From Eye Concepts (NM, AT, J-CL, SF, RK, PB, MH), Doheny Eye Institute, Los Angeles, California; Doheny Retina Institute (NM, AT, MH), Los Angeles, California; the Division of Ophthalmology and Visual Science (NM), Graduated School of Medical and Dental Sciences, Niigata University, Niigata, Japan; the Department of Ophthalmology (AT), Federal University of São Paulo, São Paulo, Brazil; and the Departments of Ophthalmology & Biomedical Engineering (MH), University of Southern California, Los Angeles, California.
Supported by Eye Concepts laboratory at the Doheny Eye Institute. Eye Concepts receives research funds that are part of an advanced royalty distribution agreement between Bausch & Lomb, Inc. and the Doheny Eye Institute. The sponsor organization had no role in the design or conduct of this research. Also supported by National Eye Institute basic research grant #EY03040.
Dr. Humayun is a consultant for Bausch & Lomb. The remaining authors have no financial or proprietary interest in the materials presented herein.
The authors thank Laurie Dustin from USC for the statistics.
Address correspondence to Mark Humayun, MD, PhD, Doheny Eye Institute, 1450 San Pablo Street DVRC 119, Los Angeles, CA 90033. E-mail: firstname.lastname@example.org
The new generations of pneumatic cutters were designed to address the limitations of conventional pneumatic cutters. When the conventional pneumatic cutters are operated at high cut-rates, the flow rate is significantly reduced. This is especially true for the smaller gauge systems with reduced diameter.1–3 On the other hand, smaller gauge instrumentation has some definite advantages, including the ability to use a minimally invasive surgical technique leading to faster postoperative patient recovery.2,4–10 Some improvements could potentially expand the applications for the small gauge systems.2,11–14
One of the primary underlying mechanisms that affects flow rates through the vitreous cutter is the duty cycle, which is defined by the percentage of time the cutter port is open relative to the complete cutting cycle (duty cycle = open time/total cycle time). At high cut-rates, the conventional pneumatic cutters have a low duty cycle, which is associated with lower flow rates2 and longer surgical time. The Millennium Vitreous Enhancer (MVE) system (Bausch & Lomb, St. Louis, MO) has a pneumatic pressure pulse and can deliver up to 2,500 cuts per minute (CPM) using a 20-, 23-, or 25-gauge vitrectomy probe with an increased port-size area and a port forward design. The port size of the 20-, 23-, and 25-gauge cutters is 0.53, 0.48, and 0.42 mm in diameter and the distance from the tip is 0.43, 0.23, and 0.23 mm, respectively. This MVE system is nearly the same material as the Adaptable Vit Enhancer (AVE) system (Mid Labs, San Leandro, CA) and is supplied from Bausch & Lomb as original equipment manufacturer. Some designs have been changed from the early type of the AVE system described in a previous article.2
The objective of this study was to analyze the performance of the MVE cutters and to study the fundamental mechanisms that limit the performance of the pneumatic cutter.
Materials and Methods
The performance of the MVE system using 20-, 23-, and 25-gauge cutters (three cutters for each gauge) was measured. The MVE console was connected to a host Millennium Microsurgical System (Bausch & Lomb). The cutters were tested across their specified cut speed range based on duty cycle and on flow rates in water and porcine vitreous.
Our methodology has been described in detail elsewhere.1–3 Briefly, each cutter was suspended in a vial of either water or vitreous. The vials were placed on a high-sampling (2 samples/second), precision (to 0.01 g) balance (Ohaus Corp., Pine Brook, NJ) that measured the weight of water or vitreous remaining throughout each trial. Using data acquisition software (LabVIEW; National Instruments, Austin, TX), the remaining mass was recorded in real time and the result was converted to volume removed as a function of time (flow rate). The density of porcine vitreous was assumed to be approximately equal to the density of water. With each cutter, four trials were conducted at different cut-rates (800, 1,000, 1,500, 2,000, and 2,500 CPM) and aspiration levels (100, 200, 300, 400, and 500 mm Hg) in both water and vitreous.
