Mean Decentration Research Articles

Relation of Visual Symptoms to Topographic Ablation Zone Decentration after Excimer Laser Photorefractive Keratectomy

The authors used computer-assisted videokeratoscopy to analyze the relation of photorefractive keratectomy ablation zone decentration to subjective patient assessments of disturbing visual symptoms. Ablation zone decentration was measured 1 month postoperatively. The study population was divided into two groups: group 1, patients whose ablation zone decentrations were less than 0.50 mm; group 2, patients whose ablation zone decentrations were greater than 0.50 mm. Visual symptoms including glare, rings or halos around lights and problems with night driving were scored preoperatively and 6 months postoperatively. The Hotelling T-square and chi-square tests were used. The mean decentration from the center of the ablation zone to the pupillary center was 0.30 mm and 190 degrees for group 1 compared with 0.66 mm and 198 degrees for group 2. The Hotelling T-square test showed a significant statistical preoperative/postoperative difference in group 1 (P < 0.03) for the halo symptom category. No other symptom category showed a significant statistical difference in either group for the mean scores. The Hotelling T-square test did not show a statistically significant difference between the two groups preoperatively to postoperatively regarding the mean scores of the individual patient differences for the three symptoms. The only significant statistical difference for the individual patient ratings preoperatively to postoperatively was for the halo symptom category (chi-square = 7.756; P < 0.03). Multivariate analysis did not show a significant statistical difference preoperatively between the two groups or postoperatively except for group 1 with regard to the halo symptom category. It appears from this study that ablation zone decentrations less than 0.89 mm from the pupillary center do not necessarily produce unwanted visual symptoms 6 months postoperatively.

Digital system measurement of decentration of Worst-Fechner iris claw myopia intraocular lens.

Topography measures the centration in refractive and corneal refractive surgery, but until recently there was no effective method to measure decentration in intraocular lens (IOL) implantation. We measured the decentration of 22 phakic eyes, in 14 high myopic patients, based upon photographs of the anterior segment using a digital analysis system (IMAGEnet Topcon Corporation 1988, Tokyo, Japan). These eyes had undergone IOL implantation for the correction of myopia with Worst-Fechner iris claw lenses. Decentration was measured with respect to the centers of both the cornea and pupil. Mean decentration of the IOL center from the cornea center was 0.51 +/- 0.25 mm (range, 0 to 0.8374 mm). When measured with respect to the center of the pupil, the average decentration was 0.47 +/- 0.29 mm (range, 0.182 to 0.9341 mm). A positive correlation between the decentration measurements of the two methods was found (p < .01 indicated statistical significance). This digital imaging system (IMAGEnet) accurately measured decentration of IOLs in myopic eyes.

Corneal Topographic Analysis after Excimer Photorefractive Keratectomy

A 2-year experience with corneal topography after photorefractive keratectomy (PRK) is reported, specifically reviewing the correlation of surface regularity index versus best spectacle-corrected visual acuity as a function of various ablation patterns. Centration and stability issues also are discussed. Excimer laser PRK for myopia was performed on 502 consecutive eyes. Corneal topographic analysis was performed at the 1-, 3-, 6-, and 12-month postoperative follow-up visits. In all patients, PRK was performed with an argon fluoride laser, and a topographic analysis was done. Topographic analysis at the first postoperative month was useful for determining decentration of the optical zone. The mean decentration from the pupillary center for all eyes was 0.34 mm. Four main patterns of ablation were noted by subtraction analysis. At the 1-month postoperative examination, a "uniform" ablation was present in 44% of eyes, "keyhole" ablations were present in 12% of eyes, "semicircular" ablations were present in 18% of eyes, and an unusual "central island" was present in the remaining 26% of eyes. However, on subsequent follow-up the central islands tended to resolve with time. At 3 months postoperatively, 18% of eyes in that group showed a central island, 8% showed a central island at 6 months, and only 2% of eyes showed a central island at the 12-month postoperative visit. Correlation of central island topography with visual acuity and progressive hyperopia are discussed. Corneal topography is essential for evaluating surface changes after excimer PRK. The surface regularity index is a good indicator of best spectacle-corrected visual acuity and is used to evaluate irregular astigmatism after PRK. Central island topographies are correlated with poor initial visual rehabilitation. Long-term stability issues are answered with continued topographic follow-up. Understanding corneal hydration changes between the central and peripheral cornea may help us understand the etiology of central islands.

