Micro Aberration Scatter Index (MASI) as an objective proxy for contrast sensitivity after keratorefractive surgery: a prospective observational study

Abstract

Purpose

To evaluate the Micro Aberration Scatter Index (MASI) as an objective proxy for visual quality after keratorefractive surgery.

Methods

This prospective study included 200 eyes from 200 patients who underwent uneventful KLEx, PRK, or LASIK. Monocular testing was performed 30 days postoperatively. MASI was derived from pyramidal wavefront aberrometry (OSIRIS-T, CSO). Contrast sensitivity was measured at 3, 6, 12, and 18 cycles per degree (cpd) under photopic conditions using sine-wave grating charts. Uncorrected distance visual acuity (UDVA) was recorded in logMAR. Correlation, regression, and tertile subgroup analyses were performed.

Results

MASI values ranged from 13.1 to 79.5 (mean 34.0 ± 13.3). MASI showed significant inverse correlations with contrast sensitivity at all spatial frequencies (r = − 0.46 to − 0.62; all p < 0.001). Strongest relationship was at 18 cpd (r = − 0.62, R²=0.39). Linear regression showed each 10-point increase in MASI predicted a 0.18 unit decrease in contrast sensitivity at 18 cpd. MASI tertiles corresponded with differences in contrast sensitivity and uncorrected acuity. Stratification by MASI tertile revealed significant differences in visual performance: low MASI (mean 21 ± 4) had 18 cpd contrast sensitivity of1.30 ± 0.24 and mean UDVA of − 0.12 ± 0.09 logMAR; mid MASI (mean 31 ± 4) had 18 cpd contrast sensitivity of1.21 ± 0.31 and mean UDVA of − 0.13 ± 0.07 logMAR; high MASI (49 ± 9) had 18 cpd contrast sensitivity of0.86 ± 0.40 and UDVA of − 0.05 ± 0.14 logMAR (both p < 0.001).

Conclusions

MASI shows early promise as a fast and clinically feasible index of optical quality. Within a single device and operator it correlated strongly with functional vision and distinguished subtle visual degradation not captured by high-contrast acuity alone. Because MASI is computed over the central 3 mm zone, its application to mesopic and night-vision symptoms should be regarded as hypothesis-generating, and inter-device, inter-operator, and multicenter reproducibility remain to be established.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s12886-026-05118-8.

Introduction

Achieving 20/20 uncorrected distance visual acuity (UDVA) is often considered the hallmark of refractive surgery success. However, many patients with technically perfect outcomes report dissatisfaction due to visual disturbances such as glare, halos, or hazy vision—particularly in dim lighting or during high-demand visual tasks. These complaints frequently stem not from residual refractive error, but from subtle optical imperfections that degrade contrast sensitivity [1].

While higher-order aberrations (HOAs) are routinely measured via aberrometry and are known to increase after LASIK, PRK, and KLEx (keratorefractive lenticule extraction), they do not fully explain functional vision loss [2]. Small-angle scatter and corneal micro-irregularities—particularly prominent in the early postoperative period—can significantly reduce contrast sensitivity despite normal Snellen acuity. For example, PRK induces temporary surface haze; KLEx has been associated with interface texturing (e.g., “orange peel” effect); and LASIK may create flap interface microfolds [3, 4]. These subtle features may not substantially alter the wavefront root mean square (RMS) but can scatter light and reduce contrast [5, 6].

Several objective methods exist for evaluating optical quality after keratorefractive surgery. Wavefront-based metrics such as HOA RMS and the Visual Strehl ratio (VSX) are widely used, relying on Zernike-based modeling to represent low-to-moderate spatial frequency aberrations. Similarly, the Objective Scatter Index (OSI), obtained via the HD Analyzer (Visiometrics, Terrassa, Spain), quantifies forward intraocular scatter based on double-pass retinal imaging. It reflects the degree of intraocular forward light scatter affecting retinal image quality and is especially useful in evaluating early cataract and tear film abnormalities [7]. Some qualitative attempts involved the use of corneal densitometry [8], whereas some non-commercial approaches revolved around segmentation of the geometrically irregular morphology [9].

