Key characteristics of phaco tip design

The four key characteristics of phaco tips that impact the efficiency and safety of the phaco procedure are: energy output, holdability, followability and surge suppression. Simple downsizing of a standard phaco tip design results in a loss of efficiency and chamber stability. When downsizing a coaxial phaco tip to fit through a sub-2-mm incision, the design must be modified in order to compensate for the loss of energy output, followability, holdability and surge suppression.

Energy output

Energy emission is a function of the area of frontal projection of the needle. When the phaco tip is downsized without modifying the design of the needle, the frontal projection area is reduced and, therefore, the output of energy decreases. The actual energy transfer is additionally dependent on the vacuum level or “power coupling.”

Followability

Followability is a function of the pump speed. The aspiration flow is limited by the inflow capacity, which is a function of the bottle height defining the infusion pressure and the tubing, and specifically the area between sleeve and phaco needle (“infusion mantle”) defining the infusion flow resistance. Followability requires that the surgeon uses high flow. If the surgeon uses an excessive pump speed where the amount of fluid transported out of the eye exceeds that supplied through the infusion line, the chamber will collapse. The broad infusing mantle provided by easyPhaco tips (Oertli Instruments AG) allows for higher flow rates by decreasing infusion resistance (Figure 1).

Figure 1: High vacuum settings of easyPhaco technology (Oertli Instruments AG) combined with a wide infusion path reduces turbulence and floating fragments while improving followability.
Source: Oertli Instruments AG

Holdability

Holdability is a function of the area of the needle orifice. When the phaco tip is downsized without adjusting its design, the area of the orifice is reduced, and thus, holdability is also reduced. Holdability allows for better transfer of the energy emitted from the needle to the lens material, reducing overall phaco energy requirement (Figure 2).

Figure 2: When using easyPhaco technology (Oertli Instruments AG), ultrasound energy is applied axially and fully absorbed by the nuclear materials instead of radiating laterally and potentially affecting the endothelium.
Source: Oertli Instruments AG

Surge suppression

Surge at a given vacuum is a function of the bore diameter of the phaco needle and the infusion supply which is proportional to the area of the infusion mantle. Downsizing the tip will inherently increase surge. The slim shaft design decreases the bore diameter while increasing the infusion mantle (Figure 3). Both help in suppressing the surge.

Figure 3: The anterior chamber remains stable when an occlusion break occurs with easyPhaco technology (Oertli Instruments AG). The capillary aspiration channel prevents sudden flow and the wide infusion path provides constant IOP.
Source: Oertli Instruments AG

Slim-shaft strong-bevel design

With the easyTip (Oertli Instruments AG) or slim-shaft strong-bevel design, the transition from a wider tip head into a waisted shaft significantly increases the area of projection as compared to a standard tip, which compensates for the loss of energy output when downsizing the tip. In addition, increasing the bevel angle increases the size of the orifice, which results in the same area of orifice as that of a standard 19-gauge 30° phaco tip when the bevel angle of a sub-2-mm CO-MICS tip is increased to 53°. By using a slim shaft while retaining the sleeve size, the area of the infusion mantle is per se increased. This augments infusion influx, which again allows for a higher pump speed and thus followability.

Combining these design criteria lead to additional advantages of the easyTip. By using the slim shaft design, the sleeve is flush with the funnel-shaped tip head and thus prevents anteriorly directed axial infusion influx while avoiding chamber efflux by better sealing the incision. This minimizes chamber turbulences and better directs the lens material toward the orifice of the phaco needle. The reduced overall bulk of the phaco tip minimizes the incision size required for inserting the phaco tip.

One downside of the strong bevel design is that a large orifice is more difficult to occlude. Thus “occludability“ is another important characteristic of a phaco needle. To compensate for that difficulty, surgeons must use higher aspiration flow to pull the lens chunk in to the opening. Once occluded, the high flow speeds up the vacuum rise to the preset high levels where power coupling and emulsification efficiency are maximal. However, high aspiration flow requires high influx capacity which, apart from an adequate bottle height, is allowed for by the augmented infusion mantle of the easyTip.

Infusion-assisted phaco

With the Oertli sub-2-mm easyTip CO-MICS, infusion supply is insufficient for high fluidics in spite of its easyPhaco design. The solution is “infusion-assisted” or “hybrid phaco,” which is derived from biaxial phaco with the exception that a sleeved easyTip CO-MICS is used instead of a sleeveless standard tip. Using such a tip and supplying additional infusion via an infusion spatula, surgeons can use full flow and vacuum settings (Figure 4). Surge is suppressed because there is enough fluid supply to counterbalance the surge that can occur when occlusion breaks. This technique only requires low-cost standard infusion tubing and an infusion handpiece of an adequate biaxial irrigation and aspiration set with no need for additional or special instrumentation.

Figure 4: Using the easyTip CO-MICS (Oertli Instruments AG) and supplying additional infusion via an infusion spatula, surgeons can use full flow and vacuum settings.
Source: Oertli Instruments AG

References:

Menapace R, Di Nardo S. How to better use fluidics with MICS. In: Alió J, Fine IH, eds. Minimizing Incisions and Maximizing Outcomes in Cataract Surgery. Berlin, Germany: Springer; 2010: 57-68.
Menapace R. Mini- and microincision cataract surgery: a critical review of current technologies. European Ophthalmic Review. 2010; 3(2):52-57.