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   Table of Contents      
ORIGINAL ARTICLE
Year : 1991  |  Volume : 39  |  Issue : 3  |  Page : 115-117

Retinal laser optical aids


L.V. Prasad Eye Institute, Hyderabad, India

Correspondence Address:
Traprasad Das
L.V. Prasad Eye Institute, Hyderabad
India
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Source of Support: None, Conflict of Interest: None


PMID: 1841883

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  Abstract 

Knowledge of optics, comparative magnification and working field of view is essential for rational use of ophthalmoscopic contact lenses for retinal photocoagulation. The three commonly used contact lenses are described and compared.


How to cite this article:
Das T. Retinal laser optical aids. Indian J Ophthalmol 1991;39:115-7

How to cite this URL:
Das T. Retinal laser optical aids. Indian J Ophthalmol [serial online] 1991 [cited 2019 Dec 13];39:115-7. Available from: http://www.ijo.in/text.asp?1991/39/3/115/24457



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  Retinal laser optical aids Top


Laser energy, is employed in the treatment of a variety of retinal disorders. For safe and optimal utilization, this energy must be transmitted to the various structures of the eye being treated. This may be activated by either lens-mirror contact lens system or by fiberoptic systems. The lens-mirror contact lens system is used in association with slit lamp delivery mode of the laser energy and fiberoptic system is used during endophotocoagulation on the operating table. The other mode of laser delivery using indirect ophthalmoscope employs different strengths of condensing lenses to view the retina.

The slit lamp is a stereoscopic microscope that is in focus about 95 mm. (4") in front of the objective lens of the microscope, or about 280 mm. (11") from the examiners eye (Haag-Streit Model 900). The slit lamp cannot focus directly on the patient's retina because of the intervening optical system of the patient's eye. If one thinks of the eye as a projector, the retina is imaged approximately at infinity by the optical system of the patient's eye. Since the biomicroscope is focussed at about 95 mm., the retina is not visible unless further optical aids, the laser contact lenses, are used for safe and effective use of contemporary ophthalmic laser systems.

Retinal laser lenses could be with or without mirrors. After the initial introduction of the Goldmann 3 mirror lens and Goldmann fundus contact lens, various laser lenses are now available with or without mirrors for retinal photocoagulation. While the list is long, some of the commonly used lenses, in addition to Goldmann 3 mirror and Goldmann fundus contact lenses, are Rodenstock panfundoscope, Mainster lens, and Mc Lean prismatic fundus lens. Precise knowledge of these lenses are useful in performing retinal photocoagulation and also minimising or eliminating errors of accidental macular photocoagula­tion while doing PRP

General considerations

Laser lens have some common characteristics. They have a concave posterior surface to conform to the P corneal curvature and a flat or convex anterior surface. Other features in specific lenses include 1) Planar mirrors that allow observation of the anterior chamber angle or peripheral retina. 2) A prism to allow visualisation of the mid-periphery of the retina. 3) A flange to stabilise the lens and prevent blinking, and 4) Knurled edge to facilitate lens manipulation. Laser lenses generally consist of a conical polymethyl­methacrylate or aluminium shell, and glass anterior surface, lenticular elements and mirrors.

Antireflection coatings are usually applied to each optical surface in a laser lens. This coating reduces reflected white light (from the slit lamp source) that could decrease contrast or the slit lamp image, and laser light (from the treatment beam) that could pose a potential hazard to an observer standing behind the laser operator. The hazard distance is 7 meters for a uncoated lens and 1.6 meters for a coated lens. Most laser lenses use broad-spectrum, multi­layer, antireflection coatings which reduce reflected light between 400 nm and 700 nm from approximately 4 per cent to less than one per cent [1].

Mirror lenses :

While mirror lenses provide high magnification as well as high resolution, only a small part of the fundus or chamber angle can be viewed at any one time. Therefore, the mirrors at various degrees of inclinations are necessary. The Goldmann 3 mirror contact lens is an excellent example. The outstanding advantage of these narrow field of view systems is the high resolution. The disadvantage is the limited field of view. The image formed in the Goldmann 3 mirror lens is the mirror image of the area focussed. It is an erect and virtual image [Figure - 1].

Lenses without Mirrors

The field of view may be increased to a variable extent by using biconcave contact lenses based on the simple Goldmann lens (without mirrors)

Another entirely different way of increasing the field of view involves using contact lenses based on the EL Bayadi lens [2]. Both Rodenstock Panfundoscope and Mainster lens are excellent examples of such lenses. All wide angle systems of this category are derived from the principle of indirect ophthalmoscopy and the common denominator of all these lenses is a large and inverted field of view. Thus both the panfundoscope and the Mainster lenses produce inverted real images[Figure - 1].