However, these tools have key limitations. Zernike-based models are inherently low resolution and miss stochastic, high-frequency, low-amplitude micro-irregularities caused by the laser–tissue interaction. OSI, while effective for assessing lens-based scatter, is less specific to corneal stromal microtexture [10, 11]. As a result, early postoperative optical degradation may go undetected by standard metrics, even as patients report “soft,” hazy, or blurred vision despite achieving 20/20 uncorrected visual acuity.

The cornea remodels at the epithelial level, while the human visual system also undergoes neuroadaptation following refractive surgery, gradually compensating for new optical conditions. Studies using functional MRI and psychophysical paradigms confirm cortical remodeling in response to optical changes [12, 13]. Yet, this adaptation takes weeks to months and cannot fully mask early postoperative degradation caused by scatter and micro-aberrations. Therefore, objective metrics that capture this transient optical degradation are essential for assessing visual quality in the early postoperative window.

To address this diagnostic gap, we developed a novel parameter—the Micro Aberration Scatter Index (MASI)—designed specifically for the Osiris-T high-resolution Hartmann–Shack aberrometer (CSO, Florence, Italy). MASI is derived from ultra-dense wavefront slope maps (> 4000 measurement points in the central 3 mm pupil zone) and captures minute deviations in gradient slopes that reflect high-frequency surface noise. Unlike conventional RMS or Strehl metrics, MASI isolates micro-aberrations likely to affect early postoperative optical quality, even in eyes with otherwise normal clinical metrics.

In this study, we aimed to validate MASI as a proxy for visual quality, focusing on evaluating the correlation with contrast sensitivity and visual outcomes. Rather than comparing surgical techniques, the aim was to verify the functional relevance of MASI as an objective measurement related to sine-wave contrast testing and provide a quantitative optical quality index suitable for clinical integration.

Methods

Study design and population

This is a prospective study approved by the ethics committee of the Svjetlost Specialty Hospital for Ophthalmology, Eye Clinic, School of Medicine, University of Rijeka (EC-2024-MASI.1). It was conducted in accordance with the tenets of the Declaration of Helsinki and written informed consent was obtained from all participants. It included patients who underwent uneventful keratorefractive surgery—KLEx, PRK, or LASIK— at Svjetlost Eye Clinic, Zagreb, Croatia, consecutively from 05/2024 to 08/2024. All eyes were evaluated monocularly at a standardized postoperative time point of 30 days. A total of 200 patients were included in the study. Each patient contributed a single, randomly selected study eye, ensuring statistical independence of all 200 observations and eliminating the need for adjustment for inter-eye correlation. Inclusion criteria were: age between 18 and 45 years, myopia with or without astigmatism, corrected distance visual acuity (CDVA) of 20/20 or better preoperatively, and clear optical media without prior surgery or corneal disease. Patients were excluded if they had irregular astigmatism, dry eye requiring treatment, or complications that might affect healing (e.g., interface inflammation, flap issues, or epithelial haze beyond Grade 1).

All surgical procedures were performed by experienced surgeons using standardized techniques. While procedure-specific differences in healing are well recognized [5, 6], the type of procedure (KLEx, PRK, LASIK) was masked to the examiners performing the postoperative contrast sensitivity and aberrometry testing to avoid assessment bias. While masking reduced the risk of bias, this analysis did not stratify outcomes by procedure type. A separate follow-up study is planned to explore MASI behavior and recovery trajectories across different surgical techniques.

All postoperative testing was performed by an optometrist, and the scans were performed by a trained ophthalmic technician.

Contrast sensitivity testing

Contrast sensitivity was assessed monocularly using sinewave grating charts (CSV-1000E, VectorVision, USA) under standardized photopic illumination (85 cd/m²). It was measured with the patient’s best correction (manifest refraction) in place. Patients underwent a 5-minute light adaptation period before testing. Measurements were obtained at four spatial frequencies: 3, 6, 12, and 18 cycles per degree (cpd). For each frequency, the final correctly identified grating was recorded and converted into log contrast sensitivity (log CS).

Each eye was tested individually, with the fellow eye occluded to ensure monocular assessment. Testing was completed within 4–6 min per eye.

