An Early Look at 27-gauge Surgery

Japanese researchers are studying new frontiers in surgical instrument design.

MARC J. SPIRN, MD

Micro-incision vitrectromy surgery (MIVS) was first introduced several years ago: In 2002, Fujii et al. introduced 25-gauge three-port pars plana vitrectomy (PPV),¹ and in 2005, Eckhart introduced 23-gauge PPV.² A major difference between 25-g and 23-g surgery vs conventional 20-g PPV is that these micro-incision techniques (ie, 25-g and 23-g) are performed via a transconjunctival approach. With 20-g techniques, the conjunctiva and sclera are opened separately and therefore must be sutured closed at the end of surgery. With 25-g and 23-g surgery, conversely, a trocar is inserted directly through the conjunctiva and sclera. The wounds in most cases are self-sealing and frequently do not require a suture.

As these micro-incision techniques have evolved and become more prevalent, specific benefits and drawbacks of MIVS have become evident. Because a peritomy is not performed and because conjunctival and scleral sutures are often not needed, operative times may be shorter. In addition, the lack of sutures may result in less postoperative inflammation, less astigmatism, and improved postoperative comfort.^3-6 However, several studies have suggested that hypotony and endophthalmitis are more likely with MIVS than with 20-g surgery.^7-9 In theory, the larger the wound, the more likely hypotony and endophthalmitis are to occur.

The rationale behind 27-g surgery is that smaller is better. Smaller wounds are more likely to self-seal and prevent hypotony. Smaller wounds are less prone to vitreous prolapse, which may act as a wick and promote endophthalmitis. Limiting hypotony and preventing endophthalmitis are some of the reasons that 27-g PPV is potentially appealing.

HISTORY OF 27-G SURGERY

The 27-g instrumentation for PPV was first introduced in 2007, when Oshima et al. introduced a 27-g chandelier light.¹⁰ Shortly after this event, Sakaguchi et al. published their experience performing 27-g nonvitrectomy for epiretinal membranes.¹¹ With this technique, a 27-g chandelier light was placed superiorly at the pars plana, and a single transconjunctival incision was made with a 27-g needle. Without performing any vitrectomy, 27-g forceps were inserted at the needle puncture site, and the epiretinal membrane was peeled. While nonvitrectomy has not gained a huge following (it has been around since approximately 1999), the use of 27-g instrumentation was interesting.

In January 2010, Oshima et al. published the first paper on the use of 27-g PPV.¹² Since 27-g PPV is not currently commercially available in the United States, much of our current knowledge of 27-g PPV is based on the experience described in this study.

In the 2010 study, Oshima et al. evaluated many different aspects of 27-g surgery (eg, infusion, aspiration, duty cycle, etc.). After comparing many of these characteristics to Alcon's (Fort Worth, TX) 25-g PPV, the authors then performed 31 cases using 27-g PPV. In order to perform these surgeries, they used a 27-g infusion line (Synergetics Inc., St. Charles, MO) with a short light pipe and high-speed vitreous cutter (Dutch Ophthalmic Research Center International BV, Zuidland, the Netherlands). Several other instruments, such as forceps and laser, were also designed to enable certain specific tasks.

In the following segment, I will discuss some of the characteristics of the 27-g system designed by Oshima et al. and how it compares to Alcon's 25-g instruments (not, however, the newer 25-g plus instruments, also by Alcon).

VITRECTOR

There are several important characteristics to consider when evaluating the efficiency and safety of a vitrector. These include its cut rate, duty cycle and size and position of the port. Cut rate is the speed at which the cutter mouth opens and closes. Recent advances in 25-g and 23-g surgery and the introduction of new vitrectomy systems have allowed for increased rates from 1,500 cuts per minute (cpm) to 5,000 cpm. Faster cut rates allow for safer peripheral vitreous dissection. Since less vitreous is pulled into the vitrector with each cut, there is less traction and therefore less likelihood of creating a peripheral tear.

