The distribution of strain in the human cornea

Experimental data on the mechanics of human cornea is meager and sometimes flawed. Moreover, questions regarding the correct material symmetry and the role of the fibrous microstructure are usually glossed over when mechanical models of the cornea and corneal shape changing procedures are presented. Accordingly, the deformation of 14 intact human corneas was measured for five pressures in the physiologic range (0, 5, 10, 25 and 45 cmH2O) by tracking small, self-adherent particles placed on their anterior surfaces. The meridional strains, calculated in five regions assuming axisymmetric deformation, are small; the average strain in the apical region being 1.14% at 45 cmH2O. Results also indicate that the strain distribution is unexpectedly nonuniform with statistically significant (p < 0.01, typical) variations between regions and a minimum occurring approximately half-way between apex and limbus. To better understand these results, a finite-element model (FEM) of the cornea was constructed and used to simulate the experiment. The heterogeneous model shows that our data may reflect the changing fiber orientation along a meridian suggested in the literature. The implications of a link between microstructure and mechanics are discussed in light of clinical procedures, such as radial keratotomy, the outcomes of which are dependent on corneal mechanical properties.

Corneal endothelial damage by air bubbles during phacoemulsification

OBJECTIVE

To characterize the mechanism by which air bubbles damage the corneal endothelium during phacoemulsification.

MATERIALS AND METHODS

A series of experiments was conducted to expose the corneal endothelium of New Zealand white rabbit and human eyes that were obtained from an eye bank to air under different conditions. Phacoemulsification at different power settings and irrigation with and without the introduction of air into the anterior chamber were performed. Corneal endothelial perfusion experiments were conducted with air bubbles that were introduced into the perfusion chamber for 2 seconds to 1 hour. Air was also injected into the anterior chambers of anesthetized rabbits for 2 minutes to 3 hours. Corneas were stained with nitrobenzo-xadiazole-phallacidin and examined with fluorescence microscopy. Selected corneas were also examined with scanning and transmission electron microscopy.

RESULTS

Intracameral air bubbles during phacoemulsification, irrigation, and perfusion studies resulted in a severe injury to the corneal endothelium in as little as 20 seconds. Intracameral air bubbles in a living rabbit resulted in a slower injury that was morphologically different from the more rapid injury.

CONCLUSIONS

Air bubbles in intraocular fluids with a high surface tension can cause a ring-shaped pattern of damage to the corneal endothelium. The mechanism that caused this pattern of damage appears to be a surface tension phenomenon.

Effect of tear lubricating solutions on in vivo corneal epithelial permeability

PURPOSE

The rabbit corneal epithelial permeability was measured noninvasively following exposure to commercially available tear lubricating solutions.

METHODS

The normal unanesthetized rabbit's corneal surface was either bathed in the tear lubricating solution for 5 min or received multiple applications of 2 drops per 30 min for 6 h for one or five days. The corneal epithelial permeability to carboxyfluorescein after a 5-min bath was measured with the Fluorotron Master. Fifteen commercially available tear preparations were tested.

RESULTS

The baseline corneal epithelial permeability was 0.0455 +/- 0.0114 nm/s. The epithelial permeability values for the corneas bathed for 5 min in control solution (BSSplus) was 0.0798 +/- 0.0074 nm/s, in the preservative-free tear lubricating solutions the range was 0.512 to 0.542 nm/s, and in the preserved tear lubricating solutions the range was 0.3518 to 11.8873 nm/s. The preservative-free tear lubricating solutions had epithelial cell permeabilities up to 6 times greater than the control solution. Whereas, the preserved tear lubricating solutions had epithelial cell permeabilities up to 149 times greater than the control solution. Multiple applications of the tear lubricating solutions for 5 days were less damaging to the epithelial permeability than the 5 min bath applications. The resulting epithelial permeabilities were up to 2 fold greater than the control with the corresponding preservative-free solution and up to 29 fold greater than control with the corresponding preserved solution.

CONCLUSION

An extended exposure of 5 min to various tear lubricating solutions demonstrated significant differences in epithelial cell permeability between preserved solutions and unpreserved solutions, whereas the multiple drop application technique demonstrated all of the preservative-free solutions and some of the preserved solutions to be insignificantly different from the control solution.