MASI measurement and derivation

Ocular wavefront measurements were performed using the OSIRIS-T pyramidal aberrometer (CSO srl, Florence, Italy). Each measurement produced a high-resolution wavefront gradient map based on a pyramidal sensor at the pupil plane, sampling across a central 6 mm optical zone. The MASI calculation focuses on the central 3 mm of the pupil, the zone most critical to foveal image quality.

The MASI calculation follows a standardized processing pipeline. Wavefront data are acquired across the pupil and centered on the pupil centroid. A Zernike decomposition is then performed within the central 3 mm analysis zone, and the corresponding Zernike fit up to the 7th order is subtracted from the measured wavefront. The remaining residual wavefront gradient field represents high-frequency optical irregularities that are not captured by conventional Zernike modeling.

MASI is defined as the root-mean-square (RMS) magnitude of this residual wavefront gradient and is calculated as:

Where and are the partial derivatives of the wavefront with respect to x and y, respectively, and and represent the corresponding partial derivatives of the Zernike fit up to the 7th order.

Consequently, MASI reflects the RMS magnitude of the residual wavefront gradient after removal of conventional Zernike-described aberrations, with higher values indicating greater residual micro-aberration scatter and optical irregularity.

MASI includes contributions from the cornea, tear film, lens, and anterior chamber.

The metric is unitless and is automatically calculated by the software with no additional acquisition time, technician input, or cost. In each eye, three MASI measurements were recorded at the 30-day visit. Only high-quality scans (good centration, absence of tear film breakup) were included. The mean MASI value across three scans was used for all analyses.

Visual acuity testing

UDVA was measured under photopic conditions at 4 m using an ETDRS chart. Visual acuity was converted to and recorded in logMAR format for statistical analysis. CDVA was not the primary endpoint, but was recorded to confirm clinical stability.

Statistical analysis

All analyses were performed using SPSS v27 (IBM Corp., USA). The primary objective was to assess the relationship between MASI and visual performance, especially contrast sensitivity.

Pearson correlation coefficients (r) were calculated between MASI and contrast sensitivity at each spatial frequency (3, 6, 12, and 18 cpd), and between MASI and logMAR UDVA.

Univariate linear regressions were conducted with contrast sensitivity (logCS) as the dependent variable and MASI as the predictor to estimate β coefficients. For each model, the 95% confidence intervals (CIs) of the β values were reported. Assumptions of linearity, homoscedasticity, and normality of residuals were evaluated using residual-vs-fitted plots and Q–Q plots. No gross violations were observed.

MASI values were divided into tertiles defined by cut-off values (< 26 for Low, 26–39 for Mid, > 39 for High MASI). These groups were used to evaluate performance differences in contrast sensitivity and UDVA using one-way ANOVA with Tukey’s post hoc comparisons.

All p-values are two-tailed, with significance defined as p < 0.05.

Results

Patient characteristics

The study included 200 eyes from 200 patients, with a mean age of 28 ± 5 years. All surgeries (KLEx: n = 100, LASIK: n = 50, PRK: n = 50) were uneventful. No eye had significant haze, interface issues, or tear film instability at the time of testing. The available postoperative parameters were: ocular-wavefront sphere + 0.43 ± 0.35 D, cylinder − 0.36 ± 0.31 D, spherical equivalent + 0.25 ± 0.34 D, HOA RMS 0.60 ± 0.30 μm (range 0.19–1.80), and mesopic pupil diameter 5.0 ± 0.8 mm (range 3.0–6.8).

The MASI values ranged from 13.1 to 79.5 (mean 34.0 ± 13.3) (Fig. 1). Contrast sensitivity (logCS) ranged from 1.17 to 2.08 at 3 cpd, 0.91 to 2.29 at 6 cpd, 0.61 to 1.84 at 12 cpd, and 0.17 to 1.55 at 18 cpd. UDVA ranged from − 0.28 to + 0.40 logMAR. Higher-order aberration (HOA) RMS ranged from 0.19 to 1.80 μm, and mesopic pupil diameter ranged from 3.0 to 6.8 mm. The data were evenly distributed across the low to high scatter range, allowing stratification into tertiles:

Fig. 1.

