The postoperative intraocular lens (IOL) performance is greatly influenced by its actual position in the eye, especially for premium ones, such as aspheric, toric, and multifocal IOLs. Holladay and Maverick1 pointed out that an effective lens position error of 0.23 mm would lead to a refractive error of 0.46 diopters (D). In addition, IOL decentration and tilt may cause postoperative glare, halos, loss of contrast sensitivity, and dysphotopsia, and thus impair postoperative optical quality overall.2–5
After cataract surgery, in-the-bag IOL fixation is completed through three processes.6 At the first step, the IOL is only supported by the haptics. Then, the anterior and posterior capsules gradually come into contact with the IOL and mechanically stabilize it. Finally, fibrous tissue and lens epithelial cells firmly adhere to the IOL, effecting final fixation. Thus, the IOL movement in the capsular bag in the early postoperative period is extremely critical for the long-term refractive outcomes and visual quality. Previous studies have reported that capsule bag shrinkage, anterior capsule fibrosis, surgical techniques, and IOL types are important factors affecting the IOL stability.7–13 Continuous curvilinear capsulorhexis is widely used in modern cataract surgery for its unique advantages. However, a well-centered capsulorhexis with appropriate size and total overlapping on the IOL optic edge is vital for the IOL stability through the symmetric capsule contraction force and contraction wrapping effect.14,15
Referring to in-the-bag IOL position, many studies have been done using the Scheimpflug method, the Purkinje method, and anterior segment optical coherence tomography (AS-OCT).16–20 Among these techniques, AS-OCT may be the most suitable device to quantify the IOL movement with its advantages of high resolution, excellent precision, and quick scanning speed. Moreover, based on the AS-OCT (a custom-developed OCT instrument) method, Marcos et al20 imaged the full anterior segment of the eye three-dimensionally and gave insights on the performance of implanted accommodating posterior chamber IOLs (Crystalens) on an accommodative stimuli. In our previous study,17 we used a commercially available SS-OCT device (Casia SS-1000; Tomey) to three-dimensionally reconstruct the full anterior segment of the eye and this new method presented high repeatability in measuring the capsule–IOL complex parameters.
To our knowledge, no study has observed the capsulorhexis and IOL movement postoperatively. Based on the three-dimensional method of SS-OCT, the current study aimed to demonstrate whether the IOL optic overlapping had a significant effect on the IOL stability and to determine the relationship between the capsulorhexis and IOL movement.
Patients and Methods
This study enrolled patients with age-related cataract ranging from grade 2 to grade 4 according to the Lens Opacities Classification System III staging system.21 The axial length was limited from 22 to 25 mm. The eyes recruited were divided into two groups according to the anterior capsule and IOL optic relationship: total anterior capsule overlap (360°, total overlap group) and partial anterior capsule overlap (< 360°, partial overlap group). The exclusion criteria were as follows: age younger than 40 years; axial length less than 22 mm or axial length greater than 25 mm; eye diseases including any corneal pathology, uveitis, and glaucoma; previous intraocular surgery; any intraoperative or postoperative complications, including any capsulorhexis problems such as capsule tear or rupture; dilated pupil diameter less than 6 mm; and failure to present for examinations or follow-up. Before inclusion in our study, all eyes underwent a complete examination including slit-lamp microscopy, noncontact tonometry, optical biometry (IOLMaster 5.0; Carl Zeiss Meditec AG), and fundus examination after pupil dilation.
This study was conducted at the Eye Hospital of Wenzhou Medical University, People's Republic of China, from June 1, 2017, to May 31, 2019. The research protocol was in accordance with the tenets of the Declaration of Helsinki and was approved by the Office of Research Ethics, Eye Hospital of Wenzhou Medical University. Written informed consent was obtained from each patient.
Surgeries and IOLs
Phacoemulsification was performed in all eyes and the IOL was implanted through a 3-mm clear corneal incision. Central continuous curvilinear capsulorhexis was performed with a diameter of approximately 5 mm. The IOL was implanted into the capsule bag; all IOLs were one-piece square-edge hydrophobic AcrySof series (Alcon Laboratories, Inc.) with a length of 13 mm. The optic of the IOL was biconvex and the diameter was 6 mm. Finally, the ophthalmic visco-surgical device (medical sodium hyaluronate gel) was completely removed.