Noninvasive measurement of corneal epithelial permeability

The Fluorotron Master fluorophotometer was used to measure the corneal epithelial permeability to carboxyfluorescein in the normal unanesthetized rabbit. This was related to the epithelial fluorescein staining grade as assessed with a slit lamp. The carboxyfluorescein concentration in the cornea was measured at 3, 60 and 120 min after bathing the cornea in 2.7 x 10(-3) M carboxyfluorescein for 5 min. The epithelial cell layer permeability was calculated from the corneal carboxyfluorescein concentration immediately after a balanced salt solution rinse (initial technique) and from the time zero intercept value extrapolated from the 60 and 120 min data (intercept technique). The slit lamp demonstrated 69% of the nonpaired rabbit eyes were free of epithelial defects, whereas, 46% of the paired rabbit eyes were free of defects. Epithelial permeability values determined from the initial or intercept technique were similar (p = 0.77, unpaired 2 tailed t-test for data < 0.165 nm/s). The initial analysis permeability values ranged from 0.0154 to 4.309 nm/s., n = 29 There was a Gaussian distribution of data between 0.015 and 0.144 nm/s with a mean of 0.0646 +/- 0.0070 nm/s. The larger values correlated with the naturally occurring epithelial defects observed with the slit lamp. The epithelial permeability values were independent of the contralateral eye or subsequent day measurements.

Corneal wound healing strength with topical antiinflammatory drugs

The objective of this study was to compare the force required to separate corneal wounds after topical applications of nonsteroidal antiinflammatory drugs (NSAIDs) or corticosteroids. Bilateral central 8-mm long corneal full-thickness incisions in 50 NZW rabbits were closed with five interrupted 10-0 nylon sutures. There were four paired-eye groups: (a) control/control, (b) control/diclofenac sodium (0.1%), (c) control/flurbiprofen sodium (0.03%), and (d) control/prednisolone acetate (1%) treated six times per day for 7 or 21 days. The wound strength was measured by determining the force necessary to separate the incision along its length. The eyes did not differ statistically from their contralateral eye for each group except control/diclofenac (7.98 g/12.32 g) and control/flurbiprofen (6.96 g/11.67 g) at 21 days. The strongest scars occurred after treatment with diclofenac and flurbiprofen, which were similar (p = 0.74). The weakest wounds for each time period were with prednisolone (1.74 g/3.21 g). The diclofenac and flurbiprofen were stronger than prednisolone-treated eyes at 7 days (p = 0.028 and p = 0.023, respectively) and at 21 days (p < 0.001). The bilateral controls were stronger than the prednisolone controls (p = 0.008 at 7 days and p = 0.001 at 21 days). Steroid treatment caused weaker corneal wound scars than did the NSAIDs. Unilateral steroid treatment adversely affected their untreated contralateral eyes. The NSAID-treated wounds were the strongest and stronger than their contralateral control eyes.

Topographic evaluation of toric soft contact lens correction

The anterior surface of the eye provides the greatest refracting surface to the optical system of the eye because of the large difference in refractive index between air and tissue. When a contact lens is placed on the eye, its anterior surface becomes the greatest refracting surface. Toric soft contact lenses have been designed to neutralize ocular astigmatism. We observed that topographic analysis of toric soft contact lenses on an astigmatic eye demonstrated residual astigmatism on the anterior surface of soft toric contact lenses. In this paper we show toric soft contact lens surface astigmatism by videokeratoscopy and interpret the origins of the astigmatism.

Nicotine on the revascularization of bone graft. An experimental study in rabbits

STUDY DESIGN

In 24 rabbits, the authors transplanted autologous cancellous bone to the anterior chamber of the eye. Half of the rabbits received nicotine and half received placebo (albumin) from mini-osmotic pumps that were implanted subcutaneously. Revascularization of the bone graft was evaluated postoperatively using ophthalmology slit-lamp and fluorescein angiography, and after sacrifice using microvascular silicone injection and histology.

OBJECTIVES

The hypothesis that nicotine inhibits the revascularization of bone graft because of its pharmacologic action on the microvasculature was tested.

SUMMARY OF BACKGROUND DATA

Pseudoarthrosis after spinal fusion occurs more frequently in smokers as compared with nonsmokers.

METHODS

Observations of the bone graft were made regarding the time after implantation when vessels within the graft were noted and the pattern of these vessels. Revascularization of the graft was graded based on the observed percent area of fluorescence after injection of fluorescein. Serum levels of nicotine were measured weekly. Colored silicone was injected at sacrifice to fix the vasculature of the bone graft. Histologic analysis of undecalcified sections was performed.

RESULTS

Nicotine, as compared with placebo, was associated with delayed revascularization within the graft, a smaller percent area of revascularization, and a larger number of grafts showing necrosis.

CONCLUSIONS

Nicotine inhibits, but does not prevent, the revascularization of cancellous bone grafts. Inhibition of early revascularization by nicotine is proposed as the pathophysiologic mechanism by which smoking may adversely affect the healing of spinal fusions.

