IOL Position and Refractive Change After Nd:YAG Laser Posterior Capsulotomy

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.