Imaging of Forward Light-Scatter by Opacified Posterior Capsules Isolated from Pseudophakic Donor Eyes

Abstract

Purpose.: Posterior capsule opacification (PCO) degrades visual function by reducing visual acuity, but also by increasing intraocular light-scatter. An in vitro model was used to elucidate the effect of PCO-morphology on light-scatter and its functional aspect, as can be assessed with straylight measurement.

Methods.: Forward PCO-scatter by opacified capsular bags was recorded with a goniometer and camera. The camera position mimicked the anatomic position of retinal photoreceptors; the camera recorded the scattered light that the photoreceptors would sense in an in vivo situation. Scattered light was recorded at different wavelengths and scatter angles, which were divided into a near (1° < θ ≤ 7°) and far (θ > 7°) large-angle domain. Using scattered light, the camera produced grayscale PCO images. The nature of the angular dependence of PCO-scatter was compared with that of scatter in the normal eye, by rescaling PCO images relative to the normal eye's point-spread function.

Results.: The scattered light images closely followed PCO severity. The angular dependence of PCO-scatter resembled that of scatter in the normal eye, irrespective of severity and PCO type. PCO shows the type of wavelength dependence that is normal for small particles: monotonically decreasing with increasing wavelength. At the near large-angle domain, the angular dependence of PCO scatter resembled the angular dependence of scatter in the normal eye less closely.

Conclusions.: Surprisingly, PCO scatter and scatter in the normal eye have similar underlying scattering processes. However, data obtained at the near large-angle domain demonstrates that, apart from scatter, PCO may also have a refractile component, which is most pronounced in pearl-type PCO.

It is well known that posterior capsule opacification (PCO) impairs visual function. Several studies have assessed the negative effect of PCO on visual acuity (VA) and contrast sensitivity (CS), but there is limited correspondence between VA and the degree of visual impairment experienced by the PCO patient. ^{1 –6} Recently, the repertoire of visual function tests was expanded by intraocular stray light measurement. ⁷ Several studies have since assessed the importance of stray light in PCO. ^6,8,9 Substantial stray light elevations were found in PCO patients with good VA, which demonstrates that PCO affects VA and stray light quite independently. ⁹ This finding confirms that visual function has two distinct functional domains: small-angle and large-angle, ¹⁰ as will be detailed later in this section. Therefore, to assess visual function properly, different parameters relating to the functional domains must be tested. Another issue is that attempts to relate PCO severity, assessed by slit lamp observation or image evaluation software, to VA and stray light have not been very successful. ^{1 –3,5,6,11 –13} This may be due to the fact that functional impairment is caused by forward light scatter, whereas slit lamp evaluation of PCO severity is based on the amount of backward scattered light. Because backscattered light does not necessarily correspond to functional impairment, forward light scatter should be assessed. ^{14 –17} We expect that both issues, (1) the independent effect of PCO on the visual function parameters VA and stray light, and (2) the lack of correspondence between PCO severity and visual function parameters, may be related to PCO morphology. Unfortunately, it is hardly possible to assess the effect of PCO morphology on visual function in vivo. 

Based on pathogenesis and cells of origin, there is a clinical differentiation between two morphologic forms of PCO, (regeneratory) pearl-type and fibrosis-type PCO. ¹⁸ In addition, there are morphologic differences within the pearl-type PCO, which adds to the morphologic heterogeneity of PCO. ¹⁹ The optical behavior of different PCO types may be diverse. Some optical characteristics may predominantly affect the small-angle domain of visual function, resulting in impaired VA, whereas others may predominantly affect the large-angle domain, resulting in an impaired stray light value. The present study focuses on stray light. 

