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T Seiler

Publications and source records attributed to T Seiler.

At least 73 records · Page 4Linked to original sources
PubMed1996

[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.

Animals↗
PubMed1996

["Central islands"--an early postoperative complication after photorefractive keratectomy].

BACKGROUND: After myopic photorefractive keratectomy undercorrected zones in the center of the treated cornea (central islands (CI)) occur early postoperatively occasionally the reason for double-vision, halos and reduction of best corrected visual acuity. PATIENTS AND METHODS: Sixty-two eyes of 33 patients were treated with an 193 nm excimer laser for correction of myopia (range: -1.0 to -12.0 D) and an optical zone of 6.0 mm. The patients were selected consecutively regarding the follow-up examination which ranged from 1 month to 21 months after surgery. RESULTS: At 1 month after surgery the incidence of central islands was 72.6%. It rarely occurs after corrections of -3.0 D and less but may be detected in every case of correction of -6.0 D and more. The correlation between attempted correction and incidence of central islands is statistically significant (p < 0.005). Approximately 21% of the patients were steroid responders whose central islands appeared more marked. CONCLUSIONS: Central islands are an early postoperative complication with a not yet decided cause. Depending on attempted correction (D) and the follow-up the central islands disappear and/or did not result in reduction of visual acuity. In contrast, in patients with increased corticosteroid response the central islands persisted longer (15 months and more). Reoperations because of central islands should not be performed before 6 months after surgery. We describe a new form of central corneal iron deposit.

Adult↗
PubMed1996

Refractive corneal surgery with lasers.

Laser refractive surgery is on one hand in a phase of consolidation regarding the routine treatment of mild to moderate myopia by means of spheric excimer laser photorefractive keratectomy. On the other hand, exciting new developments are taking place in the field of high myopia, myopic astigmatism, and new alternative lasers.

Cornea↗
PubMed1996

Laser Scheimpflug videography.

Conventional Scheimpflug photography uses slits with a constant width of 80 microns. This parameter limits the resolution as sharp contours are imaged with a basic uncertainty. In order to reduce this basic uncertainty we developed an illumination slit with a width of 20 microns and less, using a green helium-neon laser (543 nm). With this slit, much sharper imaging of ocular contours can be achieved. Since this illumination is coupled to on-line videographic detection of the Scheimpflug image, any adjustment of the target is done under visual control at the monitor. Using this device, the slope of a densitogram of contrasted contours (for example the corneal surfaces) is steeper, which guarantees a more accurate detection of corneal thickness and curvature.

Cornea↗
PubMed1996

Determination of excimer laser ablation rate of the human cornea using in vivo Scheimpflug videography.

PURPOSE: To determine in vivo the amount of human corneal tissue removed by each excimer laser pulse, the so-called ablation rate, during photorefractive keratectomy (PRK). There is confusion in the literature because the experimentally determined ablation rate of 0.4 to 0.5 microns per pulse differs from the nominal ablation rate of 0.23 to 0.3 microns per pulse, which is the value used in clinical procedures. METHODS: Eleven eyes of 11 patients were treated with PRK for correction of myopia. The corneal curvature was determined by Scheimpflug videography before and immediately after surgery. Starting from this curvature change, the authors calculated the real ablation rate. RESULTS: The real ablation rate is coincident with the nominal ablation rate and differs significantly from the ablation rate derived from deep keratectomy experiments. CONCLUSIONS: The outer layers of the cornea show significantly different ablation behavior than the deeper stroma. This information has clinical relevance for the predictability of intrastromal excimer laser procedures.

Cornea↗
PubMed1996

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.

Animals↗
PubMed1995

Erbium:YAG laser photoablation of human cornea.

PURPOSE: To determine whether the healing response of the human cornea after photoablation with a midinfrared solid-state laser was more pronounced than after excimer laser keratectomies because of the increased thermal damage in adjacent tissue. METHODS: We treated five blind human eyes with an erbium:YAG (Er:YAG) laser prototype and followed up the patients for nine months. RESULTS: Regarding epithelial healing and corneal clarity, the patients had a healing response very similar to that of excimer laser. Further clinical and experimental studies are necessary to estimate healing and regression.

Aged↗
PubMed1995

[Current evaluation of myopia correction with the excimer laser].

