[Hydrodynamics of aqueous humor in chronic simple glaucoma : Mechanisms of pressure normalization by an artificial outflow system]. / Fluiddynamik des Kammerwassers beim chronischen einfachen Glaukom : Mechanismen der Drucknormalisierung durch ein künstliches Abflusssystem.

To help elucidate the interplay of physical forces, in particular pressure and flow, controlling the distribution and absorption of aqueous humor in subconjunctival tissue, a recently published computational model was considered where the fluid production in the eye, its removal via the trabecular/uveoscleral pathways and a surgical pathway are taken into account. The target quantity is the intraocular pressure. The surgical outflow pathway is linked to a fluid bleb that is positioned below the subconjunctival tissue and is modeled as a porous medium. The computational study was conducted on the basis of the geometry and relevant parameters characterizing fluid production, the surgically formed fluid pathway as well as absorption by the subconjunctival vessels, the hydraulic and geometrical characteristics of the bleb and the outflow facility. Clinical observations can be physically interpreted on the basis of parametric studies.

Cortical mechanisms of normal and abnormal processing in the visual system, Part 1. Spatial vision, amblyopia, hyperacuity, modal assumptions: a review.

Vernier thresholds rise much more rapidly in the periphery than do grating thresholds. A similar dissociation between acuity and hyperacuity has been shown to be present in strabismic but not in anisometropic amblyopia, thus leading to the suggestion that the strabismic fovea is similar to the normal periphery. Here it is shown that a quantitative model for spatial pattern discrimination, which accounts for foveal hyperacuity data, can be extended to encompass both the periphery and amblyopia if appropriate alterations are made. For the periphery it is necessary to increase the size of model receptive fields and to introduce both spatial undersampling and position irregularity (i.e. irregularity in the location of cortical filters). The strabismic fovea also requires spatial undersampling and position irregularity but no change in receptive field size. Defects in the good eye of strabismus can be explained by spatial irregularity. Finally, the anisometropic fovea requires a reduction in mechanism sensitivity but neither spatial untersampling nor position irregularity.

Some aspects of wave aberrations of the human eye and supervision: a review.

Supervision is defined by a visual acuity of 20/10 or 20/8 and may be attained by custom-correcting the aberrations of higher order of the human eye. Higher order aberrations are those aberrations which are left in the eye after having corrected lower order aberrations, i.e., defocus (myopia, hypermetropia) regular astigmatism, and which can be corrected by ordinary spectacle lenses or contact lenses. Higher order aberrations are found to a higher or lesser degree in normal or pathological human eyes and in eyes having undergone conventional corneal surgery. According to custom keratorefractive surgery limits, given by the neural visual apparatus and the receptor mosaic, supervision (i.e., 20/10 or even 20/8) may be attained. A number of dedicated sensors have been developed in recent years that are able to detect and measure aberrations of the wave front which is a sensitive procedure for the determination and surgical control of the optical quality of the eye. Not every custom keratorefractive procedure results in supervision, however. This is because not every "normal" eye is able to reach such limits because of its basic design (anatomy or function) and also because keratorefractive procedures neglect the plastic behaviour of the cornea. The plasticity of the central neural system may furthermore interact with corrected or non-corrected visual function.

[Supernormal sight and high-resolution retinal imaging: a review]. / Supernormales Sehen und hochauflösende retinale Bildgebung -- Eine Ubersicht.

BACKGROUND: An analysis of the wavefront in human eyes has yielded new findings with respect to the cones and other elements of the retina. But the correction of higher-order wavefront aberrations in photorefractive processes is still associated with basic difficulties. MATERIALS AND METHODS: The world literature relating to wavefront corrections in experimental research and the correction of photorefractive aberrations in the human eye is surveyed. RESULTS AND CONCLUSIONS: The correction of higher-order aberrations has permitted a vast improvement in the resolution of the cone-pattern picture existing in vivo. As a consequence, new findings with regard to colour physiology have been forthcoming. On the other hand, the application of wavefront corrections in photorefractive procedures has been only partially successful.

Transpupillar laser phototherapy for retinal and choroidal tumors: a rational approach.

