[Principles of corneal cross-linking : Presentation based on the development of the various treatment protocols]

BACKGROUND

Corneal cross-linking (CXL) is used to treat corneal ectatic diseases. The aim is to improve the reduced consolidation of the cornea in order to halt further corneal protrusion and therefore subsequent deterioration of the optical imaging proportions.

MATERIAL AND METHODS

In this article the principles of corneal cross-linking based on riboflavin and UV light are presented including recent research results. Furthermore, the most important treatment protocols including standard CXL (S-CXL), accelerated CXL (A-CXL), transepithelial CXL (TE-CXL) and the approach of the CXL procedure for thin corneas are explained.

RESULTS

The CXL method depends on four major components, the riboflavin solution, oxygen, UV light and the availability of cross-linking sites on the collagen tissue. According to the present state of knowledge, the photochemical process of the CXL method induces covalent bonds between the fibrils and proteoglycans and thus stabilizes the collagen fibers, resulting in corneal consolidation. In addition to the S‑CXL, which has proven its effectiveness and safety in a large number of studies, there are other treatment protocols that have been developed based on the Bunsen-Roscoe law of reciprocity. The A‑CXL protocol has the advantage of having a shorter irradiation time but it seems to be less effective than the S‑CXL protocol concerning the increase in corneal stiffness. The use of TE-CXL has so far not yet gained acceptance in the clinical practice.

CONCLUSION

The CXL procedures primarily aim to stabilize the cornea. In the future, in addition to stabilization of the cornea, simultaneous improvement of visual acuity will be the main focus.

The Distribution of Retinal Venous Pressure and Intraocular Pressure Differs Significantly in Patients with Primary Open-Angle Glaucoma

INTRODUCTION

Until now, venous pressure within the eye has widely been equated with intraocular pressure (IOP). Measurements with dynamometers calibrated in instrument units or in force showed that the retinal venous pressure (RVP) may be higher than the IOP in glaucoma patients. In this study, the RVP was measured with a contact lens dynamometer calibrated in mmHg.

METHODS

Study type: cross-sectional.

SUBJECTS

Fifty consecutive patients with primary open-angle glaucoma (POAG) who underwent diurnal curve measurement under medication. Age: 69 ± 8 years. Measurement of RVP: contact lens dynamometry. IOP measurement: dynamic contour tonometry.

RESULTS

Pressures are given in mmHg. In all 50 patients, the IOP was 15.9 (13.6; 17.1) [median (Q1; Q3)], and the RVP was 17.4 (14.8; 27.2). The distribution of the IOP was normal and that of the RVP was right skewed. In the subgroup of 34 patients with spontaneous pulsation of the central retinal vein (SVP), the IOP and therefore, by definition, the RVP was 16.5 (13.7; 17.4). In the subgroup of 16 patients without SVP, the IOP was 14.8 (13.3; 16.4), and the RVP was 31.3 (26.2; 38.8) (p ≤ 0.001). In systemic treatment, the prescribed drugs were (the number of patients is given in parentheses): ACE inhibitors (20), β-blockers (17), angiotensin II-receptor blockers (13), calcium channel blockers (12), diuretics (7). No difference in RVP was observed between patients receiving these drugs and not receiving them, except in the β-blocker group. Here, the 17 patients with systemic β-blockers had a median RVP of 15.6 mmHg and without 20.2 mmHg (p = 0.003). In the 16 patients with a higher RVP than IOP, only one patient received a systemic β-blocker. The median IOP was 15.7 mmHg with systemic β-blockers and 16.1 mmHg without (p = 0.85).

CONCLUSION

In a subgroup of 16 of the 50 patients studied, the RVP was greater than the IOP by a highly statistically and clinically significant degree. According to the widely accepted thinking on the pathophysiology of retinal and optic nerve head circulation, the blood flow in these tissues may be much more compromised in this group of patients than has been assumed. They may be identified by a missing SVP. Topical and systemic medications showed no statistically significant influence on the RVP, except for the systemic β-blockers, in which the RVP was lower by 4.6 mmHg than for the patients who did not receive these drugs (p = 0.003).

Biomechanical efficacy of contact lens-assisted collagen cross-linking in porcine eyes

PURPOSE

Contact lens-assisted corneal cross-linking (CACXL) has been proposed for the cross-linking treatment in thin corneas. The aim of this study was to assess the biomechanical efficacy of this treatment.

METHODS

Post-mortem porcine eyes were treated with standard cross-linking and with cross-linking placing a contact lens soaked with isoosmolar riboflavin solution on the debrided cornea with or without an adherent precorneal riboflavin film of up to 100 μm thickness. Three soft contact lenses (Air Optix Aqua, SofLens and Galifa) with different degrees of hydrophilic properties were tested. After cross-linking with a surface UVA irradiance of 3 mW/cm² for 30 min (fluence 5.4 J/cm²), a 400 μm deep anterior corneal flap was created using a lamellar rotating microkeratome. Biomechanical stress-strain measurements and thermal shrinkage tests were performed.

