Approximately 40 years ago, Barraquer first experimented with the implantation of plastic materials within the corneal stroma to alter the radius to curvature of the iront surface of the cornea.1 Because the plastic materials which he had available to him at that time were impermeable to water, necrosis and opacification of the corneal stroma anterior to the implant occurred in all cases. This problem was seen several decades ago with the plastic inlays of Dohlman and Brown2 and again demonstrated in the 1980s with the use of solid polysulfone corneal inlays in humans by Choyce in England and was confirmed in laboratory animal experiments by several investigators in the USA.3,4
In this article, we examine the behavior of myopic hydrogel intracorneal lenses (ICL) in humans. They differ from the above mentioned impermeable plastic materials in that they are water permeable, so that glucose and other metabolites freely pass through the substance of the lens, allowing nutrition of the anterior corneal stroma and epithelium. A number of investigators including McCarey and Andrews,5 Binder et al,6 and Werblin et al7 have looked at hydrogel intracorneal lenses for refractive purposes. Previously, we have demonstrated longterm stability,8,9 with these hydrogel implants in monkeys as well as the creation of significant hyperopic and myopic refractive changes.10·11 Because of the long-term success seen in nonhuman primates, small, well-controlled clinical trials of myopic hydrogel implants have been undertaken in humans. This is the first reported data on the initial five myopic hydrogel intracorneal lenses placed in human subjects, followed for over 18 months.
Figure 1: Four-month postoperative appearance of hydrogel intracorneal lens implant in a human subject. The hydrogel appears as an optically void area at mid stromal depth (dark arrows). One or two faint interface opacities can be seen (open arrow), but the general appearance of the cornea is crystal ctear.
MATERIALS AND METHODS
All surgeries were done by one of the authors (JXB.) in Bogota, Columbia. The surgical procedure was initiated with a microkeratome section of the recipients' cornea. The microkeratome was used to cut a piano lamellar section at a depth of 0.25 to 0.30 mm, with a diameter of about 8.5 mm. A #5 ring with a 20 or 25 base plate was most commonly used. After successful lamellar resection, the anterior corneal cap was sutured in place initially with a single 10-0 nylon running antitorque suture. The antitorque suture was left untied, allowing the hydrogel lens to be placed between the corneal cap and the bed centered with respect to the edges of the corneal cap. After implantation of the ICL, the initial suture was tied and then a second antitorque suture was placed. All sutures were removed by 1 month postoperatively. Topical steroids and antibiotics were used postoperatively. No vision-threatening intraoperative or postoperative complications occurred in any of the five patients.
CooperVision Permalens® material was used for all implants. The implant powers ranged between - 10.00 and - 15.00 diopters. Preoperative and postoperative refractions were performed to determine the refractive effect of the implant. In all but one eye (V.l.), emmetropia was the goal of the surgery, and lens implants close to the preoperative myopic correction (spherical equivalent) of the patients were used.
Excellent corneal clarity was characteristically demonstrated with hydrogel intracorneal lenses within a few days following surgery and persisted throughout the follow-up period (Fig 1). No movement (ie, decentration) of the lenticule after implantation was observed.
As with any refractive technique, one of the major areas of concern is the accuracy of the procedure. This can be evaluated by examining the spherical equivalent of the difference between the preoperative refraction (Table) and the implant power, a difference that represents the expected residual refraction. Comparing that number to the actual refractive result at 18 months showed an under correction of 5.70 + 2.10 D (39%). Figure 2 demonstrates the spherical equivalent as a function of time in these five eyes. Two cases (R.M., M.M.) showed long-term stability, two (V.l., CH.) showed loss of effect with time, and one (D.F.) was equivocal.
On the average, 0.6 level of visual acuity were lost postoperatively. Actually, one patient (CH.) lost spectacle corrected visual acuity and that patient lost three lines. We did not differentiate between irregular astigmatism at the corneal surface and interface changes as to the cause of this loss of best corrected acuity.
There are two main reasons for using hydrogel intracorneal lenses for refractive purposes. First, hydrogel materials can be manufactured to precise specifications, and their power and configuration can be evaluated thoroughly before implantation. This is in contrast to corneal tissue lenses such as those used in epikeratoplasty, keratomileusis, or keratophakia where the parameters that govern the final shape of the tissue lens cannot be determined until after the lens has healed in place on or within the recipient cornea.12 Second, the hydrogel material does not heal or scar in place within the corneal stroma. Therefore, at any time postoperatively the lens can be exchanged for better accuracy or simply removed returning the cornea close to its original configuration. Thus far, this possibility has been demonstrated in experimental animals by Binder et al and makes the procedure theoretically reversible and modifiable in humans.13 Third, the supply of these materials is limitless.
Data on Five Myopie Eyes Receiving Minus Power Hydrogel Intracorneal Lenses
Throughout this work, we assumed that the cornea would conform to the shape of the hydrogel implant and basically would mimic the refractive behavior of a hydrogel lens as if it were sitting on the front surface of the cornea rather than within the stroma. Historically, most refractive surgical procedures tend to achieve less of a refractive result than predicted by their mathematical models. The mechanical properties of the cornea, the structure of the anterior stromal lamellae, and Bowman's membrane all undoubtedly influence the ability of the cornea to conform to the shape of the implant lens. In addition, the effects of the implanted lens on the shape of the posterior corneal surface have not been considered throughout the theoretical considerations of lens implantation. Clinically, one sees some slight deformation of the posterior corneal surface, yet this is extremely hard to quantitate. Other parameters dealing with the physical properties of the lens, such as dehydration or compression, have also not been critically evaluated. In any event, it is not surprising that some deviation from predicted correction was seen in the five eyes reported.
We have, in fact, demonstrated safety and general efficacy during the course of this preliminary clinical study, which basically confirms the information derived from nonhuman primate data.8»9 Predictability remains a significant shortcoming at this time. However, the undercorrection observed thus far should enable a more accurate procedure in the future and lens exchange using a higher power lenticule should show correction of existing or future inaccuracies. In the future, an empirically derived lenticular power much higher than the preoperative spherical equivalent would be used at the onset. On average, a less than one line decrease of best corrected spectacle visual acuity was observed in these patients. Lamellar refractive procedures have often generated some irregular corneal astigmatism that may resolve over a long period of time. Additionally, possible changes at the interface, which were noted clinically, may influence the final refractive result. These different possibilities could be differentiated clinically with hard contact lens and overrefraction, but this has not been done to date.
Figure 2: Graphic demonstration of refractive stability. Postoperative overrefractions (spherical equivalent) are plotted against the time following surgery.
Although these initial clinical studies show significant undercorrections, we are encouraged by the lack of complications to the operative procedures, and in all cases the significant myopic correction achieved. Because the initial intent of the surgery was to provide a procedure that could be modified easily, lens exchange procedures should enable the correction of the undercorrection observed, if they remain stable. Further monitored clinical tests on a small number of patients will continue to yield information. We hope an empirical relationship between lens power and its effect on the corneal surface will be generated eventually.
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Data on Five Myopie Eyes Receiving Minus Power Hydrogel Intracorneal Lenses