The vitreous used for the experiments was carefully removed en bloc from porcine eyes enucleated within 12 hours postmortem (Sierra for Medical Science, Whittier, CA). All eyes were kept at 4°C before use. Each trial conducted in water was 15 seconds in duration. Each trial conducted in vitreous was 30 seconds in duration.
A stop-action camera (1M150; Dalsa, Waterloo, Ontario, Canada) was used to capture high-speed video of the cutter action. Frame-by-frame analysis of the video was used to determine duty cycle as a function speed. All videos were analyzed by two investigators.
The water and vitreous flow rate means, standard deviations, and minimum and maximum values were calculated by gauges, aspiration levels, and cut-rates. Repeated measures analysis of variance (ANOVA) tested mean water and vitreous flow rates across cut-rates were used. Mixed models with repeated measures were used to obtain regression equations for predicting mean water and vitreous flow rates. SAS V9.1 programming language (SAS Institute Inc., Cary, NC) was used for all analyses. Accepted level of significance for all tests was a P value of less than .05.
Water Flow for 20-, 23-, and 25-Gauge Cutters
The water flow rate for the 20-, 23-, and 25-gauge cutters at various cut-rates and different aspiration levels is depicted in Figure 1. The results are summarized in Table 1. Water flow rates for all three gauges peaked at 1,500 CPM and dropped moderately with increasing cut-rate. This trend was consistent at all tested aspiration levels.
Figure 1. Water Flow Rates of 20-Gauge (a), 23-Gauge (b), and 25-Gauge (c) Cutters. Water Flow Rates for All Three Gauges Peaked at 1,500 Cuts per Minute and Dropped Moderately with Increasing Cut-Rate. CPM = Cuts per Minute.
Table 1: Water Flow Results of Cutters with Different Cut-Rates and Aspiration Levels
For the 20-gauge cutter, the flow increased proportionality 0.062 mL/sec for every 100 mm Hg of aspiration level increase (P < .001) and decreased proportionality 0.03 mL/sec for every 500 CPM increase (P < .001). For the 23-gauge cutters, the flow increased proportionality 0.066 mL/sec for every 100 mm Hg of aspiration level increase (P < .001) and decreased proportionality 0.009 mL/sec for every 500 CPM increase (P < .001). For the 25-gauge cutters, the flow increased proportionality 0.030 mL/sec for every 100 mm Hg of aspiration level increase (P < .001) and decreased proportionality 0.005 mL/sec for every 500 CPM increase (P < .001). Analysis of variance (ANOVA) across cut-rates was statistically significant (P < .05) at all aspiration levels and gauges.
As aspiration level increased each 100 mm Hg, the water flow rate increased 1.15 to 1.90 times for 20-gauge, 1.14 to 2.03 times for 23-gauge, and 1.14 to 2.00 times for 25-gauge. Water flow rates of 20-gauge were between 1.39 to 2.00 times to the rates of 23-gauge at each CPM at all tested aspiration levels and the rates of 23-gauge were from 1.39 to 2.16 times to the rates of 25-gauge at each CPM at all tested aspiration levels.
Vitreous Flow for 20-, 23-, and 25-Gauge Cutters
The vitreous flow rate for the 20-, 23-, and 25-gauge cutters at various cut-rates and different aspiration levels is depicted in Figure 2. The results are summarized in Table 2. Vitreous flow rates of all three gauges peaked at 2,000 or 2,500 CPM and increased moderately with increasing cut-rate.
Figure 2. Vitreous Flow Rates of 20-Gauge (a), 23-Gauge (b), and 25-Gauge (c) Cutters. Vitreous Flow Rates for All Three Gauges Showed a Slightly Increased Trend with Increasing Cut-Rate. CPM = Cuts per Minute.
