Cover Stories | Nov 2005
The Advantages of a Rounded Tip
This phaco tip efficiently and safely removes nuclei of grades 3+ or lower.
Steven H. Dewey, MD
For about 1 year, I have had a secret weapon to assist me in refractive lens exchange. I have specifically rounded both the inner and outer edges of the distal tip of a standard, 30º Kelman 19-gauge phaco tip for use in coaxial phacoemulsification. No changes have been made in either my technique or the phaco machine's parameters. For refractive lens exchange, the advantages of a tip without sharp edges are tremendous. Perhaps surprising is how effective the rounded phaco tip (Microsurgical Technology, Redmond, WA) is at removing even dense cataracts.
TESTING A THEORY
My first success in improving capsular safety came in 1993. I began using a J-cannula to flush cortex from regions of poor visualization, especially the subincisional space. This technique avoids the notorious “vacuum break” when I/A occludes the aspirating tip. In April 2001, I tested the efficiency of a J-cannula for irrigating cortex from the subincisional space with the expert assistance of David Apple, MD; Liliana Werner, MD, PhD; Suresh Pandey, MD; and several others at the Storm Eye Institute in Charleston, South Carolina. In three of the Miyake-Apple specimen eyes, I purposefully latched onto the capsule with the J-cannula while aspirating as firmly as I could with a 5-mL syringe. I simply occluded the tip with the capsule while applying aggressive aspiration to the plunger and then dragged the fully occluded tip across the capsule. Despite this aggressive force, the cannula did not break the capsule, and manipulating the cannula did not visibly disrupt the zonules. I reasoned that capsular damage in similar situations requires a cutting feature or effect such as a sharp edge, a simple burr, or cavitation.
Micropulses and Cavitation
Approximately 3 years ago, micropulsed phaco technology became available. It is effective at removing a nucleus with less power than previous technologies for many reasons. First, within each duty cycle, the millisecond off periods or rests allow heat to dissipate. Second, these off periods improve the anterior chamber's stability by decreasing the chatter occurring with a nuclear fragment against a phaco tip that is always on. Third, the microbursts of ultrasound create transient cavitation, an effect described by Mark Schafer, PhD,1 and found to deliver more energy than the stable cavitation created by continuous power.
Despite continuing controversy, I feel the evidence2 supports cavitation as a major factor in dismantling the nucleus into fragments small enough for efficient aspiration. Although many would dismiss cavitation as secondary to the physical effects of the titanium needle itself, cavitation is a mechanism that erodes ships' propellers and destroys the inner workings of pumps. The multifactorial effects of ultrasound at the end of the phaco needle will make the isolation of a single component a challenge. These microscopic explosions undoubtedly play a significant role in nuclear removal, especially at higher phaco powers for dense nuclei.
I had the idea of rounding the edges of the phaco needle to improve procedural safety with an expected cost of decreased cutting efficiency.
Phaco Technique and Outcomes
I conducted a study in 2002 that compared a form of chopping to a divide-and-conquer approach for nuclear removal.3 Whether it was a 2+ or 3+ nucleus (on a scale of 1+ to 4+), the chopping technique used only one-third the power of the divide-and-conquer approach. The former reduced my case time by 1 full minute. I was surprised at how many lenses of 2+ nuclear sclerosis vacuum alone could remove and by how many lenses that I had graded clear or 1+ required power for removal, despite their clinical appearance at the slit lamp.
One day postoperatively, patients in the phaco-chop group had clearer corneas, and I felt the micropulsed technology had contributed significantly to these results. After 1 week, however, the two groups of patients were indistinguishable.
As other clinicians and researchers had noted, power usage in phacoemulsification was safe within a rather large range, and lessening power simply for the purpose of reducing it did not necessarily improve surgical outcomes, except in the short term. It was then that I had the idea of rounding the edges of the needle to improve procedural safety at a modest cost to efficiency due to decreased phaco power. These results confirmed for me that a modest increase in power (eg, from decreasing cutting efficiency by rounding the phaco needle) was unlikely to lessen the safety of phacoemulsification for the patient.
RESULTS TO DATE
Since July 2004, I have performed well over 500 cases with the rounded tip, and the difference in power usage compared with a sharp tip has been virtually unnoticeable. As I had observed before, several lenses that I had considered to be clear at the slit lamp required phaco power for their removal. Other patients' cataracts required less power than I had expected.
Moving from refractive lens exchange into the true cataract category, I found no difference in power usage for 2+ nuclear sclerosis whether I used a rounded or a sharp tip. For 3+ cataracts, the differences in power are a bit more apparent, with the sharp tip being 20% to 30% more efficient.
