October 2009

 

CATARACT/ IOL

 

Defining the edge of square-edged intraocular lenses


by Liliana Werner, M.D., Ph.D.

   

Posterior chamber IOLs with a square posterior optic edge have been associated with better results in terms of posterior capsule opacification (PCO) prevention, regardless of the material used in their manufacture. Although this IOL design feature can be appropriately assessed in morphological studies using scanning electron microscopy (SEM), such studies of new IOLs have generally focused on the quality of the optic surface or optic finishing. At the Berlin Eye Research Institute, Berlin, directed by Manfred Tetz, M.D., a series of three studies was done to define the edge of square-edged IOLs. I had the opportunity to participate in the last two studies of this series, which aimed to define the edge of commercially available lenses. The results obtained were very interesting, as they demonstrated not all square edges in the market are the same.

Source: Liliana Werner, M.D., Ph.D.

Defining the square edge, part 1: experimental IOLs

Drs. Tetz and Wildeck made the first attempt to evaluate and quantify at the microscopic level how sharp the optic edge must be to effectively prevent lens epithelial cells (LECs) from growing onto the posterior capsule. Plano +0.0 D PMMA IOLs with 11 defined edge designs were especially manufactured for use in this in vitro preliminary study. To obtain different edge designs, the IOLs were removed from the tumble-polishing machine at different times. To evaluate the optic edges, standardized SEM pictures with an enlargement of x500 were taken of one IOL in each group. A digital computer system (EPCO 2000 program) was used to evaluate the area above the edges on the SEM photographs. To achieve this, the area had to be defined as the deviation from an ideal rectangular projection (Figure 1). The edge’s ability to stop cell growth was evaluated by placing each IOL into cell culture and observing bovine LEC growth over 18 days on average. Only three groups of IOLs, those with the sharpest edge design (smallest deviation area from an ideal rectangular projection), prevented the growth of LECs onto the visual axis of the IOL (Figure 2). Defining the square edge, parts 2 and 3: commercially available IOLs Commercially available lenses manufactured from hydrophobic acrylic, silicone, and hydrophilic acrylic materials were obtained for use in this series of studies through letters sent to IOL manufacturers. All of them were marketed as having a square optic edge for PCO prevention. Generally, two IOLs of each design were evaluated: a +20.0 D and a +0.0 D whenever available for a particular design. In case a +0.0 D was not available, the lowest dioptric power was used for that particular design. We used an improved methodology to evaluate the optic microedge structure of currently available lenses. The methodology was as follows.

Each IOL was carefully removed from its original packaging with toothless forceps and mounted on a support for SEM analysis. During SEM examination, the analysis of each optic edge was done from a perpendicular view. Photographs of the optic edge of each IOL were obtained at three magnifications: x25 or x100, x300, and x1000. The first two magnifications were used to document the overall orientation of the specimen, and the x1000 magnification photographs were used for the microedge analysis. The SEM photographs of each IOL were saved as electronic, high-resolution JPEG files. They were then imported into the AutoCAD LT 2000 system (Autodesk, San Rafael, Calif.). This program, which is commonly used in engineering and architecture, allows accurate area calculations. The first step was to adjust the scale of the photograph into the program using the reference bar incorporated on the bottom right corner of each SEM photograph. After the scale on each photograph was confirmed by measuring the reference bar and obtaining the corresponding value, a reference circle of known radius, divided into four quadrants by two perpendicular lines passing through its center, was projected onto the photograph. The position of the circle was adjusted so that the end of both perpendicular lines touched the lateral and posterior IOL optic edges. The area of the lateral–posterior IOL edge deviating from a perfect square defined by the two perpendicular lines inside the reference circle was easily delineated using the computer mouse. The measurement of the area was then calculated by the program and provided in square micrometers. This was done using two reference circles with a different radius: 40 μm and 60 μm. The minimum radius size of 40 μm was chosen as a function of the size of the human LEC (Figure 3). It is important to highlight that an environmental SEM technique was used for the hydrophilic acrylic lenses in order to evaluate them under low vacuum conditions, preventing dehydration. The microedge structure of modern hydrophilic IOLs, most of which have a water content in the vicinity of 26%, may be significantly modified during the vacuum required in standard SEM procedures.

The commercially available IOLs were compared with an experimental square-edged PMMA IOL (reference IOL) manufactured for use in the preliminary in vitro study.

The edge design of culture

The edge design ofculture. Two silicone IOLs (+20.0 D and +0.0 D) manufactured with round optic edges were used as controls. For the square-edged PMMA IOL, the value of the area measured with the AutoCAD system with the 40 μm radius circle was 34.0 μm2. The respective value for the +20.0 D control silicone IOL was 729.3 μm2 and for the +0.0 D control silicone IOL, 727.3 μm2.