RESULTS

In the Air Optix Aqua group (30% hydration) without riboflavin film, Young's modulus and stress at 8% strain were increased significantly versus untreated controls and the effect was 92.4% respectively 86.35% of the standard CXL value. In the SofLens group (59% hydration) without riboflavin film, Young's modulus and stress at 8% strain were increased significantly versus untreated controls and the effect was 67.04% respectively 65.28% of the standard CXL value. In the Galifa group (72% hydration) without riboflavin film, Young's modulus and stress at 8% strain were increased significantly versus untreated controls and the effect was about 68.48% respectively 75.52% of the standard CXL value. In all samples with a precorneal riboflavin film under the contact lens, there was no significant biomechanical effect compared to the untreated controls. Similarly, in the hydrothermal experiments at 70°C, there was a typical mushroom pattern with increased resistance to thermal shrinkage in the anterior stroma after standard CXL, a markedly reduced mushroom effect using a riboflavin-soaked contact lens only and no effect with the use of a riboflavin-soaked contact lens plus a precorneal riboflavin film.

CONCLUSION

The biomechanical effect of CACXL in porcine corneas is about one-third less than after standard CXL. The efficacy of CACXL might be improved by reducing or omitting the riboflavin film on the contact lens. Further risk assessment studies are necessary.

Biomechanical efficacy of corneal cross-linking using hypoosmolar riboflavin solution

Purpose:

The use of hypoosmolar riboflavin solution has been suggested for cross-linking thin corneas. The aim of this study was to compare the biomechanical efficacy of corneal cross-linking using hypoosmolar dextran-free riboflavin solution (HCXL) versus isoosmolar standard corneal cross-linking treatment (CXL).

Methods:

A total of 24 postmortem porcine eyes with debrided corneas were subdivided into three treatment groups: Controls, the isoosmolar group with isoosmolar 0.1% riboflavin-20% dextran solution and the hypoosmolar group with dextran-free, 0.1% riboflavin solution. The samples were irradiated with UVA light of 365 nm wavelength and an irradiance of 3 mW/cm² for 30 min (dose 5.4 J/cm²). For the biomechanical measurements, 400-µm-deep anterior corneal flaps were created using a lamellar rotating microkeratome. Uniaxial stress–strain measurements were performed.

Results:

In the isoosmolar treatment group, stress and Young’s modulus at 8% strain were significantly increased by 67.97%, respectively, 62.62% versus the controls. In the hypoosmolar treatment group, stress and Young’s modulus at 8% strain were significantly increased by 81.21%, respectively, 51.40% versus the controls. There was no significant difference between the iso- and hypoosmolar groups in biomechanical efficacy. On histology, there was no edema in the anterior 200 µm of the corneas after stromal swelling by the hypoosmolar riboflavin solution.

Conclusion:

Corneal cross-linking using isoosmolar or hypoosmolar riboflavin solution induces a comparable biomechanical effect. This is explained by the localization of the maximum cross-linking effect in the anterior 200 µm of the cornea which are not affected by the swelling effect of hypoosmolar riboflavin solution.

Molecular processes of corneal collagen cross-linking in keratoconus therapy

Corneal collagen cross-linking (CXL) with riboflavin and UVA light is a therapeutic procedure to restore the mechanical stability of corneal tissue. The treatment method applies to pathological tissue changes, such as keratoconus. It induces the photochemical formation of new collagen cross-links. Although therapeutic effects are indisputable, the exact molecular process of CXL and how cross-links are formed is still unclear. In this work, Fouriertransform infrared (FT-IR) spectroscopy is used to investigate the cross-linking process. For that purpose, in-situ experiments with porcine corneas are carried out using attenuated total reflection (ATR) spectroscopy. Furthermore, IR micro-spectroscopic imaging in transmission mode is used to investigate thin tissue sections of the cornea and initial approaches for the distinction of cross-linked and untreated tissue by IR microspectroscopic imaging were performed. Multivariate methods are applied to access changes that occur as a result of CXL. It is shown that spectral changes after cross-linking are caused predominantly by an increase of methyl- and methylene groups as well as primary and secondary amines. In addition, a decrease of carbonyl groups could be observed.

Keratokonus: heutige Therapieoptionen

Die Inzidenz des Keratokonus wird in aktuellen Studien deutlich höher geschätzt als bisher angenommen, somit wird diese Hornhautektasie nicht mehr zu den seltenen Erkrankungen gezählt. Als einer der bedeutendsten Risikofaktoren für die Entstehung des Keratokonus wurde neben Atopie und Down-Syndrom das chronische Augenreiben definiert. Dies ist der durch Verhaltenstraining und konsequente Aufklärung einzig wirklich modifizierbare Risikofaktor. Weitere Therapieoptionen beinhalten Maßnahmen zur Visusverbesserung wie die Anpassung von Brillen und formstabilen Kontaktlinsen, die Implantation kornealer Ringsegmente oder phaker intraokularer Linsen. Im Falle einer Krankheitsprogression bietet das korneale Crosslinking ein effektives und sicheres Verfahren, sodass das Erfordernis einer Hornhauttransplantation in dieser Indikation immer seltener wird. Neuere Studien sehen auch einen positiven Effekt einer Kombination der bisher bei Keratokonus kontraindizierten photorefraktiven Keratektomie und dem Crosslinking. Die perforierende Keratoplastik stellt allerdings weiterhin den Goldstandard der chirurgischen Behandlung des Keratokonus im fortgeschrittenen Stadium dar, wobei in den letzten Jahren auch die lamellären Verfahren immer mehr in den Vordergrund rücken.

