Features

Technology: How it works - Lasers

Ophthalmic lenses Dispensing
Dr Douglas Clarkson discusses the basics of lasers and their application within ophthalmology

Optical technologies have played a highly significant role in the development of clinical services within ophthalmology and where the key role of lasers as a mode of treatment has often been made possible by the use of lasers within diagnostic/imaging technology. It is the specific use of lasers as therapeutic tools, however, that is the specific area of application subsequently described.

Laser technology provides a fundamentally unique means of generation of light and where a typical laser resonator (something needed to build up light energy in the beam) is indicated, see figure 1. In this example, laser diodes activate a rod of Nd:YAG crystal.

Figure 1: Schematic of laser resonator

Stimulated emission of photons is favoured for photons reflected between the mirror surfaces where the mirror on the left is totally reflecting and the mirror on the right is partly transmitting, which allows the light to escape from the resonator. The light from the resonator can be considered to exist as ‘flat’ wavefronts, which allows optics to focus the beam, for example, into optical fibres of 100 microns diameter.

A ‘non-linear’ optical material (typically KTP) within the indicated resonator effectively ‘combines’ two photons of equal energy to create a photon of higher energy where the typical example is translating a wavelength of 1064nm to 532nm. The mechanism of 532nm photon generation is summarised in figure 2.

 

Figure 2: Mechanism of generation of 532nm wavelength for laser applications

Another key attribute of laser technology is the ability to deliver energy within a vast range of time scales – from continuous delivery to pulses as short as 100 femtoseconds (10-13 seconds). The combination of short pulse durations and small focal spot sizes provides the ability to achieve ultra-high power densities. In the case of the laser capsulotomy laser, a 3ns pulse of 5mJ in a volume of side 10 microns represents a power density of 1.67 1010 W/mm2.

The selection of wavelength can specifically relate to the intended target tissue. In retinal photocoagulation, a key consideration is reduced absorption by macular pigments. Medical lasers are very much at the low end of power delivery systems compared with systems for industrial applications.

Laser capsulotomy

It is modern practice in cataract surgery to leave intact the posterior capsule as a means of strengthening the vitreous barrier. In a certain percentage of cases, the posterior capsule will degrade and impair vision and which is considered to be the result of displaced lens epithelial cells1 clustering on the posterior capsule.

Rather than necessitate an additional surgical procedure, the technology was developed using Nd:YAG lasers at a wavelength of 1,064nm to break up the capsular tissue sufficiently to restore adequate vision and where the technique was first reported in the early 1980s.2 The mechanical disruptive effect is achieved by focusing a laser beam into a small volume of typical dimension 10 microns in a two to three nanoseconds (10-9 seconds) time scale, which results in plasma generation and the creation of an acoustic shock wave.

A typical individual pulse energy used is around 5 mJ (milli Joule), though this will depend on the nature of the target tissue and on extent of energy absorption along the laser beam path. It is important to target the point of focus beyond the posterior capsule surface of the lens implant to direct the energy into the capsular tissue.

This is usually undertaken by means of the ‘advancement’ setting on the specific lasers used and where values of 150 microns are typically applied. Delivery of laser energy too close the surface of the lens can result in the pitting of its surface. It is established practice to create an aperture of sufficient size to restore visual acuity but without total removal of the posterior capsule in order to retain structural integrity of the lens implant.

Over time, laser capsulotomy has been established as an ‘extended role’ of nursing staff and optometrists in the UK, with similar developments for optometrists in the USA.3 While the procedure is typically straightforward, all cases require to be undertaken within an extensive documented procedure, which takes account of contraindications, patient engagement, core guidelines, on-going clinical audit and management of
complications.

 

Laser Iridotomy

This laser treatment uses the same mode of laser treatment as laser capsulotomy in the treatment of closed angle glaucoma where the drainage pathway of aqueous humour is restricted by the aperture of the iris/cornea alignment. The process of laser iridotomy effectively opens apertures in the iris to provide additional drainage channels in order to reduce intraocular pressure.

