Improving Wound Construction Techniques for MIVS
Expert advice on avoiding complications.
PETER K. KAISER, MD
The migration of vitreoretinal surgery from traditional techniques employing 20-gauge technology to a less invasive, more patient-friendly microincisional procedure was one of the retina community's biggest success stories of the previous decade. Early concerns about instrument flexibility, poor lighting and subpar fluidics have been addressed and overcome. Our current challenges involve refinement of our surgical techniques to reduce risk of adverse perioperative and postoperative events. Careful attention to wound construction and closure is particularly critical in this effort.
Wound construction problems in microincisional vitreoretinal surgery (MIVS) lead to hypotony, endophthalmitis and subconjunctival migration of gas and oil. Furthermore, wound leak can require sutures, particularly in patients with uveitis or high myopia. Below are a few concepts and strategies that have served me well in refining my MIVS protocols.
|Peter K. Kaiser, MD, is director of the Digital OCT Reading Center of the Cole Eye Institute. A consultant for Alcon and Genentech, he has received research grant support from Novartis, QLT and Genentech. Dr. Kaiser can reached at firstname.lastname@example.org.|
ANGLED WOUND CONSTRUCTION
When displacing the conjunctiva, it is important that the hole through the conjunctiva not be aligned with the hole through the sclera so when the cannula is removed the holes do not line up.
To achieve this, the surgeon should displace the conjunctiva before inserting the trocar. In addition, to make the chord length in the sclera as long as possible, I like to flatten the sclera with a broad stabilizing device on insertion of the trocar (Figures 1 and 2). This can be done with a cotton swab or with inserters. Finally, make the angle as oblique as possible to the sclera. The result is a wound that is not even visible, compared to the obvious wound gap and leakage found with straight incisions.
Figures 1 (above) and 2 (below). Flattening the sclera with a broad stabilizing device on insertion of the trocar can help in the lengthening of the wound.
We examined this difference in wound structure in a dynamic fashion using anterior segment spectral-domain OCT and cadaver eyes that allowed for evaluation of wounds immediately after surgery in the setting of varying eye pressures; the result is that, when comparing a 23-g straight incision to a 23-g angled incision, there remains an open straight incision but a closed angled incision, regardless of IOP.
The next part of the experiment involved placing India ink on the wounds to determine where ink particles reach in differently shaped incisions. In a straight incision (Figure 3), even with 25-g cannulas, ink particles are found both in the wound and inside the eye. Conversely, there were no ink particles inside the eye when using an angled incision (Figure 4). As the size of ink particles is comparable to that of bacteria, this is an important finding, and illustrates the importance of an angled incision.
Figure 3. When straight incisions are stained with India ink, ink particles are found inside the wound.
Figure 4. Angled incisions were successful in keeping India ink outside the wound and eye when stain was used.
Initially, with the introduction of smaller-gauge instrumentation, some researchers concluded that there was a higher endophthalmitis rate with smaller-gauge instruments than with 20-g surgery. However, most of the patients who developed endophthalmitis in those early studies had straight incisions, whereas when an angled incision was made, endopthalmitis rates between 20-g surgery and smaller-gauged surgery were nearly identical.
Furthermore, current trocar blade shape is a significant improvement over earlier hypodermic needle trocars. The flatter, MVR-style blade of the newer trocars — as opposed to the earlier, beveled shape — allows for reduced penetration forces (Figure 5 and 6). The newer blade is also superior in that it produces a linear incision, as opposed to the biplanar incision made by the earlier, beveled blade (Figure 7). At our practice, sclerotomy suture rates have declined significantly when using the new, flatter trocars. Another improvement has been the introduction of valved cannulas. Normally, the introduction of the cannula into the eye after core vitrectomy can cause fluid to leak when the cannula is withdrawn. By placing a valve on the cannula, fluid is prevented from exiting the wound (Figures 8 and 9). While placement of the valves on the cannula could stand to be improved, their introduction alone is a great improvement in preventing fluid egress and vitreous incarceration.
