[Pupil physiology after cataract surgery]. / Pupillomotorik nach Kataraktoperation.

BACKGROUND: The goal of a cataract operation is to achieve an optimal outcome, which includes a round and functioning pupil. The goal of this study was to analyze the influence of cataract operations on pupils that appear to be normal on slit-lamp examination postoperatively. PATIENTS AND METHODS: A videopupillography was performed on 47 eyes of 47 patients after phacoemulsification without complications in the first eye, and on 12 eyes of 12 patients after phacoemulsification without complications in the second eye. RESULTS: After surgery, the first eyes showed a significantly more constricted pupil compared to the non-operated eye independently of the stage of irritation. After surgery of the second eye, this was only observed under near dark conditions. CONCLUSION: Even the morphologically normal pupil shows a change in motility postoperatively. We believe that the reason for this is the mechanical manipulation of the iris during cataract operation. During this, the sensitive dilator muscle of the pupil is affected, leading to temporary or permanent changes in pupillary function. The change in motility of the contralateral, non-operated eye cannot be easily explained. The pupil size is important for the visual process and alteration of its function allows an evaluation of the quality of the operation.

Scheimpflug records without distortion--a mythos?

The Scheimpflug principle was recommended as allowing distortion-free imaging; however, a detailed analysis reveals geometrical errors as well as distortions arising from absorption of light along the optical pathway. Correction formulas and factors will be presented and applied to the biometry of the eye.

Correcting postoperative astigmatism using curved lamellating keratotomy.

PURPOSE: To ascertain whether the change in refraction caused by paired lamellating corneal incisions in cadaver eyes could be achieved in living eyes and whether wound healing influences this effect. SETTING: Virchow Hospital Eye Clinic, Humboldt University, Berlin, Germany. METHODS: This prospective study included 45 patients who had a follow-up of 15 months. All had astigmatism of at least 2.00 diopters (D) induced by previous cataract surgery. Optical zones of 7.0 and 8.0 mm and chordal lengths of 3.0 and 5.0 mm were used. All incisions were paired. RESULTS: Mean preoperative astigmatism measured with a keratometer was 3.64 D +/- 0.89 (SD). After 15 months, it was 1.00 +/- 0.76 D, and mean induced astigmatism was 3.18 +/- 1.30 D. The 7.0 mm group had a mean induced astigmatism of 3.93 +/- 1.52 D, which was significantly higher than that in the 8.0 mm group (mean 2.60 +/- 0.74 D) at a uniform chordal length of 3.0 mm. Mean uncorrected visual acuity was 0.26 +/- 0.14 preoperatively and 0.50 +/- 0.28 after 15 months; mean corrected visual acuity improved from 0.58 +/- 0.21 to 0.76 +/- 0.21. Mean endothelial cell count was 1799 +/- 595 cells/mm2 preoperatively and 1784 +/- 589 cells/mm2 after 6 months; the difference was not significant. CONCLUSION: Curved lamellating keratotomy gives stable postoperative refractive results without significantly affecting endothelial cell count or visual acuity.

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.

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.

The influence of the cutting procedure on the loss of corneal endothelial cells of the recipient eye in mechanically guided trephination.

BACKGROUND: Excessive damage to the endothelial monolayer during corneal trephination may contribute to transplant failure. For this reason, we performed several trephining experiments to determine the influence of various cutting conditions (sharpness, rotation rate, and cutting power) on the endothelial damage in the recipient bed. METHODS: A mechanically guided motor trephine (Mikro-Keratron) was used for cutting experiments in fresh, enucleated porcine eyes. Eight different, reproducible combinations of cutting conditions were applied, and ten epithelial, full-thickness trephinations were performed for each combination. The endothelial defect was measured with a light microscope (60 x). In addition, the bend radii of the porcine cornea were measured. RESULTS: The mechanically guided Mikro-Keratron used with a sharp blade, high rotation rate (840 rev/min), and moderate cutting power (49 mN) caused a 31-micron-wide band of damaged endothelium in the recipient bed. Decrease of the rotation rate to 30 rev/min resulted in an increase of the endothelial defect to 168%. A cutting power of 392 mN combined with a rotation rate of 840 rev/min increased the defect to 126%. The system allowed full-thickness 360 degrees trephination. The corneal bend radii in porcine eyes measured 8.28 mm horizontally and 8.20 mm vertically. CONCLUSION: Our preliminary results indicate that the mechanically guided motor trephine, used under optimal conditions, is a suitable cutting device and may improve the outcome of penetrating keratoplasty; however, further investigation of this new approach is necessary.

The sharpness of incision instruments in corneal tissue.

Because breadths of surgical blades are very difficult to measure directly, we designed a method to measure sharpness of a variety of instruments by determining the power they required to incise corneal tissue to a uniform depth. Diamond knives were the sharpest, followed by razor blade pieces individually prepared by a special new method. Ruby knives varied considerably. The method of measuring sharpness described may allow manufacturers to improve and stabilize production methods, as well as enable surgeons with such knowledge in advance of surgery to optimize wound healing and maximize cost-effectiveness.

[The conditions of motor trephining and their influence on time and velocity in cutting the isolated cornea (author's transl)]. / Der Einfluss der Bedingungen bei der Trepanation von isolierten Schweinehornhäuten auf die Schnittgeschwindigkeit und Schnittzeit.

A special experiment was constructed to measure the influence of factors such as trephine diameter, revolutions per second, sharpness, and cutting pressure on the incision velocity and incision time for a number of trephine specimens. These measurements show the characteristics of trephines. Different degrees of sharpness were determined by comparison of the characteristics. The dependency of rotation moment transferred to tissue, of revolutions per second, and of incision power is discussed. The results are discussed, and proposals are made to increase safety by application of motor trephines and to develop them further.