The cornea is a very delicate tissue: it’s thin, clear, avascular, and susceptible to damage, particularly from the heat that is generated by the phaco probe. Think of the corneal tissue like the white of an egg: with heat these clear proteins denature and they become opaque and they contract. In the cornea, application of heat will cause the tissue to opacify and contract, which will induce a large degree of astigmatism and prevent the incision from sealing well.
The phaco probe generates heat due to friction. Ultrasound means that the frequency of the vibration is above the range of human hearing which is in the range of 20 Hz to 20,000 Hz (this higher limit declines with age). The Alcon phaco probes vibrate at about 40,000 Hz while the B&L phaco probes are at 28,500 Hz, both in the longitudinal direction.
The phaco tip moves back and forth at this fixed high rate of speed with the “phaco power” setting being a way of increasing the stroke length while the frequency stays the same. The stroke creates a mechanical impact, cavitation and implosion at the tip, and a fluid and particle wave. But the phaco probe stroke also creates heat from friction.
Here is a simple test: get the phaco probe at your normal settings and disconnect the infusion line to the hand-piece. Now pinch the phaco needle with your fingers and step on the pedal all the way to position 3. The heat will build up in just a second or two — enough to burn your finger, so be careful.
To mitigate this:
- we use a silicone sleeve around the phaco needle
- we have the infusion of BSS come into the eye bathing the phaco needle
- we intentionally use slightly leaky incisions to allow fluid egress & corneal cooling
Some of the factors which put us at risk for a corneal wound burn include:
- using a high amount of ultrasound energy, such as for a dense nuclear cataract
- lack of phaco power modulations (burst mode, pulse mode, lower duty cycle, etc)
- making too tight of an incision
- an improperly fitted silicone sleeve for the phaco needle
- not floating in the incision and pushing the phaco needle against the sides, roof, or floor of the corneal incision
Once you notice a phaco wound burn it is already too late. At this point care should be taken to finish the surgery, not cause more corneal burning or trauma, and then suture the incision closed. Keep in mind that it may require more than one suture due to the irregular surfaces that are created from the burn. These sutures should be left in the eye for many months until wound healing is sufficient to keep the tissues closed.
Fortunately, phaco wound burns are uncommon, but keep in mind that they can happen, especially when we do cataract surgery on very dense nuclear cataracts. Optimize your phaco ultrasound settings, make your incision precise, and be sure to pivot and float within the incision. That will limit the risks and help ensure a great visual outcome for our patients.
All text, figures, and content © 2018 Uday Devgan MD.