Comparison of Low- Versus High-Level Angle Kappa Compensation Strategies in SMILE Using the VisuMax 800: A Randomized Clinical Trial

The goal of this clinical trial is to compare two intraoperative kappa-angle compensation strategies during small incision lenticule extraction (SMILE) using the VisuMax 800 platform in patients with relatively large kappa-angle offsets.

Angle kappa represents the difference between the visual axis and the pupil center. In patients with a large offset, pupil-centered treatment may lead to decentration of the optical zone, which can induce higher-order aberrations and affect postoperative visual quality. The VisuMax 800 system allows intraoperative adjustment of the treatment center based on the measured offset, making different compensation strategies possible.

The main questions this study aims to answer are:

Does low kappa-angle compensation (50%) result in lower postoperative higher-order aberrations than high compensation (100%)? Does the compensation strategy influence visual acuity, refractive outcomes, contrast sensitivity, and subjective visual quality after SMILE? Are the measurements obtained from different diagnostic devices consistent when evaluating kappa-related parameters?

Researchers will compare SMILE with low compensation to SMILE with high compensation to determine which strategy provides better optical quality and visual performance.

Participants will:

Undergo SMILE surgery using either low or high kappa-angle compensation Complete scheduled postoperative examinations at 1 month, 3 months, and 6 months Receive measurements of visual acuity, refraction, higher-order aberrations, contrast sensitivity, and patient-reported visual outcomes Be monitored for safety outcomes and adverse events during follow-up

Study Overview

• Status: Not yet recruiting
• Conditions: Myopia; Refractive Ametropia
• Intervention / Treatment: Procedure: Apply low compensation to the Kappa angle.; Procedure: Apply high compensation to the Kappa angle.

Detailed Description

Angle kappa refers to the offset between the visual axis and the pupil center and is an important factor influencing the centration of the treatment zone during corneal refractive surgery. In patients with relatively large offsets, pupil-centered treatment may lead to decentered lenticule extraction, which can induce higher-order aberrations, especially coma, and may reduce postoperative visual quality.

Small incision lenticule extraction (SMILE) is a widely used corneal refractive procedure with good safety and predictability. The new-generation VisuMax 800 femtosecond laser system incorporates machine vision-assisted alignment, allowing the surgeon to adjust the treatment center according to the measured displacement between the corneal vertex and the pupil center. This enables intraoperative compensation based on the kappa-angle offset.

Previous studies in excimer laser surgery have evaluated different proportions of kappa-angle compensation, but prospective randomized studies in SMILE are limited, particularly in patients with relatively large offsets. In addition, the optimal amount of compensation during SMILE has not been clearly determined.

This study is a single-center, randomized, parallel-group clinical trial designed to compare two predefined compensation strategies during SMILE using the VisuMax 800 platform. Patients aged 18-40 years with stable myopia or myopic astigmatism and a kappa-related displacement of at least 0.20 mm will be enrolled and randomly assigned to one of two groups.

Participants in the low-compensation group will undergo SMILE with the treatment center shifted by 50% of the measured offset coordinates, while participants in the high-compensation group will undergo SMILE with 100% compensation of the measured offset. All surgeries will be performed using the same surgical protocol, and postoperative management will be identical in both groups.

Participants will undergo routine ophthalmic examinations before surgery and at scheduled follow-up visits after surgery. The primary evaluation will assess postoperative higher-order aberrations at 3 months. Secondary evaluations will include visual acuity, refractive outcomes, refractive stability, contrast sensitivity, patient-reported visual quality, optical zone centration, and safety outcomes. In addition, measurements obtained from different diagnostic devices will be compared to assess consistency.

The results of this study will help determine the optimal intraoperative compensation strategy for SMILE in patients with large kappa-angle offsets and may contribute to improving postoperative visual quality and refractive accuracy.

