For the phaco chop technique, the nucleus is held stationary by the vacuum level of the phaco probe. This is why we choose high degrees of vacuum (300 to 500 mmHg) in order to immobilize the nucleus so that the chopper can then split the cataract into fragments. With a peristaltic fluid pump, the tip of the phaco probe must be fully occluded in order to build up to the present maximum vacuum level.
When the resident surgeon attempts phaco chop without achieving the high vacuum holding power, the chop fails and the nucleus does not split. But when the phaco tip is pushed into the dense part of the nucleus so that total occlusion is achieved, the vacuum level quickly rises and the nucleus is held firmly. Now the chop technique is easy.
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