Pretutorial
Tutorial
Introduction
Causes and Preventative Measures
Consequences
Summary
References
Scott E. Burk, MD, PhD · Robert H. Osher, MD
Incisional burns may range in severity from mild collagen shrinkage to severe whitening with gaping of
the wound (Slide 1). Thermal injuries appear to be more common with clear corneal
incisions, either because mild thermal damage is more easily noticed or because more distortion of the wound
occurs during phacoemulsification. The incidence of corneal burns appears to be rising, due in part to the
adoption of smaller clear corneal incisions; the introduction of more retentive viscoelastic agents;
surgeons' tendency toward occlusion-based techniques; and surgeons' expanded ability to emulsify very hard
lenses.
Causes and Preventative Measures
Ultimately, the cause of a corneal burn, or incisional burn, is heat transfer from the phacoemulsification needle to the cornea; the degree of corneal damage is directly related to the total heat absorbed. The amount of heat absorbed is a function of the temperature of the needle, the efficiency of energy transfer, and the time of contact.
When translating these variables to clinical practice, several risk factors for inducing corneal burns and modifications to help prevent corneal burns can be identified. The first variable to consider is the use of phaco power and time. Higher phaco power and a longer duration of phaco power act to increase the needle temperature. Therefore, the judicious use of phaco energy at lower power settings decreases the risk of a corneal burn, whereas prolonged use of high phaco power (i.e., when removing a brunescent cataract) increases the risk of a corneal burn.
Phaco needle cooling is achieved by the flow of
irrigation fluid both inside the barrel of the phaco
needle (aspiration flow rate) and outside the needle
(irrigation). In addition to cooling the needle, the
aspiration flow is intimately involved in the
phacoemulsification process. The aspiration flow rate
determines the rapidity with which lens fragments (and
other intraocular tissues) are drawn toward the phaco
tip. When a fragment of nucleus is engaged and the tip
is occluded, the aspiration flow rate decreases to
zero. The next step involves the removal of this
fragment. Modern phaco machines have two synergistic
mechanisms to remove lens material: phaco power and
vacuum. Fortunately, there are many newer technologies
aimed at maintaining some aspiration flow such as the
Aspiration Bypass System (Alcon, Ft. Worth, Texas) (Slide 2). In addition, the newer
phaco machines are capable of using higher vacuum power
with a minimum of phaco energy to remove nuclear
fragments by limiting the phaco "on" time, such as with
pulse and burst modes.
Irrigation flow is another part of the cooling equation for the phaco needle. Irrigation fluid flows between the phaco needle and the silicone sleeve. Irrigation, in addition to cooling the phaco needle, maintains the anterior chamber pressure. The anterior chamber pressure equilibrates with the water pressure produced by the height of the irrigation bottle above the eye. Once at equilibrium, additional irrigation fluid will not enter the eye unless fluid or other materials, such as viscoelastic or the lens material, leave the eye.
It is important to watch for the production of "lens
milk," which suggests poor irrigation and aspiration
and, therefore, poor cooling (Slide 3). As the lens is emulsified,
generally, the lens particles are quickly aspirated or
at least vigorously stirred by irrigation, preventing
the development of a cloudy or milky appearance.
However, if aspiration and irrigation are restricted,
the emulsified lens particles will collect, producing a
cloudy, milk-like appearance. This lens milk is an
important warning sign indicating that fluid exchange
is restricted and that a corneal burn may ensue if
phacoemulsification is not stopped and the problem is
not corrected.
Fluid exchange can also be restricted by some of the newer highly retentive viscoelastics that can limit inflow of irrigation fluid, especially during the early sculpting phase of cataract removal. The danger is not only an incisional burn; if the viscoelastic is heated, severe endothelial damage may result. When using a highly retentive viscoelastic, it is imperative to intentionally sculpt a pocket for fluid exchange. A surgeon can ensure fluid exchange by checking that the irrigation bottle is dripping.
The trend toward smaller, watertight incisions that restrict fluid from escaping has resulted in increased anterior chamber stability. However, small, watertight incisions increase the risk of developing a corneal burn in two ways. First, if fluid does not escape at the wound, irrigation can only occur during aspiration. Thus, with a small, watertight incision, both aspiration and irrigation are restricted when the phaco tip is occluded.