Histogram with kernel density overlay showing the distribution of MASI values (n = 200 eyes). MASI ranged from 13.1 to 79.5 (34.0 ± 13.3), right-skewed; parametric estimates confirmed by non-parametric sensitivity analyses, enabling tertile classification for subgroup analysis

• Low MASI (< 26)

• Mid MASI (26–39)

• High MASI (> 39)

Repeatability and normality analysis of MASI measurements

Repeatability within the MASI tertile groups (Low, Mid, High) was evaluated using within-subject standard deviations (SD). Repeatability decreased with increasing MASI:

• Low MASI group: SD = 1.17

• Mid MASI group: SD = 1.55

• High MASI group: SD = 2.52

The Shapiro–Wilk test showed no significant deviation from normality in the Low MASI group (p = 0.071), but significant deviations from normality were observed in the Mid (p = 0.005) and High groups (p < 0.001).

Additionally, Levene’s test confirmed significantly unequal variances across the three groups (p < 0.001), indicating increased variability with higher MASI values.

From these within-subject standard deviations, the coefficient of repeatability (CR = 2.77 × Sw) was 3.24, 4.29, and 6.98 MASI points in the Low, Mid, and High tertiles, with 95% Bland–Altman limits of agreement of approximately ± 3.24, ± 4.29, and ± 6.98 points. The within-subject coefficient of variation was low and stable across tertiles (5.6%, 4.9%, 5.1%).

To define a normal clinical reference range, the Low MASI group was used as the baseline. The mean MASI in this group was 21.70 ± 3.81, yielding a normality range of 14.08 to 29.32 (mean ± 2 SD).

This range may serve as a reference threshold to distinguish between normal MASI values and potentially abnormal measurements indicative of increased optical scatter or visual degradation.

Correlation between MASI and contrast sensitivity

MASI showed statistically significant negative correlations with contrast sensitivity at all spatial frequencies tested (Table 1; Fig. 2). The strength of correlation increased with spatial frequency, peaking at 18 cpd (r = − 0.62; 95% CI, − 0.74 to − 0.60; p < 0.001). Correlation at other frequencies was similarly strong: 3 cpd (r = − 0.46; 95% CI, − 0.57 to − 0.35), 6 cpd (r = − 0.57; 95% CI, −0.65 to − 0.46), and 12 cpd (r = − 0.61; 95% CI − 0.69 to − 0.51).

Table 1.

Pearson correlation between MASI and contrast sensitivity (n = 200)

Spatial Frequency (cpd)	Pearson r	R²	p-value
3 cpd	–0.46	0.21	< 0.001
6 cpd	–0.57	0.32	< 0.001
12 cpd	–0.61	0.37	< 0.001
18 cpd	–0.62	0.39	< 0.001

Fig. 2.

Scatterplots of MASI versus contrast sensitivity (log CS) at 3, 6, 12, and 18 cpd (n = 200). Linear regression lines with 95% confidence bands are shown. MASI exhibited statistically significant inverse correlations with contrast sensitivity at all frequencies (r = −0.46 to − 0.62; p < 0.001), with the strongest correlation at 18 cpd (r = − 0.62, R²=0.39)

Linear regression: MASI as a predictor of contrast sensitivity

Linear regression models indicated MASI strongly predicted contrast sensitivity (Table 2). Specifically, each unit increase in MASI predicted a decrease in contrast sensitivity at 18 cpd by 0.018 log CS (β = − 0.018; 95% CI, − 0.021 to − 0.014; p < 0.001). Similar relationships were found at 3 cpd (β = − 0.008; 95% CI, − 0.011 to − 0.006), 6 cpd (β = − 0.012; 95% CI, − 0.015 to − 0.010), and 12 cpd (β = − 0.017; 95% CI, − 0.020 to − 0.014).

Table 2.

