Accurately predicting the ELP after phacoemulsification is still difficult, but it plays an important role in reducing the postoperative refractive error. Measurement errors of AL, ACD and LT are important factors contributing to errors in ELP estimation [9]. In this paper, we compared the differences between Lenstar and A-scan ultrasonography in measuring the AL, ACD and LT of cataract patients, and analyzed the possible causes of errors when the US and the OLCI are inconsistent in measuring LT.
According to previous reports, there has been no significant statistical difference between Lenstar and A-scan US in the measurement of LT [10, 11]. In this study also found no significant statistical difference in the measurement of LT by the two different techniques. Bland-Altman analysis showed that the two techniques had good consistency in LT measurement, which was consistent with previous studies. It thus seems that the measurement of LT by these two methods could be clinically interchangeable [12]. For the measurement of AL and ACD, there were large differences between the two techniques. Bland-Altman analysis also showed that the agreement of AL and ACD measurements was poor, which was consistent with previous studies [13, 14]. The reason for this difference could be the difference in the detection principle of the two instruments. Lenstar, an optical biometry instrument based on the principle of low coherent-light reflection, uses lasers to reach the surface of each ocular structure along the visual axis that are then reflected back separately. After receiving the data, the detector analyzes and processes it. Lenstar can complete the biological measurement of the ocular structure and observe the anterior and posterior surfaces of the lens and the density of the lens clearly, with high resolution and many repetitions [15–17]. A-scan US is a traditional biological measurement method, mainly based on the echo reflection principle of ultrasound to obtain the measurement values of ocular tissues. The calculation methods of AL and ACD are also different between the two technologies. Lenstar uses the interference degree of light to take measurements and conduct linear processing, and calculated through an A-scan algorithm, while US used segmented sound velocity to measure each tissue.
In order to explore the effects on LT measurement of the difference between AL and ACD as measured by the US and the OLCI, we further studied the individuals beyond the 95% consistency limit of AL and ACD, and assessed their agreement in LT measurement by a Bland-Altman analysis. The results showed that when the difference of AL and ACD measured by the two techniques was large, the consistency of LT was poor. This difference has a certain influence on the calculation of IOLs because it would have led to a different IOL power selection when using a newer formula, which predicts the ELP based on LT, such as Olsen or Barrett Universal II formulas [18]. During the operation of the contact US, when the ultrasonic probe was tilted and not along the visual axis, a larger result was often obtained, while a smaller result was often due to the compression of the ultrasonic probe on the eyeball. Our results showed that when ALUS was greater than ALOLCI or ACD US was greater than ACD OLCI, Bland-Altman analysis showed a poor agreement of LT measurement, suggesting that the tilt of the ultrasonic probe can affect the measurement of AL, ACD and LT. When ALUS was smaller than ALOLCI or ACD US was smaller than ACD OLCI, Bland-Altman analysis showed that the measurement of LT had good consistency between the two techniques, suggesting that when the ultrasound probe pressed the eyeball, the measurement of AL and ACD would be affected, but the measurement of LT was not affected.
Compared with samples from related articles published earlier, our study had a larger sample size. Most of the previous studies only compared the consistency of AL, ACD, and LT measurements between different instruments, but few studies paid attention to individuals with poor consistency. Little literature has reported the impact on LT measurement when there were large differences in AL and ACD measurements taken by different instruments or on the influence of deviation during ultrasonic operations on LT measurements. However, the current study also has limitations, such as its exclusive focus on cataract patients, and the degree of opacification of cataract may induce a certain deviation in A-scan results.