When it comes to incisions in the eye, it would seem that smaller is better. But is that always the case? When we moved from a large 10 mm incision for extra-capsular manual extraction to a 3 to 4 mm incision for phacoemulsification, there was a large benefit in terms of healing, astigmatic effect, and patient experience. But does the move from 2.8 to 2.2 mm make a meaningful difference? And what about going from 2.2 to 1.8 or even smaller?
Our main goal is to give the patient the very best visual outcome. In that regard, the primary issues to consider when changing phaco size are:
How well does the incision seal and heal?
What is the astigmatic effect of the incision?
Is the flow through the smaller phaco tip restricted?
Do we have a high quality lens which will go through the smaller incision?
The move from a width of 3.5 mm to 2.8 mm made an appreciable difference in the ability of the incision to seal without sutures. About 20 years ago, lenses were primarily designed to be foldable which meant that a 6 mm optic could be folded in half to go through a 3.5 mm incision (not 3 mm since the thickness of the IOL and the forceps tips have to be taken into consideration). When we moved towards lenses going through injectors, the move down to 2.8 mm was very logical and easy. We now have good quality IOLs that can be injected via a 2.2 mm incision, so is that the next step?
It can be a good step, but the 2.2 mm incision does require some mild adjustments: the flow of fluid is slightly less, so if you tend to use high flow rates (50 to 60 cc/min), you may want to cut this down by 10 cc/min. In addition, keep in mind that some of the higher power IOLs (such as +27.0 D and above) have thicker optics and will not go through the smaller cartridges. If a larger cartridge is used, a wound-assist technique to inject the IOL may be required. Finally, remember that a 2.2 mm incision does not cause as much astigmatic effect as a 2.8 mm incision, which can be good or bad, depending on the specific patient and eye.
Most of our cataract patients have against-the-rule (ATR) astigmatism which means that their steep axis is on or about 180 degrees. In that case, the larger 2.8 mm phaco incision will help address the patient's astigmatism at the time of cataract surgery. In addition, the slightly larger incision will allow greater flow of fluid during surgery, more room for movement without distorting the cornea, and better ease of injection of all powers of IOLs.
Is there a lower limit to the acceptable size? Yes, it becomes an issue of diminishing returns: Poiseuille's Law tells us that the flow of fluid is related, in an exponential manner, to the size of the tubing. Sure, we can even remove the entire cataract through bimanual split-handpiece phaco via incisions of 1 mm, but slower and inefficient. Companies have tried to use a smaller phaco incision for marketing efforts, but the ophthalmologists have stuck to using 2.2 to 2.8 mm incisions for the past decade.
Surgeons should be familiar and comfortable with multiple different phaco tip sizes so that they can tailor the surgery to the patient in order to deliver the best visual outcomes.