The Fourier-domain SS-OCT device (Casia SS-1000; Tomey) is a commercially available SS-OCT with a swept-source laser wavelength of 1,310 nm. It scans with a high speed of 30,000 A-scans/second and 512 lines A-scan/image sampling. Each three-dimensional radial scanning session takes only 2.4 seconds and 128 cross-sectional images of the anterior segment are captured.
SS-OCT Imaging and Three-Dimensional Reconstruction
All patients were seated with the headrest and chin-rest after pupil dilation of at least 7 mm with 1% tropicamide and 2.5% phenylephrine hydrochloride. They were then asked to focus on an internal fixation target. In addition, participants were instructed to pull down the lower eyelid against the lower orbital rim to expose the lower limbus while the technician elevated the upper eyelid against the upper orbital rim to expose the upper limbus, to exclude any eyelid artifact. Once the patient had been optimally positioned, each eye was scanned with the radial three-dimensional angle analysis scan. This high-speed scanning took only 2.4 seconds and 128 cross-section tomograms of the anterior segment were obtained. During the scanning, no eye movement was allowed.
Three standard SS-OCT scans were performed by the same experienced operator (XD) on each eye at 1 day, 1 week, 1 month, and 3 months postoperatively. The best one (all of the images should be of high quality or the three-dimensional construction will fail) was chosen for analysis. Then the 128 images obtained by each scan were used for three-dimensional reconstruction and analysis using the custom-built software reported in our previous study.17 The procedure of three-dimensional analysis was as follows.17 First, the boundaries of the cornea and IOL were detected through a semi-automatic algorithm. Second, the three-dimensional data of radical scanning were interpolated and resampled and the surfaces were fitted by the Zernike polynomial. Then all parameters were calculated after optical distortion correction. The inner edge of the iris was set manually and fitted by an ellipse based on the least-squares method. Thus, the orientation and shape of the pupil plane can be obtained. Then the pupil axis was defined as the vector that was perpendicular to the pupil plane and the pupil center was treated as the origin. The IOL axis was then defined as a vector from the anterior IOL center to the posterior IOL center. In addition, anterior capsulorhexis was achieved by ellipse fitting from the positions that were extracted from the inner edges of the anterior capsule. After 16 images were integrated through semi-automatic drawing, the three-dimensional anterior segment of the eye was reconstructed. Finally, the anterior segment parameters were all output automatically.
The anterior capsule and IOL optic relationship was defined as total anterior capsule overlap (360°) or partial anterior capsule overlap (< 360°). Partial overlap was confirmed when the anterior capsule failed to overlap the IOL optic edge (exposing) on one or more images. Total overlap was confirmed when the IOL optic edges overlapped on all 128 SS-OCT images (Figure A, available in the online version of this article).
The intraocular lens optic edges overlapping and exposing.
The parameters of IOL stability (details in Table A, available in the online version of this article) in both groups were output by the special analysis software including postoperative aqueous depth, IOL tilt, and horizontal and vertical decentration of the IOL (IOLDecen-X, IOL-Decen-Y).
Parameters Definition After Three-Dimensional Construction of Anterior Segment
Anterior capsulorhexis in eyes with partial overlap could not be three-dimensionally reconstructed because the anterior capsule always hid behind the iris despite the pupil being well dilated (Figure 1). Therefore, these parameters indicate that the anterior capsulorhexis could only be measured in the total overlap group rather than the partial overlap group. The parameters of the anterior capsule opening (details in Table A) were output by the special analysis software including horizontal and vertical decentration of the anterior capsule opening (Cap-Decen-X, Cap-Decen-Y) and IOL-anterior capsule opening (Cap-IOL-Decen-X, Cap-IOL-Decen-Y).
The right side of the anterior capsule was completely shielded by the iris in an eye from the partial anterior capsule overlap (< 360°) group.
Statistical analysis was performed using SPSS software for Windows version 19.0 (SPSS, Inc). Data are presented as mean ± standard deviation. Differences between the two groups were tested by the independent sample t test. The chi-square test was used to compare categorical data. For comparison of the differences at different postoperative time points, repeated measures analysis was used. A P value of less than .05 was considered statistically significant.
