Journal of Refractive Surgery

Original Articles 

Eight Years Experience With Permalens® Intracorneal Lenses in Nonhuman Primates

Theodore P Werblin, MD, PhD; Robert L Peiffer, DVM, PhD; Perry S Binder, MD; Bernard E McCarey, PhD; Anil S Patel, PhD

Abstract

ABSTRACT

Background: For the past 8 years, three independent laboratories have been researching the biocompatibility and performance of Permalens® intracorneal lens implants in the corneas of nonhuman primates. Both myopic and hyperopic corrections have been achieved. This article describes the evolution of the intracorneal lens design and manufacturing process.

Methods: During this time period, 63 surgeries were performed on various species of nonhuman primates. Follow-up examination extended between 30 months and 8.2 years. Objective measures of refractive performance, as well as biocompatibility were made using slit lamp, retinoscopy, autorefractor, specular microscope, etc. Additionally, histopathology was performed on many of the specimens, both acute and chronic.

Results: Surgically successful implants were achieved in between 60% and 100% of eyes in the various series of lens implants outlined in the article. Levels of contamination in the preparation of hydrogels were felt to be responsible for many of the surgical failures. The removal of silicone and other contaminants seems to have significantly improved the biocompatibility of these materials within the cornea. The major histopathological finding was that there appeared to be some epithelial thinning over the implants, but in general excellent biocompatibility was obtained over the 8-year period outlined in this paper.

Conclusions: Although extensive studies of biocompatibility have been completed, the future of the performance of these materials remains to be proven in the human subject. Additionally, empirical relationships between lens implant power and refractive results will have to be determined in humans, prior to their general clinical usage. (Refract Corneal Surg 1992;8:12-22.)

Abstract

ABSTRACT

Background: For the past 8 years, three independent laboratories have been researching the biocompatibility and performance of Permalens® intracorneal lens implants in the corneas of nonhuman primates. Both myopic and hyperopic corrections have been achieved. This article describes the evolution of the intracorneal lens design and manufacturing process.

Methods: During this time period, 63 surgeries were performed on various species of nonhuman primates. Follow-up examination extended between 30 months and 8.2 years. Objective measures of refractive performance, as well as biocompatibility were made using slit lamp, retinoscopy, autorefractor, specular microscope, etc. Additionally, histopathology was performed on many of the specimens, both acute and chronic.

Results: Surgically successful implants were achieved in between 60% and 100% of eyes in the various series of lens implants outlined in the article. Levels of contamination in the preparation of hydrogels were felt to be responsible for many of the surgical failures. The removal of silicone and other contaminants seems to have significantly improved the biocompatibility of these materials within the cornea. The major histopathological finding was that there appeared to be some epithelial thinning over the implants, but in general excellent biocompatibility was obtained over the 8-year period outlined in this paper.

Conclusions: Although extensive studies of biocompatibility have been completed, the future of the performance of these materials remains to be proven in the human subject. Additionally, empirical relationships between lens implant power and refractive results will have to be determined in humans, prior to their general clinical usage. (Refract Corneal Surg 1992;8:12-22.)

Lamellar refractive surgical techniques have been in a process of evolution for more than 30 years. Jose Barraquer pioneered the field with the procedures of keratomileusis and keratophakia, which, are still in clinical usage.1 However, these procedures are cumbersome in that they require expertise in lathing tissue as well as in the maintenance of complex and delicate equipment. More recently, a related surgical technique, epikeratoplasty, was developed by Werblin and Lyce and Werblin and Kaufman.24 This inlay technique utilizes a lens lathed from human donor cornea! tissue and distributed commercially. Although this approach greatly simplifies lamellar refractive surgery, the technique has suffered from a lack of predictability and a number of postoperative complications.5,6

