Since the introduction of the laser in the 1960s, the use of lasers in ophthalmology has become increasingly widespread. In the late 1980s, Puliafito et al. first reported a laboratory report on the application of diode lasers for retinal photocoagulation in rabbit eyes, followed by a gradual expansion of diode laser studies. in 1990, Jacobson first performed a laboratory study of diode laser iridotomy in rabbit eyes and also applied it clinically. In 1990, Jacobson first made a laboratory study of diode laser iridotomy in rabbit eyes, and also applied it to clinical practice. At the same time, McHugh et al. began clinical studies of diode laser trabeculoplasty. Schuman et al. and Peyman et al. began laboratory studies of ciliary photocoagulation in rabbit eyes. In recent years, reports of laboratory and clinical studies have emerged, and the clinical results have gradually been confirmed.
I. Introduction to diode laser
The diode laser emits radiant light with a high electro-optical efficiency (50%) and requires only a standard current output to achieve the clinically required energy level. The diode laser device is made of n-type and p-type semiconductor coupling, the laser working material is solid state gallium aluminum arsenide (GaAlAs), the emitted light has coherence and monochromatic, but compared to other lasers are more disperse, its wavelength is 780nm ~ 850nm. 810nm wavelength is commonly used in clinical practice, is a kind of near-infrared light, this wavelength of radiation light is easily conducted by the eye medium. This wavelength is easily conducted by the medium . Diode laser structure is simple , cheap , small size , easy to carry , only standard current output , air cooling , no external circulation cooling system . Its clinical application is wide, not only for glaucoma treatment, but also for retinal choroidal diseases. Whether it is transpupillary, transscleral or intraocular for retinal, choroidal and ciliary body photocoagulation, or laser iridotomy, laser trabeculoplasty, trabeculectomy combined with mitomycin post-operative laser suture release, etc., all have shown promising results.
II. Clinical applications of glaucoma surgery
(A) Ciliary body photocoagulation (Cyclophotocoagulation)
1.Transscleral cyclophotocoagulation (TSCPC)
(1) Diode laser related optical characteristics and biological effects
The application of TSCPC depends on the strong penetrating power of the light wave through the sclera and the high absorption rate of the ciliary pigment tissue. The wavelength of diode laser is 780nm~850nm, which is near infrared light. Its scleral penetration is strong (35%), second only to the 1064nm Nd:YAG laser (1.0:1.5), and hardly absorbed by the sclera (6%). When in contact with the sclera, its scleral penetration increases by 100%, while the Nd:YAG laser increases by only 50%. The diode laser conducts with a positive narrow angle distribution, and its pigment absorption is three times higher than at 1064 nm. These characteristics make it one of the best choices for ciliary body photocoagulation, and the contact method is more effective than the non-contact method.
The effect of the diode laser on the ciliary body is expressed as a thermal effect. Laboratory studies in rabbit and human eyes have shown uniform whitening of the ciliary process, wrinkling and pigment dispersion under magnification after scleral coagulation; histological examination revealed coagulative necrosis and destruction of epithelial cells in the pigmented ciliary epithelium, nonpigmented ciliary epithelium, and stroma with separation of the ciliary epithelium from the stroma. The contact method has a more pronounced thermal damage effect and deeper damage sites than the non-contact method.
Most authors found similar histological changes with the diode laser compared to the Nd:YAG laser, except for the lower energy threshold used with the diode laser, which is thought to be related to the higher absorption rate of the diode laser pigment . Some authors also found that the extent of tissue damage differed between the two lasers. simmons et al. and Monsour et al. found that the diode laser caused coagulative necrosis of the stromal layer (including ciliary blood vessels) and ciliary muscle, whereas the Nd:YAG laser only caused coagulative necrosis and destruction of the ciliary epithelium, with little response from the ciliary muscle. At the same energy level, 97% of the diode laser group showed ciliary photocoagulation, while only 78% of the Nd:YAG laser group showed a reaction. The shape and size of the photocoagulation spots were slightly different.
(2) Clinical applications
a. Contact transscleral ciliary photocoagulation
Gaasterland et al. used the IRIS Oculight SLX Diode laser system with a G-fiber probe base shaped to fit the curvature of the eye, allowing the beam to be accurately positioned 1.2 mm beyond the corneoscleral rim and parallel to the visual axis. The laser energy is adjusted during operation according to the blast sound of the tissue response. The general energy used is 1.75W (1.5W~2W) for 2 seconds, and the irradiation range is 270~360 , with a total of 17~19 points. The short-term (6 weeks) and long-term (2 years) observation of treatment-refractory glaucoma showed that the success rate of IOP reduction was 52%-77%, and the long-term effect was slightly worse than the recent effect. 70% had visual acuity improvement.