While this treatment mode is currently implemented using pulsed Nd:YAG lasers, an early reference to the clinical technique4 identified the use of a ruby laser at a wavelength of 694.3nm where the laser pulse was triggered by activation of a flashlamp. While the procedure is identified as effective in cases of acute closed angle glaucoma, a specific study by He et al indicated on balance that widespread prophylactic laser peripheral iridotomy for primary angle-closure suspects is not recommended.5

 

Selective Laser Trabeculoplasty (SLT)

This is a mode of treatment6 appropriate for open angle glaucoma where the trabecular meshwork can be targeted by disruptive laser energy fired in the gap between the cornea and the upper surface of the iris. The typical wavelength used is 532nm and where tissue containing the pigment melanin is targeted.

This is a more specialist procedure than laser capsulotomy and is usually undertaken by ophthalmologists with targeting of the trabecular meshwork by means of a gonioscopy lens. The spot size tends to be larger at around 400 microns, with pulse energy around 1mJ.

The recent Light study7 undertaken at Moorfields Eye Hospital of comparative use of SLT and drop medication of over 500 patients within a six-year period indicated generally improved patient outcomes of the SLT group compared with the drop medication group, although compliance with prescribed medication in the drop medication group may be a variable factor.

While techniques generally adopt a process of matching the pulse energy to the observed patient response, Sacks et al propose a single energy device to simplify the procedure without affecting its relative effectiveness.8 This concept is based on evidence that similar clinical results are observed from SLT lasers within a range of delivered energies, though in the clinic environment, no measurements are made of the effective pulse width of such systems.

Often the function of SLT and Nd:YAG photo disruption can be combined in the one system, such as in the Lumenis Selecta Duet system as indicated in figure 3. Recently this system is now available as a ‘digital’ version where treatment procedures can be captured by image recording within the anterior chamber to provide an in depth patient record.

 

Figure 3: Lumenis Selecta Duet systems with combined SLT and Nd:YAG functionality. Image: Lumenis

 

Transscleral Cyclophotocoagulation

This is a laser treatment9 where energy is primarily delivered to the ciliary muscle to degrade the production of aqueous humour and hence reduce the intraocular pressure of the eye. The wavelength typically used is 810nm. Delivery of energy is via a specially designed probe as outlined in figure 4 and where the procedure is typically undertaken within a theatre session in the United Kingdom to ensure sterile operating conditions.

 

Figure 4: Iridex G-Probe for Transscleral Cyclophotocoagulation. Image: Credit Iridex

 

The specially designed probe is an excellent example of the importance of the use of a consistent method of delivery of laser energy where non-contact delivery modes would likely have considerably higher levels of variability of outcome. The procedure invariably utilises single use probes and hence is a more expensive procedure than SLT and laser iridotomy.

While, initially, the available procedure used continuous power delivery, an additional option of micropulse delivery is also available, which is considered to prevent elevated temperatures in adjacent targeted tissues. The relative merits of the two modalities – continuous and micropulse are discussed by Sanchez et al.10

NICE guidance,11 however, indicates that there is presently insufficient evidence to recommend the micropulse modality as a base-line treatment compared with the continuous mode of treatment.

 

Corneal Refractive Surgery & Femtosecond Laser Technology

A range of laser technologies have been involved in reshaping the corneal profile to create improved refraction. Initially, the process of photorefractive keratectomy (PRK)12 was used to ‘sculpture’ the corneal profile using a 193nm excimer laser after a central area of epithelial cells on the corneal surface had been removed by a debridement process.

Subsequently, laser-assisted in situ keratomileusis (Lasik) was developed where a flap of cornea cut by a microkeratome was ‘folded back’ to allow laser sculpting of the cornea using the 193nm excimer laser technology. The utilisation of ultra-short femtosecond (10-15 second) laser technology at a wavelength of around 1,060nm subsequently allowed the reshaping of the cornea by excising and removal of a ‘lenticule’ of corneal tissue as a procedure known as small incision lenticule extraction (Smile).

The ultra-short femtosecond laser technology has also been used to cut the corneal flap for Lasik and the wider uses of this technology are further discussed by Crotti et al in, for example, cataract surgery and intracorneal ring segment implantation as a treatment for keratoconus.13

In cataract surgery, the femtosecond laser technology is used to provide greater precision in specific elements of the procedure, such as incising the anterior capsule. While there are specific elements of advantage in femtosecond laser technology in cataract surgery, the further uptake of the technology14,15 is probably dependent on further refinement of the procedure.

