Biomedical subjects

M M Kus

Publications and source records attributed to M M Kus.

At least 37 records · Page 2Linked to original sources

PubMed1999

"Orientation teeth" in non-mechanical laser corneal trephination for penetrating keratoplasty: 2.94 microm Er:YAG v 193 nm ArF excimer laser.

BACKGROUND/AIMS: "Orientation teeth" at the donor trephination margin and correspondent "notches" at the host margin facilitate graft orientation and avoid "horizontal torsion" induced by asymmetric suture placement. In this study the quality and reproducibility of these structures created by non-mechanical laser corneal trephination were compared using two laser emissions. METHODS: The procedure was performed in 20 enucleated pigs' eyes using open metal masks with eight "orientation teeth/notches" (0.3 x 0.15 mm, base x height), an automated globe rotation device, and either a 193 nm ArF excimer laser or a Q switched 2.94 microm Er:YAG laser. "Teeth/notches" were analysed by planimetry and scanning electron microscopy (SEM). RESULTS: Mean size was 0.30 (0.027) x 0. 16 (0.017) mm for "teeth" and 0.30 (0.035) x 0.15 (0.021) mm for "notches" (excimer), and 0.31 (0.022) x 0.16 (0.015) mm and 0.30 (0.031) x 0.14 (0.021) mm respectively (Er:YAG). Overall, variability of notches was higher than that of teeth. By SEM, comparable cut regularity and sustained ablation profile were observed with both lasers. However, the corneal surface at the cut edge appeared slightly elevated (</=35 microm) in the Er:YAG group. CONCLUSION: Orientation teeth/notches resembling those obtained with the excimer laser can be created using the Q switched Er:YAG laser, with potential advantages of lower costs, convenient equipment size, and solid state safety.

Animals↗

PubMed1999

Zernike representation of corneal topography height data after nonmechanical penetrating keratoplasty.

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.

Adolescent↗

PubMed1998

Stromal thermal effects induced by nonmechanical (2.94-mum) erbium:YAG laser corneal trephination.

OBJECTIVE: To determine stromal thermal changes after erbium (Er):YAG laser corneal trephination with the use of 2 open masks. METHODS: Corneal trephination was performed in 89 enucleated pig eyes with an Er:YAG laser (400-microsecond pulse duration), 4 open masks (2 metallic and 2 ceramic) for both donors and recipients, and an automated globe rotation device. Different combinations of laser settings were used: pulse energy, 100, 200, and 400 mJ; repetition rate, 2 and 5 Hz; and spot size, 1.3 and 3.2 mm. Thermal effects in corneal stroma and regularity of the cut edges were quantitatively assessed by light microscopy, transmission and scanning electron microscopy. RESULTS: Best regularity and minimal thermal effects of the cut were observed with the use of ceramic masks at 200 mJ, 2 Hz, and 3.2-mm spot size, with middepth thermal changes of 18 +/- 2 microm. Effects increased with cut depth and were lower in donor corneas and with the use of ceramic masks (P<.001). Regularity of the cut was higher in the donors (P = .05) with lower repetition rates (P<.001). CONCLUSIONS: Even with the "free-running" Er:YAG laser mode, features of the trephination cut resembling those created by the 193-nm excimer laser along metal mask were achieved. Ceramic masks may be more suitable than metal masks. The Er:YAG laser seems to have the potential to be a compact and low-cost alternative in nonmechanical trephination for penetrating keratoplasty. CLINICAL RELEVANCE: Thermal effects after corneal trephination with the free-running Er:YAG laser (2.94 mm) are limited and predictable.

Animals↗

PubMed1998

[Immunologic transplant reactions after non-mechanical corneal trepanation with the excimer laser].