Brancato et al. used another fiberoptic probe (EOS 3000, Optikon, Italy) with a rounded tip and a diameter of 3 mm, the beam of which lies exactly 1.5 mm outside the scleral edge when the edge of the probe is tangential to the scleral edge. It acts on the ciliary body without damaging the surrounding tissue. The energy used is 2.6W, the time is 1.5 seconds to 2.5 seconds, the spot diameter is 500μm, the irradiation range is 360, and the total number of points is 16 to 20. The success rate of IOP lowering is 70%~76%, pain relief 100%, and no vision loss in more than 1~1.5 years of follow-up for refractory glaucoma.
b. Non-contact transscleral ciliary photocoagulation
Hennis et al. and Hawkins et al. used a Microlase laser system (Keeler Corp, Broomall, Pa.) with a slit lamp as an auxiliary device, without contact lenses, to focus the laser beam parallel to the visual axis, starting at the scleral surface 1 mm beyond the corneoscleral rim and then scattering 1 mm deeper into the sclera with an energy of 1.2 W and a time of 1 second. The spot diameter is 100 μm and the irradiation range is 360, with a total of 40~45 points. The success rate of lowering IOP in refractory glaucoma was 71.4% for six months and 56% for one year, and most patients had pain relief.
Kida et al. used a contact lens with a laser focus 0.5 mm beyond the corneoscleral rim and 3 mm deep on the conjunctival surface. The energy was 1.5 W for 1 second, the spot diameter was 400 μm, and the irradiation range was 360. The average IOP decreased from 51.8 mmHg to 28.5 mmHg in the 8 eyes during 1 month of observation, and the average number of surgical procedures was 1~4 times.
(3) Surgical complications
Both contact and non-contact methods have shown that diode laser ciliary body photocoagulation surgery has few complications, is safe, effective, and repeatable. However, some complications exist.
a. Conjunctival burns: The vast majority of people develop conjunctival surface burns , the extent of which occurs in relation to conjunctival pigment changes, which are mostly not visible after one day and all disappear after one week.
b. Uveitis: most show mild inflammation or none, a few (19%) have more pronounced inflammation, which disappears with corticosteroid
It disappeared after treatment with corticosteroids.
c. Pain : Intraoperative pain was relieved with post-ball anesthesia, and some had mild pain or discomfort after surgery (22%-48%), which disappeared within a week.
d. Vision loss : Most people’s vision remained unchanged or improved, a few had vision loss (16%~23%) due to macular cystic degeneration, or uncontrolled intraocular pressure and cataract development, and a very small number of people with vision loss had manual or light perception before surgery.
e. Low intraocular pressure: the incidence was 3%~5%, and no one case of ocular atrophy occurred in all reports.
f. Other: One case was reported to have scleral perforation after surgery due to aphakic eye with thin sclera. There were no other complications such as bleeding.
(4) Surgical related influencing factors
The surgical effect of diode laser TSCPC is certain, but due to the influence of various factors, the response of each individual varies, and the influencing factors are
a. Energy size: the minimum energy to cause therapeutic tissue damage is 2.7 J. Below 2.7 J, the treatment effect is poor; above 6 J, it can cause the destruction of normal tissue. Some authors also believe that the minimum energy is 2.25J.
b. Spot size: Hennis et al. observed from laboratory studies in human eyes that the size of the spot does not affect the quality of tissue damage, and it is possible that the increase in spot size from 100 μm to 500 μm does not reduce the energy density of the laser beam, but only changes the position in the ciliary body, and this change is less than 0.5 mm. In contrast, Shepps’ study in rabbit eyes found that the damage to tissue was more pronounced with a 400-μm spot than with a 600-μm spot.
