Hazard [Health Devices Nov 1996;25(11):426-31]
In December 1988 (Health Devices 17), ECRI published a Hazard
Report addressing the occurrence of scleral and corneal burns during
phacoemulsification—a delicate and complex procedure performed to remove cataracts.
Our investigations of such incidents revealed that the injuries were caused by overheating
of the phacoemulsification probe tip. However, malfunctioning equipment was not to blame
in any of the cases reported to us; instead, the complications experienced could be traced
to issues related to surgical technique—issues that we addressed in our published
Following publication of our article, the frequency with which we received
reports of complications during phacoemulsification declined. However, since early 1995,
we have again noted an increase in the number of hospitals inquiring about and reporting
incidents of corneal burns from phacoemulsification. Injuries in the cases reported to us
range from minor dimpling of the tissue to severe damage. Therefore, we revisit this topic
below, presenting a new discussion of phacoemulsification technique to illustrate why such
injuries occur and how they can be avoided.
We suspect that the current increase in reports can be attributed to the
marked increase in the number of cataract cases performed by phacoemulsification (up from
28% of all cases in 1988 to over 86% in 1994),(1) the tendency to use new techniques that
enter the eye in clear cornea and/or that core the lens nucleus with the tip of the
phacoemulsification probe, and new incident reporting requirements. Although some of the
inquiries we receive question the safety of certain phacoemulsification units and
handpieces, our experience is that any phacoemulsification unit
can cause thermal
lesions. We do not have information indicating that one device is safer, or, conversely,
more susceptible to causing damage, than the others.
The Phacoemulsification Procedure
Phacoemulsification is performed to remove a
cataractous lens from the patient's eye. The traditional phacoemulsification
procedure is performed by the surgeon making a 2.8 to 3.5 mm limbal incision
(where the cornea meets the sclera) to gain access to the eye's anterior chamber (see figure). A viscoelastic
material is then infused into the anterior chamber to deepen the chamber and protect the
After removing the anterior lens capsule, the surgeon inserts a
phacoemulsification probe, which consists of a hollow, cylindrical tip surrounded by an
irrigation sleeve. (When activated, the probe tip oscillates rapidly, creating ultrasonic
waves that disrupt the lens.) The surgeon emulsifies the cataractous lens using shaving or
scooping motions with the probe tip. The lens fragments are then aspirated from the eye
through the hollow tip.
Usually, after removing the cataract, the surgeon enlarges the incision
(anywhere from 3.0 to 8.0 mm) and implants an intraocular lens (IOL). However, foldable
IOLs are now available that can be inserted through the smaller (i.e., 2.8 to 3.5 mm)
As the popularity of phacoemulsification has increased over the years, new
more aggressive techniques have been developed. For example, many surgeons now enter the
anterior chamber through a "clear cornea" incision (i.e., directly through the
cornea). When this method is used, minor tissue deformations or discolorations become much
more noticeable and problematic than when a limbal incision is used. In other techniques,
the surgeon removes the lens from the capsular bag by gripping (and sometimes skewering or
coring) the lens with the tip of the phacoemulsification probe. Then, with the lens in the
iris plane or anterior chamber, clear of any delicate capsular structure, it is emulsified
in a more aggressive manner than that used in the traditional method.
Controlling the Phacoemulsification Unit
The ophthalmic surgical systems that can be used to perform
phacoemulsification integrate into a single handpiece the irrigation, aspiration, and
ultrasound capabilities needed to break up and remove cataractous lenses from the eyes.
The surgeon activates these capabilities in succession, typically by depressing a single
- First, irrigation is provided (by gravity feed from
a bottle) to flush the surgical site, maintain pressure in the anterior
chamber of the eye (to keep it from collapsing when aspiration is applied),
and cool the probe tip (when it starts to oscillate).
- Next, aspiration is activated to draw fluid and lens
fragments toward, and then through, the probe tip and into a collection
container. The aspiration systems employed by different ophthalmic surgical
systems can differ significantly from one system to the next.
- Finally, ultrasound is initiated to emulsify the lens.
Maintaining control of the device in the eye requires that the surgeon be
able to achieve a balance between irrigation and the two aspiration parameters: flow and
vacuum. Flow describes the rate at which fluid and lens fragments travel toward (and
through) the probe tip. Vacuum describes the suction force that holds material at the
probe tip. During surgery, aspiration flow draws the lens and lens fragments toward the
probe tip; the vacuum then holds the lens or fragments at the tip, while the ultrasonic
waves push them away. The effects of both cavitation and mechanical impact cause the lens
material to break apart. When small enough, the fragments are aspirated through the probe
tip at a rate determined by the aspiration flow. Too high a flow rate will cause fragments
to move too fast, creating turbulence in the eye. Too high a vacuum can cause a flow surge
after an occluding lens piece is quickly emulsified.
