[Consistent IOL calculation].
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Biometry↗
Biomedical subjects
P R Preussner
Publications and source records attributed to P R Preussner.
[Accommodative lens implant].
PROBLEM: To date, approaches to restore accommodation following cataract surgery have been based on the principle of a liquid or solid implant that moves within the capsular bag and thereby changes the optical characteristics of the eye. However, these implants have been associated with shrinking and fibrosis of the capsular bag, a problem that will not be solved in the foreseeable future. APPROACH: If accommodation is effected by shifting the whole capsule including its contents on the optical axis, capsular fibrosis will virtually no longer play a role. In order to execute this shift via the neuronal feedback mechanisms of the ciliary muscle in a controlled way, radial zonular forces or movements must be transformed into axial ones, which is possible by means of tiny permanent magnets. This paper describes the basic aspects of such an implant. The dependence of accommodation on all relevant parameters is quantified in a computer simulation. CONCLUSION: The implant described appears to be practicable with justifiable technical and surgical efforts.
Accommodation, Ocular↗
[Consistent numerical calculation of optics of the pseudophakic eye].
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Anterior Chamber↗
[Analgosedation (managed anesthesia care--MAC) with propofol and piritramide for controlled cyclophotocoagulation of the eye].
The new method of controlled cyclophotocoagulation of the eye is an example of a short, non-invasive procedure that is still too painful to be done under local anaesthesia alone. The risks associated with general anaesthesia, on the other hand, seem to be inappropriately high compared to the risks associated with the procedure itself. Therefore, for this procedure, we combined local anaesthesia with 0.5% proxymetacain and 10% cocaine and sedation with propofol and analgesia with piritramide. Our experiences with this method have been positive. We have applied our method to 42 patients undergoing a total of 53 procedures and we have seen no major changes in haemodynamics and only two cases of momentary slight ventilatory depression. Therefore, we conclude that our method of managed anaesthesia care is suitable for patients undergoing cyclophotocoagulation of the eye, combining patient comfort with haemodynamic stability and minimal risk for the patient.
Adult↗
[Controlled cyclophotocoagulation].
BACKGROUND: In the past, the main problem of transscleral cyclophotocoagulation was related to the energy dosage of the individual effect. Since the target tissue is not directly observable, no information about the coagulation process was available to the surgeon. High inter- and intraindividual variations of the tissue properties often led to under- or overdosage. Underdosage is therapeutically useless, while overdosage causes the so-called "popeffects" which may induce severe damage to the eye. SOLUTION: A small fraction of the impinging laser radiation passes the ciliary body after multiple scattering, is reflected from the fundus, leaves the eye through the optical media and can be recorded by a photodetector outside the eye. The time dependence of this detector signal directly monitors the change in transmission of the coagulated tissue, because all other parameters influencing the signal are constant in time. Using this information, the surgeon or a computer can interrupt the laser process. CLINICAL EVALUATION: In the eyes treated since September 1996, a mean reduction of intraocular pressure was achieved comparable to the values for uncontrolled cyclophotocoagulation reported in the literature. So far, no case of severe complications has been observed. CONCLUSION: The accuracy and safety of transscleral cyclophotocoagulation are improved by this real-time control procedure. Reduction of risk may allow broader application of the method.
Ciliary Body↗
Real-time control for transscleral cyclophotocoagulation.
