Zernike representation of corneal topography height data after nonmechanical penetrating keratoplasty. (1/329)

PURPOSE: To demonstrate a mathematical method for decomposition of discrete corneal topography height data into a set of Zernike polynomials and to demonstrate the clinical applicability of these computations in the postkeratoplasty cornea. METHODS: Fifty consecutive patients with either Fuchs' dystrophy (n = 20) or keratoconus (n = 30) were seen at 3 months, 6 months, and 1 year (before suture removal) and again after suture removal following nonmechanical trephination with the excimer laser. Patients were assessed using regular keratometry, corneal topography (TMS-1, simulated keratometry [SimK]), subjective refraction, and best-corrected visual acuity (VA) at each interval. A set of Zernike coefficients with radial degree 8 was calculated to fit two model surfaces: a complete representation (TOTAL) and a representation with parabolic terms only to define an approximate spherocylindrical surface (PARABOLIC). The root mean square error (RMS) was calculated comparing the corneal raw height data with TOTAL (TOTALRMS) and PARABOLIC (PARABOLICRMS). The cylinder of subjective refraction was correlated with the keratometric readings, the SimK, and the respective Zernike parameter. Visual acuity was correlated with the tilt components of the Zernike expansion. RESULTS: The measured corneal surface could be approximated by the composed surface 1 with TOTALRMS < or = 1.93 microm and by surface 2 with PARABOLICRMS < or = 3.66 microm. Mean keratometric reading after suture removal was 2.8+/-0.6 D. At all follow-up examinations, the SimK yielded higher values, whereas the keratometric reading and the refractive cylinder yielded lower values than the respective Zernike parameter. The correlation of the Zernike representation and the refractive cylinder (P = 0.02 at 3 months, P = 0.05 at 6 months and at 1 year, and P = 0.01 after suture removal) was much better than the correlation of the SimK and refractive cylinder (P = 0.3 at 3 months, P = 0.4 at 6 months, P = 0.2 at 1 year, and P = 0.1 after suture removal). Visual acuity increased from 0.23+/-0.10 at the 3-month evaluation to 0.54+/-0.19 after suture removal. After suture removal, there was a statistically significant inverse correlation between VA and tilt (P = 0.02 in patients with keratoconus and P = 0.05 in those with Fuchs' dystrophy). CONCLUSIONS: Zernike representation of corneal topography height data renders a reconstruction of clinically relevant corneal topography parameters with a marked reduction of redundance and a small error. Correlation of amount/axis of refractive cylinder with respective Zernike parameters is more accurate than with keratometry or respective SimK values of corneal topography analysis.  (+info)

Changes in corneal wavefront aberrations with aging. (2/329)

PURPOSE: To investigate whether corneal wavefront aberrations vary with aging. METHODS: One hundred two eyes of 102 normal subjects were evaluated with videokeratography. The data were decomposed using Taylor and Zernike polynomials to calculate the monochromatic aberrations of the cornea for both small (3-mm) and large (7-mm) pupils. RESULTS: For a 3-mm pupil, the amount of total aberrations (Spearman rank correlation coefficient r(s) = 0.145; P = 0.103) and spherical-like aberrations (r(s) = -0.068; P = 0.448) did not change with aging, whereas comalike aberrations exhibited a weak but statistically significant correlation with age (r(s) = 0.256; P = 0.004). For a 7-mm pupil, total aberrations (r(s) = 0.552; P < 0.001) and comalike aberrations (r(s) = 0.561; P < 0.001) significantly increased with aging, but spherical-like aberrations showed no age-related changes (r(s) = 0.124; P = 0.166). Simulated pupillary dilation from 3 mm to 7 mm caused a 38.0+/-28.5-fold increase in the total aberrations, and the extent of increases significantly correlated with age (r(s) = 0.354; P < 0.001). Pupillary dilation influenced the comalike aberrations more in the older subjects than in the younger subjects (r(s) = 0.243; P = 0.006), but such age dependence was not found for spherical-like aberrations (r(s) = 0.141; P = 0.115). CONCLUSIONS: Comalike aberrations of the cornea correlate with age, implying that the corneas become less symmetrical along with aging. Spherical-like aberrations do not vary significantly with aging. Pupillary dilation markedly increases wavefront aberrations, and those effects are more prominent in older subjects than in younger subjects.  (+info)

Evaluation of corneal thickness and topography in normal eyes using the Orbscan corneal topography system. (3/329)

