The lens thickness inspection prevents an erroneous interpretation of a posterior capsule
FLACS was performed for 1070 eyes of 734 patients in this sequential study. Three hundred and seventy-seven were men and the mean age was 65.86 ± 12.71 (range 17.7–94.1). The average axial length of all patients was 24.85 ± 2.10 mm (range 21.28–34.03) as measured by IOL Master.
First, we classified all cases into three groups based on the relationship between the lines defining the PC and the intraoperative OCT images (Fig. 1). Among the total 1070 patients, 1047 cases (97.9%) had an appropriate PC line. In nineteen cases, the PC was not clearly visualized due to mature cataracts and they belonged to the undescribed group (1.8%). Second, we classified all eyes into two groups the day after PCR due to laser irradiation since we started the lens thickness inspection, which involves comparing the lens thickness acquired with other devices preoperatively to the intraoperative measurement (Fig. 2). The cases until PCR were classified as the No inspection group (474 eyes). The appropriate PC line was found in 459 (42.9%) cases and another 11 eyes (1.0%) were undescribed PC line because of mature cataracts. The remaining 4 cases had an inappropriate PC line (0.4%). The LT inspection group after the case of PCR had 596 eyes accordingly. Five hundred and eighty-eight (55.0%) of them had an appropriate PC line and the other 8 eyes had an undescribed PC line (0.7%). No one had an inappropriate PC line in the LT inspection group.
There was no significant difference in the rate of the undescribed PC line to the visualization between the No inspection and the LT inspection groups (11/463 eyes, 2.4% in the No inspection, 8/588 eyes, 1.4% in the LT inspection, respectively; p = 0.25, Fisher exact test). On the other hand, the rate of the inappropriate PC line in the No inspection group (4/459 eyes, 0.9%) was significantly higher than that in the LT inspection group (0/588 eye) (p = 0.03, Fisher exact test).
Comparison of intra- and pre-operative biometric measurements for lens thickness
Inspection of the lens thickness before femtosecond laser irradiation reduced the ratio of the inappropriate PC line. To evaluate the impact of LT inspection, we depicted scatter plots of the lens thickness measured by IOL Master, CASIA2®, and CATALYS®, respectively (Fig. 3). Although the lens thickness measured by each device was highly correlated, the lens thickness measured by CATALYS® with the No inspection (red dots, Fig. 3A) was slightly dispersed (p = 0.90). The mean lens thickness acquired with IOL Master, CASIA2®, and CATALYS® was 4.39 ± 0.54 mm (range 1.71–6.39), 4.42 ± 0.52 mm (range 2.79–6.11), and 4.48 ± 0.56 mm (range 2.5–5.9), respectively.
In the No inspection group, there was a significant difference among the lens thickness by these three biometric measurements (p < 0.001, Kruskal-Wallis test). The lens thickness measured by CATALYS® was significantly higher than those measured by others (IOL Master versus CATALYS®: p = 0.003, CASIA2® versus CATALYS®: p = 0.002, Steel-Dwass test). In the LT inspection group, there was also a significant difference between the lens thicknesses measured by IOL Master and CATALYS® (p = 0.0013, Kruskal-Wallis test). However, there was no significant difference between the lens thicknesses measured by CATALYS® and CASIA2® (p = 0.063). To summarize, inspection of the lens thickness reduced the difference between the lens thicknesses as measured by CATALYS® and CASIA2®. The lens thickness measured by CATALYS® was significantly higher than that measured by IOL master.
Cases of inappropriate PC line and a possible case to avoid PCR
Four cases were classified as the inappropriate PC line in this study. All of them were performed without the inspection of lens thickness. In three of the four cases, the intraoperative lens thickness was higher than that of the preoperative lens thickness. Hence, all these three cases had a risk of PCR due to laser irradiation (Fig. 4). In fact, the 474th case had the complication of PCR due to laser irradiation10.
After experiencing this complication, we started comparing lens thicknesses between intra- and pre-operative measurements before femtosecond laser irradiation. We encountered a possible case to avoid the complication of PCR by LT inspection (the 1029th case). In this case, the high OCT intensity area behind the PC misled the femtosecond laser system into defining the inappropriate PC line at first. Because of the lens thickness comparison, the surgeon immediately noticed the inappropriate PC line and manually adjusted the PC line to correct the segment on the OCT image (Supplementary Video S1).