Refractive keratoplasty in monkeys using intracorneal lenses of various refractive indexes

Intracorneal Inlays for the Correction of Ametropias

Various materials have been placed in the corneal stroma to modify a spherical refractive error (usually hyperopic). As the surgical procedures evolved to simplify the insertion process so has the quality and biocompatibility of the implanted materials. Failures have been due to excessive inlay dimensions, bioincompatibility of the materials used, poor choice of test model, or combinations of the above. We present a thorough analysis of the history of the intracorneal inlay and the materials and techniques used up to and including the current materials and techniques available for the correction of spherical refractive errors.

Hydrogel intracorneal inlays for the correction of hyperopia: outcomes and complications after 5 years of follow-up

PURPOSE

To evaluate safety and efficacy of an intracorneal inlay for the correction of hyperopia.

DESIGN

A prospective, nonrandomized, noncomparative, 2-center study.

PARTICIPANTS

Thirty-four hyperopic eyes were implanted with a hydrogel intracorneal inlay (Permavision, Anamed, Lake Forest, CA). Preoperative hyperopia was +3.9 diopter (D; range, +2 to +7). Uncorrected visual acuity (UCVA) was the logarithm of the minimum angle of resolution (logMAR; the decimal logarithm of decimal visual acuity with a minus sign) 0.6 +/- logMAR 1, and best-corrected visual acuity (BCVA) was logMAR 0.1 +/- 0.7.

METHODS

Corneal flaps were created with a mechanical microkeratome (M2 [Moria, Anthony, France] or Amadeus [Advanced Medical Optics Inc, Santa Ana, CA]; 180 microm), followed by inlay implantation onto the stromal bed over the pupillary center and covered by the corneal flap. Follow-up was 5 years.

MAIN OUTCOME MEASURES

We measured UCVA and BCVA; patients underwent, slit-lamp examination, pachymetry, and confocal microscopy. The follow-up was up to 6 years.

RESULTS

The UCVA improved during 3 months and was stable for up to 2 years. There was a loss of > or =2 lines of spectacle-corrected visual acuity in 35% of eyes at 2 years, and a loss of > or =2 lines in 55.5% of the eyes at 5 years. Refractive predictability was poor, with 60% of the eyes having +/-3.00 D of emmetropia. A decentration of the inlay occurred in 29.4%, progressive perilenticular deposits were observed in 88.2%, haze was seen in 73.5%, and the inlay was explanted in 58.8%, with a cumulative survival rate of 58.4%.

CONCLUSIONS

An intracorneal inlay may be an option to treat hyperopia, but the tested inlay caused significant visual loss and scarring and had to be explanted in the majority of cases.

Refractive surgery: through the looking glass

Historically, surgical procedures designed to change the refractive power of the cornea have been controversial at their inception, gaining acceptance only with time and study. From intraocular lenses to corneal remodelling with the excimer laser, and numerous procedures in between, including radial keratotomy, epikeratophakia, incisional keratotomy for astigmatism, and hydrogel and polysulfone implants, ophthalmologists have balanced the risks and benefits for their patients to produce the maximum benefit at the least cost. The present status and future potential of each of these are assessed as they appear today. There is no question that progress will continue, and that our patients will be the beneficiaries of our ongoing interest in improving older procedures and developing new techniques for the correction of refractive errors.

Intraoperative crystallization on the intraocular lens surface

PURPOSE

To report a physician survey, laboratory studies, and clinical observations of intraoperative crystallization on the surface of the intraocular lens (IOL).