[Cross-Linking in Keratoconus: "Epi-off" or "Epi-on"?]

Corneal cross-linking can halt the progression of keratoconus, but what is the best approach for treatment? There are a number of treatment options for keratoconus, but only corneal cross-linking (CXL) appears to halt the progression of the disease. To guarantee effective cross-linking, CXL treatment involves removal of the corneal epithelium prior to riboflavin application and ultraviolet light illumination - "epi-off" CXL. Several methods of "epi-on" (transepithelial) CXL have been proposed, such as keeping the corneal epithelium intact which should be less painful and help avoid other CXL-associated adverse events. The evidence so far is that epi-off CXL remains the most effective method of strengthening the cornea and slowing keratoconus progression - but transepithelial methods are gaining ground.

[Influence of corneal biomechanical properties on myopic regression after laser in situ keratomileusis]

BACKGROUND

Laser in situ keratomileusis is a safe and accepted method for correcting myopia. The operational results in terms of accuracy as well as the subjective acceptance of patients for corrections to - 8 D are now considered to be promising (Seiler, Refraktive Chirurgie der Hornhaut, 2000); however, postoperative results show individual patient problems in long-term stability. It is believed that the preoperative condition of the cornea (e.g. thickness, biomechanical properties) could have an influence on postoperative problems such as myopic regression.

METHOD

This study included a total of 46 eyes from 25 patients. At 3 months postoperatively, 15 patients (19 eyes) showed a SEQ of -0.50 D or more. Within this group, 11 patients (15 eyes) developed a regression (regression group) within the first 3 postoperative months. The remainder of the total group did not show any regression (stability group). The subjects of this study were on average 33 ± 8 years (stability group) and 31 ± 7 years old (regression group). The corneal thickness was tested and refractive error, visual acuity (BCVA/UCVA) and intraocular pressure was measured. In addition, the corneal hysteresis (CH) and corneal resistance factor (CRF) were determined.

RESULTS

The mean preoperative spherical equivalent refraction was -3.14 D ± 1.41 D (SE) in the stability group and - 6.47 D ± 1.40 D (p = 0.001)in the regression group. Also, the postoperative spherical equivalents were statistically significant different (p < 0.05). In contrast, the mean preoperative corneal thickness showed no differences in both groups (p = 0.96) (stability group 563 ± 36 µm and regression group 563 ± 28 µm).

CONCLUSIONS

The aim of the study to detect a possible causal relationship between myopia regression after LASIK and the biomechanical properties of the cornea and corneal thickness could not be clearly identified.

Interlamellar cohesion after corneal crosslinking using riboflavin and ultraviolet A light

AIMS

Collagen crosslinking treatment of progressive keratoconus using the photosensitiser riboflavin and ultraviolet A light of 370 nm wavelength has been shown to increase significantly the tensile strength of corneal collagen by about 300%. In keratoconus, interlamellar and interfibrillar slippage have been proposed as pathogenetic mechanisms. Therefore, the aim of this study was to assess the impact of collagen crosslinking on the interlamellar cohesive force.

METHODS

72 post mortem porcine eyes were divided into six different treatment groups: the untreated control group, the standard crosslinking group, the hypo-osmolar crosslinking group, the stromal swelling group, the formaldehyde group and the α-amylase group. An anterior 9×4 mm strip of 400 μm thickness was prepared using a lamellar rotating microkeratome. For interlamellar cohesive force measurements a splitting plane was created at 50% depth. Force-distance profiles were recorded using a microcomputer-controlled biomaterial testing machine.

RESULTS

The mean interlamellar cohesive force was 0.24 N/mm in the untreated control group, 0.26 N/mm in the standard crosslinking group, 0.25 N/mm in the hypo-osmolar crosslinking group, 0.23 N/mm in hydrated corneas, 0.27 N/mm in the formaldehyde group without statistically significant difference. Only the values of the α-amylase group were statistically significantly lowered by 31.5% to 0.16 N/mm.

CONCLUSIONS

Surprisingly, corneal crosslinking does not increase the interlamellar cohesive force. In the α-amylase group the cohesive force was mainly decreased because of the digestion of proteoglycans. Crosslinking seems to stabilise only inter- and intrafibrillar, but not interlamellar cohesion.

[Collagen cross-linking with riboflavin and UVA light]

PURPOSE

Since 1998 collagen cross-linking is used for patients with progressive keratoconus. Today it can almost be considered as the standard therapy.

METHODS

Patients with progressive keratoconus and minimal corneal thickness of 400 µm have been cross-linked within a study - (EK 310 499) approved by the ethical committee since 1998. An increase of the maximum K-value by ≥ 1 D within the last year, patient's statement of deteriorating visual acuity or the necessity of new contact len fitting more than once in 2 years were considered as progression.