Brancato et al. observed that the maximum threshold for diode laser exposure time was 1.5 to 1.7 times that of the Nd:YAG laser. At moderate (5.2 J to 6.6 J) and high (above 7.8 J) energy levels, ciliary thermal damage with the diode laser appears to be more extensive than that with the Nd:YAG laser.
d. Focal position: Hennis et al. found that with the non-contact method, if the focal point was 2 mm outside the corneoscleral rim, only the flattened part of the ciliary body was damaged, not the ciliary process. A distance of 0.5 mm to 1 mm is required to produce damage to the ciliary process (0.5 mm is better). A focal point scattered 1 mm deeper into the sclera is more effective than a focus on the scleral surface or a scatter of 2 mm. Ultrasound localization shows that the depth of focus is effective within 2 mm in front of and behind the ciliary body. Schuman et al. used the contact method to cause damage to the iris root when the focus is 0.5 mm outside the corneoscleral rim; 1.0 mm ~ 1.25 mm apart can cause damage to the ciliary epithelium and stroma; 1.5 mm apart, only the epithelium of the flattened part of the ciliary body is damaged without damage to the ciliary process.
e. Direction of irradiation: When the laser probe is oriented parallel to the visual axis, it acts on the anterior part of the ciliary body flat and the posterior part of the ciliary body crown; when the probe is oriented perpendicular to the sclera, it acts close to the middle of the ciliary body crown and may damage the equatorial part of the lens.
f. Scleral thickness: The ratio of the minimum energy threshold to cause a thin sclera to normal scleral ciliary response is 1:1.2 (2.9:3.5 J). The lower energy required for the contact method and the increased penetration of the laser compared to the non-contact method may be related to the thinning of the sclera by the pressure applied by the probe. The thinning of the sclera and the shortening of the distance between collagen fibers can reduce the interference from light scattering between collagen fibers, thus increasing light transmission.
g. Structural variation: Uram has observed endoscopic ciliary fluorescence imaging in patients who failed to undergo transscleral ciliary photocoagulation and found that in 20 eyes with 360 photocoagulation, none of the ciliary disruptions exceeded 120, with 9/20 eyes having disruptions of 90-120 and 11/20 eyes having less than 90. It is thought that this may be related to the natural anatomical diversity of the ciliary body or secondary variation in ciliary body structure due to previous surgery.
h. Other: the difference in laser absorption in the ciliary body of each patient; the influence of the corneoscleral rim conjunctiva and scleral pigment; the error in judging the size of laser energy based on tissue bursting sound; the type of glaucoma (neovascular glaucoma and congenital glaucoma have poor results); the possibility of pathological repair and ciliary protrusion regeneration to recreate the secretory function, etc. can affect the effect of transscleral ciliary photocoagulation.
2. Endoscopic cyclophotocoagulation (ECP)
ECP is a new technique recently developed with a laser microendoscope (Endo Optike Inc, Little silver, NJ) in two sizes: an 18-type endoscope (field of view 110, focal length 2mm) and a 20-type endoscope (field of view 75, focal length 1mm). The procedure can be viewed on a monitoring screen while the laser energy is delivered in a controlled manner to a precise ciliary process for photocoagulation. Because the procedure allows photocoagulation of the ciliary epithelium under direct vision, accurate positioning, and no damage to adjacent tissues, it should be the best surgical method, but the high technical requirements of the procedure require specialized training, and the expensive surgical equipment limits the development of the procedure.
(1) Surgical methods
a. Combined crystalline vitrectomy: After the crystalline vitrectomy is completed, a laser microscope is inserted in one of the incisions (if a crystalline vitrectomy has been performed, only one incision is made). Photocoagulation is performed when the tip of the probe is 0.5mm ~ 0.6mm away from the ciliary process to see the ciliary process clearly. The usual energy level is 0.2W ~ 0.3W and the exposure time is 1 second ~ 2 seconds. Two photocoagulation points are made for each ciliary process, and each photocoagulation point shows ciliary process wrinkling and whitening, without pigment dispersion or vacuole formation. The photocoagulation range is 90-180.
b. Combined cataract ultrasound-emulsification IOL implantation: After cataract ultrasound-emulsification, a viscoelastic substance is injected into the anterior chamber to posteriorly shift the posterior capsule of the lens. The laser endoscope is inserted from the corneoscleral rim and extended between the iris and the capsule at a distance of 1.0 mm~3.0 mm from the iris. The energy is 0.2W ~ 0.5W and the exposure time is 0.5 seconds to continuous wave. Photocoagulation was performed on the anterior and posterior parts of each ciliary process, and ciliary process wrinkling and whitening were seen. The photocoagulation range exceeded 180.5 mm. If the endoscope cannot be extended in front of the capsular bag for technical reasons, it can be changed to inside the bag, which requires slightly higher energy and slightly poorer visibility.
c. Direct corneoscleral rim incision: A direct corneoscleral rim incision of 2.8 mm is made, the anterior chamber is injected with a viscoelastic substance, and the laser endoscope is passed through the pupil into the iris until the ciliary process is seen on the monitoring screen. The energy was 0.8 W and the exposure time was controlled by a foot switch for less than 1 second. The energy of each ciliary process is within 0.8 J and ranges from 120 to 210, with a total of 18 to 45 points.