Imperfections of the eye's optical media cause aberrations and light scatter, even in young, healthy eyes. As a result, the retinal image will not be identical with the original visual stimulus, and the quality of the retinal image suffers. A common way to address this visual degradation is to suppose that the visual stimulus is a single point of light. In the presence of optical media imperfections, the light intensity of its retinal projection will be spread out over the retina. The retinal projection will be a bright light spot in the center, surrounded by a zone of diminished light intensity (Figs. 1A, 1B). This light distribution on the retina is called the “point-spread function” (PSF). It comprehensively describes the eye's optical quality. Figure 1C shows the PSF for a healthy, young, Caucasian eye, according to the Commission Internationale de l'Eclairage (CIE) ²⁰ ; an International Commission on standards in vision (see http://www.cie.co.at). In this study, the PSF for a healthy, young, Caucasian eye will be referred to as the “normal PSF.” The relative light intensity of the light point is plotted as a function of visual angle (θ). The two functional domains mentioned above are shown in Figure 1C. The central, small-angle domain is indicated by the speckled area. The large-angle domain comprises visual angles beyond 1.0° and is indicated by the striped area. The small-angle domain is affected by aberrations, resulting in diminished sharpness of the retinal image and loss of contrast. ¹⁰ This domain can be assessed by functional tests such as VA (VA of decimal 1.0 corresponds to θ ≈ 0.02°) and contrast sensitivity (spatial frequencies of >3.0 cycles per degree correspond to θ < 0.3°) and by optical approaches such as aberrometry and double-pass techniques. The large-angle domain is affected by light scatter. Scatter affects vision predominantly because in the retinal image, light from bright areas in the visual scene spreads toward dark areas. It results in a reduced retinal contrast, which is experienced by the subject as hazy vision and blinding. The corresponding functional impairment is called disability glare. Note that basic contrast sensitivity tests that are used in the clinical setting only assess contrast reduction caused by small-angle effects, such as aberrations. Contrast reduction caused by large-angle effects such as light scatter, is not assessed by contrast sensitivity tests used in the clinical setting. ^10,21,22 By international consensus, disability glare is defined as stray light, because stray light was proven to predict disability glare precisely. ²³ Stray light can be assessed clinically (C-Quant; Oculus GmbH, Wetzlar, Germany). 

Figure 1.

 

A retinal image of the outside world with a bright light originating from a car headlight, which is degraded by light scatter in a normal (young, healthy) eye (A) and in an eye with media imperfections such as PCO (B). Part of the bright light is scattered in all forward directions; its retinal projection is a bright spot surrounded by a spreading of light over the entire retina. The functional effect is a veil of light, which is called stray light, projected all over the retinal image of the outside world. (C) The Point-Spread Function (PSF) for a normal eye, according to the CIE standard. There are two domains of visual function, a small-angle domain and a large-angle domain (θ beyond 1°). In this study, the large-angle domain is divided into a near large-angle domain (1° < θ ≤ 7°) and a far large-angle domain (θ > 7°).

In vivo it is difficult to isolate the optical characteristics of PCO from the optical influence of other parts of the eye. In addition to this, capsular bags with a homogeneous coverage of a single PCO type are scarce. In the present study, the light-scattering characteristics of PCO were studied in vitro with an optical set-up, which allows isolation of homogeneous PCO areas with a specific morphology. This optical set-up has been used previously to investigate forward light scattering by the crystalline lens. ^24,25 A camera was used to document the light scatter pattern, which in vivo would have been sensed by the retinal photoreceptors. The purpose of our study was to document the scattering characteristics in different PCO types, and to elucidate the impact of PCO morphology on the large-angle domain (stray light domain, visual angles beyond 1°) of visual function. 

Methods

Human donor bulbi were used for this study. They were obtained from the Cornea Bank Amsterdam; only pseudophakic donor bulbi were selected. Information on the donor, such as ophthalmologic history, was not available. Only specimens with an intact capsule were included. As will be detailed later in this section, the specimens had to be representative of the in vivo situation. Because the aim of this study was to document forward scattered light by PCO, there were no inclusion criteria concerning intraocular lens (IOL) type or dioptric power. Note that the refractive design of the IOL affects only the earlier mentioned small-angle domain. Small-sized irregularities, such as diffractive design, glistenings, or Nd:YAG laser lesions of the IOL optic, could be expected to affect the large-angle domain. However, such irregularities were not present in the used specimens. IOLs potentially scatter due to less easily recognizable processes, of a more diffuse nature. It cannot be excluded that such processes played a role in specimens with clear capsules. Clear capsules always showed low level recordings. The precise level of these recordings cannot be assumed to faithfully represent the clear capsule. However, in the presence of PCO, PCO areas showed much more scatter compared with clear areas, and PCO scatter dominated over IOL processes. 

