The optical and mechanical design of an intraocular lens (IOL) to provide ideal performance includes many parameters.1 Among those that have been evaluated are optic shape (equiconvex, asymmetric biconvex, convexplano, planoconvex, and meniscus), optic and haptic material, haptic angulation, and optic diameter. One parameter that has received no theoretical and little clinical evaluation is the edge design of the optic.
The IOL's edge design can significantly affect its optical and mechanical performance after implantation. The optical effects are usually referred to as edge glare or unwanted optical images,2–4 and the mechanical effects may influence posterior capsule opacification (PCO) rates (D.J. Apple, MD, Q. Peng, MD, N. Visessook, MD, R.J. Schoderbek, MD, “Enhancement of the IOL Optic Barrier Effect Against Posterior Capsule Opacification Using a Truncated Optic,” presented at the XVIth Congress of the European Society of Cataract & Refractive Surgeons, Nice, France, September 1998). The purpose of our study was to explain the nature of edge glare and unwanted optical images as a function of the edge design.
Materials and Methods
The interaction of light rays as a function of edge design was evaluated using an eye model and the OptiCAD 3-D radiometric ray-tracing program (Opticad Corp.). The pseudophakic eye model with a glare source (Figure 1) consisted of the following physiologic parameters: corneal power, 42 diopters (D); external anterior chamber depth, 4.5 mm (corneal vertex to anterior vertex of IOL); IOL power, 20 D; optic diameter, 6 mm; axial length, 23 mm; and pupil diameter, 5 mm. The glare source consisted of a collimated light source at a 35 degree angle to the optical axis, consistent with nighttime glare conditions. Preliminary ray-trace analysis indicated that a 35 degree angle maximized the intensity of the reflected glare image. The image of the glare source and the reflected glare image were formed on opposite sides of the peripheral retina (Figure 1).
Four IOL edge designs were evaluated: nonlenticular biconvex with sharp or rounded edges and lenticular biconvex (edge extension) with sharp or rounded edges (Figure 2). In all cases, the IOL body diameter remained 6 mm. The optic diameter was reduced by 0.74 mm in the lenticular designs. An edge radius of 0.068 mm was used for the rounded-edge designs. For each design, the analysis traced 160 000 rays from the glare source through the pseudophakic eye model to the produced retinal image. The large number of rays used ensured accurate peak intensity, spatial location, and energy distribution of the reflected glare image.
Analysis of the results revealed 3 classes of rays in the vicinity of the optic edge (Figure 3). The first class missed the IOL entirely, causing an aphakic crescent located near the image of the glare source. The second class was reflected internally by the optic edge. The third class avoided the optic edge and was refracted by the anterior and posterior optic surfaces to form the image of the glare source.
Figure 4, A shows the energy distributions and retinal images formed by the nonlenticular sharp- and rounded-edge designs. The sharp-edge design formed a distinct arc-like pattern and the rounded-edge design, a diffuse image. Rounding a nonlenticular biconvex lens reduced the peak intensity of the reflected glare image from 44.9 to 4.0 relative units, a 91% reduction. Figure 4, B shows the energy distributions and retinal images formed by the sharp-edge nonlenticular and lenticular designs. Both sharp-edge designs formed distinct arc-like patterns. Adding the lentical reduced the peak intensity of the reflected glare image from 44.9 to 34.9 relative units, a 22% reduction. Figure 4, C shows the energy distribution and retinal image formed by a sharp-edge nonlenticular design and compares them with those formed by a rounded-edge lenticular design. The sharp-edge nonlenticular design formed a distinct arc-like pattern and the rounded-edge lenticular design, a diffuse image. The combination of edge rounding and lentical reduced the peak intensity of the reflected glare image from 44.9 to 6.0 relative units, an 87% reduction.
The data suggest that a rounded edge provides the greatest reduction in potential glare or unwanted optical images. Rounding disburses reflected edge glare rays (Figure 3, left and right), reduces the intensity on the retina, and reduces the potential for unwanted optical images. A lenticular edge design provides potential glare reduction for designs with sharp edges. The flat anterior and posterior surfaces of the lenticular edge design (Figure 3, right) have no effect on the path of the reflected edge-glare rays. However, the anterior and posterior surfaces of the nonlenticular edge design (Figure 3, left) provide some focusing of the reflected edge-glare rays, increasing the intensity on the retina (Figure 4, B).