Figure 5. Recently introduced trocar blades reduce penetration force while allowing linear incisions.
Figure 6. The flatter edges of the newer trocars provide for a better-constructed wound than the older beveled edges.
Figure 7. While older trocar blades created biplanar incisions, the newer blades' incisions are linear.
Figure 8. Valved cannulas can help prevent fluid loss.
Figure 9. Prevention of fluid loss using newer blades can be seen under magnification.
Certain changes in surgical technique can also prevent wound complications during MIVS. For instance, withdrawal of a cannula with nothing to plug the wound can cause a vitreous wick because of the suction vector created upon withdrawal. Thus, when withdrawing cannulas, the use of a solid instrument can prevent vitreous wick. The light pipe can serve this purpose; by pulling the cannula up over the light pipe and then putting the cotton tip over and pulling the light pipe out over the instrument, the surgeon can prevent vitreous wick (Figure 10).
Figure 10. Proper placement of a cotton swab while pulling the light pipe out can help prevent vitreous wick.
An important point to remember, however, is that, when the instruments are withdrawn, there will be very low IOP holding the wound closed since the other cannula is open, so it is vital that the light pipe be inserted into the other side to pressurize the eye and allow the wound to close.
Leaving a small air bubble in the eye at the end of the case can also prevent wound complications such as leakage. If a wound is poorly closed, the resulting bleb of air is an indicator of wound instability that can alert the surgeon to the need for a suture. In addition, an experiment performed by Baker Hubbard, MD at Emory also used India ink to determine migration of ink into an air-filled eye. He found that with air left in the eye, there was no ink migration and that there was no vitreous in the wound — thus preventing vitreous wick and decreasing the risk of endophthalmitis.
An additional point is that increased IOP can help close surgical wounds. IOP can be raised surgically, from 40 mm Hg to 60 mm Hg, while pressing with a cotton-tipped applicator, to help close the internal lip of the wound.
NOVEL COLLAGEN PLUG
One solution to wound leakage currently under investigation is a non-cross-linked, dissolvable plug made of ultra-pure (99%) collagen (96% to 98% type 1 collagen, with the remainder made of type 3 collagen). The speed of biodegradation of the plug is adjustable, from two to four weeks, and by placing the plug into a surgical cannula and then pulling the cannula over it, the plug will swell and close the wound. It then dissolves over time, as would be expected of a collagen implant.
The procedure for placing the plugs is rapid (less than two minutes) and has been observed to prevent inflammation in animal experiments (Figure 11). India ink experiments performed similarly to the above-described tests showed that, when the plug was placed into a poorly constructed straight 23-g incision, ink particles were not observed inside the eye (Figure 12).
Figure 11. Novel collagen plugs have been shown to prevent inflammation in animal experiments.
Figure 12. The angled insertion of the plug leaves a wound that was not penetrated by India ink.
The plugs can be used in any gauge procedure. Furthermore, the possibility exists that, in the future, the plugs can be impregnated with other drugs (eg, antibiotics, anti-inflammatory agents, antiangiogenesis drugs) that will aid in both wound healing and treatment of underlying conditions.
Before such an application emerges, however, another bulwark against a key wound complication — namely, endophthalmitis — is the use of subconjunctival antibiotics in all cases. Opinions on agents and injection sites vary. My protocol is to use cefazolin, creating a bleb directly over the cannula sites. Surgeons who instead use an aminoglycoside would be well advised to inject away from the cannula sites to avoid antibiotic ingress.
Recent refinements to instrumentation, surgical technique, and antibiotic delivery methods have given retinal surgeons more sophisticated tools for MIVS that have, in turn allowed for improved outcomes and fewer intraoperative mishaps. Looking ahead, continued technological progress will bring us 27-gauge surgical instruments, with incrementally greater benefits to surgeon and patient alike. RP