Currently, one of the most accurate refractive surgical procedures is intraocular lens (IOL) implantation as part of cataract surgery, which has an accuracy of ± 1.00 diopter standard deviation.7 This is in contrast to the 3.00 to 7.00 D standard deviation of epikeratoplasty.5 The high level of accuracy in IOL surgery is related at least partly to the use of a synthetic rather than a biological lens. The resultant refractive accuracy could be improved if the synthetic implanted lens was easily removable and/or exchangeable. In an effort to minimize dependence on mechanisms of corneal healing and to eliminate the necessity for donor corneal tissue and intraocular surgery, attempts have been made to develop an extraocular surgical technique - intracorneal lens (ICL or hydrogel keratophakia), which relies on synthetic materials placed within the cornea to achieve myopic and hyperopic refractive corrections by altering the shape of the anterior corneal surface. A synthetic lens can be precisely manufactured and the lens implant can be removed and exchanged, since the lens itself does not adhere to the stromal tissue,8 Potentially, this gives the ICL technique a high level of predictability and adjustability.

Although hydrogel keratophakia has not yet undergone extensive clinical evaluation, there have been several investigators studying these materials in nonhuman primate models.9-11 Both hyperopic9,12-14 and myopic15-18 corrections were achieved with hydrogel ICLs and encanvasing further research to determine if this technique is equal or superior to that of other currently available refractive surgery procedures.

In this article, we review the clinical and histopathological behavior of Permalens® intracorneal lens implants in nonhuman primates over an 8-year period, the longest reported follow up on hydrogel ICLs.

MATERIALS AND METHODS

Three different nonhuman primate models were used in this current study, Macaca memestrina (two ICLs - B.E.M.), Macaca Papio cynocephalus (20 ICLs- P.S.B.), and Macaca rhesus (41 ICLs-T.P.W.). The initial surgical attempts were performed using an intrastromal "pocket" dissection as described by McCarey and Andrews.9 More recently, the microkeratome has been used to achieve midstromal placement of the hydrogel lens by Binder and Werblin. These techniques have been well described previously,9,19-20 and will be briefly summarized here.

All intracorneal lens implants were made from Permalens material. The permeability, glucose flux, and other characteristics of these materials have been previously described.9 The hyperopic implants were 6 mm diameter, 0.25 mm central thickness, 0.05 mm edge thickness, 7.4 mm base curve, and 4.7 mm optic zone.21 The myopic implants varied in diameter from 5.5 to 6.75 mm. Their powers varied from - 5.00 to - 20.00 D.16 Further characterization of the myopic lens design is proprietary.

Postoperatively, slit-lamp microscope, photokeratagraphic, and keratometric examinations were performed on animals at regular intervals. Photokeratagraphs were analyzed by measuring the first ring diameter (5.5-rnillimeter diameter implants) or second ring diameter (6.75-millimeter diameter implants), using gold-plated ball bearings of known radii of curvature as standards. The largest ring which gave an interpretable reflex was used and this was proportional to the diameter of the implant. Sequential endothelial cell counts, photographing through the implant, were also performed, using the Keeler Konan endothelial microscope. Black-and-white photographs were projected on a calibrated grid and at least 100 cells counted, which after conversion computes cells per mm2. Keratometry readings were performed using either the Bausch & Lomb Keratometer or the American Optics Ophthalmometer.

Histopathology was performed by removing only the anterior corneal lamellae and hydrogel implant, leaving the animal with an exposed stromal bed,22 or by sacrificing the animal or performing penetrating keratoplasty to obtain a full-thickness corneal specimen (animals with 5-digit numbers, Table I).23 Both light and electron microscopic examinations were performed on all tissues as previously reported.22,23 Quantitative evaluation of epithelial thickness and keratocyte density was performed by counting mean epithelial cell thickness at both stromal margins of the implant and overlying the center of the implant. Keratocyte density was determined above and below at the center and the periphery of the implant. Repeated counts of keratocytes seen in several fields under high power light microscope were averaged for this study. Quantitative results were compared with "normal" species-matched controls.

