Nucleus Disassembly and Removal
Most methods of nucleus disassembly and removal consist of several distinct steps: sculpting, cracking, chopping, grasping, and emulsifying. With modern phaco machines, all parameters (power levels and intervals of delivery, aspiration flow rate, and vacuum) can be adjusted for each step of the procedure as well as for the density of the cataract, giving the surgeon maximum control of the process.
Two main modes or settings are used during phacoemulsification: (1) sculpt (if using a “divide and conquer” technique) or chop (if using a chopping technique; see the section Techniques of Nucleus Disassembly) and (2) segment removal. Although exact settings will depend on the machine and can be optimized for a surgeon’s technique, preliminary relative settings for the major steps of nucleus removal are shown in Table 8-2.
With the sculpt setting, the central nucleus is debulked. This process involves a shaving maneuver in which the phaco tip is never fully occluded in order to generate minimal vacuum. Thus, the portion of the phaco needle that is in contact with the lens passes through it without grabbing, and the lens material can be emulsified and aspirated in a controlled fashion. Because of the scaphoid shape of the posterior lens, the sculpted groove should be deeper centrally and shallower peripherally to avoid sculpting through the peripheral posterior capsule. Sculpting is usually performed with low vacuum, low aspiration flow rate, and linear continuous or pulsed ultrasound mode (with high duty cycle and high pulses per second) with a relatively high maximum power setting.
The chop setting is used to impale and hold the nucleus with the fully occluded phaco tip, allowing for mechanical chopping of the nucleus with a second instrument. This can be effectively performed with burst-mode longitudinal phacoemulsification, high vacuum, and a 0° to 30° bevel phaco tip (which is easier to occlude to achieve full vacuum than is a 45° or 60° bevel phaco tip).
The segment removal setting is employed once the nucleus has been divided (using one of the techniques described later in this chapter); the resulting fragments are grasped using moderately high vacuum and pulled centrally for emulsification. Full occlusion of the phaco tip is required for vacuum to build to the desired level. Once this level has been reached, ultrasound power may be applied. After the nucleus has been emulsified, the epinuclear material may be removed with a lower aspiration flow rate setting with either the phaco handpiece or the irrigation/aspiration (I/A) instrument (discussed in the section Irrigation and Aspiration later in this chapter).
Alternatively, nonphaco mechanisms for nuclear disassembly include fracturing the lens mechanically with a nitinol loop or prechopper or segmenting and softening the nucleus by femtosecond laser (see the section Alternative Technologies for Cataract Extraction later in this chapter).
Table 8-2 Preliminary Relative Settings for the Steps of Nucleus Removal
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.