Ophthalmic Surgery, Lasers and Imaging Retina

Experimental Science 

Fluidics Comparison Between Dual Pneumatic and Spring Return High-Speed Vitrectomy Systems

Rodrigo A. Brant Fernandes, MD; Bruno Diniz, MD; Paulo Falabella, MD; Ramiro Ribeiro, MD; Anderson G. Teixeira, MD; Octaviano Magalhães, MD; Nilva Moraes, MD; Andre Maia, MD; Michel E. Farah, MD; Mauricio Maia, MD; Mark S. Humayun, MD

Abstract

BACKGROUND AND OBJECTIVE:

To compare the water and vitreous flow rates and duty cycle (DC) between two ultrahigh-speed vitrectomy systems: pneumatic with spring return (SR) and dual pneumatic (DP) probes.

MATERIALS AND METHODS:

The flow rate was calculated using a high-sampling precision balance that measured the mass of water and vitreous removed from a vial by a vitreous cutter. Frame-by-frame analysis of a high-speed video of the cutter was used to determine the DC. Three cutters of each gauge (20, 23, and 25 G) were tested with an SR and a DP system using the standard DC setting (biased open) at 0 (water only), 1,000, 2,000, 3,000, 4,000, and 5,000 cuts per minute (CPM) with aspiration levels of 100, 200, 300, 400, 500, and 600 mm Hg.

RESULTS:

The DC was slightly higher with the SR system using most parameters and gauges although without statistical significance. The water flow rate was somewhat higher with the SR system, except for 25 G with 4,000 and 5,000 CPM. The vitreous flow rate was similar using most parameters, with the SR system showing higher flows at lower cut rates (1,000–3,000 CPM).

CONCLUSIONS:

SR and DP systems produced similar water and vitreous flow rates. Additional studies in human eyes are necessary to confirm these findings.

[Ophthalmic Surg Lasers Imaging Retina. 2015;46:68–72.]

From the Department of Ophthalmology and Visual Sciences, Federal University of São Paulo, São Paulo, Brazil (RABF, BD, PF, RR, AGT, OM, NM, AM, MEF, MM); Department of Ophthalmology, Hospital Evangélico de Curitiba, Curitiba, Brazil (RR); and Department of Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles, California (PF, RR, MSH).

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Mark S. Humayun, MD, 1355 San Pablo Street, DVRC 119, Los Angeles, CA 90033; email: humayun@med.usc.edu.

Received: August 04, 2014
Accepted: October 06, 2014

Abstract

BACKGROUND AND OBJECTIVE:

To compare the water and vitreous flow rates and duty cycle (DC) between two ultrahigh-speed vitrectomy systems: pneumatic with spring return (SR) and dual pneumatic (DP) probes.

MATERIALS AND METHODS:

The flow rate was calculated using a high-sampling precision balance that measured the mass of water and vitreous removed from a vial by a vitreous cutter. Frame-by-frame analysis of a high-speed video of the cutter was used to determine the DC. Three cutters of each gauge (20, 23, and 25 G) were tested with an SR and a DP system using the standard DC setting (biased open) at 0 (water only), 1,000, 2,000, 3,000, 4,000, and 5,000 cuts per minute (CPM) with aspiration levels of 100, 200, 300, 400, 500, and 600 mm Hg.

RESULTS:

The DC was slightly higher with the SR system using most parameters and gauges although without statistical significance. The water flow rate was somewhat higher with the SR system, except for 25 G with 4,000 and 5,000 CPM. The vitreous flow rate was similar using most parameters, with the SR system showing higher flows at lower cut rates (1,000–3,000 CPM).

CONCLUSIONS:

SR and DP systems produced similar water and vitreous flow rates. Additional studies in human eyes are necessary to confirm these findings.

[Ophthalmic Surg Lasers Imaging Retina. 2015;46:68–72.]

From the Department of Ophthalmology and Visual Sciences, Federal University of São Paulo, São Paulo, Brazil (RABF, BD, PF, RR, AGT, OM, NM, AM, MEF, MM); Department of Ophthalmology, Hospital Evangélico de Curitiba, Curitiba, Brazil (RR); and Department of Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles, California (PF, RR, MSH).

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Mark S. Humayun, MD, 1355 San Pablo Street, DVRC 119, Los Angeles, CA 90033; email: humayun@med.usc.edu.