Linear regression: contrast sensitivity vs. MASI

Spatial Frequency (cpd)	Intercept	MASI Slope (β)	95% CI for β	R²	p-value
3 cpd	2.207	–0.008	–0.011 to − 0.006	0.21	< 0.001
6 cpd	2.211	–0.012	–0.015 to − 0.010	0.32	< 0.001
12 cpd	2.065	–0.017	–0.020 to − 0.014	0.37	< 0.001
18 cpd	1.769	–0.018	–0.021 to − 0.014	0.39	< 0.001

Residuals for all regression models were normally distributed with no heteroskedasticity or major outliers, as confirmed by Q–Q plots and residual-vs-fitted analyses.

Contrast sensitivity and visual acuity by MASI tertile (Table 3; Figs. 3 and 4)

Table 3.

Visual performance by MASI tertile

MASI Group	MASI (mean ± SD)	3 cpd	6 cpd	12 cpd	18 cpd	UDVA (logMAR)
Low MASI	21 ± 4	1.98 ± 0.20	1.90 ± 0.20	1.63 ± 0.24	1.30 ± 0.24	–0.12 ± 0.09
Mid MASI	31 ± 4	1.98 ± 0.16	1.89 ± 0.17	1.56 ± 0.28	1.21 ± 0.31	–0.13 ± 0.07
High MASI	49 ± 9	1.74 ± 0.27	1.53 ± 0.31	1.16 ± 0.39	0.86 ± 0.40	–0.05 ± 0.14

Fig. 3.

Mean contrast sensitivity (± 1 SD) across spatial frequencies for three MASI tertile groups: Low (< 25.92), Mid (25.92–39.22), and High (> 39.22). Statistically significant differences in contrast sensitivity were observed across all tertiles (p < 0.001, ANOVA with Tukey post hoc), with the largest differences at 12 and 18 cpd

Fig. 4.

Heatmap displaying pairwise Pearson correlation coefficients between MASI, logMAR uncorrected visual acuity (UDVA), and contrast sensitivity at 3, 6, 12, and 18 cpd. MASI demonstrated moderate to strong negative correlations with contrast sensitivity (r = −0.46 to − 0.62) and a weaker correlation with UDVA (r = + 0.41). Color intensity indicates correlation strength

Patients were divided into tertiles based on MASI values. Table 3 shows the mean MASI for each group together with the mean contrast sensitivity and UDVA.

MASI correlation with aberrations and optical parameters

Beyond its strong inverse relationship with contrast sensitivity, MASI also showed statistically significant correlations with other clinical and optical variables in the dataset (Table 4).

Table 4.

MASI correlation with aberrations and optical parameters

Variable	Pearson r	p-value
HOA RMS	0.37	< 0.001
Optical wavefront SPH	-0.13	0.064
Ocular wavefront CYL	0.08	0.25
SEQ	-0.12	0.10
UDVA logMAR	0.41	< 0.001

The strongest correlation was observed between MASI and HOA (r = + 0.37, p < 0.001), confirming that residual micro-aberrations captured by MASI are partially linked to structured wavefront distortion. However, this correlation was moderate, suggesting that MASI captures additional noise or irregularity beyond what is described by conventional HOA metrics.

MASI showed no statistically significant correlation with optical wavefront SPH (spherical power, r = − 0.13, p = 0.065), ocular wavefront CYL (cylinder, r = + 0.08, p = 0.25), or spherical equivalent (SEQ) (r = − 0.12, p = 0.10). These results confirm that MASI is not a surrogate for refractive accuracy and is only moderately associated with higher-order aberrations. This supports the idea that MASI captures a distinct component of optical degradation—likely related to residual micro-irregularity or surface noise—not explained by conventional wavefront metrics. Together, these findings suggest that MASI reflects a composite of subtle surface irregularity and wavefront complexity—distinct from refractive or low-order optical metrics.

Key findings:

• High MASI eyes had 0.44 log CS lower contrast sensitivity at 18 cpd compared to Low MASI eyes (p < 0.001).

• UDVA was slightly worse in High MASI eyes (–0.05 logMAR vs. − 0.12, p < 0.01).

Discussion

This study provides preliminary clinical validation of the Micro Aberration Scatter Index (MASI) as a fast, objective, and clinically meaningful proxy for visual quality following keratorefractive surgery. In a sample of 200 eyes, MASI demonstrated a strong, consistent inverse relationship with contrast sensitivity across spatial frequencies, especially at higher frequencies. MASI also moderately correlated with UDVA, supporting its relevance to functional visual outcomes beyond traditional high-contrast acuity.

