J CATARACT REFRACT SURG - VOL 32, JULY 2006

Effect of incisional friction

and ophthalmic viscosurgical devices

on the heat generation of ultrasound

during cataract surgery

Michael Floyd, MD, Jeremy Valentine, MD, Jamie Coombs, MD, Randall J. Olson, MD

PURPOSE: To determine the thermal features of the Legacy (Alcon) and Sovereign (Advanced MedicalOptics) phacoemulsification machines in a cadaver eye and with 7 ophthalmic viscosurgical devices(OVDs).

SETTING: In situ and in vitro study.

METHODS: Temperature without occlusion was recorded at the sleeve placed in the wound of a ca-daver eye, and temperature over baseline was determined after 60 seconds. The result was then com-pared with the results in a previous study that used balanced salt solution (BSS) in artificial chambers.In the second portion of the experiment, with irrigation and aspiration lines occluded, temperaturewas recorded at the sleeve placed in an artificial chamber filled with sodium hyaluronate 2.3%(Healon5), sodium hyaluronate 1.4% (Healon GV), sodium hyaluronate 1.0% (Healon), sodium hyalur-onate 1.6% (Amvisc Plus), sodium hyaluronate 1.0% (Provisc), sodium hyaluronate 3.0%–chondroitinsulfate 4.0% (Viscoat), or hyaluronate 3.0% (Vitrax). Temperature over baseline was also determinedafter 60 seconds. These results were compared with each set of OVD data and with the results inthe prior BSS study.

RESULTS: In the eye-bank model, the Legacy machine had a 62% temperature increase from incisionalfriction and the Sovereign machine had a decrease of 8.6% over results in an artificial anterior chamber.The OVD temperature increases were greater for the Sovereign (P<.001) and followed the same gen-eral trend for the Legacy. The least temperature increase was with Amvisc Plus, Healon, and Healon GV;the intermediate increases were with Provisc and Vitrax; and the greatest increases were with Viscoatand Healon5. The OVD findings did not correlate with viscosity or pseudoplasticity.

CONCLUSIONS: Incisional friction alone increased heat generation in the Legacy, a stroke-lengthdriven instrument, more than in the Sovereign, a power-driven instrument. Ophthalmic viscosurgicaldevices are not only a concern due to outflow occlusion but can also add up to 6 times the heat incomparison with BSS. The need to aspirate the OVD before using ultrasound is thus verified.

J Cataract Refract Surg 2006; 32:1222–1226 Q 2006 ASCRS and ESCRS

Phacoemulsification-induced wound burn has been shownto be related to the machine used, surgical technique,

power modulation, aspiration line occlusion, and workload

(incision friction and nuclear hardness).1,2 While ophthal-

mic viscosurgical devices (OVDs) have been implicated as

an additional wound burn risk,3 little has been written

about the differences between the OVDs.

This article explores incisional friction and the effect of

different OVDs as they relate to heat buildup duringphacoemulsification.

Q 2006 ASCRS and ESCRS

Published by Elsevier Inc.

1222

MATERIALS AND METHODS

The Legacy (Alcon) and Sovereign (Advanced Medical Op-tics) phacoemulsification machines were used in cadaver eyeexperiments. A 72-hour-old fresh, unfrozen human eye was ob-tained from a 72-year-old donor. Straight 0.9 mm, 30-degree bev-eled tips covered with the manufacturer’s standard sleeve wereused. After priming and tuning, the vacuum limit was set at100 mm Hg, aspiration at 20 cc/min, and 60% continuous power.Balanced salt solution (BSS) was used and the bottle readjusted toa fluid height of 70 cm above the handpiece prior to each run. Amicrothermistor sensor (Physitemp Instruments Inc.), accurate

0886-3350/06/$-see front matterdoi:10.1016/j.jcrs.2006.01.107

LABORATORY SCIENCE: EFFECT OF INCISIONAL FRICTION AND OVDs ON US HEAT GENERATION

to G 0.1�C, was glued with corneal glue (Isodent) to each phaco-emulsification sleeve so the sensor was 1.0 mm from the tip. Theother end was plugged into an isothermal block.