In part 2 of the study, we evaluated hydrophobic acrylic and silicone lenses. There was a large variation in the deviation area from a perfect square, not only between different IOL designs but also between different powers of the same design. Considering the measurements done with the 40-radius circle, the values for hydrophobic acrylic (N = 19) and silicone (N = 11) lenses were 183.38 +/– 82.18, and 74.39 +/–88.54 μm2, respectively (all dioptric powers evaluated included). The hydrophobic IOLs used, labeled as square-edged IOLs, had an area of deviation from a perfect square ranging from 4.8 to 338.4 μm2 (40 μm radius reference circle). Of the 30 commercially available square-edged, hydrophobic IOLs evaluated, only seven silicone lenses of five designs had area values that were smaller than, or close to, those of the reference square-edged PMMA IOL.

In part 3 of the study, we evaluated hydrophilic acrylic lenses. The study lenses had an area of deviation from a perfect square ranging from 60.84 to 871.51 μm2 for the +20 D lenses (379.01 +/–188.26; N = 24), and from 35.52 to 826.55 μm2 for the low diopter lenses (281.71 +/–241; N = 23), as measured with the 40-micron circle (P = 0.12; not significant). The area of deviation from a perfect square ranged from 35.52 to 826.55 μm2 for the single-piece lenses (280.44 +/–189.85; N = 33), and from 130.2 to 871.51 μm2 for the three-piece lenses (451.51 +/–242.29; N = 14), as measured with the 40-micron circle (P = 0.01; significant).

Considering all lenses included in the study (N = 47), the area of deviation from a perfect square ranged from 35.52 to 871.51 μm2 (331.39 +/– 218.90). We found that the area measurement values of hydrophilic acrylic lenses, as a group, were higher than the values reported for hydrophobic acrylic or silicone lenses in part 2. The differences among the three groups of materials were found to be statistically significant (Figures 4 and 5).

Other edge studies

The group of David Spalton, M.D., London, also performed a SEM study comparing the edge profile of commercially available square-edged IOLs. Their study included a total of 17 square-edged designs of +20.0 D, with five hydrophobic acrylic, seven hydrophilic acrylic, and five silicone lenses. Perpendicular images with a magnification of x500 were obtained and analyzed by using a purpose-designed software to produce a line tracing of the edge profile of the lenses. The sharpness of the edge profile was then quantified by measuring the local radius of curvature at the point on the posterior edge with the smallest radius. Their conclusions are comparable to ours in that as a group, hydrophilic acrylic lenses appear to have relatively rounder edges in comparison to hydrophobic acrylic and silicone lenses. This is probably due to the manufacturing process of hydrophilic acrylic lenses, which involves lathe cut from dehydrated blocks, which are then re-hydrated. Water absorption by the IOL material may render the final aspect of the edge rounder as the IOL swells.

Clinical implications

The factor that may play the most important clinical role in evening out the differences in the micro-edge profiles observed in our study is shrink-wrapping of the IOL by the capsular bag, which enhances contact between the posterior IOL surface and the posterior capsule. However, this factor may not even out large differences in edge profile. The results of all above-mentioned studies are interesting in the light of some clinical studies comparing square-edged IOLs manufactured from different materials reporting higher rates of PCO with hydro-philic acrylic lenses. In many instances the authors concluded that this was related to a “material” effect; however, the edges of the lenses included were perhaps just not comparable. Our study confirms that all square edges in the market are not the same, and perhaps large variations in edge profile may account for differences in clinical outcomes of post-op PCO.

The methodology used in such studies can help optimize the edge profile of IOLs. As examples, through proprietary changes in manufacturing and/or polishing techniques the area deviating from a perfect square from hydrophobic acrylic lenses manufactured by Hoya Optics (San Jose, Calif.) changed from 329.7 μm2 to 39.1 μm2. Oculentis (Berlin, Germany) is optimizing the edges of their hydrophilic acrylic lenses to an area deviating from a perfect square down to less than 2 μm2 (evaluated using the environmental SEM technique described in part 3).

In summary, analysis of the microstructure of the optic edge of currently available, square edge IOLs revealed a large variation of the deviation area from a perfect square, as well as values that were in mean higher for hydrophilic acrylic lenses in comparison to values reported for hydrophobic acrylic and silicone lenses. Only existing and future clinical data will help us better understand the effect of microedge structure and design on reducing PCO. Perhaps a cutoff value to clinically label an IOL as square-edged should be sought.

References

Tetz M, Wildeck A. Evaluating and defining the sharpness of intraocular lenses: Part 1: Influence of optic design on the growth of the lens epithelial cells in vitro. J Cataract Refract Surg 2005; 31:2172-2179.

Werner L, Müller M, Tetz M. Evaluating and defining the sharpness of intraocular lenses. Microedge structure of commercially available square-edged hydrophobic lenses. J Cataract Refract Surg 2008; 34:310-317.

Werner L, Tetz M, Feldmann I, Bücker M. Evaluating and defining the sharpness of intraocular lenses. Microedge structure of commercially available square-edged hydrophilic lenses. J Cataract Refract Surg 2009; 35:556-566.

Nanavaty MA, Spalton DJ, Boyce J, et al. Edge profile of commercially available square-edged intraocular lenses. J Cataract Refract Surg 2008; 34:677-686.

Defining the edge of square-edged intraocular lenses Defining the edge of square-edged intraocular lenses
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