Real-Time Assessment of Intraoperative Vaulting in Implantable Collamer Lens and Correlation with Postoperative Vaulting

Introduction

Phakic intraocular lens (pIOL) implantation is the procedure of choice in eyes with high amplitude of refractive errors not amenable to corneal ablative procedures. It has been found to be safer than excimer laser surgical correction for moderate to high myopia, with a lower chance of loss of best spectacle-corrected visual acuity, better contrast sensitivity, as well as better patient satisfaction and preference (1).

Implantable collamer lens (ICL) (Visian; STAAR Surgical Co., Monrovia, CA, USA) is a posterior chamber pIOL that was approved by the Food and Drug Administration in 2005 for correction of moderate to high myopia (2). Safety and success of ICL implantation depends on adequate vaulting of the ICL; extremes of vaulting are associated with increased incidence of complications such as anterior subcapsular cataract and glaucoma (3–4–5). Disparity between the ICL size and the sulcus-to-sulcus diameter is the single most important determinant in estimating the ICL vault (6). Other factors such as anterior chamber depth, lens thickness, and presence of ciliary microcysts also affect the final vaulting (7). Postoperatively, clinical assessment of vaulting is done with the help of slit-lamp biomicroscopy and objectively with the help of anterior segment optical coherence tomography (AS-OCT).

Intraoperative optical coherence tomography (iOCT) is an emerging entity with potential benefits in both anterior and posterior segment surgeries (8). It has the potential to increase our understanding of the tissue alterations that occur during surgical manipulations. The use of handheld iOCT has been described in cases of endothelial keratoplasty, lamellar keratoplasty, penetrating keratoplasty, glaucoma surgeries, and other posterior segment surgeries (9–10–11–12–13). There is no study available in the literature that has evaluated the intraoperative relationship between the ICL and the anterior lens capsule using iOCT. We herein describe the use of microscope integrated with iOCT to assess the intraoperative vaulting in patients undergoing ICL implantation and compare it with the postoperative vaulting.

Methods

In this prospective study, we evaluated 40 eyes of 22 consecutive patients who underwent ICL V4c (Visian) implantation at a tertiary care ophthalmology setup. Informed consent was taken from all patients. Ethical clearance was obtained from the institutional review board and the study conformed to the tenets of the Declaration of Helsinki.

Inclusion criteria for ICL implantation were age ≥21 years, myopia ≥-6.0 D, stable refraction for at least 1 year preoperatively, intraocular pressure (IOP) <21 mm Hg, anterior chamber depth (ACD) from the anterior crystalline lens surface to the corneal endothelium ≥2.9 mm and endothelial cell density ≥2,500 cells/mm². Patients with preexisting ocular pathology and previous ocular surgery were excluded from the study. A thorough preoperative workup was done, including Snellen uncorrected distance visual acuity and best-corrected distance visual acuity, manifest and cycloplegic refractions, applanation tonometry, slit-lamp biomicroscopy, corneal topography (Orbscan IIz; Bausch & Lomb, Rochester, NY, USA), examination of the fundus, and noncontact specular microscopy (SP-8000; Konan Medical, Inc., Nishinomiya, Hyogo, Japan). Horizontal white-to-white distance (WTW) and ACD were measured using a scanning-slit topographer (Orbscan IIz). The size and power of the ICL were determined on the basis of the manifest refraction, keratometry, and the WTW measurements. The calculations were done on the online calculator (STAAR online calculation and ordering system). Our intraoperative measurements did not lead to any adjustment in ICL selection for implantation in our series of cases.

Peripheral iridotomy was not performed as the V4c series of implantable collamer lenses were implanted in all cases.

Surgical technique

Integrated microscope with intraoperative OCT (OPMI LUMERA 700 and RESCAN 700; Carl Zeiss, Jena, Germany) has an integrated spectral-domain OCT with a wavelength of 840 nm, which acquires 27,000 A-scans per second with an axial resolution of 5.5 µm OCT and an A-scan depth of 2,000 µm. The OCT does not have measuring calipers to allow direct assessment of the intraoperative vaulting. A 9-mm cube was acquired for analysis. The height of the OCT cube displayed on the CALLISTO eye is 2,000 μm and it was used as a measure to assess the intraoperative vaulting. A ruler was used to measure the vaulting from the OCT scan displayed on the screen of the CALLISTO eye and the reading in millimeters was converted into microns (Fig. 1, A and B). The minimum marking on a ruler is 1 mm; hence the minimum resolution of the ruler is 7.5 μm when measurements are done on full screen of 26.5 cm. Hence the maximum error of measurement by this method would be ±7.5 μm.

