Holmium laser thermokeratoplasty.

PURPOSE: Corneal curvature can be altered by shrinking stromal collagen with a pulsed solid-state holmium: YAG laser in a procedure termed laser thermokeratoplasty. METHODS: The authors performed laser thermokeratoplasty in 40 human cadaver eyes using a ring pattern of 32 spots, each spot having a diameter of 300 microns. RESULTS: The amount of induced corneal steepening decreased as ring diameter was increased in 1 mm increments, with 22.2 +/- 3.3 and 3.7 +/- 2.0 diopters (D) of central steepening with diameters of 3 and 7 mm, respectively. Results of histologic examination showed a cone-shaped zone of increased stromal hematoxylin uptake extending posteriorly for 90% of stromal thickness. Energy levels greater than those needed to induce topographic changes produced limited endothelial injury in rabbit corneas and, in some cases, intraocular inflammation. A computerized, finite element model of the globe demonstrated central corneal steepening as a result of heat-induced stromal contraction to a depth of 75% corneal thickness. CONCLUSIONS: These data support previous studies indicating that central corneal topography can be modified by heating corneal stroma in a controlled fashion with the mid-infrared holmium:YAG laser.

Computer modeling of wound gape following radial keratotomy.

BACKGROUND: The edges of radial keratotomy incisions remain separated by epithelium or new collagen. To determine if the amount of gape of the incision edges agrees with the induced corneal flattening, two mathematical models of the cornea were studied. METHODS: A finite element model of the eye was constructed; radial incisions through Bowman's membrane and stroma were simulated, as well as incisions confined to the stroma but not involving Bowman's or Descemet's membranes. Geometrical calculations were also performed in a second morphologic model of a spherical shell. RESULTS: The finite element and geometrical models agree closely on the corneal curvature change for a given incision gape. Both models predict that linear intrastromal incisions that do not involve Bowman's layer would not produce significant corneal flattening. CONCLUSIONS: Corneal flattening after radial keratotomy is closely related to incision gape, mediated through structural weakening of the cornea. Purely intrastromal radial incisions, as might be created with Nd:YAG lasers, prevent wound gape and would not appear to be a promising technique for flattening the cornea.

Optimal astigmatism to enhance depth of focus after cataract surgery.

A small amount of myopic astigmatism can enhance the depth of focus of the pseudophakic eye, optimally providing at least 20/30 visual acuity for both near and distance fixation. For given spherocylindrical refractive errors and fixation distances, the cross-sectional area of Sturm's conoid at the retina was calculated for a schematic eye. These data were used to determine the optimal astigmatic error needed to obtain maximum depth of focus and least theoretical blur for any given spherical equivalent refractive error. Optimal depth of focus was obtained when the plus cylindrical component equaled negative sphere - 0.25 diopters. The near and distance visual acuities of ten pseudophakic patients with induced refractive errors were highly correlated with this model. Low myopic astigmatism after cataract surgery may represent an alternative to multifocal intraocular lenses by providing spectacle independence.

Tissue addition theory of radial keratotomy: a geometric model.

We examined the use of a geometric model to explain the effects of radial keratotomy (RK) on corneal topography based on wound gape of radial incisions. Histopathologic studies of healed human RK incisions revealed incision gape in Bowman's membrane and anterior stroma, with intervening epithelial ingrowth or fibrous scarring. For six human RK incisions examined centrally, mid-peripherally, and peripherally, the linear separation of the cut ends of Bowman's membrane averaged 0.012 mm, 0.018 mm, and 0.027 mm, respectively. The effect of postoperative corneal flattening of the "addition of tissue" within incisions was mathematically predicted by calculating the effects of inducing radial gape on a spherical shell. The predicted change in corneal curvature agreed closely with data from a human case report. The "tissue addition" theory may partly explain the change in corneal topography following RK and subsequent wound healing.

Effect of extraocular muscle surgery on corneal topography.

A computerized videokeratoscopy system was used to evaluate changes in corneal topography after muscle surgery in 36 eyes of 18 rabbits. Topographic analysis revealed a significant flattening of the cornea in the superior and superotemporal octants after superior rectus recession (mean +/- SE, -1.78 +/- 0.16 diopters) compared with control eyes undergoing a sham procedure (-0.17 +/- 0.18 D; P less than .05). Excision of all rectus muscles caused a generalized corneal flattening (-1.42 +/- 0.13 D; P less than .001). A computerized, finite element model of the globe, including the rectus muscles, demonstrated corneal deformation as a result of extraocular muscle tension; recession of an extraocular muscle in this model caused corneal flattening in the quadrant of the recessed muscle. These data suggest that corneal topography is affected by extraocular muscle tension, corroborating clinical reports of refractive change after strabismus surgery.

Implantation of scleral-fixated posterior chamber intraocular lenses.

