Technology Available

Core Messages

ßDecreasing the downsizing the incision size for coaxial phacoemulsification does not necessar-

ily mean that the efficiency and quality of the fluidics and the emulsification properties decrease.

ßIn this chapter, we will show how a tip/sleeve design, which is optimized according to the

physical laws, allows for excellent fluidics and emulsification.

ßDown-sizing clear corneal incisions to less than 2mm significantly enhances corneal topograph-

ical neutrality and wound deformation stability.

ßA funnel-shaped wound configuration reduces tissue stress by oar locking during phaco and

facilitates instrument insertion. This is especially true for the CO-MICS 2-style phaco tip with its narrow shaft and its sleeve running flush with the protruding head.

5.1.1 Physical Considerations

Historically, cataract operations have been performed using many different techniques and instruments. Till date, there is no standardized method: Incision architecture and size, choice of phaco tip and irrigation sleeve, and method of removing the nucleus and cortex are three examples where the surgeon can choose between various options. It depends on the surgeon’s

R. Menapace ( ) Department of Ophthalmology

Medical University Vienna, Vienna General Hospital Waehringer Guertel 18-20, A 1090 Vienna, Austria e-mail: rupert.menapace@meduniwien.ac.at

skills and preferences, but also on non-medical arguments such as tradition and teaching.

A good understanding of the physical properties may help each surgeon to choose his/her own appropriate technique and to develop the most efficient way for cataract surgery. Understanding the fluidics is a prerequisite for the efficient use of the evolving microphacoemulsification instrumentation. In the following sections, the physical dependencies of aspiration efficiency, chamber stability, and holdability are detailed.

5.1.1.1 Aspiration Efficiency

There are two different pumps in use for cataract surgery: vacuum-controlled pumps (e.g. Venturi) and

flow-controlled pumps (e.g. peristaltic). The Venturi pump induces a vacuum in a rigid vacuum container, while with the peristaltic pump the vacuum is produced by a wheel containing ball bearings that milks fluid out of the tubing. The vacuum “propagates” through the tube and induces a flow if the needle is not occluded.

The total flow depends on the applied vacuum pasp, the anterior pressure pa.c., and the flow resistance R. Mathematically the flow F can then be expressed by

the following formula:

pa.c − pasp = Φ.

R

The flow resistance R depends on the design of the particular instrument and the tubing. For smaller diameter tips, for example, the resistance increases. Therefore, a higher vacuum is needed in order to obtain the same flow. On a Venturi pump the vacuum can be set to a higher value, and on a peristaltic-pump it may be necessary to increase the vacuum limit to obtain the same flow rate. In Fig. 5.1, the relation between the vacuum (Y-axis) and the induced flow (X-Axis) is plotted for two different needles. The data of the red curve have been taken with a traditional 19-gauge needle, and the blue curve has been recorded with a CO-MICS microincision needle from Oertli Instruments. The slope of the two curves corresponds to the resistance R, and the data confirm the higher resistance value for the smaller needle. If, with a peristaltic pump, the vacuum limit is not adequately raised, the pump sensor will prematurely

Flow Rate [ml/min]

Fig. 5.1 The relation between the flow in the non-occluded case and the necessary vacuum is plotted

stop the pump, as a result of which the preset flow rate will not be reached.

In order to achieve the same efficiency of aspiration, the vacuum has to be raised by 100–150 mmHg during quadrant removal.

5.1.1.2 Chamber Stability

The stability of the chamber is one of the most important issues in cataract surgery, since chamber instability or collapse may result in endothelial and capsular damage. To avoid this, it is important that the pressure inside the eye never falls below atmospheric pressure. In such a case, the anterior chamber will collapse and, as a result, the corneal endothelial will be damaged. The natural anterior overpressure of up to 20 mmHg ensures a certain reserve for chamber stability. Ideally, this limit is never underrun, not even during an operation. The course of overpressure over time graphically corresponds to the area beneath the actual pressure and the critical pressure and may be termed “stability band” for better clearness.