RESULTS

Surgical Outcome

The technical success rate of hydrogel keratophakia in the nonhuman primate model was high, approaching 100% in our later series of lenses (Table 1). An example of uncontrolled factors24,25 was the animals' manipulation of the eye and wound due to inflammation and irritation irrespective of etiology. Approximately 21 eyes showed technical failure due to this mechanism (resulting in epithelial ingrowth in the bed). Nevertheless, we observed an overall technical success rate of 44% in our model. Previous reports have described a 50% or greater failure rate in nonhuman primates.24

Between 1983 and 1984, several series of ICLs (61 lenses in all) were implanted (Table 1). The different lens series are summarized in Table 2. Early problems with contamination of the ICL with polishing compound and silicone resulted in a high incidence of acute and chronic surgical failure manifested by aseptic necrosis of the corneal cap (series I, two eyes, 22%) and progressive scarring in the stroma surrounding the implant (series I, two eyes, 22%). Recent modifications of manufacturing techniques have resulted in a high rate of technical and clinical success (series VI, five eyes, 100%).

Table

Table 1Results of Hydrogel Myopic Intracorneal Lenses in Nonhuman Primates

Table 1

Results of Hydrogel Myopic Intracorneal Lenses in Nonhuman Primates

Table

Table 1Results of Hydrogel Myopic lntracorneal Lenses in Nonhuman PrimatesTable 2Summary of Intracorneal Lens Surgery

Table 1

Results of Hydrogel Myopic lntracorneal Lenses in Nonhuman Primates

Table 2

Summary of Intracorneal Lens Surgery

To judge the overall technical success with the more recent, better manufactured hydrogel lenses (Table 2), the percent of successful implants was calculated. In this tabulation, acute surgical failures (within about 1 month of surgery and all cases of epithelial ingrowth) were tabulated but not computed with percent of successful implants. Generally, these difficulties arise due to wound related problems where a sheet of epithelium grew across the stromal bed from the wound area. Those implants that developed cloudiness several months after surgery were classified as chronic failure. Those implants that maintained clarity throughout the observation period were termed successful. In the last series (VI), 100% of the implants were successful, and in the series just before that, almost 90% were successful. For all of the surgeons involved, no alterations in surgical technique or instrumentation occurred during the course of this study.

Figure 1: Nonhuman primate, 7 years postoperative with hyperopic Permalens ICL in place. The deep stromal position of the optically void hydrogel is demonstrated (between arrows). The surrounding cornea is crystal clear with minimal interface debris.Figure 2: Three-year, postoperative, myopic hydrogel implant demonstrating debris (arrows) at the interface behind and in front of the intracorneal lens. Some of the early lens preparations utilized polishing compounds, which left a residue of particulate matter. These particles were stable throughout the follow-up period.

Figure 1: Nonhuman primate, 7 years postoperative with hyperopic Permalens ICL in place. The deep stromal position of the optically void hydrogel is demonstrated (between arrows). The surrounding cornea is crystal clear with minimal interface debris.

Figure 2: Three-year, postoperative, myopic hydrogel implant demonstrating debris (arrows) at the interface behind and in front of the intracorneal lens. Some of the early lens preparations utilized polishing compounds, which left a residue of particulate matter. These particles were stable throughout the follow-up period.

Clinical Observations

A total of 63 surgical procedures were performed using Permalens, hydrogel intracorneal lenses between 1980 and 1985 (Table 1). The mean follow-up period for successful surgeries was 47 months, with a range from 30 months to 8.2 years. Surgically successful procedures were defined as those corneas that were crystal clear at the end of the observation period (28 out of 63 eyes, 44%). Acute failures were due either to the animal's manipulation of the surgical wound which often led to epithelial ingrowth in the bed (21 eyes) or to chemical contamination of the implant (early manufacturing difficulties seen in 25 eyes, 40%). Mean follow up in the acute failures group was 1.7 months. Six animals in this group were followed long-term, even though surgical failure was detected within 3 months following surgery. Chronic failures were defined as those eyes where chronic opacification of the corneal stroma was noted, (10 eyes, 16%). Mean follow up of this group was 7 months. This was felt to be due to a chronic reaction to the implant caused by chemical contamination of the lens material.