RESULTS

The analysis includes 153 eyes of 111 patients with a minimal follow-up of 12 months and a maximum follow-up of 6 years. The keratectasia significantly decreased in the 1 (st) year by 2.28 D. The visual acuity improved significantly by at least one line or, respectively, remained stable (i. e., no line loss) in the 1st year in 73 %. The results remained stable over the next two years. Despite the low number of patients with a follow-up longer than 3 years and therefore limited statistical statement power, our results still indicate a long-term stabilisation or, respectively, improvement after collagen cross-linking. We saw no severe side-effects. Three patients with an exacerbated neurodermitis showed continuous progression of keratoconus and were cross-linked again.

CONCLUSIONS

To date there have been numerous promising publications on collagen cross-linking for keratoconus. The results of this study indicate that collagen cross-linking appears to be an effective therapeutic option for progressing keratoconus. Besides the clinical there are enormous economical and psychosocial benefits. Cross-linking is an out-patient, minimally-invasive, cost-effective treatment with minimal strain for the persons concerned.

[Biomechanical condition of the cornea as a new indicator for pathological and structural changes]

AIM

Several methods permit the measurement of geometric parameters of the cornea, but until now biomechanical conditions of the cornea have been ignored (e.g. in refractive corneal surgery). Besides the geometric condition, biomechanical properties of the cornea have been shown to influence applanation measurement of intra-ocular pressure (IOP) and epidemiological studies have identified corneal thickness as an independent risk factor for the development and progression of glaucoma. The aim of this investigation was to characterize the biomechanical properties of the cornea using the ocular response analyzer (ORA).

METHODS

The ocular response analyzer (ORA) is a new method available for non-contact measurement of the biomechanical properties of the cornea. We evaluated the reproducibility of measurements, the difference between static and dynamic factors and the impact of independent factors (e.g. IOP, age, CCT, swelling of the cornea) on 2,500 measurements of corneal hysteresis (CH) and corneal resistance factor (CRF).

RESULTS

In a large sample size we observed changes in CH and CRF after refractive surgery procedures (LASIK, UV-A cross-linking, keratoplasty) and in other corneal disorders (keratoconus, corneal dystrophies).

CONCLUSIONS

CRF and CH changes may reflect structural changes of the cornea. Thus, the ORA provides valuable information for a better understanding and characterization of the biomechanical condition of the cornea, especially with regard to diseases such as keratoconus and glaucoma.

[Measurement of the disc area by indirect ophthalmoscopy]

PURPOSE

The aim of this study was to assess the accuracy of measuring the optic disc area by indirect ophthalmoscopy.

PATIENTS AND METHODS

In a prospective clinical trial, 57 eyes of 29 subjects (age 57.3+/-12.1 years) were examined. The refractive error was -0.67+/-2.69D (+3.75D to -8D).The vertical and horizontal disc diameters (DD) were measured using a Haag-Streit slit lamp and 60D-, 78D-, 90D-, and Super Field lenses (Volk Optical, Mentor, USA). Afterwards the disc area was calculated by an ellipse formula (horizontal DD x vertical DD x pi/4). The magnification factor given by the manufacturer was taken into account for each lens: 1.15x (60D lens), 0.93x (78D lens), 0.76x (90D lens), and 0.76x (Super Field lens), respectively. As reference for the disc size, the same eyes were examined by HRT II (Heidelberg Engineering, Heidelberg, Germany). Bland-Altman plots were used to assess the agreement between measurements obtained by indirect ophthalmoscopy and HRT.

RESULTS

The results of the disc estimate compared with the HRT measurements were as follows: 0.119+/-0.51 mm(2) (60D lens), 0.224+/-0.57 mm(2) (78D lens), 0.10+/-0.51 mm(2) (90D lens), and -0.07 s+/-0.47 mm(2) (Super Field lens). Differences were not statistically significant for the 60D (p=0.083), 90D (p=0.147), or Super Field lenses (p=0.257). However, the difference between the 78D lens and HRT was statistically significant (Student's t-test; P=0.004).

CONCLUSION

Measuring the disc size by indirect ophthalmoscopy is possible. The 90D lens showed the smallest and the 78D lens the largest deviation.

[Corneal aberrations before and after corneal and corneoscleral small incision cataract surgery]

BACKGROUND

The corneal as well as the corneoscleral incisions are considered to be standard in cataract surgery. However, both incision techniques have their advantages and disadvantages. We investigated whether the way of performing the incisions has different effects on the corneal wavefront, especially on higher-order aberrations.

METHODS

Each of 26 patients (6 men, 20 women), aged between 62 and 85 years (average 74.9 +/- 6.69 years), received a standard phakoemulsification by performing a corneal incision on the right eye (group 1) and a corneoscleral incision on the left eye (group 2). The corneal surface was preoperatively investigated by using the corneal wavefront instrument Optikon Keratron Scout, (Schwind Company, Germany). After 6 to 8 weeks the same parameters were measured again.

RESULTS

The statistical analysis of the pre- and postoperative data within the same group indicated significant differences (p < 0.05) in group 1 for Z (3 +/- 3) = Trefoil and in group 2 for Z (2 +/- 2) = Astigmatism. On comparing the postoperative data of the corneal incision with those of the corneoscleral incision, there was only a significant difference (p < 0.05) for Z (3 +/- 3) = Trefoil.

