The Visian Implantable Collamer Lens (Staar Surgical Co.) is a foldable phakic intraocular lens (pIOL) designed to be placed in the posterior chamber behind the iris with the haptic zone resting on the ciliary sulcus.1 To prevent damage to intraocular tissues and achieve a stable refractive outcome, pIOLs must maintain a safe distance from the corneal endothelium and the crystalline lens.2 A pIOL is more likely to come in contact with the crystalline lens in the peripheral area as a result of the pIOL’s design, which tends to be thicker at the optic–haptic junction.3 The leads to an increased risk for cataract formation. These factors make clinical assessment of peripheral vault and long-term follow-up an important part in the evaluation of safety of pIOL implantation. However, the long-term, accurate behavior of peripheral vault has not been fully elucidated.
Ultrasound biomicroscopy (UBM) is the most ideal method for visualizing peripheral vault because anterior segment optical coherence tomography and other optical devices are not able to penetrate the iris pigment epithelium; thus, they cannot evaluate the structures behind the iris.4 The purpose of the present study was to use UBM to evaluate longitudinal changes in central vault, peripheral vault, and the endothelium–anterior pIOL distance in patients with pIOLs.
Patients and methods
All eyes in this study had implantation of a pIOL (ICL V4 model) for myopia by the same surgeon (Y.S.) at the Department of Ophthalmology, First Affiliated Hospital, College of Medicine, Zhejiang University, from December 2008 to September 2010. At the time of surgery, patients were fully informed of the details and possible risks of the surgical procedure. All patients provided written informed consent before surgery in accordance with the tenets of the Declaration of Helsinki. The university’s ethics committee approved the study.
The inclusion criteria for pIOL implantation were age between 21 years and 45 years, stable refraction, corneal astigmatism lower than 1.00 diopter (D), a clear lens, and at least 24 months of follow-up. Exclusion criteria were keratoconus, refractive surgery, glaucoma, cataract, uveitis, synechiae, anterior chamber depth (ACD) less than 2.8 mm, and angle depth less than grade II (20 degrees) by the Spaeth grading system.5,6
Preoperative Examination and Phakic Intraocular Lens Power
The preoperative examination included visual acuity, manifest and cycloplegic refractions, keratometry, corneal topography, pachymetry using scanning-slit corneal topography (Orbscan II, Bausch & Lomb), endothelial cell count (ECC), A-scan ultrasonography, slitlamp microscopy, applanation tonometry, and dilated indirect fundoscopy. The pIOL diameter was individually determined based on the horizontal white-to-white (WTW) distance and the ACD measured with the scanning-slit corneal topography system following the manufacturer’s recommendations (ie, adding 0.5 mm or 1.0 mm to WTW depending on the ACD). Power calculation for the pIOL was performed using the software provided by the pIOL manufacturer and a modified vertex formula.7
Preoperatively, 2 peripheral iridectomies were created with a neodymium:YAG laser. The surgical technique has been described in detail.8
Postoperative examinations were scheduled at 1 day, 1 week, and 1, 3, 6, 12, 18, and 24 months. The evaluations included visual acuity, manifest refraction, applanation tonometry, ECC, slitlamp microscopy, and UBM.
Ultrasound biomicroscopy was performed by the same examiner (Y.W.) using the SW-3200L full-scale 50 MHz digital system (Tianjin Suowei Electronic Technology Co. Ltd.). All patients were scanned in the supine decubitus position under standard illumination with nonaccommodation. Topical anesthesia of oxybuprocaine hydrochloride 0.4% was administered, and an eyecup filled with sterile normal saline was used. The patient was asked to fixate on a target located on the ceiling with the fellow eye to maintain fixation. For central measurements, a central section of the anterior chamber was taken through the corneal apex. It was centered on the pupil and included the pIOL and the anterior crystalline lens surface.4
As described by Du et al.,4 the following parameters were obtained from each examination with the calipers provided by the manufacturer9,10:
- Endothelium–anterior pIOL distance. This distance was measured between the first reflection of the ultrasound (US) between the aqueous humor and the corneal endothelium and the first reflection of the pIOL surface at the horizontal meridians of the central section (Figure 1).