MASI and contrast sensitivity: a strong link

The strongest correlations observed were between MASI and contrast sensitivity at 12 and 18 cpd (r = − 0.61 and − 0.62, respectively). These spatial frequencies represent finer visual detail and are particularly vulnerable to small-angle scatter and micro-irregularities—exactly the phenomena MASI is designed to capture. Regression models showed that a 10-unit increase in MASI predicts a 0.18 log CS drop in contrast sensitivity at 18 cpd, a clinically significant decline.

This aligns with clinical experience: patients with elevated postoperative scatter often report soft or hazy vision and struggle in low-contrast environments (e.g., night driving), even when Snellen acuity is normal. MASI quantifies this impact objectively and automatically from standard wavefront scans.

MASI correlation with aberrations and optical parameters

MASI showed a moderate correlation with higher-order aberrations (r = + 0.37), supporting its relevance as a scatter-sensitive metric partially linked to traditional wavefront complexity. MASI did not significantly correlate with ocular wavefront SPH, ocular wavefront CYL, or spherical equivalent, confirming that it reflects optical quality rather than residual refractive error. Because contrast sensitivity was best-corrected, the absence of correlation between MASI and sphere/cylinder/SEQ is stronger evidence that MASI is independent of residual refractive error. These findings support MASI as a distinct, complementary index in assessing postoperative vision quality.

Why MASI complements HOA RMS

Traditional HOA RMS metrics often fail to explain patient dissatisfaction after surgery. While they quantify systematic optical distortions like coma or spherical aberration, they miss irregular, high-frequency fluctuations in the wavefront—especially those from stromal healing, surface haze, or tear film instability [2].

MASI fills this gap by specifically isolating residual, high-frequency slope deviations. These micro-aberrations may not significantly increase total RMS error but can degrade retinal image contrast and scatter light across the photoreceptor array, reducing contrast sensitivity. Our results confirm this: MASI explained up to39% of the variance in contrast sensitivity at 18 cpd, far exceeding the explanatory power typically seen with HOA metrics in similar studies [1]. Although this represents a substantial proportion of the observed variability, approximately 61% of the variance remained unexplained, indicating that visual performance is influenced by additional factors beyond those captured by MASI, including neural processing, tear film dynamics, retinal function, and other optical characteristics.

Clinical Implications: Screening, Monitoring, and Triage

MASI’s integration into standard aberrometry makes it well-suited for everyday clinical use. Its advantages include:

• Zero additional test time (automatically computed)

• Applicability to any refractive platform

• Ability to track healing (e.g., post-PRK haze or KLEx interface quality)

• Patient counseling aid (e.g., explaining “20/20 unhappy” vision)

In our tertile analysis, eyes with High MASI (mean 49 ± 9) had contrast sensitivity at 18 cpd nearly 0.44 logCS worse than those with Low MASI (mean 21 ± 4). These patients would likely perceive meaningful degradation in night vision, contrast, and subjective clarity.

Thus, MASI could be used for:

• Flagging eyes at risk of poor visual quality despite normal acuity

• Guiding enhancements or prolonged steroid use

• Validating symptoms (e.g., glare, halos) with objective data

• Benchmarking surgical technique performance across platforms or energy settings

While MASI showed strong correlations with contrast sensitivity under standard photopic testing conditions, caution is warranted when extrapolating its findings to dim-light environments. The peripheral corneal zone, which contributes more prominently under mesopic or scotopic conditions, may introduce additional aberrations or scatter not captured in the current analysis. Therefore, although MASI may help flag eyes at risk of functional complaints, its application to night vision symptoms should be considered hypothesis-generating rather than conclusive.

Importantly, repeatability declined progressively across MASI tertiles, with higher MASI groups showing increased within-subject variability. This supports the notion that increased scatter reflects a more unstable or irregular optical system. Thus, MASI not only quantifies the magnitude of optical scatter but also reflects optical stability, reinforcing its role as a reliable diagnostic anchor.

