Most of the phaco machines currently in use make it possible to carry out bimanual phaco. It is therefore not necessary to change machines in order to alternate techniques. Certain technological developments now facilitate the decision. Ancillary systems enable safe optimisation of the bimanual procedure. In this chapter, we will analyse the most pertinent systems. Becoming aware of these options will enable the ophthalmologist to update his or her machine for the transition to bimanual phaco.
There are two specific problems connected with microincisions in the transition to bimanual phaco: On the one hand, there is ultrasound emission modulation, enabling emulsification of the crystalline lens without burning the corneal tunnel, and, on the other, the fluidics. These issues must be resolved before embarking upon this surgical technique.
Ultrasound emission
BURST mode
Most machines today are produced with ultrasound power modulation technologies such as the PULSE and BURST modes, which enable discontinuous US emission. These two modes enable nuclear emulsification without generating thermal burns in the corneal tunnel. The effect of US can be subdivided into two components: mechanical and acoustic. The mechanical effect causes fragmentation of the nucleus and is accompanied by a production of heat that can generate burns. The acoustic effect is responsible for cavitation phenomena that pulverize the nucleus, but also for turbulence and repulsion of nucleus fragments.
To optimise ultrasound power without adverse effects, the pulse mode consists in discontinuous delivery of US pulses with an active (ON) and an inactive (OFF) cycle of equal duration. In practice, a frequency of 15 pulses per second (PPS) is used. This pulse mode enables the practitioner to reduce the heat emitted by the US tip and the rebound effect of the nucleus thanks to the relaxation time intervening between US bursts.
In the burst mode, the planned duration variation and the proportion of ON and OFF cycles can be managed by means of the US software to reflect the position of the pedal. The more the surgeon presses on the pedal, the more frequent the pulses. In the burst mode, it is possible to increase the US relaxation time without modifying the PPS, which enables optimum use of US without undesirable side effects.
The US probe
In standard phacoemulsification, the cornea is protected from thermal burns by a silicone irrigation sleeve that is coaxial to the US probe and has three functions: introducing BSS (Balanced Saline Solution) in the anterior chamber, cooling the US probe and insulating the cornea against direct friction from the US probe. Finally, the sleeve also prevents the occurrence of corneal burns. The corneal incision required for passage of the US probe and the silicone sleeve is of 2.7 mm.
In bimanual phaco, the use of discontinuous mode US allows the temperature of the US probe to be maintained below the corneal tissue burn threshold. In this way, instead of being coaxial to the US probe, irrigation can now be separated onto a second chopper or manipulator-irrigator instrument. The standard diameter of the US probe is 1.1 mm, producing a microincision of appropriate size.
To avoid the formation of a spray of liquid projected onto the cornea by the US probe, which would interfere with visualisation during bimanual phaco, the silicone infusion sleeve is cut at 2/3 and screwed onto the handpiece handle. It surrounds only the proximal portion of the probe, near the thread, leaving the intraocular end free.
The US probe design contributes to an improvement in the stability of the anterior chamber by reducing chamber collapse upon removing obstructions from the US probe, called "surge effect". When the probe is plugged up by a fragment, tubing is crushed by the effect of the aspiration. When the US emission removes the obstructions from the probe, a brutal aspiration effect follows, causing chamber collapse. The longer and thinner the tip, the higher the resistance to flow, thus minimising the risk of surge. This is why microtips (e.g. Microtip, Thin tip) with a diameter of 0.9 mm are particularly useful. They also help reduce microincision size.
ABS (Aspiration Bypass System) type probes have a small orifice on the US probe tube whose anti-occlusion action is linked to the short-circuiting effect of the main aspiration system. This short-circuiting also helps cool the probe.
Probes with thermal protection (Microflow, Mackoll) are fitted with grooves or a carbon layer applied under vacuum, making it possible to limit probe heating to protect the cornea from burns at the incision site.
Fluidics
The air pump
Improved fluid circulation enables the surgeon to work safely with high vacuum and aspiration levels while avoiding chamber collapse. High vacuum enables US emissions to be limited during surgery, but requires a high infusion flow rate to ensure that incoming intraocular fluid compensates for the exiting fluid. It is therefore paradoxical to try to increase infusion flow rates with smaller diameter instruments in rendering it dependent solely on gravity. Further to the work of S. Agarwal, the use of an air pump to force additional pressure into the infusion bottle has become an interesting alternative in bimanual phaco. A set of tubing equipped with an air filter is connected to an air pump on one end and to the infusion bottle on the other. Infusion pressure can thus be managed in a precise way without needing to raise the irrigation bottle. The settings I use on the air pump are as follows: infusion pressure 60 mm/Hg, and a head of 40 cm; this corresponds to the same infusion flow rate as that obtained with a bottle set at a height of 120 cm.
The anti-surge flow reducer
This chamber collapse prevention system comprises a disposable tip that can be fitted to the aspiration tubing of any machine. It consists of a 0.3 mm diameter flow reducer placed between the aspiration outlet of the handpiece and the aspiration tubing. The interior of the reducer comprises a two-chamber system separated by a cylindrical filter. Nuclear fragments are retained at the distal end of the central chamber while the aspirated BSS flows into the aspiration tubing through the proximal end of the filter. Thus it is not possible for the aspiration tubing to be obstructed by a nuclear fragment, and this clever device prevents the surge effect that destabilises the anterior chamber.
The Dual Linear Pedal
To control the circulation of intraocular fluid and reduce surge, a dual linear control pedal setting is available on certain machines. This separates infusion and aspiration from ultrasound emission. With this setting, irrigation and aspiration are triggered by means of vertical pressure on the pedal (Pitch), while US emission takes place by means of a horizontal movement of the practitioner’s foot (Yaw). This enables linear control of the aspiration function during US emission. In these conditions, aspiration can be kept low during emulsion of a fragment occluding the US probe, while it is not possible to produce this effect with the pedal’s standard operation. The surgeon can drive the probe into a nuclear fragment following a US burst and nil aspiration, then increase aspiration to re-center the fragment in a safe area and emulsify it with the Divide-and-Conquer technique. The surgeon may also select strong aspiration and maximum US power to impale the probe into the nucleus in the phacochop technique.
