MILESTONES IN RETINA

Expert perspectives on the evolution of retina practice, procedures, technologies and instrumentation.

MILESTONE

25-gauge Vitrectomy

Rohit R. Lakhanpal MD, FACS, FASRS, and Eugene de Juan, Jr., MD, FASRS

Poiseuille’s Law: A physical law that gives the pressure drop and an incompressible and Newtonian fluid in laminar flow flowing through a long cylindrical pipe of constant cross section. The law states that the velocity of a liquid flowing through a capillary is directly proportional to the pressure of the liquid and the 4th power of the radius of the capillary and is inversely proportional to the viscosity of the liquid and the length of the capillary.

Micro-incisional transconjunctival vitreous surgery was introduced approximately 25 years after Robert Machemer, MD, and Jean-Marie Parel, Ing.ETS-G, PhD, developed and reported closed, pars plana vitrectomy using a vitreous cutter with infusion and aspiration (VISC) with controlled intraocular pressure. Drs. Machemer, Parel, and Gholam Peyman were among early developers of closed vitrectomy with a suction cutter technology. Subsequent additions included endoillumination, membrane peeling, operating lens, x-y microscope controls, endolaser, and improvements in scissor technology.[1)]

In the late 1990s, de Juan et. al [2] described a new, 25-gauge vitrectomy system that operated through self-sealing wounds. 25-gauge vitrectomy traded conjunctival dissection and sutured sclerotomies for transconjunctival trocar/cannula systems. This facilitated repeated insertion and removal of tools as well as improved wound self-sealing, thus reducing the need for sutures and in turn, offering decreased postoperative inflammation, faster healing, and reduced post-operative astigmatism, which in combination resulted in a more rapid visual recovery and improved patient experience. Smaller gauge instrumentation also allowed for greater fluidic stability and thus, safer, more controlled, removal of vitreous.

However, the miniaturization of instrumentation brought along several engineering challenges. The smaller instrument diameter and lumen had counterproductive effects on instrument flexibility and endoillumination. These problems were mitigated with shortened instrument shafts which increased rigidity and advances in illumination including chandelier lighting.

Comparison of 20-gauge, 23-gauge and 25-gauge vitreous cutters. The smaller gauge instrumentation has smaller ports but the port is closer to the tip of the probe.

Smaller instrumentation also impacted flow rates and thus, efficiency. Specifically, the reduction in internal diameter of smaller gauge systems resulted in a substantial decrease in flow, as per Poiseuille’s law, wherein flow is inversely proportion to the fourth power of the radius. Reduced flow meant both less infusion inflow and less efficiency in vitreous removal. To compensate, newer small gauge vitrectomy platforms created a pressure gradient whereby the machine sensed flow rates and compensated by increasing infusion. Infusion pressure could therefore be adjusted based on flow rate to maintain the desired IOP during surgery, and this IOP-compensated infusion was accurate to within 2 mmHg.

Higher aspiration vacuums in the range of 400to 600 mmHg were employed to counter the higher-pressure gradient head loss with the smaller vitrector probe diameters. Likewise, increasing cut rates for vitrectomy probes had the potential to improve fluidics by balancing out the loss of flow associated with smaller lumen diameter. High cut rates also reduced the “bite” size and thus the effective viscosity of non-Newtonian fluids such as vitreous. Flow rates and efficiency of vitreous removal could therefore be maintained at high cut rates and pulsatile traction was minimized. This was a huge advantage over earlier systems. However, with traditional pneumatic cutters, increased cut rates were associated with reduction in duty cycle, converging at 50% for both open biased and closed bias duty cycles. The development of dual pneumatic cutters in which the cutter was controlled independently of port opening and closing allowed for the realization of the benefits of very high cut rates by allowing high duty cycles. More recently, even higher cut rates have been achieved with dual blade (2 blades in a single port), dual port (2ports), or bidirectional cutting (cutting in both directions), thus doubling previous cut rates. These technologies increase aspiration flow, reduce “surge” turbulence at the aspiration port, and reduce traction on surrounding tissue, a perceived advantage over single motion cutters.[3]

Other concerns with 25-gauge technology were similarly improved. For example, improved wound construction decreased concerns about perceived increased risk of hypotony and endophthalmitis as compared to 20-gauge systems. Initial perpendicular incisions were exchanged for angled two-step beveled, self-sealing incisions. Subsequent large multicenter studies determined that there was no increased risk of either hypotony or endophthalmitis with changes in entry and exit wound construction. Risk of endophthalmitis is low and on the order of 20-gauge systems.[4,5] Decreased concerns over safety and performance along with improved patient satisfaction allowed 25-gauge to become the most widely used platform in the United States. Recent American Society of Retina Specialists (ASRS) Preferences and Trends (PAT) Surveys indicate 90% of retina surgeons had used small gauge systems routinely.

The development of 25-gauge vitrectomy has been a natural progression from 20-gauge systems as laparoscopy was a progression in general surgery. Smaller, self-sealing incisions that would self-seal seemed a very attractive concept, along with the potential for improved fluidics and safety with smaller-gauge systems. Initial setbacks were addressed with improved two-step entry, improved endoillumination, faster cut rates, and decreased flexibility of the instrumentation. As a result, surgical results improved and complications decreased. Thus, surgeon comfort levels increased dramatically and the 25-gaugeplatforms are now the most widely used in vitreoretinal surgery.

References

1. Machemer R, Parel JM, Norton EW. Vitrectomy: a pars plana approach.Technical improvements and further results. Trans Am Acad OphthalmolOtolaryngol. 1972 Mar-Apr;76(2):462-6. PubMed PMID: 4667660.

2. de Juan Jr., E, Shelley TH, Barnes AC, Jensen PS;  United States Patent# 7,077,848: Sutureless ocular surgical methods andinstruments for use in such methods

3. Watanabe A, Tsuzuki A, Arai K et al. Treatment of a droppednucleus with a 27-gauge twin duty cycle vitreous cutter. Case Reports inOphthalmology, vol.7, no. 1, pp.44-48, 2016.

4. Govetto A, Virgili G, Menchini F, et al. A systematic reviewof endophthalmitis after microincisional versus 20-gauge vitrectomy.Ophthalmology, vol. 120, no.11, pp. 2286-2291, 2013.  

5. Kunimoto DY, Kaiser RS, Wills Eye retina Service. Incidenceof endophthalmitis after 20- and 25-gauge vitrectomy. Ophthalmology, vol. 114,no.12, pp.2133-2137, 2007.

(Milestone essay published 2022)