Surface topography of soft contact lenses for neutralizing corneal astigmatism

We compared the surface topography of 38 soft contact lenses in situ to the corneal topography of 17 patients. The analyzed lenses consisted of patients' spherical and toric lenses as well as additional spherical lenses of various thicknesses. Thirteen brands of contact lenses were worn; corneal astigmatism ranged from 0.12 to 3.81 D. Our hypothesis was that toric soft contact lens neutralization of corneal surface astigmatism occurred through the creation of a more spherical anterior surface. Videokeratoscopy was used to analyze corneal surface changes with contact lens wear. The correction of astigmatism for toric corneas (toricity > 0.75 D) fit with toric lenses (i.e., the difference between the surfaces of the cornea and anterior lens) showed surface astigmatism neutralization of only 34 +/- 38%. The greater the astigmatism, the greater the neutralization. Spherical lenses not only failed to mask corneal toricity, but actually increased it. The topographic map can be a valuable asset for analyzing and possibly improving contact lens fitting of toric contact lenses.

Assessment of the long-term corneal response to hydrogel intrastromal lenses implanted in monkey eyes for up to five years

The biocompatibility of hydrogel intracorneal lenses (ICLs) implanted in monkey eyes was evaluated for periods ranging up to five years. Seventy-three plus or minus powered ICLs made of Lidofilcon A (68% water) or Lidofilcon B (79% water) were implanted following lamellar dissection with a microkeratome. Ten sham surgical procedures were performed without ICL implantation as controls. Eyes were followed for up to five years by slitlamp biomicroscopy and specular microscopy. Light and transmission electron microscopic evaluations of enucleated eyes were performed at various intervals. Minimal tissue reaction was noted; both hydrogel materials appeared to be equally well tolerated. Failures usually occurred as a result of microkeratome problems encountered during surgery. Histopathological changes to the cornea included epithelial thinning anterior to the thickest portion of the ICL, fibroblastic activity along the ICL-stromal interface, and deposition of an amorphous extracellular material adjacent to the ICL. These observations did not appear to be clinically significant as the eyes were quiet by slitlamp examination. Removal of three ICLs eight to ten months prior to enucleation restored the normal histological characteristics of the cornea. The endothelial cell density of ICL-implanted eyes decreased by 4.3% (n = 17) six months after surgery but remained stable thereafter. The variation in endothelial cell area and percentage of hexagonal cells did not change over 50 months. The results appear to demonstrate that high water content synthetic ICLs can be well tolerated in the monkey cornea for up to five years.

Intrastromal crystalline deposits following hydrogel keratophakia in monkeys

Intrastromal corneal crystals developed in 11 of 49 monkey eyes with hydrogel intracorneal lenticules followed over a 4 year period. All of the eyes, except one, underwent synthetic keratophakia via a microkeratome incision. The diagnosis of crystal formation was initially made by slit-lamp examination and confirmed with specular microscopy (from 21 to 150 days post-hydrogel keratophakia). The crystals were noted to lie in a plane adjacent to the hydrogel lenticule and specular microscopy revealed polychromatic, hyperrefringent crystals with either a rectangular or linear morphology. Prior to intrastromal crystal formation, these corneas developed early postoperative stromal edema (< 3 weeks). We hypothesize that early stromal edema in combination with the hydrogel intracorneal lenticule induced the production of lipid crystals by stromal keratocytes.

A biomechanical evaluation of rabbit corneas in M-K and Dexsol

We compared the average force required to separate normal corneas at a 50% stromal depth, with the force required to separate corneas stored for 2, 5, 7, 10, and 14 days in either McCarey-Kaufman or Dexsol corneal storage medium. The required interlamellar separation force was calculated by standardizing the width of the test strips. The average required force for 35 fresh rabbit corneas (no storage) was 9.1 +/- 1.5 g/mm. There was no significant change in the required separation force after storage in either medium for up to 14 days.

Practical aspects of a corneal topography system

We used the EyeSys Corneal Topography System to examine several issues relating to corneal topography systems and the interpretation of their results. Interferometry measurements of EyeSys calibration spheres indicated that they deviated from suggested values by 0.05 +/- 0.13 D (mean +/- 1 standard deviation). The EyeSys unit reliably determined the calibration spheres to be spherical with differences between the flat and steep axis of 0.10 +/- 0.09 D. The data for the 3 mm chord circle was the least reliable. The spherical equivalent values for the calibration spheres were constantly greater than the 0.25 D reproducibility suggested by the manufacturer. Furthermore, the precision of the outputted values (0.01 D) is beyond the capability of the instrument. This gives the impression that the topography unit cannot consistently reproduce measurements of the calibration objects or chrome plated steel spheres. Image centralization and focus were found to be critical in obtaining accurate results. A target off center by more than 0.25 mm resulted in unreliable data; increasing the focal distance by greater than 1 mm beyond the focal point resulted in a sharp decrease in accuracy (a decrease in the focal distance was even more critical). When measuring aspheric contact lenses, it was found that the keratometer and EyeSys unit provided a close approximation of the surface characteristics of the lenses. A realistic view of the limitations of the topography system being used is critical for proper interpretation of the data.

Eight years experience with Permalens intracorneal lenses in nonhuman primates

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.