For intracorneal ring segment implantation, there is growing confidence in the procedure to provide long-term improvement and stability in visual and topographic results, though in some younger patients the procedure may not arrest disease progression.16

Femtosecond laser technology is also being developed for new application areas rather than refinements of existing techniques. A treatment modality of femtosecond laser image-guided high-precision trabeculotomy (Flight)17 has been developed in the USA by Vialase where laser energy is directed using OCT image technology and which is currently undergoing clinical trials.

A new treatment modality of laser-induced refractive index change (Liric)18 treatment of the cornea uses femtosecond laser pulses at relatively low pulse energies to change refractive index values of the cornea rather than sculpture its refractive profile. Although a technology very much in its infancy, researchers at Rochester University in the USA anticipate the ability to also modify the refractive properties of specially developed intra ocular lens implants to ‘top up’ vision requirements.

 

Retinal Photocoagulation

Retinal photocoagulation remains one of the key laser treatments within ophthalmology, which essentially progressed with the commercial availability of argon ion lasers in the 1970s. The initial discovery of the argon laser had taken place in 196419 and with an early description of its clinical use20 appearing in 1968.

A significant technology development was where the laser energy could be coupled to the slit lamp delivery system using optical fibres rather than using reflecting mirrors. While the initial spectral output of the argon laser contained wavelengths at 488nm and 514nm, there has been a progression to longer wavelengths through to 532nm from frequency doubled Nd:YAG wavelengths to yellow wavelengths and 577nm produced from frequency doubling of 1,154nm.

This has allowed more effective treatment of areas closer to the macular region where there is a higher concentration of the protective macular pigments, lutein, zeaxanthin and mesozeaxanthin. Figure 5 indicates how lasers for retinal photocoagulation have been developed with reduced absorption with respect to macular pigment. This move to longer wavelengths has also reduced the potential discomfort experienced during retinal photocoagulation.

 

Figure 5: Indication of relative macular pigment absorption coefficient as a function of wavelength

Often the targeting of the laser in retinal photocoagulation is critical. Leaking retinal vessels can be identified using fluorescein injection with retinal imaging using a scanning laser ophthalmoscope where the site of leakage can be identified for targeted retinal photocoagulation as indicated in figure 6. The fluorescein is typically excited by a scanning laser at 488nm and subsequent light detected at around a peak wavelength at around 525nm.

 

Figure 6: Site of retinal vessel leakage identified using a scanning laser ophthalmoscope

 

While the core practice of retinal photocoagulation in the outpatient scenario has been the creation of visible lesions, the introduction of so called ‘pattern scanning’ lasers has altered treatment modalities where they are used. For the Pascal pattern scanning laser, patterns of non-visible lesions can be generated rapidly where boundaries of treated areas can be highlighted by ‘marker’ pulses of higher energy.

In this implementation, the individual pulse energy is determined by the associated pulse width.21 In figure 7 the first five pulses of a 3x3 pattern of around 10mS pulse width are followed by four ‘marker’ pulses of 20mS pulse width.

 

Figure 7: Pulse profile of sequence of pattern scanning Topcon laser pulses for a 3x3 pattern


There is some evidence22 of less ‘thermal spread’ from lesions created with ‘pattern scanning’, with resultant sparing of photoreceptors. Difficulties arise, however, in conducting comparative studies due to the increased number of variables introduced into treatment parameters.

Pattern scanning lasers present a more complex modality of retinal photocoagulation compared with single spot technologies and where care is required, for example, to map the plane of focus of laser energy to the retinal profile of the patient. A specific user control screen of a Pascal laser is indicated in figure 8 where a range of available patterns are indicated.

 

Figure 8: Control screen of pattern scanning Pascal laser system

 

Delivery Modalities: Retinal Photocoagulation

Within the majority of outpatient sessions of retinal photocoagulation, laser energy is delivered via slit lamp delivery systems. Some treatments can also be undertaken using a laser indirect ophthalmoscope (LIO), where a narrow laser beam from a headset is directed through a hand held 20 dioptre lens onto the patient retina.

