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Laboratory science

Acrylic intraocular lens damage after folding using a forceps insertion technique

Erie, Jay C. MD; Newman, Brant; Mahr, Michael A. MD; Khan, Amir R. MD; McIntosh, Malcolm MS

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Journal of Cataract & Refractive Surgery: March 2010 - Volume 36 - Issue 3 - p 483-487
doi: 10.1016/j.jcrs.2009.09.037
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Abstract

Cataract surgery performed using small-incision foldable intraocular lenses (IOLs) minimizes postoperative astigmatism and accelerates visual recovery. The main methods of implanting a foldable IOL are using a folding forceps or a cartridge injector system. Several reports describe the vulnerability of acrylic IOLs to folding or forceps defects 1–6 or damage or deposits from cartridge injector systems.7–9

We recently observed occasional linear abnormalities on the surface of the AcrySof MA60AC acrylic IOL (Alcon, Inc.) after it was folded and inserted with a metal forceps (Figure 1). The purpose of our study was to use high-magnification microscopy to evaluate the IOL optic surface after folding using a forceps technique.

Figure 1
Figure 1:
Slitlamp photograph of linear boxcar-like abnormalities on the anterior optic surface of a 3-piece acrylic IOL created by using a forceps insertion technique. Significant iris loss, zonular dehiscence, and a capsular tension ring are seen in this previously traumatized eye, which had previous surgical repair for a scleral rupture.

MATERIALS AND METHODS

AcrySof MA60AC 3-piece acrylic IOLs were received as implantation samples from the manufacturer for inclusion in the study. Before folding, the IOL optic surfaces were inspected using an Axio Imager microscope and supporting Axio Vision software (Carl Zeiss MicroImaging, Inc.). Micrographs were taken with a ×10 objective (×100 total magnification). Differential interference contrast, brightfield reflected light, and darkfield reflected light imaging techniques were used as necessary. The software was used to measure the length and width of abnormalities that were observed.

Before the IOL was folded, each folding and insertion forceps was cleaned and sterilized using the standard procedure at the hospital surgical center. The procedure consisted of rinsing the instruments with warm water and placing them in a Gentinge washer/disinfector (model 4656, Gentinge, Inc.) for a light-soil cycle. The instruments were air dried and then steam autoclaved using a Pre-Vac autoclave (model 733HC, Gentinge, Inc.) for a 10-minute exposure and 40-minute dry cycle.

In the first experiment, 3 IOLs were analyzed. The first IOL was folded in half using a new folding forceps (#7722, Duckworth/Kent). The IOL surface was wetted with a balanced salt solution, after which the IOL was grasped for insertion using a new titanium insertion forceps (#7740, Duckworth & Kent). The IOL was held by the insertion forceps for 20 seconds at room temperature (70°F) and then released into its original container for transport to the laboratory. Approximately 1 hour later, the IOL surface was inspected by microscopy. The exact process was repeated for 2 additional IOLs except that for the second IOL, a titanium insertion forceps of intermediate-length use (1 to 3 years) was used. The titanium insertion forceps was chosen from the operating room stock. For the third IOL, a titanium insertion forceps of extended-length use (>3 years) was used.

The second experiment evaluated the effect of temperature on folding-induced IOL surface abnormalities. Three IOL samples were warmed to 98°F for 10 minutes. In the same manner used in the first experiment, each IOL was folded in half using a folding forceps, grasped with 1 of the 3 insertion forceps, held for 20 seconds, and released into its original container. Within 1 hour of handling, the IOL was transported to the laboratory for microscopic examination.

In the third experiment, the effect of time on folding-induced IOL surface abnormalities was evaluated. Three IOLs at room temperature (70°F) and 3 IOLs warmed to 98°F for 10 minutes were studied. In the same manner as in the other 2 experiments, each IOL was folded in half with a folding forceps, grasped with 1 of the 3 insertion forceps, and held for 20 seconds. The IOL surfaces were photographed using the microscope 1 hour and 72 hours after folding. Seventy-two hours was chosen as the endpoint based on previous force-decay data of acrylic haptics over time in a study10; the data imply that no further stress relaxation of acrylic material occurs after 72 hours and that any observed changes can be considered permanent.

The fourth experiment evaluated the effect of coating the IOL surface with an ophthalmic viscosurgical device (OVD) before grasping it with the insertion forceps. Five IOLs at room temperature (70°F) were studied. After each IOL was folded in half with the folding forceps, the IOL surface was coated with sodium hyaluronate 1.0% (Provisc) rather than balanced salt solution. Each IOL was then grasped with 1 of the 3 insertion forceps and held for 20 seconds. The IOL surfaces were photographed using the microscope 1 hour after folding.

In the final experiment, debris that was visible on some optic surfaces after folding was observed. Under high magnification, some IOLs in the previous experiments had deposits on the anterior optic surface; a medium-pressure stream of deionized water was directed to the surface of these IOLs for 10 seconds using a spray bottle. The IOLs were allowed to air dry overnight and were then rephotographed using the microscope.

RESULTS

Before folding, all 17 IOL samples had clear, smooth, homogeneous surfaces without visible defects on microscopic examination (Figure 2, top). Varying degrees of symmetrical parallel depressions were seen on the anterior optic surface under high-magnification microscopy in all 12 IOLs 1 hour after folding in experiments 1, 2, and 3 (Figure 2, bottom). No abnormalities were seen on the posterior optic surface.