Hydrogel keratophakia: long-term morphology in the monkey model

After 8.5 years a rhesus monkey with bilateral hydrogel (Permalens) intracorneal implants was evaluated by light microscopy and transmission electron microscopy to determine the long-term effects of a synthetic plastic upon the primate cornea. Light microscopy revealed that the hydrogel lenticules were implanted between 80 and 90% of stromal depth, causing posterior bulging of the stroma into the anterior chamber; this was also seen clinically with slit-lamp biomicroscopy. Some minor pathologic changes were also found elsewhere in the cornea, including epithelial remodelling anterior to the hydrogel implant and a few vacuolated endothelial cells posterior to the edges of the implant. Histologically, no inflammatory reaction was seen in either cornea. This primate demonstrates the long-term biocompatibility of hydrogel intracorneal lenses.

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.

Refractive predictability of myopic hydrogel intracorneal lenses in nonhuman primate eyes

The refractive predictability of myopic hydrogel intracorneal lenses made of lidofilcon A was evaluated in 24 monkey eyes following a microkeratome dissection. All eyes were examined monthly for refractive alteration and clinical appearance during a follow-up period of 7 to 33 months. The refractive yield was measured by retinoscopy and found to be a function of the implant depth. The mean +/- SD percent of the desired correction achieved averaged 70% +/- 24% at a 36% to 60% depth (n = 8), 54% +/- 22% at a 61% to 79% depth (n = 7), and 12% +/- 14% in implants deeper than at a 79% depth (n = 9). For eyes with implants at a 36% to 60% depth, the predicted vs achieved refractive alteration yielded a correlation coefficient of .97 (88% within +/- 3 diopters of the correlation). The refractive outcome remained stable throughout 7 months of follow-up, whereas deeply placed lenses (greater than or equal to 80%) exhibited significant regression in power during this time.

Interlamellar adhesive strength in human eyebank corneas

The interlamellar biomechanical properties of stromal collagen are relatively unknown, yet may be highly significant with respect to wound healing and the efficacy of certain keratorefractive surgical procedures. Interlamellar adhesive strength was measured as the tearing force required to separate corneal lamellae at a 50% stromal depth in 16 human eyebank corneas. The mean value for the central cornea was found to be 14.2 (+/- 0.5 SEM) g-wt/mm of tissue width. Histology showed a smooth separation between the lamellae along the tearing plane in the central cornea. We believe that the adhesive strength measured in the central cornea may be primarily the force needed to break interlamellar proteoglycan bonds between collagen lamellae, because no torn lamellae were found in this region. The mean adhesive strength and the SEM increased toward the periphery in a symmetrical fashion. The mean adhesive strength in the far periphery was 31.6 (+/- 3.7 SEM) g-wt/mm at 5 mm nasally, and 28.4 (+/- 3.2 SEM) g-wt/mm at 5 mm temporally, and was approximately twice the mean central value. The rising value of the mean adhesive strength with increasing distance from the central cornea was believed to be due to a more highly disorganized collagen network in which greater numbers of lamellae passed obliquely in depth through the tearing plane. These lamellae would contribute their tensile strength to the adhesive strength measurement along the tearing plane. Histology from the peripheral cornea confirmed the existence of depth-varying collagen lamellae and the torn ends of lamellae that passed across the tearing plane.

Lipid deposits posterior to impermeable intracorneal lenses in rhesus monkeys: clinical, histochemical, and ultrastructural studies

Synthetic materials are being evaluated for their potential long-term use in corneal refractive surgery. Clinical and histopathologic studies were performed with polymethylmethacrylate (PMMA) and polysulfone intracorneal lenses in rhesus monkey eyes that were followed for up to 3 years. The 5 mm diameter lenses were placed in the deep posterior corneal stroma of four eyes. Fine, polychromatic crystalline deposits formed a layer posterior to the implants. Enucleated eyes had the corneas either frozen for histochemistry or fixed for electron microscopy. Special stains included oil red 0 and filipin on fresh frozen tissue. The cornea, with a PMMA intracorneal lens showed myriad crystalline aggregates in the deep corneal stroma behind the implant. These crystalline deposits stained positively with oil red 0 and with filipin indicating the presence of neutral fat as well as unesterified cholesterol. The polysulfone implant showed no evidence of crystalline deposits histologically but was surrounded by homogeneous aggregates that could represent nonspecific reaction to the lens material or protein deposits. Electron microscopy of all four corneas revealed dissolved lipid aggregates and laminated electron dense material that were most abundant posterior to the implant where the keratocytes appeared disintegrated. The PMMA lens appeared to induce lipid keratopathy.