AIMS: To map the thickness, elevation (anterior and posterior corneal surface), and axial curvature of the cornea in normal eyes with the Orbscan corneal topography system. METHODS: 94 eyes of 51 normal subjects were investigated using the Orbscan corneal topography system. The anterior and posterior corneal elevation maps were classified into regular ridge, irregular ridge, incomplete ridge, island, and unclassified patterns, and the axial power maps were grouped into round, oval, symmetric bow tie, asymmetric bow tie, and irregular patterns. The pachymetry patterns were designated as round, oval, decentred round, and decentred oval. RESULTS: The thinnest point on the cornea was located at an average of 0.90 (SD 0. 51) mm from visual axis and had an average thickness of 0.55 (0.03) mm. In 69.57% of eyes, this point was located in the inferotemporal quadrant, followed by the superotemporal quadrant in 23.91%, the inferonasal quadrant in 4.35%, and the superonasal quadrant in 2.17%. Among the nine regions of the cornea evaluated (central, superotemporal, temporal, inferotemporal, inferior, inferonasal, nasal, superonasal, and superior) the central cornea had the lowest average thickness (0.56 (0.03) mm) and the superior cornea had the greatest average thickness (0.64 (0.03) mm). The mean simulated keratometry (SimK) was 44.24 (1.61)/43.31 (1.66) dioptres (D) and the mean astigmatism was 0.90 (0.41) D. Island (71.74%) was the most common elevation pattern observed in the anterior corneal surface, followed by incomplete ridge (19.57%), regular ridge (4.34%), irregular ridge (2.17%), and unclassified (2.17%). Island (32.61%) was the most common topographic pattern in the posterior corneal surface, following by regular ridge (30.43%), incomplete ridge (23. 91%), and irregular ridge (13.04%) patterns. Symmetric bow tie was the most common axial power pattern in the anterior cornea (39.13%), followed by oval (26.07%), asymmetric bow tie (23.91%), round (6. 52%), and irregular (4.53%) patterns. In the pachymetry maps, 47.83% of eyes had an oval pattern, and round, decentred oval, and decentred round were observed in 41.30%, 8.70%, and 2.18% of eyes, respectively. CONCLUSION: The information on regional corneal thickness, corneal elevation and axial corneal curvature obtained with the Orbscan corneal topography system from normal eyes provides a reference for comparison with diseased corneas. The Orbscan corneal topography system is a useful tool to evaluate both corneal topography and corneal thickness.  (+info)

Proposed classification for topographic patterns seen after penetrating keratoplasty. (4/329)

AIMS: To create a clinically useful classification for post-keratoplasty corneas based on corneal topography. METHODS: A total of 360 topographic maps obtained with the TMS-1, from 95 eyes that had undergone penetrating keratoplasty (PKP), were reviewed independently by two examiners in a masked fashion, and were categorised according to a proposed classification scheme. RESULTS: A high interobserver agreement (88% in the first categorisation) was achieved. At 12 months post-PKP, a regular astigmatic pattern was observed in 20/85 cases (24%). This was subclassified as oval in three cases (4%), oblate symmetric bow tie in six cases (7%), prolate asymmetric bow tie in six cases (7%), and oblate asymmetric bow tie in five cases (6%). An irregular astigmatic pattern was observed in 61/85 cases (72%), subclassified as prolate irregular in five cases (6%), oblate irregular in four cases (5%), mixed in seven cases (8%), steep/flat in 11 cases (13%), localised steepness in 16 cases (19%), and triple pattern in three cases (4%). Regular astigmatic patterns were associated with significantly higher astigmatism measurements. The surface asymmetry index was significantly lower in the regular astigmatic patterns. CONCLUSIONS: In post-PKP corneas, the prevalence of irregular astigmatism is about double that of regular astigmatism, with a trend for increase of the irregular patterns over time.  (+info)

Reassessment of the corneal endothelial cell organisation in children. (5/329)

AIM: To assess uniformity of the corneal endothelial cell mosaic in children. METHODS: 36 healthy children (5-11 years old, 16 boys, 20 girls) were assessed by specular microscopy. Endothelial cell density (ECD) was calculated from measured cell areas, and the number of sides/cell noted. RESULTS: Average values for ECD and cell areas were 3987 cells/mm(2) (95% CI 3806 to 4168 cells/mm(2)) and 278 (SD 85) mm(2) respectively, with normal distribution (COV 28. 2%, range 17.4 to 39.2%) and with the average percentage of six sided cells being 66.6% (8.8%). Cell area was positively correlated to number of cell sides (p <0.01, r(2)=0.993), but the percentage of six sided cells was negatively correlated to ECD (p <0.01, r=0.493). CONCLUSION: A high ECD occurs in children, but this does not mean there is a high percentage of "hexagons".  (+info)

Pterygium-induced corneal astigmatism. (6/329)

