of vision by inducing astigmatism, and delay visual rehabilitation for several months [9–11].

Corneal thermal injuries during cataract surgery are characterized by whitening of the transparent corneal tissue followed by stromal contraction that may result in a misfit of the incision lips, with a loss in the watertight closure [12]. In extreme cases, coagulation necrosis can occur with potential corneal decompensation.

The increase in corneal temperature provokes a contraction of the alpha helix with the maintenance of the triple helix tertiaris structure. These alterations appear to be corneal wrinkles and are reversible. A further increase in temperature creates a destruction of the collagen matrix tertiaris structure. These alterations create a loss of the corneal clarity and determine nonreversible damage [13].

The surgeon often becomes aware of this when the damage is already irreversible.

The denaturization of the collagen fibers occurs after 18 s at a temperature of 63°C (145°F), after 1 s at 100°C (212°F), and 0.001 s at a temperature of 200°C (392°F) [13].

The first thermal changes in the cornea do not occur below a temperature of 40°C [please provide reference].

6.3.3 Heat Generation

The standard ultrasonic handpiece produces a longitudinal movement of the tip. The power generated by the tip is the product of the frequency and the stroke length.

The frequency is the number of tip movements within unit time and is measured in Kilohertz (KHz; 1,000 cycles per second).

All of the phacoemulsifiers today operate at a frequency between 35 and 45 KHz; the higher the frequency, the higher is the risk of thermal damage [14]. The frequencies used today represent the best compromise to achieve emulsification efficiency and thermal safety.

The frequency can vary during phacoemulsification due to the changing load at the tip (i.e., the size, the shape and the density of lens material occluding the tip). All of the modern machines have a software that regulates either the resident frequency or the stroke length.

The stroke length is the length of the tip movements and is measured in thousandths of an inch (mil). Today

all the machines operate in a range between 2 and 4 mil which again, represent the best compromise between emulsification efficiency and thermal safety.

The longitudinal movements emulsify the lens material by the application of mechanical, cavitational, thermal, and acoustical energy. The mechanical energy (jackhammer) is due to the hit of the needle against the lens material. The cavitational energy results from the gas bubbles which are formed by the backstroke of the tip, creating a relative vacuum. With vaporization of some of the fluid, the gas bubbles are the fuel that generates cavitational energy. The next forward movement of the tip compresses the bubbles resulting in their implosion.

There are two types of cavitation energy. First is the transient cavitation, which is characterized by highenergy implosion of the cavitation bubbles. Second is the stable cavitation,which occurs when the bubbles begin to vibrate with fewer collapses and dramatically reduced release of energy.

6.3.4Factors that Contribute to Thermal Incision Damage

The following factors can contribute to an increase in the temperature of the tip and to the genesis of the incision burns:

(a)Energy emission – amount and pattern of how the energy is delivered

(b)Incision – incision construction and possible constriction of the sleeve

(c)Viscoelastic devices and possible occlusion of the aspiration line

(d)Irrigation flow

(e)Position of the tip inside the incision

(f)Tip design

(g)Surgical technique

6.3.4.1Energy Emission: Amount and Pattern of How the Energy Is Delivered

Over the past 10 years, the evolution of technology with power modulations and pulse shaping (Fig. 6.31) has allowed us to work utilizing less energy.

Continuous phaco mode produces the highest energy, pulse and burst modes reduce the energy, and finally, the ultrapulse mode reduces the energy significantly.

Micropulse settings (6 ms on, and 12 ms off) produces less frictional heat at the incision site than the pulse setting (50 ms on, and 50 ms off), which maintain the same emulsification efficiency [15].

This is true in vivo even when in micropulse mode, the phaco time off is equal to phaco time on. In vitro, there isn’t a significant difference in temperature between ultrapulse (6 ms on, 6 ms off) and pulsed energy (50 ms on, 50 ms off), when the ratio of phaco on/phaco off, is 1 [16].

The concept of thermal inertia means that the shorter pulse has a slower propagation in biological tissues [17]. This effect reduces the penetration of heat by only 10% and so does not explain the limited increase in temperature generated by ultrapulse mode. Therefore, other factors have also been involved, specifically the features of how ultrasound energy works.

It has been hypothesized that shorter phaco on time produces a decrease in repulsion of the nuclear fragments from the tip at each pulse. This allows an increase in followability and holdability, with an enhancement of the jackhammer effect due to the near continuous contact of the fragments with the tip.

Another hypothesis is that micropulse generates only transient cavitation, which is the most energetic, and therefore an efficient component of cavitational energy. In fact, when the pulse lasts for more than a few microseconds, the transient cavitation gives way to stable cavitation. Stable cavitation is less efficient than transient cavitation and a bigger part of energy is dissipated as heat [18].

During recent years, we have moved from the concept of pulse modulation to the concept of shape modulation.