MASI is not just a corneal metric

While MASI is highly sensitive to corneal irregularities, it reflects total ocular wavefront micro-aberration, including contributions from the tear film and lens. In our dataset of young, phakic patients (mean age 28), lenticular impact was minimal. However, in older populations, MASI might help flag early dysfunctional lens syndrome (DLS) or lens vacuoles before they manifest on slit-lamp exam or OSI.

Tear film instability can transiently increase MASI. We minimized this effect by ensuring all eyes had stable tear films at testing. Nonetheless, MASI’s sensitivity to subtle optical instability—including potential contributions from tear film irregularity—suggests potential utility in the early detection of dry eye disease. Similar to the diagnostic role of OSI [10, 11], MASI could serve as a non-invasive tool in evaluating subclinical or early-stage dry eye. Future studies correlating MASI with tear film breakup time, osmolarity, or non-invasive tear stability metrics are warranted to validate this application.

Positioning MASI among optical quality metrics

MASI should be viewed as complementary to, rather than a replacement for, established objective measures of optical quality. Different metrics quantify different aspects of optical performance and therefore provide distinct but potentially complementary information.

The Visual Strehl ratio (VSX) is a neurally weighted image-quality metric derived from wavefront measurements and is closely linked to retinal image sharpness and visual acuity. Because VSX is based on reconstructed wavefront error and neural weighting functions, it primarily reflects the impact of low-to-moderate spatial-frequency aberrations on image quality [14–16]. In contrast, MASI is derived from residual high-frequency wavefront slope variations after subtraction of conventional Zernike modes and is intended to characterize micro-aberration scatter and optical irregularity at a finer spatial scale.

Similarly, the Objective Scatter Index (OSI) quantifies total forward intraocular scatter using a double-pass imaging system and is particularly sensitive to scatter arising from the crystalline lens and other ocular media [7]. While OSI provides a global assessment of forward scatter, MASI is derived from high-density wavefront measurements and reflects residual micro-irregularities within the ocular optical system. Because the two metrics are based on different physical principles, they should be regarded as complementary rather than interchangeable.

Conventional HOA RMS metrics quantify structured optical aberrations represented by Zernike polynomials and are valuable for characterizing low- and mid-spatial-frequency wavefront distortions. However, by design, they are less sensitive to stochastic high-frequency residual variations that are not well represented by Zernike decomposition. MASI was developed to characterize these residual wavefront features, although the clinical significance of this additional information remains to be fully established.

Importantly, direct comparisons between MASI and established metrics such as OSI and VSX were not performed in the present study. Consequently, the current data do not support claims of superiority or greater comprehensiveness. Future studies directly comparing MASI with OSI, VSX, HOA RMS, and other objective optical quality metrics will be necessary to determine the incremental clinical value of MASI and define its role within the broader landscape of visual quality assessment.

Real-world relevance

A 0.44 log CS reduction in contrast sensitivity at 18 cpd—as observed between high and low MASI tertiles—represents a clinically meaningful degradation. Such a drop is consistent with patient complaints of difficulty reading in dim environments, problems with night driving, and overall hazy or “soft” vision, despite achieving excellent UDVA.

One important consideration is that MASI is calculated from wavefront measurements based on monochromatic (typically infrared) light. This differs from real-world visual tasks, which involve polychromatic light under varied spectral and contrast conditions. Although MASI correlates strongly with contrast sensitivity, its performance under polychromatic conditions may be influenced by wavelength-dependent scatter, chromatic aberration, and retinal processing. Future refinements to MASI may consider spectral weighting or correction factors to enhance its applicability to real-world visual perception.

Limitations

Device- and site-related limitations

The study was conducted at a single center using a single OSIRIS-T pyramidal aberrometer operated by a highly trained team. While intra-operator repeatability was high, these findings may not generalize to other devices or less experienced clinical settings. Broader validation using different aberrometry platforms and independent centers is essential.

MASI is a novel, derived metric without previously published normative values, thresholds, or multicenter reference ranges. Although repeatability in our center was high—with consistent intra-patient values across repeated measurements—the analysis was conducted using a single OSIRIS-T aberrometer operated by a highly trained team. Broader external validation is required.