Table 2: Vitreous Flow Results of Cutters with Different Cut Rates and Aspiration Levels
For the 20-gauge cutter, the flow increased proportionality 0.020 mL/sec for every 100 mm Hg of aspiration level increase (P < .001) and increased proportionality 0.003 mL/sec for every 500 CPM increase (P < .001). For the 23-gauge cutters, the flow increased proportionality 0.009 mL/sec for every 100 mm Hg of aspiration level increase (P < .001) and increased proportionality 0.002 mL/sec for every 500 CPM increase (P < .001). For the 25-gauge cutters, the flow increased proportionality 0.004 mL/sec for every 100 mm Hg of aspiration level increase (P < .001) and decreased proportionality 0.001 mL/sec for every 500 CPM increase (P < .001). ANOVA across cut-rates was statistically significant (P < .05) at all aspiration levels and gauges except at a few test points: 20-gauge at 300 mm Hg (P = .07), 23-gauge at 300 and 500 mm Hg (P = .27 and .08, respectively), and 25-gauge at 500 mm Hg (P = .22).
As aspiration level increased each 100 mm Hg, the vitreous flow rate increased 1.33 to 2.80 times for 20-gauge, 1.17 to 2.80 times for 23-gauge, and 1.05 to 3.08 times for 25-gauge. Vitreous flow rates of 20-gauge were from 1.74 to 2.68 times the rates of 23-gauge at each CPM at all tested aspiration levels and the rates of 23-gauge were from 1.49 to 3.76 times to the rates of 25-gauge at each CPM at all tested aspiration levels.
Duty Cycle for 20-, 23-, and 25-Gauge Cutters
For all three cutter gauges, the duty cycle peaked at 1,500 CPM and dropped moderately as the cut-rate increased, ranging from 78.0% ± 3.0% to 63.0% ± 6.0% for 20-gauge, 78.0% ± 1.0% to 63.0% ± 1.0% for 23-gauge, and 76.6% ± 1.5% to 60.9% ± 2.4% for 25-gauge in Figure 3. These duty cycles were significantly consistent for the three gauge cutters. Incomplete port closing and opening, which was reported in the conventional 20-gauge pneumatic cutter at 2,500 CPM,2 was not observed for all three cutter gauges in our high-speed video results.
Figure 3. Duty Cycle of 20-Gauge, 23-Gauge, and 25-Gauge Cutters. All Three Gauge Cutters Maintained More than 60% Duty Cycle at All Tested Cut-Rates. CPM = Cuts per Minute.
The flow rate is dependent on several factors, such as the viscosity of the aspirated fluid, the inner diameter of the vitrectomy probe and aspiration tube, the size of the cutting port, the structure of the instruments, the aspiration level, the cut-rate, and the duty cycle. The viscosity of the aspirated vitreous depends on the initial viscosity of the gel and the change in viscosity induced by the vitrectomy probe’s fragmentation action, which is a measure of the cut-rate, the cutter’s quality,15 and the character of the vitreous. Current vitreous cutters of the same gauge size have similar physical dimensions, with the exception of the cutting port area.16 Physical instrument parameters such as size and shape cannot be changed during the surgery. The only two parameters that can be changed during the surgery are the cut-rate and the aspiration level.
Our water flow rate curves are similar to the duty cycle curves, with both peaking at 1,500 CPM. This can be explained because water is a low-resistance fluid that is readily aspirated without cutting.3,15,17 In contrast, the vitreous flow rates showed a slight increasing trend for all gauges tested. Vitreous is classified as a gelatinous fluid made up of 98% liquid and 2% protein3,18 and larger vitreous chunks can obstruct the cutter port. As the cut-rate increases at the same aspiration level, the vitreous chunks that are being cut become smaller and less resistant to aspiration.3,18 Our high-speed video results showed that all cutters opened and closed completely even at high cut-rates, which is likely to result in smoother cutting with less obstruction and surging. The vitreous flow rates would become more similar to the water flow rates at much higher cut-rates.