Received: August 04, 2014
Accepted: October 06, 2014

Introduction

Surgical instruments for vitreoretinal surgery have seen many improvements in the last decade. These improvements include the decrease in the diameter and size of probes, scissors, and forceps as well as advanced vitrectomy systems that reach high-speed cutting rates up to 5,000 cuts per minute (CPM). Improved fluidics, duty cycle (DC) control, and high cut rates allow the removal of vitreous gel in a more controlled manner, avoiding iatrogenic tears and other complications.1–4

These technologies minimize surgical trauma and also decrease surgical time, leading to better patient recovery with less discomfort and more convenience for both the patient and the surgeon. On the other hand, the reduction of the size and diameter of probes limits the functionality and efficiency of surgical instruments.5–10

Earlier versions of 23-G and especially 25-G instruments were extremely fragile, resulting in deformation during surgical manipulation and thus compromising fluidic efficiency.6,7,10 These limitations resulted in a lower vitreous flow rate and a longer surgical procedure compared with the traditional 20-G probes. Therefore, efforts were made to optimize the efficiency of these smaller instruments by modifying the design of probes and ports, such as maximizing the cut port surface area and increasing the internal lumen diameter to improve the flow rate, moving the cut port closer to the tip of the probe to allow better vitreous shaving, and creating new cutters that allow cut rates up to 5,000 CPM.5,10

Regarding vitrectomy machines, some parameters can be manipulated in order to increase efficiency during surgical procedures, such as a higher cut rate, modifiable DC of the guillotines, increased aspiration rates, and balanced saline solution infusion with pressure control.

Most vitrectomy systems present a constant DC with adjustable aspiration and cut rates. When regular pneumatic with spring return (SR) probes are used at higher cut rates, the time the port remains open tends to decrease, resulting in a reduced flow rate and, consequently, an increased surgical time.6 To address this situation, manufacturers have attempted to create a DC control for vitreous cutter probes, especially at higher cut rates.

Recently, a vitrectomy system with DC control was developed with a dual pneumatic (DP) lumen probe to offer three DC modes.11,12 With this new feature, the surgeon can choose which DC is better indicated for each surgical maneuver (eg, 80% of the time with the port opened for vitreous core vitrectomy or 30% of the time with the port opened for vitreous base shaving).

The probe design was also changed to achieve better control. Whereas the previous systems used a spring-controlled guillotine with a pneumatic push to close the port and a spring-controlled gradual return, this novel technology uses a DP system to control both the closing and opening of the port. Despite those changes, previous studies have shown that both machines were able to reach similar DCs at higher cut rates.13,14

This current study aims to evaluate the performance of two ultrahigh-speed vitreous cutters, an SR probe and a DP probe, comparing their flow rates and duty cycles within an in vitro model.

Materials and Methods

The performance of the new 20-, 23- and 25-G cutters (three cutters of each gauge) was tested with an SR vitrectomy system (Stellaris PCTM System; Bausch & Lomb, St. Louis, MO) and a DP system (Constellation Vision System; Alcon Labs Inc, Fort Worth, TX) using the standard DC setting (biased open) according to methodology previously developed by our group.15,16

Briefly, for the flow tests, each cutter was suspended in a vial of either water or porcine vitreous. The vials were placed on a high-sampling (two samples per second), precision (0.01 g) balance (Ohaus Corp, Parsippany, NJ) that measured the weight of the remaining water or vitreous throughout each experiment. Using data acquisition software (Lab-VIEW; National Instruments, Austin, TX), the remaining mass was recorded in real time, and the results were converted to volume removed as a function of time (ie, flow rate).

A stop-action camera (Motionscope M1; Red Lake, Tokyo, Japan) with 1,000 frames per second resolution was used to capture high-speed video of the cutter action. Three investigators, who were masked to the system parameters, independently analyzed the videos, frame by frame, to determine the DC as a function of speed. The DC was calculated using the following equation: DC = open time + (opening time + closing time)/complete cycle duration.

Statistical Analysis

The average and standard deviation of the water and vitreous flow rates were calculated for each size, aspiration level, and cut rate. To compare these two systems (SR and DP), sizes (20, 23, and 25 G), aspiration levels (0–600 mm Hg), and cut rates (1,000–5,000 CPM), mixed linear regression models were used to evaluate the influence of these variables in water and porcine vitreous. The models were adjusted using the flow rate as the dependent variable. Aspiration level and the type of vitrectomy machine were the explanatory variables for every combination of gauge and cut rate. SAS V9.2 programming language (SAS Institute, Cary, NC) was used for all analyses. The accepted level of significance for all tests was P < .05.