CONCLUSION

In accordance with the induced higher-order aberrations both surgical techniques can be considered as being equivalent.

Statistische Verfahren zum Vergleich zweier Messmethoden und zur Kalibrierung: Konkordanz-, Korrelations- und Regressionsanalyse am Beispiel der Augeninnendruckmessung

BACKGROUND

The comparison of two or more quantitative measurement procedures applied to the same patient is often carried out by unsuitable statistical methods. An orientation for the choice of methods has been given for such problems.

METHODS

Graphical methods, the concordance correlation coefficient and some regression straight lines are presented for the judgement of concordance of measurement values, and they are exemplary explained for the measurement of the intraocular pressure with lid tonometry and the ocular blood flow system (OBF).

RESULTS

The t-test for paired samples and tests of equivalence are invalid in this context. The Pearson correlation coefficient measures to what degree two variables are associated over a linear relation with reference to the population. It ignores location shift as well as scale shift, and it says nothing about the intraindividual concordance. A suitable procedure is the concordance correlation coefficient which can decomposed into precision, location shift, and scale shift.

CONCLUSIONS

An assessment of concordance of intraindividual measurement values should include an x-y-graphic of the measured points, a Bland-Altman plot, and the concordance correlation coefficient.

Augeninnendruckmessungen im Tages- und Nachtverlauf bei Glaukompatienten und Normalprobanden mittels Goldmann- und Perkins-Applanationstonometrie

OBJECTIVE

Our aim was to evaluate intraocular pressure (IOP) levels in primary open angle glaucoma (POAG) patients and healthy controls during both the day and night while measuring in an upright as well as in a supine position.

METHODS

In a prospective clinical trial, 30 glaucoma patients on topical treatment and 50 healthy controls received IOP measurements every 4 h for a 24 h period starting at 8 am. Additionally, blood pressure and heart rate were measured and perfusion pressures were calculated. At 12 am IOP was initially measured in a sitting position and then, after 20 min, in a supine position. At midnight this was carried out conversely. At 4 am IOP was measured in a supine position; all other measurements were performed in a sitting position. Measurements in the sitting position were performed by Goldmann and Perkins tonometry and in a supine position by Perkins tonometry.

RESULTS

IOP was 1 mmHg lower in Perkins tonometry measurements compared to Goldmann tonometry. There was no difference between the two patient groups. In a supine position, IOP measured by Perkins tonometry was higher than in an upright position. At 12 am the difference was 1.8 mmHg+/-2.7 mmHg (p=0.001) in healthy subjects and 1.3+/-2.7 mmHg (p=0.013) in the POAG patients. At 12 pm the increase of IOP in the supine position was even more pronounced with 2.4+/-3.4 mmHg in healthy subjects and 5.6+/-3.2 mmHg in the POAG patients (p=0.001). The blood pressure and the perfusion pressure were lowest during night measurements.

CONCLUSIONS

During diurnal IOP measurements in an upright position there were no statistically significant differences in IOP changes between groups. However, in a supine position IOP was significantly higher than in a sitting position and increased more in the glaucoma patients than in healthy controls. This observation might be due to a faulty regulation of the fluid shift in glaucoma patients and could cause progression of glaucomatous damage.

[Applanation tonometry in "normal" patients and patients after LASIK]

BACKGROUND

Until now it was thought that morphological parameters of the eye such as corneal thickness, corneal curvature and axial length do not affect tonometry results. However, the aim of this study was to find out whether there actually is an influence of these parameters on applanation tonometry.

PATIENTS AND METHOD

In this prospective study we examined 125 eyes of 125 normal patients with a corneal thickness of 568.8 +/- 43.79 microm, a corneal curvature of 7.72 +/- 0.27 mm and an axial length of 23.62 +/- 2.05 mm. Before performing a phacoemulsification, the anterior chamber was temporarily punctured. With a closed system the intraocular pressure (IOP) was manometrically set at 20, 35 and 50 mmHg using an H (2)O column. The IOP was then measured with a Perkins tonometer. With these patients we compared 102 eyes that had undergone LASIK due to a myopia of 6.3 +/- 2.17 D. Before and 6 months after surgery, IOD, k-values and central corneal thickness of these patients were measured.

RESULTS

At all set pressure levels there was a highly significant correlation of measured IOP and corneal thickness. At all set pressure levels the measured IOP significantly depended on corneal thickness (r(2) = 0.78 - 0.83). After LASIK, IOP was reduced from 16.5 +/- 2.1 to 12.9 +/- 1.9 mmHg. There was a significant correlation between IOP and corneal curvature as well as corneal thickness (r(2) = 0.631; P < 0.001). The biomechanical characteristics of the cornea are changed so that the measured IOP has to be corrected by an additional 0.75 mmHg.

CONCLUSION

Since corneal thickness does affect Goldmann applanation tonometry we recommend to use the "Dresden Correction Table" (Tab. ) to achieve the real IOP. Pressure measurements after LASIK are inaccurate because of a change in corneal biomechanics, corneal thickness and curvature and they should be corrected as follows: IOP (real) = IOP (measured) + (540 - CCT)/71 + (43 - K-value)/2.7 + 0.75 mmHg.