BACKGROUND: Previous work has suggested an association between increasing size of pterygium and increasing degrees of induced corneal astigmatism. OBJECTIVES: To assess the quantitative relation between pterygium size and induced corneal astigmatism using a computerized corneal analysis system (TMS II) and slit-lamp beam evaluation of pterygium size, and to conclude whether corneal astigmatism is an early indication for surgical intervention. METHODS: We evaluated 94 eyes of 94 patients with unilateral primary pterygium of different sizes, using TMS II and slit-lamp beam measurements of the size of the pterygium (in millimeters) from the limbus to assess parameters of pterygium size with induced corneal astigmatism. Best corrected visual Snellen acuity was performed. RESULTS: Primary pterygium induced with-the-rule astigmatism. Pterygium extending > 16% of the corneal radius or 1.1 mm or less from the limbus produced increasing degrees of induced astigmatism of more than 1.0 diopter. Significant astigmatism was found in 16.16% of 24 eyes with pterygium of 0.2 up to 1.0 mm in size, in 45.45% of 22 eyes with pterygium of 1.1 up to 3.0 mm in size (P < or = 0.0004), and in 100% of 3 eyes with pterygium of 5.1 up to 6.7 mm in size (P = 0.0005). We found that visual acuity was decreased when topographic astigmatism was increased. CONCLUSIONS: When primary pterygium reaches more than 1.0 mm in size from the limbus it induces with-the-rule significant astigmatism (> or = 1.0 diopter). This significant astigmatism tends to increase with the increasing size of the lesion. Topographic astigmatism tends to be improved by successful removal of the pterygium. These findings suggest that early surgical intervention in the pterygium may be indicated when the lesion is more than 1.0 mm in size from the limbus.  (+info)

Effect of disagreement between refractive, keratometric, and topographic determination of astigmatic axis on suture removal after penetrating keratoplasty. (7/329)

BACKGROUND/AIMS: Post-keratoplasty astigmatism can be managed by selective suture removal in the steep axis. Corneal topography, keratometry, and refraction are used to determine the steep axis for suture removal. However, often there is a disagreement between the topographically determined steep axis and sutures to be removed and that determined by keratometry and refraction. The purpose of this study was to evaluate any difference in the effect of suture removal, on visual acuity and astigmatism, in patients where such a disagreement existed. METHODS: 37 cases (from 37 patients) of selective suture removal after penetrating keratoplasty, were included. In the first group "the disagreement group" (n=15) there was disagreement between corneal topography, keratometry, and refraction regarding the axis of astigmatism and sutures to be removed. In the second group "the agreement group" (n=22) there was agreement between corneal topography, keratometry, and refraction in the determination of the astigmatic axis and sutures to be removed. Sutures were removed according to the corneal topography, at least 5 months postoperatively. Vector analysis for change in astigmatism and visual acuity after suture removal was compared between groups. RESULTS: In the disagreement group, the amount of vector corrected change in refractive, keratometric, and topographic astigmatism after suture removal was 3.45 (SD 2.34), 3.57 (1.63), and 2.83 (1. 68) dioptres, respectively. In the agreement group, the amount of vector corrected change in refractive, keratometric, and topographic astigmatism was 5.95 (3.52), 5.37 (3.29), and 4.71 (2.69) dioptres respectively. This difference in the vector corrected change in astigmatism between groups was statistically significant, p values of 0.02, 0.03, and 0.03 respectively. Visual acuity changes were more favourable in the agreement group. Improvement or no change in visual acuity occurred in 90.9% in the agreement group compared with 73.3% of the disagreement group. CONCLUSIONS: Agreement between refraction, keratometry, and topography was associated with greater change in vector corrected astigmatism and was an indicator of good prognosis. Disagreement between refraction, keratometry, and topography was associated with less vector corrected change in astigmatism, a greater probability of decrease in visual acuity, and a relatively poor outcome following suture removal. However, patients in the disagreement group still have a greater chance of improvement than worsening, following suture removal.  (+info)

Corneal topography by keratometry. (8/329)

AIMS: To investigate the ability of a telecentric keratometer to describe the asphericity and curvature of convex ellipsoidal surfaces and human corneas. METHODS: 22 conicoidal convex surfaces and 30 human corneas were examined by conventional keratometry. Additional keratometric measurements were made when the surface was tilted in the horizontal plane relative to the instrument optical axis. This resulted in a series of radius measurements derived from different regions of the surface. These measurements were used to determine the apical radius and the p value of the horizontal meridian of each surface. The results were compared with those derived from measurements using the EyeSys videokeratoscope and form Talysurf analysis. The method was repeated on 30 human corneas and the results compared with those of a videokeratoscope. RESULTS: For the aspheric buttons, the keratometric and the EyeSys results tended to give higher values for both apical radius and the p values than those of the Talysurf analysis. The best agreement was between the Talysurf and the keratometer where the results were not significantly different. For the human corneas, the apical radii were significantly different comparing the keratometer with the videokeratoscope but the p values were not significantly different. CONCLUSION: The keratometric method for assessing curvature and asphericity appears to hold promise as a method for quantifying the corneal topography.  (+info)

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