BACKGROUND: Nonmechanical trephination has been established as the standard procedure in penetrating keratoplasty (PK) for avascular corneal diseases at our institution. The purpose of this study was to analyze the incidence and reversibility of immunologic graft reactions after nonmechanical trephination and to detect potentially causative factors. PATIENTS AND METHODS: Out of a total series of 400 nonmechanical PKs, 286 consecutive procedures with sufficient follow-up performed between 07/1989 and 09/1997 were included in the study (104 x keratoconus, 78 x Fuchs' dystrophies, 31 x bullous keratopathies, 28 x ulcers, 25 x avascular scars, 12 x stromal dystrophies, 4 x buphthalmos, 4 x others; 202 x PK only, 84 x combined procedures; 276 first PK). The age of the 138 females and 148 males at the time of surgery ranged from 16 to 89 (mean 55 +/- 19) years. The recipient and donor trephinations were performed from the epithelial side using an 193-mm excimer laser (MEL50 or MEL60, Aesculap-Meditec, 1.5 x 1.5 mm spot mode, 16-24 mJ/pulse, repetition rate 30 or 25/s; metal masks). The shape of the recipient trephination was either circular with four or eight "orientation teeth" (n = 251; 5.0-8.0 mm diameter) or elliptical (n = 35, 6.0 x 7.0 to 7.5 x 8.5 mm diameter). In 62% of procedures fresh or short-term-preserved donor tissue was used, and in 38% of procedures the donor tissue was organ-culture-preserved. RESULTS: During a mean follow-up of 22 +/- 18 months (maximum 7.7 years), 10 acute diffuse (3 irreversible; 1.0%) and 3 chronic focal endothelial graft reactions occurred (4.5%) not earlier than 4 months and not later than 35 months after PK. Elective procedures (3.5%) resulted in significantly (P = 0.01) less reactions than acute corneal ulcers (14.3%). After 1, 2 and 3 years, the cumulative reaction rates (Kaplan-Meier values) were 1.3%, 6.3% and 13.9% in elective procedures, none of which, however, occurred after 26 elliptical trephinations. With fresh or short-term-preserved donor tissue (4.2%), graft reactions did not happen more frequently but earlier (12 +/- 6 months) than with organ-culture-preserved donor tissue (2.2%, 30 +/- 6 months). In patients with keratoconus (4.9%), reactions occurred more frequently (P = 0.05, LogRank) and earlier than in patients with Fuchs' dystrophy (1.3%). CONCLUSIONS: In addition to well-established optical advantages, nonmechanical trephination seems to have no immunologic drawbacks.

Adolescent↗

PubMed1998

[Correlation between semi-quantitative classification of corneal topography and the coefficient of Fourier transformation after non-mechanical perforating keratoplasty].

BACKGROUND: Semiquantitative classification of corneal topography after penetrating keratoplasty has the potential for focusing information about the areal dioptric power of the cornea. The purpose of this study was to objectify the procedure of manual semiquantitative classification using a Fourier transform of corneal topography power data and to correlate both methods. PATIENTS AND METHODS: Fifty patients each (30 keratoconus, 20 Fuchs dystrophy) underwent nonmechanical trephination (excimer laser MEL60, Aesculap-Meditec, Jena) in penetrating keratoplasty. All procedures (7.5-mm trephination diameter in Fuchs, 8.0 mm in keratoconus, double-running 10-0 nylon suture) were done by one surgeon. Pre-, intra- and postoperative treatment were identical. At the follow-up examinations, the keratometric astigmatism, qualitative and quantitative criteria of the automatic videokeratography, visual acuity and refraction were assessed. Corneal topography was classified both manually and based on Fourier coefficients. RESULTS: After a mean follow-up of 24 +/- 5 months, keratometric net astigmatism was 3.0 and 2.7 D with keratoconus and Fuchs dystrophy. Corneal topography analysis showed a higher orthogonality of the bow-tie shape and less asymmetry between opposite hemimeridians with increasing follow-up after keratoplasty. The semiquantitative classification showed a statistically significant correlation with the classification based on Fourier coefficients, especially with higher astigmatism and after suture removal (P = 0.04/0.01 before/after suture removal). DISCUSSION: After nonmechanical trephination, the semiquantitative classification of corneal topography can be synthetized using Fourier analysis of corneal dioptric power data. In the future, this method may be favored for prediction of potential best-corrected visual acuity after penetrating keratoplasty.

Adult↗

PubMed1998

Experimental correction of irregular corneal astigmatism using topography-based flying-spot-mode excimer laser photoablation.

PURPOSE: To present a novel experimental approach for treating irregular corneal astigmatism. METHODS: After decomposition of topographic analysis data into orthogonal Zernike polynomials, a regular target surface was defined, and the ablation profile was calculated. In a polymethylmethacrylate with an irregular surface, computer-controlled ablation was performed using a 193-nm excimer laser in "flying-spot mode." RESULTS: The difference between flattest and steepest hemimeridians in the 3-mm zone, surface regularity index, and surface asymmetry index were reduced from 8.7 to 1.1 diopters, 1.19 to 0.39, and 1.84 to 0.14, respectively. CONCLUSION: Tying corneal topography into laser software offers a straightforward concept for correction of irregular corneal astigmatism in an experimental model.

Astigmatism↗

PubMed1998

[Transplant vertical tilt after perforating keratoplasty--comparison between non-mechanical trepanation with excimer laser and motor trepanation].

