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

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.

Inclusion criteria

• 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.
• Normal corneal morphology and corneal thickness meeting the surgical requirements.
• Best corrected visual acuity (CDVA) before surgery >= 1.0.
• 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.
• Voluntary to undergo SMILE surgery on the VISUMAX800 platform, understanding the random grouping scheme and signing the informed consent form.

Exclusion criteria

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