METHOD

We sent a survey to all ophthalmologists in the states of Wyoming, Idaho, Montana, Utah, and Colorado asking whether crystallization on the IOL surface had occurred in any of their patients and what viscoelastics, IOLs, and other solutions were used. All returned surveys were tabulated and analyzed by standard statistical means. A sample of crystallization from an IOL on a glass slide submitted by a physician was analyzed to ascertain the relative elemental composition. During in vitro laboratory studies, BSS Plus (Alcon Surgical, Fort Worth, Texas) and BSS (Alcon Surgical) were measured and analyzed for precipitation. Healon GV (Pharmacia/Upjohn, Kalamazoo, Michigan) and calcium chloride were combined in various solutions and examined for precipitate formation. Silicone IOLs and plate silicone were placed in different BSS and BSS Plus solutions with different viscoelastics and varying calcium concentrations. In seven patients, prominent crystallization on an IOL surface was examined, photographed, and followed for up to 3 years.

RESULTS

Two hundred six surveyed ophthalmologists returned 181 surveys (88%) and reported 29,609 cataract surgeries, with IOL implantation with 22 eyes (0.07%) (22 patients) in which intraoperative crystallization was observed on the IOL surface during 1993. The survey indicated there was a correlation with BSS Plus (chi-square = 4.9, P = .0268) and silicone IOLs (chi-square = 6.8, P = .0093). The sample showed the cation to be calcium.

CONCLUSION

Crystallization on the IOL surface during cataract surgery is a rare occurrence that may be associated with calcium as the cation related to an osmotic gradient around the IOL with increased calcium concentration. If encountered surgically, the lens should be exchanged in the operating theater after irrigating the anterior chamber with BSS and completely filling the capsular bag with a low molecular weight viscoelastic.

Clear Lensectomy and Intraocular Lens Implantation for Hyperopia from +7 to +14 Diopters

BACKGROUND

Our results of clear lens extraction and intraocular lens implantation to correct hyperopia from +6.75 to +13.75 D are presented, as well as evaluation of two intraocular lens calculation formulae.

METHODS

Clear lens extraction and posterior chamber intraocular lens implantation was performed in 35 normally sighted eyes of 21 patients with a mean baseline hyperopic spherical equivalent refraction of +9.19 +/- 0.34 D (range +6.75 to +13.75 D). The refractive goal was -1.50 D, using the SRK II formula in 17 eyes and the SRK-T formula in 18 eyes. Follow-up was up to 5 years.

RESULTS

Mean uncorrected visual acuity after surgery was 0.8 (range 0.5 to 1.0). Stability of refraction was noted from the second month after surgery. No eyes lost any lines of spectacle-corrected visual acuity. Using the SRK II formula, 100% of eyes were within +/-1.00 D of emmetropia and with the SRK-T formula, 83.3% for a combined 91.4% of eyes within +/-1.00 D of emmetropia. One eye required intraocular lens exchange and another eye required photorefractive keratectomy for myopia. Both procedures were necessitated by an intraocular lens miscalculation of more than +/-2.00 D. Nineteen eyes (54.2%) developed posterior capsular opacification and were treated with Nd:YAG laser capsulotomy.

CONCLUSION

Clear lens extraction is a safe, effective, and predictable procedure for the treatment of hyperopia from +6.75 to +13.75 D. The SRK II formula proved slightly superior to the SRK-T in intraocular lens calculation when using the "-1.50 D rule."

Tissue reactions induced by modified poly(vinyl alcohol) hydrogels in rabbit cornea

The purpose of this study was to compare the tissue reactions induced by modified poly(vinyl alcohol) (PVA) hydrogels with other materials implanted in the rabbit cornea. PVA hydrogels with and without covalently bonded collagen were compared to ethylene-vinyl alcohol copolymer, and polysulfone discs. These materials were implanted in the rabbit corneal stroma. The resultant tissue reactions were histologically and clinically studied. The PVA hydrogels had high flux rates of glucose and L-lactic acid in vitro. Covalent immobilization of collagen onto their surface was found to render them more biocompatible. However, excessive ulceration and vascularization of the cornea still occurred in vivo, possibly because of extremely little blinking of the rabbits, which might cause desiccation of the cornea.