After the Cornea Bank Amsterdam had removed the corneoscleral disc for transplantation purposes, we isolated the specimens, which were capsular bags enclosing an IOL, from the bulbi. As capsular bags are fragile and easily damaged during preparation, many specimens had to be discarded. During the study, damage of the capsular bags was reduced by immersion of the bulbus in a fixative before preparation and by improvement of the preparation technique. Initially, an immersion medium of phosphate-buffered saline (PBS) only was used, because it was unclear whether a fixative would affect the optics of the capsular bag tissue. It is important to assure tissue stability of unfixated specimens during the recording process. Therefore, at the end of the entire recording procedure the first recording was repeated and served as a quality control. ²⁴ During the study, the effect of a 1% paraformaldehyde and PBS fixative on the appearance of opacified capsular bags was studied. Immediately after immersion and also after 2, 4, 6, 8, and 24 hours, the specimen's appearance was carefully monitored using the darkfield microscopy set-up shown in Figure 2 (report in preparation). After it was found that there were no discernible changes, the 1% paraformaldehyde and PBS solution was used to fixate the bulbi. 

Figure 2.

 

Darkfield microscopy set-up (side view, not to scale) used to examine and photograph the specimens. A digital camera was mounted on the microscope. The specimens were placed under the microscope with its posterior side down and illuminated by a darkfield ring light.

To isolate the capsular bag, the donor bulbus was put in an eye holder and positioned under a microscope. The initial preparation technique included removal of the iris, and cutting of the zonular fibers over 360°, at a position close to the ciliary body. The capsular bag was carefully lifted out using a spoon-shaped surgical spatula, and at the same time vitreous adhesions were removed. Later, we used a different preparation technique, which resembles the Miyake-Apple technique. ^26,27 After removal of the iris, a continuous 360° pars plana incision was made parallel to the limbus. This yielded a specimen of a capsular bag with IOL, attached by zonula fibers to a scleral rim approximately 2 mm wide. The specimen was carefully lifted out to avoid vitreous traction, and vitreous adhesions to the posterior capsule were removed. The specimen was then transferred to a Petri dish and was examined for free iris pigment and residual vitreous. Any pigment present was removed by rinsing the specimen in several Petri dishes with fresh PBS. After residual vitreous had been removed, the specimen was put in a clean Petri dish. 

Because it was important that the specimens corresponded closely to in vivo capsular bags, all specimens obtained were carefully examined by experienced ophthalmologists (IJEM and BLMZ). For this purpose, the specimen was put under a microscope (Stemi SV 11 stereomicroscope, Zeiss, New York) (Fig. 2). This set-up also included slit lamp illumination (not shown), corresponding to the observation technique used by ophthalmologists when examining capsular bags in vivo. To ensure that the in vitro microscope view corresponded to the in vivo ophthalmologist view, the specimen was placed with the posterior side down. It was illuminated from an angle of 30° using a darkfield ring light (Schott AG, Mainz, Germany) fed by a cold light source (KL 1500 LCD, Schott AG). To prevent small surface imperfections of the Petri dish producing artifacts in the photomicrograph, the specimen was raised slightly by placing it on a small rubber ring (outer diameter 15 mm, inner diameter 10 mm, thickness 2.5 mm). Color photomicrographs of the specimens were made using a digital camera (DSC-S75, Sony Electronics Inc., Oradell, NJ), mounted on the microscope. The white balance of the camera was carefully set using the illumination light reflected off a white standard (Spectralon Diffuse Reflectance Standard SRS-99-010, Labsphere Inc., North Sutton, NH). Camera settings, such as shutter speed and focus distance were fixed. 

To image the light scattered by the specimen it was transferred to a goniometer set-up, which was previously used for studies on the crystalline lens. ^24,25 A top view representation of the set-up is shown in Figure 3. As a rule, the intensity of scattered light in human eyes is low. Therefore, it is essential to carefully control the amount of light scattered by sources other than the specimen, such as dust particles. A cell (Hellma 700.000-OG, height 50 mm × width 50 mm × depth 10 mm; Hellma GmbH, Müllheim, Germany) was meticulously cleaned with a cleaning concentrate diluted in distilled water (Hellmanex concentration 1%; Hellma GmbH). Next, the cell was filled with PBS and placed in the goniometer set-up. A light-scatter baseline was measured. The maximum accepted value was log[s] = −1, corresponding to approximately 1% of the average amount of stray light in healthy, young eyes. ²⁵ The definition of “log[s]” is described elsewhere. ^25,28 The specimen was immersed in the PBS-filled cell. To remove the effects of surface reflections of the cell from the measurement, it was rotated 13° from the θ = 0° position (Fig. 3). As may be expected, this had virtually no effect on the recordings of the specimen itself. To ensure that the amount of light scattered by the PBS-filled cell had not altered during the process of image acquisition, after each experiment the specimen was carefully removed and the light-scatter baseline measurement was repeated. ²⁴  

Figure 3.