The current study included 46 eyes of 34 patients divided into two groups: total anterior capsule overlap (360°, the total overlap group, n = 29) and partial anterior capsule overlap (< 360°, the partial overlap group, n = 17). In this study, it was found that the majority of IOL optic edges were still overlapped by the anterior capsule and the exposing range was smaller than three clock hours in all eyes in the partial overlap group. The details of the two groups are shown in Table 1. Table 2 shows the postoperative decentration of the IOL and capsulorhexis.
Profile of Eyes in the Study
Decentration of IOL and Capsulorhexis (mm)
Figure 2 shows that postoperative aqueous depth significantly decreased in the first week after surgery (P < .001) and then remained stable for 3 months (all P > .05). However, there was no difference of postoperative aqueous depth between the two groups (P = .499).
The postoperative aqueous depth (PAD) changes in Group-T (total anterior capsule overlap, 360°) and Group-P (partial anterior capsule overlap, < 360°) at 1 day, 1 week, 1 month, and 3 months. Symbols and bars represent means and 95% confidence intervals, respectively. *The changes were statistically significant.
Figure 3 shows that IOL tilt remained relatively stable during 3 months postoperatively (all P > .05), but it was much greater in the partial overlap group than in the total overlap group (P = .014).
The intraocular lens (IOL) tilt changes in Group-T (total anterior capsule overlap, 360°) and Group-P (partial anterior capsule overlap, < 360°) at 1 day, 1 week, 1 month, and 3 months postoperatively. Symbols and bars represent for means and 95% confidence intervals, respectively.
The IOL moved significantly from the inferior nasal to the superior temporal quadrant in the first week postoperatively in both groups (IOL-Decen-X, P < .001; IOL-Decen-Y, P < .001) (Figure 4). After 1 week, the IOL movement horizontally and vertically was slight and there was no statistical difference (all P > .05). Additionally, horizontal IOL movement was significantly greater in the partial overlap group than that in the total overlap group (P = .024) and vertical IOL movement was comparable in both groups (P = .137).
The intraocular lens (IOL) movements (IOL-Decen changes) in the x and y axes in Group-T (total anterior capsule overlap, 360°) and Group-P (partial anterior capsule overlap, < 360°) at 1 day, 1 week, 1 month, and 3 months postoperatively. Symbols represent means. *The changes were statistically significant. IOL-Decen = IOL decentration relative to the pupil center
The anterior capsule openings of the partial overlap group cannot be three-dimensionally reconstructed for the iris shield in the positions. The IOL and capsulorhexis of the total overlap group (n = 29) both sharply moved from the inferonasal to superotemporal quadrant in the postoperative 1 week, respectively (IOL-Decen-X, P = .001; IOL-Decen-Y, P < .001; Cap-Decen-X, P = .010; P < .001) (Figure 5). After that, they moved slightly back toward the inferior nasal direction for 1 month, but there were no significant differences (all P > .05) except the movement of capsulorhexis horizontally (Cap-Decen-Y, P < .001). From 1 to 3 months after surgery, both the IOL and capsulorhexis remained stable (all P > .05). Obviously, the IOL moved extremely steadily with capsulorhexis during 3 months postoperatively. Figure 6 shows the decentration of the IOL and capsulorhexis was small and steady in the 3 months after surgery (all P > .05 for Cap-IOL-Decen-X and Cap-IOL-Decen-Y).
The capsulorhexis and intraocular lens (IOL) movements (Cap-Decen & IOL-Decen changes) in the x and y axes in Group-T (total anterior capsule overlap, 360°) at 1 day, 1 week, 1 month, and 3 months postoperatively. Symbols represent means. *The changes were statistically significant. Cap-Decen = capsulorhexis decentration relative to the pupil center; IOL-Decen = IOL decentration relative to the pupil center
The capsulorhexis movement relative to IOL (Cap-IOL-Decen changes) in the x and y axes in Group-T (total anterior capsule overlap, 360°) at 1 day, 1 week, 1 month, and 3 months postoperatively. Symbols represent means. IOL = intraocular lens; Cap-IOL-Decen = capsulorhexis decentration relative to the IOL center
With the increasing application of toric and multi-focal IOLs, more patients are becoming satisfied with the excellent visual quality after surgery. However, there are still some visual complaints that cannot be explained clearly. To our knowledge, many visual disturbances can be attributed to postoperative IOL position, especially in eyes with multifocal IOLs. In addition, significant changes in kappa and alpha angles may play an important role in the visual quality. Thus, it may be necessary to measure the IOL movement and true position in the capsular bag after the surgery.
