Corneal Shape Changes Research Articles

Granular Dystrophy of the Cornea

We performed a phototherapeutic keratectomy using excimer laser 193-nm ablation in a patient with granular dystrophy of the cornea. The hyperopic shift caused by the 5-mm diameter tissue ablation could effectively be corrected by a high oxygen-transmissible, aspheric, rigid contact lenses 1 week after treatment. Corneal topographic analysis was used to monitor the changes in corneal shape after excimer treatment. The 3.5-D hyperopic shift did not change after 4 months, but corneal shape altered during this period. It was not necessary to adjust lens parameters. The central corneal area was clear, and visual acuity improved from 20/40 to 20/20.

Computerized Statistical Analysis of Corneal Topography for the Evaluation of Changes in Corneal Shape After Surgery

We examined 20 patients for the effect of a 3-mm sutureless temporal clear corneal incision cataract operation on corneal topography, as recorded with the computer-assisted video-keratoscope TMS-1 (Computed Anatomy, Inc., New York, New York). Each topographic image was cut into 178 fields in eight concentric rings. The refractive values of these fields were stored in a database. The mean differences between the preoperative and one-month postoperative findings were calculated and transformed into color-coded maps as output for the statistical analysis. The patients operated on showed a mean flattening of 0.4 to 1.0 diopter in the temporal region. This was statistically significant by paired Wilcoxon signed-rank tests. There was no significant vertical steepening or nasal flattening, except in two isolated fields. For control purposes, we twice examined 20 age-matched volunteers who were not operated on. The paired Wilcoxon tests of this group did not show significant changes. When compared with Cravy's vector analysis, induced topographic change analysis is better at evaluating surgically induced corneal shape changes.

Corneal topographic analysis of superolateral incision cataract surgery

Using corneal topography, we compared the corneal shape changes following superolateral incision cataract surgery with those following conventional superior incision surgery. In our superolateral incision procedure, a 6.5 mm limbal incision was made from the 9 o'clock to 11 o'clock meridians to avoid incising the superior limbus. One hundred four patients were divided into two groups; Group 1 comprised 66 patients who had surgery using the superolateral approach; Group 2 comprised 38 patients who had surgery using the superior approach. The corneas were examined by keratometer and topographic modeling system preoperatively, and at one week and one, three, and six months postoperatively. Keratometric measurements showed that surgically induced astigmatism (SIA) in Group 1 was significantly less than that in Group 2 throughout the six-month observation. The standard SIA deviation in Group 1 was also smaller than that in Group 2, indicating a smaller degree of variability in the superolateral incision surgery. In the corneal topographic analysis, a color-coded map averaging all the Group 1 corneas at each interval showed a slight steepening in the central cornea in the 10 o'clock meridian one week after surgery. This surgically induced steepening disappeared by one month, and the corneal shape recovered its preoperative shape. In contrast, the Group 2 averaged map showed a marked steepening of the upper and lower corneas. The steepening gradually decreased but remained until three months after surgery. Superolateral incision surgery induced a smaller degree of change in the corneal shape, as well as in SIA, than superior incision surgery. The postoperative corneal shape changes disappeared rapidly after the superolateral incision, and the corneal shape soon stabilized and recovered its preoperative shape.

Über die mathematische Darstellung des postoperativen regulären Hornhautastigmatismus

There are several contradictory approaches to describe a change in corneal astigmatism induced by operations. Assuming a rotational ellipsoid for the outer surface of the cornea we derived formulas that represent the change in astigmatism. Also, the change of the corneal power in the axis of the intervention (e.g. operation) has been determined. The exact derivation yield formulas for the absolute value of the astigmatism change identical to those of Naylor and Jaffe but includes, in addition, algorithms for "with-the-rule" and "against-the-rule" components. The spherical change of the cornea is also obtained. The models of Cravy and Naeser are mathematically inconsistent and should be avoided. The presentation of the change in astigmatism should depend on the primary question. In many cases, the complicated non-linear vector calculation is not necessary since the actual changes in corneal shape may be demonstrated by simpler formulas. The results of these formulas are much easier to interpret.

Noninvasive Monitoring of Excimer Laser Ablation by Time-Resolved Reflectometry

Current excimer laser photorefractive procedures use empiric etch rates to determine specific changes in corneal shape. A real-time analytic method for monitoring the tissue ablation process may be useful in tailoring energy delivery to a specific patient and in detecting detrimental phenomena such as corneal desiccation. We monitored excimer laser ablation by studying the amplitude and temporal characteristics of ArF laser pulses reflected from the ablation site. Two target materials were used: polymethylmethacrylate (PMMA, a synthetic polymer that undergoes an incubation phase where no ablation occurs for an initial finite number of laser pulses), and bovine cornea. Observed reflectivity changes during irradiation of PMMA were compared to profilometric ablation depth measurements. Corneal ablation was performed both with and without nitrogen gas flow at the ablation site to study the effect of tissue desiccation. For ablation of PMMA at 160 mJ/cm2, the incubation phase included the initial eight laser pulses. For corneal tissue ablation at a fluence of 125 mJ/cm2, flowing nitrogen gas caused significant shortening and amplitude reduction in the reflected laser signals. Noninvasive time-resolved reflectometry provided real-time information about target ablation. This technique may have diagnostic utility during laser corneal surgical procedures.

Changes in corneal shape after suture cutting using the argon laser for postoperative astigmatism following cataract extraction

We studied the changes in corneal shape after suture cutting with an argon laser to reduce corneal astigmatism following cataract extraction. Sixty-two patients who exhibited high with-the-rule astigmatism (>3 diopters [D]) following extracapsular lens extraction had argon laser suture cutting. The patients were classified into three groups: Group A comprised 30 patients whose sutures were cut two to three months after planned extracapsular cataract extraction (p-ECCE); Group B consisted of eight patients who had the same treatment five to nine months after p-ECCE; Group C comprised 24 patients who had the treatment one to two months after phaccemulsification (PE). The dioptric reduction of corneal astigmatism (the percent reduction of astigmatism) was 1.83 ± 0.98 D (37.4 ± 18.3%) in Group A, 3.20 ± 2.35 D (55.6 ± 34.4%) in Group B, and 2.08 ± 1.20 D (41.4 ± 20.4%) in Group C. There was no statistical significance in the dioptric reduction and the percent reduction among Groups A, B, and C. This suggests that the wound size and time of cutting are not directly correlated to the effect of argon laser suture cutting. To examine the relationship between the effect and changes in corneal shape from suture cutting, we divided the patients into two subgroups; one was Subgroup (+) in which the percent reduction of astigmatism was above 25%; the other was Subgroup (-) in which the percent reduction was below 25%. Keratometric measurement demonstrated an elongation of corneal curvature in the steeper meridian, a shortening in the flatter meridian, and a shift of axis in Subgroup (+) that were statistically significant compared with Subgroup (-). Corneal topographical analysis demonstrated a flattening of the upper cornea that corresponded to the site of the cut suture and a concomitant steepening of the other part of the cornea in Subgroup (+). In contrast, corneal configuration remained virtually unchanged in Subgroup Our results indicated that a change in the overall corneal shape by argon laser suture cutting was necessary to reduce postoperative astigmatism.

Topographic Changes in Contact Lens-induced Corneal Warpage

Twenty-one eyes of 12 patients with contact lens-induced corneal warpage were followed prospectively using computer-assisted topographic analysis. Sixteen eyes had worn rigid contact lenses (13 eyes, polymethylmethacrylate; three eyes, gas-permeable), and five eyes had worn soft contact lenses. Initial corneal topographic patterns were characterized by the presence of central irregular astigmatism, loss of radial symmetry, and frequent reversal of the normal topographic pattern of progressive flattening of corneal contour from the center to the periphery. A correlation was noted between the initial corneal topography and the resting position of the contact lens on the cornea for nine of the 16 eyes with rigid contact lenses. Initial topography for each of these corneas showed relative flattening of the corneal contour underlying the resting position of a decentered contact lens. Superior-riding lenses produced a topography that simulated early keratoconus. After cessation of contact lens wear, 16 of 21 eyes had a change in corneal shape to a topography that was consistent with a normal pattern. Five corneas stabilized with an abnormal topographic pattern. A much longer time without contact lenses than had been previously reported, up to 5 months, was required for a return of a stable corneal topography in eyes with contact lens-induced corneal warpage caused by rigid lenses.

One-Year Refractive Results of Central Photorefractive Keratectomy for Myopia in the Nonhuman Primate Cornea

Photorefractive keratectomy for the correction of myopia was performed in 32 eyes of 16 green monkeys. The corneas healed satisfactorily, with normal formation of basal lamina and hemidesmosomal attachments visible in 14-week histologic specimens. No recurrent erosions were observed clinically. After a transient period of faint haze, all corneas were clear at 17 weeks and remained clear through the 1-year follow-up. In terms of accuracy, all corneas demonstrated a significant flattening compared with preoperative values, but no significant difference was seen between the groups with different intended corrections (1.5 and 3 diopters). The changes in corneal shape stabilized by 17 weeks, as measured by keratometry. The clinical results suggest that mechanical removal of the epithelium is preferable to laser ablation of the epithelium. Overall, the results demonstrate that excimer laser ablation of the corneal stroma can produce a stable diptric change in the primate cornea with good healing and long-term corneal clarity.

Prospective Analysis of Photokeratoscopy for Arcuate Keratotomy to Reduce Postkeratoplasty Astigmatism

Seven eyes of seven patients with visually disabling postkeratoplasty astigmatism underwent arcuate keratotomy to reduce astigmatism. Keratotomy length was prospectively determined by comparing the corneal shape change needed by each patient with shape changes in cadaver eyes induced by arcuate keratotomy. Keratograph ring ovality quantified corneal shape change. Keratotomies were placed in the wound interface using a diamond knife with blade length set to 100% of corneal thickness. Prekeratotomy keratometric astigmatism was 9.27 +/- 2.14 D (mean +/- SD), and at 2.0 to 18.4 months after keratotomy, astigmatism was 2.75 +/- 2.25 D (mean +/- SD). All corneas had decreased keratometric astigmatism and less oval keratoscope rings. Five corneas had 3.00 D or less residual keratometric astigmatism. Change in ring ovality was significantly correlated with keratotomy length and keratometric astigmatism change. The utility of keratoscope ring ovality as a corneal topography metric was demonstrated by systematically investigating the response to arcuate keratotomy.

Computer simulation of arcuate and radial incisions involving the corneoscleral limbus.

We have created a computer model of the eye that improves upon previous models with two assumptions: (1) the cornea is considered anisotropic, and (2) the strain tensor is non-linear for large displacements. Values used include those for Young's modulus, Poisson's ratio, and three coefficients of anisotropia. A finite element computer program was used to stimulate the behaviour of the eye. Four simulations were done: (1) a full thickness arcuate limbal cataract incision, (2) a partial thickness arcuate corneal relaxing incision, (3) a partial thickness radial incision extending across the limbus, (4) a partial thickness radial incision confined to the cornea. A full thickness corneal arcuate incision for cataract induced more change in corneal shape than a limbal incision. However, the gaping is more important in the limbal incision. Partial thickness arcuate corneal relaxing incisions were found to be more effective with a clear zone 3-5 mm in diameter. Radial incision simulation demonstrated little enhancement of the effect of the incisions when across the limbus.

The Future Direction of Refractive Surgery

ABSTRACT: Over the next 3 years, I believe that we will see modifications of the tissue lenses and surgical procedure that will improve the accuracy of donor tissue epikeratophakia, as well as the emergence of synthetic epikeratophakia, increased use of hydrogel intracorneal lenses, further development of excimer laser ablation for the correction of refractive error, and more sophisticated analysis of changes in corneal shape after refractive surgery. Ten years from now, I believe that two refractive surgical procedures, for the most part, will be performed: one tissue addition procedure (synthetic epikeratophakia) and one tissue subtraction procedure (excimer laser sculpting of the cornea). Topographic analysis will be an integral part of intraoperative evaluation during refractive and other surgical procedures, as well as diagnosis and decision-making in the office.

Corneal endothelial polymegathism after the extended wear of rigid gas-permeable contact lenses.

To determine if there are any corneal endothelial cell size and shape changes in subjects wearing rigid gas-permeable contact lenses for cosmetic extended wear, six unadapted lens wearers were fitted using four different lens materials. Each subject wore one lens extended-wear and the other daily-wear for 3 months. Before lens wear and after 3 months, noncontact endothelial photographs were taken. Cell tracings were analyzed by a digitizing computer. This study showed significant endothelial polymegathism, an increase in maximum cell area to minimum cell area, and a significant decrease in percent frequency of hexagonal cells in the extended-wear eyes. There existed in these subjects a significant correlation between changes in cell size (polymegathism) and cell shape (pleomorphism).

Method for Quantitative Evaluation of Corneal Shape Changes by Contact Lens Molding

Method for Quantitative Evaluation of Corneal Shape Changes by Contact Lens Molding

Radial Keratotomy-Reply

In Reply. —We do not perform A-scan ultrasound measurements of the axial length of the globe as part of the preoperative diagnostic studies in the PERK study for many reasons. Axial length measurements are not necessary to rule out progressive pathologic myopia, since the stability of refraction must be proved in every patient and indirect ophthalmoscopy performed preoperatively to search for the signs of pathologic changes in the posterior pole. 1 Although A-scan ultrasound performed with a water bath stand-off could be used to show changes in axial length produced by changes in corneal shape after radial keratotomy, these measurements can be made more

New Corneal Shapes in Keratorefractive Surgery

The corneal shape change as measured by the Humphrey keratometer and the corneascope after radial keratotomy in 50 patients was compared. The Humphrey keratometer analyzes a unique curve fit algorithm of the aplanatic corneal surface and designates this comparison as a shape factor. Both the corneascope photographs and the Humphrey keratometer demonstrated central corneal flattening after successful radial keratotomy. In addition, the normal “plus” shape factor was routinely converted to a “negative” shape factor after the procedure. This is analagous to converting the aplanatic central cornea, which normally approximates the end of an ellipse, to a topography which appears in cross section as the side of an ellipse. This alteration in topography is directly correlated to the magnitude of cycloplegic refractive change seen with this operation. The corneascope map topography of the cornea confirms this alteration in curvature.