this value as a vacuum or as a negative pressure of 300 mmHg. The two points of view are equally correct. As a conclusion: A vacuum is a negative pressure. Therefore, the pressure difference pa.c.−pasp

can also be written as pa.c. + vacasp, where vacasp is the aspiration vacuum.

5.1.2 Surgical Considerations

What can be done by the surgeon to fully exploit the emulsification potential of phaco tips specifically designed for microincisional cataract surgery (MICS), while preserving maximum incisional and corneal stability, avoiding tissue trauma or incisional leak, and provide optimal chamber stability for the protection of the endothelium and the posterior capsule?

With biaxial phaco tips, exact dimensioning of the incision size is crucial, since tissue trauma will ensue when incision size is too small, and collateral leak will result when it is too wide. The advantage of the coaxial phaco approach is the protection of the peri-incisional tissue from mechanical distension and thermal shrinkage [1, 2], and the lack of fluid loss through the space between the rigid metal tip and the incisional slit. With a stab incision, any tilting of the phaco instrument will jeopardize the safety of the procedure: With a biaxial instrument, the metal tip will either compromise the integrity of a tightly fitting incision or result in significant and variable collateral infusion leak. Such leak will not only jeopardize the chamber stability, but also endothelial integrity: The turbulences quickly wash away the protective viscoelastic layer from the endothelium, exposing the latter to the bombardment by emulsified nuclear particles carried along with the fluid stream. On the other hand, tilting a coaxial instrument may cause chamber collapse by pinching off the sleeve.

To avoid this, special attention must be paid to the details of incision construction:

5.1.2.1 Incision Configuration

Generally, incisions have to be designed in such a way as to provide for maximum topographical stability of the cornea and deformation resistance of the incision when digitally massaged.

•Corneal stability can be increased by reducing the incision width and/or by moving the incision entrance backward into or behind the limbus. The latter has lost ground because of cosmetic concerns with, though only transient, occasionally prominent postoperative redness due to conjunctival bleeding and foreign body sensations caused by conjunctival scarring, even though minimal. With clear corneal incisions (CCIs), however, significant sectorial flattening has been found which may permanently affect the optical 5 mm zone [3].

•Incision stability is dependent upon the width and length of the incision. It increases as the incision narrows and the tunnel lengthens. Inherently, CCIs cannot be made as long as limboor sclerocorneal incisions. Apart from the design, deformation resistance is also dependent upon the incision location: Incorporating limbal tissue has been demonstrated to considerably increase deformation resistance [4]. This is due to the folding-grille-like arrangement of scleral fibres which better absorbs localized indentation of the globe outside the incision. Also, the vascularized conjunctiva covering the incision entrance heals quickly, protecting the latter from gaping when massaged. In contrast, corneal incisions are inherently shorter, lack the mechanical properties of limbal and scleral tissue, and heal protractedly.

In order to provide topographical neutrality in the optically relevant central corneal zone and sufficient deformation stability, defined as the patient’s inability to reopen the incision tunnel even with intense digital massaging in the temporal palpebral fissure, CCIs must be downsized as much as possible.

Detailed attention must be paid to the design of small CCIs in order to not only optimize the postoperative deformation stability, but also to allow for easy and atraumatic insertion of instrumentation (phaco and injector tip), to avoid damage of the tissue and specifically of the corneal lip when a non-sleeved phaco tip is used, and to provide unimpeded infusion inflow when a sleeved phaco tip is moved to the sides or downwards.

In a parallel-walled tunnel, any deviation of a coaxial phaco tip from the tunnel axis will inevitably compress the sleeve, jeopardizing chamber stability, especially after occlusion break. With a non-sleeved biaxial phaco tip, any off-axis manipulation of the tip

will result in mechanical deformation and potential damage of the incision by oarlocking or, with a wider incision, in varying fluid leakage depending on the angle of offset between instrument and incision axis. To avoid this, CCIs must be designed funnel-shaped. The narrow end of the funnel will then act as the pivot point providing for a tight fit of the sleeve while offering freedom of tip movement without deforming the incision or sleeve, respectively. Since it is the diameter of the smallest section of the tunnel rather than a small diameter of the whole tunnel that is critical, the deformation stability of a funnel-shaped incision is not reduced compared to a parallel-walled tunnel.

In principle, the funnel may open up internally or externally. Though both may be applied, there are well-defined advantages and disadvantages of both options to be considered:

•Internally widening funnel: It is created by fully entering the chamber with the blade. After partially retracting it, the inner aspect of the tunnel is extended towards both sides in a funnel-shaped manner by again advancing the blade and pushing the cutting edge to the left and right while sparing the tunnel entrance. This incision design has the advantage of avoiding pinching of the sleeve of a coaxial phaco tip and maximally protecting the inner corneal lip from overextension and mechanical damage when a nonsleeved instrument is used. As the main downside, however, such an incision configuration impedes the insertion of instruments, particularly that of injector cartridge tips for lens injection, that are not adequately conical and bevelled.