The sensor, sleeve, and tip setup were inserted into a2.85 mm square incision beginning at the limbus created by a2.85 mm steel keratome. Before each run, it was ascertainedthat the sensor remained in the wound. The handpiece was setat a position parallel to the table and slightly raised by usingtowels to maintain the relationship with the height of the eyeand to prevent tip torque in the incision. The foot pedal was de-pressed to position 2 (irrigation and aspiration) until the temper-ature equilibrated to 23.6�C G 0.4�C. At the start of the timer, thepedal was fully depressed to position 3, allowing irrigation, aspi-ration, and ultrasound. Temperature readings were recorded every10 seconds for 60 seconds. After each run, irrigation and aspira-tion were continued by maintaining the machine pedal in position2 and the incision was cooled using ice water until the tempera-ture returned to baseline. Five runs were done first with the Sov-ereign and then with the Legacy phacoemulsification machine inthe same incision and precisely in the same manner.

The OVDs tested were sodium hyaluronate 2.3% (Healon5),sodium hyaluronate 1.4% (Healon GV), sodium hyaluronate 1%(Healon), hyaluronate 3.0% (Vitrax), sodium hyaluronate 1.6%(Amvisc Plus), sodium hyaluronate 1.0% (Provisc), and sodiumhyaluronate 3.0%–chondroitin sulfate 4.0% (Viscoat).

The Sovereign and Legacy machines were calibrated and as-sembled as previously described. Before each run, the aspirationline was occluded 4 cm from the handpiece by a mosquito clampto prevent aspiration of the OVD.

Due to the small amount of OVD supplied in the standardvial, an Alcon test chamber was cut to 34 mm length and thesame chamber was used throughout. The chamber held 1.5 mLand contained 1.0 mL when the tip and sleeve were inserted. AnOVD was used to fill the chamber for each run.

When the timer was started, the pedal was depressed to po-sition 3 and temperature results were registered every 10 secondsfor 60 seconds. Each OVD was run at 20% power for 5 runs andthen at 40% power for another 5 runs. To prevent handpiece de-struction, once the temperature exceeded 60�C, the run was con-cluded. If there were fewer than 60 seconds in any run,comparisons were always with the same time (ie, 40 seconds to40 seconds, 20 seconds to 20 seconds, etc.).

Accepted for publication January 9, 2006.

From the Department of Ophthalmology and Visual Sciences,University of Utah Health Sciences Center, Salt Lake City, Utah,USA.

Supported in part by a grant from Research to Prevent Blindness,Inc., New York, New York, to the Department of Ophthalmologyand Visual Sciences, University of Utah.

Dr. Olson is a consultant to Advanced Medical Optics. No authorhas a financial or proprietary interest in any material or methodmentioned.

Sara E. Simonsen, RN, MSPH, assisted with the statistical analysisof the data.

Reprint requests to Randall J. Olson, MD, Department of Ophthal-mology and Visual Sciences, John A. Moran Eye Center, 50 NorthMedical Drive, Salt Lake City, Utah 84132, USA. E-mail: randall.olson@hsc.utah.edu.

J CATARACT REFRACT SUR

The beginning temperature was subtracted from the finaltemperature for each run, and the 5 results were averaged andcompared.

The data were also compared with the in vitro data obtainedusing exactly the same parameters with BSS.2 For analysis, 2-tailedt tests were used and significance was set at P Z.0008 with a Bon-ferroni correction (61 comparisons were made).

RESULTS

The increases in temperature in all of our studies are

listed in Table 1. These data were then compared to our

in vitro results performed with exactly the same parameters

(Table 2). The table represents ratios of the mean increase

in temperature of the experimental runs to the results using

BSS in vitro. It is apparent that in the eye-bank model, Leg-

acy had a 62% increase in heat generation from incisional

friction, while Sovereign had a decrease of 8.6% in theeye-bank model.