- Central vault. This measurement comprised the central distances at the horizontal meridians between the posterior reflection of the pIOL and the first reflection of the crystalline lens surface within a 1.0 mm radius from the pupillary center (Figure 1).
- Peripheral vault. This measurement was obtained along the major axis on which the bilateral haptic footplates were located at the temporal sulcus and nasal sulcus. To perform the measurements, a line was drawn from the scleral spur parallel to the pupillary margin of iris and a perpendicular line was drawn through the midpoint. By following this last line, the measurement was performed between the final reflection of the US from the pIOL and the first reflection from the anterior capsule of the crystalline lens (Figure 2).
Statistical analysis was performed using SPSS software (version 16.0, SPSS, Inc.). Descriptive statistics were obtained. Visual acuity changes postoperatively were described as percentages. The spherical equivalent refraction, intraocular pressure (IOP), endothelial cell density (ECD), and the UBM parameters were expressed as the mean ± standard deviation and range, the latter indicating the minimum and maximum values. The Mann-Whitney U test and the least-significance-difference (LSD) test were used for quantitative data analysis; a P value less than 0.05 was considered statistically significant.
The study enrolled 32 eyes of 31 patients. No patient had preoperative astigmatism greater than 1.00 D. Table 1 shows the patients’ baseline demographic data and 24-month postoperative results for outcomes. All the patients had uneventful surgery. Six eyes (9.7%) had no change in corrected distance visual acuity, 24 eyes (38.7%) gained 1 line, 15 eyes (24.2%) gained 2 lines, 15 eyes (24.2%) gained 3 lines, 2 eyes (3.2%) gained 4 lines, and no eye lost 1 line or more. There were no significant differences between preoperative and 24-month postoperative IOP (mean 15.66 ± 3.13 [SD] versus 15.71 ± 3.13 mm Hg; P=.078) or ECD (mean 2810.92 ± 304.43 cells/mm2 versus 2756.75 ± 301.75 cells/mm2; P=.069). No pigmentary glaucoma, pupillary block, cataract, or other vision-threatening complications occurred during the follow-up.
Changes in the Endothelium–Anterior Phakic Intraocular Lens Distance
Table 2 and Figure 3 show the mean values and ranges of the postoperative endothelium–anterior pIOL distance over time. The mean endothelium–anterior pIOL distance increased from 1 month to 3 months; however, the difference was not significantly different (P=.454, LSD test). After 3 months, there was a trend toward a decrease in the endothelium–anterior pIOL distance over time; however, multiple comparisons showed no significant differences between any 2 periods (all P > .05).
Table 3 shows the mean change in the endothelium–anterior pIOL distance between 2 consecutive measurements over time. An increase in endothelium–anterior pIOL distance occurred between 1 month and 3 months, and this change was statistically significant when compared with the change between 3 months and 6 months (P=.000). Thereafter, the changes between intervals were not statistically significantly different (all P > .05).
Changes in Central Vault
Table 2 and Figure 4 show the mean values and ranges for postoperative central vault over time. There was a trend toward a decrease in central vault over time. Multiple comparisons showed statistically significant differences between 1 month and 12 months (P = .015), between 1 month and 18 months (P=.007), and between 1 month and 24 months (P=.003); there were no statistically significant differences between any other 2 periods (P > .05 for each).
Table 4 shows the mean change in central vault between 2 consecutive measurements over time. The largest change occurred between 1 month and 3 months, and this change was statistically significant when compared with the change between 3 months and 6 months (P=.009). Thereafter, the changes between intervals were not statistically significantly different (all P > .05).