Safety

One animal in this study has been followed for over 8 years. Slit-lamp photographs and observations performed during this period have demonstrated no change in corneal clarity, no interface opacification, and no epithelial changes (Fig 1). There was no evidence of epithelial erosion or breakdown at any point throughout the study in any of the surgically successful animals. In some of these eyes, there has been isolated particulate matter noted at the interface, both in front of and behind the intracorneal lens (Fig 2). These particles may represent talc, polishing compound used in early implants (series I, Table 1), and metallic particles from the microkeratome. These were inert and did not create any cellular reaction. In fact, no reactions to the hydrogel (for example, cellular infiltrate and vascularization) have been seen. In all successful cases, the anterior segment of the eye, including anterior chamber, iris, and lens, were not adversely effected.

Figure 3: (A) Preoperative endothelial photomicrograph in nonhuman primate (3000 cell/mmp 2). (B) Thirty-five month, postoperative, corneal endothelium beneath a myopic intracorneal lens (animal shown in 3A). Endothelial morphology is easily depicted in this nonhuman primate eye (2800 cell/mmp 2). (C) Seven-year, postoperative appearance of endothelium beneath hyperopic hydrogel inlay. Clearly discernible endothelial morphology is evident with no apparent deleterious effects (2500 cell/mmp 2). No preoperative cell count was available.Figure 4: Photograph of the optic nerve taken through 4-year postoperative hydrogel implanted eye. Vascular detail is clearly discernible through the intracorneal lens.Figure 5: A hydrogel intracorneal inlay with an extremely thick edge (arrow). At the point of contact between the edge of the implant and stroma, an accumulation of whitish material is noted (arrow). This material is felt to accumulate in the area created by separation of the anterior and posterior cornea lamellae, because of the thickness of the edge of the implant.

Figure 3: (A) Preoperative endothelial photomicrograph in nonhuman primate (3000 cell/mmp 2). (B) Thirty-five month, postoperative, corneal endothelium beneath a myopic intracorneal lens (animal shown in 3A). Endothelial morphology is easily depicted in this nonhuman primate eye (2800 cell/mmp 2). (C) Seven-year, postoperative appearance of endothelium beneath hyperopic hydrogel inlay. Clearly discernible endothelial morphology is evident with no apparent deleterious effects (2500 cell/mmp 2). No preoperative cell count was available.

Figure 4: Photograph of the optic nerve taken through 4-year postoperative hydrogel implanted eye. Vascular detail is clearly discernible through the intracorneal lens.

Figure 5: A hydrogel intracorneal inlay with an extremely thick edge (arrow). At the point of contact between the edge of the implant and stroma, an accumulation of whitish material is noted (arrow). This material is felt to accumulate in the area created by separation of the anterior and posterior cornea lamellae, because of the thickness of the edge of the implant.

Table

Table 3Central Endothelial Cell Counts (Cells/mmp 2) in Corneas With Minus Power Hydrogel Intracorneal LensesTable 4Change in Corneal Curvature (D)* in Cornea With Minus Power Hydrogel Intracorneal Lens

Table 3

Central Endothelial Cell Counts (Cells/mmp 2) in Corneas With Minus Power Hydrogel Intracorneal Lenses

Table 4

Change in Corneal Curvature (D)* in Cornea With Minus Power Hydrogel Intracorneal Lens

The appearance of the cornea was crystal clear in all successfully implanted animals. As can be seen in Figure 3, photographs taken of unoperated and operated corneas at several time periods show the endothelial cells with excellent clarity. Tb evaluate corneal transparency, we also took photographs of the optic nerve through a corneal contact lens placed over the intracorneal lens. These photographs were compared to those taken in a similar manner through an unoperated cornea. Both photographs showed excellent fundus detail (Fig 4).