The OVD results showed great variability, with greater

temperature elevation in general with the Sovereign than

with the Legacy (P!.0001). The OVDs generally separated

into 3 categories. The least temperature increases were with

Amvisc Plus, Healon, and Healon GV; the intermediate in-

creases were with Provisc and Vitrax; and the greatest

increases were with Viscoat and Healon5. This rangedfrom 1.51 to 6.3 times greater than the results obtained

using BSS alone.

Giving Amvisc Plus results a rating of 1, all the OVDs at

20% and 40% power runs with both Legacy and Sovereign

are listed in Table 3. The same OVD grouping trend is

noted, which does not correlate with viscosity in that the

most viscous (Healon5) and the least viscous (Viscoat) pro-

duced the most heat, nor does it correlate with pseudoplas-ticity in that Vitrax was significantly different from Viscoat

even though they have similar pseudoplasticity.

DISCUSSION

Moderate to severe wound burns occur in about 1 in

1000 phacoemulsification cases.4 Although many can be

appropriately treated, the results can be serious, with the

potential of induced cylinder and loss of best corrected

vision.5–7 Ultrasound-related heat generation is, therefore,an important safety issue, and this additional work gives us

further clues about burn creation and prevention.

Our eye-bank work confirms our theoretical analysis

of in vitro temperature production of the Sovereign and

Legacy phacoemulsification units.2 Due to the stroke-

length-determined foot pedal of the Legacy, with increased

friction, power is increased to maintain the stroke length

and therefore the Legacy had a 62% increase of energy pro-duction due to incisional friction. The results were very

consistent with the same parameters used for both instru-

ments. Lifting, pushing, or torquing the probe in the wound

G - VOL 32, JULY 2006 1223

LABORATORY SCIENCE: EFFECT OF INCISIONAL FRICTION AND OVDs ON US HEAT GENERATION

Table 1. The mean increase in temperature for all OVDs based on 5 runs at each power. The beginning temperature was subtracted from the final and the

results then averaged. The OVD data are from an occluded system, and the cadaver eye used free-flowing BSS (some runs stopped prior to 60 seconds as

specified when the temperature exceeded 60�C).

OVD Power

LegacyMean TemperatureIncrease (�C G SD)

SovereignMean TemperatureIncrease (�C G SD)

Eye-bank eye 60% 10.2 G 0.16 14.6 G 0.26Viscoat 20% 7.46 G 0.594 42.4 G 3.96*

40% 29.9 G 3.09 49.9 G 2.07*Healon5 20% 11.6 G 2.30 33.9 G 2.25z

40% 27.5 G 1.81 41.2 G 2.73*Provisc 20% 6.7 G 0.418 38.7 G 3.03{

40% 19.0 G 1.17 35.5 G 1.81†

Vitrax 20% 7.1 G 1.68 33.1 G 5.4140% 19.0 G 1.56 40.0 G 1.56z

Healon GV 20% 6.16 G 1.35 26.0 G 1.5940% 17.8 G 1.90 25.6 G 1.83z

Healon 20% 5.96 G 0.167 22.3 G 0.90940% 13.9 G 1.29 38.3 G 1.22

Amvisc Plus 20% 5.92 G 0.517 19.5 G 3.2940% 16.5 G 0.626 42.3 G 2.22

OVD Z ophthalmic viscosurgical device

*20-second data†30-second dataz40-second data{50-second data

(in our experiments the probe was not held) is likely to in-

crease this temperature result substantially.8 The Sovereignresults were also consistent but actually were less than what

was produced in vitro.

Having less heat production with the Sovereign, know-

ing that there is increased friction, may seem confusing;

however, the explanation is very simple and relates to inci-

sion leakage around the probe and out of the wound, which

was always seen in the eye-bank eyes but never seen in our

in vitro work. This leakage around the external aspect ofthe sleeve cooled our thermister sitting on the sleeve.