Duty cycle is a measure of the percentage of time the vitrector mouth is open at any given cut rate. In their study, Oshima et al. evaluated the duty cycle of their 27-g system compared to Alcon's Accurus 25-g system. At a cut rate of 1,000 cpm, the duty cycle was equal between 27-g and 25-g (61% vs 62%). At a cut rate of 1,500 cpm, the duty cycle for 27-g outperformed 25-g (38% vs 28%). At cut rates higher than 1,500 cpm, the duty cycle dropped precipitously for 27-g: 21% at 2,000 cpm and 7% at 2,500 cpm. Since the 25-g cut rate maximum was 1,500 cpm when using the Accurus, comparative duty-cycle measurements could not be performed at 2,000 cpm and 2,500 cpm. Constellation, the current state-of-the-art vitrectomy system from Alcon, can achieve cut rates of 5,000 cpm.

The port's size and its distance from the end of the cutter are also vital considerations when evaluating a vitrector. Compared to Alcon's 25-g probe, the 27-g probe designed by Oshima et al. was slightly larger (0.079 mm² vs 0.066 mm²), and the port was slightly closer to the tip (0.211 mm vs 0.330 mm). Having the port closer to the tip may be advantageous when performing complex maneuvers such as membrane segmentation or delamination (Figure 2).

Figure 2. A 27-gauge vitreous cutter with shaft length of 25 mm (A). Tip of the 27-g cutter in high magnificiation (B and C). IMAGE WITH PERMISSION OF ELSEVIER. ORIGINALLY APPEARED IN: OSHIMA Y, WAKABAYASHI T, SATO T, OHJI M, TANO Y. A 27-GAUGE INSTRUMENT SYSTEM FOR TRANSCON JUNCTIVAL SUTURELESS MICRO INCISION VITRECTOMY SURGERY. OPHTHALMOLOGY. 2010;117:93-102.E2.

ASPIRATION AND FLOW

Aspiration is an essential component that affects the efficiency and function of the vitrector. Aspiration ability helps determine the rate at which vitreous is pulled into the cutter and can facilitate certain tasks such as lifting the posterior hyaloid or removing blood. Oshima et al. found that at a cut rate of 1,000 cpm and a vacuum of 600 mm Hg, the 27-g aspiration rate was 70% that of 25-g. At a cut rate of 1,500 cpm and a vacuum of 600 mm Hg, the aspiration rate was 80% that of 25-g.

This reduced aspiration rate is likely a minor issue during many cases. However, there are certain times when high aspiration ability is helpful. For example, in some cases of macular hole or vitreomacular traction repair, the hyaloid may be strongly adherent, requiring high levels of aspiration to free the adhesions between the hyaloid and the optic nerve. If the aspiration capacity is insufficient, detaching the hyaloid with the vitrector may not be possible. In fact, in their series, Oshima et al. mechanically detached the hyaloid using a membrane spatula rather than the vitrector. It is not clear if this was their preferred method or if they chose this method because the 27-gauge vitrector lacked the requisite aspiration capacity to detach the hyaloid.

Flow is a parameter that is directly related to aspiration. In order to maintain a constant intraocular pressure, inflow into the eye must be equal to outflow out of the eye. In other words, the infusion rate must equal the aspiration rate. Oshima et al. found that infusion pressures between 20 and 30 mm Hg were optimal for preventing pressure surges. Because aspiration rates are lower for 27-g than for 25-g, inflow rates are also lower. Lower flow rates may be gentler and less traumatic to the eye.

RIGIDITY

Rigidity of instrumentation is dependent on a number of factors: the material it is fabricated from, the thickness, the diameter (or gauge in this case) and the length. When comparing 25-g to 23-g instrumentation, there is a large difference in the rigidity of the instruments, with 23-g instruments being much more rigid. Having rigid instruments is helpful in a number of settings, including when performing peripheral dissection, during endophotocoagulation (particularly near the 12 o'clock position), and when removing dislocated intraocular lenses, among other settings. One reason many vitreoretinal surgeons prefer 23-g instruments over 25-g is directly related to the extra rigidity of 23-g instruments. Alcon recently introduced 25-g plus product line, which incorporates several advances, including increased rigidity compared to standard 25-g.

