Smoothing out the sharp edges

April 15, 2006

For almost 2 years, I have pioneered a change in phaco needleconfiguration to improve the safety of cataract surgery. The DeweyRadius Tip, manufactured by Microsurgical Technologies (MST,Redmond, WA) is a standard phaco needle with a simpleexception-no sharp edges. This rounded design can be appliedto any phaco needle of standard gauge, bevel, or bend.

Using this rounded needle has required no changes to 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 it is in removing even denser cataracts.

My interest in improving capsular safety has an early beginning. In 1993, I began using a J-cannula to flush cortex from regions of poor visualization, especially the subincisional space. This avoids the notorious "vacuum break" of occluding the aspirating tip with the I&A apparatus.

In three of the specimens, 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. While the sample size was small, this experiment satisfied some of my curiosity as to the resilience of the capsule and the zonules. I reasoned that capsule damage in similar situations was not due to a "vacuum break" in most cases, but required a cutting feature or effect, such as a sharp edge or burr.

Micropulsed phaco

About 3 years ago, micropulsed phaco technology became available. For many reasons, it is effective at removing a nucleus with less power than previous technologies.

First, within each duty cycle, the millisecond off periods, or "rests," allow heat to dissipate. Second, these rests 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. This effect was described by Dr. Mark Shafer ("Cavitation Generation and Cavitational Effects in Phacoemulsification," presented at the American Society for Cataract and Refractive Surgery [ASCRS] annual meeting in San Francisco, Apr. 2003) and found to deliver more energy than the stable cavitation created by continuous power.

Despite the continuing controversy, I feel the evidence supports cavitation (Packer M, Fishkind WJ, Fine IH, Seibel BS, Hoffman RS. The physics of phaco: A review. J Cataract Refract Surg 2005;31:424-431) as a major factor in dismantling the nucleus into fragments small enough for efficient aspiration. Although many would dismiss cavitation as far secondary to the physical "jackhammer" effect 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, but these microscopic explosions undoubtedly play a significant role in nuclear removal, especially at higher phaco powers.

I conducted a study in 2002 that compared a form of chopping with a divide-and-conquer approach for nuclear removal ("Transitioning to Chop: A Non-Impaling Technique," presented at both the ASCRS and European Society of Cataract and Refractive Surgeons [ESCRS] meetings in 2003). 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 also reduced my case time by more than a minute. I was surprised at how many lenses could be removed by vacuum alone and by how many lenses that I had graded clear required power for removal, despite their clinical appearance at the slit lamp.