A Closer Look at Trocars
Improvements in design facilitate more desirable wound construction.
Dr. Packo: Dr. Pollack and Dr. Rizzo, you have a strong interest in wound construction. Are there any noteworthy changes in trocars?
Dr. Pollack: There are two major design improvements in the Constellation trocar system that I believe will make it easier for us to create reliably leak-free wounds.
We are learning that a flatter insertion of the trocar gives us a better chance of achieving leak-free wounds, which is probably also important in reducing the risk of endophthalmitis.
To create reliably leak-free 23-gauge wounds, I use what I call the 5°/30° insertion technique. This is performed by displacing the conjunctiva with the tip of the trocar, creating a flat 5° intrascleral tunnel with the trocar tip (Figure 1), and then raising the handle slightly to about 30° before completing the insertion of the trocar/cannula system through the sclera (Figure 2). When performed correctly, the cannula hub lies flat against the sclera (Figure 3). The nose of the old trocar was short, and the ridge on the handle would frequently catch on the upper part of the lid speculum, making insertion more difficult. The long nose of the new trocar eliminates this issue.
Figure 1. The initial scleral tunnel is created with the trocar blade tip at a shallow angle, approximately 5°.
Figure 2. After completion of a 1.5 mm scleral tunnel, the trocar blade is raised to a 30° angle and the cannula insertion is completed.
Figure 3. The cannula hub lies flat against the sclera after insertion using a 5°/30° biplanar insertion technique.
The second improvement involves the tip. The tip of the older Alcon blade has a single bevel across the diameter of the trocar, and the length of the cutting edge is approximately 1.1 mm. The cutting edge of the new blade is longer than 2 mm. It also has a slimmer profile (Figure 4) with tapering of both the top and the bottom aspects of the blade (Figure 5), similar to the MVR design that we know from 20-gauge surgery and the blade used in the DORC 2-step 23-gauge vitrectomy system (Dutch Ophthalmic Research Center, The Netherlands). These design modifications should decrease tissue resistance to the blade, making trocar insertion even smoother.
Figure 4. Note the thin profile of the new EdgePlus blade.
Figure 5. The new EdgePlus trocar blade is tapered on top and bottom.
I recently studied 23-gauge wound construction using the new Alcon trocars in rabbit eyes (unpublished data). High-resolution ultrasound was performed immediately after the vitrectomy and demonstrated the ability of these new blades to produce nice wound sealing with impressive scleral apposition (Figure 6).
Figure 6. This high-resolution ultrasound Image shows the postvitrectomy wound made with an EdgePlus trocar in a rabbit model.
Dr. Packo: We all have seen the C-shaped scleral puncture with the previous 25-gauge and 23-gauge entries. What did the entry on the sclera look like in your series using this new blade design?
Dr. Pollack: My experience in human eyes has been even more impressive than in the rabbit model, presumably because of differences in the elasticity of the scleral fibers in these two models. In the rabbit model, the wound site was frequently visible. In humans, however, I found the insertion site is difficult or impossible to see after cannula removal. Visible wounds appear as a thin line. Interestingly, whether using the old or new version of the trocar, the flatter the entry, the less prominent the wound. I have found that with a flat insertion, the wound closes so completely that, even using the previous-generation blade, wound shape does not play a significant role in the integrity of the wound. I think the insertion angle is the bigger issue. In my experience, a relatively flat insertion angle will produce leak-free wounds, regardless of which of these two blade designs is used.
Dr. Packo: Dr. Rizzo, you have thought a lot about wound construction, particularly entry with small-gauge surgery. Have you changed how you put the trocar into the eye with the new system?
Dr. Rizzo: When using the old trocar, I was concerned about the effect of the chevron incision on the anatomy of the sclera. With the new trocar, the incision is linear and smaller, causing less disruption of the scleral fibers. I also found that the new trocar forgives a poor incision. You can perform a simple oblique incision, and it is completely sealed at the end of the surgery.
The Alcon cannula is metal, as opposed to plastic, which is used in other 1-step systems, such as those manufactured by Synergetics and Bausch & Lomb. The blades also differ in shape. The new Alcon blade is different from the older 23-gauge in that it is flatter with a ridge on the back; the B&L blade is the shape of a solid needle and the Synergetics system presents a very flat knife. All three are 1-step systems in which the cannula overlies the trocar with which it is inserted, as opposed to the 2-step systems, such as that manufactured by DORC, which involve an initial incision with a stiletto blade followed by the insertion of the cannula. RP