Phacoemulsification with aspiration has become the preferred means of removing cataracts. One possible issue with phacoemulsification is the generation of free radicals, likely due to effects caused by the ultrasonic movement of the emulsifying.¹ A free radical is a chemical species with an unpaired electron in its outermost shell, which makes the free radical very reactive with some other molecules, including some biomolecules. The consequences of free-radical reactions include destruction of lipids in membranes, inactivation of critical enzymes, and induction of DNA damage.

Our laboratory previously identified the hydroxyl radical as one by-product of phacoemulsification² The hydroxyl radical is a potent oxidant that reacts with the first organic molecule that it encounters. In order to study this potentially damaging species, we developed a method to quantitate the production of the hydroxyl radical during phacoemulsification. This quantitation allows us to compare factors that might influence hydroxyl radical generation and/or subsequent hydroxyl radical reaction with biomolecules. The goal was to learn if particular surgical methods could provide additional protection for the eye from exposure to hydroxyl radicals.

Radical generation

The generation of hydroxyl radicals should be a function of the amount of ultrasonic power applied, and may vary with different phacoemulsification modalities and tips. Additionally, since a hydroxyl radical will react with the first organic molecule that it encounters, it is likely that the chemical content of an irrigation solution used during phacoemulsification may alter or limit reactions of the hydroxyl radical with biomolecules. Therefore, these were the variables we examined in order to find preferred parameters to protect the eye from hydroxyl radicals.

Using a laboratory test cell and the Infiniti Vision System (Alcon Laboratories), we studied the effects of different modalities, tips, and irrigation solutions on hydroxyl radical concentration sampled from the aspirate during the application of phacoemulsification ultrasonic energy. The concentration of hydroxyl was quantified using a well-established deoxyribose method.³ The hydroxyl radical reacts with deoxyribose to form malondialdeyde (MDA). Malondialdehyde then reacts with thiobarbituric acid (when acid and heat are added) to produce a bright pink colour that can be quantitated spectrophotometrically. For our experiment, the deoxyribose was added to the irrigation solution and samples were taken from a port in the collection bag during phacoemulsification; these samples then were assayed for MDA.

Longitudinal ultrasound v. torsional mode

Figure 1: Malondialdehyde Production following Phacoemulsification at 100% Power using Different Modalities and Tips.

The comparison of different lens removal technologies pitted longitudinal ultrasound against the torsional mode using the OZil handpiece (Alcon Laboratories). Both modalities used a 0.9 mm Tapered Kelman phaco tip (Alcon Laboratories) and both were carried out in balanced salt solution (BSS). The results were clear: torsional mode generated only 30.1 nM of MDA, half as much as longitudinal ultrasound (63.1 nM) (Figure 1). The amount of MDA was further reduced by using different tips with the torsional phacoemulsification: 13.2 nM of MDA was produced when using torsional ultrasound with the Mini-Flared 45° Kelman tip, similar to the results of torsional ultrasound with the OZil-12 tip (14.3 nM).

Figure 2: Malondialdehyde Production following Phacoemulsification at 100% Power in various Irrigating Solutions.

Adding organic substances to the irrigation solution decreased the availability of the hydroxyl radical to react with biomolecules in the eye. This was best demonstrated by comparing BSS, which contains organic molecules such as citrate and acetate, against doubly deionized water (ddH₂O), which is virtually free of organic molecules. The amount of MDA produced was 87.9 nM in ddH₂O versus 63.1 nM in BSS, as shown in Figure 2. The amount of MDA in the aspirated irrigation solution was even less (45.5 nM) with BSS Plus, which contains dextrose and glutathione, and was smallest (23.2 nM) when using NAVSTEL, a balanced salt solution supplemented with dextrose, glutathione, and hydroxypropylmethylcellulose (HPMC).

Best protection

In conclusion, oxidative stress on the eye during phacoemulsification may be minimized by using modalities and tips that produce fewer free radicals, and irrigation solutions which can compete for reaction with the hydroxyl radical. With BSS as the irrigation solution, using the torsional modality with either the 0.9 mm Kelman Mini-Flared tip or the OZil-12 tip reduced MDA production by approximately 50%. Using irrigation solutions that contained organic molecules also decreased the concentration of MDA. Using torsional mode, appropriate tips, and irrigation solutions with organic content can potentially reduce a surgeon's concerns about exposing the eye to oxidation by the hydroxyl radical during phacoemulsification.

References

1. A. Holst, et al, Curr Eye Res, 1993; 12:359-365.

2. M.D.Cameron, et al, J Cataract Refract Surg, 2001; 27:463-470.

3. K.H. Cheeseman, et al, Biochem J, 1988; 252:649-653.