BACKGROUND: In transscleral cyclophotocoagulation, the surgeon cannot directly observe the applied laser effects. Overdosage, possibly resulting in unwanted pop effects, or underdosage with no therapeutic effect therefore often occur. METHOD AND MATERIALS: Laser radiation passing through the sclera and ciliary body is partly reflected from the fundus and can be monitored from outside the eye by a detector system. Since all other parameters influencing the intensity of the recorded radiation are constant in time during one laser exposure, the time dependence of this radiation directly reflects the change of transmission of the treated tissue. The laser exposure therefore can be stopped by a computer when certain criteria of the recorded curves are fulfilled. In addition, the transmission curves are displayed on a monitor in real time, permitting the surgeon to interrupt the exposure. A Nd:YAG laser and a diode laser are connected to the device. After successful tests in enucleated porcine eyes which were evaluated histologically and in human cadaver eyes this method is applied to patients suffering from refractory glaucoma. RESULTS: The transmission curves from enucleated porcine eyes show that the 810-nm diode laser is more appropriate for this method than the 1064-nm. Nd:YAG laser, because the radiation output of the diode is more stable in time and the tissue absorption is higher, both resulting in a larger dynamical range of useful signal. The curves from the porcine eyes, the human cadaver eyes and the curves from patients show a typical shape which allows interruption of exposure either by the surgeon or by the computer program before a pop effect occurs. CONCLUSIONS: This new method increases the precision and safety of transscleral cyclophotocoagulation.
Adult↗
New device for artificial increasing intraocular pressure.
BACKGROUND: For certain special investigations, particularly in glaucoma diagnostics, intraocular pressure has to be increased. Mostly, a suction cup apparatus is used for that purpose. These devices, however, have some disadvantages. They are at least uncomfortable, sometimes risky for the patient and induce a strong astigmatism which can influence the results of the corresponding investigation. METHOD: A very simple mechanical device has been constructed and tested, by means of which the intraocular pressure can be increased for diagnostic purposes. A ring on a lever, which is moved towards the eye by small weights, compresses the eye through the lids without the need of even local anesthetics. The device can be easily mounted to any head fixation which is normally used in combination with ophthalmological equipment. e.g. slit lamp, visual field analyzer or fundus camera. Tests have been performed in four different groups: one group of 19 healthy volunteers, one of 21 patients with Graves' disease, one of 21 patients with high myopia and one of 20 patients with open-angle glaucoma. RESULTS: Pressure elevations could be achieved in all patients with very good reproducibility with respect to the applied forces. However, in patients with exophthalmos (Graves' disease), the elevation effect was somewhat higher than in normal eyes. The induced astigmatism was only about 0.5 dpt for pressure elevations of about 20 mmHg. CONCLUSION: The tests performed showed that the new device is easy to handle and comfortable for the patients. If very high accuracy is needed concerning the pressure values achieved, an individual calibration should be performed.
Adult↗
Steps to optimize transscleral photocoagulation.
BACKGROUND: In transscleral photocoagulation, the desired effect is coagulation of parts of the ciliary body or of the peripheral retina. However, the application is often limited by the unwanted effect of coagulation of the sclera. To reduce this effect, the ratio of incident radiation flux to radiation flux transported through the sclera (and able to coagulate the target tissue) should be minimized by the incident beam characteristics. METHODS: Monte Carlo simulations for the radiation transport problem of multiple scattering in the sclera were used to calculate the ratio of transported to incident radiation for different parameter settings of beam diameters, optical thicknesses of the sclera and beam angles. To verify the theoretical calculations, an simple optical device utilizing a bulb instead of a laser source was constructed and applied to enucleated porcine eyes. RESULTS: The theoretical calculations showed that the ratio of incident to transported radiation flux can typically be decreased by a factor of three by increasing the beam radius from 0.35 mm (as used in state-of-the-art laser devices) to 2 mm. This was confirmed by the experiments. Coagulations of the ciliary body or of the peripheral retina were possible with power densities an order of magnitude below the values normally applied with laser sources. CONCLUSION: To improve transscleral photocoagulation, beam diameters should be increased.
Animals↗
Automatic exposure control in retinal laser therapy.
Retinal laser spots are often over- or underexposed due to inhomogeneities of fundus pigmentation or variations in the radiation power impinging on the retina. We developed a method to control the exposure time of each individual spot of retinal laser photocoagulation in real time. The laser light reflected from the spot area is recorded by a detector. The intensity of this light increases during the exposure due to bleaching of the retinal pigment. If the reflected light reaches a predefined level, the laser apparatus is immediately stopped by a computer, which controls the whole process. The procedure was tested in enucleated porcine eyes. After verification of the proper function, a pilot study in a small collective of 19 patients (20 eyes, 25 sessions) with clear optical media was performed. In all cases, the apparatus worked safely. Visible indications for overexposed spots, particularly bleeding in extreme cases, did not occur. However, about 2%-5% of the spots were underexposed, mainly due to eye movements of the patient during the coagulation. Large variations in the automatically controlled exposure time confirm the difficulty in properly adjusting this parameter without such an automatic device. However, to be applicable to patients with opaque optical media, the method must be improved.