The critical point for chamber stability is the moment where the occlusion breaks. This situation may be simulated on an artificial “eye”. (A rigid plastic chamber with an integrated sensor to measure the intraocular pressure is used.) For the experiment, a vacuum of 600 mmHg is built up during the simulated occlusion, using a peristaltic pump. Then, the occlu-

sion is broken at the time tbrk and the pressure is measured inside the “eye” as a function of time. The

maximum vacuum inside the eye pstb is a measure of chamber stability. In Fig. 5.2, the data are shown for a

traditional 19-gauge tip and the CO-MICS microincision tip from Oertli Instruments, both used in a coaxial instrument.

The measurement shows that the maximum drop of the pressure is higher for the traditional 19-gauge tip (approximately 135mmHg below atmospheric pressure) as compared to the CO-MICS tip (55mmHg). This measurement demonstrates that the smaller tip has a better chamber stability than the larger one when using the same vacuum settings. This result is explained by the fact that not only the irrigation line but also the aspiration line does have a considerably larger flow resistance. A high vacuum does not induce poor chamber stability when using a small aspiration tip, because the aspiration flow is kept small by the large flow resistance.

19G, Excellerator 1 CO-MICS 2

Vacuum [mmHg]

Fig. 5.2 The dynamics of the pressure in an artificial “eye” is plotted. The atmospheric pressure has been subtracted and corresponds to 0 mmHg in this figure. The critical pressure of 20 mmHg approximately matches the natural pressure inside the human eye

Fig. 5.3 The time-integrated vacuum (chamber stability level) is plotted on the Y-axis for various vacuum settings of the pump (X-axis). The blue line corresponds to the CO-MICS tip, and the red line is a traditional 19-gauge tip

The curve further shows that the dynamics of the traditional 19-gauge needle is much faster than the one of the CO-MICS. The irrigation line in the traditional 19-gauge system provides sufficient BSS-flow to restore the anterior chamber pressure within the shortest time (steeply rising slope between tirr and t2). In the case of CO-MICS, it takes longer to re-establish this pressure. This is explained by the cross section of the space between the tip and the sleeve, which is greater with a 19-gauge as compared to a CO-MICS system.

Apart from determining the highest vacuum value pstb, there is another measure for chamber (in-)stabil- ity. It corresponds to the time-integrated vacuum inside the eye. For a given stability-limit, the (negative) pressure is “summed up in time” whenever it is lower than the critical limit. In this measurement, the critical limit has been set to 20 mmHg, close to the natural eye pressure. The integrated vacuum corresponds to the area between the curve and the critical limit, which is the shaded area in Fig. 5.2 for the traditional 19-gauge tip. The value of this area is measured for various vacuum settings of the pump. The results are shown in Fig. 5.3.

For all vacuum settings of the pump, the chamber stability level, (defined in the way described above), is clearly better for the CO-MICS tip than for the traditional 19-gauge tip. For the CO-MICS tip, the vacuum can be increased by approximately 100 mmHg to obtain the same amount of stability.

As a conclusion one can state that the chamber stability of the small diameter needle is better when working with the same vacuum limit. In order to achieve a certain flow rate, the vacuum has to be raised by approximately 100–150 mmHg. Then the chamber stability is comparable to the traditional 19-gauge instrumentation.

5.1.1.3 Holdability

The holdability describes a property of the system during an occlusion. The holdability is defined as the force with which the nuclear fragment is firmly attached to the tip. It must be distinguished from the followability, which describes the streaming characteristics towards the tip.

The holding force F depends on the vacuum in the aspiration line, on the size and geometry of the phaco tip, and on the degree of occlusion (Fig. 5.4).

The following calculations apply for a fully occluded phaco tip. Partial occlusion will reduce the holdability considerably. In case of perfect occlusion, the pump does not induce a flow but a constant vacuum pasp that equals the vacuum generated at the location of the pump. The general formula for calculating the force F that results from a pressure p acting on a surface with area A, is as follows:

Fig. 5.4 Scheme of a phaco hand piece with a nuclear fragment in the state of complete occlusion (a) and partial occlusion (b), respectively

F F

F = p · A.