Effect of Anterior Capsulotomy Shape and Intraocular Lens Design on Lens Decentration

OBJECTIVE: To determine whether the anterior capsulotomy shape (capsulorhexis or envelope) or the IOL design (polyHEMA, one-piece PMMA open and closed loops, three-piece polypropylene and PMMA loops) influence the lens decentration after intercapsular cataract surgery. STUDY DESIGN: Retrospective study of postoperative results 18 months after surgery. SETTING: University Eye Clinic of Trieste, Italy. PATIENTS: 150 patients divided into two groups: in 75 cases anterior circular continuous capsulorhexis (CCC) was performed and in 75 cases the envelope technique was used. Only patients with uncomplicated eyes and absence of posterior capsular opacification were included. MAIN OUTCOME MEASURES: Degree of IOL decentration was measured by projecting a slide of the anterior segment of the eye, taken after maximum dilatation, and calculating the distance between the circles formed by the corneoscleral limbus and the edge of the IOL. RESULTS: Including all IOLs, mean decentration was 0.28 ± 0.21 mm in the capsulorhexis group and 0.55 ± 0.32 mm in the envelope group; the difference is statistically significant ( P < 0.01). One-piece IOLs decentred less than three-piece lenses when capsulorhexis was performed. There were no significant differences found with different types of IOLs implanted by the envelope technique. CONCLUSIONS: The loop material did not appear to influence the IOL centration. The loop design appears to influence the centration only when capsulorhexis is performed. In the envelope technique, the decentration relates more to the irregular opening of the capsular bag than to the loop design.

Corneal topography following excimer photorefractive keratectomy for myopia

Corneal topographic analysis was performed on 97 consecutive eyes with at least one month follow-up after excimer laser photorefractive keratectomy for myopia. Centration of the ablated zone was documented in all eyes at the one month postoperative examination with the Topographic Modeling System (Computed Anatomy Inc., NY). Thirty-seven percent of eyes were within 0.25 mm of the pupillary center (PC); 48% of eyes were between 0.25 and 0.50 mm, 13% were between 0.50 and 1.00 mm, and 2% were more than 1.00 mm from the PC. The greatest amount of decentration was 1.50 mm. The mean decentration was 0.36 mm. The surface regularity index (SRI), which is a measure of the optical performance of the cornea, was significantly less at the sixth postoperative month than at the first postoperative month (P = .013). No correlation between the SRI and the amount of decentration was found. Four main patterns of ablation were noted by differential topographic analysis at the one month interval. A central uniform ablation was present in 45% of eyes; “semicircular ablations” were present in 33% of eyes, a “keyhole” ablation pattern in 12%, and a curious “central bump” ablation pattern in 10%. Corneal topographic analysis was a useful tool for documenting and quantitating results following excimer photorefractive surgery.

Decentration of intraocular lenses implanted after intercapsular cataract extraction (envelope technique)

Pseudophakos decentration produces symptoms such as glare, halos, and other visual aberrations. We have reviewed 161 eyes of patients who had an intercapsular cataract extraction with intraocular lens (IOL) implantation with a follow-up of at least six months. Mean decentration was 0.54 mm. It was 0.46 mm for the lenses implanted symmetrically in the bag, 0.65 mm for those implanted in the suleus, and 0.91 mm for those implanted asymmetrically (bag-sulcus). The differences between the lenses fixated in the bag and those asymmetrically situated were statistically significant (P < .001). Circular looped lenses (360-degree encircling loop) decentered less (mean 0.25 mm) than modified C-loop lenses (mean decentration 0.63 mm) and modified J-loop lenses (mean decentration 0.60 mm). These differences were statistically significant (P < .001). The differences were also significant if we considered only the lenses implanted in the bag (mean decentration 0.20 versus 0.59 and 0.54, respectively) (P < .0001).