Changes in Peripheral Vault
The temporal peripheral vault and the nasal peripheral vault were not statistically significantly different at any time period (P > .05). For example, at 12 months the mean temporal vault was 0.258 ± 0.173 mm (range 0.00 to 0.830 mm) and the mean nasal peripheral vault was 0.241 ± 0.180 mm (range 0.000 to 0.670 mm) (P=.590). Therefore, the values of nasal peripheral vault were used to represent the peripheral vault. Table 2 and Figure 4 show the mean values and ranges for postoperative central vault over time. There was a trend toward a decrease in the peripheral vault over time, although multiple comparisons showed no statistically significant differences between any 2 periods (all P > .05).
The peripheral vault was 0.0 mm in 1 eye at the 1-month examination and 3-month examination and in 3 eyes at the 6-month examination, 12-month examination, and examinations thereafter. The 3 eyes did not develop anterior subcapsular cataract until the time of writing this paper. Also, the central vault in the 3 eyes was 200 μm or less, and the initial peripheral vault 1 month after surgery was less than 100 μm.
Table 5 shows the mean change in peripheral vault between 2 consecutive measurements over time, and this change was statistically significant when compared with the change between 1 month and 3 months (P=.000). Thereafter, the changes between intervals were not statistically significantly different (all P > .05).
We observed a mean increase in endothelium–anterior pIOL distance of 30 ± 50 μm between 1 month and 3 months postoperatively, although the difference was not statistically significant (P=.454). After 3 months, there was a trend toward a decrease in the endothelium–anterior pIOL distance over time, although multiple comparisons showed no statistically significant differences between any 2 periods (P > .05 for each). Several reasons might explain this phenomenon. First, the pIOL footplates might be supported by the ciliary body rather than in the ciliary sulcus.11 Second, the dynamic interactions between the pIOL and the back surface of the iris during accommodation or iris movement might help the pIOL settle back toward its definitive position near the sulcus.12 On the other hand, an age-related increase in ciliary muscle anteroposterior thickness in phakic patients aged 2 to 91 years was reported by Strenk et al.13 This might somewhat affect the position of the pIOL over time, theoretically by a forward shift of the pIOL, and may explain why the endothelium–anterior pIOL distance decreases in the later periods after pIOL implantation.
In this study, we also evaluated longitudinal changes in central vault and peripheral vault after pIOL implantation. The largest change occurred between 1 month and 3 months (mean 47 ± 17 μm and 21 ± 14 μm, respectively) (P=.009 and P=.000, respectively). This agrees with results in a study by Alfonso et al.,14 who observed a trend toward a decrease in mean subjective central vault over time. However, almost all previous studies evaluated central vault and not peripheral vault. In our study, we found that the central vault decreased with time. The peripheral vault was decreased, but to a smaller extent.
Several reasons might explain the changes in vault over time that occurred in our study. As we know, a thicker crystalline lens may contribute to anterior protrusion of the front surface of the crystalline lens15 and thus change the amount of vault.16,17 In addition, dynamic factors, including accommodation and pupil constriction, might affect the lens vault in the posterior chamber. Petternel et al.12 report a significant reduction (−73 ± 50 μm) in vault under photopic conditions, which is supposed to reflect the effect of pupil miosis on Implantable Collamer Lens position. Du et al.4 confirmed a significant decrease of central vault after induced pharmacologic accommodation. They also found a significant reduction in peripheral vault during pilocarpine-induced accommodation, although the changes were not significantly different from those in the control group. The explanation for this result was that the steepening of the peripheral area was smaller relative to the central area. This might explain the slighter decrease in peripheral vault than central vault in our study. In addition, using 50 MHz UBM, we found that not all the pIOL haptics were placed ideally in the ciliary sulcus; some were located at the ciliary body and in contact with the zonular fibers. This finding is in line with what Choi et al.11 reported. It is believed that the zonular fibers in highly myopic eyes are structurally weak because of the stretching of the fibers after globe elongation with no proportional change in crystalline lens size.18 This, combined with posterior chamber pIOL (PC pIOL) implantation, may lead to zonular dehiscence19 and spontaneous dislocation of the PC pIOL. This might cause a decrease in the peripheral vault.