•Externally widening funnel: It is created by not fully advancing a blade with a total width that is larger than that of the inner incision. There are blades available with a cross-mark of a defined length. The blade is advanced until the visible penetration line in Descemet´s membrane coincides with the mark on the front side of the blade below. This incisional architecture has the advantage of facilitating docking or insertion of instruments, especially during lens injection. The downside is that – though the Descemet´s membrane is very robust and elastic – it may be overextended or torn, compromising the sealing properties of the internal corneal valve. Also, endothelial damage may result from direct stripping or indirectly by creating radiating stress folds.

The current popularity of externally widening funnels is mainly nourished by the lack of instrumentation with appropriate tip designs making it cumbersome to snugly thread them into the incisional canal. The gradual introduction of strongly bevelled micro-incisional phaco tips and injector cartridges with an oval cross section will eventually allow to take full advantage of the benefit from the Descemet’s membraneand endotheliumpreserving properties of internally widening funnels.

Conventional coaxial phaco tips require a larger incision due to the addition of a soft infusion sleeve. The overall diameter has been reduced by downsizing the metal tip. This, however, reduces phacoemulsification efficiency as explained above. When using modified designs with a trumpet-shaped and strongly bevelled head and a narrow shaft accommodating a sleeve with a diameter hardly larger than that of the metal tip head itself (e.g. CO-MICS 2 tip), phacoemulsification efficiency comparable to a standard 20-gauge tip can be achieved while only requiring a 1.4mm funnelshaped incision for unimpeded infusion flow. The increased flow resistance of the narrow shaft inherently acts as surge break which allows working with extremely high vacuum settings. Biaxial phaco instrumentation cannot profit from such a design since the incision size required depends on the segment with the largest diameter, which is the tip head. Reducing the size of the shaft without adding a sleeve would cause considerable leakage and, thus, chamber turbulences and instability. Therefore, a separate surge protector must be interpolated into the infusion line (e.g. “StableChamber®”, Bausch & Lomb).

With the large front opening of strongly bevelled tips and the particular fluidics coming with narrowshaft designs, the surgical technique must also be adjusted for maximum efficiency.

5.1.2.2 Phaco Technique

Generally, highly efficient phacoemulsification requires permanent full tip occlusion. With a peristaltic pump, a high flow rate provides for quick attraction and apposition of nuclear chunks to the phaco tip and for quick subsequent vacuum rise. However, full circumferential sealing of the tip opening by the nuclear chunk is required if the maximum vacuum level has to be reached within the shortest time possible and maintained during phacoemulsification. In order to achieve

such docking, low phaco power should be used to allow for full circumferential coupling. With suboptimal sealing between the chunk and the tip, the chunk may be pushed away from the tip when using higher phaco power, which has been termed “chatter”.

For the physical reasons detailed above, a strong bevel compensates for the reduced holdability of the smaller diameter of MICS phaco tips. This has been implemented in the Oertli CO-MICS 2 tip. However, increasing lumen and bevel inherently counteracts ease of occlusion (“occludability”). Therefore, Such a tip design requires surgical adaptation of the surgical technique:

Nuclear disintegration (“divide”): For the popular chop technique, tips with a larger diameter and a weaker bevel are most adequate. The former guarantees for a strong grasp of the nucleus required during the chopping manoeuvre. The latter avoids tilting or slipping of the nucleus chunk. By simply reducing the diameter of a conventional tip for micro-incisional phacoemulsification, holdability is inherently reduced, since the latter is proportional to the area of the tip opening. This can be compensated by increasing the bevel angle of the tip. However, the latter is counterproductive regarding the chopping procedure, since the chunk may easily slip off the bevel when an oblique vector force is exerted by the chopper. Therefore, the chopping technique is less appropriate for such a tip.

With a medium to hard nucleus, a modified dividing technique is recommended that may be termed “direct crack” in analogy to the “direct chop” technique: With the pump set at a low vacuum level and the bevel directed to the side and away from the phaco spatula, the tip is directed downward right towards the centre of the nucleus. No more energy is used than necessary to allow the tip to penetrate the nucleus without pushing on it. A second or third pass may be done when the tip gets stuck in a hard nucleus. After having reached the bottom of the nucleus, the phaco spatula is deeply inserted in between the phaco tip and the trough wall, and both instruments are then moved sideward which usually results in a nasal crack, which is then extended temporally (with surgeon sitting on temporal side) and usually ends up in the complete division of the nucleus into two halves (Fig. 5.7a). During this combined manoeuvre, the sideward tilted tip acts like a sharp blade that easily cuts through the nucleus, and as a blunt spatula when dividing the nucleus. Then, the well-hydrodissected lens contents are rotated by 90° increments, and two halves are split into quadrants in a similar way.