 

Goniometer set-up (top view, not to scale) used to obtain grayscale images of PCO. The posterior part of the specimen was oriented toward the camera. The camera served as the in vitro counterpart of the in vivo retinal photoreceptors and recorded scattered light. The scattered light was used to produce grayscale images of PCO.

In the goniometer set-up, light was emitted by a halogen light source and passed through an infrared blocking filter and a narrowband interference filter. Narrowband interference filters transmit a narrow range of selected wavelengths. Three narrowband (full width at half hight [FWHH], 10 nm) interference filters were used: red (peak wavelength of 661 nm), green-yellow (peak wavelength of 561 nm), and blue (peak wavelength of 440 nm) (Optics Balzers AG, Balzers, Liechtenstein). The 561 nm data are given unless otherwise noted. A circular area of the specimen with a diameter of 4 mm, which corresponds to an average photopic/mesopic pupil diameter, was illuminated. The posterior part of the specimen was oriented toward the camera. So, the incident light first reached the anterior capsule and then the posterior capsule, as it does in vivo. The part of the incident light that is scattered in the forward direction by the specimen, was collected by a charge-coupled device (CCD) camera. ²⁹ The camera represents the in vitro counterpart of the in vivo retinal photoreceptors, which detect the light scattered toward the retina. The images obtained were grayscale images of scattered light. The amount of light collected by the camera is limited by the aperture diameter. In the set-up an aperture diameter of 3 mm was used and the distance from aperture to the specimen was 14 cm, this resulted in an aperture of 1.2° diameter. 

If the camera is positioned at θ = 0° (Fig. 3), the light transmitted directly (nonscattered light) is collected. This can be compared with a clinical retrograde slit lamp image, although the direction of the light is reversed (because the fundus is used as a reflector). In a retrograde image, the differences in light intensity can result from differences in the amount of light scattered. At opacified areas, some of the incident light is scattered, so only a reduced amount of light will be detected at θ = 0°. As a consequence, opacified areas will appear as less intense, shadowy patterns in the retrograde image. 

The specimen scattered the incident light in different forward directions. The scattered light was recorded by rotating the camera in the horizontal plane around the specimen and acquiring images at fixed scatter angles (θ_{in air} = −30°, −20°, −15°, −10°, −7°, −4°, +4°, +7°, +10°, +15°, +20°, and +30°). In vivo the scattered light projects toward the retina through the vitreous body, a medium with a refractive index of 1.336. ³⁰ The recording angles were corrected for this refractive index, resulting in visual angles of θ = −22°, −15°, −11°, −7°, −5°, −3°, +3°, +5°, +7°, +11°, +15°, and +22°, as indicated by the black dots in Figure 1C. All these visual angles are beyond 1.0° and therefore concern the large-angle domain only. The visual angles θ = −7°, −5°, −3°, +3°, +5°, and +7° are nearer to the small-angle domain than the angles θ = −22°, −15°, −11°, +11°, +15°, and +22°. In this study the angles θ = −7°, −5°, −3°, +3°, +5°, and +7° are indicated as the “near large-angle domain,” and the angles θ = −22°, −15°, −11°, +11°, +15°, and +22° are indicated as the “far large-angle domain” (Fig. 1C). The visual angles can be used to compare in vitro and in vivo light scattering characteristics. ²⁵ For each specimen, all images obtained were combined in an image-set. 

For each scatter angle θ, the corresponding PSF value was calculated by dividing the amount of light collected at angle θ (I _θ) by the total amount of light passing through the specimen (I _total), or PSF (θ) = I _θ / I _total (1/steradian). ²⁵ I _total was measured by positioning the camera at θ = 0° (Fig. 3), and using a wide aperture of 10° diameter. As the light intensity collected at θ = 0° is high, there is a risk that the image will be saturated. To reduce the intensity by a factor of approximately 10³, a calibrated neutral density filter was inserted in front of the infrared blocking filter. ²⁵  