Phacoemulsification units allow surgeons to control the aspiration
parameters using either a fixed or linear mode of operation. In fixed modes, the unit
provides aspiration at a set level (as specified on the control panel) when the surgeon
depresses the foot pedal. In linear modes, the surgeon's increasing depth of foot pedal
depression controls one of the aspiration parameters. Operating the unit in a fixed mode
is relatively straightforward; however, achieving the desired clinical performance also
requires an understanding of the unit's linear mode of operation.
The number and type of aspiration controls, as well as the implementation
of the linear modes of operation, depend on the type of pumping mechanism the unit uses to
generate flow and vacuum:
- Peristaltic pump—Systems with peristaltic pumps have two aspiration controls:
aspiration flow and vacuum limit. The aspiration flow control determines the
speed at which the pump turns; the faster the pump turns, the greater the
resulting flow rate. By comparison, the vacuum limit is simply a safety
setting that stops the pump when the vacuum reaches the set limit.
Peristaltic systems can have either linear flow or linear vacuum (vacuum
limit) modes. (The availability of these modes for each machine function
[e.g., phacoemulsification, irrigation/aspiration, vitrectomy] depends on
the device manufacturer and model.) In the linear flow mode, the flow rate
is controlled by the foot pedal, and the vacuum limit is constant. This
allows the surgeon to adjust the speed with which fluid and objects move
toward the tip. In the linear vacuum mode (sometimes called the variable
vacuum mode), the pump speed remains constant, but the vacuum level at which
the pump shuts off varies depending on the depth to which the foot pedal is
depressed (i.e., as the pedal is depressed further, the vacuum limit allowed
before pump shutoff increases).
- Venturi or diaphragm pump—On systems
using either a venturi or diaphragm pump, the only aspiration control is
vacuum. This vacuum setting is the actual negative pressure applied to the
collection container and aspiration tubing. For a given vacuum setting, the
flow rate is determined by the dimensions of the tubing, fluid viscosity,
and the degree of occlusion (i.e., typically, the flow rate will be
proportional to the applied vacuum). These systems have only a linear (or
variable) vacuum mode. In this mode, the applied vacuum is controlled by the
foot pedal. With this type of pumping mechanism, adjusting the vacuum
directly affects the flow rate.
It is important to understand that the vacuum setting on a peristaltic
system does not control the same aspiration characteristic as the vacuum setting on a
venturi or diaphragm system.
The Problem: Overheating of the Probe Tip
In the cases reported to us, thermal injuries at the location where the
probe entered the eye were caused by overheating of the probe tip. During extended use of
the probe, the rapid oscillation of the probe tip and the friction generated are known to
cause excessive heating. However, our tests with porcine eyes and egg albumin have shown
that overheating of the tip can also occur very rapidly (within 1 to 3 sec) and can cause
injury even if present for only a short time. Furthermore, our tests have shown that
excessive heating does not occur when both irrigation and aspiration flow are
present. However, operating the ultrasound generator without irrigation and aspiration
flow will cause burns.
The Cause: Insufficient Irrigation and Aspiration Flow
Our investigations of reports of thermal eye injury revealed that neither
the phacoemulsification unit nor its handpiece had malfunctioned in any of the cases.
Instead, the burns were caused by a lack of sufficient irrigation and aspiration
flow—both of which help cool the probe tip—that could have been avoided if
proper surgical technique and procedures were observed.
Insufficient irrigation or aspiration can have
many causes. For example, irrigation can be blocked or inhibited if the
irrigation fluid bottle is empty, if the bottle is positioned too low for
adequate flow, or if the irrigation tubing or sleeve is crimped or compressed.
Similarly, aspiration flow can be inhibited or stopped if the probe tip becomes
occluded (e.g., by viscoelastic substances, by the lens nucleus),
if the vacuum limit is set too low, if the aspiration tubing becomes crimped, or if the
cassette/tubing set is loaded improperly. Factors that contribute to the use of these
devices under such conditions are discussed below.
Insufficient Knowledge of the Equipment Used
Because maintaining control of a phacoemulsification unit requires
achieving a delicate balance between irrigation and aspiration flow and vacuum, the use of
unfamiliar equipment can lead to undesirable results. For example, surgeons often learn a
procedure on one machine, memorizing that system's settings; however, if they try to use
those settings on another supplier's system—one that employs a different aspiration
system—the likelihood of surgical complications will increase.
Surgeons also must understand how new phacoemulsification units differ
from their predecessors. For example, at least three new systems use a peristaltic pump
that automatically adjusts the pump's speed in relation to the achieved vacuum and the set
vacuum limit. As the actual vacuum exceeds approximately 50% to 80% (varies by
manufacturer) of the set limit, the system automatically starts to slow the pump. This
reduces the vacuum rise time after the tissue is captured. This feature has been
implemented to avoid undesirable vacuum overshoot. To achieve the flow rate
characteristics they are accustomed to, surgeons can set the vacuum limit higher than on
previous systems. This is because the flow rate on this unit would be significantly lower
than expected at vacuum limit settings that they have used on other systems.