Biomechanical changes in the anterior lens capsule after trypan blue staining

PURPOSE

To examine the effect of trypan blue staining on the biomechanical behavior of the porcine anterior lens capsule.

SETTING

Department of Ophthalmology, Technical University of Dresden, Dresden, Germany.

METHODS

Fifty-five anterior lens capsules from porcine cadaver eyes were used. Two parallel 8.0 mm x 4.0 mm large capsule strips were prepared from each capsule. After trypan blue staining for various time intervals combined with exposure to white light (6000 lux) or with no light exposure, biomechanical stress-strain measurements were performed using an automated material tester. Untreated specimens and specimens treated with glutaraldehyde 0.1% were used as controls. The absorption spectrum of trypan blue 0.1% solution and the emission spectrum of the light source were measured.

RESULTS

After treatment with light and trypan blue, at 25% strain, there was a statistically significant increase in stress of up to 70.1% and in elastic stiffness of 47% and a decrease in the ultimate mechanical strain of up to 13%. There were no biomechanical changes in capsules with trypan blue staining in the absence of light or after a short illumination time of 30 seconds, indicating a light-dependent process. After 30 minutes of glutaraldehyde 0.1% treatment, there was an increase in stress of 321.6% at 25% strain and a decrease in the ultimate strain of 47.6%. The emission spectrum of the light source included the absorption peak for trypan blue at 580 nm.

CONCLUSIONS

Trypan blue staining of the lens capsule combined with light irradiation for at least 1 minute led to an increase in elastic stiffness at 25% strain and a reduction in the ultimate extensibility. This effect is probably due to the photosensitizing action of trypan blue, leading to light-induced collagen crosslinking of the capsule collagen similar to age-related crosslinking. Nucleus expression might be impeded by the increased capsule stiffness. Continuous curvilinear capsulorhexis is facilitated.

Corneal Endothelial Cytotoxicity of Riboflavin/UVA Treatment in vitro

Recently, we have developed collagen crosslinking induced by combined riboflavin/UVA treatment, thus increasing the biomechanical rigidity of the cornea to treat progressive keratoconus. The present safety study was performed to evaluate possible cytotoxic effects of combined riboflavin/UVA treatment on the corneal endothelium in vitro. Endothelial cell cultures from porcine corneas were treated with 500 microM riboflavin solution, exposed to various endothelial UVA irradiances (370 nm) ranging from 0.1 to 1.6 mW/cm2 for 30 min and evaluated 24 h later using trypan blue staining and Yopro fluorescence staining. The effect of either treatment alone (UVA irradiation ranging from 0.2 to 6 mW/cm2) was also tested. An abrupt cytotoxic threshold irradiance level was found at 0.35 mW/cm2 after combined treatment with riboflavin plus UVA irradiation and at 4 mW/cm2 with UVA irradiation alone. Riboflavin alone was not toxic. A cytotoxic effect of the combined riboflavin/UVA treatment on corneal endothelial cells is to be expected with a corneal thickness of less than 400 microm. Therefore, pachymetry should be routinely performed before riboflavin/UVA treatment to exclude patients at risk.

[Treatment of keratoconus by collagen cross linking]

BACKGROUND

We were able to show a significant increase in corneal stiffness of rabbit and porcine eyes after combined riboflavin/UVA-induced collagen cross-linking. In this study,we tried to treat keratoconus patients with this method to stop the progression of corneal ectasia.

PATIENTS AND METHODS

We treated 16 eyes of 15 patients with progressive keratoconus and mostly moderate keratectasia (48-56 dpt). After removal of the epithelium (7 mm X), riboflavin solution was applied on the cornea, which was irradiated with UVA (370 nm,3 mW/cm(2)) at a distance of 1 cm for 30 min.Post-operative follow-up controls were conducted every 3 months in the first year and then every 6 months, always including visual acuity testing, corneal topography and measurements of endothelial cell density. The follow-up time was between 1 and 3 years.

RESULTS

Progression of keratectasia was stopped in all patients. Best corrected visual acuity and the maximal keratometry values improved slightly in about 50% of the cases. In all patients corneal transparency, the degree of keratectasia registered by corneal topography and the density of endothelial cells remained unchanged within the follow-up time. No negative side-effects were observed.

CONCLUSIONS

Our results show that collagen cross linking might be a useful conservative treatment modality to stop the progression of keratoconus. By this means the need for keratoplasty might be significantly reduced. Given the simplicity of the technique and minimal costs of the treatment it might also be well suited for developing countries.Further studies are envisaged to exclude long-term side effects and to evaluate the long term durability of the mechanical stiffness effect.

[Irradiation of cornea with ultraviolet light and riboflavin administration as a new treatment for erosive corneal processes, preliminary results in four patients]

BACKGROUND

Corneal melting is a serious condition and in many cases maybe managed only by surgery. UV-radiation with Riboflavin is a new method that may stop the melting process of the cornea.

PATIENTS AND METHOD

Four patients suffering from melting ulcera of the cornea of various origin underwent a single UV-radiation. Two ultraviolet diodes were used emitting light with an wavelength of 370 nm with an energy of 2.5 mW/cm2. The radiation time was 30 minutes. Before he radiation was performed 2-3 drops of 0.11% Riboflavinsolution were applied to the cornea. The treatment area was 6-7 mm in diameter.