In the normal PSF, the intensity diminishes as a function of θ, approximately as 1/θ² (Stiles-Holladay approximation for θ > 1°). ²⁰ Because of the steeply declining scatter intensity, much light is collected at the smallest angles of this domain and the amount collected diminishes substantially with increasing angle. For example, at θ = 5° (1/θ² = 1/25) the light intensity would be about a factor 20 times higher than the light intensity at θ = 22° (1/θ² = 1/484). Without rescaling, the grayscale images collected at large angles would be very dark. To obtain images of similar brightness, the light intensities of the images obtained were rescaled using the normal PSF. ²⁰ So, at each angle the recorded PCO intensities were divided by the corresponding PSF intensity (black dots in Fig. 1C). If, after this rescaling, intensity differences exist between the images of an image-set, these differences indicate how PCO scatter differs from that of the healthy, young eye (normal PSF). Areas of relatively bright intensity indicate more light scatter than the normal PSF, whereas dark areas indicate less light scatter than the normal PSF. 

The angular and the wavelength dependence of scattered light can be used to assess the size of small scattering particles. ¹⁵ In this study, the type of wavelength dependence of PCO scatter is assessed using the three different peak wavelengths mentioned earlier in this section. The intensity of scattered light depends on particle size. Scattering by particles much smaller than the light wavelength has strong wavelength dependence and weak or no angular dependence, especially for the angular range investigated in this study. Larger particles have weaker or no wavelength dependence and strong angular dependence. In the earlier mentioned in vitro studies on the human crystalline lens, the wavelength and angular dependence found was of intermediate strength, corresponding to particles of the same order of magnitude as wavelength. ^24,25 It should be noted that the wavelength dependence of retinal stray light has long been controversial. Although early in vivo studies found no significant wavelength dependence, ^31,32 later in vitro and in vivo studies did demonstrate wavelength dependence. ^15,33,34 The weak effect found in the early in vivo studies was clarified as the result of opposing wavelength dependencies and relatively imprecise measurement techniques. ^15,33,34  

Results

In total, 59 specimens with capsule were obtained. As mentioned earlier, it was important that the specimens closely corresponded to in vivo capsular bags, according to the assessment of experienced ophthalmologists (IJEM and BLMZ). This assessment identified 25 representative PCO specimens. Figure 4 shows photomicrographs of eight of these specimens. The eight specimens represent the morphologic variation that was found among the included 25 specimens. Specimen number 8 in Figure 4 shows a nearly clear posterior capsule. The posterior capsule of specimen number 3 is covered by a mild, diffuse PCO. The posterior capsule of specimen number 6 has areas of mild, diffuse PCO and relatively clear areas. It also shows posterior capsule folds. Specimen number 1 shows fibrosis-type PCO, whereas specimen numbers 2, 4, 5, and 7 show pearl-type (regeneratory) PCO. According to the ophthalmologists, the PCO severity in specimen numbers 2, 4, and 7 would have justified Nd:YAG laser posterior capsulotomy. Specimen numbers 2 and 7 show mild fibrosis of the anterior capsule and specimen numbers 3, 4, and 6 show severe fibrosis of the anterior capsule. 

Figure 4.

 

Photomicrographs of eight representative specimens, obtained with the darkfield microscopy set-up shown in Figure 2.

Figure 5 shows a comparison between images obtained in capsular bags numbers 1 to 4 shown in Figure 4. The first column shows the photomicrographs obtained with darkfield microscopy, the other columns show images obtained with the goniometer. The second column shows the “retrograde” images (θ = 0°), the third column the scattered light images captured at the near large-angle domain (θ = 5°) and the fourth column shows the scattered light images captured at the far large-angle domain (θ = 22°). For proper comparison with the images from the goniometer, the photomicrographs were inverted, rotated, and size-adjusted. Note that the photomicrographs show an image that is the complement of the “retrograde” image. As mentioned in Methods, the photomicrographs were obtained using the technique of darkfield microscopy. This technique collects scattered light only, which is why light-scattering parts of the capsule show up brightly against a dark background. On the contrary, a “retrograde” image shows only directly transmitted light. Because in light-scattering areas some light is scattered and therefore lost from direct transmission, the light intensity of scattering areas is reduced and they show up as “shadow-like” areas. For example, image 5.3A shows the enhanced light intensity of the anterior capsule and image 5.3B shows its complement, a “shadow-like” area of diminished light intensity. Similarly, the Elschnig pearl edges in image 5.2A have an enhanced light intensity, as opposed to the corresponding edges in image 5.2B, which have a diminished light intensity. See also the detailed images in Figure 6. 

Figure 5.