BACKGROUND: Besides decentration of the graft/host trephination and "horizontal torsion" "vertical tilt" is an important factor for reduced visual outcome after penetrating keratoplasty (PK). The purpose of this study was to evaluate the time course of vertical tilt in absolute value and direction and to correlate it with functional results after PK. PATIENTS AND METHODS: Fifty patients each (20 primary dystrophies, 30 keratoconus) underwent nonmechanical trephination (NMT) (excimer laser MEL60, Aesculap-Meditec, Heroldsberg, Germany) or mechanical motor trephination (MT) (Geuder, Heidelberg, Germany) in penetrating keratoplasty. All procedures (7.5 mm in dystrophies, 8.0 mm in keratoconus, 8 orientation teeth in NMT, double-running 10-0 nylon suture) were performed by one surgeon (GOHN). At a postoperative gate of 6 weeks, 6 months, before partial suture removal and after complete suture removal, corneal topography analysis (TMS-1, Tomey, Tennenlohe, Germany) was performed. After a Gram-Schmidt-orthogonalization corneal topography height data of 25 noncentric rings in 256 hemimeridians were decomposed into Zernike components of radial order n = 16 in the sense of minimizing the root mean square error. The tilt of the surface relative to the videokeratoscope axis was calculated from the Zernike components Z1(1) and Z1(-1). The meridional power at the cardinal meridians was derived from all parabolic Zernike terms. Tilt and the difference between both meridians of the Zernike representation (ZA) were correlated with the results of Zeiss keratometry (KA). Simulated Keratometry (SimK) of the TMS-1, subjective refraction (RZ) and best-corrected visual acuity. RESULTS: After NMT, vertical tilt of the graft was 3 degrees without significant change over time. Following MT, an equivalent time course could be observed before partial suture removal. However, after complete suture removal, a significant increase of the tilt was measured to 5 degrees (p = 0.02). No significant difference could be detected comparing keratoconus and Fuchs' dystrophy both in NMT and MT. The direction of the vertical tilt component piled up to the hemimeridian defined by the knot of the first running suture. At all postoperative follow-up examinations, the ZA of the Zernike decomposition showed a good correlation to the RZ, whereas the KA and the SimK did not. At the end of the follow-up, best-corrected visual acuity after NMT was 2 decimal lines better than after MT. CONCLUSIONS: The Zernike decomposition of topographic height data is a suitable tool for extraction and quantifying vertical tilt of the graft following penetrating keratoplasty. In contrast to conventional keratometry with its 4-point measurement, a decomposition of topographic height data into orthogonal polynomials enables a detection of both cardinal meridians even in corneas with a high degree of local irregularities.

Adolescent↗

PubMed1998

[Interpretation of corneal topography after penetrating keratoplasty with wave-front parameters--comparison between non-mechanical trepanation with excimer laser and motor trepanation].

BACKGROUND: Besides irregular astigmatism characterized by the asymmetric components of the corneal surface, the aberration of the cornea from an ideal sphere degrades the optical performance of the "optical system eye". Best-corrected visual acuity may be markedly decreased with an increasing aperture diameter. The purpose of this study was to evaluate the time course of the symmetrical part of the aberration from an ideal sphere and to correlate it with functional results after penetrating keratoplasty (PK). PATIENTS AND METHODS: Fifty patients each (20 primary dystrophies, 30 keratoconus) underwent nonmechanical trephination (NMT) (excimer laser MEL60, Aesculap-Meditec, Heroldsberg, Germany) or mechanical motor trephination (MT) in penetrating keratoplasty. All procedures (7.5 mm in dystrophies, 8.0 mm in keratoconus, 8 orientation teeth in NMT, double-running 10-0 nylon suture) were performed by one surgeon (GOHN). At a postoperative gate of 6 weeks, 6 months, before partial suture removal and after complete suture removal, corneal topography analysis (TMS-1, Tomey, Tennenlohe, Germany) was performed. After a Gram-Schmidt-orthogonalization, corneal topography height data of 25 noncentric rings in 256 hemimeridians were decomposed into Zernike components of radial order n = 16 in the sense of minimizing the root mean square error. The symmetrical part of the deviation from an ideal spherical surface was calculated from the Zernike components Z4(0), Z6(0), ..., Z16(0). From the Zernike components, the longitudinal focus distribution and its standard deviation (SDF) was determined. SDF was correlated with the surface asymmetry index (SAI), the surface regularity index (SRI), the potential visual acuity (PVA) of the TMS-1 and the spectacle-corrected visual acuity. RESULTS: In the time course after PK, SDF decreased from the 6 weeks follow-up examination to the end of the follow-up from 1.27 mm to 0.77 mm in NMT (p = 0.01) and from 1.29 mm to 1.20 mm following MT (p = 0.24) within the central corneal region of 3 mm in diameter. The SAI did not depend on SDF, whereas the SRI correlated significantly inversely with the SDF within the 3 mm zone immediately before (p = 0.01 and p = 0.02) and after suture removal (p = 0.01 each) after NMT. After MT, only a mild inverse correlation was observed before (p = 0.05) and after suture removal (p = 0.04). In the time course after the 6 months follow-up the SDF within the 3 mm central area correlated inversely with the best-corrected visual acuity, more with NMT than with MT (p = 0.005 and p = 0.04 after suture removal). Best-corrected visual acuity was approximately 2 decimal lines better following NMT. CONCLUSIONS: Zernike decomposition of corneal topography height data allows a separation and quantification of aberration of corneal graft surface from an ideal sphere. Although corneal surfaces with a high degree of local irregularities can be decomposed due to the orthogonality condition. Following NMT, SDF was markedly lower after suture removal.