Figure 2
Figure 2:
Top: Smooth, defect-free anterior optic surface of the acrylic IOL before folding. Bottom: Depressions of the anterior optic surface seen 1 hour after folding and grasping with an insertion forceps. The depressions have a boxcar configuration of 350 to 400 μm in width, corresponding to the 500 μm width contact area of the insertion forceps. Debris of unknown etiology is seen (original magnification ×100 magnification).

The anterior surface abnormalities corresponded to the contact location of the insertion forceps. No abnormalities were seen in the contact area corresponding to the folding forceps. The measured widths of the abnormalities ranged from 350 to 400 μm, similar to the width of the contacting surface of the insertion forceps (500 μm).

The boxcar configuration of the forceps surface depressions seen under high magnification (Figure 2, bottom) resembled the IOL surface abnormalities seen clinically in the index case (Figure 1). Forceps with greater signs of oxidative and mechanical damage (intermediate and long-term use) (Figure 3) introduced more profound depressions and greater amounts of surface debris. Surface debris, when present, was removable by vigorous rinsing with water (Figure 4). Energy-dispersive x-ray spectroscopy of the content of the debris was inconclusive. The length and width of the IOL surface depressions were greater in IOLs warmed to 98°F than in IOLs folded at room temperature (70°F).

Figure 3
Figure 3:
High-magnification photographs of a new titanium insertion forceps (top), a titanium insertion forceps with intermediate wear (middle), and a titanium insertion forceps with advanced signs of oxidative and mechanical damage (bottom).
Figure 4
Figure 4:
Left: Surface debris on an IOL after folding. Right: An IOL after medium-pressure rinsing with deionized water for 10 seconds shows no surface debris.

The IOL surface abnormalities persisted up to 72 hours after folding. Shallow depressions seen 1 hour after folding were somewhat less noticeable by 72 hours after folding, with no negative or positive effect conferred by warming to 98°F. Deeper depressions, however, were unchanged up to 72 hours compared with 1 hour after folding. Coating the anterior IOL surface with OVD before grasping it with the insertion forceps eliminated IOL surface abnormalities in the 5 IOLs studied.

DISCUSSION

We found that using a titanium forceps insertion technique for at least 1 acrylic IOL model (AcrySof MA60AC) can create linear abnormalities on the anterior IOL optic surface. These abnormalities were surface depressions associated with occasional debris and rare microscratches. The introduced surface depressions were consistent in location and in width with the contacting surface of the insertion forceps but not with that of the folding forceps. The surface depressions persisted for up to 72 hours after folding, were more prominent when the IOL was warmed to 98°F before folding, and were more prominent when a forceps with a high degree of wear was used. Coating the anterior IOL surface with OVD before grasping it with the insertion forceps prevented the surface abnormalities.

Acrylic IOLs are popular because of their ease of use, excellent optic quality, and superior uveal biocompatibility. Intraocular lens implantation using an injector system is more common than implantation using a forceps technique; however, forceps insertion is useful in complicated cases, such as scleral wall or iris suture fixation. Previous studies show that metal implantation forceps can injure the surface of poly(methyl methacrylate)11 and acrylic1–6 IOLs. Nonhydrogel acrylic IOLs with lower water content may be more vulnerable to surface damage during folding than methacrylates with higher water content.2 Intraocular lenses with silicone optics are more resistant to surface damage after folding with a forceps.12

The outstanding optical performance of foldable acrylic IOLs is well known. The mechanical properties of acrylic IOLs, however, become important during folding and insertion. During folding, an acrylic optic must be able to safely carry loads in the form of pressure that do not alter its structure after unfolding. The response of acrylic to applied loads or stresses is nonlinear and is a function of temperature and duration of stress application. The yield point of acrylic is the amount of applied stress at which the deformation of a material undergoes irreversible changes in shape in response to the applied force. Once the yield point is exceeded, acrylic will not return to its original shape when the applied stress is removed. Studies suggest that changes in acrylic material that are present after 72 hours can be considered irreversible and permanent.10 The insertion forceps in our study created permanent depressions on the anterior optic surface that were more prominent by warming the IOL and by using an insertion forceps with signs of wear. This suggests that the yield point of the acrylic optic was exceeded in these cases. Coating the anterior acrylic IOL surface with OVD before grasping it with the insertion forceps prevented irreversible depressions, presumably by preventing the stress load from reaching the yield point of the acrylic polymer.

The nature of the surface debris on the anterior optic surface on some IOLs after folding is unknown. The debris may represent particulate matter transferred to the IOL from the insertion forceps, especially if the forceps' contacting surface is worn and irregular. The surface debris is easily eliminated by rinsing with water, suggesting that it is also removed in vivo when OVD is aspirated from the anterior chamber after IOL insertion.

The visual impact of surface abnormalities of the magnitude seen in our study is unknown but is likely small. In general, we believe the best option for patients is the implantation of IOLs with smooth, defect-free surfaces. Our study found areas for improvement in the design of insertion forceps that will reduce stress loads applied to the acrylic optic surface. In the meantime, we recommend coating acrylic IOLs with an OVD before grasping it with any forceps because our results suggest that this reduces the likelihood of subsequent IOL surface abnormalities.

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