Intraocular Lens Position Stability, Anterior Segment Parameters, and Refractive Change Following Nd:YAG Laser Posterior Capsulotomy: A Prospective Multi-Device Assessment Using Swept-Source OCT Biometry (IOLMaster 700), Anterior Segment OCT (CASIA2), and Scheimpflug Tomography (Pentacam AXL)
Intraocular Lens Position Stability, Anterior Segment Parameters, and Refractive Change Following Nd:YAG Laser Posterior Capsulotomy: A Prospective Multi-Device Assessment Using Swept-Source OCT Biometry (IOLMaster 700), Anterior Segment OCT (CASIA2), and Scheimpflug Tomography (Pentacam AXL)
After cataract surgery, the thin natural membrane that holds the artificial lens in place can slowly turn cloudy. This is called posterior capsule opacification. It makes vision blurry and can cause glare. The standard treatment is a short, painless laser procedure called Nd:YAG capsulotomy, which opens a small clear window in the cloudy membrane.
Opening this membrane changes the support structure around the artificial lens. Eye doctors do not agree on what happens next. Some studies found that the lens shifts slightly backward in the eye after the laser treatment, some found that it shifts forward, and some found no movement at all. It is also unclear whether such a small shift changes how well a person sees or the strength of their glasses. One reason for these mixed results may be that each study used a different imaging device, and these devices measure the depth of the front chamber of the eye in different ways.
This study measures the position of the artificial lens in the same eye with three different imaging devices, before and after the laser treatment. The devices are called IOLMaster 700, CASIA2, and Pentacam AXL. All three take detailed pictures of the front of the eye without touching it. The study team wants to find out whether the artificial lens moves after the laser treatment, how much it moves, whether any movement changes vision or the glasses prescription, and whether the three devices give matching results.
Who can take part
People who had cataract surgery at least three months ago, who now have a cloudy membrane behind the artificial lens, and whose eye doctor has already decided that the laser treatment is needed.
What taking part involves
Taking part does not change the laser treatment in any way. The decision to perform the treatment, the technique, and the laser settings are made by the treating doctor for medical reasons and are not influenced by the study. The study only adds extra measurements before and after the treatment.
There are three study visits:
At each visit the study team checks how well you can see, measures the strength of glasses that gives you the sharpest vision, examines your eye with a slit lamp, measures the pressure inside your eye, and takes images with the imaging devices. All of these are standard eye examinations used in everyday clinical practice. Nothing is inserted into the eye, and there are no injections, no medicines, and no radiation. Drops to widen the pupil are not needed for the study.
Taking part is voluntary. You can stop at any time, without giving a reason and without any disadvantage for your medical care. Your laser treatment and all routine follow-up visits go ahead either way. There is no payment for taking part, and taking part brings no direct medical benefit. However, the detailed examination may reveal something that would not be noticed during routine follow-up, such as raised eye pressure. If this happens, you will be told and any necessary care will be arranged as part of your normal treatment at no extra cost.
Study data are stored under a study code instead of your name.
BACKGROUND AND RATIONALE
Posterior capsule opacification (PCO) is the most common long-term complication of cataract surgery, and Nd:YAG laser posterior capsulotomy is its established standard treatment. Disruption of the posterior capsule alters the mechanical environment of the capsular bag in which the intraocular lens (IOL) is fixated and may therefore affect IOL positional stability. Because IOL position determines the effective lens position, which is the dominant source of residual refractive prediction error in modern IOL power calculation, any clinically relevant positional change could translate into a measurable refractive shift.
The published evidence is conflicting. Individual studies report posterior IOL displacement with hyperopic shift, anterior displacement with myopic shift, or no measurable change, while the most comprehensive meta-analysis to date (850 eyes from 18 publications) reports population-level stability of anterior chamber depth (ACD) and spherical equivalent up to one month after capsulotomy. Subgroup-specific effects have been described in relation to corneal health status, IOL haptic design, the interval between cataract surgery and capsulotomy, and capsulotomy size.
A methodological limitation common to this literature is that every published study quantifies ACD with a single measurement device. Different technologies define ACD differently, using optical versus geometric measurement principles, corneal epithelium versus endothelium as the anterior reference surface, and device-specific segmentation algorithms. Cross-study comparison is therefore unreliable, and it remains unresolved whether the contradictory findings reflect genuine biological heterogeneity or device-dependent measurement artefact. In addition, automated three-dimensional quantification of IOL tilt and decentration by anterior segment swept-source OCT has not been systematically applied to the post-capsulotomy question, and the IOL platforms routinely implanted at European cataract centres are underrepresented in the available data.
