Year : 2004 | Volume
: 52 | Issue : 1 | Page : 67--71
Self retaining contact lens system for vitreous surgery
Kakarla V Chalam, Vinay A Shah
Department of Ophthalmology, University of Florida College of Medicine, Jacksonville, FL 32209, USA
Kakarla V Chalam
Department of Ophthalmology, University of Florida College of Medicine, Jacksonville, FL 32209
We describe the principle and design of a new self-retaining contact lens system for vitreous surgery. The system has three lenses: the plano-concave, prism and magnifying lens. This system is based on the principle of a direct imaging contact lens, designed for a 150-200mm focal length operating microscope. The contact lenses are designed to have an inferior concave surface [radius of curvature (ROC) 7.7mm], modified by the addition of four footplates to provide stability and centration during vitreous surgery. The lenses are used with a drop of viscoelastic material placed between the concave surface of the contact lens and cornea. This induces negative suction and helps retain the lens in position during surgery. These specially designed lenses provide a stable, well-centered, high-resolution, magnified view of the fundus. This system eliminates the need for a skilled assistant or for suturing the lens to the sclera during vitreous surgery.
|How to cite this article:|
Chalam KV, Shah VA. Self retaining contact lens system for vitreous surgery.Indian J Ophthalmol 2004;52:67-71
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Chalam KV, Shah VA. Self retaining contact lens system for vitreous surgery. Indian J Ophthalmol [serial online] 2004 [cited 2019 Sep 19 ];52:67-71
Available from: http://www.ijo.in/text.asp?2004/52/1/67/14624
The use of corneal contact lenses during vitreous surgery is popular as they afford excellent visualisation of the fundus. To maintain a clear view of the posterior vitreous, the contact lens must neutralise the refractive power of the cornea, permit visualisation of vitreoretinal pathology, allow corneal contact on rotation of the globe, and eliminate the accumulation of blood or air bubbles between the contact lens and the cornea.
Many different contact lens systems exist. Plano-concave lenses provide a non-magnified view of the posterior pole. These lenses require a host of supplemental lenses (i.e., prismatic lenses, biconcave lenses) to view the entire fundus in stages during vitreous surgery., Alternatively, the panoramic wide-angle fundus viewing system provides enhanced visualisation of the peripheral fundus during surgery. Though the wide-angle system has the advantage of a larger field of view, resolution and stereopsis are compromised. This creates the need for a conventional contact lens for the pathology located in and around the macular area.
Surgeons often require higher magnification for precise surgical manoeuvres, such as removal of epiretinal or subretinal membranes around the macula, drainage of intraretinal or subretinal blood or exudates, or internal limiting membrane peeling for macular hole surgery. Magnifying contact lenses are suitable for these procedures. These lenses either require a sutured lens ring or the presence of an assistant to hold the irrigating handheld lens system for stability.
We have designed a new self-retaining contact lens system with footplates for vitreous surgery that has many advantages over existing lens systems.
Design of the Self-retaining Contact Lenses
Common self-retaining features
The lens system contains three lenses, the plano-concave lens, magnifying lens and 15° prism lens. These are one-piece lenses made of autoclavable acrylic material. The refractive index is 1.51 at 550 nm wavelength. The plastic is impervious to common chemical sterilising agents. The total weight of the lens is 0.3 gm. All lenses are one-piece aspheric plastic lenses with four footplates, each footplate 3 mm long. The optical part of the lens is 4 mm high, while the total height of the lens on a flat surface is 7.75 mm [Figure 1]. The diameter of the optical portion of the lens is 10.0 mm. The diameter of the contact portion including the footplates, is 16 mm. The radius of curvature (ROC) of the inferior concave surface is 7.7 mm.
Plano-concave lens. The superior surface of the plano-concave lens is plano [Figure 1], giving it a refractive power of 0 D. Thus the total refractive power of the lens is -66.3 D.
Magnifying lens. The superior surface of the magnifying lens is convex (high magnification) with ROC of 21.62 mm [Figure 1]. This gives the superior surface a power of 23.8 D, resulting in a net power in-air of -38.5D (23.8 D - 66.3 D) for the magnifying lens.
Prism lens. The superior surface of the prism has a 15° angulation [Figure 1]. The inferior surface refractive power in air is -66.3 D, while that of the superior surface is 0 D, with a net power in air of -66.3 D.