Although the pathogenesis of cataract development, except for surgical trauma, has not been fully elucidated, it is thought to involve direct physical contact between the pIOL and the crystalline lens or malnutrition of the lens resulting from poor circulation of the aqueous humor.15 Furthermore, low pIOL vault was thought to be the most important factor.3 Findings in a study by Gonvers et al.20 suggests that a central vault greater than 0.09 mm protects the crystalline lens from cataract formation. Long-term observation showed that a minimum central vault of 230 μm is necessary to ensure total clearance of the Implantable Collamer Lens.16 Choi et al.11 describe the ideal pIOL vault as between 250 μm and 750 μm. In our study, the mean central vault was 0.505 ± 0.205 mm at 24 months, much greater than these recommended values. This might explain why no lens opacification occurred in our study.
The Implantable Collamer Lens (model V4) thickness is less than 50 μm at the central optical zone, 500 to 600 μm at the optic rim, and approximately 100 μm at the haptic footplates.4 As a result of this design, the pIOL is more likely to come in contact with the crystalline lens in the peripheral area, leading to an increased risk for cataract formation. Schmidinger et al.16 report a reduction in central vault over a 10-year period after implantation of the Implantable Collamer Lens, and eyes that developed cataract had midperipheral contact between the pIOL and the crystalline lens. In contrast, Lackner et al.19 found that the Implantable Collamer Lens (model V4) vaulting did not correlate with the risk for lens opacification. In a study by Trindade et al.,21 there was no lens opacification for more than 2 years in eyes with a Implantable Collamer Lens despite contact between the pIOL and crystalline at the optic–haptic junction in the midperiphery. Similarly, the 3 eyes in our study with peripheral contact between the pIOL and the crystalline did not develop anterior subcapsular cataract for more than 24 months postoperatively. Therefore, we consider low vault to be a risk factor for anterior subcapsular cataract rather than as an inevitable factor leading to cataract. We also found that the initial peripheral vault values in the 3 eyes after surgery were less than 100 μm.
One limitation of our study is that we measured pIOL vault while the patient was supine, which may differ when measured with the patient upright. Further study should analyze the effect of the posture on pIOL vault measurements. In this study, some pigment deposits were observed on the surface of the pIOLs; however, the deposits were not associated with symptoms and were of no clinical significance. Long-term studies are necessary to determine the role of peripheral vault on pigmentary glaucoma and metabolic cataract formation.
In summary, we believe that this is the first study of highly myopic patients to assess the longitudinal changes in central and peripheral distances between the Implantable Collamer Lens pIOL and the crystalline lens using UBM for 2 years after implantation. Further long-term observation is required to confirm whether the tendency toward a slight decrease in lens vault remains in the late postoperative period. Additional studies with a larger number of cases and a longer follow-up are needed for longitudinal assessment of the behavior of the pIOL and to fully understand long-term complications, such as glaucoma and cataract.
What Was Known
- The PC pIOL was designed to vault anteriorly to the crystalline lens. It can cause cataract due to contact with the crystalline lens, loss of corneal endothelium due to the small distance between the pIOL and the corneal endothelium, and glaucoma due to dispersion of iris pigment and closure of the pupil opening. Thus, positional stability of the pIOL is of great importance.
- It has been reported that central vault has a tendency to decrease over time; however, there is little information on the time course of the changes in central vault when accurately measured by UBM.
What This Paper Adds
- There was a significant increase in endothelium–anterior pIOL distance between 1 month and 3 months postoperative, after which a slight decrease occurred over time. A tendency toward a decrease in central vault and peripheral vault was observed over time.
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