The only opacity which has been seen in implanted corneas was found where the lens edge contacted the stroma - a white opacification was sometimes observed (Fig 5). This was particularly evident with earlier ICL designs, which were less tapered at the edge. This deposit became less evident in lens designs where there is a very delicate thin peripheral taper. This scar did not adversely affect the overall clarity of the cornea, because of its paracentral location.

The lack of toxic effect on the corneal endothelium by the hydrogel implants was confirmed by sequential endothelial counts; there was a slight decrease of 7.5% loss of cell density in four eyes followed for 18 months (Table 3).

Efficacy, Predictability, and Stability

Both hyperopic14,26,27 and myopic corrections16,18 are achievable with hydrogel intracorneal lenses. In this study, we describe only myopic refractive results in four eyes where long-term topographic data were obtained (Table 4). We have previously demonstrated the correlation between myopic implant power and observed changes of curvature on the anterior corneal surface.28

Figure 6: Monkey 324 G OD, series Vl, 10 months postoperatively. Epithelium (E) at the margin of implant is slightly thickened, with thinning overlying the implant cavity (I). Fibroblasts are noted at the Implant margin (F). Anterior stroma (S) is slightly compacted; Descemet's (D) membrane and endothelium are unremarkable. Hematoxylin and eosin, original magnification, × 200.

Figure 6: Monkey 324 G OD, series Vl, 10 months postoperatively. Epithelium (E) at the margin of implant is slightly thickened, with thinning overlying the implant cavity (I). Fibroblasts are noted at the Implant margin (F). Anterior stroma (S) is slightly compacted; Descemet's (D) membrane and endothelium are unremarkable. Hematoxylin and eosin, original magnification, × 200.

Histopathology

The general appearance of the epithelium and its attachments to the basement membrane was unremarkable; however, the epithelium axially over the implant was thinned to an average of three cell layers (normal is four), which is statistically significant (P = .000006) (Fig 6). The peripheral epithelium overlying the implant was not statistically different in thickness from the normal control.

Normal mean keratocyte densities were; anterior axial cornea, 16.8 ± 2.55 cells/0.29 mmp 2; posterior axial cornea, 14.3 ± 4.2; anterior peripheral cornea, 15.1 ± 3.2; and posterior peripheral cornea, 11.5 ± 4.4. There was a statistically significant decrease in keratocyte density comparing the normal control and experimental axial measurements (P - .002). The keratocyte density at the periphery of the anterior corneal cap was not significantly different from the control (Table 5). The keratocytes and collagen lamellae themselves were normal morphologically. No signs of inflammation were noted. Epithelial cysts or rests were occasionally seen in the stromal bed. At the edges of the lenticule, accumulations of collagen and keratocytes were seen filling the void space between the stromal surfaces. A thin layer of collagen was noted in the stromal bed surrounding the lenticule.

DISCUSSION

We have demonstrated that hydrogel ICLs have excellent tissue biocompatability over an 8-year period of observation based on persistent corneal clarity, lack of inflammatory reaction, absence of vascularization, stable epithelium, and normal appearing endothelium. Although only a subjective assessment of corneal clarity can be made in the nonhuman primate model, evidence confirming persistent transparency has been gained from examination of sequential endothelial photographs, retinal photographs, as well as routine slit-lamp microscopy.

Early in the development of hydrogel corneal lenses, there were difficulties created by chemical contaminants of the hydrogel lens. Chemical analysis showed that silicone-coated hydrogel lenses were used in our early experiments (see next paragraph). Although silicone is oxygen permeable, making it suitable for use in contact lenses placed on the surface of the cornea, it is impermeable to water and metabolites.