Magnification and field of view

Magnification and field of view are critical parameters for determining which lens is best for a particular clinical problem. The three main lenses - Goldmann three mirror lens, Panfundoscope and Mainster lens - are compared in [Table - 1][Table - 2][Table - 3] [3]. The Three mirror Goldmann lens [4]

This has been a standard ophthalmoscopic lens since biomicroscopic laser photocoagulation began, providing excellent magnification of the posterior pole and magnification as well as peripheral retinal ob­servation through the mirrors. It has three mirrors, inclined at various angles : a) 59 degrees for gonios­copy and visualisation of the ora serrata in eyes with large iridectomies. b) 67 degrees for observation of the peripheral fundus from the equator to the ora and c) 73 degrees for viewing the fundus from the equator to the posterior pole.

The Goldmann lens has a flat anterior surface and produces an erect, virtual ophthalmoscopic image located near the posterior surface of the crystalline lens. The chief disadvantage of the Goldmann lens, is its limited field of view. Rodenstock Panfundoscope [5]

Introduced in 1969, the Panfundoscope lens provides a wide angle view of the fundus up to the equator without rotation of the lens and the patient's eye. The lens has two optical elements : a high power concave meniscus lens (interfacing with the patients cornea) and a spherical field lens. It produces an inverted, real image located in its spherical biconvex anterior lens element. Spot size of the retina is one and half times that of the photocoagulator setting. The panfundoscope lens has a working field of view 84 per cent greater than that of the Goldmann lens.

Mainster lens

Introduced in 1986 the Mainster lens provides a higher magnification but a smaller field of view than the panfundoscope lens. The Mainster lens has a biconvex, aspherical anterior lens element and a concave lens to fit to the corneal curvature. It produces an inverted, real image located in front of its biconvex aspheric anterior lens element. It has a working field of view 58 per cent greater than that of a Goldmann lens and 14 per cent less than the Panfundoscope.

The primary advantage of both the Panfundoscope and Mainster lens is that large areas of the fundus may be treated in pan retinal photocoagulation without lens rotation. Furthermore, visualisation of the optic disc and macula during peripheral treatment prevents disorientation. Unfortunately, reflexes compromise the retinal image, and in the panfundoscope peripheral distortion can produce marked laser beam astigma­tism when treating the peripheral retina., To avoid anterior segment irradiance the photocoagulator spot setting should not exceed 500 micron size. Also the working distance on these lenses are greater than of the Goldmann lens.

Mc Lean Prismatic Fundus lens

This lens has a 25 degree prism to improve obser­vation of the mid-peripheral retinal field from 25 - 35 degrees. This area is frequently difficult to treat with a standard 3 mirror lens, particularly in elderly patients with poor ocular rotation.


  Conclusion Top


Photocoagulation tasks are less demanding than photodisruption. Optical aids with high resolution are essential for photodisruptive methods. Even a very modest amount of image degradation may make photodisruptive tasks impossible and even hazardous. When using the thermal laser energy sources for photocoagulation, however, such demands are critical only in special situations like macular photocoagula­tion. For other applications as in performing panretinal photocoagulation outside the posterior pole, high resolution, though desirable, is not mandatory. For panretinal photocoagulation use of either Goldman three mirror lens or Panfundoscope or Mainster lens is recommended, although the image resolution is poorer in the latter two lenses. For macular photocoagulation both Goldmann fundus contact lens and Goldmann Three-mirror lens are useful, though magnification is greater in the former.

Appendix

List of various laser lenses for both anterior and posterior segment available commercially:

Anterior segments laser lenses

Abraham iridectomy lens Abraham YAG laser lens Hoskins Nylon suture laser lens Ritch trabeculoplasty laser lens

Schirmer peripheral iridectomy laser lens Single mirror gonioscopy lens Thorpe four mirror gonioscopy lens Posterior segment lenses Goldmann three mirror lens Goldmann fundus laser lens Karickoff laser lens

Mc Lean prismatic fundus lens Rodenstock panfundoscope

 
  References Top

1.
Dickert. JP, Mainster A.. HO PC. Contact lenses for laser applications. Ophthalmology. 1984 : 91(S) : 79-87.  Back to cited text no. 1
    
2.
EL Bayadi G. New method of slit lamp micro ophthalmoscopy. Br. J Ophthalmol. 1953, 37 : 625-628.  Back to cited text no. 2
    
3.
Mainster MA., Crossman JL, Erickson PJ. Heacock G. Retina laser lenses : Magnification, spot size and field of view. Br. J Ophthalmol. 1990. 74:177-179.  Back to cited text no. 3
    
4.
Goldmann H. Biomicroscopy of the eye. Am. J. Ophthalmol. 1968:66:789-804.  Back to cited text no. 4
    
5.
Lobes LA, Benson W, and Grand MG. Panfundoscope contact lens for argon laser therapy. Ann Ophthalmol. 1981 : 13 : 713-714.  Back to cited text no. 5
    


    Figures

  [Figure - 1]
 
 
    Tables

  [Table - 1], [Table - 2], [Table - 3]



 

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