Results

Duty Cycle

The DC peaked at 1,000 CPM and dropped as the cut rate increased with both systems. The DC was slightly higher for the SR system using all gauges and cut rates, except for the 20-G cutter at 5,000 CPM, although the analysis did not have enough statistical power to determine a significant difference between the systems. The closed port time was the main difference between the machines and was longer on average in the DP system (Figure), which might be the explanation for the differences in DC.

The ratios of cutting phases between the systems. The average closed phase (20, 23, and 25 G) of the duty cycle was largest with the dual pneumatic system, which may be responsible for the differences between the cycles.

Figure.

The ratios of cutting phases between the systems. The average closed phase (20, 23, and 25 G) of the duty cycle was largest with the dual pneumatic system, which may be responsible for the differences between the cycles.

Based on the cutting phase duration (opened, closing, closed, opening), the cut rate was calculated for each probe. The observed cut rate of both systems showed disparities when compared with the cut rate displayed on the machine (SR: −7.7% to +20%, DP: −16.7% to +22.5%).

Water Flow Rate Comparison

The average water flow rates were used to compare the two systems.

20-G Cutters

The water flow rate was higher with the 20-G SR cutter when compared with the DP system at the following cut rates: 0, 1,000, 2,000, and 3,000 CPM. There was no statistically significant difference between 20-G SR and DP cutters in the water flow rate at 4,000 and 5,000 CPM.

23-G Cutters

The water flow rate was higher with the 23-G SR cutter when compared with DP at all cut rates (0, 1,000, 2,000, 3,000, 4,000, and 5,000 CPM).

25-G Cutters

The water flow rate was higher with the 25-G SR cutter when compared with DP at 1,000, 2,000, and 3,000 CPM. Conversely, the DP cutter showed a higher water flow rate at 4,000 and 5,000 CPM.

A comparison of water flow performance of both machines is summarized in Table 1 based on mixed linear regression models (Appendix A).

Performance Comparison Between DP and SR Vitrectomy Systems in Water Flow for Different Gauges (20, 23 and 25) and Cut Rates (1,000 to 5,000)

Table 1:

Performance Comparison Between DP and SR Vitrectomy Systems in Water Flow for Different Gauges (20, 23 and 25) and Cut Rates (1,000 to 5,000)

Vitreous Flow Rate Comparison

The average vitreous flow rates were used to compare the two systems.

20-G Cutters

The vitreous flow rate with the 20-G SR cutter was higher than DP at 1,000 CPM, whereas the DP showed a higher flow rate at 5,000 CPM. There was no statistically significant difference between 20-G SR and DP cutters in the vitreous flow rate at 2,000, 3,000, and 4,000 CPM.

23-G Cutters

The vitreous flow rate was higher with the 23-G SR cutter when compared with the DP cutter at the following cut rates: 1,000, 2,000, and 3,000 CPM. There was no statistically significant difference between 23-G SR and DP cutters in the vitreous flow rate at 4,000 and 5,000 CPM.

25-G Cutters

The vitreous flow rate was higher with the 25-G SR cutter when compared with the DP cutter at 2,000 CPM. For the remaining cut rates (1,000, 3,000, 4,000, and 5,000 CPM), there was no statistically significant difference between the two systems with 25-G cutters.

A comparison of the vitreous flow performance of both machines is summarized in Table 2 based on mixed linear regression models (Appendix B).

Performance Comparison Between DP and SR Vitrectomy Systems in Vitreous Flow for Different Gauges (20, 23 and 25) and Cut Rates (1,000 to 5,000)

Table 2:

Performance Comparison Between DP and SR Vitrectomy Systems in Vitreous Flow for Different Gauges (20, 23 and 25) and Cut Rates (1,000 to 5,000)

Discussion

The present analysis is a comparison between two of the most commonly used vitreoretinal surgical systems. Our group has previously published a separate analysis of the fluidic performance of SR and DP systems. The water and vitreous flow rates and DC of both systems have been evaluated independently, but a closer and direct comparison between them has not yet been addressed. We believe that this comparison is extremely important for surgeons because it will allow them to understand the differences and similarities between the machines as well as the potential advantages and disadvantages.

The SR system provides an air pulse that closes the cutter tip, and when the air is released, a spring returns the tip to the opened position, completing the cycle. This physical process of closing and opening has a characteristic constant time. As the cut rate increases, the duration of opened time decreases, whereas the closed time remains relatively constant. This feature reduces the DC as the cut rate increases.