RESULTS

After the treatment in three of the four patients the melting process of the cornea stopped. At least temporarily, a surgical procedure could be delayed.

CONCLUSION

Because of the absence of any side-effects in serious cases the method is recommended prior to surgery.

[Studies on the stabilization of the cornea in rabbits]

BACKGROUND

The mechanical stabilization of the cornea in keratoconus may delay progression of this disease. The cross-linking techniques optimized in corneas of enucleated porcine eyes were investigated under in vivo conditions in rabbits to estimate the biocompatibility and duration of the stiffening effect.

METHODS

Twenty-eight rabbits were treated monocularly, the fellow eye serving as control. The epithelium was mechanically removed and 19 eyes were treated with riboflavin plus ultraviolet irradiation (365 nm, 2 mW/cm2) for 45 min and 9 eyes with 0.075% glutaraldehyde for 20 min. After treatment, the eyelids were sutured for 3 days. The healing process was controlled by slit-lamp examination and photographically documented. After 1 month, 20 animals and after 3 months 8 animals were sacrificed, the eyes enucleated, and the stress-strain relation of the corneas measured and compared to the fellow eye.

RESULTS

The epithelium was closed after 4-5 days. The transparency of the corneas remained clear during follow-up, and there were no signs of inflammatory reaction. Stress for a strain of 6% was higher in the treated corneas by a factor of 1.3 +/- 0.66 (P = 0.319) in the glutaraldehyde group and by a factor of 1.6 +/- 0.75 (P = 0.0408) in the riboflavin group at 1 month, and by 1.3 +/- 0.48 (P = 0.07) at 3 months after treatment.

CONCLUSIONS

The cross-linking technique using riboflavin plus UV irradiation is suitable for at least temporarily stiffening the cornea in vivo and seems to be a promising method for conservative treatment of keratectasia.

[Increased rigidity of the cornea caused by intrastromal cross-linking]

PURPOSE

To increase the stability of the cornea by artificial cross-linking (radiation or chemical agents) and to investigate a future therapy for keratoconus.

MATERIALS AND METHODS

The epithelium of enucleated porcine eyes was removed. Ten eyes in each of eight test groups were treated with UV light (lambda = 254 nm), 0.5% riboflavin and UV light (365 nm), blue light (436 nm) and sunlight, and the chemical agents glutaraldehyde (1% and 0.1%, 10 min) and Karnovsky's solution (0.1%, 10 min). Strips of 5 mm in width and 9 mm in length were cut from each cornea and the stress-strain behaviour of the strips was measured. For comparison, eight groups of ten untreated corneas each were measured by the same method.

RESULTS

Compared to untreated corneas riboflavin and UV irradiation as well as glutaraldehyde and Karnovsky's solution treatment resulted in significantly increased stiffness of the cornea (p < 0.05).

CONCLUSIONS

The biomechanical behaviour of the cornea can be altered by low-concentration glutaraldehyde, Karnovsky's solution, and by riboflavin and UV irradiation, which offers potential conservative treatment of keratoconus. To optimize this effect further investigation is necessary regarding the dose-effect relation and the in-vivo conditions.

[Laser-induced pressure waves in the eye. Propagation characteristics]

BACKGROUND

The characteristics of shock waves during photoablation were investigated for an IR and a UV laser. These stress waves may be harmful to ocular structures.

MATERIAL AND METHODS

The amplitude of shock waves was measured by a needle-shaped hydrophone in enucleated porcine eyes during excimer laser (193 nm, 23 ns, diameter of ablation 1.5-7.5 mm) and Er:YAG laser photoablation (2.94 microns, 200 microseconds, 1.2 mJ/cm2, diameter of ablation 4 mm).

RESULTS

With the excimer laser at ablation zones larger than 4.5 mm, a pressure focus occurs at a distance of 4-6 mm behind the cornea. The pressure amplitudes are smaller than 80 bar for a fluence of 180 mJ/cm2 and decrease steadily to values below 10 bar towards the retinal level. Higher fluences produce higher pressure values; in the range of 60 to 220 mJ/cm2 the relation is linear. For the Er:YAG laser, pressure amplitudes are smaller than 0.5 bar.

CONCLUSIONS

Mechanical damage of the retina is unlikely during excimer-or Er:YAG-laser ablation. The existence of a pressure focus may result in mechanical damages of the posterior lens or anterior vitreous at large ablation diameters. During Er:YAG laser ablation, shock waves could not be detected with our measurements. Theoretical estimations yield values of less than 700 mbar at a fluence of 1.2 J/cm2. The pressure load of the endothelium is independent of diameter but dependent on fluence.