 

A comparison between photomicrographs obtained with the darkfield microscopy set-up (Fig. 2) and grayscale images obtained with the goniometer set-up (Fig. 3) at, θ = 5° and θ = 22° in four different specimens. In each row, the photomicrograph and the “retrograde” image (θ = 0°) complement each other.

Figure 6.

 

A comparison between detailed photomicrographs and complementary grayscale images (θ = 0°) obtained in specimen number 1 and 2 (Fig. 4).

Figures 7 and 8 show the angular dependence of PCO-scatter. They are complete image-series of scattered light obtained in specimen numbers 1 and 2 shown in Figure 4. There seems to be little difference between positive and negative angles, so scattering seems to be symmetrical to opposite sites. As mentioned in Methods, intensity differences between the images of the image sets shown in Figures 7 and 8 can be used to assess the nature of the angular dependence of PCO-scatter. The intensity differences within the image sets are small. Put differently, the stray light part of the PCO-PSF has more or less the same course as the stray light part (large-angle domain) of the normal PSF. Figure 9 shows quantitative data on the angular dependence of PCO-scatter. Figure 9A shows the average difference in shape between the PCO-PSF obtained from the complete set of representative specimens, and the normal PSF as a function of visual angle (short-dashed curve, right vertical axis). Note that if the slope of the short-dashed curve would have been zero, it would indicate an exact correspondence between angular dependence of the PCO-PSF and the normal PSF. The error bars represent standard deviations over all representative specimens. From these bars it is clear quantitatively that the difference in angular dependence among all specimens is small. Figure 9A also shows the same average result, but on an absolute PSF scale (solid line, left vertical axis). The long-dashed curve represents the normal PSF (corresponding to Fig. 1). The two curves show a close correspondence. 

Figure 7.

 

The angular dependence of scattered light images obtained in specimen number 1 (Fig. 4) with the goniometer set-up (Fig. 3). The overall light intensity is higher in images obtained at the near large-angle domain (1° < θ ≤ 7°) than in those obtained at the far large-angle domain (θ > 7°). Note that images collected at the far large-angle domain (e.g., θ = −22°, θ = +22°) are somewhat elliptical, due to the oblique angle of observation.

Figure 8.

 

The angular dependence of scattered light images obtained in specimen number 2 (Fig. 4) with the goniometer set-up (Fig. 3). The PCO area shows an increased overall light intensity, especially at the near large-angle domain (1° < θ ≤ 7°). Again, the images collected at the far large-angle domain are somewhat elliptical.

Figure 9.

 

Quantitative data on the angular (A) and wavelength dependence (B) of PCO-scatter. (A) The short-dashed curve (right vertical axis) shows the average difference in shape between the PCO-PSF, obtained from the complete set of representative specimens, and the normal PSF as a function of visual angle. The error bars represent standard deviations over all representative specimens. The same average result is also shown on an absolute PSF-scale (solid line, left vertical axis). The long-dashed curve represents the normal PSF. (B) PCO-PSF values obtained with a blue light filter (440 nm, blue curves), a green-yellow light filter (561 nm, green curves), and a red light filter (661 nm, red curves) as a function of visual angle. PCO-PSF values obtained in fibrosis-type PCO (specimen number 1 in Fig. 4) are given as solid curves and those obtained in pearl-type PCO (specimen number 2 in Fig. 4) are given as long-dashed curves. For clarity, the long-dashed curves are displaced 0.30 upward along the vertical axis.

However, close scrutiny of Figures 7 and 8 reveals slight intensity differences. The overall light intensity in the images captured at the near large-angle domain is higher than in the images captured at the far large-angle domain, e.g., the brightness differences between images 7.2 and 7.13 of the set in Figure 7. The brightness differences between images 8.2 and 8.13 of the set in Figure 8 are most pronounced. The intensity differences also show up in Figure 9. The slope of the short-dashed curve in Figure 9A is slightly steeper than that of the normal PSF. Also note the on average steeper slope of the three dashed curves corresponding to the pearl-type specimen in Figure 9B, compared with the slope of the three solid curves corresponding to the fibrosis-type specimen. Another finding is that the light intensity observed in pearl-type PCO (Fig. 8) is enhanced compared with fibrosis-type PCO (Fig. 7). 