Refractive results of hyperopic hydrogel intracorneal lenses in primate eyes

Hyperopic hydrogel intracorneal lenses were successfully implanted into 27 of 33 primate eyes. All eyes were evaluated preoperatively and postoperatively at monthly intervals for clinical appearance and refractive alteration. In a preliminary surgical series, several factors, such as tight sutures and implant design, resulted in a poor refractive yield. The final surgical series used a microkeratome with a pediatric microkeratome ring for smooth interface cuts, interrupted suturing with sufficient tension to align the wound without compression, a suture through the lens to prevent its dislocation, and intraoperative keratometry to reduce postoperative cylinder. The predicted vs measured refractive alteration for a range of 6 to 20 diopters had a correlation coefficient of .95. Keratometry changes correlated to the refractive changes with a coefficient of .97 but understand the change in refraction created by the surgery.

Glucose consumption in cultured corneal cells

The rate of glucose consumption in cultured epithelium, endothelium, and keratocytes was measured; and the effect of reduced glucose availability on the consumption rate of these three cell lines was delineated. All three cell types exhibited an asymptotic decrease of glucose over time while being incubated in Krebs-Ringers solutions of varying glucose concentrations. At a concentration resembling that of the aqueous, epithelium (EPI), endothelium (ENDO), and keratocytes (K) consumed 6.7, 7.4, and 9.0 micrograms/cm2/hr respectively. Each cell type consumed glucose at a rate that was related to the amount of available glucose. As glucose concentration was reduced from 90 to 30 mg%, which was a 66% reduction in available glucose, the consumption of EPI, ENDO, and K dropped 74%, 61%, and 44% respectively.

Effect of diameter and depth on the response to solid polysulfone intracorneal lenses in cats

Impermeable, solid polysulfone intracorneal lenses (ICLs) can change corneal refractive power but will not allow diffusion of nutrients from the aqueous to the anterior stroma. Lenses of 4, 5, 6, and 7 mm in diameter were implanted in 50 cat eyes at depths ranging from 45% to 95% of corneal thickness to determine the effect of depth and diameter. Two types of stromal opacities occurred during follow-up: peripheral amorphous opacity (100%) and crystalline-granular lipid central opacity (79%). No eyes implanted with a 4-mm ICL ulcerated, but ulceration occurred in 28% of the eyes overall. Because opacification occurred with all lens diameters and implantation depths, metabolic demands of the cornea are not met. No safe limit exists at which these ICLs can be predictably used in cat eyes.

Refractive keratoplasty in monkeys using intracorneal lenses of various refractive indexes

The refractive power of the cornea can be altered with an intracorneal lens when it is inserted within a freehand lamellar-pocket incision. The effectiveness of the implant in altering the refractive power of the cornea depends on the refractive index of the intracorneal lens, since little change in the anterior corneal curvature occurs. Thirteen nonhuman primate eyes received intracorneal lenses made of synthetic polymers with refractive indexes ranging from 1.3686 to 1.633. The lidofilcon-B hydrogel (refractive index, 1.3686) lenses created no significant alteration in the refractive power of the cornea. The polymethylmethacrylate (refractive index, 1.4900D) and polysulfone (refractive index, 1.633) lenses created a predictable and significant alteration of corneal power that increased with each implant with a successively higher refractive index. The polymethylmethacrylate lens created a refractive alteration that was approximately 31% of the lens power in air. The polysulfone lens created a corneal power alteration that was approximately 40% of the lens power in air.

Complications of hydrogel intracorneal lenses in monkeys

We performed 32 hydrogel keratophakia procedures in rhesus monkey corneas. Surgery was technically successful in 20 eyes. Toxic stromal keratitis occurred in five eyes because of impurities on the lenticules. Fibrosis appeared around the implant in five eyes. Two eyes showed aseptic necrosis of the inferior cornea with extrusion of the implant, attributed to exposure keratitis. Complications during surgery included faulty microkeratome incisions (three eyes) and epithelial implantation in the lamellar bed (one eye). Three lenticules decentered, one extruding through the wound, probably because the monkeys rubbed their eyes excessively. Nine of the 11 eyes with postoperative refraction data showed overcorrection or undercorrection of more than 1.00 diopter. Most of these complications could have been prevented by better technique, design, and manufacture of the lenticule or cooperation of the subjects.

Corneal epithelial Cl-dependent pump quantified

Pairs of rabbit corneas were perfused and corneal thickness monitored. The epithelial side of the preparation, and after stabilization also the endothelial side, was covered with silicone oil. A constant thinning of the cornea was observed at a rate of 11.1 microns hr-1 (= 1.0 micron L h-1 cm-2) S.D. 2.1, n = 9. This thinning was also present after endothelial removal: 12.3 microns hr-1, S.D. 1.6 (n = 5). Epithelial abrasion virtually abolished the thinning (2.5 microns hr-1, S.D. 1.89, n = 4), as did cooling from 34 to 0 degrees C. When the preparation was perfused with a Cl-free solution (SO4(2-) instead of Cl-, corrected for osmolarity with sucrose) no significant thinning of the preparation was observed (2.4 microns hr-1, S.D. 2.49, n = 4) after covering both surfaces with silicone oil. This simple set of experiments quantified the epithelial pump mechanism. The epithelial pump rate of about 1.2 microns L hr-1 cm-2 has to be taken into account when endothelial pump rates are measured in the in vitro preparation with intact epithelium.