A reasonable aim of optical manipulation in vitreoretinal surgery is to place the image of the fundus inside the eye. This is achieved by this direct imaging lens, designed for a 150-200mm focal length operating microscope. [Figure 2] depicts the lens optics. The contact surface curvature almost equals the curvature of the anterior corneal surface, eliminating the refractive power of the cornea. This does not change the image formation but eliminates the reflection on the inferior surface of the lens and the cornea, which increases the image resolution and stereopsis. Through the contact lens, the refractive power of the cornea is eliminated. The effective refractive element left in the eye is the far less powerful crystalline lens. The retina is located within the focal length of the crystalline lens, thus forming a virtual image of the fundus (F') in a plane behind the globe. As F' is seen through the acrylic and vitreous, to the observer, in air F' appears at F". This is due to the difference in index of refraction between the two media. This is the reason for the microscope to be focused on the plane inside the globe at F" [Figure 3].
Magnification factor for the plano-concave and prism lenses is 1 x, i.e., there is no change in the image size with these lenses . Thus magnification of the fundus depends on the operating microscope used.
Magnification of the magnifying lens
The refractive power of the inferior surface of the contact lens in air is -66.3D, while that of the superior surface is 23.8D, giving a net power in-air of -38.5D. The normal emmetropic eye has a refractive power of approximately +58.64 D. Thus magnification can be calculated according to the following equation:
f lens sin a
M = _________
f eye sin a
This simplifies to
f lens D eye 58.64
M = ____ ____ = _____ = 1.52 x
f eye D lens 38.5
M = magnification; f = focal length, D = dioptric power
Thus, the lens provides approximately 1.5 x magnification.
The field of view is approximately 18-20° with the plano-concave and prism lenses with a pupil diameter of 6 mm; the field of the view with the magnifying lens is 15°. The field of view is also dependent on the diameter of the pupil [Figure 4]. [Figure 5] shows the fundus as seen through the plano-concave lens.
Using the Lenses
This is a contact lens system with a self-retaining design. After placing a drop of viscoelastic material between the concave surface of the lens and the cornea, the lens is pressed downward on to the cornea, and is kept in place by negative suction [Figure 6]. The inferior concavity created by the surrounding footplates, and the lightweight (0.3 gms), and small size of the lens allows stability and easy rotation of the globe during surgical manipulation, obviating the need for an assistant or a sutured ring on the eye. While positioning the lens on the cornea, one of the footplates is usually kept at the 12 o'clock position. This aligns the gaps between the footplates with the sclerotomies (10 and 2 o'clock positions) providing better manipulation and easier entry and exit of instruments through the sclerotomies. The prism lens provides a good view of the mid-peripheral fundus in the area opposite to the base of the prism.
This system has an added advantage during combined anterior segment and posterior segment surgeries. Surgical problems related to anterior segment surgery such as wound leaks can be addressed immediately by lifting the lens from the cornea. There is no sutured ring that needs removal during such a manoeuver or at the end of the procedure.
The lens is reusable and can be sterilised by autoclaving, gas sterilisation or by other chemical sterilising agents including 2% glutaraldehyde.
Resolution of the image and the stability of the lens are concerns with all lenses used in vitreoretinal surgery. We have tried to address both these concerns with this new system. We have previously described the self-retaining plano-concave lens. This is the first description of a comprehensive self-stabilizing lens system for vitreous surgery [Figure 7]. This system provides wide versatility and stability for the surgeon.
Higher magnification can be achieved by increasing the magnification of the surgical microscope; however, resolution decreases as magnification is increased with the microscope. This is undesirable while performing surgery in and around the macula, which warrants maximum resolution. The newly designed magnifying lens provides 1.5 X magnification. The image is virtual and erect, without reducing resolution or stereopsis. Wide-angle panoramic viewing systems provide a large field of view but the image is reduced, lower in resolution and inverted. A stereo-diagonal inverter is required to re-invert the image . This adds additional reflecting surfaces to the system, which further sacrifices image clarity. Thus most surgeons still prefer the contact plano-concave lens systems while performing precise macular surgery. Magnifying prismatic lenses for peripheral vitreous surgery have also been described.
Use of a magnifying contact lens requires less magnification through the microscope and thus improves resolution and maintains good stereopsis. The self-retaining prism remains well centered on the cornea and provides a stable view of the opposite mid peripheral retina.
Various contact lens stabilising systems for vitreous surgery have been described in the literature ranging from the modified Goldmann contact lens, held by a curved haemostat described by Machemer to more recent self-stabilising, lightweight lenses. The sutured lens rings with two tabs located 180° apart for scleral fixation, are among the commonly used lens stabilising systems. The ring requires suturing for stability. Anchoring sutures predisposes to slow perilimbal haemorrhage, which can accumulate between the contact lens and the cornea, thereby obscuring the surgeon's view. The possibility of globe perforation due to suturing also exists. The ring obscures the surgeon's view while performing anterior chamber procedures and is more time consuming.