Figure 7 also shows that the fibrosis-type specimen has fiber structures with different orientations; a predominantly vertical orientation at the three and nine o'clock positions, indicated by the ellipses, and a predominantly horizontal orientation at the six o'clock position, indicated by the square. Closer scrutiny reveals that the vertically-oriented fiber structures have a markedly brighter appearance than those oriented horizontally. Because of their convex surface, the fiber structures are expected to have a rod-like behavior. It was realized before that structures with a rod-like shape, such as posterior capsule folds, behave optically like rods. ^18,35 This is called the “Maddox-rod phenomenon”. ^18,36 We expect that other structures with a convex surface, such as the fiber structures in fibrosis-type PCO, will behave similarly. In these structures, incident light is focused as a line perpendicular to the axis of the fiber structure. So, vertically-oriented fiber structures focus incident light as a horizontal line, which is recorded by the optical set-up of this study. Because light deflected in other planes is not recorded by this set-up, only the vertically-oriented fiber structures have a brighter appearance. These brightness differences between the two orientations are most distinct at the near large-angle domain (Fig. 7). 

Figures 10 and 11 show the wavelength dependence of the scattered light images obtained in specimen numbers 1 and 2 in Figure 4, using the three different wavelengths (661 nm, 561 nm, and 440 nm). In both specimens, the scattered light images obtained with the three wavelengths appear quite similar. Closer inspection reveals some brightness differences between the images in a set recorded at identical angles but using different wavelengths, for example, differences between images 10.1B, 10.2B, and 10.3B of the set in Figure 10. The images at wavelengths of 561 nm and 661 nm are less bright than those at 440 nm. As described in Methods, all data were normalized with respect to the direct beam. This was done for each wavelength independently. So, with decreasing wavelength the fraction of the light that is scattered increases. This is in correspondence with the color photomicrographs in Figure 4 showing a blue hue. Close scrutiny of the image sets of Figures 10 and 11 reveals that the wavelength dependence in pearl-type PCO is weaker than that of fibrosis-type PCO. Note the minimal brightness differences of the pearly area in images 11.1B, 11.2B, and 11.3B, compared with those of the fibrosis area in images 10.1B, 10.2B, and 10.3B. Figure 9B shows quantitative data on wavelength dependence of light scattered by pearl-type PCO and fibrosis-type PCO. Note the narrowly-spaced dashed curves in blue, green, and red obtained in pearl-type PCO, compared with the more widely spaced solid curves obtained in fibrosis-type PCO (Fig. 9B). 

Figure 10.

 

The wavelength dependence of scattered light images obtained in specimen number 1 (Fig. 4) with the goniometer set-up (Fig. 3). The images obtained with a blue light filter (440 nm, third row) are brighter than those obtained with the green-yellow (561 nm, second row) or the red light filter (661 nm, first row).

Figure 11.

 

The wavelength dependence of scattered light images obtained in specimen number 2 (Fig. 4) with the goniometer set-up (Fig. 3). The pearl area is only slightly brighter when the blue light filter (440 nm, third row) is used, compared with the green-yellow (561 nm, second row) or the red light filter (661 nm, first row).

The mildly opacified anterior capsule (images 11.1B, 11.2B, and 11.3B) shows a slightly stronger wavelength dependence compared with the pearl- and fibrosis-type areas. 

Discussion

In this study, the scattering characteristics that are important in PCO were documented. The nature of the angular dependence of light scattered by different PCO-types was assessed by comparing it with that of a normal eye. In addition, the wavelength dependence of light scattered by different PCO types was visualized. 

We found that the angular dependence of PCO scatter is similar to that of scatter in the normal eye. In vitro crystalline lens studies already found that the angular dependence of light scattered by the crystalline lens is similar to that of scatter in the normal eye, ^24,25 which corresponds to the finding of an early study that in the normal aging eye, the crystalline lens is the dominant stray light source. ²³ We must conclude that the angular dependence of PCO scatter and lenticular scatter is similar to that of scatter in the normal eye. This is surprising, because PCO has a different morphology in comparison with lenticular opacification. It should be noted that, despite the similar angular dependence, in PCO the scatter intensity can be much higher than in the normal eye. The morphologic heterogeneity of PCO showed up in details; it was found that the angular dependence of light scattered by pearl-type PCO is stronger than light scattered by fibrosis-type PCO. As a consequence, pearl-type and fibrosis-type PCO may have a different functional effect on visual quality, as will be described further in this section. 