ESCA analysis performed by Buddy D. Ratner, PhD, University of Washington, Seattle, Wash, showed series I lenses to have 34 atom percent silicone contamination whereas series IVc had only 7 atom percent, which was felt to be insignificant, making this contaminant extremely detrimental to an intrastromal lens. An impermeable barrier placed within the stroma of the cornea causes necrosis of the stroma anterior to the inlays.9,24,29 This may have been a major factor in the toxic effects of some hydrogels seen histopathologically by one author.30 Once the Permalens hydrogel material was carefully cleaned of chemical contamination, its performance as an intracorneal lens appeared to be excellent without any significant reaction within the cornea (Tables 1-3).

Intracorneal inlays can correct hyperopic and myopic refractive errors.9,12-17 These corrections are stable for at least a 2-year period of observation (Table 4). Unfortunately, the nonhuman primate model can generate only semiquantitative information about the precision of the correction achieved at the corneal surface. A number of factors make these observations less than ideal, including lack of fixation and lack of blinking with sedation. Keratoscopic rather than retinoscopic images were felt to be more reproducible with these myopic implants. Only refractive data from human subjects can provide detailed information about refractive accuracy, which is critically needed at this time.

Table

Table 5Light Microscopic Findings in Corneas With Minus Power Hydrogel Intracorneal Lens

Table 5

Light Microscopic Findings in Corneas With Minus Power Hydrogel Intracorneal Lens

Histopathologic studies have demonstrated several changes within the cornea in association with hydrogel implantation.21,22 The etiology is unknown but could be nutritional or mechanical. Epithelial thinning and even epithelial irregularity are not at all unusual in refractive surgical procedures. For example, in epikeratoplasty, several clinical specimens have shown alteration of normal epithelial morphology far more severe than that demonstrated in this study.31,32 The pathological and clinical significance of the minor alteration in epithelial thickness found over hydrogel implants is not at all clear at this time. Certainly, epithelial stability, as monitored clinically, was found throughout the entire time course of these experiments.

The stroma anterior to the hydrogel inlay demonstrates a slight decrease in keratocyte density. Keratocyte density and repopulation has also been seen to vary in other refractive surgical procedures and has not been found to be deleterious to the cornea.33,34 An example of this is the entire lack of keratocytes seen in healed keratophakia lenticules years after surgery.34 These corneas, with a cellular tissue lenses in place, do not appear to be physiologically optically abnormal. Similarly, in epikeratoplasty, there is a prolonged keratocyte repopulation time with marked irregularity to the keratocyte repopulation, which again does not seem to be detrimental to the optical function of the cornea.32

The stroma surrounding the implant generally shows little or no reactivity. Occasionally, a slightly higher density of keratocytes or fibroblasts was seen in this area, but clinically, this does not appear to adversely affect the optical properties of the cornea. This has been described with other polymers implanted within the cornea.24 At times, particulate matter from several sources has also been seen at the interfaces around the implants. Initial implants were coated with debris from polishing compounds (series I, Table 1). This problem has been eliminated. Other particulate contamination may represent intraoperative debris or particles liberated from the blade of the microkeratome (Fig 3). Previous descriptions reported no serious clinical consequences.34,35 Epithelial proliferation surrounding the implant may represent true epithelial ingrowth or implantation of isolated clusters of cells at the time of surgery. They can be removed surgically;16 however, they can probably be prevented by careful attention to surgical technique at the time of the microkeratome section.

There was a 7.5% decrease in endothelial density associated with surgery. This seems to be a common factor in other refractive surgical procedures such as radial keratotomy36 and keratomileusis.37 The concern of progressive endothelial loss, however, does not appear to be supported by our data.

Apple et al demonstrated in a rabbit model a significant reaction to hydrogel implants placed in the corneal stroma.30 A possible explanation for these observations might be related to the poor choice of the rabbit as a model or to an inflammatory reaction to hydrogel lenses, which were prepared in a chemically impure form, with chemical monomer contaminants in particular as was seen in our earlier series of lenses. Caution needs to be exercised in evaluating various sources of ICLs for possible chemical contamination.