BENEFITS OF A ROUNDED TIP
The advantages of a phaco needle with a rounded tip are more than theoretical. With passive contact, this tip has no sharp edge that can damage the ocular structures it touches—whether the iris, the capsule, or the incision. Occasionally during insertion, a sharply edged needle may catch on the internal structure of the incision. This problem is less frequent with a rounded needle. Of greater significance, incidental contacts of the rounded tip with the anterior capsule or capsulotomy's edge have not yet resulted in a radial tear of the anterior capsule.
During surgery, the rounded tip has grabbed both the iris and capsule during aspiration and left no signs of damage to either. At first, I incidentally aspirated the capsule while going after residual cortex without damaging the capsule. Since then, I have purposefully aspirated the capsule into the needle during a handful of cases without resulting damage (Figures 1 and 2). I thus confirmed my earlier work on cadaveric eyes and dispelled (for me, at least) the myth of the vacuum break due to a fundamental weakness in the average capsule. Of course, capsules do not all have the same intrinsic strength, and zonules are not all equally resilient. Based on my experience, however, I think of the rounded edge as I think of my bicycle helmet. The margin of safety either one provides me with amply merits its use. I certainly do not test the safety of the rounded tip on every capsule, just as I do not test my helmet's ability to resist impact on every ride I take.
Even without a sharp tip, the phaco needle can damage tissue, or it would not be useful for removing the nucleus. On occasion, the iris may enter the lumen of the needle, usually when it has cored through a piece of nucleus before the surgeon eases off the foot pedal. A pleasant and unexpected benefit of the rounded tip is that it does not damage the iris during a brief grab, even with ultrasound applied (Figure 3). Previously, I would observe a characteristic notching created in the sphincter of the iris by a sharp-edged tip. Although the rounded tip can disrupt a bit of pigment during an incidental grab with ultrasound applied, the pupil remains nicely round (pathological irides, such as floppy irides, undoubtedly will remain a challenge).
Three times in my experience, the capsule has touched the rounded tip and occluded it while I was in foot position 3. Twice, the capsule was released without injury. Unfortunately, the overlay for the Sovereign cataract extraction system (Advanced Medical Optics, Inc., Santa Ana, CA) was not properly set up to record the power used at that time, although I expect it was fairly low because I was easing off the foot pedal. In one case, the capsule bounced into the tip of the needle with the phaco power at 14%, according to the overlay, and ruptured. The case involved a dense posterior subcapsular cataract in a patient with significant pseudoexfoliation and 20/200 vision. Intriguingly, the capsulotomy that the tip created was round.
It is important to note that the rounded needles change shape over time. In the case of the ruptured capsule, a restriction in the availability of rounded phaco tips had me using needles I no longer considered safe due to burrs. Titanium flows with the tip's movement and leads to the formation of burrs. I have been told anecdotally that the linear marks appearing on a phaco needle after several uses are due to contact with other metal items during surgery. Although I am certain that metal-to-metal contact will worsen the process, these marks appear on the tips and sides of the rounded needles after a dozen or so uses without any observable instrument in the cases I reviewed. At what point these burrs are large enough to break the capsule by mere contact needs research.
As far as I can tell, a sharp-edged phaco tip is of no benefit in eyes with soft nuclei. Rather, it can cause significant damage to the capsule, the iris, and even the incision. Rounding the tip's edge improves procedural safety without sacrificing the efficiency of nuclear removal or requiring a change in surgical equipment or technique. For firm lenses, the difference in power usage becomes evident, but whether it will result in a significant clinical difference remains to be seen. Were it not to provide the basis for a comparative study, I would have given up a sharp tip for any case of less than 4+ nuclear sclerosis in favor of the safety that the rounded edge provides.
Steven H. Dewey, MD, is in private practice with Colorado Springs Health Partners in Colorado. He has applied for a patent for the modification to the phaco needle tip discussed in this article. Dr. Dewey may be reached at (719) 475-7700; firstname.lastname@example.org.
1. Schafer ME. Cavitation generation and cavitational effects in phacoemulsification. Paper presented at: The ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; April 13, 2003; San Francisco, CA.
2. Packer M, Fishkind WJ, Fine IH, et al. The physics of phaco: a review. J Cataract Refract Surg. 2005;31:424-431.
3. Dewey SH. Transitioning to chop: nonimpaling technique. Paper presented at: The ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; April 15, 2003; San Francisco, CA.