAV/sulcus IOL), and one of subluxated IOL (PAV/sulcus IOL). The authors concluded that photolysis was a safe and effective alternative to phacoemulsification in softer cataracts.

In a separate clinical trial, Huetz and Eckhardt47 reported 100 cases with photolysis with a six-month follow-up. Cataracts were divided into three groups (I-III), depending on the density of nuclear sclerosis (the LOCS III system). The mean total energy used in group I was 1.97 ± 1.43 J, in group II 3.37 ± 1.59 J, and, in group III, 7.7 ± 2.09 J. There was no significant difference in pachymetry in groups I and II preor postoperatively, however, group III experienced an average increase in pachymetry to 1.84% on the second postoperative day. Of note, at six months, there was no significant difference in pachymetry from preoperative values in any of the three groups.

Surgical techniques

Much like phacoemulsification, Dodick photolysis can either remove cataracts in pieces, or ablate the lenticular cataract in toto. There are several techniques available to facilitate cataract extraction using the Dodick photolysis system.

The classic technique used for photolysis is similar to early phacoemulsification, using the creation of a ‘bowl’ prior to removal of the lens from the capsular bag. Two 1.4-mm incisions are placed within the clear cornea. After the completion of a continuous circular capsulorrhexis, hydrodissection of the lens nucleus is performed. The bullet-shaped irrigation handpiece and the laser/aspiration handpiece are placed on the central anterior portion of the cataract. While using the irrigation handpiece to keep the nucleus down in the bag, the aspiration/ laser handpiece touches down to ablate pieces of the cataract, lifting off periodically to allow clearing of the port. Only once the central plate of the cataract is ablated, and the nucleus is substantially debulked and the residual shell aspirated into the anterior chamber. This technique has several advantages over other techniques, the most significant of which is that it does not require the development of several other skills while mastering photolysis. It also allows the surgeon to continue to work centrally for the bulk of the ablation, as well as to utilize the posterior capsule to provide mild counter-trac- tion to the laser and keep the nucleus against the probe. The major disadvantage of this technique is that it is slightly slower than other techniques, and the beginning surgeon must be cautioned to resist the temptation to lift the nucleus out of the capsular bag prior to its being de-bulked.

As with all other techniques, the Dodick technique involves the creation of two 1.4-mm incisions through which the cataract extraction is performed. After the creation of a continuous circular capsulor-

rhexis and hydrodissection of the lens nucleus, the lens is pre-chopped into two hemispheres using two Dodick-Kammen choppers in a horizontal chopping technique. One or both of the residual hemispheres is then re-chopped into quarters. The quadrants are then photolysed using the separate aspiration-laser and irrigation handpieces. The advantages of this technique are that the lens is broken into pieces early in the case, less energy is required (this is also true of pre-chopping using phacoemulsification), and a smaller capsulorrhexis may be used (which may provide for better centration of the IOL). Disadvantages of this technique involve the learning curve necessary for mastering this skill.

The Wehener technique necessitates the creation of a large capsulorrhexis to allow the entire nucleus to be lifted into the anterior chamber, using high aspiration with the port turned downwards towards the nucleus (Fig. 5). After the nucleus has been raised out of the capsular bag with the laser/aspiration handpiece, the irrigation handpiece is placed beneath the nucleus with the irrigation probe directed posteriorly towards the posterior capsule. The nucleus is then brought down and ‘back-cracked’ over the irrigation handpiece (Fig. 6). The residual pieces are then photolysed or further lifted and ‘back-cracked’ into quarters. In order to facilitate this process, a specialized irrigation handpiece, called a Wehener spoon, can be utilized. The Wehener spoon has a pointed end, which is angled upwards towards the nucleus. A possible advantage of this technique is the potential protection of the posterior capsule as the irrigation port is directed downwards towards it and away from the aspiration/ laser handpiece. Potential disadvantages include the difficulty in passing the pointed, angled Wehener spoon into the eye, the need for a large capsulorrhexis, and the requirement for an ability to safely ‘back-crack’.

Just as phacoemulsification requires the occasional conversion to conventional extracapsular cataract extraction techniques, photolysis may require the conversion to phacoemulsification. As with conversion to extracapsular cataract extraction, how often this is required depends on experience with appropriate patient selection, the experience and expertise of the surgeon, and factors endogenous to the eye. Conversion to phacoemulsification is relatively easy, and does not appear to pose any greater risks to the eye than the initial use of phacoemulsification without conversion. After the handpieces have been removed from the eye, viscoelastic is used prior to the enlargement of one of the incisions with a 2.75-mm blade. Lens phacoemulsification is then performed in the conventional fashion.