Lack of Experience Performing the Procedure or Using the Equipment
Surgeons performing the phacoemulsification procedure gain proficiency
with the technique over time. Studies show that complications (e.g., thermal burns) occur
at a significantly higher rate during a surgeon's first 50 to 100 cases. However, because
phacoemulsification is such a delicate and complex procedure, thermal burns can—and
occasionally do—occur even when the operating surgeon has a great deal of experience
performing the procedure.
Similarly, when surgeons start using a new surgical technique or a new
phacoemulsification unit, they are again operating at a level of reduced proficiency. As
such, the incidence of complications they experience could increase. Therefore, it is
important that they take the time to master both the surgical technique and control of the
new unit when making such changes.
However, in many of the incidents we investigated, the surgeons had
performed the procedure hundreds of times without incident. Also, many report that the
thermal injuries happened unexpectedly, with no obvious change in procedure between cases.
The Use of More Aggressive Techniques
With many of the new, more aggressive techniques, the potential for fully
occluding the handpiece's tip while in the ultrasound mode is increased. This is
especially true of techniques in which aspiration flow and vacuum levels are used to grip,
skewer, or core the lens with the tip of the handpiece.
The Use of Smaller Incisions and Smaller-diameter Probe Tips
A trend in cataract surgery is to use lower flows and smaller incisions.
Following this trend, some suppliers have marketed and others are planning to market a
smaller-diameter phacoemulsification tip. A smaller-diameter tip allows for smaller
incisions; however, clinicians should be aware that the smaller diameter will restrict
aspiration flow more and be easier to occlude than standard tips. We believe that surgeons
should carefully compare the benefits of these smaller tips to the potential drawbacks.
Avoiding the Problems
To avoid the problems discussed in this report, surgeons and nurses must
understand how the fluidic systems (i.e., irrigation and aspiration) operate on the
phacoemulsification units they use. Specifically, surgeons need to understand how fluid
flow and vacuum affect the clinical performance they are trying to achieve. Also, they
need to be sure to allow appropriate safety margins.
Many irrigation problems can be minimized by
using incisions long enough to prevent irrigation sleeve crimping and to allow
wound leakage. (Crimping
can also be avoided by using a rigid irrigation sleeve offered by some suppliers.)
Aspiration problems can be minimized by not coring the lens with the tip, or, if such a
technique is to be used, by not activating ultrasound while the tip is embedded in the
lens. Many systems have warning signals (e.g., audio signal, vibrating foot pedal,
automatic ultrasound mode change from continuous to pulse) that indicate a full occlusion;
however, the surgeon must recognize the signal and manually stop the ultrasound mode.
In addition, performing all recommended pre-use tests of the irrigation
and aspiration systems will help ensure that the equipment is set up properly. Such checks
can help prevent problems with tubing placement, cassette loading, and irrigation bottle
The following recommendations should reduce the risk of thermal injury:
- Make sure all operating room personnel are
completely familiar with the operation of the phacoemulsification unit to be
used. The surgeon should specifically become familiar with the unit's
aspiration characteristics and should follow the manufacturer's
recommendations, which might include different parameter settings than those
used with other models.
- Perform all pre-use irrigation and aspiration tests
recommended by the handpiece manufacturer. Contact the manufacturer if you
are not sure which tests are required to ensure adequate flows.
- Verify that the incision is large enough (e.g., at
least 3.2 mm when using a standard phacoemulsification probe tip) throughout
its entire depth to avoid pinching the irrigation sleeve and to allow some
- On machines with peristaltic pumps, use audible
vacuum indicators and alarms to call attention to blockages of aspiration.
(Machines with venturi or diaphragm pumps supply a constant vacuum level,
regardless of occlusions.)
- When using a viscoelastic material, aspirate the
area around the anterior lens capsule before phacoemulsification to ensure
that proper irrigation and aspiration are established while leaving a
sufficient layer of viscoelastic material on the endothelium.
- Before activating the ultrasonic generator, have the
circulating nurse watch the drip chamber to verify that aspiration and
irrigation are well established.
- Avoid overtorquing the wound. Excessive probe
manipulations can narrow the incision and increase friction.
- Do not use excessive ultrasound power. Apply power only while shaving
the nucleus, not while the tip is imbedded in the nucleus or while moving
the tip away from the nucleus. Reducing the amount of phacoemulsification
power where possible will also help limit heat generation. To reduce the
duty cycle of the tip while maintaining lens adherence to the tip, the use
of ultrasound in a pulse mode may also be recommended when performing
aggressive techniques that are likely to occlude the tip.
- Leaming DV. Practice styles and preferences of ASCRS members--1994
survey. J Cataract Refract Surg 1995 Jul;21(4):378-85.
- Cataract Extraction Units, Phacoemulsification
- Aqueous/Vitreous Humor Replacement Media [16-844]
Cause of Device-Related Incident
User errors: Failure to perform pre-use
inspection; Failure to read label; Incorrect clinical use; Incorrect control settings
Support system failure: Failure to train and/or credential
Mechanism of Injury or Death