Adult↗

PubMed1998

[Topography-assisted correction of superficial irregularities of the cornea with the excimer laser].

BACKGROUND: A retinal image performance distorted by an asymmetric or irregular corneal surface cannot be compensated for with spherocylindric glasses completely. The best-corrected visual acuity is markedly decreased and contact lens fitting often impossible. The purpose of this study was to calculate the differential height between corneal topography raw data and any regular surface with mathematical methods in order to ablate the differential height with a computer-controlled laser beam, thereafter. METHODS: A Zernike decomposition of radial degree n = 16 was realized within a clinically relevant central corneal area of 8 mm in diameter based on corneal topography raw height data of a commercially available topographer (TMS-1, Tomey, Erlangen). Any target surface could be defined by varying weighting of the Zernike coefficients. The calculated differential height ablation between the raw data and the target surface given in a polar grid was transformed to a Cartesian grid to evaluate the sleeping time at each grid position considering the characteristic ablation curve for the intended ablation of the height difference. Subsequently, differential height ablation was simulated using an automated laser beam control for a modified excimer laser (MEL60, Aesculap-Meditec, Jena). We developed software tools for Zernike decomposition of corneal topography raw height data and time-regulated automatic laser beam control of the grid positions in the higher programming language C (Borland C++ 3.1, Borland Inc., München). RESULTS: Definition of a target surface can be realized alternatively by selecting a set of Zernike coefficients or defining a spherical or spherocylindrical surface by superposition of parabolic terms in a fixed proportion creating a best-fit target surface to the raw data. In originally "relatively flat" areas, the differential height profile indicates a "relatively deep" ablation resulting in relative steepening towards the periphery of the ablation zone. The resolution of the mechanical unit of the laser beam control consisting of two linear stepping motors is 9 microns in the focal plane with a reproducibility of 5 microns. The software unit is guiding the laser beam in a meandering fashion within the ablation area considering the calculated sleeping time for each grid position. Mean overlap of the 1 mm laser spots is 70%. The laser beam diameter of 1 mm effects a peripheral transition zone of 0.5 mm. CONCLUSIONS: Zernike decomposition of corneal topography height data is an efficient tool for localizing and quantifying superficial irregularities and for directly calculating an ablation profile from created differential height data. With an automatic laser beam control a well-defined laser ablation of superficial corneal irregularities is possible, subsequently.

Astigmatism↗

PubMed1998

[Design and preliminary results of the Erlanger non-high-risk penetrating-keratoplasty study].

PURPOSE: The purpose of this prospective study is to examine the non-high-risk penetrating keratoplasty clinically, tyndallometrically and immunological-serologically in order to learn more about the pathogenesis, early symptoms and therapy monitoring of corneal graft rejection. PATIENTS AND METHODS: Since february 1997, patients undergoing non-high-risk-PK have been enrolled in this prospective study. Examinations are done preoperatively and in well-defined postoperative gates (6 weeks, 3 months, 6 months, etc.) and include clinical assessments, corneal topography, laser-tyndallometry, corneal endothelial cell count and corneal pachymetry. Preoperatively as well as at each postoperative examination, 10 ml serum and perioperatively corneoscleral donor tissue are collected and frozen (-80 degrees C) to allow immunological and serological examinations in case of graft rejection later. A relational data base (MS Access) guarantees complete and homognous standardized clinical and serological data. We randomised our patients with respect to duration of topical steroid therapy in standard situations (short-time vs. long-time) as well as for systemic steroid therapy (bolus group vs. tapering group) in case of a graft rejection in 2 treatment groups. RESULTS: Up to March 1998, 99 patients have been enrolled in this prospective study. Our report includes the first consecutive 55 patients (25 female, 30 male, recipient age 56.9 +/- 19.4 years, donor age 57.0 +/- 19.9 years). Up to now (follow-up 2.5 +/- 2.1, maximum of 7.3 months) we did not observe immunologic graft rejections. In one patient a primary graft failure occurred. Laser tyndallometry showed a reduction of blood-aqueous barrier breakdown from 6 weeks postoperatively (9.3 +/- 5.7 photon counts/msec) to 3 months postoperatively (6.8 +/- 3.5 photon counts/msec). CONCLUSION: This prospective randomized clinical and immunological study after non-high-risk-PK is supposed to give more information about pathogenesis, early symptoms and therapy monitoring of graft rejection after elective PK. The concept and design of this study are described. Preliminary data may give significant results in a few years.