Intraocular lenses

To date, none of the IOLs that can be inserted through a 1.4-mm incision have yet been FDA ap-

Fig. 5. Using aspiration only, the photolysis probe has elevated the nucleus in toto out of the capsular bag (in blue).

Fig. 6. After the nucleus has been successfully elevated from the capsuar bag, the irrigation probe is slipped between the posterior surface of the lens and the capsular bag. Its tip is used to pierce the posterior surface of the lens. Please note that the irrigation is directed towards the capsular bag while the reflecting surface faces the nucleus.

Fig. 7. With the irrigation probe posterior to the lens and the photolysis probe remaining on aspiration, the lens has been back-cracked into hemispheres.

Fig. 8. The right hemisphere of the bisected nucleus is rotated 90 degrees, to that the cut edge faces the cornea. The irrigating spoon rests beneath this hemisphere, supporting the hemisphere. Please note that the photolysis wave is now being propagated parallel to the nuclear fibers, which enhances its efficiency.

proved. Kanellopoulos et al.48 reported implanting a dehydrated prefolded Acritec acrylic lens (model H44-1C-1, Acritec, Berlin, Germany) in three cases in which he used the Dodick photolysis system. The synthetic lens was prefolded to an external diameter of 1.2-1.3 mm and implanted through the original photolysis incision. The lens fully unfolded within the capsular bag within 30 minutes postoperatively. This lens achieves it small size through dehydration, but unfortunately requires some time to unfold. Of note, the entire case was completed through incisions no larger than 1.3 mm.

Another lens that has been implanted in Europe is the ThinOptX ultra-thin lens. This plate lens design passes easily through a 1-mm incision and unfolds within the eye to a total length of 11.2 mm. This lens technology can be used to create a lens that is either hydrophobic (acrylic) or hydrophilic (current models). The current A constant is 118.94. The lens is designed to use a series of refractive rings concentric to the optical center. These rings result in one focal point, which is what distinguishes this lens from a Fresnel prism (prismatic lens), which utilizes multiple successive focal points. The average lens has three concentric rings (not including the outside edge of the lens). It is folded in half and subsequently rolled and placed (through the 1.0- mm incision) within the capsular bag. Warm balanced saline solution is used during lens folding and curling, in order to facilitate more rapid unfurling within the capsular bag. In none of the lens implanted to date has any patient complained of glare or halos, even under low-light conditions.

Conclusions

Given that the technology is still being developed, the question of whether to be an early adopter of technology, or to wait until further refinements have been made, will largely depend on the style of the surgeon. Early adopters of technology have the advantage of developing familiarity with machines, fluidics, and techniques before they become the expected knowledge of the average surgeon, as well as being able to meet the ever-increasing demands of patients for the latest technologies. Disadvantages of adopting techniques early involve the utilization of technology that may not yet be fully perfected.

When Kelman first pioneered the concept of phacoemulsification, the small-incision surgical revolution had yet to occur. Without the development of IOL advances to take advantage of the new technology, the scope of the transformation that would take place within ophthalmology was unclear. Lasers in cataract surgery hold the promise of continuing the ongoing advance to true micro-incision surgery. The potential for greater safety with this technology is not only limited to the size of the incision, but also laser cataract surgery virtually

eliminates the risks of corneal burns, reduces endothelial cell loss, ablates smaller pieces of tissue at a time (possibly providing greater safety than phacoemulsification during the training process), and may inhibit the development of lens epithelial cells (reducing the risk of posterior capsular opacities).

Current laser systems are restricted in their treatment of dense cataracts, similarly to the early limitations of phacoemulsification technology. Given the rapid advances made in this technology to date, it is likely that we will soon overcome these boundaries. Laser technology holds the promise of introducing true micro-incision surgery, with IOLs able to fit through increasingly smaller incisions. This may be an important development in the march towards endocapsular surgery. The surgery of the future may involve a puncture on the anterior capsule with in-the-bag photolysis and implantation of a synthetic injectable lens material into the largely intact capsular bag. This may preserve accommodation, as well as eliminate posterior capsular opacities, and render current technology obsolete.

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