Changing the cut-rate changes the duty cycle for pneumatic cutters. The duty cycle of the conventional pneumatic cutters drops significantly at higher cut-rates (20-gauge = 800 to 2,500 CPM and 25-gauge = 800 to 1,500 CPM) and exhibits a steeper decline in duty cycle from 800 CPM to the maximum cut-rate (72.7% to 6.3% and 59.3% to 18.5%).2 The MVE system, however, provides a new paradigm, with a high flow rate at a high cut-rate for all three cutter gauges because of its stable duty cycle, which remains greater than 60% at 2,500 CPM and varies between 60% and 80% in Figure 3.
Figures 1 and 2 demonstrate that using a larger cutter gauge and a higher aspiration level results in higher flow rates for both vitreous and water at the tested CPM. All water and vitreous flow rates had a relatively constant pattern across the cut-rate range, with higher values for water flow. At each cut-rate tested, the vitreous flow rate was approximately doubled (from 1.05 to 3.08 times) for each 100-mm Hg increase in the aspiration level. Similarly, at each cut-rate tested, a one size increase in the cutter gauge (25- to 23-gauge or 23- to 20-gauge) approximately doubled the vitreous flow rate.
The flow rate of the MVE system is stable and predictable for all three cutter gauges. These findings should aid the surgeon to easily select the optimal probe and parameters for the situation by weighing the various advantages and limitations, the cutting place (core or close to the retina), and the target material.
- DeBoer C, Fang S, Lima LH, et al. Port geometry and its influence on vitrectomy. Retina. 2008;28:1061–1067. doi:10.1097/IAE.0b013e3181840b64 [CrossRef]
- Fang SY, Deboer CM, Humayun MS. Performance analysis of new-generation vitreous cutters. Graefes Arch Clin Exp Ophthalmol. 2008;246:61–67. doi:10.1007/s00417-007-0672-8 [CrossRef]
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- Fujii GY, De Juan E Jr, Humayun MS, et al. Initial experience using the transconjunctival sutureless vitrectomy system for vitreoretinal surgery. Ophthalmology. 2002;109:1814–1820. doi:10.1016/S0161-6420(02)01119-3 [CrossRef]
- Fujii GY, De Juan E Jr, Humayun MS, et al. A new 25-gauge instrument system for transconjunctival sutureless vitrectomy surgery. Ophthalmology. 2002;109:1807–1812. doi:10.1016/S0161-6420(02)01179-X [CrossRef]
- Ibarra MS, Hermel M, Prenner JL, Hassan TS. Longer-term outcomes of transconjunctival sutureless 25-gauge vitrectomy. Am J Ophthalmol. 2005;139:831–836. doi:10.1016/j.ajo.2004.12.002 [CrossRef]
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- Lahey JM, Francis RR, Kearney JJ. Combining phacoemulsification with pars plana vitrectomy in patients with proliferative diabetic retinopathy: a series of 223 cases. Ophthalmology. 2003;110:1335–1339. doi:10.1016/S0161-6420(03)00454-8 [CrossRef]