Adult↗

PubMed1998

Experimental 193-nm excimer laser trephination with divergent cut angles in penetrating keratoplasty.

PURPOSE: To study the morphologic properties of divergent cut angles after nonmechanical trephination for penetrating keratoplasty in donors and recipients and to assess its implications for donor-recipient stability in an experimental porcine model. METHODS: An excimer laser 193-nm corneal trephination was performed in 30 enucleated porcine eyes by using a modified "open mask" (stainless steel sheet 10CrNi18), to obtain round donor buttons and corresponding recipient beds 7 mm in diameter at the level of Bowman's layer. An automated globe-rotation device allowed different cut angles toward the optical axis. Three cut angles were intended: 0 degrees, 15 degrees divergent, and 35 degrees divergent. Quality of the cuts was assessed by means of histology and scanning electron microscopy (SEM). In a different setting, after alignment of the donor button and corresponding recipient bed in an artificial anterior chamber, intracameral pressure was increased stepwise, modifying the height of a bottle of balanced saline solution connected to the chamber. Criterion for "instability" was the movement of the donor button in all four quadrants during observation with a surgical microscope (x12). RESULTS: Reproducible cut angles and smooth cut surfaces of donors and recipients were confirmed by histologic and SEM evaluation. Macroscopically, a good alignment of a divergent donor button in a corresponding recipient bed was achieved. Instability of the donor buttons occurred with a water column niveau at 2 cm in the 0 degrees cut, at 3 cm in 15 degrees divergent cuts, and at 4.5 cm in 35 degrees divergent cuts. CONCLUSION: The stability of a corneal donor button in a recipient bed seems to increase in proportion with the degree of divergence of the trephination angle. Considering the good histologic quality and macroscopic alignment obtained, divergent cut angles may allow a reduction of sutures, assuring a water-tight wound closure in penetrating keratoplasty.

Animals↗

PubMed1998

The regularity of laser keratectomy depth in nonmechanical trephination for penetrating keratoplasty.

BACKGROUND AND OBJECTIVES: To study the regularity of laser keratectomy depth in nonmechanical trephination for penetrating keratoplasty (PK) and to assess its implications on astigmatism and visual acuity (VA). PATIENTS AND METHODS: In this retrospective clinicopathologic study, the authors enrolled 26 women and 32 men (mean age 54 +/- 20 years) with either keratoconus (n = 27) or Fuchs' dystrophy (n = 31), who underwent excimer laser (193 nm) trephination using a manually guided beam. Donor cornea trephination was performed using an artificial anterior chamber and either an automated rotation device (n = 27) or a manually guided beam (n = 31). The minimum residual corneal thickness (MRCT) and the random residual corneal thickness (RRCT), requiring division with scissors in excised patient buttons or corneoscleral donor rims, were assessed in a masked fashion using histologic sections. The postkeratoplasty keratometric net astigmatism, the subjective cylinder, and the VA were evaluated before and after suture removal. RESULTS: In patients with keratoconus, the mean MRCT (13% +/- 14%) and RRCT (38% +/- 20%) were significantly higher than in the patients with Fuchs' dystrophy (7% +/- 11% and 26% +/- 15%, respectively) (P < .01). When an automated rotation device for trephination of the donor cornea was used, the mean MRCT (2% +/- 4%) and RRCT (14% +/- 11%) were significantly smaller than when a manually guided laser beam was employed (15% +/- 12% and 38% +/- 15%, respectively) (P < .001). After suture removal, the VA increased significantly with automated trephination (P = .04), but not with manually guided trephination of the donor cornea (P = .24). However, after a mean follow-up of 30 +/- 8 months, the differences in the mean keratometric astigmatism, refractive cylinder, and VA after automated trephination (3.0 D, 2.6 D, 20/29, respectively) compared with those after manually guided trephination of the donor cornea (4.3 D, 3.9 D, 20/33, respectively) did not reach statistical significance. CONCLUSIONS: The regularity of donor trephination depth can be significantly improved by using an automated rotation device instead of a manually guided beam. Although residual postkeratoplasty astigmatism was not directly related to trephination depth, the improvement of the functional results after suture removal seems to be promoted by automated trephination.