Thermomechanical behavior of the cornea

Shrinkage of corneal collagen is used during thermokeratoplasty, a method to remodel the corneal curvature. The goal of our investigations was to determine the optimal temperature range for maximal shrinkage of the collagen fibers with minimal damage. By means of a commercially available stress-strain-measuring device including a paraffin oil bath at temperatures varying from 35 degrees to 120 degrees C, strips of pig cornea measuring 5 mm in width and 9 mm in length were investigated in the stress range of sigma = (0.5-12.5) x10(4) N/m2 by stress-strain, stress-relaxation, and creep measurements. The biomechanical properties of the cornea remained unchanged in the temperature range of 30-50 degrees C. Starting at 60 degrees C, shrinkage occurred that increased up to a temperature of 90 degrees C. The maximal rate of shrinkage of 57 +/- 12% was determined at temperatures of 75 degrees and 80 degrees C. At above 100 degrees C the shrinkage effect decreased because of the destruction of intermolecular bonds between the collagen fibers. The stress-strain curves generated for shrunken corneas were flatter than those generated for native corneas, which means that Young's modulus is significantly reduced. For the achievement of optimal shrinkage during thermokeratoplasty, temperatures of 70-85 degrees C should be reached in the tissue to be coagulated. Higher temperatures are capable of causing a shrinkage effect but also induce the destruction of tissue.

[Thermo-mechanical behavior the the cornea]

BACKGROUND

Shrinkage of corneal collagen is used during thermokeratoplasty, a method to remodel the corneal curvature. The goals of our investigations were to determine the optimal temperature range for maximal shrinkage of the collagen fibers with minimal damage.

MATERIALS AND METHODS

By means of a commercially available stress-strain-measuring device with a paraffin oil bath of temperatures varying from 35 degrees C to 120 degrees C strips of pig cornea 5 mm in width and 9 mm in length were investigated in the physiological stress range from sigma = (0,5-12,5) . 10(4) N/m2 by stress-strain, stress relaxation and creep measurements.

RESULTS

Biomechanical properties of the cornea remain unchanged in the temperature range from 30 to 50 degrees C. Starting at 60 degrees C shrinkage occurs that increases up to 90 degrees C. The maximal rate of shrinkage of (57 +/- 12)% was measured at temperatures of 75 to 80 degrees C. Above 100 degrees C this effect is reduced by the destruction of intermolecular bonds between the collagen fibers. The stress-strain curves of the shrunk corneas are flatter than that of native corneas, which means, the Young's modulus is significantly reduced.

CONCLUSIONS

In order to realize optimal shrinkage during thermokeratoplasty temperatures of 65-85 degrees C should be achieved in the coagulated tissue. Higher temperatures cause also a shrinkage effect but also a destruction of tissue.

[Cryotherapy of malignant eyelid tumors]

The treatment of tumors of the eyelids must extend not only to complete removal or destruction of the neoplasm, but also to the maintenance of physiology functions. In addition, cosmetic aspects should be taken into consideration. In a prospective study, 278 cryosurgical operations performed for lid tumors in the years from 1980 to 1985 were followed up for 5 years to decide whether cryotherapy fulfilled these demands. We used spray freezing with liquid nitrogen, because the low temperatures and high freezing rates required for cell destruction are achieved only by this technique. The 5-year recurrence rate was 3.5%. The liquid nitrogen therapy is especially suitable when the eye lids are operated on, because the lacrimal system remains functional. Gas expansion cryotherapy should not be used in in the treatment of tumors, as it may lead to stimulation of growth.

[Combined cryocoagulation of ciliary body and retina in neovascular glaucoma. Initial experience]

Therapeutic success in neovascular glaucoma with conservative and surgical methods is sadly inadequate. Based on the encouraging results we have obtained with transconjunctival cyclocryothermy in primary glaucoma and with peripheral cryopexy (transconjunctival cryotherapy), we applied these two methods in combination to treat neovascular glaucoma.

PATIENTS

A total of 25 eyes in 23 patients were treated with the above method. The average preoperative bulbar pressure was 43 mmHg.

RESULTS

The highest average reduction of pressure was obtained in weeks 2-4 after treatment (20 mmHg); 40% of the eyes were still compensated after 4 months (below 22 mmHg), 35% showed bulbar pressures between 26-28 mmHg, and 25% showed bulbar pressures about 35 mmHg. These patients had no troublesome symptoms. In all cases, a good improvement of rubeosis iridis was observed. The combined cryotherapeutic method of treating neovascular glaucoma is efficient in controlling the internal bulbar pressure.

[Cryoenucleation in malignant melanoma of the choroid]

The works of Zimmermann, Fraunfelder et al. and Migdal offer reasons for postulating atraumatic procedures for the enucleation of malignant melanoma of the choroid. Cryosurgery represents an effective method of blocking the melanoma prior to enucleation in such a manner that an intraoperative dissemination of tumor cells is unlikely. The technology which is at present widely used for this purpose (Fraunfelder et al.) has considerable disadvantages. For examples cryoapplication is only possible with favorable, i.e., peripheral tumor localization, since the cryoapplicator cannot be introduced into the orbit. Therefore, a cup-shaped cryoapplicator chilled by liquid nitrogen has been developed which can be introduced into the orbit. The operating temperature of -196 degrees C causes the half of the eyeball in which the tumor is located to freeze within 100 seconds. Subsequently the applicator is removed and the enucleation is continued in the traditional manner. The technical suitability of this procedure has meanwhile been demonstrated in 12 enucleations.