Lewis and colleagues designed a contact lens support ring that requires only a single suture for fixation and provides improved stability and centration of a sutured contact lens. This system is safer and saves time. With our system, sutures are not required.
A free-floating contact lens is kept on the cornea with a drop of balanced salt solution or viscoelastic material. The lens does not require an assistant to constantly hold it, but is unstable and requires frequent repositioning. O'Connor described the use of plastic forceps with silicone-covered tips to position and reposition the lens without scratching the plastic of the lens .
Hand-held irrigating lens systems require constant attention to maintain the lens in the appropriate position by a skilled assistant, making the procedure tedious. As the eye rotates during surgery, it is important for the assistant to follow the eye movement with the lens and simultaneously maintain proper contact with the cornea to avoid blurring of the retinal image. While working under high magnification (i.e., macular surgery), instability of the hand held-lens increases the risk of complications. In addition, excessive pressure on the cornea increases the intraocular pressure to dangerous levels, can cause corneal epithelial oedema due to constant irrigation and pressure, resulting in wrinkling of the corneal stroma and Descemet's membrane, ultimately blurring the view . Different types of lens holders (flexible hand-fixed and lid-retractor fixed lens holder) have been described for better stability and centration. A benefit of the irrigating contact lens design is that it has an infusion handle, which provides a stream of irrigating solution to prevent blood or air bubbles from accumulating under the lens. A combination contact lens retaining ring with its own irrigating system has also been described. This ring also requires sutures to maintain its position. In our system the lens is held by suction, as it is pressed onto the globe over a drop of viscoelastic material. This prevents blood, debris or air bubbles from accumulating between the lens and the cornea and assures visibility.
Ikuno and colleagues recently introduced a new suture-free contact lens system. It is a stainless steel ring that has two upright tabs 180° apart with a hook on the top and two elastic bands 8 mm in diameter to fix the ring to the eyelid speculum. This system improves upon many of the drawbacks (suturing and assistance) mentioned above. However, rotation of the globe is difficult due to the silicone bands in this system. Stenkula designed a contact lens with an inner cylindrical optic piece that fits smoothly in an outer scleral piece, which is fixed to the sclera with three sutures. The outer part keeps the eye steady for anterior vitreous and anterior segment surgery. During posterior segment surgery the inner cylinder piece is inserted over a methylcellulose medium. The cylinder can be rotated and tilted within the scleral part. However, this system is not widely used because of its complexity. Conventional quartz contact lenses are heavier (0.9gm) with an effective weight of 1.4 gms when assembled with the stabilising ring. Our self-retaining lens is three times lighter (0.3gm) than the conventional contact lens and more than four times lighter than the combined lens and ring assembly.
An additional benefit of this self-retaining design has been in combined cataract surgery (phacoemulsifi-cation) or penetrating keratoplasty with vitrectomy due to its lighter weight and minimal pressure onto the cornea. In these surgeries, an anterior wound leak during posterior segment surgery can be addressed easily by simply lifting the lens. Migration of blood in between the lens and the cornea is less likely with this lens system due to viscoelastic and suction holding the lens, providing an uninterrupted view of the fundus during the vitrectomy.
Another situation is when silicone oil is used during surgery. The traditional hand-held irrigating lens uses aqueous solutions for infusion. Silicone, a hydrophobic agent, forms a mixture of oil and water between the lens and the cornea and interferes with the visibility of the retina. Using methylcellulose solution in the infusion has been proposed for this situation. We use a viscoelastic agent between the lens and the cornea. This stabilises the lens through suction preventing migration of silicone oil, blood, debris or air bubbles between the lens and the cornea.
All the three lenses in the system are made of a high refractive acrylic material rather than the usual material like quartz, PMMA or optical crown glass . This makes the lens lighter in weight.
A major limitation with this lens system is the field of view. Different lenses have to be used according to the stage of surgery. The posterior pole plano-concave lens is needed for core vitrectomy, the magnifying lens for macular surgery and the prism lens is needed for peripheral vitreous surgery. Due to the limitation of the prism lens to view the ora serrata, indentation is required for dealing with certain peripheral retinal pathologies.
This is the first report of a self-stabilising contact lens system for vitreous surgery. We have previously described this concept in wide-angle lens, plano-concave and prism lenses.,, We have used the self-retaining contact system in over 2500 vitreoretinal procedures. This lens system provides high resolution and magnification. It does not require an assistant or a ring to maintain.
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