Undesired light images associated with ovoid intraocular lenses

Undesired light images associated with ovoid intraocular lenses Samuel Masket, M.D., Edward Geraghty, B.S., Alan S. Crandall, M.D., James A. Davison, M.D., Stephen H. Johnson, M.D., Douglas D. Koch, M.D., Stephen S. Lane, M.D.

ABSTRACT Ovoid intraocular lenses (IOLs) accounted for approximately 35% of the lenses implanted in 1991 . Despite their popularity among surgeons, patient observations and complaints suggest that ovoid lenses are associated with undesired optical images postoperatively. A pilot clinical study was carried out to test that hypothesis. A multicentered investigation of 289 cases with well-centered IOLs determined that 45% of 168 patients with ovoid lenses observed unwanted optical phenomena at some time after surgery, whereas 17% of 121 patients with round IOLs had similar complaints. Based upon the preliminary clinical study a laboratory investigation was designed to assess optical performance differences between ovoid and round IOLs. One testing system used scatterometry and a second, ray tracing analysis. Both methods determined that the truncated, thickened edge of ovoid lenses was associated with significant light scattering, possibly accounting for the extraneous light images observed by some patients. The results of the clinical and laboratory investigations suggest that undesired optical images produced by ovoid IOLs are due to lens shape rather than to the reduced dimension of the optic. Key Words: capsulorhexis, extraneous light image, intraocular lens, ovoid intraocular lens, ray tracing analysis, reduced dimension intraocular lens, scatterometry, small incision cataract surgery

Ovoid lenses, generally 5.0 mm x 6.0 mm in optic dimension (Figure 1), represent a significant portion of the intraocular lenses (lOLs) used in the United States. Preference for ovoid lenses by members of the American Society of Cataract and Refractive Surgery has been as high as 35%.1 The improved centration afforded by the capsulorhexis technique and the "astigmatism neutral" management of the small cataract incision and closure have fostered the concept that reduced-dimension ovoid lenses offer the patient a more rapid and stable return of vision after surgery, without any loss of visual function, than larger round lenses. No subjective patient response to these devices had been reported to, or investigated by, the U.S. Food and Drug Administration before their release for unrestricted use with premarket approval status. The initial surgeon response to ovoid IOLs was enthusiastic. As a result, one of us (S.M.) placed these devices in the fellow eyes of patients who had full-size,

round lenses implanted in the first eye. Several patients offered unsolicited complaints about light streaks, spots, and halos in the recently implanted eye, whereas they had no such complaints about the previously implanted eye. The initial patient complaints were in cases with well-centered IOLs and physiologic pupils; the complaints appeared different from the observations of earlier patients with decentered posterior chamber lenses implanted during the era of can-opener capsulotomy. The experiences of the patients with ovoid IOLs generated interest in the present study, which sought to determine whether ovoid lenses were associated with an increased likelihood of undesired optical images after surgery and to identify the cause of the visual observations. The study was divided into clinical and laboratory investigations. The pilot clinical portion was based on objective surgeon evaluation of postoperative eyes and

Drs. Masket, Crandall, Davison, Johnson, Koch, and Lane have no financial interest in any products described in this study. Mr. Geraghty is employed by Kabi Pharmacia Ophthalmics, Inc., Monrovia, California. Reprint requests to Samuel Masket, M.D., Suite 204, 7230 Medical Center Drive, West Hills, California 91307. 690