This not only explains the Sovereign results but also sug-

gests that actual Legacy heat generation is greater because

the incision leakage would also result in increased cooling

of the Legacy tip versus our in vitro results. We can use our

in vitro results2 from the weight experiments to estimate

the actual Legacy heat increase in the eye-bank eye as about

120%, which is equivalent to hanging a weight of 204 gfrom the tip. This does confirm the clinical relevance of

the weight that was used in our in vitro experiments (100

and 200 g).

The risk for wound burn with an anterior chamber full

of OVD has generally been recognized as caused by loss of

flow. All our comparisons were with outflow occluded, and,

therefore, it is clear that the OVD-induced temperature

increase is much more than a problem of loss of fluidflow. Furthermore, due to the fact that OVD-induced

J CATARACT REFRACT1224

Table 2. Comparison of results of Table 1 to our in vitro results2 per-

formed with BSS using the same parameters. (Except where specified,

comparisons are with 60-second data and P values less than 0.0001

when comparing Legacy and Sovereign).

Legacy SovereignOVD Power Ratio Ratio

Eye-bank eye 60% 1.62 0.91Viscoat 20% 1.90 7.14{

40% 3.53 6.30{

Healon5 20% 2.97 3.86†††

40% 3.25 5.20{

Provisc 20% 1.71 4.00††††

40% 2.25 3.87x

Vitrax 20% 2.31z 2.8840% 2.25 3.72†††

Healon GV 20% 1.57* 2.2640% 2.11 2.38†††

Healon 20% 1.52 1.9840% 1.64 3.56

Amvisc Plus 20% 1.51 1.70†

40% 1.95 2.77

OVD Z ophthalmic viscosurgical device

*P Z .008†P Z .002zP Z .004{20-second comparisonx30-second comparison†††40-second comparison††††50-second comparison

SURG - VOL 32, JULY 2006

LABORATORY SCIENCE: EFFECT OF INCISIONAL FRICTION AND OVDs ON US HEAT GENERATION

temperature increase ranged from 1.5 to 6.3 times the BSS

result, we need to not only explain the increase associated

with OVDs but also the variability between the different

OVDs.This OVD-induced temperature increase is not work-

load related because the Sovereign results were worse

than the Legacy results. If it were workload related, due

to the differences in how power is delivered, we would ex-

pect the reverse.1 This must be cavitational energy and sec-

ondary reactions (ie, from free radicals9) trapped by the

OVD near the tip with minimal heat dissipation and a prob-

able heat gradient tapering off quite dramatically as youmove away from the tip. Unfortunately, the wound and

the OVD near the tip are in juxtaposition, which would

accentuate any wound burn problem.

The Sovereign generated more temperature rise, and

this difference needs to be explained. We feel, again, that

the explanation is relatively straightforward and comes

from our observation of how cavitation bubbles are rapidly

generated and trapped in the OVD around the phaco tip, sowithin a matter of seconds, the tip is largely working in an

air environment. Our earlier work,1 looking at the power

input of Legacy and Sovereign, shows that when working

in air, being a power-controlled instrument, the Sovereign

actually races, especially on a low power setting, almost

like a car engine that is not tied to the wheels through

a clutch. Such a surge in power in air would increase cavita-

tional energy and thereby increase free radical formation.This is further confirmed by the fact that the OVD temper-

ature enhancing effect is clearly not related to viscosity;

Viscoat caused the greatest temperature increase but is

the least viscous. If workload related, we would expect

a correlation with viscosity, which we did not find.

While the Sovereign results indicate an increased

wound burn risk with an anterior chamber full of OVD,

we reported4 on overall wound burn risk and Sovereignat 0.056% was significantly less likely to create a burn

Table 3. Ratio of results of Table 2 where Amvisc plus/BSS ratio is given

a value of 1.0 (from results of 60-second data except where specified).