Keratectomies with the excimer laser demonstrate extraordinarily smooth edges. Thermal and actinic side effects are negligible. Currently two techniques are in clinical use: the wide-field approach and the scanning-slit approach. A third one, the flying-spot approach is being tested in clinical studies. Postoperative refraction is mainly determined by the amount of correction. For corrections up to 6.0 D we found success rates of more than 90% and the refraction stabilizes during the first year after surgery. If the correction is higher, the success rate is decreased to 50% or less and the regression takes years. The complication rate is also dependent on the amount of correction: with corrections of more than 6.0 D it exceeds 1% and in corrections of 9.0 D and more, manifest corneal scars occur in more than 10% of the cases. Other complications are: eccentric ablation, IOP elevation due to steroid medication, and ptosis. Rheumatic diseases are absolutely contraindicated because of potential corneal melting ulcers, and subretinal neovascularization. In addition, corrections of more than 6.0 D are relatively contraindicated. More recent techniques like astigmatic PRK, LASIK, and hyperopic PRK are discussed.

Follow-Up Studies↗
PubMed1995

[Scar formation in the cornea after photorefractive keratectomy].

The morphologic correlate of the so-called subepithelial haze after photorefraktive keratectomy (PRK) has not yet been decided. Excimer laser keratectomy 100 microns in depth was performed in an eye that was scheduled for enucleation. Three months after surgery a dense scar was diagnosed with the slit lamp. After enucleation the cornea was investigated by light microscopy. The thickness of the scar was less than 1 micron in most sections with a maximum of 5 microns. This surprising finding explains the predominantly scattering power of the subepithelial haze, which results in a relatively good visual acuity of 20/30 to 20/25 in most clinical cases.

Aged↗
PubMed1995

Dynamic mechanical spectroscopy of the cornea for measurement of its viscoelastic properties in vitro.

Cylindric samples (3 mm in diameter) of explanted human corneas were biomechanically characterized in the frequency range from 0.1 mHz to 100 Hz with a specially modified viscoelasticity spectrometer. Such spectra of the shear compliance J (the reciprocal value of the shear modulus G) were measured as a function of corneal hydration and temperature. Variation of the hydration from 0.20- to 1.00-mm sample thickness (5-fold) changed the shear compliance 600-fold. Such a strong effect means that the shear-compliance spectra are highly sensitive to changes in the biomechanical properties of the cornea. This is demonstrated by three examples. A myopic cornea (-3D) was significantly softer (by a factor of 7 at lower frequencies and a factor of 4 at higher frequencies) than an emmetropic cornea. An increasing post mortem interval decreased the shear-compliance values (stiffening) obtained at higher frequencies (by a factor of 0.7 per day), whereas the values obtained at lower frequencies were reproduced. The biomechanics of thermal coagulation were studied in detail. The temperature was increased step by step, and at 48 degrees C a first irreversible decrease in the compliance was recorded at lower frequencies (0.8-fold). A further stiffening by a factor of 1/6 at lower frequencies and 1/5 at higher frequencies ended at 64 degrees C with a minimum in the compliance. To provide a new diagnostic tool, the biomechanical spectra of the cornea have to be measured in vivo. Equipment suitable for the megahertz range has been developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical Phenomena↗
PubMed1995

[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.

Aged↗
PubMed1994

Complications of myopic photorefractive keratectomy with the excimer laser.

BACKGROUND: Although many thousands of myopic eyes have been operated on by excimer laser photorefractive keratectomy (PRK), the safety of this procedure is still a concern. METHODS: The results and complications of PRK have been studied for up to 2 years in a prospective trial including 193 eyes in 146 patients. In addition, specific complications of PRK are described that occurred in patients outside the prospective study. RESULTS: Two eyes (1.2%) lost two lines of best-spectacle-corrected visual acuity 1 year after surgery, but at 2 years one of these eyes had regained baseline visual acuity. At 1 year, 12 eyes (7.1%) had lost more than two lines of visual acuity under glare conditions. Significant complications such as manifest scarring, overcorrection, undercorrection, and continued regression are dependent on attempted refraction. Eccentric ablations with resultant induced astigmatism are rare and the incidence is dependent on the experience of the surgeon. Progressive hyperopia did not occur. CONCLUSION: Except in corrections greater than 6 diopters, complications after PRK are rare. Assuming an appropriate patient selection, PRK may be considered a relatively safe procedure compared with other refractive procedures.

Adult↗
PubMed1994

[Doppler echocardiography assessment of hemodynamic values and additional heart defects in atrial septal defects].

Due to the rapid progress in (Doppler-) echocardiography "one must question whether cardiac catheterisation remains a necessary prelude to cardiac surgery in atrial septal defects" (8). Although the estimation of the magnitude of the intracardiac shunt and the anatomical size of the defect ist possible by (Doppler-) echocardiography there remains the problem of associated disorders (e.g. anomalies of the pulmonary venous connection) which may not be detected by transthoracic echocardiography. In 25 children submitted to cardiac catheterisation studies in the period between 1990 and 1992, the anatomical size of an atrial septal defect was measured echocardiographically. The haemodynamic parameters Qp/Qs and Rp/Rs were employed to calculate the so-called effective resistance of the defect (Rd/Rs), which was derived from an electrical analogue and represents the ratio of the resistance of the defect (Rd) to the systemic vascular resistance (Rs). The echocardiographically measured anatomical size (expressed as the ratio of the area of the defect to the cross-sectional area of the ascending aorta) was related to the effective resistance of the defect. A significant (non-linear) correlation was found between Rd/Rs and the anatomical size of defect. Based on these data we developed a nomogram describing the relationship of the size of the defect to the haemodynamic parameters (Qp/Qs and Rp/Rs). If the data of a patient do not comply with this nomogram there is strong evidence of an additional cardiovascular malformation, necessitating further evaluation. A second cohort of four patients with associated defects (partial and total anomalous pulmonary venous connection and a corresponding sinus venosus defect) was clearly identified by the nomogram method.

Adolescent↗
PubMed1994

[Moist ablation of the corneal surface with the Er:YAG laser. Results of optimizing ablation].

The Er:YAG laser, emitting light at 2.94 microns, may be an alternative to the 193 nm excimer laser for photorefractive keratectomy. Compared to the excimer laser, the ablation rate is very high. Surface roughness is also more pronounced than for the excimer laser. Using a precorneal liquid film, these two factors can be reduced, as shown in ablation experiments performed on porcine corneas. Thermal damage of the remaining corneal tissue is another side effect. There is no significant decrease in the amount of thermal damage with this new technique,--not even when the pulse length is reduced.

Animals↗
PubMed1994

[Standardized wound construction for no-stitch cataract surgery with maximal incision to 11 mm. Experimental and clinical results].

We use the trapezoidal lamellar incision as standard construction for sutureless self-sealing wound closure in our clinic: a 7 mm incision for phacoemulsification and implantation of a 6.5 mm optic diameter PMMA posterior chamber less (PCL) and an incision up to 11 mm for ECCE with standard PCL. The operation can be performed in a nearly closed system because of the self-sealing wound construction. Experiments in cadaver eyes showed that the wound closure of a 7 mm incision with the no-stitch technique ruptured at a pressure five times greater than the cross-stitch-sutured corneoscleral incision and in an 11 mm incision four times greater (602 +/- 149 mmHg and 150 +/- 29 mmHg). Due to the high stability of the wound closure, no sutures are necessary. Complications arising in connection with sutures are therefore avoided. Clinically, two typical postoperative complications were observed: hypotony occurred in 1% and anterior chamber hemorrhage in 5%. These complications are exclusively caused by imperfection of the internal corneal opening.

Cataract Extraction↗
PubMed1994

[Effect of incision form and tunnel length on induced astigmatism with the no-stitch technique].

The frown incision with horizontal suture can induce minimal astigmatism. This method is also currently used with the no-stitch technique. We assess whether it is better than other incisions. Two prospective studies were conducted, altogether including 95 operations in 95 patients. Entry criterion was preoperative astigmatism < 0.75 D. Astigmatism was assessed at 1 week and 4 weeks postoperatively. First study: induced astigmatism with three incision types--frown (A), horizontal arc (B) and limbus-based (C). Second study: induced astigmatism with trapezoid incision--2-mm or 4-mm tunnel. The induced astigmatism did not differ significantly in the three incision types: A 0.82, B 0.67, C 0.8 D. Using the trapezoid incision with 2-mm and 4-mm tunnel the induced astigmatism was 0.3 and 0.38 D, respectively. This difference was also not significant. Thus, neither incision type nor preparation of a longer tunnel has any influence on postoperative induced astigmatism.

Astigmatism↗
PubMed1993

Effective spherical aberration of the cornea as a quantitative descriptor in corneal topography.

Following excimer laser photorefractive keratectomy and other refractive surgical procedures, complaints of halos, glare, and monocular diplopia are common. These procedures increase the asphericity of the cornea, which may cause the optical distortions. We used ray tracing techniques to estimate the longitudinal monochromatic aberration of the cornea from the measured corneal topography (effective spherical aberration) in 15 normal eyes with varying degrees of astigmatism and in ten eyes after photorefractive keratectomy. Best spherical corrected visual acuity in the astigmatic eyes was highly correlated with effective spherical aberration (r = -0.9527, P < .001). In the eyes that had photorefractive keratectomy, the effective spherical aberration was highly correlated with measured glare visual acuity (r = 0.875, P < .002). These results suggest that effective spherical aberration is a valuable topographic measure that provides information about the optical performance of aspheric corneas.

Astigmatism↗