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Fig. 1

Intraoperative and postoperative assessment of implantable collamer lens vaulting. (A) Height of the optical coherence tomography (OCT) cube displayed on the CALLISTO eye as measured by a ruler corresponds to 2,000 μm and is used for calibration of the measurement values. (B) Intraoperative vaulting as assessed by a ruler on the CALLISTO eye; the value in millimeters is converted to microns. (C) Intraoperative vaulting measured by the image processing software (ImageJ 1.46r). (D) First postoperative day vaulting measured by anterior segment OCT (Visante).

Subsequently, the vaulting was also measured using image processing software (ImageJ 1.46r; National Institutes of Health, Bethesda, MD, USA) and the height of the captured OCT cube was used for calibration (Fig. 1C). The intraoperative ruler-assisted vault measurements corresponded with the imageJ measurements within 5 μm in all cases. The intraoperative ruler-derived measurements are reported in this study.

The V4c ICL was injected into the anterior chamber through the 3.2-mm temporal corneal incision under 1% sodium hyaluronate (Healon; AMO Inc., Santa Ana, CA) and the 4 haptics of the ICL were positioned beneath the iris with the help of the Pallikaris ICL manipulator (Duckworth and Kent Ltd., Hertfordshire, UK). Bimanual irrigation aspiration was done to remove the residual ophthalmic viscosurgical device (OVD) and the corneal wounds were hydrated. An integrated microscope with iOCT (OPMI LUMERA 700 and RESCAN 700; Carl Zeiss) was used during surgery that projects real-time high-definition OCT images directly onto the surgical field as well as on the screen of the CALLISTO eye. It was used to assess the relationship between the ICL and anterior capsule of lens throughout the surgery and the ICL vaulting at the end of surgery. All patients were amenable to intraoperative measurements. The intraoperative vaulting was assessed after complete OVD removal and adequate hydration of the corneal incisions. The corneal incisions were made watertight to ensure an optimal anterior chamber fill.

Postoperatively, AS-OCT (Visante; Carl Zeiss) was done to measure the vaulting on the first postoperative day and after 1 month (Fig. 1D). Uncorrected and best-corrected Snellen visual acuity, IOP, and anterior and posterior segments were assessed in all cases. Postoperatively, all patients received topical antibiotic and steroids for 1 month.

Statistical analysis was done using Statistical Package for the Social Sciences (SPSS 11.0; SPSS Inc., Chicago, IL, USA). Normally distributed continuous variables were expressed as mean ± SD. A paired sample t test was used to assess the correlation between the intraoperative and postoperative vaulting. A p value of less than 0.05 was considered significant.

Results

Forty eyes of 22 patients were evaluated. The mean age of the patients was 24.3 ± 2.1 years. The mean central vaulting noted intraoperatively after removal of OVD was 558.4 ± 122.8 µm (range 346-792 µm). Postoperative mean vaulting was 576.0 ± 131.2 µm (range 350-820 µm) on day 1 and 551.1 ± 122.5 µm (range 345-800 µm) on day 30. The paired samples correlation between intraoperative and postoperative day 1 vaulting was 0.969 (p<0.001) and between intraoperative and postoperative day 30 vaulting was 0.945 (p<0.001). The paired difference between intraoperative and postoperative day 1 vaulting was 17.7 ± 32.9 (95% confidence interval [CI] 7.1, 28.2; p = 0.002) and between intraoperative and postoperative day 30 vaulting was 7.2 ± 40.7 (95% CI -5.8, 20.3; p = 0.268). Intraoperative relationship between the ICL and the anterior lens capsule was assessed during ICL implantation, positioning of the haptics beneath the iris, and irrigation-aspiration. An ICL-lenticular touch was not noted at any time during the surgery. No case had vaulting less than 250 µm. Vaulting more than 750 µm was noted in 2 cases intraoperatively (780 µm and 792 µm). There was no intraoperative irido-corneal touch and both cases had a deep anterior chamber. Hence, an on-table explantation of the ICL was not deemed necessary. Postoperatively, there was no appositional angle closure in these 2 cases and IOP was <21 mm Hg. All patients had uncorrected visual acuity ≥20/25 (range 20/20-20/25) after 1 month. The mean IOP on postoperative day 1 was 14.3 ± 3.5 mm Hg. The postoperative course was uneventful in all cases. No patient developed raised IOP or cataract by the last follow-up. An ICL explantation was not required in any case.