Astigmatism↗

PubMed1998

Graft decentration in penetrating keratoplasty: nonmechanical trephination with the excimer laser (193 nm) versus the motor trephine.

BACKGROUND AND OBJECTIVE: Graft decentration is an obvious cause of postkeratoplasty astigmatism. The purpose of this study was to compare graft decentration after nonmechanical trephination with the excimer laser (193 nm) with that after mechanical motor-trephination in 50 consecutive patients with Fuchs' dystrophy and 50 patients with keratoconus. PATIENTS AND METHODS: To determine decentration in absolute values and clock hours, a postoperative slide was projected with a fixed magnification onto a pattern with circles corresponding to the trephination margin. Using a second transparent and movable pattern with concentric circles and ellipses, the authors measured the amount and direction of decentration relative to the limbus and to the pupil. In addition, the keratometric astigmatism and the refractive cylinder were assessed. In this prospective study, the patients were assigned randomly to either method of trephination. RESULTS: The decentration was significantly lower (P < .002) with excimer laser trephination (0.23 +/- 0.26 mm, relative to the limbus; 0.33 +/- 0.26 mm, relative to the pupil) than with mechanical trephination (0.58 +/- 0.23 mm, relative to the limbus [P < .01]; 0.64 +/- 0.24 mm, relative to the pupil [P < .005]). There was no significant difference between the results obtained in patients with Fuchs' dystrophy and those of patients with keratoconus. The preferred direction of decentration relative to the pupil was the lower quadrants. There was a mild correlation between net astigmatism and the absolute value of decentration. However, with sutures in place, there were no significant differences in the keratometric net astigmatism between mechanical and nonmechanical trephination (P = .16) or between Fuchs' dystrophy and keratoconus (P = .18). CONCLUSIONS: The results indicate that the amount of decentration can be reduced by specific techniques associated with nonmechanical trephination. This might have a favorable impact on the residual astigmatism after suture removal.

Adolescent↗

PubMed1998

Ellipsoidal fitting of corneal topography data after arcuate keratotomies with compression sutures.

BACKGROUND AND OBJECTIVE: After paired arcuate keratotomies and compression sutures (AK) for treatment of high postkeratoplasty astigmatism, corneal topography tends to be irregular. The purpose of this study was to demonstrate a mathematical method for approximation of discrete corneal topography power data with an ellipsoid for better appreciation of the clinical outcome after AK. PATIENTS AND METHODS: Thirty-one eyes of 28 consecutive patient who underwent AK for excessive postkeratoplasty astigmatism were studied. Regular keratometry, corneal topography (TMS-1), subjective refraction, and best-corrected visual acuity (VA) were assessed preoperatively and at 1 week and 1 year postoperatively. A simplex algorithm was applied for fitting an ellipsoidal surface to raw corneal topography power data. A set of parameters (meridional power, axis, and asphericity) were calculated. The cylinder of subjective refraction was correlated with the keratometric readings, the simulated keratometry (SimK) of the topography system, and the respective parameters of the model surface. RESULTS: Keratometric astigmatism and the cylinder of the model surface decreased from 8.1 +/- 3.2 and 7.9 +/- 2.9 D preoperatively to 4.5 +/- 2.1 and 5.3 +/- 2.0 D after 1 year, respectively. The asphericity in both meridional cross sections changed from a prolate ellipse preoperatively to an ablate ellipse at the early postoperative follow-up stage. Regarding the cylinder axis, there was a significant correlation of the model surface with the refractive cylinder at all examinations (P < .05), whereas there was no significant correlation of the SimK axis and the refractive cylinder axis. CONCLUSION: The approximation of corneal topography power data with an ellipsoidal model surface renders reconstruction of clinically relevant corneal topography parameters, including corneal asphericity with a marked data compression. Even in markedly irregular corneal surfaces, such as after AK, the correlation of amount/axis of refractive cylinder with the model surface parameters is more accurate than it is with respective SimK values of corneal topography analysis.

Adult↗

PubMed1997

[Reproducibility and validity of a new automatic method of specular microscopy analysis of corneal endothelium].

