Cause of Device-Related Incident
Support system failures; User errors

Clinical Specialty or Hospital Department
Clinical/Biomedical Engineering; Ophthalmology; OR / Surgery

Device Factors
*Not stated

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death
Blindness

Support System Failures
Failure to train and / or credential

Tampering and/or Sabotage
*Not stated

User Errors
Failure to perform pre-use inspection; Failure to read label; Incorrect clinical use; Incorrect control settings

UMDNS
Cataract Extraction Units, Phacoemulsification [17-596]; Aqueous/Vitreous Humor Replacement Media [16-844]

Scleral and Corneal Burns during Phacoemulsification



Hazard [Health Devices Nov 1996;25(11):426-31]

Problem Summary

In December 1988 (Health Devices 17[12]), 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 report.

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.

Background

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 corneal endothelium.

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) incision.

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 foot pedal:

  • 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.

Discussion

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 height.

Recommendations

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 fluid leakage.
  • 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.

Note

  1. Leaming DV. Practice styles and preferences of ASCRS members--1994 survey. J Cataract Refract Surg 1995 Jul;21(4):378-85.

UMDNS Terms

  • Cataract Extraction Units, Phacoemulsification [17-596]
  • 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

Blindness


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