JCATARACf REFRACfSURG-VOL 19, NOVEMBER 1993

signed to discern differences between ovoid and round lenses in their propensity to scatter light. Two laboratory systems were used. O~e investigational method used scatterometry,2 a techmque for determining the character of light reflection from optical surfaces. With this system (Figure 2), monochromatic light with a 2.0 mm beam diameter is projected from a laser toward the optic in air at a variety of oblique angles, varying by ten degrees between 40 and 70 degrees. All test lenses were 20.0 diopters and were one-piece and three-piece designs; they were provided by several manufacturers. In the test system, the optic was held in a test chamber without provision for a pupillary aperture. The projected light was designed to strike the edge ofthe optic, with half the beam passing beyond the lens without refracting through the lens. The portion of Fig. 1. (Masket) One-piece all-PMMA lens with ovoid op- the light passing beyond the lens without refracting was tical portion. The optic measures 5.0 mm in its short diameter and 6.0 mm in the axis ofthe loop supports. used as a reference. The pattern oflight scatter from the refracted light was computer analyzed over a range of angles from 0 to 170 degrees from reference. A portion of the refracted light passing through the a subjective postoperative questionnaire. The laboratory lens will be incident on the vertical edge of the optic from investigations were performed independently using scat- within the lens. For some light rays, the internal edge of terometry and ray tracing analysis. the l~ns will act as a mirror and reflect the beam, causing the llght to disperse and focus in a location other than SUBJECfS AND METHODS that intended; these aberrant rays may fall upon the Two hundred eighty-nine postoperative patients of retina (Figure 3). In the test apparatus the resultant several clinical investigators were examined by slitlamp tracings or scatterplots indicate the presence and degree to determine the centration of the poly(methyl meth- of aberrant light scatter. The second laboratory method used ray tracing analacrylate) (PMMA) optic IOL, confirm the shape of the ysis, a computerized modeling system for determining optic, and ascertain the condition of the iris and pupil. of bundles of light rays passing through optical the fate Patients were selected for the study retrospectively and 3,4 With this·method, the theoretical focus and systems. were not randomized. Inclusion criteria required that the original cataract surgery was uncomplicated, in- scatter of light is plotted with the prediction software. duced no iris or pupil sphincter damage, and that IOL The ray tracing program modeled the experimental setplacement was within the confines of a continuous cur- up. Ray bundles oflight were directed at both round and vilinear capsulotomy (capsulorhexis). Additional crite- ovoid lenses from a 40-degree angle. The program calria for inclusion were a clear cornea, physiologic pupil, culated the amount and direction of scatter based upon and absence of any ocular disease that might induce the physical properties of the test system and by applying glare symptoms. Two groups of eyes, those with ovoid lenses and those with round lenses, were established retrospectively. The ovoid lenses were 5.0 mm x 6.0 mm; the round lenses were 6.0 mm or 6.5 mm in diameter. Patients were asked to respond to a questionnaire that asked whether unexpected optical effects of the implant surgery were observed after surgery. Besides recording voluntary observations of patients, the questionnaire asked about awareness oflight streaks, halos, arcs, spots, etc., and also whether these visual forms were noticed under bright light, dim light, or dark conditions. Visual phenomena noted during darkness were considered ent?Ptic ~nd not IOL related. To reduce bias, the questlOnmure was administered by technicians unaware of the IOL type used in each eye. The technicians were not Fig. 2. (Masket) Schematic representation of the laboratory informed of the purpose of the study. mo?el. ~blique light is aimed near the edge of the The laboratory investigations were performed by optIC. FIfty percent of the light is used as reference' the remainder is refracted/scattered by the lens. ' TMA Industries (Bozeman, MT). The studies were deJ CATARACf REFRACf SURG-VOL 19, NOVEMBER 1993

691

Table 1. Incidence of undesired postoperative optical images. Intraocular Lens Shape Ovoid Round

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statistical probability. The statistical probability is based on a bidirectional scatter distribution function.

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RESULTS As noted in Table 1, the clinical study indicated that 75 of 168 patients (45%) with ovoid IOLs experienced light streaks, spots, halos at some time after surgery. Similar observations were noted by 21 of 121 patients (17%) with round IOLs. The scatterometry analysis developed two distinct patterns that appeared consistently. Figure 4 demonstrates a single scatter tracing from a thin-edged 5.0 mm round optic; oblique illumination was at 50 degrees to

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(Masket) Scatterplot recorded for oblique light at 50 degree incident angle to a round, 5.0 mm lens optic. The reference intensity at 40 to 60 degrees is over 1,000 times as bright as light falling on the retina located at approximately 20 degrees.

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(Masket) Composite scatterplot for oblique light between 40, 50, 60, and 70 degrees incident to ovoid lens optic. Note the consistent scatter peak near the center of the curve.

the optic. The central aspect of the curve is relatively smooth. In contrast, Figure 5 represents a scatter curve obtained at 50 degrees of oblique illumination from the truncated edge ofa 5.0 mm x 6.0 mm ovoid IOL. Near the center of the tracing there is a light scatter peak not present in the round lens pattern. Composite curves of light at incident angles between 40 and 70 degrees demonstrate a consistent scatter peak for the oval lens

J CATARACT REFRACT SURG-VOL 19, NOVEMBER 1993

Scatter Data tor SM Round Lens 188

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(Masket) Composite scatterplot from both edges ofa single-piece ovoid lens optic. The scatter peak emanates from the truncated edge (red).

