Analysing anterior segment parameters plays a significant role in choosing the appropriate PIOL size.[8–13].As we know,now most doctors choose lens size using WTW through online calculation system. The ICL lens size usually by adding the WTW distance with 0.5 to 1.0 mm.It may lead to cataract when choosing a smaller lens,
and increase the risk of angle-closure glaucoma when choosing a bigger lens. Inappropriate size can lead to toric lens instability,develop ocular hypertention,iritis,or lesions in the corneal endothelium[17, 18]. Theoritically more parameters like the ATA distance and ciliary STS diameter play great role in the design the size of lens.Many studies compared the horizontal WTW and vertical WTW values using different devices, the average values of the two were 11.5 ~12.5 mm[19–23] and 10.6 mm.[24, 25]
Our study showed that the horizontal WTW values of between 10.8 mm and 12.3 mm which just similar as previous studies[26, 27].Baikoff et alfound that in 74% of eyes, vertical ATA distance was at least 100 mm greater than the horizontal ATA distance,50% of eyes was greater more than 300 mm,but still controversial and need proved in further studies.In our study,we found that vertical STS distances to be higher than the horizontal values which also proved in previous studies. Oh et alfound the ciliary sulcus shape was vertically ovoid, which makes the issue more complicated. We found there was also a significant relationship between horizontal and vertical STS measured by UBM(y=0.9855x-0.0178,R² = 0.1979).This finding just suggest that the position of different implant lens in horizontal and vertical can be affected by the shape of sulcus,so we could regulate the vault by adjust the position of lens. Unfortunately, we did not use UBM to measure the position of the ICL haptic in the current study, which will need to be done in the future to confirm the possible relationship between the position of the ICL haptic and the shape affecting the vault.
Comparing WTW and ATA,Pin˜ero et al found that the WTW distance was bigger than the ATA distance, Baikoff found the WTW distance similar to ATA distance,Reinstein et al found the WTW distance to be smaller.Our data agree with the results of Reinstein et al.Aanterior segment parameters measured by different machines and different method of measurement all affect much to the results.In our study significant correlation was found between the WTW distance and the horizontal ATA distance ( y= 0.9605x±0.1491,R² = 0.9148), similar to results of Reinstein et al.
Different results for the relationship between ATA and WTW also been studied,Dinc et al found that WTW measured by IOLMaster 500 and WTW messured by Scheimpflug camera showed significant difference.But insignificant difference were found between ATA obtained from anterior segment OCT among WTW obtained by IOLMaster 500 or WTW messured by Scheimpflug camera.Dinc concluded that ATA can be used interchangeably with WTW.Nemeth et al found that WTW and ATA measured by partial coherence interferometry biometer had a quite weak correlation (r=0.51). He demonstrated that ATA cannot be used interchangeably with WTW.In our study,we agree with Nemeth since our results proved that Bland-Altman plots show statistically significant differences between AS-OCT ATA and Pentacam WTW with a mean difference close to zero(-0.66 mm). Despite the relatively high correlation (intraclass correlation co-efficient =0.689), the range of agreement is quite broad (1.33 mm).It was not clinically acceptable for designing the size of lens.(Figure 6). This indicates that ATA measured by AS-OCT and WTW measured by Pentacam cannot be used interchangeably.
Our study found that although there was a significant correlation between ACD the WTW distance, there was no correlation between ACD with hSTS and vSTS measured with the UBM device, no correlation between the ACD and the ATA distance measured with the AS-OCT device, just because the reproducibility of UBM is not very good,and all measured manually except WTW measured by the system automatically.
The results showed in our study,only when ACD was less than 3.5 mm there was a significant correlation between WTW and hSTS diameters.So when using WTW diameters to predict sulcus sizes may not be accurate especially ACD greater than 3.5mm.The ICL lens size are currently sized to the nearest 0.5 mm, with increasing ACDs, the potential error lead to lens size related complication such as cataract,glaucoma,or iris pigment dispersion also increase.Considerable variation between WTW diameter and ciliary STS diameter were also found in many studies[35–37].They concluded the reason for this variation is unclear, it may be affected by the light conditions、may be related to the basic information(age,refraction, eg),or the variation between WTW and ACD diameters.
There are several limitations.Firstly, UBM has poor repeatability and relies heavily on inspectors.So even experienced inspector might affect the result.Secondly, when the Scheimpflug camera automatically obtains the WTW value. The different light codition has an impact on the system measurement especially WTW defined as grey-scale photography of the eye and sclera-cornea boundary (limbus), the reflectivity of the tissue might affect results.Thirdly,samples were small,further studies still need to confirm our data.