In this case, the ultrasound emission is characterized by the so called “kick,” which is an impulse that lasts for 1mm sec and which is 12% more powerful than the maximum power of the rest of the pulse. Therefore, the “kick” can be defined as an increase in power in the first microsecond of the duty cycle. The power of the “kick” can be the same in all duty cycles, or can be programmed to produce an increase or a decrease throughout the “on” time.

The “kick” physically separates the nuclear material from the phaco tip allowing the creation of a microvoid between the occluded tip and the nuclear material. The microvoid allows fresh balanced salt solution (BSS) to get between the phaco tip and the nuclear material, and results in the generation of transient cavitation and an increased release of energy without an increase in temperature. Therefore, the efficiency of the emulsification is increased.

In the year 2006, a new concept of ultrasonic movements was introduced to decrease the heat generation while maintaining or increasing the phaco efficiency by torsional movements of the tip at a frequency of 32KHz.

Many years ago, a similar idea of lateral movements was introduced, but the low frequency of the sonic movements (100–400 Hz) was insufficient to guarantee efficiency.

The reduction of the frequency by 20% and of the stroke length by 50%, make it possible for the torsional movements to use less energy and generate a lower increase in temperature [19] which is estimated to be 60% of the temperature rise with standard longitudinal movements of the tip [20].

The lens is efficiently emulsified by a mechanical effect that is different from the jackhammer effect,

since the tip continuously shears the lens without repelling it. However, the role of cavitation in torsional emulsification requires further investigation [21].

Recently, a software has been introduced that combines the lateral tip movement with the longitudinal movement resulting in an elliptical cutting surface. This maintains the continuous advantage of cavitational energy.

The temperature at the tip and at the incision site have been studied in vivo, initially using a thermocoupled thermometer and then thermal imaging. The thermocoupled thermometer (Fig. 6.32) is characterized by a digital thermometer connected to a sensor which can be placed on the incision or directly connected to the tip. Despite being accurate, these thermometers do not allow the full range of movement during surgery. Therefore, in recent years, infrared cameras and specific software (Fig. 6.33) have been used, creating thermographic, color images of the ocular surface, where it is possible to detect minimal temperature changes and precise temperature measurements in real time (Figs. 6.34 and 6.35).

Fig. 6.35 Tip temperature over 45°C

Using these instruments, many authors have tried to compare different machines at different settings and different surgical conditions. This is impossible because parameter settings on different machines do not correlate with each other [14, 22, 23] (See Sect. 5.2 in Chap. 5). Other factors such as incision outflow are impossible to standardize.

6.3.4.2Incision: Incision Construction

and Possible Constriction of the Sleeve

Generally, incisions need to be of the appropriate width to minimize incision outflow (with microincision cataract surgery) without compressing the sleeve, and appropriate length (usually 2.0 mm) to allow selfsealability. Trapezoidal-shaped incisions allow more

maneuverability of the tip without stressing or tearing the incision.

Prior to entering foot position 3, surgeons should aspirate some viscoelastic divice from the front and the top.

6.3.4.3Viscoelastic Devices and Possible Occlusion of the Aspiration Line

There are no indications that the use of different ophthalmic viscodevices (cohesive, dispersive, etc.) is more often linked to higher incision temperatures. The aspiration line may become clogged by an ophthalmic viscodevice, which would then block irrigation flow. When this occurs with the phaco power on, the heat is no longer dissipated (Fig. 6.36a, b). Increased heat may also result from cavitation bubbles being trapped in viscoelastic divice and blocking out flow, as well as, perhaps, the liberation of free radicals during phacoemulsification, which may have a minimal, theoretical impact [24].

a

b

6.3.4.4 Irrigation Flow

Cooling of the irrigating fluid should have some effect on reducing the temperature at the tip; however, this practice has been discontinued in the US with the advent of power modulations. Increasing the bottle height may result in some increased incisional out flow at the cost of increasing intraocular pressure, which may, under some conditions, exceed intraluminal profusion pressure in the ophthalmic artery [25].

6.3.4.5 Position of the Tip Inside the Incision

Movement of the phacoemulsification handpiece during the surgical procedure can result in compression of the tip against the sleeve, with a resultant transmission of heat to the incision itself. Therefore, movements have to be appropriate and ergonomic with a consideration of the possibility of transmitting heat to the incision (Fig. 6.37–6.38).

6.3.4.6 Tip Design

Increasing the lumen of the sleeve, compared to the size of the phaco tip, may result in greater amounts of

Fig. 6.37 A well centered tip inside the incision (the space between the tip and the sleeve and the space between the sleeve and the incision-wall are maintained)

Fig. 6.36 (a, b) Corneal burn caused by a viscoelastic device, which clogs the aspiration line

Fig. 6.38 When the tip is pushed toward the incision wall, the tip is in contact with the sleeve and the sleeve, with the incision-wall