The major remaining uncertainty with hydrogel ICLs is their accuracy for correcting refractive errors. Animal modeling, such as the data presented in this article, can only give an approximate evaluation of predictability. Clinical studies in humans can precisely and uniquely define the accuracy of this procedure as well as its human clinical safety and overall efficacy. Initial studies to determine the precise empirical relationship between lens power and refractive change may necessitate lens exchange. However, this is easily done with hydrogels.8 Other investigators38 have proposed, and we support their conclusion, that limited, carefully controlled, and monitored human clinical studies are now necessary to evaluate this exciting refractive procedure.

REFERENCES

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2. Werblin TP, Lyce SD. Epikeratophakia: the surgical correction of aphakia. I. Lathing of corneal tissue. Curr Eye Res. 1981;1:123-129.

3. Werblin TP, Kaufman HE. Epikeratophakia: the surgical correction of aphakia. ?. Preliminary results in a nonhuman primate model. Curr Eye Res. 1981;1:131-137.

4. Werblin TP, Kaufman HE, Friedlander MH, Granet NS. Epikeratophakia: the surgical correction of aphakia. ??. Preliminary results of a prospective clinical trial. Arch Ophthalmol. 1981;11:1957-1960.

5. McDonald MB, Kaufman HE, Aquavella JV, et al. The nationwide study of epikeratophakia for myopia. Am J Ophthalmol. 1987;103:375-383.

6. Binder PS. Optical problems following refractive surgery. Ophthalmology. 1986;93:739-745.

7. Holladay JT, Prager TC1 Ruiz RS, et al. Improving the predictability of intraocular lens power calculations. Arch Ophthalmol. 1986;104:539-541.

8. Binder PS, Zavala EY, Deg JK. Hydrogel refractive keratoplasty: lens removal, and exchanges. Cornea. 1983;2:119-125.

9. McCarey BE, Andrews DM. Refractive keratoplasty with intrastromal hydrogel lenticular implants. Invest Ophthalmol Vis Sci. 1981;July:107-115.

10. Binder PS, Deg JK, Zavala EY, Grossman RG. Hydrogel keratophakia in nonhuman primates. Curr Eye Res. 1981/ 2; 1:535-542.

11. Werblin TP, Blaydes JE, Fryczkowski AW, Peiffer RL. Alloplastic implants in nonhuman primates. I. Surgical technique. Cornea. 1982;1:331-336.

12. Werblin TP, Blaydes JE, Fryczkowski AW, Peiffer RL. Stability of hydrogel intracorneal implants in nonhuman primates. CLAO Journal. 1983;9:157-161.

13. Binder PS, Zavala EY, Deg JK, Baumgartner SD. Hydrophilic lenses for refractive keratoplasty: The use of factory lathed materials. CLAO Journal. 1984;10:105-111.

14. McCarey BE, McDonald MD, Van Rij G, et al. Refractive results of hyperopic hydrogel intracorneal lenses in primate eyes. Arch Ophthalmol. 1989;107:724-730.

15. Werblin TP, Fryczkowski AW, Peiffer RL. Alloplastic implants in nonhuman primates. III. Myopic correction, preliminary report. Ann Ophthalmol. 1984;16:1127-1130.

16. Werblin TP, Peiffer RL, Fryczkowski AW. Myopic hydrogel keratophakia: preliminary report. Cornea. 1985;3:197-204.

17. Werblin TP, Peiffer RL, Patel AS. Myopic hydrogel keratophakia: improvements in lens design. Cornea. 1987;6(3):197-201.

18. McCarey, BE, Storie BR, Van Rij G, Knight PM. Refractive predictability of myopic hydrogel intracorneal lenses in nonhuman primate eyes. Arch Ophthalmol. 1990;108:1310-1315.

19. Werblin TP, Blaydes JE, Fryczkowski AW, Peiffer RL. Alloplastic implants in nonhuman primates. II. Modifications of Barraquer microkeratome. Cornea. 1983;2:127-131.