Hydrogel keratophakia: a microkeratome dissection in the monkey model

High water content intracorneal implants were fabricated from Vistamarc hydrogel (Vistakon, Inc.) at 58%, 68%, and 72% water content and a range of powers from +7.25 to +17.00 dioptres. The Barraquer microkeratome technique was used to implant the lens at 59.0 +/- 9% (+/- SD) depth in the corneas of 14 rhesus monkey eyes. The contralateral eye served as a control. Three eyes were lost to the study because of complications. The remaining 11 animals were followed up for 51 +/- 2 weeks with the refractive yield being 118 +/- 34% and the keratometric yield being 92 +/- 30%. The measured and theoretically expected refractive changes have a linear regression line correlation coefficient of 0.74, whereas the respective keratometric data had a correlation coefficient of 0.04. The measured refraction became stable within 2 to 3 dioptres after 20 postoperative weeks. The hydrogels were well tolerated within the corneal tissue. There was a minimum of interface problems except along the edge of the implant. Implants with abruptly cut edges versus a fine wedge tended to have more light scattering collagen at the implant margin.

Hydrogel keratophakia: a freehand pocket dissection in the monkey model

High water content hydrogels can be made with water and solute permeabilities comparable to those of the corneal stroma, thus making them feasible as intrastromal implants for refractive keratoplasty. The materials have been shown to be compatible with the cornea tissue, but for a lenticule of hydrogel to be effective in a refractive keratoplasty procedure it must alter the anterior curvature of the cornea. In this investigation hydrogel lenticules were implanted by a free-hand pocket dissection in eight Macaca mulatta (rhesus) and two Macaca nemestrina (pigtail) primate eyes. The results of pre- and postoperative keratometry and subjective retinoscopy as well as biomicroscopy were recorded. The alteration in refractive power was calculated in relation to the hydrogel lenticule parameters such as base curve, refractive index, etc. The corneal refractive change had a yield of +3 +/- 27% (+/- SD). The central keratometric change had a yield of +6 +/- 16%. The hydrogel plus power lenticule implanted in a free-hand intrastromal pocket created no significant steepening of the anterior cornea surface and therefore little change in refraction.

Polysulfone corneal lenses

Polysulfone is a thermoplastic compound first synthesized in 1965. A unique characteristic of polysulfone is its high refractive index (1.633), which allows very thin optical lenses to be manufactured. Over the last five years, D. Peter Choyce has surgically implanted over 40 polysulfone lenses into eyes of his patients. Analysis of his data indicates that polysulfone intraocular lenses are capable of correcting large refractive errors. Based on his work, a multicenter study was undertaken to evaluate the safety and efficacy of polysulfone as an intracorneal lens material in laboratory models. Four monkeys, eight baboons, and 24 cats were used as laboratory models; 5.0-mm to 6.0-mm diameter hyperopic (+28.5 diopter) and myopic (-17.0 and -25.5 diopter) lenses were surgically implanted within the corneal stroma in one eye of each of the laboratory models while a sham lamellar dissection was performed in the other eye. One hundred percent (4/4) of monkey eyes, 12.5% (1/8) of baboon eyes, and 70.0% (18/24) of cat eyes maintained clear media by ophthalmoscopic examination at follow-ups ranging from three to six months. Complications included both visually and nonvisually significant interface opacities, lens extrusion, anterior corneal necrosis, refractile particles, and epithelial thinning.

Hydration stability of intracorneal hydrogel implants

A hydrogel intracorneal lens for refractive keratoplasty must have predictable and stable optics when implanted in the corneal stroma. A series of experiments was performed to evaluate the hydrogel hydration stability when in the corneal stromal environment. Hydrogel ICLs of 54%, 63%, 66% and 71% water content showed no loss of hydration (by weight) after one week in the rabbit corneal stroma. In vitro experiments with hydrogel discs of 56%, 65%, 69.5% and 75% water content were subjected to swelling pressures ranging from 55 to 150 mmHg in a suction chamber. Only the hydrogel of 75% water content showed a significant loss of hydration at the physiologic swelling pressure of 55 mmHg. This study shows that hydrogel materials with up to 69.5% water content can be expected to be dimensionally stable when used in keratorefractive surgery.