DESIGN
Single-centre, prospective, non-interventional diagnostic cohort study using a paired within-eye pre-post comparison, in which each eye serves as its own control. Consecutive pseudophakic eyes with visually significant PCO scheduled for clinically indicated Nd:YAG capsulotomy are enrolled without pre-selection for IOL model, axial length, PCO morphology, or interval since cataract surgery. Measurement device, operator, and measurement conditions are held constant across time points, so that the paired design isolates the effect of capsulotomy from inter-device and inter-session variability. Simultaneous acquisition on three platforms at each time point additionally permits an inter-device agreement analysis for ACD change that is not achievable in a single-device study.
STUDY VISITS
Visit 1 (baseline) takes place on the day of capsulotomy, before the procedure. Visit 2 follows at one week (plus or minus 3 days) and Visit 3 at one month (plus or minus 7 days) after capsulotomy.
At the two full visits (Visit 1 and Visit 3) the following measurements are acquired in a fixed sequence designed to minimise measurement interference: uncorrected distance visual acuity; IOLMaster 700 optical biometry; CASIA2 anterior segment swept-source OCT; Pentacam AXL Scheimpflug tomography; autorefraction followed by subjective manifest refraction with Jackson cross-cylinder refinement and best-corrected distance visual acuity; and slit-lamp examination including intraocular pressure measurement. Mydriasis is not required by the protocol, and all measurements are performed under natural pupil conditions; if dilation is clinically indicated for other reasons, it is performed only after all study measurements are complete.
At Visit 2, visual acuity, manifest refraction, IOLMaster 700 biometry, slit-lamp examination and tonometry are mandatory, whereas CASIA2 and Pentacam AXL imaging is optional in order to limit patient burden at the interim visit; where performed, it follows the identical standardised protocol and contributes to the agreement analyses. At both post-capsulotomy visits the refractionist is masked to the previous refraction values to avoid anchoring bias.
PARAMETERS
CASIA2: ACD referenced to the posterior corneal surface, anterior chamber angle and volume, IOL tilt (horizontal and vertical) and decentration acquired with the post-operative cataract application, anterior, posterior and total/real keratometry, central corneal thickness, pupil parameters, quality score and analysis area.
IOLMaster 700: ACD referenced to the corneal epithelium, keratometry and total keratometry, axial length, central corneal thickness, white-to-white, pupillometry.
Pentacam AXL: ACD, SimK, total corneal refractive power at the 3.0 mm zone, true net power, posterior keratometry, pachymetry mapping, corneal irregularity indices (ISV, IVA, IHD, KI, CKI), chamber angle and volume, axial length.
Scans that do not meet the manufacturer's quality criteria are repeated once and otherwise documented as failed with the reason specified.
Historical data are extracted from the medical record for each enrolled eye: date and details of the cataract surgery including incision location and technique, implanted IOL model, power, material, optic and haptic design, optic edge geometry and, for toric IOLs, planned cylinder and alignment axis, together with the preoperative biometry and the documented postoperative course.
Capsulotomy parameters are documented immediately after the procedure: laser device, posterior offset, starting and maximum energy level, total energy delivered, number of shots, capsulotomy shape and diameter, contact lens used, peri-procedural medication, and intraprocedural observations including IOL pitting. The study neither prescribes nor modifies the capsulotomy technique, the energy settings, or the capsulotomy size, all of which remain at the discretion of the treating ophthalmologist.
ANALYSIS
The primary analysis set comprises one eye per patient, selected deterministically as the first treated eye within the study period; if both eyes are treated on the same day, the eye is selected at random using a reproducible seed. Bilateral eyes are analysed in a pre-specified sensitivity analysis using generalised estimating equations or linear mixed models to account for within-patient correlation.
The primary endpoint is tested against the null hypothesis of no change using a paired t-test, or a Wilcoxon signed-rank test where the normality assumption is not met, at a two-sided alpha of 0.05. It is the only confirmatory hypothesis specified. All secondary endpoints are explicitly classified as exploratory and hypothesis-generating; each is reported with a two-sided p value and a 95% confidence interval, without family-wise error correction, because they address heterogeneous mechanistic, descriptive and device-agreement questions rather than competing tests of a single hypothesis. Inter-device agreement is reported as estimation only, with bias, 95% limits of agreement and intraclass correlation coefficients, and without hypothesis testing. Pre-specified sensitivity analyses include restriction to corneally stable eyes, exclusion of eyes with Fuchs endothelial corneal dystrophy, exclusion of visits outside the protocol time window, and an alternative clinical relevance threshold for the primary endpoint.
The target sample size is based on the paired comparison of the primary endpoint. Published data report mean ACD changes after capsulotomy between 0.00 and 0.06 mm with within-eye standard deviations of approximately 0.05 to 0.10 mm. At a within-eye standard deviation of 0.08 mm and a two-sided alpha of 0.05, 57 eyes provide at least 80 percent power to detect a mean change of 0.03 mm, and the study is substantially overpowered for the pre-specified clinical relevance threshold. An attrition rate of approximately 10 percent is anticipated.
Reporting follows the STROBE statement for cohort studies, with STARD elements applied to the inter-database agreement analyses.
Inclusion Criteria:
Exclusion Criteria: