Corneal topographic changes after radial keratotomy

Patterns of Corneal Topography after Penetrating Keratoplasty

Background

The topography of corneas after penetrating keratoplasty is highly variable. We classify the topography into five groups.

Methods

We performed videokeratography on 45 clear compact penetrating keratoplasties, with all sutures removed. Three ophthalmologists classified the keratographs independently into five previously defined topographic groups, based on the pattern of the normalized color-coded videokeratograph.

Results

The five topographic patterns included: prolate bow tie, 14 (30%); oblate bow tie, 14 (30%); mixed prolate and oblate bow tie, 8 (17%); asymmetric, 3 (9%); and steep/flat, 6 (14%). The three ophthalmologists agreed in their initial classification in 87% of the cases and after discussion, in 96%.

Conclusion

The topography of the cornea after penetrating keratoplasty can be classified into five qualitative groups by trained observers, with good clinical reliability.

Long-term changes in corneal surface configuration after penetrating keratoplasty

PURPOSE

To examine the long-term longitudinal changes in corneal surface configuration as determined by Fourier series harmonic analysis of videokeratography data and of visual acuity and refraction after penetrating keratoplasty (PK).

DESIGN

Interventional case series.

METHODS

One hundred thirty eyes of 130 consecutive patients who were scheduled for PK using 16 interrupted 10-0 nylon sutures were recruited. Spherical equivalent power, regular astigmatism component, irregular astigmatism (asymmetry and higher-order irregularity) component of the central cornea as determined by Fourier analysis of videokeragraphic data, spectacle-corrected visual acuity, and spherical equivalent were examined at 1 week, and at 1, 3, 6, 9, 12, 18, and 24 months after PK.

RESULTS

Spherical equivalent power increased considerably for up to 1 month after PK, but thereafter showed no further appreciable change up to the final follow-up at 24 months. The regular astigmatism component decreased markedly for up to 6 months after PK, while the total irregular astigmatism (sum of the asymmetry and higher-order irregularity) component decreased considerably up to approximately 3 months, and then these showed no further relevant change for up to 24 months. Spectacle-corrected visual acuity also improved markedly until approximately 3 months after PK, after which it was virtually stable. Furthermore, important correlations were found between regular and irregular astigmatism and the spectacle-corrected visual acuity.

CONCLUSIONS

Corneal surface configuration after PK appears to be stable by approximately 6 months after PK, concurrent with postkeratoplasty stabilization of visual acuity.

Corneal Asphericity in Eye Bank Eyes Implanted with the Intrastromal Corneal Ring

OBJECTIVE

To evaluate the effects of the intrastromal corneal ring, a device developed to reduce myopia, on corneal asphericity in a large set of eye bank eyes.

METHODS

Forty-one deturgesced eye bank eyes were implanted with intrastromal corneal rings of five different thicknesses, ranging from 0.25 mm to 0.45 mm. Corneal asphericity, before and after implantation, was examined using two different metrologies. Corneal asphericity profiles were produced from dioptric power data collected from videokeratography. To statistically assess the corneal asphericity differences between exam times for each intrastromal corneal ring thickness, dependent sample confidence intervals (95%) were calculated for the mean differences between preoperative and postoperative measures for each topographic diameter zone. Laser holographic interferometry was used to inspect corneal asphericity in one eye bank eye case study for four intrastromal corneal ring sizes. Wave unit map and geometric zonal spot ray tracing analyses derived from laser holographic interferometry topography were surveyed.

RESULTS

Videokeratographic analysis suggested that preoperative corneal shape was prolate, i.e., flattened from central to paracentral cornea. Corneal shape became more prolate with intrastromal corneal ring implantation for all intrastromal corneal ring thicknesses. Laser holographic interferometry demonstrated that prolate asphericity was preserved with the intrastromal corneal ring sizes tested and that optical collection efficiency of the cornea was not diminished.

CONCLUSION

Using two different measurement techniques, this eye bank eye study demonstrated that intrastromal corneal rings maintain prolate corneal asphericity.

Disparity between refractive error and visual acuity after photorefractive keratectomy: multifocal corneal effects

PURPOSE

To determine the relationship between postoperative refractive error and uncorrected visual acuity (UCVA) after photorefractive keratectomy (PRK) and compare the results to those in unoperated control eyes with different degrees of myopic refractive error.

SETTING

Academic cornea and refractive surgery subspecialty practice.

METHODS

Uncorrected visual acuity and manifest refraction were recorded for 52 consecutive patients who had PRK for myopia. Eight control eyes that did not have PRK and in which artificial myopia was induced were also studied to ascertain the association of UCVA with myopia in untreated eyes. Uncorrected visual acuity in postoperative eyes was compared with that in control eyes.

RESULTS

Of the 46 eyes with a myopic spherical equivalent postoperative refraction, 44 (96%) had better UCVA than control eyes with equivalent myopic refractions. Twelve of 13 (92%) eyes with refractions of -1.00 diopter or more had a UCVA of 20/40 or better.

CONCLUSION

After excimer laser PRK, patients achieved better Snellen visual acuity than might be expected from their residual refractive error, perhaps as a result of a multifocal postoperative corneal topography. Nonuniformity of the corneal surface following PRK may create "focal areas of emmetropia" that allow patients to achieve better visual acuity than the refraction may predict.

Laser in situ keratomileusis to treat myopia: early experience

PURPOSE

To present our early experience in methodology and results of laser in situ keratomileusis (LASIK) in treating moderate and high myopia.

SETTING

Vardinoyannion Eye Institute of Crete and the Cornea and Refractive Surgery Service, Department of Ophthalmology, Heraklion University Hospital, Crete, Greece.

METHODS

Forty-three moderately to highly myopic eyes has LASIK. Follow-up was between 12 and 24 months. The Draeger's rotor microkeratome was used to create a 150 microns thick 8.5 x 9.5 mm corneal flap, and the stromal bed was ablated for the myopic correction using the Munnerlynn photorefractive keratectomy algorithm. The preoperative spherical equivalent ranged from -8.50 to -25.87 diopters (D). Attempted correction ranged from 8.00 to 16.00 D. Four eyes developed complications (i.e., anterior chamber perforation and keratoconus, intrastromal epithelial cells, macular lacquer cracks, and transient endothelial decompensation) and were excluded from the study. The remaining 39 eyes were retrospectively divided into two groups--A (21 eyes) and B (18 eyes)--according to the preoperative spherical equivalent (higher or lower than -14.00 D, respectively).

RESULTS

Refraction and corneal topography stabilized between 4 and 12 weeks postoperatively. Best spectacle-corrected visual acuity was within one Snellen line in all eyes. At 24 months, 19 eyes (79.2%) were within 2.00 D of intended correction. Attempted correction (12.20 D +/- 2.30 [SD]) was very close to mean achieved correction at 12 and 24 months (11.60 +/- 2.65 D). Mean postoperative astigmatism at 24 months (1.41 +/- 0.87 D; range 0 to 3.50 D) was close to the mean preoperative value (1.52 +/- 1.08 D; range 0 to 4.00 D). An average 2.43% endothelial cell loss was observed at 24 months.

CONCLUSION

With limitations, LASIK could be considered as a treatment for moderate and high myopia.

Assessment of the power and height of radial aspheres reported by a computer-assisted keratoscope

PURPOSE

The two purposes of this study were (a) to assess the accuracy with which a keratoscope, the Topographic Modeling System (TMS-1), calculated the heights and powers of rotationally symmetric, radially aspheric test surfaces and (b) to determine whether the TMS-1 used an axial solution for radius of curvature to determine the power of a sphere that would produce the same semichord as would the test surface on a keratograph.

METHODS

The TMS-1 heights and powers were studied for four test surfaces that had radial profiles similar to those of normal corneas. The powers of the surfaces were calculated from the local radius of curvature derived from the surfaces' manufacturing formulas. The heights and powers that would result from an axial solution were calculated in a TMS-1 simulator. TMS-1 data were compared with data from the surfaces' formulas and with data from the simulation.

RESULTS

The TMS-1 data were almost identical to the heights and powers calculated from the simulated axial solution. The TMS-1 data were similar to the heights and powers calculated from the mathematical formulas from the apex to 2 mm from the apex but differed by up to 85 microns of height and 10 diopters of power in the periphery.

CONCLUSIONS

The TMS-1 appeared to use the axial solution that does not calculate power from local radius of curvature. Clinicians should use caution when inferring corneal shape from power maps based on an axial solution, especially outside the central 2-mm radius of a normal cornea, because such power does not depict corneal curvature.

Dynamic shifts in corneal topography after radial and transverse keratotomy

PURPOSE

The authors aimed to quantitate the dynamic patterns of change in corneal topography after multistaged radial and transverse keratotomy using digitized video-keratography.

METHODS

Single and paired radial and transverse keratotomies, with videokeratoscopy between each stage and at the end of the procedure, were performed on fresh animal cadaver eyes using an artificial orbit system.

RESULTS

All incisions led to central flattening. A single radial keratotomy caused flattening adjacent to the incision, and steepening 180 degrees away. A paired radial keratotomy caused increased flattening in the meridian of the incisions, and less flattening 90 degrees away. A single transverse incision caused steepening adjacent to the incision and diffuse flattening elsewhere. A paired transverse incision caused flattening near the optical center along the meridian bisecting the incisions and steepening 90 degrees away.

CONCLUSION

The authors have demonstrated that computerized videokeratography can be used successfully to systematically quantitate dioptric shifts in multiple hemimeridians and measurement zone diameters after refractive surgery.

Computerized topographic analysis of the healing graft after penetrating keratoplasty for keratoconus

After penetrating keratoplasty, visual rehabilitation can be slow and is largely a function of corneal surface configuration. Computerized topographic analysis allows the detailed study of corneal surface factors that determine the optical function of the graft. We performed a prospective, longitudinal study of eight patients with keratoconus by using computerized topographic analysis to determine the rate and pattern of postoperative surface normalization and stabilization. Study data included Snellen visual acuity, contrast sensitivity function, central keratometry, photokeratoscopy, and computerized topographic analysis. Data were collected preoperatively and at one week, one month, two months, three months, and six months postoperatively. Results demonstrate that the greatest configurational changes both topographically and functionally occur in the first month after keratoplasty. The computer-generated surface asymmetry index and the surface regularity index correlated well with improvement in Snellen visual acuity measurements. Contrast sensitivity function was depressed initially but improved to well above preoperative values by one month postoperatively and paralleled the improvement in the surface indices and visual acuity. The axis of astigmatism stabilized by one month postoperatively. Our data indicate that topographic analysis provides a good indication of the rate and course of optical stabilization during the early healing process after keratoplasty and correlates well with visual function in the otherwise normal eye.

Topographical analysis of the centration of excimer laser photorefractive keratectomy

A major advantage of myopic photorefractive keratectomy (PRK) is the precision with which the excimer laser ablates corneal tissue. But like other refractive surgery procedures, PRK must solve the problem of accurately centering the treatment zone. We present our technique for PRK centration with postoperative corneal topographic data on 110 patients from Phase IIB and III of the clinical trials. The distance between the center of the post-PRK flat zone and the corneal vertex was determined by topography in millimeters and meridian degrees. On average, treatment zones were decentered down and right 0.52 mm at 196.74 degrees; 92.73% were centered within 1.00 mm, while 57.27% were within 0.50 mm. The centration data were correlated to postoperative visual acuity as well as treatment zone diameter. Mean uncorrected visual acuity was 20/20 for decentrations up to 1.00 mm but fell to 20/30 for deviations greater than 1.00 mm. Best corrected acuity was also preserved below 1.00 mm but compromised above this level. No difference in decentration was found between 4.5 mm and 5.0 mm ablation zones. Our findings indicate that PRK centration is accurate within 1.0 mm in over 92% of cases and that visual acuity is relatively preserved despite deviations from perfect centration. Further technical improvements will enhance the accuracy of PRK.

Changes in corneal topography after excimer laser photorefractive keratectomy for myopia

Computer-assisted analysis of corneal topography was performed in 17 normally sighted human eyes during the first year after excimer laser photorefractive keratectomy (PRK) for myopia. Laser ablation of the central cornea produced an optical zone with a smooth power transition to the peripheral cornea. Decentration of the ablation was noted in some eyes (less than 0.5 mm in 3 eyes, 0.5 to 1.0 mm in 10 eyes, 1 to 1.5 mm in 3 eyes, and 2.1 mm in 1 eye), suggesting that careful alignment of the laser beam is critical. Improved methods to align the ablation within the center of the entrance pupil are needed. In 12 of 17 eyes, the topographic pattern appeared to stabilize between 3 and 7 months after PRK. In the remaining five eyes, central ablation power changed by more than 0.5 diopters (D) between the 6- and 12-month examinations. Regression was more common and more pronounced in eyes with intended corrections more than 5 D, whereas the majority of eyes with intended corrections of 5 D or less showed good correspondence between the final change in central ablation power and the attempted correction. Two eyes had a loss of at least two lines of best spectacle-corrected visual acuity that was attributable to irregular astigmatism, decentration of the ablation, and/or corneal opacification.

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.