The DP surgical system supplies intermittent air pulses, alternating between each line. The first drive line acts to close the port, whereas the second line opens the port. This was designed to maintain a constant duty cycle even at high cut rates.

Despite those theoretic differences, in our experiment, both machines showed similar DCs and, surprisingly, even a slight trend for a higher DC with the SR cutter. This was also reflected in the higher water flow observed in the majority of the tests performed with the SR cutter.

Nevertheless, we believe that a refinement of the air pulse duration in the DP system could decrease the closed port time and reduce even more those differences between cycles (Figure).

This new evaluation compared the water and vitreous flow, confirming the similar performance in DC of the SR and DP cutters previously published.13,14 DP and SR produced similar results in terms of water and vitreous flow although some small differences were observed in this experiment. The SR system presented a trend of higher water flow with the majority of gauges and parameters, except for the 25 G with 4,000 and 5,000 CPM. The differences in water flow ranged from 0.005 to 0.055 mL/s (Table 2). The vitreous flow rate was similar in most parameters tested, with a trend of higher flow for the SR system at lower cut rates (1,000–3,000 CPM). The differences in vitreous flow ranged from 0.0026 to 0.0123 mL/s.

Port geometry differences between these two cutters could also have influenced the results because the port area of the 23-G SR system is larger than the 23-G DP system (0.328 mm2 vs 0.313 mm2).

The two new vitrectomy systems mentioned previously were released in sequence in the market, presenting the following new features: better fluidics, higher cut rates, smaller gauges, improved illumination, and new designs for the port and tip of the cutter. However, the main innovation was the improved DC control that aimed to provide the same surgical efficiency as previously observed with the 20-G cutter but with the advantages of microincision vitrectomy (23- and 25-G cutters).

The introduction of a DP probe along with DC control provided, for the first time, the opportunity to use a different DC at distinct moments of the surgery depending on the cutter’s distance from the retinal surface. This advance theoretically results in a safer surgical procedure with a biased closed port when the surgeon works close to the retinal surface (named vitreous shaving DC) and a biased open port when removing the vitreous core or when there is a need for more aggressive vitreous removal (named core DC), which is similar to all standard vitrectomy systems. Although the influence of the DC control was clear in a previous study,17 the difference in the vitreous flow was reduced at higher cut rates (3,000 and 4,000 CPM), probably because of a convergence of the DC at around 50% (Table 2).

Although a difference between the set cut rates and the observed cut rates was observed in both systems, this article focused on comparing the set cut rates because those are the ones available for surgeons. Further analysis between the observed cut rates in both systems is still necessary.

In conclusion, the present study showed that these two systems presented similar results in terms of the water and vitreous flow rate sand DC. The improvements provided by these systems over previous vitrectomy machines offer important advantages regarding the efficiency in fluidics and should be used to achieve a safer surgical procedure and better visual outcomes.

Future studies evaluating the newer generation of vitrectomy systems with even faster cut rates and smaller probes (27 G) may guide further technologic improvements and optimize the results of vitreoretinal surgery.

References

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Performance Comparison Between DP and SR Vitrectomy Systems in Water Flow for Different Gauges (20, 23 and 25) and Cut Rates (1,000 to 5,000)

Cut Rate (CPM)Best PerformanceFlow Rate Difference (mL/sec)P Value
20-gauge0SR0.043.002
1,000SR0.028.01
2,000SR0.055< .001
3,000SR0.035.019
4,000--.692
5,000--.095
23-gauge0SR0.026< .001
1,000SR0.023< .001
2,000SR0.031< .001
3,000SR0.031< .001
4,000SR0.022.003
5,000SR0.014.044
25-gauge0--.422
1,000SR0.005.02
2,000SR0.009< .001
3,000SR0.007.01
4,000DP0.013.003
5,000DP0.030< .001

Performance Comparison Between DP and SR Vitrectomy Systems in Vitreous Flow for Different Gauges (20, 23 and 25) and Cut Rates (1,000 to 5,000)

Cut Rate (CPM)Best PerformanceFlow Rate Difference (mL/sec)P Value
20-gauge1,000SR0.01233.009
2,000--.372
3,000--.454
4,000--.230
5,000DP0.00422< .001
23-gauge1,000SR0.00681< .001
2,000SR0.00518.048
3,000SR0.00523< .001
4,000--.910
5,000--.943
25-gauge1,000--.179
2,000SR0.00261.008
3,000--.510
4,000--.688
5,000--.135

10.3928/23258160-20150101-11

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