• Study Type: Interventional
• Enrollment (Estimated): 100
• Phase: Not Applicable

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

1. Age range: 18 - 40 years old. Patients with stable refractive status for >= 1 year, having myopia or myopic astigmatism, with refractive power within the range of SMILE procedure.
2. Normal corneal morphology and corneal thickness meeting the surgical requirements.
3. Best corrected visual acuity (CDVA) before surgery >= 1.0.
4. The kappa angle-related offset coordinates (x, y) obtained before surgery, with the absolute value of either |x| or |yl| of any eye >= 0.20mm.
5. Voluntary to undergo SMILE surgery on the VISUMAX800 platform, understanding the random grouping scheme and signing the informed consent form.

Exclusion Criteria:

1. History of prior ocular surgery, significant ocular trauma, or presence of corneal or ocular diseases that may affect the safety of refractive surgery.
2. Severe systemic diseases that may affect ocular healing or surgical safety.
3. Females who are pregnant or lactating.
4. Inability to cooperate as required to complete the procedure or follow-up visits.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: Double

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Active Comparator: Low-Level compensation group; Subjects underwent SMILE surgery on the VisuMax 800 platform. Preoperatively, decentration coordinates (x, y) were measured. While preserving the directional sign, the absolute value of the planned compensation was set to 50% of the measured decentration; the values were then rounded to the nearest increment permitted by the device input precision and entered into the system. All other surgical procedures, as well as postoperative medication and follow-up protocols, were performed according to a standardized regimen.	Procedure: Apply low compensation to the Kappa angle.; SMILE PRO is an upgraded version/procedural designation of SMILE (Small Incision Lenticule Extraction) performed on the ZEISS VisuMax 800 femtosecond laser platform. It is essentially an all-femtosecond corneal refractive procedure. Its basic principle is to use a femtosecond laser to create a lenticule within the corneal stroma and a small incision, and then remove the lenticule through the incision, thereby reshaping the corneal curvature to correct myopia and astigmatism. In the low-level compensation group, a low level of kappa-angle compensation was applied during the preoperative parameter input.
Experimental: High-Level compensation group; Subjects also underwent SMILE surgery on the VisuMax 800 platform. Decentration coordinates (x, y) were measured, and the theoretical compensation was set at 100% of the measured decentration. While preserving the directional sign, the measured values were entered directly into the device without any additional adjustment. Except for the different compensation strategy, all other surgical procedures and postoperative management were identical to those of the low-compensation group.	Procedure: Apply high compensation to the Kappa angle.; SMILE PRO is an upgraded version/procedural designation of SMILE (Small Incision Lenticule Extraction) performed on the ZEISS VisuMax 800 femtosecond laser platform. It remains, in essence, an all-femtosecond corneal refractive procedure. Its basic principle is to use a femtosecond laser to create a lenticule within the corneal stroma together with a small incision, and then remove the lenticule through that incision, thereby reshaping the corneal curvature to correct myopia and astigmatism. In the high-level compensation group, a high level of compensation for the kappa angle was applied during the preoperative parameter input.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Coma aberration	Coma aberration measured by wavefront aberrometry to evaluate the effect of intraoperative kappa-angle compensation on postoperative optical quality.	3 months after surgery

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Higher-order aberrations	Total higher-order aberrations, spherical aberration, and trefoil aberration measured using wavefront aberrometry under standardized conditions to evaluate postoperative optical quality.	1 month, 3 months, and 6 months after surgery
Optical zone decentration	Decentration of the optical zone measured using corneal topography by determining the displacement between the optical zone center and the corneal vertex.	3 months after surgery
Manifest refraction	Postoperative sphere, cylinder, and spherical equivalent measured by manifest refraction.	1 month, 3 months, and 6 months after surgery
Refractive accuracy	Proportion of eyes within predefined ranges of target refraction to evaluate refractive predictability.	1 month, 3 months, and 6 months after surgery
Refractive stability	Refractive stability will be assessed as the change in manifest spherical equivalent refraction between postoperative visits.	1 month to 6 months after surgery
Contrast sensitivity	Contrast sensitivity will be measured using a standardized contrast sensitivity testing system (CSV-1000, VectorVision, USA) at multiple spatial frequencies.	3 months after surgery
Quality of Vision (QoV) score	Visual symptoms will be assessed using the Quality of Vision (QoV) questionnaire, which evaluates the frequency, severity, and bothersome nature of visual disturbances. Scores are recorded on ordinal scales, with higher scores indicating worse visual quality.	3 months after surgery
Patient satisfaction score	Patient satisfaction will be assessed using a 5-point rating scale. Scores range from 1 to 5, with higher scores indicating greater satisfaction.	3 months after surgery
Adverse events	Incidence of intraoperative or postoperative complications, including loss of corrected distance visual acuity and other clinically significant events.	Up to 6 months after surgery
Visual Acuity	Binocular uncorrected distance visual acuity and corrected distance visual acuity measured using standardized visual acuity charts to evaluate visual performance and safety.	1 month, 3 months, and 6 months after surgery

Collaborators and Investigators

• Sponsor: Zhongshan Ophthalmic Center, Sun Yat-sen University

Publications and helpful links

General Publications

• McAlinden C, Pesudovs K, Moore JE. The development of an instrument to measure quality of vision: the Quality of Vision (QoV) questionnaire. Invest Ophthalmol Vis Sci. 2010 Nov;51(11):5537-45. doi: 10.1167/iovs.10-5341. Epub 2010 May 26.
• Doostparast A, Ghandhari M, Rastegar M, Khosronejad AH, Ghandhari M, Eslampoor A. Distribution and inter-device agreement of chord mu/alpha lengths and axes between Pentacam HR, Sirius, and IOLMaster 700. Photodiagnosis Photodyn Ther. 2025 Dec;56:105275. doi: 10.1016/j.pdpdt.2025.105275. Epub 2025 Nov 6.
• Shao T, Wang Y, Ng ALK, Chan TCY, Hao W, Zhang J, Cheng GPM, Jhanji V. The Effect of Intraoperative Angle Kappa Adjustment on Higher-Order Aberrations Before and After Small Incision Lenticule Extraction. Cornea. 2020 May;39(5):609-614. doi: 10.1097/ICO.0000000000002274.
• Lai X, Liu X, Zeng T, Huang Y, Yang X. Comparison of visual outcomes and optical aberrations after SMILE with intraoperative Kappa angle adjustments between small and large Kappa angles. Sci Rep. 2024 Jun 24;14(1):14551. doi: 10.1038/s41598-024-65366-w.
• Deng M, Zhou D, Li M, Shi Y, Sun L, Zou J, Ma X. Evaluation of corneal asymmetry and higher-order aberrations after small incision lenticule extraction for moderate and high myopia. Int Ophthalmol. 2025 Jun 1;45(1):218. doi: 10.1007/s10792-025-03576-5.
• Vingopoulos F, Zisimopoulos A, Kanellopoulos AJ. Comparison of effective corneal refractive centration to the visual axis: LASIK vs SMILE, a contralateral eye digitized comparison of the postoperative result. J Cataract Refract Surg. 2021 Dec 1;47(12):1511-1518. doi: 10.1097/j.jcrs.0000000000000687.
• Chang JS, Law AK, Ng JC, Chan VK. Comparison of refractive and visual outcomes with centration points 80% and 100% from pupil center toward the coaxially sighted corneal light reflex. J Cataract Refract Surg. 2016 Mar;42(3):412-9. doi: 10.1016/j.jcrs.2015.09.030.

Study record dates

Study Major Dates

• Study Start (Estimated): April 1, 2026
• Primary Completion (Estimated): May 30, 2027
• Study Completion (Estimated): May 30, 2027

Study Registration Dates

• First Submitted: March 21, 2026
• First Submitted That Met QC Criteria: March 31, 2026
• First Posted (Actual): April 6, 2026

Study Record Updates

• Last Update Posted (Actual): April 6, 2026
• Last Update Submitted That Met QC Criteria: March 31, 2026
• Last Verified: March 1, 2026

More Information

Terms related to this study

• Keywords: SMILE; Randomized Clinical Trial; myopia; Refractive Surgery; Angle Kappa; VisuMax 800
• Additional Relevant MeSH Terms: Eye Diseases; Refractive Errors; Myopia
• Other Study ID Numbers: IIT2025193

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No