a

b

Fig. 5.7 (a) “Direct crack”: With the bevel directed to the side and away from the phaco spatula, the tip is directed downward right toward the centre of the nucleus. After having reached the bottom of the nucleus, the phaco spatula is deeply inserted in between the phaco tip and the trough wall, and both instruments are then moved sideward which usually results in a distal crack that is then extended proximally. (b) The sideward turned tip opening is approached to the flank of the quadrant and docked to it by aspirating it with high flow and vacuum, slightly activating the phaco power to make the tip edge dig into the lens material. When full occlusion is obtained, the phaco power is linearly modulated to emulsify the quadrant, using not more power than necessary for aspiration

Quadrant aspiration (“conquer”): When all four quadrants are separated, phacoaspiration is initiated. Other than with conventional tips the bevel of the CO-MICS 2–style tip is turned sideward. Instead of engaging a nasal quadrant at its apex, it is approached to its flank and docked to it by aspirating it with high flow and vacuum, slightly activating the phaco power to make the tip edge dig into the material. When full occlusion is finally obtained, the phaco power is linearly modulated to emulsify the quadrant, using not more power than necessary for aspiration (Fig. 5.7b).

With a soft lens mass, a trough is created step by step with the bevel oriented upward, as with the conventional technique, which allows good visual control of the depth reached. If the mass is too soft and cohesive to allow subsequent nucleus division, the bevel may be turned sideward and docked to each half which is then aspirated with flow and vacuum set at a high level, usually resulting in a collapse of the lens contents. Alternatively, the tip may be docked to the lens equator with the bevel pointing centrally, and the whole lens mass aspirated with low phaco power while it rotates in a spiral fashion.

It is important that, when working with the bevel turned sideward, the fluid outlets of the sleeve must be aligned with the bevel so as to direct the outflow horizontally in order to preserve the adherent viscoelastic layer, and thus protect the endothelium.

5.1.2.3Infusion-Assisted High-Flow HighVacuum Phacoaspiration (Hybrid Phaco)

As stated, emulsification efficiency is highly dependent on the coupling force between the phaco tip and the nuclear chunk. If not appropriate, emulsification efficiency is suboptimal and nuclear chatter may ensue. Setting the vacuum at a very high level carries the risk of chamber flattening due to the surge following occlusion break. This is mitigated by the surge-reducing effect of a narrow segment placed within the tubing. This can be inserted behind the phaco handpiece as realized in the Bausch & Lomb Stellaris® phaco equipment (“StableChamber®”), or integrated in the phaco tip as in the Oertli® CO-MICS 2 tip. Surge may also be counteracted by elevating the infusion bottle. However, excessive infusion pressure exerts considerable stress on ocular structures and blood flow [5]. Therefore, an additional infusion line may be used that complements

the coaxial inflow. An anterior chamber maintainer has been used to accomplish this. However, an additional paracentesis is required, and the tubing interferes with free globe mobility. Alternatively, an infusion handpiece of a biaxial phaco or I&A set may be used as an “infusion spatula” inserted through the pre-existing paracentesis instead of a conventional phaco spatula. Since this approach combines micro-coaxial and biaxial features, it may be termed “Hybrid phaco”. The additional influx provided by the infusion spatula supplements the flow provided by the sleeve to a considerable extent. The abundant surplus of infusion allows for vacuum levels as high as 600 mmHg and flow rates of up to 40 mmHg even with a moderately elevated bottle, while the chamber stays rock-solid when occlusion breaks. The tip of the infusion handle is inserted only when nuclear division is completed. It serves as a blunt spatula to manipulate the nuclear pieces towards the tip opening, and is positioned between the posterior capsule and the tip when occlusion break is imminent in order to mechanically protect the posterior capsule should chamber flattening occur (Fig. 5.8a). The additional use of an appropriate high-flow infusion spatula (Fig. 5.8b) has proven to significantly enhance phacoemulsification efficiency during the phase of quadrant conquering: Energy consumption as expressed by the integrated phaco time displayed by the machine was reduced by more than one half (by 53.3%). Fluid consumption from the bottle was increased by one fourth (25.8%) which reflects the enhanced followability provided by the high flow. (Preliminary data from prospective randomized intraindividual comparison study with 30 patients [6, 7]. Fluidic settings: 20ml/min and 400 mmHg without, and 35 ml/min and 600 mmHg with infusion spatula, respectively). Turbulences during aspiration are confined to a smaller space around the phaco tip, and nuclear chunks or emulsified lens material are immediately re-grasped or aspirated by the tip, which should minimize the risk of endothelial trauma.

In conclusion: With MICS, special attention has to be paid to the details of incision construction. The reduced holdability of a smaller-diameter tip can be compensated by increasing the bevel angle. The trum- pet-shaped head of the CO-MICS 2 coaxial tip additionally boosts phaco energy transfer by increasing the projection area of the frontal plane without requiring a larger incision for the sleeve, since the reduced shaft diameter allows the sleeve to run flush with the head