20. Binder PS, Akers PH, Zavàla EY, Deg JK Refractive kerato plasty. Microkeratome evaluation. Arch Ophthalmol 1982;100:802.

21. McCarey BE, Andrews DM, Hatchell DL, Pederson H. Hydro gel implants for refractive keratoplasty: corneal morphology. Curr Eye Res. 1982;2:29-38.

22. Peiffer RL, Werblin TP, Fryczkowski AW. Corneal hydroge alloplastic implants: histology. Ophthalmology. 1985; 92:1294-1304.

23. Binder PS, Baumgartner SD, Deg JK, Zavala EY. Hydrophilic lenses for refractive keratoplasty, The use of factory lathed materials. CLAO Journal. 1984;10:105.

24. Deg JK, Binder PS, Kirkness C. Unfenestrated polysulfone implants are incompatible with the baboon and human cornea. Invest Ophthalmol Vis Sci. (supp) 1987;28:276.

25. Werblin TP. Epikeratophakia: techniques, complications and clinical results in refractive corneal surgery. Int Ophthalmol Clin. 1983;23:45-48.

26. Watsky MA, McCarey BE. Alloplastic refractive keratophakia: a comparison of predictive algorithms. CLAO Journal. 1986;12:112-117.

27. Binder PS, Baumgartner SD. Morphology of hydrogel implants used for refractive keratoplasty. Invest Ophthalmol Vis Sci. 1984;25:843.

28. Werblin TP, Peiffer RL, Patel AS. American Academy of Ophthalmology 1986 - Synthetic keratophakia for the correction of aphakia. Ophthalmology. 1987;94:926-934.

29. Barraquer JI. Queratoplastia refractiva. Barcelona. Estudios E Informaciones Oflalmolgicas. 1949;ll:chl0.

30. Apple DJ, Ohrloff C, Duffin RM, Olson RJ. Opacification, vascularization, and chronic inflammation produced by hydrogel corneal lamellar implants. Am J Ophthalmol. 1984;95:422-425.

31. Goodman GL, Werblin TP, Peiffer RL. Failed epikeratophakia for keratoconus. Cornea. 1986;5:29-34.

32. Binder PS, Zavala EY. Why do some epikeratoplasties fail? Arch Ophthalmol. 1987;105:63-69.

33. Jester JV, Rodrigues MM, Villaseñor RA, Schanzlin ?1 Keratophakia and keratomileusis: histopathologic, ultrastructural, and experimental studies. Ophthalmology. 1984;91:793-805.

34. Swinger CA, Barker BA. Prospective evaluation of myopic keratomileusis. Ophthalmology. 1984;91:785-792.

35. Tucker DN, Barraquer JI. Refractive keratoplasty: clinical results. Ann Ophthalmol. 1973;March:335-351.

36. Jester JV, Steel D, SaIz J, et al. Radial keratotomy in nonhuman primate eyes. Am J Ophthalmol. 1981;92:153171.

37. Aquavella JV, Barraquer F, Rao GN, Ruiz LA. Morphological variations in corneal endothelium following keratophakia and keratomileusis. Ophthalmology. 1981;88:721-723.

38. Beekhuis WH, McCarey BE, van Rij G, Waring III GO. Complications of hydrogel intracorneal lenses in monkeys. ArcA Ophthalmol. 1981;105:116-122.

Table 1

Results of Hydrogel Myopic Intracorneal Lenses in Nonhuman Primates

Table 1

Results of Hydrogel Myopic lntracorneal Lenses in Nonhuman Primates

Table 2

Summary of Intracorneal Lens Surgery

Table 3

Central Endothelial Cell Counts (Cells/mmp 2) in Corneas With Minus Power Hydrogel Intracorneal Lenses

Table 4

Change in Corneal Curvature (D)* in Cornea With Minus Power Hydrogel Intracorneal Lens

Table 5

Light Microscopic Findings in Corneas With Minus Power Hydrogel Intracorneal Lens

10.3928/1081-597X-19920101-07

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