This modality can provide improved targeting of peripheral retinal areas and can also be a modality delivered in operating theatre sessions. Where the retinal surface is obscured by bleeding, the vitreous can be removed with a vitrectomy procedure and retinal photocoagulation undertaken using endo probes where the laser energy can be directly coupled to the retinal surface. Such probes can have their own intrinsic light source and have probe tip orientation controlled externally.

 

Safe Working Environments

While there is always emphasis on the core technology of therapeutic laser systems, there are also a range of safety considerations involved in the safe use of such systems. One key consideration in outpatient facilities is that of allocation of adequate space for their safe use. In the rare event of patient collapse during treatment, there is a basic requirement for access of emergency teams and space for effective clinical intervention.

These considerations can be lost sight of with the proliferation of laser systems within a finite allocation of clinical treatment areas. Also, while only one laser at a time will be used in a specific laser room environment, often more than one laser type will be present which can give rise to confusion about the use of appropriate laser safety eyewear.

These and other considerations require the provision of a recognised laser protection advisor service to structure a documented system of laser use and provide training for staff groups and monitor compliance with safety practices.

 

In Summary

It is useful to review the key characteristics of lasers in use in ophthalmology. Figure 9 summarises the presentation of wavelength with pulse duration for the key laser ophthalmic applications with pulse duration represented on a logarithmic scale.

 

Figure 9: Variation of wavelength with pulse duration for the key laser ophthalmic applications

The dominating feature of excimer applications is the high photon energy at this wavelength, which is capable of disrupting molecular bonds within corneal tissue. For retinal photocoagulation applications an important consideration is of selected wavelength to minimise the degree of absorption within macular pigments.

There is no advantage in reducing pulse duration significantly since this would increase the risk of tissue disruption. For the application of SLT, the wavelength of 532nm provides useful tissue absorption while the short pulse duration provides mechanical ‘agitation’ to the target tissue. For the cyclodiode, the wavelength provides useful light absorption for the ciliary muscle.

For the laser capsulotomy (and laser iridotomy) a key consideration of the wavelength is a high level of optical transmission to the target site while the short pulse duration provides the required disruptive effect. For the femtosecond laser technology, the wavelength is used to provide access to the target tissue and the ultrashort laser pulse creates the necessary vaporisation of tissue at the beam focus.

It is an observation that where clinical practice utilises a specific laser technology for a specific clinical issue, it becomes a challenge to introduce a new technology that may have superior clinical outcomes in the medium and long term. It is possibly simpler, therefore, to introduce a novel laser technology in a completely new area of clinical application. 

  • Dr Douglas Clarkson is a research fellow at UHCW NHS Trust in Coventry with a wide interest in optical technologies applied to diagnostic specialties and modes of treatment, which include lasers and non-coherent light sources.

 