Legacy Sovereign

OVD 20% Power 40% Power 20% Power 40% Power

Healon 1.01 0.84 1.16 1.29Healon GV 1.04 1.08 1.33 1.06z

Vitrax 1.53 1.15 1.69 1.65z

Provisc 1.13 1.15 2.38{ 1.84†

Healon5 1.97 1.67 2.51z 3.13*Viscoat 1.26 1.81 4.73* 3.80*

*Comparison with 20-second Amvisc plus/BSS ratio†Comparison with 30-second Amvisc plus/BSS ratiozComparison with 40-second Amvisc plus/BSS ratio{Comparison with 50-second Amvisc plus/BSS ratio

J CATARACT REFRACT SU

than Legacy with a 0.116% wound burn rate (P Z .014).

This indicates that there are many features involved in

wound burn risk, and the OVD only one of them.

We often noticed a smoky layer in the OVD near the

tip. Clinical cases of wound burn with an anterior chamber

full of OVD have presented, with a change in the appear-ance of the OVD around the tip heralding the wound

burn. All of this, we feel, is evidence that there is probably

an oxidative exothermic reaction occurring that is quite

variable and OVD to OVD. This variability is also seen by

the fact that while Legacy is generally more forgiving of

this effect than Sovereign, such is not the case for Amvisc

Plus and Healon, which were the 2 OVDs not significantly

different with either machine.The OVD sheer rate is markedly increased due to the

mechanical and cavitational forces of ultrasound near the

tip. The actual viscosity, due to variable pseudoplasticity,

may correlate better with our results than 0 sheer rate vis-

cosity. Unfortunately, we have no way to measure what is

happening from a viscosity standpoint near the tip and try-

ing to do so would be very difficult. Still, we do not think

any viscosity change with the increased sheer rate is likelyto explain our findings because Healon GV, which is very

viscous in comparison with Healon and Amvisc Plus, is

generally statistically similar to these 2. Furthermore, Vi-

trax and Viscoat, which are markedly less pseudoplastic

than the other OVDs, were also different from each other.

There must be other elements we do not understand that

promote the exothermic reaction that we propose. Possibly

the chondroitin sulfate in Viscoat is a factor in thisreaction.

Provisc was a surprise in that it behaved clinically and

rheologically like Healon and Amvisc Plus. It, however,

produced a greater temperature elevation than either of

the other 2 and also greater than Healon GV, which is

much more viscous than Provisc. Therefore, our proposed

variable exothermic reaction gives us no obvious character-

istic we can point to that will determine an OVD heat re-sponse to ultrasound energy. Further differentiating how

OVDs respond, other than measuring the response, will

have to be determined through future experimentation.

Fortunately, the clinical lesson from all of this is clear

and relatively straightforward. At the beginning of all pha-

coemulsification cases, an appropriate period of irrigation

and aspiration without ultrasound should be carried out

to produce a sizable fluid cavity inside the OVD-filled ante-rior chamber. Dispersive OVDs take longer, and this re-

moval is enhanced with dispersive OVDs by moving the

tip from side to side. Anecdotal reports of wound burn

within seconds of starting the case while using ultrasound

in an OVD-filled anterior chamber are due both to irrigation

occlusion and the heat generating potential of the OVD. We

are not aware of any such study; however, it would be

RG - VOL 32, JULY 2006 1225

LABORATORY SCIENCE: EFFECT OF INCISIONAL FRICTION AND OVDs ON US HEAT GENERATION

interesting to try to determine whether such early-onset

wound burn is related to the use of Viscoat or Healon5.

Incisional friction alone increases heat generation

more in a stroke-length driven instrument such as Legacy

than a power-driven instrument such as Sovereign. We

also conclude that OVDs are not only a concern due tooutflow occlusion but also because they variably can add

up to 6 times the heat creation of ultrasound in comparison

to BSS. The lesson is that surgeons should aspirate OVDs to

produce a fluid cavity prior to using ultrasound.

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