Based on the new method using SS-OCT, this is the first study to investigate the effect of anterior capsule overlap on the stability of the IOL and the relative movement of the IOL and anterior capsule opening during 3 months postoperatively.
The postoperative axis movement of IOLs results from many factors, including fibrotic reaction of the capsular bag and mechanical characteristics of the IOL in the capsular bag.11,12,22,23 In the current study, postoperative aqueous depth of the two groups decreased consistently in the first week, which was in line with the study by Petternel et al.24 There was no difference in IOL axis location between eyes with or without total overlap in this study. The reason might be that the majority of IOL optic edges were still overlapped by the anterior capsule and the exposing range was smaller than three clock hours in all eyes in the partial overlap group. Therefore, this limited asymmetric traction of capsule shrinking was not enough to significantly influence the IOL axis position. Additionally, in accordance with previous literature,25–27 this study found that postoperative aqueous depth became much more stable after the first week. Thus, this result might provide a theoretical basis for an appropriate time point of postoperative spectacle prescription.
Previous studies28–30 demonstrated that the partial anterior capsule overlap might lead to IOL tilt or decentration. The asymmetrical force of capsular bag shrinking would play an important role in IOL instability in the capsular bag. Guyton et al31 reported that the visual quality can be impaired when IOL tilt is greater than 5°. There was a significant correlation between IOL tilt and ocular coma-like aberrations, and the quality of the retinal image improved when the IOL tilt was reduced.3,32 In addition, Altmann33 pointed out that the aspheric advantage can be lost in the case of IOL decentration greater than 0.5 mm. Holladay et al34 indicated that the performance of aspheric IOLs was worse than spherical IOLs when decentration was greater than 0.4 mm. Therefore, the IOL tilt and decentration of both groups in this study were so small they had no significant effect on the visual quality of aspheric IOLs.
In this study, the IOL tilt was relatively stable in the early postoperative term of 3 months, but the tilt was much greater in the partial overlap group than in the total overlap group within 3 months. Therefore, the asymmetric capsulorhexis determined the IOL tilt during the early term rather than the subsequent limited asymmetric traction of capsule shrinkage in this study.
We also found that the IOL centers of both the total overlap group and the partial overlap group moved sharply from the inferonasal to the superotemporal position during the first week after surgery and then both became relatively stable. IOL decentration in the x-axis was much greater in the partial overlap group than that in the total overlap group, which was consistent with the fact of asymmetric traction. However, the rules of IOL movements were shown to be the same in both groups regardless of total overlap. Even the location of exposure was not associated with the rules of IOL movements. The underlying reasons for it are still unknown and future work is required to discover them.
Moreover, this study was the first to analyze the movement of the IOL and anterior capsule opening at the same time in eyes with total anterior capsule overlap on the three-dimensional level. We found that both the centers of the IOL and the anterior capsule opening moved together and remained stable from 1 day to 3 months postoperatively. Therefore, we hypothesized that the IOL movement was a type of following movement driven by the anterior capsule opening changes induced by the capsular bag shrinkage.
There are several limitations in the current study. First, to eliminate confounding factors, only one type of IOL was investigated. Although this study has provided some useful information for cataract surgeons, it was a small sample because the postoperative pupils of many patients could not be dilated up to 6 mm. Thus, another prospective study with a large number of different types of IOLs should be conducted. Second, the current study solely involved eyes with normal axial length, excluding eyes with short and long axial lengths for the capsular bags. The zonule fibers varied with axial lengths and IOL position may be obviously influenced by the axial length of eye. Therefore, the effects of different axial lengths on the IOL stability should be compared in the next study. The long-term visual quality should also be evaluated to analyze the effects of IOL displacement.
With the new three-dimensional method of SS-OCT, we found IOL movement in the capsular bag to be statistically significant in the first week after cataract surgery and then it became steady. The eyes with total anterior capsule overlapping presented much better IOL centrality and stability than those with partial overlapping. In addition, IOL movement was consistent with capsulorhexis, which may have been driven by the capsular bag contraction and fibrosis. Therefore, it has also been proven that proper size of the central continuous curvilinear capsulorhexis is vital for postoperative IOL position and IOL performance.