References

  1. Hollick EJ, Spalton DJ, Ursell PG, Pande MV. Lens epithelial cell regression on the posterior capsule with different intraocular lens materials. Br J Ophthalmol. 1998 Oct;82(10):1182-8
  2. Aron-Rosa D, Aron JJ, Griesemann M, Thyzel R. Use of the neodymium-YAG laser to open the posterior capsule after lens implant surgery: a preliminary report. J Am Intraocul Implant Soc. 1980 Oct;6(4):352-4
  3. Lighthizer N, Johnson S, Holthaus J, Holthaus K, Cherian B, Swindell R et al. Nd:YAG Laser Capsulotomy: Efficacy and Outcomes Performed by Optometrists. Optom Vis Sci. 2023 Oct 1;100(10):665-669
  4. Perkins ES. Laser iridotomy. Br Med J. 1970 Jun 6;2(5709):580.
  5. He M, Jiang Y, Huang S, Chang DS, Munoz B, Aung T et al. Laser peripheral iridotomy for the prevention of angle closure: a single-centre, randomised controlled trial. Lancet. 2019 Apr 20;393(10181):1609-1618
  6. Alon S. Selective Laser Trabeculoplasty: A Clinical Review. J Curr Glaucoma Pract. 2013 May-Aug;7(2):58-65
  7. Gazzard G, Konstantakopoulou E, Garway-Heath D, Adeleke M, Vickerstaff V, Ambler G et al; LiGHT Trial Study Group. Laser in Glaucoma and Ocular Hypertension (LiGHT) Trial: Six-Year Results of Primary Selective Laser Trabeculoplasty versus Eye Drops for the Treatment of Glaucoma and Ocular Hypertension. Ophthalmology. 2023 Feb;130(2):139-151
  8. Sacks Z, Katz LJ, Gazzard G, Van Tassel SH, Blumenthal EZ, Lerner FS et al. A Proposal for the Use of a Fixed Low-Energy Selective Laser Trabeculoplasty for Open Angle Glaucoma. J Glaucoma. 2024 Jan 1;33(1):1-7
  9. Anand N, Klug E, Nirappel A, Solá-Del Valle D. A Review of Cyclodestructive Procedures for the Treatment of Glaucoma. Semin Ophthalmol. 2020 Aug 17;35(5-6):261-275
  10. Sanchez FG, Peirano-Bonomi JC, Grippo TM. Micropulse Transscleral Cyclophotocoagulation: A Hypothesis for the Ideal Parameters. Med Hypothesis Discov Innov Ophthalmol. 2018 Fall;7(3):94-100
  11. NICE, Repetitive short-pulse transscleral cyclophotocoagulation for glaucoma, Interventional procedures guidance [IPG692]Published: 28 April 2021 available at:- https://www.nice.org.uk/guidance/ipg692/chapter/3-Committee-considerations
  12. Stein R. Photorefractive keratectomy. Int Ophthalmol Clin. 2000 Summer;40(3):35-56
  13. Crotti C, Deloison F, Alahyane F, Aptel F, Kowalczuk L, Legeais JM et al. Wavelength optimization in femtosecond laser corneal surgery. Invest Ophthalmol Vis Sci. 2013 May 9;54(5):3340-9
  14. Zhou KJ, Huang Y, Wang Y, Pan AP, Shao X, Tu RX et al. Safety and efficacy of cataract surgery performed with a low-energy femtosecond laser compared with conventional phacoemulsification in Chinese patients: a randomized clinical trial. Eye Vis (Lond). 2023 Jul 2;10(1):31.
  15. Kecik M, Schweitzer C. Femtosecond laser-assisted cataract surgery: Update and perspectives. Front Med (Lausanne). 2023 Mar 2;10:1131314.
  16. Warrak EL, Serhan HA, Ayash JG, Wahab CH, Baban TA, Daoud RC et al. Long-term follow up of intracorneal ring segment implantation in 932 keratoconus eyes. J Fr Ophtalmol. 2020 Dec;43(10):1020-1024
  17. Nagy ZZ, Kranitz K, Ahmed IIK, De Francesco T, Mikula E, Juhasz T. First-in-Human Safety Study of Femtosecond Laser Image-Guided Trabeculotomy for Glaucoma Treatment: 24-month Outcomes. Ophthalmol Sci. 2023 Apr 17;3(4):100313.
  18. Zheleznyak L, et al. First-in-human laser-induced refractive index change (LIRIC) treatment of the cornea. Invest Ophthalmol Vis Sci 2019;60:9:5079-5079
  19. Bridges WB. Laser oscillation in singly ionized argon in the visible spectrum. Applied Physics, 1964 Lett. 4, 128
  20. L’Esperance FA Jr. An ophthalmic argon laser photocoagulation system: design, construction, and laboratory investigations. Transactions of the American Ophthalmology Society, 1968;66:827-904
  21. Clarkson DM, Makhzoum O, Blackburn J. Determination of pulse profile characteristics of multi spot retinal photocoagulation lasers. Med Eng Phys. 2015 Oct;37(10):1027-31
  22. Lavinsky D, Cardillo JA, Mandel Y, Huie P, Melo LA, Farah ME et al. Restoration of retinal morphology and residual scarring after photocoagulation. Acta Ophthalmol. 2013 Jun;91(4):e315-23