- Lakhanpal RR, Humayun MS, de Juan E, et al. Outcomes of 140 consecutive cases of 25-gauge transconjunctival surgery for posterior segment disease. Ophthalmology. 2005;112:817–824. doi:10.1016/j.ophtha.2004.11.053 [CrossRef]
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Water Flow Results of Cutters with Different Cut-Rates and Aspiration Levels
|Aspiration (mm Hg)||800 CPM||1,000 CPM||1,500 CPM||2,000 CPM||2,500 CPM|
|100||0.146 ± 0.004||0.075 ± 0.003||0.041 ± 0.007||0.148 ± 0.006||0.077 ± 0.004||0.041 ± 0.006||0.154 ± 0.005||0.082 ± 0.009||0.041 ± 0.006||0.0135 ± 0.005||0.079 ± 0.013||0.040 ± 0.007||0.118 ± 0.015||0.063 ± 0.010||0.036 ±0.008|
|200||0.269 ± 0.008||0.140 ± 0.003||0.077 ± 0.012||0.274 ± 0.009||0.144 ± 0.005||0.078 ± 0.012||0.282 ± 0.006||0.153 ± 0.007||0.080 ± 0.010||0.258 ± 0.008||0.149 ± 0.015||0.078 ± 0.012||0.218 ± 0.009||0.270 ± 0.011||0.072 ± 0.017|
|300||0.369 ± 0.014||0.225 ± 0.04||0.106 ± 0.012||0.374 ± 0.016||0.229 ± 0.04||0.108 ± 0.012||0.389 ± 0.017||0.236 ± 0.03||0.111 ± 0.01||0.322 ± 0.109||0.211 ± 0.005||0.106 ± 0.013||0.308 ± 0.012||0.211 ± 0.047||0.100 ± 0.021|
|400||0.458 ± 0.014||0.278 ± 0.04||0.131 ± 0.03||0.471 ± 0.019||0.285 ± 0.04||0.133 ± 0.002||0.485 ± 0.024||0.294 ± 0.03||0.138 ± 0.001||0.441 ± 0.018||0.132 ± 0.002||0.287 ± 0.035||0.367 ± 0.015||0.263 ± 0.05||0.116 ± 0.003|
|500||0.545 ± 0.021||0.330 ± 0.04||0.157 ± 0.03||0.548 ± 0.027||0.340 ± 0.036||0.160 ± 0.002||0.561 ± 0.024||0.343 ± 0.034||0.166 ± 0.002||0.540 ± 0.027||0.330 ± 0.042||0.157 ± 0.03||0.459 ± 0.045||0.307 ± 0.065||0.142 ± 0.004|
Vitreous Flow Results of Cutters with Different Cut Rates and Aspiration Levels
|Aspiration (mm Hg)||800 CPM||1,000 CPM||1,500 CPM||2,000 CPM||2,500 CPM|
|100||0.0065 ± 0.002||0.0030 ± 0.001||0.0014 ± 0.001||0.0084 ± 0.002||0.0039 ± 0.001||0.0020 ± 0.001||0.0112 ± 0.002||0.0059 ± 0.001||0.0038 ± 0.003||0.013 ± 0.003||0.0070 ± 0.002||0.0044 ± 0.001||0.0135 ± 0.002||0.0074 ± 0.002||0.0044 ± 0.002|
|200||0.0183 ± 0.004||0.0075 ± 0.002||0.0043 ± 0.002||0.0190 ± 0.004||0.0104 ± 0.003||0.0051 ± 0.001||0.0251 ± 0.005||0.0118 ± 0.003||0.0067 ± 0.002||0.0292 ± 0.007||0.0123 ± 0.003||0.0074 ± 0.001||0.0270 ± 0.004||0.0127 ± 0.002||0.0079 ± 0.002|
|300||0.0365 ± 0.011||0.0209 ± 0.007||0.0075 ± 0.003||0.0426 ± 0.011||0.0200 ± 0.006||0.0087 ± 0.003||0.0493 ± 0.012||0.0233 ± 0.005||0.0103 ± 0.004||0.05221 ± 0.006||0.0221 ± 0.006||0.0111 ± 0.00||0.0485 ± 0.014||0.0225 ± 0.005||0.0111 ± 0.003|
|400||0.575 ± 0.015||0.0221 ± 0.008||0.0098 ± 0.004||0.057 ± 0.012||0.0268 ± 0.006||0.0127 ± 0.005||0.0783 ± 0.020||0.0342 ± 0.009||0.0139 ± 0.005||0.0803 ± 0.014||0.0317 ± 0.009||0.0177 ± 0.006||0.0715 ± 0.009||0.0311 ± 0.008||0.0173 ± 0.004|
|500||0.0921 ± 0.022||0.0368 ± 0.018||0.0179 ± 0.004||0.0878 ± 0.019||0.0431 ± 0.014||0.0182 ± 0.008||0.1074 ± 0.032||0.0400 ± 0.018||0.0202 ± 0.012||0.1095 ± 0.02||0.0513 ± 0.014||0.0251 ± 0.012||0.0959 ± 0.013||0.0472 ± 0.008||0.0240 ± 0.008|