A watertight incision increases the risk of a corneal burn in another way. The irrigation sleeve provides a cooling "jacket" of fluid around the phaco tip. When the incision is tight, this sleeve is compressed against the phaco needle. With the sleeve compressed against the phaco needle, there is no cooling fluid separating the needle from the corneal tunnel. Heat is transferred through the sleeve directly to the cornea. To solve this problem, a surgeon may create an incision slightly larger than the phaco tip, or the tip may be modified to reduce heat transfer and ensure fluid passage. The Mackool phaco tip (Alcon) has a rigid outer sleeve designed to decrease heat buildup and transfer, whereas the Barrett phaco tip has a series of longitudinal grooves that ensures infusion even if the sleeve is compressed. (An excessively large incision results in poor chamber stability and undue irrigation fluid use.)
A similar mechanism may result in thermal injury when the phaco needle is pressed against the wound due to an excessively steep approach. The ideal approach to phacoemulsification should cause the least wound distortion. Less distortion to a wound is produced when a phaco needle follows the path of the blade that created it. In most cases, the phaco tip is nearly horizontal. As the phaco tip is angled down for lens removal, significant deviation from a horizontal angle causes compression of the sleeve and greater contact between the phaco needle and the wound. The use of modified phaco tips is one step in solving this problem, but there are several techniques to minimize severe wound distortion. First, a temporal approach prevents a surgeon's operating over the brow where surgeons are forced to work at an angle significantly above horizontal in all but the most proptotic eyes. Second, a surgeon should avoid an excessively long corneal tunnel. Third, minimizing excess lifting or deflection of the handpiece will prevent focal compression of the sleeve. Finally, the use of a downward angulated tip such as the Kelman tip when the orbit is deep will enable lens removal while the intra-incisional portion of the needle remains nearly horizontal.
The consequences of a corneal burn depend primarily on its severity, extent, and location. Generally, the burn associated with phacoemulsification involves primarily the anterior lip, or roof, of the incision. Most likely, the anterior lip is involved because the heat dispersing effects of fluid in the anterior chamber protect the internal portion of the incision and because of increased contact of the anterior wound lip with the phaco needle as it is angled downward during cataract removal.
The mildest clinically evident corneal burn is a slight
graying or whitening of the incision roof, with loss of
normal corneal transparency. These injuries may
slightly delay postoperative recovery, but the injuries
rarely have any lasting effect on vision. More
prolonged or intense thermal injury results in collagen
shrinking and extensive whitening of the anterior lip
of the wound. The collagen shrinkage results in wound
gaping and a fish-mouth appearance. This gaping can
lead to poor wound closure, astigmatism, and problems
with reepithelialization. If the contracted lip is not
sutured, the patients often experience a foreign body
sensation and are at increased risk for epithelial
ingrowth. We have developed a "gape stitch" technique
for closing a burnt incision by bringing the posterior
portion of the roof to the anterior portion of the
floor with either a radial or a horizontal suture
(Slide 4A, Slide 4B and Slide
5).1,2 This technique minimizes
postoperative astigmatism induced by the collagen shrinkage.
Severe burns from intense, repeated, or prolonged
thermal injury may even require penetrating
keratoplasty. Of course, the best solution is to avoid
incisional burns by adopting preventative techniques
and immediately stopping the phacoemulsification at the
earliest signs of decreased fluid circulation — when
the irrigation bottle is not dripping or when lens milk
is observed.
Corneal burns result from the interplay of a number of factors including phaco power, time, cooling, and heat transfer. Often, it is not a single factor that results in thermal injury; rather, it is usually a combination of several factors that produce a corneal burn. By understanding each of the factors and anticipating situations in which unfavorable circumstances present, one can consciously adopt preventative techniques to avoid thermal injury.
Posted April 2001
1. Osher RH. Video Journal of Cataract and Refractive Surgery. IX; 1993.
2. Osher RH. Video Journal of Cataract and Refractive Surgery. VI; 1990.