Adult↗
[Automatic tracking system for laser surgery of the human cornea].
An automatic, closed loop, real-time tracking device is described. The purpose of this device is, despite movements of the eye, to keep the laser beam correctly on target while performing corneal surgery. This type of laser surgery, is employed specifically to ablate some of the cornea to correct myopia, hyperopia and astigmatism.
Cornea↗
The principle of retinal rheometry.
A novel technique termed retinal rheometry, which is used to quantify the microcirculation of the human retina, is detailed. The applicability of different types of technological equipment (cameras and laser scanning ophthalmoscopes, among others) to the proposed technique are compared. The individual steps of the data reduction process are presented, and initial results obtained in a small population of healthy volunteers are given.
Blood Flow Velocity↗
[Continuous registration of the filling volume of the human urinary bladder].
A sensing system for continuous recording of bladder volume is described. The system is intended for use in particular in patients with paraplegia or bladder plastique. Owing to the very simple measuring procedure employed the implantable components can be designed for very low power consumption. Also, there is no need for an additional data transfer from inside the body to the exterior, because measurement and telemetry are physically the same procedures.
Equipment Design↗
[Automatic compensation for the effect of eye movements in laser keratotomy].
In refractive laser surgery of the human cornea, especially in radial keratotomy, one major problem comes arises as a result of the patient's eye movements. If drugs to paralyze the muscles should be avoided, the laser beam must correspond with the eye movements. Here we present a closed loop tracking system, which uses an additional infrared optics to record the movements and a two-dimensional mechanically driven mirror to follow them. The functional principle and performance data are presented. The influence of different kinds of tracking on the therapy results are briefly discussed.
Eye Movements↗
[Determination of the circulation in the human retina using image analysis of video-angiograms].
A method is described to determine the total retinal blood flow. Intravenous fluorescence angiography is the clinical procedure used, together with digital image processing methods. Our system consists of a standard fundus camera, a highly sensitive sensor with a low-light-level intensifier (micro-channal plate) and a CCD, a computer system including a real-time memory, an image display subsystem, and other standard peripherals (disk, magtape, terminal, etc.). The recording speed is 100 frames (noninterlaced), the image size 256 X 256 pixels, and the depth 8 bit. In order to properly track the images against the patients eye movements, it is necessary to have absolutely sharp pictures. The fundus camera is therefore equipped with a stroboscopic bulb, which gives light pulses of 0.01 ms. The stroboscope is synchronized by the electronics of the camera. After recording the images, further evaluation is done.
Blood Flow Velocity↗
Quantitative measurement of retinal blood flow in human beings by application of digital image-processing methods to television fluorescein angiograms.
A method is presented that allows the quantitative determination of the blood flow in retinal arteries in human beings. Television fluorescein angiograms are used as input. This method does not need any gauge procedures since all the necessary information is taken from the image itself. Also, the patients' eye movements do not introduce errors because their influence is removed by the computer program.
Fluorescein Angiography↗
[Expert system for automatic, subjective determination of refraction].
An automatic system for determining a patient's subjective refraction for the fitting of glasses or contact lenses, is described. The system works without any intervention on the part of the ophthalmologist or optometrist. Directions are provided and questions asked of the patient via a loudspeaker or headphones with the aid of a computer. This computer also controls the positioning of different lenses before the patient's eyes using a phoropter, and the projection of the chart figures for visual acuity testing. The patient responds via a dedicated graphic tactile answering unit. The principles involved are described, and the experience so far gained with the method discussed.
Artificial Intelligence↗