Surgical and technological advances drive the development and improvement of knives to meet cataract surgeons’ evolving needs.
The knives used to create initial incisions are extremely important in cataract surgery, as the construction of these incisions impacts the success of all subsequent surgical steps. Without an appropriate incision, surgeons may find the following moves more difficult, with more potential for complications. Moreover, construction of an incision is critical for its proper sealing. With a leaking clear corneal incision, the risk of developing endophthalmitis after cataract surgery increases dramatically.
Each surgical knife has its own pros and cons, and all cataract surgeons choose and become accustomed to their own favorite knives. The common goal, though, is to create a precise, predictable incision.
The Main Incision
The form a primary cataract incision takes is usually dictated by its location, in the sclera, limbus, or clear cornea. Prior to the advent of phacoemulsification, surgeons used scleral incisions: making an entry at partial thickness and dissecting the interdigitated scleral fibers to reach and enter through Decemets membrane. Most surgeons choose a crescent blade for dissecting the scleral tunnel, and a different keratome for incising Decemets membrane. Today, such non-phaco cases have become so rare that scleral incisions are very seldom used. I only use a scleral incision when implanting a primary anterior chamber lens, in which case I make a Hoffman pocket to suture the sclera without disturbing the conjunctiva.1
Today, most cataract surgeries begin with clear corneal incisions. While the creation of a scleral incision requires more than one blade, clear corneal incisions can be accomplished with a single blade. First described in the 1990s, clear corneal incisions were constructed on a single plane in a simple in-and-out manner.2,3 Surgeons would follow the plane of the cornea in partial thickness and then dimple down slightly to enter through Descemets membrane. Charles Williamson adapted the incision by adding a perpendicular groove before entering the anterior chamber. It was then further modified by David Langerman, who deepened the perpendicular groove for greater stability.2 As it turned out, these grooved incisions often resulted in persistent foreign body sensation and did not provide the meaningful benefits expected of them.3
Early clear corneal incisions were about 2 mm long and 4 mm wide, initially made with straight-sided knives. These were then replaced by knives with a trapezoidal shape, with surgical technique evolving toward smaller incisions believed to provide greater stability. In addition to making them smaller, it has been found advantageous to create clear corneal wounds with a square architecture.4,5 My favored cataract incision is single-plane, paracentesis-style clear corneal incision, and I always try to achieve a square or near-square architecture (with a chord length of at least 2 mm).
Paracentesis incisions are no less critical than main incisions. Measurements have shown that 22 mL balanced salt solution (BSS) are lost per minute from a 1-mm paracentesis, which can cause increased turbulence and higher risks for complications. For this reason, I always keep paracentesis incisions very small (less than 0.5 mm). Many surgeons use a 15-degree supersharp metal blade for the paracentesis; but these incisions can be inconsistent in size, often exceeding 1 mm.
Assorted Knife Choices
Cataract surgeons today have many surgical knives to choose from, available in different materials, designs, and sizes. Most are keratomes designed to make an incision of one particular size, while some have a shoulder that enables them to make incisions of different sizes, depending on how far the entry is.
Metal or diamond knives are most commonly used, but knives of other materials—including ruby and porcelain—are also available, though rare. Some ceramic knives have a small retractable shield designed to provide extra protection and reduce the risk for injury, but for a cautious, high-volume surgeon, the chance of such injury is extremely low.
Diamond knives, available in various shapes and dimensions, offer exceptional sharpness and consistency and create highly reproducible incisions. Although the quality of diamond knives is unsurpassed, they require high maintenance. Without proper care, diamond blades may become very fragile and break more often, adding to their already high initial cost. For this reason, at most teaching institutions where no central personnel take the responsibility of sterilization and special care, surgeons are more likely to use metal blades.
Unlike diamond knives, which tend to move right through tissue without much noticeable friction, metal blades provide the surgeon with more tactile awareness of the cut. This may be particularly helpful to surgeons who operate less often, but will not be of much benefit to high-volume cataract surgeons.
Cost is another important consideration. No doubt the initial price of diamond knives is much higher than that of metal blades, but diamond blades also last much longer if maintained properly. For high-volume surgeons who would like to minimize their use of disposable metal knives to reduce costs, diamond blades can be a cost-effective choice in the long run.
The Triamond Blade
My favorite knife is the Triamond diamond blade (Arbisser-Fine) by Mastel Precision, and I use it for both main and paracentesis incisions. Howard Fine first developed the Triamond blade with Mastel Precision, and then I made a small change: adding a little angle to the left side of the tip (which was absolutely flat and straight across by original design). This modification enables the knife to enter more delicately and smoothly (Figure 1).
Unlike keratomes which make incisions in one maneuver, the Triamond blade needs to be advanced into the anterior chamber and then moved to the side to create an incision of particular size. Marking the epithelium with a scoring device enables the surgeon to predict incision size with certainty.
The greatest advantage of the Triamond blade is that it can make a variable incision. The initial incision is 0.3 mm on the inside and 0.5 mm on the outside. By moving the blade to the side, the surgeon can then expand the incision to any size. In addition, since the Triamond blade is very thin, tissue distortion common with thicker keratomes is minimal; so it is not necessary to hydrate the stroma much to close the wound at the end of the case. I usually irrigate the wound so there is nothing visible left between the lips of Descemets valve.
Due to its precision and reproducibility, the femtosecond laser has the potential to revolutionize cataract surgery by improving incisions, capsulorhexis, and nuclear fragmentation. Using a femtosecond laser, surgeons can make incisions of any shape or size while reliably achieving ideal wound architecture, eliminating the variability of manual incisions. The laser should serve to standardize the procedure more thoroughly, making surgically induced astigmatism more predictable. Investigations are ongoing to determine the ideal shape and location of femto-cataract incisions, as well as whether they are truly safer and more stable.
Eventually, the femtosecond laser may replace surgical knives, at least in some contexts. But as clear as its advantages are, the laser’s cost will likely be a barrier for many institutions.
THE BOTTOM LINE
Knife selection is important in cataract surgery: proper wound construction not only facilitates the following surgical steps but also reduces the risk for complications. Cataract surgery knives are being continually refined to meet evolving surgical needs, and are now available in a variety of materials, designs, and dimensions. Though all surgeons have favored blades for different reasons, constructing a stable, strong, and safe incision is a mutual goal.
Lisa B. Arbisser, MD, is an adjunct associate professor at the University of Utah Moran Eye Center in Salt Lake City, UT, and an ophthalmologist at Eye Surgeons Associates PC in Bettendorf, IA. She has no financial disclosures related to this article. Refractive Eyecare associate editor Ying Guo, PhD, assisted in the preparation of this manuscript.
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2. Fine IH. Self-Sealing corneal tunnel incision for small-incision cataract surgery. Ocular Surgery News. 1992:38-9.
3. Fine IH, Hoffman RS, Packer M. Profile of clear corneal cataract incisions demonstrated by ocular coherence tomography. J Cataract Refract Surg. 2007;33(1):94-7.
4. Ernest PH, Fenzl R, Lavery KT, Sensoli A. Relative stability of clear corneal incisions in a cadaver eye model. J Cataract Refract Surg. 1995;21:39-42.
5. Masket S, Belani S. Proper wound construction to prevent short-term ocular hypotony after clear corneal incision cataract surgery. J Cataract Refract Surg. 2007;33(3):383-6.