(Masket) Composite scatterplot for oblique light at 40, 50, 60, and 70 degrees incident to round lens optic. Note that the center of the curve does not have the scatter peak seen with the ovoid optic in Figure 6.

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(Figure 6) and an absence of this phenomenon for the round lens (Figure 7). Subsequent scatter analyses compared the rounded edge with the truncated edge of several different lens types made by several manufacturers. Typical scatterplots (Figures 8 and 9) revealed a consistent pattern of increased scatter from the thicker, straighter, truncated edge of the ovoid lenses, whereas additional tests performed on a round lens (Figure 10) failed to demonstrate a difference between the scatter of two loci on the edge of the lens. The results of the ray tracing analyses (Figure 11) for 40-degree incident light are noted in Figures 12 and 13; as in the case of the scatter analyses, they show a tendency for increased light scatter from the truncated edge of the tested ovoid lens.

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(Masket) Composite scatterplot from two sides ofa one-piece, 5.0 mm round lens. Note that the two resultant curves are virtually identical.

DISCUSSION

Fig. 8.

(Masket) Composite scatterplot from round and truncated edge of a three-piece ovoid lens. Note the scatter peak originating from the thickened, truncated edge of the optic (red).

The limited clinical data of this study suggest that there is an increased likelihood for patients with wellcentered ovoid lenses (5.0 mm x 6.0 mm) to be aware of extraneous and undesired light images; 45% of patients with ovoid lenses and 17% of those with full-size, well-centered round lenses (6.0 mm or larger) reported unwarranted optical images. Since 17% of patients with round lenses noted the extraneous images, the phenomenon may be due in part to the surface reflective quality of PMMA. Other IOL materials were not evaluated in this study. The results of the laboratory tests indicate that the increased tendency for undesired optical images with ovoid lenses is shape rather than size dependent. However, the limited clinical investigation compared patients with full-size lenses to those with lenses with one

J CATARACT REFRACT SURG-VOL 19, NOVEMBER 1993

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reduced dimension. Richard Lindstrom, M.D., (personal communication, 1991) suggested that ovoid lenses, when oriented with their long axis horizontally, tend to decenter superiorly, in the short rather than in the long axis. As a result, the effective size of the short side of the optic is further reduced. In the clinical situation of superior decentration of an ovoid optic, extraneous light symptoms are more likely. Nevertheless, the cases in the present study were required to have wellcentered lenses and normal pupillary function and iris architecture. As a result, the undesired optical observations do not appear to be due to lens edge exposure in the pupil, but rather to lens shape. Both forms of laboratory analysis suggested a different optical performance for the truncated edge of ovoid lenses than for the

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thinner round edge of either round lenses or the round portion of ovoid lenses located 90 degrees away from the truncated edge. The truncated edge was noted to scatter light in a predictable fashion when analyzed with scatterometry or ray tracing analysis. While it may not be possible to conclude that the laboratory models accurately depict the clinical problem, there seems to be evidence that the scattered oblique light emanating from the truncated edge may fall upon the retina and be visually significant. Additional clinical studies comparing the observations of patients with 5.0 mm and 5.5 mm round lenses and those of patients with 5.0 mm x 6.0 mm lenses would be useful to characterize the role of lens optic shape and size in producing unwanted light images. Minimizing incision size must be weighed against the optical implications of implanting lenses that produce undesired light images. REFERENCES 1. Learning DV. Practice styles and preferences of ASCRS

members-1991 survey. J Cataract Refract Surg 1992; 18:460-469 2. Bender JA, Henning TD, Bernt ML. Near specular measurements of integrated scatter. In: Stover JC, ed, Optical Scattering: Measurement and Analysis. New York, McGraw-Hill, Inc, 1990 3. Herzberger M. Modem Geometrical Optics. Melbourne, FL, RE Krieger, 1980 4. Kashiwagi T, Khu PM. Ray tracing error correction in ophthalmic optics. J Cataract Refract Surg 1991; 17: 194198

J CATARACT REFRACT SURG-VOL 19, NOVEMBER 1993