(2) Surgical results
Several entry routes have shown good IOP-lowering effect of ECP, with no significant visual acuity loss.
(3) Surgical complications
No serious surgical complications were reported, the most common being transient inflammatory reaction and vitreous hemorrhage. There was no loss of vision due to surgical complications, and no eye atrophy was found; no cataract development was found in crystalline eyes; no foraminal retinal detachment due to internal eye surgery Sawtooth margin detachment Severe vitreous hemorrhage, etc. However, due to the small number of surgical cases and the short observation period, it is not yet possible to conclude the riskiness of the procedure.
(ii) Laser trabeculoplasty (LTP)
The treatment of glaucoma by LTP was first reported by Wise and Witter. The commonly used laser is the blue-green argon ion laser (488nm-514.5nm). Krypton red laser (647 nm) has also been reported (43) and Nd:YAG laser (1064 nm). The IOP-lowering effects of these lasers are the same. The mechanism of action of LTP is not yet clear, but it is believed that the collagen fibers of the trabecular meshwork are pulled to widen the intertrabecular space and open the Schlemm canal. Another theory is that the degeneration of the trabecular fibers and the cell division, followed by the widening of the intertrabecular space, are the cause of the effect. It has also been suggested that this is a biochemical effect, and it has been hypothesized that the Schwalbe wire cells produce a phospholipid that increases the drainage of atrial fluid through the trabecular meshwork. Diode laser trabeculoplasty (DLT) has recently been investigated.
1. Laboratory studies
McHugh et al. conducted electron microscopic scanning studies on the diode laser and argon ion laser photocoagulation points in human eyes and found that the tissue damage of both lasers was the same, manifesting as shrinkage and swelling of the trabeculae and destruction of tissue structure at high energy. McMillan et al. found that the diode laser had an energy parameter of 0.4W ~ 1.2W, time 0.1 sec ~ 0.2 sec, and 100 μm spot diameter, while the argon laser had an energy parameter of 0.5W ~ 1.0W, time 0.1 sec, and 50 μm spot diameter. The diode laser showed little tissue response, while the argon laser showed whitening and pigment dispersion, trabecular fragmentation and fusion. Electron microscopy revealed that the trabecular fusion ring was observed only at diode laser energy greater than or equal to 0.12 J, while the argon laser required only 0.05 J energy. The authors suggest that this may be due to the lower pigment absorption and scleral refractive index of the diode laser compared to the argon ion laser 4Ë in addition, the minimum spot diameter of the diode laser is 100 μm, while that of the argon ion laser is 50 μm, and the former must require more energy per unit area than the latter. The authors also found that the damage sites of the two lasers were the opposite of what McHugh et al.(50) found, with the damage sites of the diode laser being limited to superficial layers and the damage sites of the argon ion laser being deeper.
2. Clinical studies
All clinical studies have shown that DLT and Argon laser trabeculoplasty (ALT) have similar IOP lowering effects, but DLT requires more energy than ALT (1.1W-1.2W for DLT and 0.6W-0.75W for ALT). In the early stage of treatment, the magnitude of IOP reduction seems to be slightly greater with DLT than with ALT. Operation: The laser targets the pigmented part of the trabecular network. The energy is adjusted so that the trabeculae are mildly whitened. The spot diameter is 100 μm, the exposure time is 0.1 to 0.2 seconds, the range is 180, and 20 to 25 points per quadrant. Surgical complications: The postoperative reaction to DLT is mild, with a slight atrial water flash within 1-2 hours after surgery, which does not require treatment. There is no significant change in visual acuity. No postoperative Peripheral anterior synechiae (PAS), ocular pain, transient IOP elevation, etc. were found. A comparative study of DLT and ALT showed that the anterior chamber flash value of ALT was significantly higher than that of DLT 1 hour after surgery, and the difference was not significant after 1 week. This indicates that the blood-atrial water barrier is less disrupted in DLT than in ALT. The study also found that 7/21 eyes (33%) in the ALT group had ocular pain, but not in the DLT group; at 8 weeks postoperatively, 4/21 eyes (19%) in the ALT group had PAS, but not in the DLT group. The authors suggest that this is related to the difference in the pigment absorption rate of the two lasers and the depth of tissue damage.