One of the potential drawbacks of 27-g surgery is its excess flexibility. To minimize excess flexibility, Oshima et al. shortened the shaft of their vitrector from 32 mm (ie, Alcon's 25-g shaft length) to 25 mm. In doing so, they created a 27-g vitrector with similar shaft rigidity to the 25-g vitrector. When a known force (0.5 N) was placed 20 mm from the vitrector base, the 25-g instrument displaced 3.3 mm while the 27-g cutter displaced 5.8 mm. The authors contend that, even with a shorter shaft length, however, they were able to perform core and peripheral PPV in eyes with axial lengths ranging from 22 to 28 mm.

ILLUMINATION

When 25-g PPV was first introduced, one of the early drawbacks was that the illumination using halogen lighting was not nearly as bright as conventional 20-g PPV. Recently, xenon and mercury vapor illumination have been introduced, allowing for improved illumination through smaller fibers. This innovation has helped propel MIVS and has enabled the proliferation of multiple illuminated instruments such as illuminated picks, illuminated lasers, and chandelier lights.

In their recent study, Oshima et al. designed a shortshaft light pipe.¹² This light was 13 mm in length and was sharp enough to enter the eye using a stab incision. The short length of their illuminator made inadvertent retinal touch unlikely. According to the authors, the light pipe gave off 20 lm of light with a xenon light source and 25 lm with a mercury vapor light source, enough to perform the necessary tasks.

INSTRUMENTATION

Over the last several years, major advances in instrumentation have occurred, and it is now possible to perform nearly all types of vitreoretinal surgery using micro-incision techniques. In their recent study, Oshima et al. developed several instruments for 27-g PPV. These instruments included a vitrector, a one-step infusion line (Figure 1), a one-step short-shaft light pipe, several forceps (asymmetric, endgripping, and pick forceps), a membrane spatula, a blunt-tip and sharp-tip endolaser probe, microvertical scissors, sharp-point vertical scissors and a trocar cannula system. Using these instruments, they were able to perform a myriad of different surgeries safely and effectively.

Figure 1. A 27-gauge one-step entry infusion line (A) and beveled sharp tip and 4 mm metallic tube (B). IMAGE WITH PERMISSION OF ELSEVIER. ORIGINALLY APPEARED IN: OSHIMA Y, WAKABAYASHI T, SATO T, OHJI M, TANO Y. A 27-GAUGE INSTRUMENT SYSTEM FOR TRANSCON JUNCTIVAL SUTURELESS MICROINCISION VITRECTOMY SURGERY. OPHTHALMOLOGY. 2010;117:93-102.E2.

In some cases, Oshima et al. used a trocar-based system such as is used with current 25-g and 23-g techniques. Trocars enable efficient exchange of instruments and prevent excessive traction on the vitreous base during entry and removal of instruments. For more difficult cases or in phakic patients, where exchanging instruments is likely, a trocarbased system is typically safer and more efficient. In some cases, however, Oshima et al. entered the eye directly without trocars. In order to do this, they developed several sharp instruments, such as a sharp laser and a sharp light pipe.

POTENTIAL INDICATIONS

When 25-g surgery first became available, its indications were limited. Most surgeons used 25-g instruments for straightforward cases such as macular holes, macular puckers or simple vitreous hemorrhages. Twenty-gauge PPV was still preferred for complex cases. It is likely that if 27-g gains popularity, it will follow a similar path. In their study, Oshima et al. performed 27-g PPV for relatively uncomplicated indications, including macular holes, macular puckers, vitreomacular traction, vitreous hemorrhage and focal tractional retinal detachment. Until there are major advances in instrumentation, rigidity, or both, it is unlikely that complex cases, such as complex retinal detachment repair, will be performed efficiently or safely with 27-g.

CONCLUSIONS

In conclusion, 27-gauge PPV is intriguing on a number of levels. On the one hand, because of its smaller wound size, it will likely help limit the cases of postoperative hypotony and endophthalmitis. Smaller wounds may also translate into improved postoperative comfort and faster wound healing. Lower flow rates are likely to result in gentler, less traumatic surgeries. These benefits, on the other hand, must be weighed against the drawbacks of increased instrument flexibility and limited aspiration capacity — drawbacks that may make some tasks difficult or impossible to perform. In the future, it is unlikely that 27-g PPV will completely replace either 25-g or 23-g, but it is certainly feasible that it will find a niche and help make MIVS safer. RP

REFERENCES

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