Edge rounding and lenticular effects are not independent; combining them reduced the intensity by 87% (Figure 4, C), whereas rounding alone (nonlenticular design) reduced the intensity by 91% (Figure 4, A). The nonlenticular rounded-edge design provides a small additional reduction in intensity because the anterior surface power contributes to the disbursement of reflected edge-glare rays, which subsequently pass through the posterior rounded edge, or rays initially passing through the anterior rounded edge and reflected internally are subsequently disbursed by the posterior surface power. The intensity difference between nonlenticular and lenticular rounded-edge designs is not clinically significant, especially compared with the effects of rounding alone.
Recent reports have suggested the sharp-edge design as a possible factor in reducing PCO (D.J. Apple, MD, Q. Peng, MD, N. Visessook, MD, R.J. Schoderbek, MD, “Enhancement of the IOL Optic Barrier Effect Against Posterior Capsule Opacification Using a Truncated Optic,” presented at the XVIth Congress of the European Society of Cataract & Refractive Surgeons, Nice, France, September 1998). At present, it is unclear whether the apparent PCO reduction is due to the IOL material, a sharp-edge design, or some combination.
The optical consequences of a sharp-edge design are not conjecture and have been demonstrated clinically and in the laboratory.2 A sharp-edge design increases the probability of a thin, ring-like image in the midperipheral retina on the side opposite the primary image of the glare source. Ovoid IOLs were made from circular IOLs that were truncated, leaving them with sharp corners. The use of ovoid IOLs declined in the mid 1990s after the edge glare was documented in the laboratory.2,5 A recent clinical report of pseudophakic glare confirms the presence of a crescent semi-ring-like visual sensation at night, which was not seen when a nonlenticular IOL with a sharp edge was replaced by a lenticular rounded-edge design (R.J. Olson, MD, “IOL Exchange to Correct Severe Glare in 5.5 mm AcrySof Patients,” presented at the Symposium on Cataract, IOL and Refractive Surgery, San Diego, California, USA, April 1998; “Pseudophakic Dysphotopsia,” presented at the XVIth Congress of the European Society of Cataract & Refractive Surgeons, Nice, France, September 1998).
Additional lens characteristics that would increase the intensity and size of the retinal ring image include increasing the edge thickness or reducing the optic diameter of the lens. Patient characteristics that may affect whether the image is noticed clinically include scotopic pupil diameter (larger pupil size when driving at night) and opacification of the peripheral capsule that may mask any edge characteristics. This latter characteristic has been postulated to explain the decrease in patient symptoms over time.
If PCO is shown to be reduced clinically with the sharp-edge design, the best optical design will be the opposite of the best mechanical design. If this dilemma exists, further refinements in edge design will be necessary to determine the optimal characteristics that minimize unwanted optical images and PCO. The optimal design must be weighed clinically against the frequency of occurrence and the complications associated with the treatment of each condition. If unwanted optical images persist from edge design, the only treatment is lens exchange. If significant PCO occurs, a capsulotomy can be performed. Both procedures have associated risk, but the intraocular procedure of lens exchange has a significantly higher risk than the noninvasive procedure of neodymium:YAG capsulotomy.
Ray tracing confirmed that the glare images from the edge of an IOL with a sharp corner appear like a thin crescent or partial ring in the periphery of the retina opposite the image of the glare source. Rounding the corners significantly reduced the peak intensity of the reflected glare image by 87% to 91% (10 times). Thus, edge rounding can significantly reduce the potential for unwanted optical images.
1. Holladay JT, Bishop JE, Prager TC, Blaker JW. The ideal intraocular lens. CLAO J 1983; 9:15-19
2. Masket S, Geraghty E, Crandall AS, et al. Undesired light images associated with ovoid intraocular lenses. J Cataract Refract Surg 1993; 19:690-694
3. Smith SG, Lindstrom RL. Intraocular Lens Complications and Their Management. Thorofare, NJ, Slack, 1988
4. Holladay JT. Evaluating the intraocular lens optic. Surv Ophthalmol 1986; 30:385-390
5. Leaming DV. Practice styles and preferences of ASCRS members—1997 survey. J Cataract Refract Surg 1998; 24:552-561