Close inspection of the image-series in Figures 7 and 8 did reveal an exception to the similarity in angular dependence of PCO scatter and scatter in the normal eye: at the near large-angle domain (1° < θ ≤ 7°), the angular dependence of PCO scatter is slightly stronger, which implies the presence of additional light-spreading pattern caused by a refractile component. The Maddox-rod phenomenon in fibrosis-type PCO, which was described in Results, appears to cause such an additional light-spreading pattern. On the basis of physics theory it can be expected that because of its morphologic appearance, pearl-type (regeneratory) PCO might also produce an additional light-spreading pattern. Although we did find that PCO scatter has some wavelength dependence, especially in pearl-type PCO it is weak. This marginal wavelength dependence implies the presence of a refractile component in the light-spreading pattern caused by pearl-PCO. 

The angular and wavelength dependence found in this study, suggest that PCO scatter is dominated by small particles. This also applies to the scattering particles of opacified anterior capsules. The size of these particles is in the order of wavelength of visible light (400 nm to 700 nm). However, apart from scatter, PCO has a refractile component caused by structures much larger than wavelength, such as rods and pearls. These refractile structures affect the near large-angle domain. This effect might be an extrapolation from the small-angle domain. Depending on the ratio between small particles and refractile structures in PCO, PCO may mainly affect the small-angle or the large-angle domain, which may elucidate the quite independent effect of PCO on the visual function parameters VA and stray light. The ratio in pearl-type PCO may be in favor of refractile structures, whereas in fibrosis-type PCO it may be in favor of small particles. As a consequence, pearl-type PCO may affect VA to a larger extent than stray light, whereas fibrosis-type PCO may affect stray light to a larger extent than VA. 

In this study, the CCD camera served as the in vitro counterpart of the in vivo photoreceptors. So, the camera recorded the scattered light that the photoreceptors would sense in an in vivo situation. Regarding this analogy, two remarks must be made. The first is that the visual scene ‘perceived’ by the camera differs from the visual scene that would have been perceived by a PCO patient. The visual scene captured by the CCD camera was the specimen (Figs. 7, 8, 10, and 11), whereas that perceived by a PCO-patient is a scene of the outer world (e.g., Figs. 1A and 1B). Second, the cone photoreceptors would not detect the exact same amount of light as captured on the images, because of the Stiles-Crawford effect. The Stiles-Crawford effect refers to the directional sensitivity of photoreceptors, particularly those in the central fovea; foveal cones are less sensitive to rays of light passing through the pupil margin. The amount of light captured on the images in this study most closely corresponds to the amount of light detected by peripheral photoreceptors. So, the effect of PCO on retinal image formation detected by the photoreceptors may depend on PCO localization. Moreover, PCO located behind the peripheral margin of the pupil has little effect on both retinal image formation and light scatter. On the contrary, centrally located PCO deteriorates retinal image formation and also increases light scatter. 

In summary, although PCO shows an increased intensity of scatter, the light-scattering characteristics of PCO are very similar to those of the normal eye. This indicates the presence of small particles in PCO. However, PCO and especially pearl-type PCO, has an additional light-spreading pattern caused by refractile components with weaker wavelength dependence, which is typical for refractile structures. We expect that the size ratio between small particles and refractile structures in PCO determines its effect on the two domains of visual function and their corresponding functional impairments. Fibrosis-type PCO may predominantly consist of small particles, which mainly affect the large-angle domain. Functionally, fibrosis may have a more important effect on stray light than on VA. In pearl-type PCO refractile structures may be relatively more important, affecting the small-angle domain. Functionally, pearls may have a more important effect on VA than on stray light. 

Footnotes

 Presented in part at the annual meeting of the European Association for Vision and Eye Research, Crete, Greece, October 2010 (program no. 4241); and at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, May 2011 (program no. 827).

Footnotes

 Supported in part by Dutch Grant ISO 62031.

Footnotes

 Disclosure: M.C.J. van Bree, None; I.J.E. van der Meulen, None; L. Franssen, None; J.E. Coppens, None; N.J. Reus, None; B.L.M. Zijlmans, None; T.J.T.P. van den Berg, None

The authors thank Cornea Bank Amsterdam, The Netherlands, for the supply of human donor bulbi; and Bastiaan Kruijt, PhD (Netherlands Institute for Neuroscience, Royal Netherlands Academy, Amsterdam, The Netherlands) for fixation of the specimens and contribution to Figures 2 3 4 5 6 7–8, 10, and 11.