This study comprised 60 eyes (60 patients) with cataracts. They were randomly assigned to have phacoemulsification using the power-free-chop technique or the phaco-chop technique. The criteria for these cases were 60-80 years old, 22.0-25.0 mm axial lengths, more than 2000 endothelial cells/mm2, anterior chamber depth beyond 2.5 mm, dilated pupil diameter beyond 6 mm, cataract nucleus grade 3 (according to Emery-Little classification8) and no other oculopathy.
All surgeries were performed by the same surgeon (L. Y.), who was experienced in these 2 techniques, with a Centurion® phacoemulsification unit (Alcon Laboratories Inc.).
In Group 1 (power-free-chop), a standard 2.75 mm clear corneal incision was made at 11 o’clock, and a side port was created with a stab knife at approximately 4 clock hours away. After continuous curvilinear capsulorhexis was made, hydrodissection and hydrodelineation were performed. The phaco tip was inserted into the anterior chamber, and the superficial cortex was removed. The phaco tip was erected and placed on the nucleus near the capsulorhexis edge at the 11 o’clock position, then it was leaned near the geometric centre of the nucleus as deep as possible using irrigation/aspiration (I/A) gear without ultrasound power (Step 1, foot pedal in position 2, Figure 1A, 1B, 2A). The bevel face of the phaco tip was upwards. At this stage, occlusion was not necessary. The chopper was placed beyond the edge of the nucleus and moved to the phaco tip horizontally. Then, the nucleus was split into 2 hemispheres by the encountered chopper and phaco tip (Figure 2B). Both hemispheres were turned 90 degrees around the horizontal axis using the chopper. The phaco tip was placed in the centre of 2 hemispheres at irrigation status. The chopper was placed beyond the hemisphere, which was opposite the main corneal incision (Step 2, foot pedal in position 1, Figure 1C, 1D, 2C). Then, the chopper moved to the phaco tip horizontally to split the hemisphere into 2 pieces (Figure 2D). Each piece was aspirated and emulsified. Then, the same process was repeated. In the whole process, the position of the chopper was crucial. It must be placed beyond the edge of the nucleus and as deep as possible. I/A was used to clear the remaining cortex. Sodium hyaluronate was injected into the anterior chamber, and a foldable intraocular lens (IOL) was inserted into the capsular bag. The last step was clearing the sodium hyaluronate by I/A, and the corneal incisions were closed by stromal hydration.
Compared with Group 1, there were 2 differences in the operation in Group 2 (phaco-chop). In steps 1 and 2, the phaco tip was buried in the centre of the nucleus with ultrasound power (Figure 3A-D).
The cumulative dissipated energy (CDE), best corrected visual acuity (BCVA), time to achieve maximum vision, corneal thickness variation, and time to return to the preoperative values (±20 μm according to the preoperative value) were recorded postoperatively. In the first week, all patients were examined every day, then at an interval of 2 or 3 days in the first month, followed by once every month thereafter.
The chi-square test and the independent-samples t-test were used to compare the groups for statistical significance. Data were analysed using SPSS software (version 13.0, International Business Machines Corp.). The level of significance was set to a P value of 0.05.