PURPOSE: This prospective study was designed to test the reproducibility of a new automated technique for analyzing the corneal endothelium and to assess the validity of the technique by comparing it with a standard method. SUBJECTS AND METHODS: We used a contact specular microscope combined with a video camera (Tomey EM-1000) and a computer (IBM compatible PC, 486DX33) with suitable software (Tomey EM-1100, version 0.94). Video images of the corneal endothelium (area: 0.312 mm2) were passed directly into the computer input by means of a frame grabber and were automatically processed. The area to be analyzed could be varied by location and size (5580-135,150 microns2), depending on the quality of the image. Healthy corneas of 67 volunteers (age: 30.9 +/- 8.6 years) were examined. One examiner measured cell density three times in each of 42 eyes (retest-stability); three different examiners made one measurement in each of 25 eyes (objectivity). We evaluated the cell density determined by the computer after automated analysis and assessed the corrected cell density. This second result was obtained after the examiner had corrected the processed image by drawing in cell boundaries that the computer had not recognized or erasing cell boundaries the computer had sketched in by mistake. Additionally, a photograph of the corneal endothelium (specular microscope Bio Optics LSM 2000 A) was obtained from 40 volunteers to be used for manual cell counting applying a "fixed-frame" technique (validity). RESULTS: The corrected values showed a high retest-stability (reliability coefficient r = 0.943) and a high objectivity (r = 0.904). The values obtained by the automated method (2415 +/- 214 cells/mm2) did not differ significantly from those obtained by manual cell counting (2431 +/- 228 cells/mm2) (P = 0.898). The uncorrected values (2252 +/- 190 cells/mm2) were on average 7.2 +/- 2.6% lower than the corrected ones (177 +/- 69 cells/mm2). Retest-stability (r = 0.856) and objectivity (r = 0.737) of the uncorrected values were satisfactory. The uncorrected value was significantly lower than the value of manual cell counting (P < 0.001). The size of the analyzed area (range 12,750-84,708 microns2; average 31,438 +/- 10,655 microns2) had no significant effect on cell density (Spearman's correlation coefficient k = -0.150, P = 0.093). CONCLUSION: The automated method for analyzing the corneal endothelium quickly produces valid, reproducible results in normal corneas, provided that the correction mode of the software is applied.

Endothelium, Corneal↗

PubMed1997

[Fourier analysis as a mathematical model for evaluating and presenting postoperative corneal topography data after non-mechanical perforating keratoplasty].

BACKGROUND: Videokeratography has given the possibility to obtain information in curvature from a much larger region of the cornea than that covered by keratometry. Fourier analysis as a mathematical model can be used to represent real physical attributes of the cornea and to divide corneal topography in its basic components: the zero-frequency component as the mean ring power, the one-cycle component as a representation of decentration and the two-cycle component as a representation of regular corneal toricity. The purpose of this study was the reconstruction of the corneal refraction after penetrating keratoplasty with a small number of characteristic parameters and the evaluation of the time course of the fourier coefficients as indices for a regular astigmatic cornea in the postkeratoplasty period including suture removal. PATIENTS AND METHODS: Fourty patients (group 1: 20 primary dystrophies, group 2: 20 keratoconus) underwent nonmechanical trephination (excimer laser MEL60, Aesculap-Meditec, Heroldsberg, Germany) in penetrating keratoplasty. All procedures (7.5 mm in dystrophies, 8.0 mm in keratoconus, 8 orientation teeth, double-running 10-0 nylon suture) were performed by one surgeon. At a postoperative gate of 6 weeks, 6 months, before partial suture removal and after complete suture removal, corneal topography (TMS1, Tomey, Tennenlohe, Germany), keratometry, visual acuity and subjective refraction were assessed. Radial approximation with a 5th order polynomial fit of the refractive data on 25 non-centric rings of the TMS, within 256 hemimeridians was performed to get data at equally spaced concentric rings. Fast Fourier transformation of the data sets in the mid periphery (1.4-1.8 mm apical distance) was done to get DC-, one-cycle and two-cycle component. Fourier coefficients were correlated with keratometric readings, subjective refractive values and visual acuity. RESULTS: Spherical equivalent was fairly constant in the postoperative interval before suture removal. After suture removal, a corneal flattening of about 3 diopters occurred. The one-cycle component before suture removal tended to be higher in group 1 compared to group 2 before suture removal. This difference was statistically significant (p = 0.01) after suture removal. Comparing keratometry, calculated meridians by the Tomey software, two-cycle component of the fourier decomposition and subjective refraction, the best correlation (p = 0.02) was observed between two-cycle-component and the refractive cylinder in amplitude and axis after suture removal. Best corrected visual acuity was inversely correlated with the amount of energy of higher harmonics compared to the whole energy before (p = 0.04) and after (p = 0.01) suture removal. CONCLUSIONS: Fourier transformation renders reconstruction of corneal topography data with a marked data reduction and a small error. From fourier coefficients conclusions may be drawn concerning potential best-corrected visual acuity and amplitude/axis of subjective cylinder, even in corneas with severe local irregularities.