Gel injection adjustable keratoplasty

Gel injection adjustable keratoplasty (GIAK) is a new refractive surgical procedure designed for the correction of myopia by injection of a gel substance into the peripheral corneal stroma. This paper describes the GIAK technique and reports the results obtained in 21 fresh cadaveric eyes using the procedure. After a deep interlamellar canal has been dissected with a helicoid spatula surrounding the visual axis, the gel is injected under keratometric control. In the 1st group of 14 eyes, the degree of correction varied from 2.2 to 12.8 D; there was a direct relationship between the amount of gel injected and the keratometric change. In the 2nd group of 7 eyes, the adjustability of the procedure was demonstrated. Through partial extraction of the gel and subsequent modification of the corneal curvature, the previously induced keratometric changes could be reversed or altered to a specific extent. Following the initial injection of gel to a targeted flattest meridian power of 35 D, an average value of 35.8 +/- 0.5 D was achieved in these eyes. We subsequently attempted to increase the flattest meridian to 40 D by partial removal of the gel and achieved a mean value of 40.2 +/- 0.4 D. Average presurgical astigmatism of 1.497 +/- 0.737 D was reduced to a postsurgical reading of 0.941 +/- 0.590 D (P = 0.005, Student's paired t-test), indicating an autocorrection by autodistribution of the gel inside the canaliculus (Laplace's law). GIAK is a simple, inexpensive procedure designed for the correction of myopia that has the added advantage of reducing preexisting astigmatism without encroaching on the visual axis.

Modeling glucose distribution in the cornea

The central cornea obtains its glucose by diffusion through the cornea from the aqueous humor to the epithelium. The diffusion of glucose in the cornea is analogous to the flow of current in an electrical resistance network. The cellular consumption of glucose can be compared to shunting a portion of the charge to electrical ground. An electrical analog model of the cornea was developed to predict the availability of glucose to the epithelium and the distribution of glucose in the stroma. The glucose constant concentration lines in the normal stroma are parallel to the corneal surface and have decreasing values from 880 to 580 micrograms/ml. The effects on epithelial glucose concentration by implanting an intracorneal lens (ICL) of varying diameter, depth, permeability and thickness can be modeled. Glucose permeability through the intracorneal lens has the most significant effect on glucose availability. The ICL profile i.e. power, can also be an important fact in determining glucose availability. A minus power design requires a thin central lens zone with a thick peripheral zone. The design results in relatively more glucose flux through the optical zone of the lens and thus improves central epithelial glucose availability.

Histopathology and clinical behavior of polysulfone intracorneal implants in the baboon model. Polysulfone lens implants

The authors performed 13 myopic intracorneal polysulfone implants in a baboon model using two different implant designs. In the initial series, they used eight lenticules 5 to 6 mm in diameter with a central thickness greater than 0.15 mm. These failed 6 days to 6 months after surgery. The second series of five implants 4 to 5 mm in diameter with a thinner central thickness and improved edge design was better tolerated initially, but only two of five corneas remained clear 27 months after surgery. Clinical failure was associated with corneal neovascularization and opacification posterior to the implant or aseptic necrosis of the cornea overlying the implant. In most eyes, the anterior stroma was hypocellular anterior to the implant. Numerous vacuolated keratocytes and scattered eosinophils were detected posterior to the implant. In one case, lipid deposition was confirmed posterior to the lenticule. The authors conclude that solid polysulfone intrastromal implants are not biocompatible with the baboon cornea.

Synthetic keratophakia for the correction of aphakia

Although the majority of aphakic patients will have intraocular lens implants (IOLs), there are several groups of patients for whom IOLs are not indicated. Synthetic keratophakia for the correction of aphakia has several inherent advantages over alternative forms of refractive surgery which use lathed corneal tissue. For example, the ability to produce a synthetic lens to precise specifications before surgery is a dramatic advantage over the use of lathed tissue lenses whose refractive properties cannot be determined until weeks or months after the lens is in place on the recipient cornea. Synthetic materials appear to behave in a predictable manner and are biocompatible. Both polysulfones and hydrogel materials can be used to correct aphakia in clinical situations where other modalities are less appropriate.