|Year : 1989 | Volume
| Issue : 2 | Page : 84-85
Glass lens implant
|How to cite this article:|
Bidaye V. Glass lens implant. Indian J Ophthalmol 1989;37:84-5
| Introduction|| |
PMMA is not the best material for the manufacture of intraocular lenses. It cannot be autoclaved and its contamination by ethylene oxide may cause post-operative sterile uveitis. Biodegradation as well as ultraviolet transmission can also damage PMMA. This led to the search for other materials for manufacturing IOLs, and glass was found to be suitable  .
| Materials and Methods|| |
Presently available designs of glass IOLs like `spider lens', polyflex-N lens, S-flex lens can be implanted both in A.C. and P.C. This reduces the inventory of IOL surgeons to a great extent. The overall length of the lens is 12.5mm or 13.5 mm. The diameter of the glass optic is 5.2 mm and that of the optic portion including polyamide rim is 5.9 mm. The optic is plano convex and the loops have a 6° angulation towards the plano side of the optic. This 6° angulation helps to keep the pupil free. When implanted in posterior chamber the convex surface is facing the posterior capsule and when implanted in the anterior chamber the convex surface is facing the cornea.
This lens can be sterilized by autoclaving or boiling at the surgeon's office prior to implantation and therefore, it does not have a sterility expiry date.
| Implantation Technique|| |
For anterior chamber implantation, the lens size recommended for use is 12.5 mm in eyes with corneal diameter (w to w) of 11.0 mm or below and 13.5 mm in bigger eyes.
The lens can be introduced into the anterior chamber through a 7.00 mm opening, with the convex side facing the cornea after the anterior chamber is deepened with 2% methyl cellulose or air. If the inferior loop tucks the iris, the lens should be withdrawn a little, to free the iris tuck and then reinserted. The superior loops are introduced in the anterior chamber angle with a Lister hook. If the superior loop tucks the iris, the loop is manipulated with the hook or the iris is depressed with a spatula at the site of tuck. After lens insertion, a peripheral iridectomy usually needs to be done in between the two superior loops.
For ciliary sulcus posterior chamber implantation, a 13.5 mm lens may be used in all cases. Alternatively, for 'in-the-bag' fixation 12.5 mm lens can be implanted in all eyes. The Cataract must be extracted by planned ECCE or endocapsular ECCE. In either case, the lens is inserted with the convex side facing the posterior capsule.
After ECCE, for sulcus fixation the capsular bag is collapsed by injecting methyl cellulose in front of the anterior capsule. After the inferior loops are positioned behind the iris and in front of the anterior capsule the superior rim of the optic is pushed down behind the upper iris border. The superior loops are then engaged either in a Lister hook or held with a Me Pherson forceps, and positioned behind the iris one by one.
For in-the-bag implantation, the capsular bag is formed with the injection of methyl cellulose in the bag. The inferior loops are slipped behind the anterior capsule and then the superior rim of the optic is placed behind the iris. The superior loops are then engaged in the upper part of the capsular bag one by one.
It is possible to implant this lens `in the bag' with one sweep using a special spider lens forceps, designed by the author, The superior loops are folded under the lens forceps and the lens is inserted in the lower part of the bag. The superior loops are released one by one in the upper part of the bag by tilting the forceps first on one side and then on the other.
| Observations|| |
In these glass IOLs the optic is made up of high quality glass and the haptic is of polyamide, a thermostable plastic developed by space technologists. These glass lenses are thermostable and hence autoclavable. They are nondegradable and more biocompatible than PMMA. The highly polished glass surface is resistant to lens precipitates (like KPs) which cluster on the PMMA lenses. The scratch-proof hard surface facilitates easy primary or secondary surgical posterior capsulotomy when needed. An added advantage of these lenses is that they can be mass produced by machine and this makes them cost effective.
Because of the high refractive index of the glass the optic is very thin. The central thickness of a standard glass lens is 0.3 mm which is almost half that of PMMA lens. The high flexibility of the polyamide loop makes for excellent lens adjustability. A polyamide loop can be made as thin as 0.08 mm compared to 0.18 mm of the prolene loop.
Due to the low wetting angle of 7° a glass lens collects less pigment cell deposits and reduces post-operative anterior uveitis.
The weight of this lens is 11.7 mg in AIR and 9.5 mg in saline which is much less than the maximum tolerable lens weight.
| Discussion|| |
The concept of using glass in IOL's is not new. Both Emmrich  and S trampelli  had earlier implanted glass lenses and Ridley  had considered this material as an alternative to acrylics. It was first introduced commercially by Lynell Medical Technology Inc  . in the mid seventies in the USA as an iris clip lens of the maltese cross style and subsequently, as a Binkhorst style lens. The glass optic proved to be good material as it was resistantto Argon Laser and successful laser coagulation of the retina could be done through the glass optic. Hence it became a lens of choice for diabetic patients.
The popularity of this lens increased till the advent of the YAG Laser. YAG shattered these lenses and that brought about the downfall of these lenses and slowly they disappeared from the American market.
The main disadvantage of glass IOL's is its inability to stand Nd-YAG Laser posterior capsulotomy. YAG shatters this lens. But posterior capsular opacification after ECCE is less common with 'in-the-bag' glass lens implant. Because of a 6° angulation of its haptic, the posterior convex surface of the glass optic snugly apposses the posterior capsule and prevents migration of residual lens epithelial cells to the central portion of the posterior capsule. If needed, secondary posterior capsulotomy can be easily done using 30 G disposable needle bent at its tip. The scratch proof glass optic is not damaged by the sharp point of the needle. The author thinks that glass lenses have a good future in India, at least for the next 20 years, and by then; he hopes, some other laser, safe for glass, may appear on the horizon.
| References|| |
|1.||Momose A : Implants in Ophthalmology Vol 1 No 1 April P 11-13,1987. |
|2.||Momose A :Implantation of Lynell iris-clip lenses with special reference to diabetic patients. Proceedings 1st Korean Japan J. meeting Ophth. p.1 32-134, 1982. |
|3.||Fechner PU and Alpar JJ : Fechner's intraocular lenses. Thieme Inc. p.52,1986. |
|4.||Emmrich K : Vorder ammerlinsen aus silikatglaz Klin Monatsbe Augenbeclkd 132: 254 1958. |
|5.||Strampeui B : Ant. Chamber lenses. Arch Ophth. 66:12 1961. |
|6.||Ridley H: Intraocularscrylic lenses. A recent deveopment in the surgery of cataract. British Journal of Ophth. 36 :113,1952. |