(iii) Laser peripheral iridotomy
Laser iridotomy has been tried since the beginning of the diode laser in ophthalmology. The strong penetration of the diode laser into the iris stroma and the strong absorption of the diode laser by the iris pigment epithelium make this laser particularly suitable for iridotomy. The mechanism of action is similar to that of the argon laser, both being laser thermal effects. The procedure is the same as with the argon laser: a circular photocoagulation of the dark iris (optimal energy parameters: 0.2 W, 0.2 s exposure time, 200 μm spot diameter), followed by a penetrating shot in the center of the ring (optimal energy parameters: 1.0 W, 0.05 s, 75 μm spot diameter). The effect of ring photocoagulation is to thin the iris to a bulging surface and to deepen the anterior chamber at the photocoagulation site to facilitate the next penetrating shot and to avoid endothelial damage. The choice of high energy, short duration, and small spot size makes penetration easier and reduces the incidence of complications. For up to 9 months, 100% of the iris holes remain open. Postoperative complications such as corneal endothelial damage are less than argon laser, transient lens clouding and pupillary distortion are more frequent than argon laser, and other complications such as transient high intraocular pressure and inflammatory reaction are similar to argon laser.
(iv) Laser sclerectomy
Diode laser sclerectomy has been studied in animal laboratories by Karp et al. (56)(57), but no clinical reports are available. Karp et al. (56) divided 25 New Zealand rabbits into three groups, and performed laser sclerectomy and conventional full sclerectomy via the external (conjunctival incision to the corneoscleral rim) and internal (anterior chamber puncture to the anterior chamber angle) respectively. The diode laser energy was 2.5W for 4 seconds. The effect of IOP reduction was similar in the three groups, but the duration was the longest in the group with external laser surgery. Histological examination revealed that the laser treatment resulted in significantly less subconjunctival scarring and less inflammatory response than conventional filtration. In a further study, Karp et al. divided New Zealand rabbits into two groups and performed diode laser transconjunctival sclerectomy alone and mitomycin C (MMC) as a control. MMC was placed on the conjunctival surface at a concentration of 0.5 mg/ml for 7 minutes, followed by a transconjunctival sclerectomy. A 2 mm conjunctival incision is then made 7 mm from the scleral limbus and a laser probe is inserted into the scleral limbus, which is struck parallel to the iris until it penetrates into the anterior chamber. The energy level is 2.5W for 4 seconds. After the probe enters the anterior chamber, an atrial angioplasty is performed with the probe in contact with the iris at an energy of 2.5W for about 1 second at three points. The IOP reduction rate was similar in the two groups (63% in the MMC group and 60% in the non-MMC group), but the duration of IOP reduction was significantly longer in the MMC group than in the non-MMC group, demonstrating that external diode laser sclerectomy is feasible and the use of combined MMC can effectively improve the success rate of filtration surgery.
(E) Laser suture lysis
At present, trabeculectomy combined with MMC is used when the scleral flap is closed, and laser suture lysis of the scleral flap can be used to remove the tight sutures to enhance filtration when the IOP rises after surgery. The Argon ion laser is generally used. Recently, a diode laser has also been used for suture release using a Mandelkorn contact lens with a 75-μm spot diameter diode laser at an energy of 1 W for 0.1 to 0.2 seconds. The focus is on the nylon thread of the scleral flap. Clinical observations showed that the diode laser did not produce thermal damage to the thin MMC follicles similar to that caused by the argon laser, but postoperative discomfort was more pronounced than after the argon laser. Laboratory reports indicate that the diode laser causes shallower and less extensive damage to the conjunctival tissue than the argon laser. The least damage to the conjunctival tissue is caused by the 585 nm and 610 nm wavelengths. Several wavelengths have no damage to the sclera and conjunctival epithelium.
Third, the development prospects
In summary, the diode laser is a near-infrared light, the effect on the tissue is thermal, with strong penetrating power, simple structure, small size and low price, high efficiency, air cooling, no high current and external cooling water. It can be used in special environments such as operating rooms and can be carried to remote mountainous areas, and has practical and effective application value. The diode laser has been used as a pump source for continuous-wave dual-frequency Nd:YAG laser, emitting 532 nm wavelength light, and has been used in animal trials for laser peripheral iridotomy, with positive results. In the future, it is expected that new materials will be used to replace argon ion lasers, krypton red lasers and dye lasers with useful diode outputs up to visible wavelengths. Diode lasers will play a greater role in the clinical application of ophthalmology.