Predicting refractive alterations with hydrogel keratophakia

Hydrogel lenticules are being used as intracorneal lens (ICL) implants for refractive keratoplasty. The experimental surgical success can be evaluated through an understanding of their effect on the optical system of the cornea. An algorithm that utilizes elementary optics can be used to calculate the total corneal power produced through intracorneal lens implantation via either pocket or microkeratome dissection. Two groups of animal experiments involving ICL implantation using both pocket and microkeratome dissections were performed on Rhesus monkeys. The predicted effects of the surgical techniques were compared with the measured effects obtained via streak retinoscopy. The mean difference and standard deviation of the measured minus the predicted values for the pocket dissection group is -0.59 +/- 1.52 D, N = 7. The respective difference for the microkeratome group is -0.19 +/- 1.07 D, N = 4. These small differences illustrate the accuracy of the algorithm in predicting the effects of refractive keratoplasty with hydrogel intracorneal lenses.

The active and passive transport of water across the corneal endothelium

The relationship between hydrostatic pressure and water flow across the corneal endothelium of the rabbit was determined at 2 degrees C and 34 degrees C and in each case the layer behaved as if it had a constant leak resistance; the value of the resistances was proportional to the viscosity of water at the temperature in question. Superimposed on this passive flow there appeared to be an active transport of water of 67 microns hr-1 in the warm preparation which completely disappeared when it was cooled.

Gentamicin diffusion across hydrogel bandage lenses and its kinetic distribution on the eye

Soft contact lenses have been used as therapeutic bandages to aid epithelial healing following pentrating keratoplasty. Often the hydrogel lenses are used in conjunction with topical medications such as gentamicin. A reported complication is the persistence of infectious ulcers even though the eye is being treated with topical antibiotics. The purpose of this study was to measure the gentamicin diffusion coefficients for some hydrogel bandage lenses and to design a kinetic model to estimate the drug distribution on an eye covered with a hydrogel contact lens. The model includes the hydrogel diffusion coefficients and literature values for tear production, tear exchange per blink around the edge of a lens, fit, etc. From the computer generated data, it can be shown that the permeability of gentamicin sulfate through the Saulfon-80 hydrogel lens on a normal eye was only 0.002% of the amount of the drug under the contact lens after 10 minute intervals of topical drug application. The important drug distribution pathway was around the edge of the lens.

Hydrogel implants for refractive keratoplasty: corneal morphology

Refractive keratoplasty procedures for large dioptric corrections utilize a lenticular addition to the cornea which is typically cut from a donor cornea, but can also be fabricated from hydrogel plastics (1). The success of an intrastromal hydrogel implant depends on the nonreactiveness of both the hydrogel to the stroma tissue and the tissue environment to the hydrogel. Permalens hydrogel (Cooper Labs, Inc.) was fabricated into 6 mm diameter, 0.24 mm thick, and +15.00 diopter lenticules and surgically implanted into intracorneal pockets of rabbit corneas for eleven months. Light and electron microscopy revealed scattered zones of abnormal fibroblast activity at the posterior interface of the hydrogel and stroma. The anterior pocket lining was covered with a single cell layer of keratocytes, but there were no irregular areas that might be light scattering. The remaining ultrastructure appeared normal. Preliminary work with primates did not reveal this type of abnormal fibroblast activity. The data indicates that hydrogels for intrastromal implantation in refractive keratoplasty have a promising future.

Refractive keratoplasty with intrastromal hydrogel lenticular implants

The feasibility of using hydrogel lenticular implants of high water content to alter the anterior corneal curvature for purposes of refractive keratoplasty has been investigated in rabbits. Lenticules (6 mm in diameter) of Permalens (Perfilcon-A) were trephined from contact lens and implanted within an intralamellar pocket in the cornea. The in vitro glucose flux across the hydrogel (0.23 mm thick) was measured at 131 +/- 7 micrograms/cm(2)/hr. For clinical comparison, non-water-permeable disks of Teflon were also implanted. The Teflon implant caused an aseptic ulcer to develop anterior and central to the implant by 9 +/- 4 days. The hydrogel lenticular implant did not cause central ulceration during the 7 month postoperative follow-up. There was a thinning and eventual erosion of the stroma anterior to the edge of the hydrogel implant, 16 +/- 7 weeks. The glycogen contents of the epithelium anterior to (1) the sham operation, i.e., lamellar pocket dissection, (2) the implanted hydrogel lenticule with or without the presence of an erosion, and (3) the control corneas were statistically from the same population. Yet there was a slight dehydration of the stroma anterior to the hydrogel implant when compared to control tissue. A thin-edged implant lenticule design should overcome the stromal thinning caused by the thick-edge implants. During the short-term follow-up, the hydrogel lenticular implant proved to be successful as a refractive keratoplasty implant material.