Discussion

There has been an increasing use of ICL for surgical refractive correction in recent years, especially for higher degrees of refractive error. Optimal vaulting of ICL is essential for long-term success of the procedure as this minimizes the development of sight-threatening complications such as glaucoma and cataract (3–4–5). An ideal vault after posterior chamber pIOL implantation may be considered to be roughly equivalent to the central corneal thickness, with an insufficient vault defined as <250 μm and an excessive vault defined as >750 μm (3–4–5). Though a comprehensive preoperative workup with repeat WTW and sulcus-to-sulcus measurements is done to avoid any fallacy, occasional surprises with ICL sizing are not uncommon. An undesirable vaulting may be a result of a thick crystalline lens, low ACD, too-small phakic IOL, and WTW measurement error (7). Vertical compression by the iris, dampening effect of the ciliary sulcus structure, or innate ICL vault are additional factors that may lead to unexpected vaulting after ICL implantation (6). Ciliary sulcus microcysts have been identified as a source of WTW sizing mismatch with ICL that may require explantation of the ICL (7, 14). In a review by Zeng et al (7) of 616 myopic eyes with previous pIOL implantation, 2.6% cases (16/616) needed a pIOL exchange. The reason for pIOL explantation was low vaulting (≤100 μm) in 50% of cases and too high vaulting (≥1000 μm) in 50% of cases.

An intraoperative real-time assessment of vaulting is the best predictor of the final outcome. Anterior segment OCT is routinely done in the postoperative period for a quantitative assessment of vaulting. The vaulting as assessed by the iOCT in our series of 40 eyes was found to correlate well with the postoperative AS-OCT vaulting and the clinical picture. It may act as a tool to aid in on-table decision-making in cases with extremes of vaulting and help decrease the incidence of pIOL explantation following development of complications.

Cases with extremes of vaulting intraoperatively are expected to fare poorly in the postoperative period as well. Persistent acute angle closure attacks unresponsive to iridotomies have been reported in the immediate postoperative period following pIOL implantation (15–16–17–18). In these cases, the postoperative AS-OCT reveals excessively high vaulting and aids in proper identification of the mechanism of angle closure. The oversized pIOL results in an inward compressive force on the ICL footplates, which results in excessive anterior pIOL vaulting that presses the iris into the angle, causing a nonpupillary block glaucoma. An explanation of pIOL is needed in these cases to prevent visual loss and lower the IOP (15–16–17–18). Intraoperative OCT helps visualize the iris-angle relationship, and a decision to explant the pIOL can be taken on-table in cases with extremely high vault leading to appositional angle closure. Timely explantation of the pIOL will prevent the development of vision-threatening complications. It may also decrease the complications related to subsequent second surgery in the postoperative period.

Long-term reduction of the central vaulting has been demonstrated and eyes with insufficient vaulting are more predisposed to secondary cataract formation (19). Localized malnutrition caused by poor circulation of the aqueous humor, and a direct contact between the pIOL and the crystalline lens, are responsible for the development of cataract (20–21–22–23–24). A phakic IOL in direct contact with the anterior lens capsule in the central optical zone is an indication for immediate explantation and iOCT will facilitate on-table decision-making, which will prevent development of cataractous changes in the crystalline lens.

Intraoperative OCT allows in vivo cross-sectional imaging of the anterior segment during surgery. From making the initial incision to final hydration of the wound, a real-time dynamic visualization of the entire surgical procedure is provided. The relationship between the ICL and crystalline lens can be visualized throughout the surgery. This is of use while positioning the haptics beneath the iris with a manipulator as the safety margin between the ICL and lens can be maintained. The inadvertent touch of instruments to the central optical zone of the ICL can be avoided specially during manipulation of the ICL and irrigation-aspiration. However, it is not possible to accurately assess the contact of ICL to peripheral crystalline lens or peripheral iris while positioning the haptics beneath the iris due to the shadowing effect of the instruments.

Handheld OCT has been used intraoperatively in various anterior and posterior segment surgeries (8–9–10–11–12–13). However, a handheld OCT requires stopping the surgical procedure to capture an image. It provides a static cross-sectional view of the anterior or posterior segment and the dynamic instrument-tissue interactions cannot be visualized. Moreover, the duration of surgery significantly increases because of the time taken to acquire the OCT image. Microscope integrated iOCT provides real-time dynamic visualization of tissue-instrument projected directly in the surgical field. Both planar and cross-sectional views of the surgical field are available to the surgeon through the eyepiece. A continuous and simultaneous acquisition and display of the OCT images ensures a safe and fast surgery.

To date, it has been possible to assess the ICL vault only in the postoperative period. This is the first study to demonstrate that it is possible to assess on-table ICL vaulting with the help of iOCT, and the measured intraoperative vaulting accurately correlates with the postoperative vaulting. The dynamic relationship between the ICL and the anterior capsule of the crystalline lens can be continuously assessed. This enhances the safety of the surgical procedure by providing a real-time display of the intraoperative manipulations.

Although an ICL explant was not needed in any of our cases, the iOCT assessment may aid in on-table exchange of ICL when indicated. A sample size of more than 1,000 patients will be needed to demonstrate definite benefit in cases with extremes of vaulting that may require an intraoperative ICL exchange.