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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.