A surgical technique for implanting a posterior chamber intraocular lens (IOL) in eyes without capsular or zonular support is presented. A 10-0 polypropylene suture attached to a standard needle is tied to the apex of each haptic and passed transclerally through the ciliary sulcus to secure the haptics at the 3 o'clock and 9 o'clock meridians. This technique produced good visual results in six aphakic patients who were contact lens intolerant, two patients in whom capsular or zonular rupture at the time of cataract surgery precluded standard nonsuture fixation techniques, one patient who had intracapsular cataract extraction for a subluxated, cataractous lens, and one patient who had secondary IOL implantation in combination with penetrating keratoplasty for aphakic bullous keratopathy. Advantages over other techniques of posterior chamber lens implantation in the absence of capsular support include technical ease, avoidance of iris fixation, and more precise placement of scleral fixation sutures, thus minimizing IOL decentration.

Collagen corneal shields enhance penetration of topical prednisolone acetate.

We used an animal model to compare the corneal and aqueous penetration of topical prednisolone acetate 1% with and without the presence of a commercially available collagen corneal shield. A significant (P less than .02) increase in penetration into cornea and aqueous humor at 30 minutes and 120 minutes was found in eyes with a collagen corneal shield in place. Increased concentrations were achieved by soaking the shield in prednisolone acetate for 15 minutes prior to application.

Endophthalmitis due to Propionibacterium acnes sequestered between IOL optic and posterior capsule.

A 68-year-old woman had delayed onset, persistent uveitis following routine extracapsular cataract extraction with posterior chamber intraocular lens implantation. The patient initially responded to topical steroids, but developed a whitish capsular plaque through to represent possible Propionibacterium acnes endophthalmitis. A vitrectomy and capsular biopsy yielded cultures positive for P. acnes only after nine days. The intraocular lens was left in place. Light and electron microscopy revealed bacteria sequestered within the capsular bag.

Topical ciprofloxacin treatment of Pseudomonas keratitis in rabbits.

Ciprofloxacin is a new quinolone antibiotic that is highly active in vitro against Pseudomonas aeruginosa. A rabbit model of bacterial keratitis was used to assess the in vivo efficacy of topical ciprofloxacin. Albino rabbits received intrastromal injections of 5 X 10(2) aminoglycoside-resistant P aeruginosa organisms. At five hours after inoculation, ciprofloxacin (3 mg/mL) therapy was initiated (one drop every 30 minutes for 12 hours). Corneal tissue was then excised for bacterial colony counts. No organisms were recovered from ciprofloxacin-treated eyes, compared with an average of 3.1 X 10(7) organisms per milliliter recovered from untreated controls. This model suggests that topical ciprofloxacin may be clinically useful in the treatment of aminoglycoside-resistant P aeruginosa keratitis.

Use of collagen corneal shields in the treatment of bacterial keratitis.

We used an animal model of Pseudomonas keratitis to compare treatment by topical tobramycin with and without the presence of a commercially available collagen corneal shield. Pilot studies showed a significant, 30-fold increase in penetration of tobramycin into the anterior chamber in eyes with a collagen shield in place. Twenty albino rabbit eyes were inoculated with P. aeruginosa to produce stromal keratitis. After 12 hours of topical tobramycin dosing, eyes with a collagen corneal shield in place had a statistically significant (P less than .01) decrease in colony forming unit counts in comparison to treated eyes without a shield and control eyes.

Use of collagen corneal shields versus soft contact lenses to enhance penetration of topical tobramycin.

We compared the corneal penetration in rabbits of topical tobramycin in the presence of collagen corneal shields and bandage soft contact lenses. A collagen corneal shield was placed on six albino rabbit eyes, while therapeutic soft contact lenses (61.4% poly-2-hydroxyethyl-methacrylate/38.6% water) were placed on six eyes. Four control eyes received no shield or contact lens. Topical tobramycin was applied to all 16 eyes every five minutes for six doses. Samples of aqueous humor were removed at 15 and 60 minutes following the last dose. Collagen corneal shields allowed a significant (P less than .05) increase in tobramycin penetration into the anterior chamber at 60 minutes compared with hydrophilic soft contact lenses or controls.

Topical imipenem therapy of aminoglycoside-resistant Pseudomonas keratitis in rabbits.

We used a rabbit model of bacterial keratitis to assess in vivo efficacy of topical imipenem, a highly potent beta-lactam antibiotic with an unusually broad spectrum of activity, including aminoglycoside-resistant Pseudomonas aeruginosa. Albino rabbits received intrastromal injections of 5 x 10(2) organisms of an aminoglycoside-resistant strain of P. aeruginosa. At five hours postinoculation, imipenem (5 mg/ml) therapy was initiated using one drop per 30 minutes for 12 hours. Corneal tissue was then excised for colony forming unit counts. Imipenem was highly effective in reducing colony forming unit counts to zero in comparison to 4.1 x 10(5) organisms for untreated controls. A second regimen beginning 24 hours postinoculation of one drop per hour for 24 hours was also successful in significantly reducing colony forming units vs controls (P less than .05). These data suggest that topical imipenem may have clinical applicability in the treatment of P. aeruginosa keratitis.