Physiological and clinical scope

MASI is computed from a 3 mm central pupil zone, yet the average mesopic pupil diameter in our cohort was 5.0 ± 0.8 mm. As a result, MASI may underrepresent peripheral aberrations and fail to detect scatter that becomes more pronounced under scotopic conditions—limiting its sensitivity for night vision complaints [17].

Although MASI strongly correlated with contrast sensitivity, we did not collect PROMs related to visual quality, such as glare, halos, or night driving difficulty. Including PROMs in future studies would strengthen the clinical interpretation of MASI and enable correlation with subjective visual experiences.

Our study population had a mean age of 28 years, primarily reflecting younger adults undergoing refractive surgery for myopia, and thus may not directly generalize to older patient populations or different refractive errors. Although KLEx, LASIK, and PRK were all included and masked from examiners performing MASI and contrast sensitivity testing, the distribution (KLEx: n = 100, LASIK: n = 50, PRK: n = 50) should be noted. MASI’s performance in older cohorts or those with different surgical profiles warrants further investigation. Additionally, since age is known to independently influence contrast sensitivity, future research should specifically evaluate how age interacts with MASI values. However, the primary intent of MASI is not to replace contrast sensitivity testing but rather to provide clinicians with a rapid, objective measure to enhance clinical decision-making without necessitating complex or prolonged tests.

Analytic constraints and future work

As mentioned earlier, direct comparison with established scatter metrics like OSI was not performed due to device unavailability (HD Analyzer). While OSI specifically quantifies intraocular scatter, MASI uniquely combines corneal micro-aberration scatter with wavefront irregularities. Although our device (OSIRIS-T) provides VSX, we used ocular wavefront metrics as proxies given their partial interchangeability for optical quality assessments based on expert consensus. However, direct comparison of MASI with VSX would still be valuable, and future studies should explicitly evaluate MASI against OSI and VSX to further validate its complementary role [15, 16].

MASI measurements were obtained at a single postoperative time point (30 days). As such, the results reflect a cross-sectional snapshot of optical quality and cannot capture dynamic healing processes or neuroadaptation over time. Longitudinal follow-up data are needed to characterize the temporal evolution of MASI and its relationship with visual recovery.

Finally, while MASI showed strong correlations with contrast sensitivity and uncorrected acuity, surgical type was not analysed in this report. Although multiple refractive procedures were included, the study was not powered for procedure-level comparisons, particularly within the PRK and LASIK subgroups. Consequently, potential procedure-specific differences in MASI could not be reliably evaluated and warrant investigation in dedicated future studies.

MASI shows strong potential as an objective and clinically practical measure of visual quality following keratorefractive surgery. In a dataset of 200 eyes, MASI demonstrated statistically significant inverse correlations with contrast sensitivity at all spatial frequencies, particularly at higher frequencies where optical degradation most affects patient perception.

Because MASI is automatically derived from standard wavefront aberrometry without additional testing time or operator input, it can be seamlessly integrated into clinical workflows. MASI captures residual micro-aberrations and optical scatter that are often missed by conventional metrics such as HOA RMS or OSI, yet are highly predictive of functional vision outcomes. It helps distinguish between eyes with similar high-contrast acuity but differing subjective visual clarity, offering clinicians a more nuanced understanding of visual performance.

MASI may serve as a practical adjunct to contrast sensitivity testing and could assist in postoperative monitoring and enhancement decisions. Its use as a benchmarking tool remains to be validated through broader studies across surgical types and patient populations.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

IG and DZR were responsible for conceptualization, methodology, data curation, formal analysis, and investigation. IG and SAM prepared the original draft, conducted review and editing, and prepared the visualizations. SAM, FV and DZR provided supervision and critical review.

Funding

No funding was received for this study.

Data availability

Data are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

Ethics approval was provided by the ethics committee of the Svjetlost Specialty Hospital for Ophthalmology, Eye Clinic, School of Medicine, University of Rijeka (EC-2024-MASI.1). This study was conducted in accordance with the tenets of the Declaration of Helsinki and written informed consent was obtained from all participants.

Consent for publication

Not applicable; all patient data were anonymized.

Competing interests

The authors declare no competing interests.