BACKGROUND: The physical laws are considered that need to be taken into account for optimal photothermal treatment of solid and vascular tumors, as well as other vascular anomalies of various etiology of the retina and choroid. Optimal irradiation therapy should take into account the distribution of both radiant and thermal energy in tumors, such as retinoblastomas, malignant melanomas and vascular malformations. Strict confinement of the extent of photothermal damage is critical since such pathological entities are frequently located close to the macula or optic nerve head. METHODS: A formal treatment of the optical quantities related to these requirements is presented. In this analysis we emphasize the following topics: Arrhenius' law, the kinetics of protein denaturation, the electromagnetic radiation field, wavelength, laser pulse duration (exposure time), optical properties of tissue, photocoagulation and thermotherapy. RESULTS: Generally, the conditions are best fulfilled when using radiation in the near-infrared range of the electromagnetic spectrum, such as that emitted from the diode (810 nm) and Nd:YAG (1064 nm) laser, because of the good optical penetration properties of this radiation in tissue. The xenon arc lamp was a very effective and particularly appropriate energy source for such purposes, and its withdrawal from the world market may have been untimely. Short wavelength sources of radiation, such as the argon ion (488, 514 nm) or the frequency-doubled Nd:YAG (532 nm) laser, are unsuitable for the irradiation of large vascular structures, as they have poor penetration depths. However, for vascular formations with a short path length (1 mm or less), short wavelength sources appear to be the most appropriate choice. Optical coupling of radiant energy to the eye by means of indirect ophthalmoscopic systems or contact lenses to the eye is crucial. Strong positive lenses may lead to severe constriction of the laser beam which leads to high irradiance within the anterior segment, increasing the chances of it being damaged; with negative contact lenses, such as the -64 D Goldmann type, this danger is reduced. CONCLUSIONS: Photothermotherapy is not without risk unless the temperature field can be well adapted to the tumorous structure, as temperature elevations outside a small therapeutic range that affect vital structures are considered to be a risk factor.

[Protein-condensation diseases--molecular basis of cataract formation]. / Protein-Kondensationskrankheiten--Molekulare Mechanismen bei der Kataraktogenese.

There are reasons to classify a number of apparently disparate diseases as "condensation" (or molecular aggregation) diseases. Examples of such condensation diseases include the late phase of diabetes mellitus, Alzheimer's disease and others. With an expanding knowledge, the list of these diseases is likely to increase. We shall describe the underlying common mechanisms, the aim being to find anticataractogenic drugs based on this insight. The common, most important denominator of various clinically differing condensation diseases derives from the interaction of the macromolecules which is in part attractive and in part repulsive. Aggregation resp. clumping of the macromolecules of the crystalline lens, the reasons for light scattering, may be prevented by introducing a number of molecules of various designs into the original macromolecular complex which reduce the tendency of aggregation. Cataract inhibitors of this category may be regarded as effective if they are able to increase the time constant of the normal aging process (i.e. the increment of scatter) by about 20%.

A new laser scalpel--a brief report.

One may consider the clad silica fibre cutting probe as a cheap alternative of the popular sapphire probe, sharing some of its properties. Made of silica, the scalpels have a fair heat tolerance and may be used without cooling by liquids or gas as is necessary with sapphire blades. The laser scalpel described here is driven (powered) with a 10 W cw Nd:YAG laser energy source (1064 nm) and possesses good cutting and haemostatic properties. It may also be used with lasers emitting other wavelengths. One may assume that a diode laser emitting in the near infrared (800-810 nm) may have very similar, although no analogous cutting properties when used with the described scalpel.

Lasers, optical systems and safety in ophthalmology: a review.

This is a review of optical methods related to biomicroscopy and laser treatment of the posterior segment of the eye. Contact lenses can be used to observe optical structures and couple laser radiation into the eye for a vast range of conditions and techniques. A small laser spot size is indispensable for photodisruptive work, though this requires a large beam diameter at the pupil and therefore optical systems and techniques such as scleral indentation which maximize the pupillary beam diameter are preferred. For coagulation work the choice of beam focusing optics is crucial for optimum safety. Vitreous replacement can be used to change the refractive power of the eye and permit new combinations of treatment and optical systems. This review covers many aspects of laser irradiation of the eye. It should be clear that, with the multitude of different procedures and optical systems involved, the laser surgeon is faced with a daunting task in assessing and meeting safety limits.

Oculoplastic surgery with a contact Nd:YAG laser silica scalpel. A case report.

Baso-cellular carcinomas involving the medial canthal regions of both eyes were resected using a 10 W cw-Nd: YAG laser scalpel. The wound bed was repaired one side by a free, split-thickness graft and on the other by a frontal pedicle flap. Surgery was, for the most part, virtually bloodless. Since laser radiation immediately seals both capillaries and lymphatics, the incidence of postoperative hemorrhage and edema was significantly less than after cold scalpel surgery. Healing was uneventful. The advantages of laser-assisted surgery are worthy of consideration: not only is the operation time markedly curtailed, but the wound healing response is excellent, with only a minimal inflammatory reaction.

Sclerostomy ab interno using long-wave laser modalities: acute morphological effects.

Sclerostomy fistulas were created ab interno in cadaver porcine eyes by an active contact method using a 200-microns-diameter silica optical fiber in conjunction with either Ho:YAG-, Nd:YAG-, or diode-laser light, and a qualitative comparison of the degree of collateral thermal damage induced in each case was undertaken at the light and electron microscopic levels. The apparent breadth of coagulated tissue observed in association with cross-sectioned fistula profiles exhibited considerable local variation, irrespective of the radiation source employed; no intrinsic difference in either the intensity or the extent of the thermal insult incurred was noted between the three laser modalities.

[The current status of sclerostomy procedures in treatment of open angle glaucoma]. / Der gegenwärtige Stand der Sklerostomieverfahren bei der Behandlung des Offenwinkelglaukoms.

METHODS: A number of ab interno and ab externo sclerostomy procedures have been described in recent years. CONCLUSION: At the present time, there is no clear indication that such interventions are superior to the classical fistulizing glaucoma operations, such as trabeculectomy.

[Sclerostomy using various laser modalities. Ultrastructure of acute effects]. / Sklerostomie mit Hilfe verschiedener Lasermodalitäten. Ultrastruktur der Akuteffekte.

The effects of Ho:YAG- (free-running mode), Nd:YAG- (continuous wave mode) and diode- (continuous wave mode) laser radiation on the degree of collateral thermal damage realized during the creation of sclerostomy canals ab interno (by an active contact method) were compared morphologically: Both the nature and extent of coagulation occurred by scleral collagen fibrils were found to be independent of the laser modality employed.

[Surgery with the laser scalpel. Physical principles and clinical effectiveness]. / Chirurgie mit dem Laserskalpell. Physikalische Grundlagen und klinische Wirksamkeit.

BACKGROUND: The conventional surgical method for cutting vascularized tissue with scalpel and scissors may now be improved by use of the laser scalpel. In particular, the frequent interruption necessitated and the poor visibility caused by bleeding may be mitigated owing to the haemostatic properties of laser light. This instrument may be of particular value for tasks involving extrabulbar structures, such as extraocular muscles and lacrimal tissue, as well as for facial surgery. MATERIALS AND METHODS: The mode of action and physical properties of a new laser scalpel are described and its characteristics compared with those of other instruments on the market. The probe consists of a clad, sculptured silica fibre with a core diameter of 0.6 mm and a conical 0.15 mm-diameter cutting tip. Radiation generated by a low cw-Nd:YAG laser module is fed into the probe, at the exit point of which a maximal power density of 57 kW/cm2 is attained. Radiated laser energy penetrates the tissue as an incision is made, thereby inducing an efficient blood flow stasis which is amplified by thermal energy diffusing from the immediate surroundings of the scalpel tip. RESULTS: In this report, the laser scalpel is implemented for the excision of a vascularized, amelanotic, facial naevus. Both the cutting and haemostatic effects were found to be excellent, occurrence of the latter phenomenon being supported by ultrastructural findings. The healing response was comparable to that observed after conventional surgery. CONCLUSION: The new laser scalpel represents an inexpensive and effective cutting and haemostatic tool powered by a standard cw Nd:YAG laser module, with a wide spectrum of potential applications.

The efficacy of cw Nd:YAG laser trabeculoplasty.

The long-term (mean follow up, 51 weeks) efficacy of cw Nd:YAG laser trabeculoplasty in reducing intraocular pressure (IOP) was studied in 106 eyes of 75 patients. Fifty patients (73 eyes) of this group had primary open-angle glaucoma; 11 (17), mixed glaucoma; 10 (12), pseudoexfoliation syndrome; and 4 (4), secondary glaucoma of varied etiology. Fifty laser impacts were delivered over 180 degrees of the circumference of the chamber angle with an Nd:YAG laser working in the cw mode (spot diameter, 100 microns; exposure duration, from 200 to 300 milliseconds; constant power level, 9 W, resulting in 1.8 to 2.7 J/pulse and a total energy dose/session of 90 to 135 J). In a few cases, an exposure duration of 400 milliseconds was used, corresponding to a pulse energy of 3.6 J/pulse and a total energy dose/session of 180 J. Success was defined as a reduction in IOP to 22 mm Hg or less without any further laser or surgical intervention. The cumulative success rate was 97.2% at 1 month, 93.3% at 3 months, 83.3% at 6 months, 78.7% at 12 months, and 71.5% at 24 months. Thus, the efficacy of cw Nd:YAG laser trabeculoplasty is comparable to that reported for trabeculoplasty performed using the argon laser (ALT), the Nd:YAG laser working in the free-running mode, and the diode laser. Biomicroscopy failed to reveal any serious inflammatory reactions persisting longer than 1 week. We conclude that cw Nd:YAG laser trabeculoplasty is a safe and effective alternative to ALT.

Versatility of the cw-Nd:YAG and diode lasers in ocular surgery.

The effectiveness of cw-Nd:YAG and diode laser light in performing transscleral chorioretinal coagulation is demonstrated in cadaver porcine eyes using a new contact probe that facilitates irradiation of these layers. Both radiation sources also may be viable alternatives to argon laser light in effecting chorioretinal and cycloendocoagulation.