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Profile of Eyes in the Study
|Parameter||Total Overlap Group (n = 29)||Partial Overlap Group (n = 17)||P|
|Age (years)||63 ± 11.7||67 ± 11.9||.349|
|IOP (mm Hg)||12.1 ± 2.2||11.1 ± 3.2||.224|
|Endothelial cells (/mm2)||2,332.1 ± 477.9||2,337.2 ± 214.2||.968|
|AL (mm)||23.6 ± 0.75||23.8 ± 0.62||.417|
|IOL (D)||20.30 ± 1.90||21.00 ± 1.00||.171|
Decentration of IOL and Capsulorhexis (mm)
|Group||Mean||SD||95% CI Lower||95% CI Upper|
|IOL-Decen-X (Group-T, n = 29)|
| 1 day||0.081||0.148||0.025||0.138|
| 1 week||−0.029||0.146||−0.085||0.027|
| 1 month||−0.019||0.196||−0.093||0.056|
| 3 months||0.014||0.252||−0.081||0.110|
|IOL-Decen-Y (Group-T, n = 29)|
| 1 day||−0.105||0.235||−0.195||−0.016|
| 1 week||0.061||0.159||0.000||0.122|
| 1 month||0.005||0.245||−0.088||0.098|
| 3 months||0.030||0.207||−0.049||0.109|
|IOL-Decen-X (Group-P, n = 17)|
| 1 day||0.180||0.191||0.082||0.279|
| 1 week||0.091||0.160||0.009||0.174|
| 1 month||0.128||0.203||0.024||0.233|
| 3 months||0.087||0.143||0.014||0.161|
|IOL-Decen-Y (Group-P, n = 17)|
| 1 day||−0.052||0.140||−0.124||0.020|
| 1 week||0.124||0.122||0.061||0.187|
| 1 month||0.111||0.148||0.035||0.186|
| 3 month||0.067||0.104||0.013||0.120|
|Cap-Decen-X (Group-T, n = 29)|
| 1 day||0.066||0.143||0.012||0.121|
| 1 week||−0.027||0.133||−0.078||0.024|
| 1 month||0.001||0.124||−0.046||0.048|
| 3 months||0.029||0.125||−0.018||0.076|
|Cap-Decen-Y (Group-T, n = 29)|
| 1 day||−0.071||0.155||−0.130||−0.012|
| 1 week||0.070||0.157||0.011||0.130|
| 1 month||−0.016||0.135||−0.068||0.035|
| 3 months||0.003||0.162||−0.059||0.064|
|Cap-IOL-Decen-X (Group-T, n = 29)|
| 1 day||−0.015||0.203||−0.092||0.062|
| 1 week||0.002||0.186||−0.069||0.073|
| 1 month||0.020||0.225||−0.066||0.106|
| 3 month||0.015||0.301||−0.100||0.129|
|Cap-IOL-Decen-Y (Group-T, n = 29)|
| 1 day||0.034||0.236||−0.055||0.124|
| 1 week||0.010||0.197||−0.065||0.085|
| 1 month||−0.021||0.268||−0.123||0.081|
| 3 month||−0.027||0.243||−0.120||0.065|
Parameters Definition After Three-Dimensional Construction of Anterior Segment
|PAD||Postoperative aqueous depth||The distance from central corneal endothelium to the anterior surface of the IOL|
|IOL-Decen-X||IOL decentration in the x-axis||The displacement of IOL center relative to the pupil center in the x-axis|
|IOL-Decen-Y||IOL decentration in the y-axis||The displacement of IOL center relative to the pupil center in the y-axis|
|IOL-Tilt||IOL tilt||The angle of IOL axis and pupil axis|
|Cap-Decen-X||Anterior capsulorhexis decentration in the x-axis||The displacement of anterior capsulorhexis center relative to the pupil center in the x-axis|
|Cap-Decen-Y||Anterior capsulorhexis decentration in the y-axis||The displacement of anterior capsulorhexis center relative to the pupil center in the y-axis|
|Cap-IOL-Decen-X||Anterior capsulorhexis-IOL decentration in the x-axis||The displacement of anterior capsulorhexis center relative to the IOL center in the x-axis|
|Cap-IOL-Decen-Y||Anterior capsulorhexis-IOL decentration in the y-axis||The displacement of anterior capsulorhexis center relative to the IOL center in the y-axis|