In the specific setting of the phacotip within the eye, the term p can be identified with the pressure difference between the two sides of the nuclear particle (the phaco tip on the one hand and the pressure in the anterior chamber on the other). In this case, the pressure in the anterior chamber equals the pressure induced by the irrigation bottle. It therefore depends on the height of the irrigation bottle measured with respect to the patient’s eye.

A is the surface of the needle opening. Therefore, the holding force can be expressed as a function of the pump vacuum pasp and the anterior chamber pressure pa.c. which is given by the height of the irrigation bottle,

F = ( pa.c − pasp ) A.

Writing the irrigation pressure as a function of bottle height h finally leads to the expression

F = (ρ g h − pasp ) A,

where r is the density of BSS and g, the acceleration of gravity. The formula shows that there are three different ways to increase the holdability:

•Increasing the bottle height h

•Increasing the vacuum settings of the pump (both peristaltic and Venturi)

•Increasing the tip area A

When increasing the bottle height, both the holdability and the chamber stability can be optimized. However, the increase of the bottle height is limited by medical aspects (high intraocular pressure and leaking of the incisions).

An increased vacuum in the aspiration line raises the holdability too. However, the vacuum cannot be set too high due to its negative influence on the chamber stability. Here, a compromise has to be found between the two effects, but priority has to be given to the chamber stability.

The third option for an increased holdability is the use of a bigger needle opening. Here, either the tip diameter or the bevel angle can be increased. The diameter of the needle is limited by the incision size. The relation between the diameter and the area (flat cut) is given by the formula for a circle

The dependence on the bevel angle α is given by the following formula

This is shown in Fig. 5.5.

Oertli Instruments has chosen a new solution for the latest generation of micro incision tips. In Fig. 5.6, two designs are illustrated: on the lower picture (b), a traditional phaco tip is shown. It has an inner diameter of 0.57 mm and a bevel angle of 30°. On the upper picture (a), the latest generation of the

bevel angle α [°]

•The reduction of the external metal tube diameter along the shaft allows adding a sleeve such that the total size of the tip–sleeve combination is the same as for a traditional CO-MICS tip.

The holding force can now be written as a function of the four parameters: bottle height, aspiration vacuum, diameter of the needle, and bevel angle (h, pasp, d and α)

Fig. 5.5 Increase of the surface area of a phaco needle as a function of the cut angle

As an example, the holdability should be calculated for the following, realistic parameters: bottle height h = 70cm, aspiration vacuum pasp = −400mmHg (−53328.8Pa). The density of water is r = 1,000kg/m3 and the gravitational constant equals g = 9.81m/s2. The values for the holdability are given in Newton.

Fig. 5.6 Difference between (a) a CO-MICS 2-tip from Oertli Instruments and (b) a traditional tip design (CO-MICS 1). Both the diameter and the bevel angle a are maximized to obtain an optimum holdability in the CO-MICS 2 design

Oertli CO-MICS tip is drawn. A staged outer tube diameter allows for an increased opening area. It maximizes the inner diameter of the tip opening (0.84 mm) while keeping the outer diameter (at the location of the elastic sleeve) at a minimum. In addition, the bevel angle of 53° leads to the same holdability as a 19-gauge tip with an angle of 30°.

This design has two advantages:

•The thin wall of the metal tubing at the head and the angled cut both maximize the area of the frontal tip opening and thus the holdability.

As a conclusion, one can say that the holdability is increased by lifting the irrigation bottle, by applying a high vacuum, and by choosing a needle with a large internal diameter and a large bevel angle. When replacing a 19-gauge or 20-gauge tip by a smaller one, the vacuum should be enhanced in order to obtain the same flow rate. This increase in vacuum further increases the holdability. An angled cut, for example, the CO-MICS 2-tip again raises the holdability.

What is “vacuum”? What is “pressure”?

In ophthalmic surgery all pressure values are measured relative to the atmospheric pressure. For example, an intraocular pressure of 15mmHg means that the pressure inside the eye is 15mmHg higher than the atmospheric pressure. Or on the other hand, the aspiration pump displays a vacuum of 300mmHg if the pressure inside the tubing is 300mmHg lower than the ambient pressure. Therefore, we could either consider