Adult↗

PubMed1997

[Limbus-parallel keratotomies with compression sutures in treatment of high astigmatism after perforating keratoplasty: a vector analysis and topographic study].

BACKGROUND: Visual acuity following penetrating keratoplasty is frequently limited by excessive astigmatism which cannot be compensated for with spectacles or contact lenses. The purpose of the study was to determine the effects of arcuate keratotomies and compression sutures on the amount and regularity of corneal astigmatism and on the visual acuity. PATIENTS AND METHODS: Between June 1989 and August 1995, 56 eyes from 56 patients (30 women, 26 men, average age 53 +/- 16 years) with excessive post-penetrating keratoplasty astigmatism were treated with paired arcuate cuts and compression sutures 4.8 +/- 3.5 years after suture removal (45% keratoconus, 30% scars, 20% dystrophies). The incisions were made along the meridian of maximum dioptric power in a sector extending for 60 +/- 15 degrees (6 mm diameter). Incision depth was standardized at 450 microns. Compression sutures were placed 90 degrees away in the flat meridian. Eight patients required more than one procedure to obtain the desired effect. Best corrected visual acuity (VA), keratometric readings and topographic power maps were analyzed pre- and postoperatively. We categorized the topographic maps into six groups: from group 1 (regular) to group 6 (irregular). For description of the astigmatic change after surgery, the formulas by Naylor and Jaffe (vector-corrected astigmatism) were applied. RESULTS: The mean preoperative astigmatism was 10.8 +/- 3.1 (4.2 to 19.2) diopters (D). After a mean follow-up of 1.1 years, the mean net astigmatism was 5.8 +/- 3.2 (0 to 16) D. The mean preoperative visual acuity (VA) was 0.38 +/- 0.31 (from 0.03 to 1.0). At the end of follow-up, the mean VA was 0.43 +/- 0.25. Non-refractive reasons for poor visual acuity included amblyopia (n = 5), macular degeneration (n = 4), glaucoma (n = 4), cataract (n = 2), and others (n = 5). Twenty-seven percent of the preoperative topographic maps were categorized into groups 1 and 2 and only 10% into groups 5 and 6. At the end of the follow-up, none of the 53 available topographic maps was categorized into group 1, 7.5% into group 2 and 30% into groups 5 and 6. The mean astigmatic change (vector-corrected astigmatism) was 12.3 +/- 5.2 (1.0 to 29.8) D with a turn of the axes ranging from -39 to 44 degrees. CONCLUSIONS: Arcuate incisions with compression sutures are easily performed and hold a low complication rate in comparison with other refractive operations. Definition of successful surgery is even broader when a significant reduction of astigmatism that usually allows the use of spectacles or contact lenses is considered. A disadvantage is that each particular case is unpredictable because of the tendency of the topography to irregularity and possible regression of the effect.

Adult↗

PubMed1996

Thermal effects in excimer laser trephination of the cornea.

BACKGROUND: Excimer laser trephination, as an alternative to mechanical trephination of the cornea in penetrating keratoplasty, is expected to reduce long-term postkeratoplasty astigmatism. Trephination with high energy densities may induce thermal epithelial alterations when metal aperture masks are used. METHODS: Ninety porcine eyes fixed in an artificial anterior chamber (20 mmHg) were trephined with a 193-nm excimer laser in order to study the effect of the temperature on the cornea and the aperture mask during and after termination of the laser exposure. Energy levels tested were in the range used in patients. A pyroelectric infrared sensor connected to a PC via an analog-digital converter was used. With a high-speed sampling routine written in C+2 it was possible to monitor on line the temperature in a focus 0.8 mm in diameter. RESULTS: In donor trephination the maximum temperature increase of the metal mask was 11 K (donor tissue 6.1 K, repetition rate 30/s). During recipient trephination the maximum temperature increase of the mask was 9.4 K (7.5 K on the cornea). An increase in the repetition rate and a decrease in the rotation speed resulted in greater temperature increase of the exposed cornea or metal mask. CONCLUSIONS: Online monitoring of cornea and mask surface temperatures during excimer laser trephination is possible using high-speed sampling equipment. Appropriate adjustment of repetition rate and rotation speed may reduce thermal effects, especially in donor trephination.

Animals↗