Specular microscopic evaluation of donor corneal endothelium

The corneal endothelium of intact isolated globes can be histologically evaluated with a specular microscope. The best observations are made within 6 to 8 postmortem hours; however, useful observations have been demonstrated up to 19 postmortem hours in a moist chamber stored eye at 4 C. The technique takes only a few minutes and enables one to see individual endothelial cell changes at magnifications of 120X to 400X. Endothelial screening can be done with this system for such changes as guttata, low cell populations, and trauma.

In vitro specular microscope perfusion of M-K- and moist chamber-stored human corneas

Human donor corneas were stored in McCarey-Kaufman (M-K) medium for 3 to 6 days; then they were mounted in the specular microscope, and the endothelial surface perfused with a glutathione-bicarbonate-Ringer solution. During storage, the corneal thickness increased 37% above an assumed normal of 0.520 mm. The corneas did not have sufficient active thinning to be accepted as viable when tested by the temperature-reversal phenomenon. Since comparable donor corneas have been used in successful penetrating keratoplasties, there must be a discrepancy between the cell viability as tested by the temperature-reversal phenomenon and clinical application.

Improved corneal storage for penetrating keratoplasties in humans

A group of 92 patients, who had penetrating keratoplasties with donor corneas that were stored for up to 7 days in M-K media, had 90% clear grafts 2 months postoperatively and about 80% at 6 months. A comparison of the postoperative corneal thickness measurements was made of the patients receiving donor corneas stored by cryopreservation for prolonged times in moist chambers for less than 24 hours, or in M-K media for several days. There was no statistical difference between the 3 storage methods during the 10 week follow-up nor was there any difference between 72 hours and 96 hours of storage in M-K media.

Clinical specular microscopy

This paper presents a clinical specular microscope for the examination and photography of endothelial cells at high magnification (X200). The instrument is used easily during routine examination without inconvenience or discomfort to the patient. We have found the specular microscope extremely useful clinically in the elucidation and documentation of corneal endothelial diseases in vivo. This instrument allows the detection of endothelial damage or disease which may not be seen by slitlamp examination. The effects of normal aging, endothelial dystrophies, trauma, and inflammation can be monitored. The instrument is especially useful in the evaluation of ocular surgical procedures. It can be used on the first postoperative day without inconvenience to the patient or damage to the eye. By the comparison of photographs taken before and after the operation, an estimate of endothelial cell loss may be made. Thus, the endothelial effects of different techniques of cataract extraction intraocular lens insertion, and penetrating keratoplasty can be observed and compared. This new technique for the clinical examination of corneal diseases has great promise in improving diagnosis as well as therapy.

The phacoemulsification procedure. I. The effect of intraocular irrigating solutions on the corneal endothelium

Irrigating solutions for intraocular purposes were evaluated for their possible damaging effects on the corneal endothelial cell structure and function. Rabbit corneal endothelium was perfused in vitro with Tis-u-Sol, Travenol Ringer's Solution, or Travenol Sodium Chloride. The irrigating solutions caused an immediate corneal swelling of 67 mum per hour +/- 5 (mean +/- standard error), which was not modified by a previous stabilization perfusion with glutathione-bicarbonate Ringer's (GBR). In contrast, the Plasma-lyte-148 solution which is used in the phacoemulsification procedure, did not cause corneal swelling for more than twenty minutes, and for more than sixty minutes if the cornea was perfused after a GBR stabilization. After more than sixty minutes of corneal swelling, endothelial intercellular junction separations appeared. This breakdown was present with the tested irrigating solutions except for Travenol Ringer's Solution, which contained calcium. Plasma-lyte was also evaluated in conjunction with the surgical phacoemulsification procedure. The complete procedure or just irrigation with ultrasound did not cause endothelial cell damage similar to a prolonged in vitro irrigation. Instead, endothelial cells were traumatically damaged in varying degrees by the surgical manipulations.

Scanning electron microscopy of postoperative M-K and moist chamber-stored corneas

In the present study, using the in vivo test situation which all corneal donor material must meet, we performed penetrating keratoplastics utilizing corneal tissue stored for five days in either M-K medium or conventional moist chambers. Cat corneas stored for five days in M-K medium had a higher incidence of postoperative graft clarity along with an intact endothelial cell layer as determined by scanning electron microscopy than corneas stored in a conventional moist chamber. The endothelium of corneas immediately after moist chamber storage revealed minimal surface alteration which contrasted to the more severely stressed endothelium immediately after M-K storage. However, the M-K stored endothelium survived the trauma of penetrating keratoplasty much better than cells stored in moist chambers.

Improved corneal storage for penetrating keratoplasties in man

Twenty-six of 28 human corneas stored for up to seven days in a simple tissue culture medium (dextran dissolved in tissue culture medium with streptomycin-penicillin mixture) remained clear and thin after keratoplasty. Temperature reversal phenomenon and electron microscopy demonstrated high quality of such tissue after storage. The method is inexpensive, requires limited technical assistance, and provides the surgeon with excellent tissue at regular operation hours.